1 University of Limerick Design, Construction & Analysis of a Pulsejet Engine Prepared by: Thomas Naughton 0542717 Under the Supervision of: Dr. Patrick Frawley Final Year Report Submitted to the University of Limerick, March, 2010 Aeronautical Engineering I declare that this is my own work and that all contributions from other persons have been appropriately identified and acknowledged i Abstract This engineering reports the design construction and analysis of a pulsejet engine to achieve static thrust. An engine was designed using available theory. Following a delay due to ignition system problems, the completed engine was tested extensively in an attempt to achieve static thrust. A detailed analysis of petal valve vibration was carried out while attempting to get the engine to resonate. This included the experimental verification of a mathematical model. Several tests were carried out using different petal valve natural frequencies. The tests resulted in the engine being capable of achieving sustained resonance without the external supply of air for up to two minutes. Petal valve failure was determined to be the cause of the short running times. The operation of the engine was analysed using available thermodynamic models but these models were determined to be inaccurate for pulsejet cycle prediction. The pressure cycles within the pulsejet engine were obtained experimentally using a high-temperature pressure transducer. The resulting pressure/time plots were compared to other plots which were obtained from published literature. The plots were found to correlate well together with peak pressures measured in three very different sized engines being within 0.07 Bar of each other. ii Acknowledgements I would like to thank my supervisor, Dr. Patrick Frawley, for his support and guidance throughout the project. Without his support this project would not exist. I would like to thank the technical staff of the M&AE Department workshop for their help with building the project. Especially Mr. Patrick ODonnell, Mr. Ken Harris and Mr. Jim Caulfield. I would also like to thank the technicians of the Aeronautical Laboratory, Mr. Jim Ryan, Mr. John Cunningham and Mr. Adrian McEvoy for their help throughout testing. I would also like to thank the technical staff of the Electronic Engineering Department, Mr. John Bird and Mr. John Clifford for their help with ignition system. Finally, I would like to thank my family for their valuable support throughout the year. iii Table Of Contents 1. Introduction 1 1.1. Brief History 1 1.2. Operation 3 2. Objectives 5 3. Literature Review 6 3.1. Jet Design 7 3.2. Reed Valve Design 7 3.3. Thermodynamics 10 4. Conceptual Design 11 4.1. Valve System 12 4.2. Choice of Fuel 13 4.3. Fuel Delivery 14 4.4. Ignition System 16 4.5. Test Stand 17 5. Theory & Design 18 5.1. Jet Design 18 5.2. Petal Valve Vibration Frequency 23 5.3. Thermodynamic Analysis 28 5.4. Material Selection 30 6. Construction 36 6.1. Jet Body 36 6.2. Intake Diffuser 37 6.3. Valve Plates 38 6.4. Valve Retainer Plates 39 6.5. Petal Valve 40 6.6. Fuel Injection Nozzles 41 6.7. Test Stand 43 7. Testing & Troubleshooting 45 7.1. Ignition System Problems 45 7.2. Fuel Mixing 48 7.3. Valve Frequency Ratio Tuning 49 7.4. Valve Frequency High-Speed Camera Test 54 iv 7.5. Data Collection 55 8. Results 57 9. Discussion 59 9.1. Jet design 59 9.2. Petal Valve Vibration Theory 59 9.3. Valve Life 59 9.4. Valve Response to Engine Forcing Frequency 60 9.5. Ignition System 61 9.6. Thermodynamic Analysis 61 9.7. Pressure Cycle Visualisation 61 9.8. Exhaust Velocity Determination 62 10. Conclusion 63 References 64 Appendices Appendix A Engineering Drawings Appendix B Ignition Circuit Diagram Appendix C Electro-chemical Etching Process Appendix D Turn-it-in Originality Report Summary Appendix E Kistler Pressure Tranducer Data Sheets (CD) Appendix F Excel Spreadsheets (CD) v List of Figures Figure 1.1 Marconnet pulsating combustor [Reynst, 1961] .............................................. 1 Figure 1.2 V-1 flying bomb with Argus AS-014 pulsejet engine [museumofflight.org, 2010] ................................................................................................................................. 2 Figure 1.3 Ignition Stage ................................................................................................... 3 Figure 1.4 Combustion/Power Stage ................................................................................ 3 Figure 1.5 Intake Stage ..................................................................................................... 4 Figure 1.6 Compression/Re-ignition Stage ....................................................................... 4 Figure 3.1 Pressure-Time plot example for 50cm valved pulsejet engine [Ordon, 2006] 6 Figure 3.2 Tharratt's mechanical valve which was claimed to withstand 25 hrs operation at full thrust [Tharratt, 1965] ... 8 Figure 3.3 Cross-section of Standard Valve (left) and Low-loss Modified Valve (right) [Bressman, 1946] . 9 Figure 4.1 Argus AS-014 Grid Valve Layout [FZG-76 Gerte-Handbuch, 1944] ... 11 Figure 4.2 Aprilia RS125 Reed Valve Assembly . 12 Figure 4.3 Petal Valve ... 12 Figure 4.4 Normally Aspirated (left) and Injected (right) Fuel Delivery [aardvark.co.nz, 2009] .. 14 Figure 4.5 Sketch of Valve Head Design . 16 Figure 4.6 Piezoelectric Oven Igniter 17 Figure 4.7 CAD Model of Test Stand Used With Previous Project .. 17 Figure 5.1 Intake Orifice Design ... 22 Figure 5.2 Final Jet Body Dimensions .. 23 Figure 5.3 Petal Valve Geometry .. 24 Figure 5.4 Simplified Petal Valve Model .. 25 Figure 5.5 Petal Valve Geometry Split for Centroid Determination . 26 Figure 5.6 Points of Interest Within the Pulsejet Engine .. 28 Figure 5.7 Ideal pulsejet cycle [El-Sayed, 2008] .. 28 Figure 5.8 Pressure Cycle plot in AS-014 pulsejet engine [Bressman, 1946] ... 31 Figure 5.9 Mechanical Properties of Grade 43A Steel at Elevated Temperatures [Bailey, 2009] .. 33 vi Figure 6.1 Combustion Chamber and Flange 37 Figure 6.2 Intake Diffuser . 38 Figure 6.3 Anodised Valve Plates; Original (left) & Modified (right) .. 39 Figure 6.4 Valve Retainer Plate . 40 Figure 6.5 Electro-chemical Etching Apparatus 41 Figure 6.6 Fuel Injection Nozzles; Internal (top) & External (bottom) . 43 Figure 6.7 Engine Mounted on Test Stand 44 Figure 7.1 Final Ignition Circuit 47 Figure 7.2 Uneven Burning in the Combustion Chamber (left) & Burning With New Nozzle Fitted (right) .. 48 Figure 7.3 Valve Motion Sign Convention (left) & Simplified Valve Motion Plot (right) .51 Figure 7.4 0.010" Deformed Shim Steel Valve Following Engine Run .52 Figure 7.5 Impact and Fatigue Damage on a 0.006" Spring Steel Petal Valve . 53 Figure 7.6 High-Speed Camera Experiment Setup ... 55 Figure 7.7 Equipment Set Up For Pressure Data Collection . 56 Figure 8.1 55mm Position Pressure/Time Plot .. 57 Figure 8.2 1075mm Position Pressure/Time Plot .. 58 vii Nomenclature Symbol Description Units A Area m2 A Mean Cross Sectional Area m2 Ac Exhaust Area m2 Av Valve Area m2 CNC Computer Numerically Controlled - Cpc Specific Heat Capacity at Constant Pressure (cold) J/kg/K Cph Specific Heat Capacity at Constant Pressure (hot) J/kg/K Ch Specific Heat Capacity at Constant Volume (hot) J/kg/K Diameter m comb Combustion Chamber Diameter m c Exhaust Diameter m E Youngs Modulus of Elasticity GPa F Thrust N FEM Finite Element Method - I Second Moment of Area m4 ID Internal Diameter m I Length m Icomb Combustion Chamber Length m Icng Engine Length m H Mach Number - NACA National Advisory Committee for Aeronautics - OD Outside Diameter m P Static Pressure Pa P0 Stagnation Pressure Pa Energy Density of Fuel J/kg R Universal Gas Constant J/kg/K I Static Temperature K I0 Stagnation Temperature K u Strain Energy J uc Exhaust Velocity m/s viii un Inlet Velocity m/s u] Jet Velocity m/s I Volume m3 Icng Engine Volume m3 b Breadth of Beam m c0 Local Speed of Sound in Air m/s Frequency Hz Heat Added Per Unit Mass J/kg k Spring Stiffness N/m m Mass of Lumped Mass kg mbcum Mass of Beam kg m u Mass Air Flow kg/s m] Mass Fuel Flow kg/s r Radius m r Internal Radius m ro External Radius m t Thickness m y Vertical Distance to Centroid m yc Specific Heat Ratio (cold) - yh Specific Heat Ratio (hot) - pb Burner Efficiency - pd Diffuser Efficiency - p Density kg/m3 o Normal Stress Pa oh Hoop Stress Pa 1. 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At the same time the Argus engine company were developing valveless pulsejet engines. Schmidt joined the Argus company and, in 1939, together they successfully developed a valved pulsejet design which was later used to power the unmanned V-1 flying bomb, (figure 1.2). Figure 1.2 V-1 flying bomb with Argus AS-014 pulsejet engine [museumofflight.org, 2010] Following the end of World War II, many of the Argus engines were captured by the US and Russia. The captured engines were reverse engineered and analysed extensively in an attempt to create a viable propulsive device for use on aircraft. The majority of this testing has been documented and published by NACA. The main concern regarding the pulsejet was the operating life of the reed valves in the front of the engine. Efforts were made to increase the operating life but these were quickly overtaken by the development of the turbojet engine. The turbo-jet engine offered increased reliability and fuel efficiency and the pulsejet was largely forgotten as a source of aircraft propulsion. Today, small pulsejet engines still find use as radio control model powerplants. This is mainly due to their simplicity and low cost when compared to a model turbojet of the same size. Pulsejets have also paved the way for Pulse Detonation Engine (PDE) technology which is a major research interest among aircraft manufacturers as a high-tech, fuel efficient form of propulsion. 3 1.2. Operation The operation of a pulsejet engine is similar to that of a modern reciprocating engine. It operates in defined cycles which draw in air/fuel mixture, compress it, ignite it and exhaust it in stages before the cycle repeats itself. The following figures 1.3 1.6 are used to explain the cycles more clearly. The first stage begins with the ignition of a fuel/air charge, figure 1.3. Ignition is provided by a spark plug during start-up and by residual combustion during normal operation. Figure 1.3 Ignition Stage The ignited fuel/air mix expands rapidly, increasing the pressure within the engine to greater than atmospheric pressure. This forces the spring valves shut which then forces the expanding gases to exit rapidly through the tailpipe producing thrust, (figure 1.4). Figure 1.4 Combustion/Power Stage Due to the Kadenacy effect, a partial vacuum is formed behind the rapidly escaping exhaust gases. The pressure at the front of the engine is now lower than atmospheric pressure. The pressure differential across the reed valves causes them to open and draws fresh fuel/air mixture into the engine, (figure 1.5). At the same time, the pressure difference within the engine slows down the momentum of the small proportion of exhaust gases which have not yet exited the tailpipe and draws them back up towards the front of the engine. 4 Figure 1.5 Intake Stage The momentum of the small piston of burning exhaust gases which has been sucked back into the engine helps to compress the fresh fuel/air charge and ignite it (figure 1.6) and the cycle repeats itself. The cycle repeats itself 40-250 times per second depending on the size and length of the engine. Figure 1.6 Compression/Re-ignition Stage The reed valves are the only moving part of the engine. The operating life of these valves can vary from 1-2 minutes up to several hours depending on factors such as valve material, valve construction and valve natural frequency. These factors are discussed in more detail in section 3.2. Partial Vacuum5 2. Objectives The main aims of the project were as follows: To design and build a working pulsejet engine to provide static thrust To analyse the operation of the engine both theoretically and experimentally so as to gain a better understanding of the principles behind the operation of these engines. To investigate the pressure cycles within the engine during operation. 3. Liter The vast between 1pulsejets fperiod wer Much resepart of Pustill ongoithis projecIn recent Universityconductedforms of ithese theswhich wapressure cfrom a preobtained f rature Revmajority o1944 and 1for aircraft re studied aearch on puulse Detonaing, most ofct. years, somy, USA, und tests on dinstrumentases were maas to be buicycle plots wessure transfrom the 50cFigure 3.1 Prview of available1970. This propulsionas part of theulsating comation Enginf this informe work hasnder the sudifferent sization to collainly valvelilt for this within the ensducer test ocm valved pressure-Time pe researchis mainly n after this e research fmbustion hane (PDE) demation is cls been carriupervision ozed pulsejetlect data anless designsproject. Hongines whicon the comppulsejet is splot example forregarding due to thetime. Manyfor this projas been carevelopmentassified andied out by of Dr Willts ranging fnd analyse ts and were owever, thech could be pleted enginhown in figr 50cm valved ppulsejet ene lack of cy research pect. rried out sint. However,d could not students ofliam L. Rofrom 8cm ttheir operatconsiderabey did provcompared wne. One of tgure 3.1. pulsejet enginengines was commercial papers fromnce the earl, since the be accessedf North Caroberts. Theto 50cm ustion. The jebly smaller vide good ewith the plothe pressure e [Ordon, 2006]6conductedinterest inm that timely 1980s asresearch isd for use inrolina Statese studentsing variousets tested inthat the jetxamples ofots obtainede-time plots] 6 d n e s s n e s s n t f d s 7 An additional source of information for the project was a final year report submitted to the University of Limerick by David Curran in 2004. This report details the construction and testing of a valved pulsejet of similar design to the engine constructed during the course of this project. Although the engine which was constructed in 2004 was not able to self sustain without an external source of air, the recommendations for future work given in the report were considered and referred to during the course of the design work. 3.1. Jet design The most comprehensive paper found on pulsejet design was C.E. Tharratts The Propulsive Duct. These were published as a series of articles in a journal entitled Aircraft Engineering and Aerospace Technology between 1965 and 1966 while the author was involved in research for the Chrysler Space Division, New Orleans. In these articles, Tharratt attempted to produce a comprehensible theoretical approach to pulsejet design and thermodynamic analysis. The first article proposed three basic equations which one could use to successfully determine the basic dimensions of a pulsejet tube as well as a theoretical analysis of the pulsejet thermodynamic cycle. Several reports were published by Cornell Aeronautical Laboratory in the late 1940s as part of the Project Squid experiment carried out for the United States Navy. These reports were studied but much of their content was decided to be unnecessarily detailed or irrelevant for the purposes of this project. Some content regarding reed valve design was used and is detailed in section 3.2. 3.2. Reed Valve Design Tharratts second article in The Propulsive Duct provides a brief overview of valve design. Mechanical spring valves such as those used in the Argus engine and aerodynamic valves used in all valveless designs are discussed in detail. Tharratt has claimed that he developed a mechanical spring valve to withstand 25hr. at full thrust, including several continuous runs of 7hr. duration. (Figure 3.2) This operating life is much higher than those experienced by NACA during their tests on captured Argus 8 engines. Tharratt provides an image of this valve in the article but does not include further details regarding its design or operating characteristics. Figure 3.2 Tharratt's mechanical valve which was claimed to withstand 25 hrs operation at full thrust [Tharratt, 1965] A report published by Cornell Aeronautical Laboratory in 1947 entitled 4x6 Pulsejet Engine Project contains a section in which reed valve material is discussed. The report tested the performance of a pulsejet engine using reed valves of varying thickness and materials and discussed the results briefly. According to this report, annealed spring steel or soft steel reeds are superior to tempered and polished spring steel reeds for longevity. The tests also showed that heavier reeds, in general, showed a longer life than thinner reeds although operation of the jet was more difficult to start and resonance of the jet was more easily upset when using heavy reeds than with the lighter reeds. These observations were taken into account when choosing the appropriate valve material for the jet. Two wartime reports published by NACA bear particular relevance to reed valve design. The first of these reports, written by Manganiello, Valerino & Breisch and published in 1945, attempts to solve the issue of poor reed valve operating life which had been observed during earlier sea-level performance tests of a 22-inch pulsejet. The average valve life was reported to be 30 minutes before the valve tips were damaged by the repeated impact with the valve grid and a severe loss of thrust was observed as a result. The authors attempted to extend the life of the valves on the same engine by coating the valve grid with a thin layer of neoprene. The reasoning behind the neoprene coating was to cushionon the valsignificantvalves wabroken ofvalves wegrid showgrid. The due to the The seconlow-loss auniform 0attempt tothick sprinthe two vasimulated although thigh-speedjet. The authe over-amade thatstandard v Figure n the impactlve tips. Folt effect on vas visible aff near the ere beginninwed a signifionly disadvreduced intnd report, wair valve on0.010 thicko improve png steel rivealves can bram pressuthe modified performanuthor concluall performat the life valve. 3.3 Cross-sectit of the valvllowing testvalve life. and after rivet holesng to split aicant improvvantage obtake area afwritten by Bn the performk reed valvperformanceeted to a lige seen in fiures to simued valves shnce of the muded that tance of theof the modion of Standardves on the vts, the reporAfter 51.6 163.6 minus, evidentlyand fray nevement on served durifter the addiBressman pumance of a ves were ree. The comghter strip ofigure 3.3. Tulate flight howed an imodified jetthe modifice engine. (dified valved Valve (left) anvalve grid art concludeminutes ofutes of opery due to fatear the trailvalve life cing the test ition of the nublished in 22-inch pueplaced by mposite valvof 0.006 thThe engine speeds of 0ncrease in t was slightcation result(Bressman, e was consnd Low-loss Mond thereby d that the nf operation nration, one tigue in fleling edgescompared towas a sligneoprene co1946, invesulsejet. In tha composives consistehick spring swas tested 0-330mph. Tthrust at lotly lower thated in only 1946) The iderably shodified Valve (rreduce impneoprene cono deterioravalve was exure, and . The modo the unmodght reductiooating. stigates thehis report, thite valve deed of a stripsteel. A comat sea leveThe tests shower flight han that of tha negligiblee observatiohorter than (right) [Bressm9pact stressesoating had aation of thecompletelythree otherdified valvedified valveon in powere effect of ahe standardesign in anp of 0.015mparison ofl at varioushowed that,speeds, thehe standarde change inon was alsothat of thean, 1946] 9 s a e y r e e r a d n f s , e d n o e 10 3.3. Thermodynamics The pulsating nature of combustion in a pulsejet engine makes it very complicated to analyse as the processes are time dependant. Due to the lack of interest in pulsejet engines as source of aircraft propulsion, not much research has been done to accurately predict the processes within them. Therefore, very little thermodynamic analysis of a pulsejet engine can be found in modern literature. The most modern analysis which was found was published as a small section in Ahmed F. El-Sayeds Aircraft Propulsion and Gas Turbine Engines. The analysis assumes that combustion takes place at a constant volume process and that the exhaust gases expand isentropically in the tailpipe. In reality, combustion in a pulsejet engine is neither a constant-volume nor a constant-pressure process and, since most pulsejets glow red-hot during operation, the expansion of exhaust gases cannot be accurately modelled as isentropic. However, the analysis could provide an estimate of the pressure and temperature conditions within the engine early on in the design process. Another source of theoretical analysis was found in Jet Propulsion, a reference text prepared by the Guggenheim Aeronautical Laboratory for the Air Technical Service Command and published in 1946. This text also makes similar assumptions about the behaviour of gases within the jet as those made in El-Sayeds text. These assumptions are bound to result in inaccuracies in calculations but were nevertheless used early on in the design process to provide a rough estimate of operating conditions in the engine. This text uses a different form of equation that El-Sayed to model the heat addition during combustion. Both analyses would be carried out and their accuracy determined from experimental results. 11 4. Conceptual Design This section outlines the process used to determine the final layout of the project jet, the choice of fuel, ignition system and test stand. 4.1. Valve System The two most common valve systems found on existing pulsejet engines are grid valves and petal valves. Grid valves are the most common type of valve used for larger engines producing more than 100N thrust. This is because they provide the least amount of intake flow resistance and they provide the most flexible layout on these bigger engines. Bigger engines require a larger intake area and it is much easier and more reliable to use several grid type valve assemblies to make up the required intake area than to design one extremely large petal valve. The reed valves in a grid layout are usually single valves for each intake orifice or sometimes grouped together so that one valve covers three or four orifices. This method of assembly is much more practical for engine maintenance. This way, if one valve fails, it is only necessary to replace that one valve or, at the most, a group of three or four. If one valve fails in a petal system, the entire set must be replaced. The main disadvantage with grid valve systems is their complexity. The grid valve systems used in large engines such as the Argus AS-014 are incredibly complex. (Figure 4.1) Figure 4.1 Argus AS-014 Grid Valve Layout [FZG-76 Gerte-Handbuch, 1944] 12 However, the grid valve systems used on smaller engines look more like the reed valve assemblies used at the crankcase inlet in modern two stroke engines. An example of the reed valve assembly used in an Aprilia RS125 engine is shown in figure 4.2. Figure 4.2 Aprilia RS125 Reed Valve Assembly Petal valve systems, on the other hand, are much simpler to design and construct. They consist of a flat plate which covers the front of the engine in which a radial pattern of holes is machined. A single spring steel valve shaped like a flower (figure 4.3) covers all the holes. A circular, curved, steel disk called the valve retainer is bolted to the valve plate behind the petal valve to limit the distance which the valves can flex when open. Figure 4.3 Petal Valve 13 The biggest disadvantage of the petal valve system is its inefficiency. Because the valve is placed perpendicular to the incoming airflow, they produce a lot of resistance. This limits their use to smaller pulsejets. Also, if one petal of the valve fails, the entire valve must be replaced. Considering both options, it was chosen to use a petal valve system for the project jet. This type of system would allow for quicker manufacturing and would also help incorporate a central fuel delivery point as outlined in section 4.3. 4.2. Choice of Fuel One of the major advantages of pulsejet engines is their ability to run on most commercially available fuels. Many small pulsejets used for model aircraft propulsion are run on liquid fuels such as methanol or nitro-methane. These fuels may be attractive for this purpose as they have a high energy density and good flammability range in air. However, nitro-methane is too expensive to be used in a larger scale engine as the fuel consumption would make it costly to run. Methanol has a very high flammability range but also has the disadvantage of burning with a clear flame. This could prove to be a safety hazard in a laboratory environment. Diesel or kerosene fuels are also a good alternative. These fuels are cheap compared to other alternatives but they do cause problems during cold starting. These fuels need to be vaporised prior to injection which requires the use of a heat exchanger coil. Engines using these fuels are usually started on a more flammable fuel and then switched over to diesel or kerosene when the engine has reached operating temperature. The use of these fuels has the added complexity of having a secondary fuel system for the starting fuel. Ordinary low-octane petrol has been used with some success in pulsejet engines. Petrol is easily ignited using a spark plug which eliminates the need for a secondary fuel system. It is also cheap and readily available. However it does need to be vaporised before combustion which makes it unsuitable for direct injection into the combustion chamber unless a heat exchanger coil is used. 14 The other disadvantage with using liquid fuels is the need for a fuel pump to provide the correct fuel pressure. This adds complexity to the project. The use of a fuel pump can be avoided if the engine is designed to be naturally aspirated. This method has the drawback of not being throttle-able as outlined in section 4.3. The problem with fuel pressure can be overcome if a gaseous fuel such as propane or butane is used. The gas would be fed from the cylinder already under pressure and the fuel flow could be regulated using a gas regulator at the cylinder exit. The use of a gaseous fuel would also facilitate direct injection into the combustion chamber without the need for a vaporiser. Although the gas cylinder is bulkier and heavier than a similar liquid fuel tank, this was considered to be unimportant as the engine was to be a purely static engine. It was decided to use propane as a fuel due to its high energy density (50 MJ/kg), ease of cold starting by spark plug, the elimination of a fuel pump system and relatively low cost. 4.3. Fuel Delivery Fuel delivery to the combustion chamber can either be normally aspirated or injected. (Figure 4.4) Figure 4.4 Normally Aspirated (left) and Injected (right) Fuel Delivery [aardvark.co.nz, 2009] A normally aspirated engine operates on much the same principle as the carburettor in a car or motorcycle. The atomiser is placed in a venturi in the intake and fuel is drawn 15 from it as high-speed air passes through the venturi. Aspirated engines are simple in construction but aspiration does pose some restrictions. Fuel flow is very dependent on the vertical placement of the fuel tank in relation to the atomiser. Aspirated engines are not throttle-able as there is no method for varying fuel flow. The venturi must be properly designed to produce the required pressure difference across the fuel system so fuel can flow. Fuel injection solves the problems associated with aspiration. Although an injection nozzle is more complex to machine, it was decided to inject the fuel in the project engine. Injection would allow throttling of the engine and would also allow higher flexibility of fuel use. It was decided to inject the fuel directly into the combustion chamber behind the valve retainer plate. A second retainer plate would be placed on the combustion chamber side of the injection nozzle. There were several reasons behind choosing this setup. The different valve retainer plates could be machined with different radii of curvature. The effect of different retainer plates on valve life could then be investigated simply by swapping them around. The channel created between the two retainer plates would guide the fuel out towards the point where the incoming air is moving over the tips of the retainer plates at a higher velocity and this would aid fuel mixing. The second retainer plate would create a secondary barrier between the valves and the hot combustion gases and would help keep the valves cooler during operation. The heat absorbed by the second retainer plate during combustion would help to preheat the fuel as it passed between the plates and this would help increase the efficiency of combustion. The fuel injector nozzle would also double as the central bolt to clamp the entire valve head assembly together. A sketch of the assembly is shown in figure 4.5. 16 Figure 4.5 Sketch of Valve Head Design 4.4. Ignition System The ignition system in a pulsejet engine is only required for starting to ignite the first charge of fuel/air mixture. After the jet has achieved successful ignition and is running correctly, the ignition system is no longer needed. The easiest method of ignition is through a spark plug situated in the wall of the combustion chamber. Normally a spark is created across the plug gap through the excitation of an induction coil and then switching off the current to the main coil which causes the magnetic field in the coil to collapse rapidly. The change in magnetic field around a secondary coil induces a very high voltage across the coil which then jumps the plug gap in the form of a spark. However, instead of using an automotive coil to produce the spark, it was decided to use a piezoelectric igniter from a gas oven. (Figure 4.6) This was an extremely simple method. It required a push-button to be pressed repeatedly until the engine fired but it was lightweight, cheap and it did not require any additional equipment like a battery and separate switch. Air Fuel Valve Retainer Plate 17 Figure 4.6 Piezoelectric Oven Igniter 4.5. Test Stand The test stand needed to fulfil the following requirements: Securely support the jet during tests Allow for the measurement of thrust Support all necessary ancillary equipment such as ignition system and measuring devices. Be mobile enough to allow easy setup for tests. Since the project engine was to be approximately the same size as the jet which a previous student built, it was decided to use the same test stand design for this project also. A CAD drawing of the test stand is shown in figure 4.7. Figure 4.7 CAD Model of Test Stand Used With Previous Project Push Button Earth Connection Electrode 18 5. Theory & Design This section details the theory used in the design, analysis and troubleshooting of the project engine. Due to safety concerns, it was decided that the engine in this project should produce no more than 90N thrust. This also corresponded with the thrust limitations associated with a petal valve design. 5.1. Jet Design The theory governing the design of the pulsejet engine in this project was adapted from C.E. Tharratts The Propulsive Duct. Tharratt developed his equations in the 1960s with the imperial system of units in mind. The equations were modified to work with SI units before being used to design the project engine. 5.1.1. Tailpipe Tharratt proposed the following basic equation governed the basic design of the pulsejet engine duct: II, = u.uuS16F 5.1Where: V = Engine Volume (cu. ft.) L = Effective acoustic length of engine (ft.) F = Thrust (lbf) Manipulating this equation to take SI unit inputs produces equation 5.2: II, = u.uuuu66F 5.2 The simplest form of pulsejet is simply a straight tube of constant cross-section. It was decided to use this as a starting point. 19 Since for a straight pipe, I = AI If this relationship is substituted into equation 5.2 and simplified, then a direct relationship between thrust and cross-sectional area is established. This cross-section area will be used as the tailpipe area and will be referred to as Ae (exhaust area) from here on. Ac = u.uuuu66F 5.3 It was decided to make use of standard seamless pipe sizes available on the market to make the tailpipe. This would reduce the complexity of having a long welded seam running the entire length of the pipe. Inputting the maximum desired thrust of 90N into equation 5.3 returned a tailpipe diameter of 87mm. The next smallest seamless mild steel pipe available on the market was 3 Sch40 pipe. This gave an internal diameter of 78mm. Using 3 Sch40 pipe as the basis for the design, the expected thrust was calculated by substituting the area of the pipe back into equation 5.3. The expected thrust returned was 72.6N. The total length of a pulsejet engine is what determines the frequency at which it operates according to another of Tharratts basic equations, 5.4. = c04I 5.4 There is still quite a bit of debate regarding the correct operating frequency for a given size of jet. Therefore it was decided to look at some existing designs, their operating frequencies and their length to diameter (L/D) ratios to determine a suitable length for this engine. The known properties of some existing pulsejet designs are shown in table 5.1. 20 Table 5.1 Known Pulsejet Properties Engine Static Thrust Output (N) Frequency (Hz) L/D ratio Argus AS-014 2,200 46 9.6 Dynajet 20 260 15 By assessing the above data and considering the intended thrust output of the project engine, it was determined that an L/D ratio of 14 would be a good starting point for the engine. This would allow trimming of the tailpipe later on during testing if needed. Using this ratio and the internal diameter of 3 Sch40 pipe, a total engine length of 1.1m was calculated. This length was substituted into equation 5.4. The operating temperature of the engine was estimated to be 1000K approx. = yRI4I 1.S6 287 1uuu4 1.1 142Ez This frequency falls within the expected range for a pulsejet of this size. 5.1.2. Valve Plate Another important relationship which Tharratt developed was that which related the intake valve area to the exhaust area. He proposed that: A = u.2SAc 5.5This equation does not take into account the inefficiencies associated with different valve layouts. It is generally assumed that a petal valve layout has an efficiency of 0.5. Therefore the equation must be modified to allow for this. To make calculation simpler, equation 5.5 can be modified to allow for the efficiency factor and to take an input of tailpipe diameter rather than area: A pctuI = u.11Snc2 5.6 A pctuI = 22uSmm2 21 A combustion chamber is not necessary in a pulsejet engine. However, due to the layout of a petal valve system, it is usually necessary to include a wider section which resembles a combustion chamber at the front of the tailpipe. This wider section will be referred to as a combustion chamber for convenience. The valve plate layout must be designed before the dimensions of this section can be determined. The valve plate was designed in ProEngineer by observing the following criteria and attempting meet the required valve area while keeping the outer diameter of the orifices as low as possible. It was desired to keep the number of petals in the valve as low as possible so that the probability of failure due to fatigue could be kept low. It was also observed by studying previous designs that the maximum diameter intake hole was 12mm to reduce deformation of the valve during the positive pressure cycle of the engine. To try and solve this, the valve orifices had to have a minimum distance of 12mm in one direction. The valve plate needed to allow for 2mm valve overlap minimum around each orifice. Due to machining constraints, the smallest radius included in the design could be no smaller than 3mm. The final design, shown in figure 5.1, had 10 intake orifices and the outer diameter of the intake orifice ring was 90mm. 5.1 To prevenfrom beinwhich wouand the codetermine During histhe intake was decidof fresh fu 1.3. Combunt the flow ong overly reuld allow foombustion cd for the cos research inphase, a puded to makeuel/air mixtuFustion Chamof air throuestricted, thor an area ochamber. Uombustion cnto pulsejetulsejet drawe the combuure. Figure 5.1 Intakmber ugh the intakhe combustiof twice the Using this guchamber. t design in tws in 15%-2ustion chamIComb =ke Orifice Desike holes andion chambevalve area tuideline, anthe 1930s, P20% of its vmber big eno= u.2Icng ign d over the tr was givento exist betwinternal diaPaul Schmidolume in frough to acc tips of the vn an internween the vaameter of 1dt observed resh fuel/aircommodate 22valve petalsal diameteralve orifices117mm wasthat duringr mixture. Itthis charge2 s r s s g t e Using equangle wasmachining The final which was 5.2. Pe During finvalves to ta pulsejet is the fvalves. Tliterature. In order tmodel neeis essentiauation 5.7, s included g constraintdesign for ts used to boetal Valve Vnal testing the pressureengine. Thefrequency oThis is a det o determineeded to be fally a cantilea combustias a reduces. the jet bodyolt on the vaFiguVibration Fof the engie oscillatione response iof the enginail of desige the naturafound to simever beam oIcomb =ion chambeer to tailpipy is shown ialve head asure 5.2 Final JFrequencyine, it becans within this a functionne and ngn which seal frequencmplify the cof varying cu.2c2Icngcomb2er length ope diameterin figure 5.2ssembly andJet Body Dimename apparee engine is n of the drivis the naturems to havy of vibraticomplex shacross-sectiong f 97mm wr. This ang2. This drawd diffuser tonsions nt that the critical to thving frequenral frequence been largion of the pape of the vn. (Figure 5was determingle was detewing includo the front oresponse ohe correct oency ratio, ncy of vibragely overloopetal valvesvalves. The 5.3) 23 5.7ned. A 30ermined bydes a flangeof the jet. of the petaloperation of/n, whereation of theoked in pasts a suitablepetal valve3 7 y e l f e e t e e 24 Figure 5.3 Petal Valve Geometry According to Singiresu S. Raos text, Mechanical Vibrations, kcq = SEII3 Where I is the second moment of area and for a simple beam: I = bt312 Therefore: kcq = Ebt34I3 5.8 If all variables in equation 5.8 are kept constant and only b is allowed to change, then it can be shown that, as b increases, so does k. Therefore k increases with distance from the root of the petal valve. If the petal were to be deflected through a small distance, then the majority of bending would occur at the root where k has its smallest value. To simplify the problem of varying k, the petal valve was modelled as a cantilever beam of constant cross-section equal to that at the root of the petal, with a lumped mass at the end which would represent the extra mass of the side lobes of the valve. The value of that extra mass was found by: Finding the mass of the side lobes Finding the centre of gravity of that mass Calculating the moments produced by this mass about the root 25 Then calculating an equivalent mass which would produce the same moment about the root if it were placed at the tip of the valve. The result is a constant-section beam with a lumped mass at its end as shown in figure 5.4 for which the natural frequency of vibration can be easily calculated. Figure 5.4 Simplified Petal Valve Model The equivalent mass of this model can be found using Raos equation: mcq = m +u.2Smbcum 5.9 The natural frequency of vibration can now easily be calculated using = _km 5.10Where k is obtained using equation 5.8 and m is calculated from equation 5.9. In order to find the position of the centroid of the side lobes, a simple 2D CAD program called QCad was used. The geometry of the valve was drawn and then split into the main beam and up to five other simple shapes as shown in figure 5.5. The areas into an Exfollowing The equiOr: The mass Figand centroxcel spreadsequation: ivalent massof the beamgure 5.5 Petal Vids of thesesheet to calcs to be place m was calc Valve Geometrye shapes weculate the py = ed at the enm = mm = ptAculated usinmbcumy Split for Centere found usposition of tAyA nd of the beamIobcsyI AIobcsyI ng: m = ptbI troid Determinasing QCad their combiam was then ation and these vined centroin calculated26values inputid using thed using: 5.115.126 t e 227 In order to tune the petal valves to the required natural frequency of vibration, it was necessary to investigate the relationship between natural frequency , spring stiffness k, length L and material thickness t. By combining equations 5.8 5.12 and simplifying, the following equation 5.13 was obtained. n = _ Ebt24pI2(AIobcsy +u.2SbI2) 5.13By varying t and L in equations 5.8 and 5.13, it can be shown that an increase in t will increase the natural frequency of vibration but will also increase the static stiffness by a larger factor. n t but k t3 If L is reduced instead, there is a smaller increase in static stiffness for the same increase in natural frequency. n _1I4 but k 1I3 Therefore it is more desirable to tune the valve frequency by reducing the effective length of the valve than by increasing the thickness. Keeping the static stiffness as low as possible is also necessary to allow the engine to produce static thrust as the valves do not have the benefit of ram-air pressure to help open them. 5.3. Thermodynamic Analysis The thermodynamic cycle of the pulsejet engine was analysed using theory from two different sources. The first is Ahmed El-Sayeds text; Aircraft Propulsion and Gas Turbine Engines and the second is the Guggenheim Aeronautical Laboratorys reference text Jet Propulsion. Both methods are examined separately. Both methods refer to conditions within the engine at certain points. These points are illustrated in the following diagram, figure 5.6. 28 Figure 5.6 Points of Interest Within the Pulsejet Engine 5.3.1. El-Sayed (2008) Ahmed El-Sayed idealises the process within the pulsejet engine as illustrated in the T-S diagram in figure 5.7 below. Figure 5.7 Ideal pulsejet cycle [El-Sayed, 2008] Due to ram effect in flight and inefficiencies due to diffuser shape and valve system, both the pressure and temperature at point 2 can be calculated as follows: P02 = Pu _1 +pd yc 12 H2] ycyc-1 5.14 I02 = I01 = I0u= Iu_1 +yc 12 H2] 5.15a 1 2 3 4 29 Where d is the efficiency of the diffuser and valve system and M is the flight Mach number. It is assumed that combustion takes place at a nearly constant volume. Therefore: P03 = P02_I03I02] 5.16T03 is determined from the energy balance in the combustion chamber, equation 5.17. (mu + m])CphI03 = m uCphI02 + pbm]R 5.17Where Cp is the specific heat capacity at constant pressure and b is the burner efficiency. The exhaust gases expand out the tailpipe to ambient pressure. This process is assumed to be isentropic and the temperature of the exhaust gases is calculated from the following relationship, equation 5.18: _I03I4 ] = _P03Pu ]yh-1yh 5.18The exhaust velocity and thrust are then calculated from equations 5.19 and 5.20 respectively. Ic = _2CphI03_1 _PuP03]yh-1yh_ 5.19 F = m u|(1 + )Ic I] 5.205.3.2. Guggenheim Aeronautical Laboratory (1946) The reference text, Jet Propulsion, makes precisely the same assumptions as El-Sayed up until the combustion stage. Although the assumptions about constant volume combustion and isentropic expansion remain the same, this text proposes different relationships to model the process of heat during combustion. This is described in equation 5.21 below. 30 = R = Ch(I03 I02) = 1yhCphI03_1 I02I03] 5.21 5.4. Material Selection 5.4.1. Jet Body In order to choose a suitable material for the jet body, it was necessary to first determine the maximum pressures which could be expected within the engine during operation. These pressures were obtained from the preliminary thermodynamic analysis in section 5.3. Also considered were pressure cycle plots obtained from existing pulsejet engines. The analysis in section 5.3 provided a reasonable estimate for the stagnation pressure in the combustion chamber as P03 = 4.1 Bar Adding in a factor of safety of approximately 2, the jet body should be capable of withstanding maximum internal pressures of up to 8 Bar. Comparing to existing pulsejet analysis as shown in figure 3.1 and fig 5.8, 8 Bar pressure appears to provide a huge factor of safety. Figure 3.1 shows a peak pressure of 27 psi or 1.86 Bar and figure 5.8 shows a peak pressure of 28 psi or 1.93 Bar. Considering that both jets are at different ends of the thrust scale with the project jet lying between them, it seemed reasonable to assume that the actual pressures experienced in the jet would be very similar. Therefore, by choosing the material to withstand 8 Bar pressure, it could be ensured that the safety concerns of the university could be comfortably met. Since the not a majosteel or staMild steel Lo Be Be Suma45ThDue to thwelded-serupturing material oapplicationA stress athick-wall78.1mm aFigure project jet wor concern. ainless steell has more aower cost anetter machinetter weld-abuperior heatade from st seconds (his problem he combinaeam structurunder operaof choice ans. analysis of led cylinderand 5.49mm5.8 Pressure Cywas to be aAmateur-bul. appeal as a pnd higher avne-ability bility t radiation painless stee(Beck, 2005is reported ation of preral steel tubating conditas this wasthe 3 Schr theory. Thm respectivelrt, = S9.uS.49Cycle plot in ASa purely statuilt pulsejetpulsejet matvailability properties. el should no5) as the boto be eliminessure and bing was untions. A106s designed h40 pipe chhe pipe hasly. uS9 = 7.1 tngine [Bressmane, the totalcommonly stainless steto one pulted staticallerheat and bld steel is usperature in as the weldeteel pipe wahigh-tempction 5.1.1 l diameter ick wollcJ an, 1946] al weight of constructedeel for severlsejet websily for morburn holes ised. the engineed seam wawas decided perature, higwas conduand wall thJ 31f the jet wasd from mildral reasons:ite, enginesre than 30 -in the steel.e, commonas at risk ofupon as thegh-pressureucted usinghickness of s d s - . n f e e g f 32 According to thick-walled cylinder theory: oh = r2Pro2 r2_1 +ro2r2_ 5.22Since the maximum stress is needed and maximum stress occurs at the outer wall, then r = ro and equation 5.22 can be simplified to: oh mux = 2r2Pro2 r2 5.23Using the dimensions of the pipe and the expected pressures then the hoop stresses were calculated as follows: Table 5.2 Max Hoop Stress in Jet Tube Pressure (Bar) 8 1.93 Max Hoop Stress, h max (MPa) 5.36 1.29 While no information was found for the yield strength of A106 steel at high temperature, the stress values in table 5.2 were compared against information found for several other weaker grades of carbon steel. The comparison showed that the stresses in the jet tube were significantly lower than the yield strengths of mild steel grades which were not designed to operate under high temperature. An example of the temperature dependant properties of common grade 43A steel is shown in figure 5.9. From the plot, the yield stress of 43A steel at 800C can be approximated to 15Mpa. 33 Figure 5.9 Mechanical Properties of Grade 43A Steel at Elevated Temperatures [Bailey, 2009] 5.4.2. Valve Plate The valve plate must be able to withstand the repeated impact of the valve tips up to 150-200 times per second. As well as being able to withstand this impacting, the valve plate should also provide a certain amount of damping force to the valve tips. This damping force should absorb some of the impact energy from the valves and thereby reduce the amount of stress the valve tip experiences during impact. This would help to increase the operating life of the valves. The concept of energy being absorbed by a material during an impact was related back to strain energy theory. The energy stored within a material when work has been done on it is termed the strain energy (Hearn, 1997). Since the work being done on the valve plate is the impact from the valve, the material which stores the most amount of strain energy will be the best material choice for the valve plate. Some of the kinetic energy of the valve tip will be converted to strain energy in the valve plate material. 34 Strain energy can be expressed as: u = o2AI2E Or: u = o2I2E 5.24 V will be constant for a given valve plate design. will also be constant for a given valve impact. If the only variable is the material of the valve plate, then: u 12E A material with smaller E will result in a smaller value of U To maximise U, aluminium with E = 70GPa was chosen for the valve plate material over steel with E = 200GPa. The downside to using aluminium is that aluminium has a very low surface hardness. This would most likely result in the repeated impact of the valves damaging the surface of the valve plate and affecting the seal between the valve plate and the valves. To avoid this, it was decided to hard-anodise the machined valve plate. The thin layer of aluminium oxide would increase surface hardness significantly without affecting the underlying material properties. A table showing Vickers hardness values for different materials is included below in table 5.3 as a comparison. Table 5.3 Material Hardness Comparison Table [Hard Anodising Ltd, 2005] Material Vickers Hardness Number Untreated Al 6082 100 120 Hard Anodised Al 6082 400 460 Mild Steel 200 220 Stainless Steel 300 350 35 Additionally, it was determined that an aluminium valve plate would conduct heat from the valves quicker than steel and help keep them from overheating. 5754 aluminium alloy was chosen as the final valve plate material due to its excellent anodising properties and local availability. 36 6. Construction This section details the manufacturing and construction of the project engine, the problems encountered and how they were overcome. The majority of manufacturing of the components was carried out in the universitys engineering workshop. Detailed engineering drawings of all components can be found in appendix A of this report. 6.1. Jet Body The tailpipe section of the jet body was made from a 1m length of 3 Sch40 seamless carbon steel pipe. The nominal wall thickness of this pipe is 5.5mm, therefore, the combustion chamber was designed to have the same wall thickness. The tailpipe was left ~150mm too long to allow tuning of the exhaust during testing. The combustion chamber was machined from a solid carbon steel piece to the dimensions shown in appendix A. The finished combustion chamber was welded to one end of the tailpipe. To allow easy assembly and disassembly of the engine, a flange was machined from 3mm mild steel plate. The flange incorporated eight 6mm holes which were designed to take M5 bolts to bolt the engine together. The flange was welded to the front of the combustion chamber. A small fitting was machined to allow the spark plug to be incorporated into the jet body. This fitting was simply a 10mm piece of 25mm diameter round bar. A 5mm step was machined in the piece so that the OD of the step was 18mm. The fitting was then drilled and tapped with an internal M14x1.25 thread to take the spark plug. A 19mm hole was drilled in the combustion chamber wall, 60mm from the front of the engine. The spark plug fitting was inserted into this hole and welded in place. The flat surface of the spark plug fitting provides a good surface for the plugs crush washer to seal against. The completed combustion chamber end of the jet body is shown in figure 6.1. 37 Figure 6.1 Combustion Chamber and Flange 6.2. Intake Diffuser The intake diffuser for the jet was initially designed to be a simple cone rolled from 1mm mild steel sheet. However, the correct facilities to roll a cone of this size did not exist in the university and, after an unsuccessful attempt to roll the cone in an external workshop, the design was abandoned for that described below. The final intake diffuser (figure 6.2) was machined from a solid block of aluminium. The design was kept simple with the ID by the valve plate being 110mm and an internal wall slope of no more than 7. A 10mm thick flange was incorporated into the design. Eight holes were drilled in the flange and tapped M5x0.8 to match up with the holes in the combustion chamber flange. 38 Figure 6.2 Intake Diffuser 6.3. Valve Plates Two different valve plates were manufactured. The first was as per the design described in section 5.1.2 and the second was a modification of the same design. The second valve plate simply extended the intake orifices towards the centre of the plate to increase the total intake area available. This was designed as a back-up in case problems were found regarding the original valve plate design during testing of the engine. The two valve plates were CNC machined from 10mm thick 5754 aluminium alloy. The OD of the valve plates were machined to the same OD of the flanges on the diffuser and the combustion chamber. Eight 6mm holes were machined in them to match the flanges. One 18mm hole was machined in the centre of the valve plates to allow the fuel delivery nozzle to pass through. Both machined valve plates then had to be polished before being sent to Marchant Engineering, Tramore, Co. Waterford to be hard anodised. The completed valve plates are shown in figure 6.3. 39 Figure 6.3 Anodised Valve Plates; Original (left) & Modified (right) 6.4. Valve Retainer Plates Three different valve plates were manufactured, each with a different radius of curvature. The largest radius of curvature was chosen so that the valve would have a maximum tip travel of about 8mm. This was determined to be the smallest tip travel allowable to allow the incoming air to flow unrestricted. A second retainer plate was chosen to have a much smaller radius of curvature which would allow the valve to open further during the intake phase. This would also increase bending stresses in the valve petals. The third retainer plate was given the same radius of curvature as the first but without any flat contact area in the centre. This would allow the valve to have total flexibility from the root. The effects of different retainer plates on valve life could then be investigated. The three valve retainer plates were CNC machined from mild steel bar stock and an 18mm hole drilled in the centre to allow fitting of the fuel jet. One of the completed retainer plates is shown in figure 6.4. 40 Figure 6.4 Valve Retainer Plate 6.5. Petal Valve The petal valve was cut from 0.006 blue spring steel sheet. The intricate shape of the petal valve cannot be cut with a snips as the material will just split. Therefore, an electro-chemical etching process detailed in appendix C of the report was used. The process involved first coating the material to be etched with an electrically insulating coating. An automotive primer was used in this case. The shape of the petal valve was then drawn on the painted surface and the lines scribed with a sharp knife to expose the metal underneath. The spring steel was placed in a saturated salt/water solution so that all the lines to be scribed were submerged. A stainless steel plate of approximately the same size was placed in the solution also with a sponge between the two pieces of metal to avoid contact between them. The spring steel was connected to the positive terminal of a 12V power supply and the stainless steel plate was connected to the negative terminal. When current was switched on, bubbles were seen to rise from the cathode. The entire apparatus was placed under an extractor hood and left for 40 mins approx until the spring steel appeared to have been eaten away at the scribed lines. The petal valve could then be popped from the rest of the material and the paint cleaned off by immersing the valve in cellulose thinners for up to 30 mins. The valve was then ready to be used in the engine with no further modifications. 41 The electro-chemical etching process had some drawbacks. Care must be taken to ensure a good even coat of masking paint is applied to the valve material. Any pinholes in the cured paint will result in pinholes being etched in the valve. These holes render the valve useless. The masking paint must be allowed to cure properly for at least 48 hours. Otherwise the etching process undercuts the paint very easily and a poor surface on the finished valve results. The valve must not be allowed to sit in the etching solution for too long or the process eats through weak points in the masking paint and the finished valve will have holes in it. Figure 4.3 shows an electro-chemically etched valve and the etching apparatus is shown below in figure 6.5. Figure 6.5 Electro-chemical Etching Apparatus During testing, shim steel was used to make valves of different thicknesses. This material was cut using a dremel tool and the burrs ground off with a grinding wheel on the dremel tool. 42 6.6. Fuel Injection Nozzles 6.6.1. Internal Injector The fuel injection nozzle was turned from 25mm mild steel bar stock. A 5.5mm hole was drilled in the centre of the piece to a depth of 45mm then six radial 2.5mm were drilled to intersect with it and form the fuel injection outlets. An internal chamfer was cut in the inlet hole which would help produce a tight seal with the nipple on the propane hose. Both sides of the piece were turned down to 18mm, leaving a 5mm wide collar at the injection holes. An M18x1.5 thread was machined on the turned down sections. A 15mm section at the inlet end was turned down to 16.6mm and an external 3/8 BSP thread was cut to allow the propane hose end to be threaded on. The completed internal fuel injection nozzle is shown in figure 6.6. 6.6.2. External Injector During testing, it was decided to move the point of injection forward into the intake diffuser. (see section 7.2) The quickest and simplest way to do this was to make a fitting which would screw directly onto the original nozzles 3/8 BSP thread. A 75mm piece of 20mm round bar was turned down to 16.6mm diameter. A 5.5mm hole was drilled from one end to a depth of 48mm. Six radial 2mm holes were drilled to intersect with the larger axial hole similar to what was done with the original nozzle. The internal chamfer was cut in the inlet also. At the opposite end, a 5mm hole was drilled to a depth of 20mm and then tapped M6x1. A spare brass propane hose fitting was then fixed to this end using an M6x15 wide-head screw bolt. The new external fuel injector can be seen in figure 6.6. 43 Figure 6.6 Fuel Injection Nozzles; Internal (top) & External (bottom) 6.7. Test Stand The test stand was constructed to the same basic design as was used for a previous pulsejet. However, to simplify the build for preliminary testing, it was decided to omit the bearings from the support straps and the bearing tracks on the frame. Instead, the jet supports would be bolted directly to the uprights in the frame. This would not allow for thrust measurement but was a secure and simple method of securing the jet until self-sustaining could be achieved. The individual components of the frame were cut from 30mm box-section steel and welded together. 9mm holes were drilled 25mm from the top of each upright before welding. To make the supporting straps for the jet, two 25mm wide straps of 2mm thick mild steel were bent around a section of the tailpipe. The ends were then bent up so that there was a gap of about 25mm between them. A 9mm hole was drilled through the tabs to allow the straps to be tightened with M8 bolts. Two 110mm lengths of 16mm round bar were welded to the outside of each strap so that the bars were in line with each other and normal to the curve of the strap. Four 9mm holes were drilled in the bars to match with the holes in the frame uprights. The straps were secured to the frame using four M8x50 bolts. Figure 6.7 shows the engine mounted in the completed test stand. 44 Figure 6.7 Engine Mounted on Test Stand 45 7. Testing & Troubleshooting The following section details the testing of the engine, the problems encountered and how they were overcome. This section also details the measurement techniques which were used at the end of the project, after successful running of the engine had been achieved. 7.1. Ignition system problems During initial testing, it was found that the spark generated by the piezoelectric igniter was inadequate to ignite the propane/air mixture. No form of ignition could be achieved using this method so it was decided to upgrade the system to use a motorcycle ignition coil and an old motorcycle battery as a power source. A coil was purchased and wired to the battery via a push-to-make switch. The circuit was then tested by connecting the coil lead to the spark plug. A spark was observed but it appeared weak and unreliable. When the circuit was connected to the engine, this spark also proved unable to ignite the fuel/air mixture in the engine. The spark plug which was being used up to this point was an NGK BM6A plug. This plug has a standard thread reach of 9.5mm. It was decided to replace this plug with one with a longer reach thread. This would place the electrode further into the engine and increase the chances of ignition. An NGK BR9EH plug was purchased as a replacement. The replacement plug had a 19mm thread reach and also had a higher heat rating which would allow the engine to withstand higher engine temperatures and therefore last longer. A 5kV power supply was connected to the spark plug as a temporary solution to the ignition problem. This system produced a continuous spark across the plug gap. A continuous spark is undesirable in a pulsejet engine as it can disrupt the pulsating combustion and cause the engine to stop. It was decided to use this method anyway to see if the engine would at least ignite with the current spark plug position. Ignition with the continuous spark was achieved but the jet did not pulsate at all. The result of this test is discussed in more detail in section 7.3. It was decided that the intermittent spark 46 which could be produced using an induction coil was much more desirable for pulsejet ignition. The motorcycle coil in the old circuit was replaced with an old-type distributor coil from a car and a short length of silicone HT lead was purchased to provide the connection to the plug. However, on testing, the spark was again weak and very unreliable. The circuit was checked over with a multimeter and the impedance of the coil and spark plug were found to be 4 k each. The total resistance of 8 k between the coil and plug electrode was much too high and the HT lead and plug were replaced. The spark plug was replaced with a non-resistor type B9ES NGK plug and the silicone HT lead was replaced with a length of standard copper-cored spark plug wire. The performance of the new system was found to be very satisfactory with a strong reliable spark being produced across the plug gap each time the switch was pushed. Although this system was adequate for ignition, it proved awkward to have to push a button each time a spark was needed. This system meant that more people were needed to run a test; one person to provide spark and a second to vary the fuel flow until ignition was achieved. An automatic system would solve this problem by allowing the operator to simply switch on the ignition circuit, vary the fuel flow until ignition was achieved and then switch off the ignition circuit. An ideal automatic system would be switched on using a toggle switch and then discharge the coil at preset regular intervals to send a steady stream of sparks across the plug gap until the system was switched off again. After contacting the Electronic Engineering department in the university to help with automating the circuit, two possible solutions were determined: 1. Use a 555 timer circuit with a large transistor which would act as a switch to cut the current to the induction coil at regular intervals which would be controlled by the 555 circuit. This had the advantage of being completely portable with all power to the circuit being provided by the motorcycle battery. The downside 47 was that the timing of the spark was dependant on the 555 circuit and could not be easily changed. 2. Use almost the same circuit as above but instead of using a 555 timer to control the transistor, a signal generator would provide a square wave signal to do the same thing. This system had the advantage that the timing of the spark was easily adjustable by varying the frequency of the output square wave on the signal generator. The disadvantage was that the signal generator needed an A/C power source and so the portability was reduced. The second solution was chosen over the first as it was simpler to set up and the ease of adjustment was attractive. The Electrical Engineering department also had such a circuit already made up for demonstration purposes which was made available to this project and could easily be integrated into the existing circuit. The final circuit provided a reliable and adjustable ignition source for the pulsejet during testing. It also allowed tests to be conducted more easily and with minimal personnel. The final ignition setup is shown in figure 7.1. A circuit diagram can also be seen in appendix B. Figure 7.1 Final Ignition Circuit Signal Generator Battery Ignition Coil On/Off Switch & Transistor Circuit 48 7.2. Fuel Mixing During the initial tests when the engine was igniting but was acting almost like a simple propane burner (section 7.3), a video clip taken looking up the tailpipe showed that the burning in the combustion chamber appeared over-rich and uneven. This can be seen in a still image from the video clip in figure 7.2 below. It was thought that the fuel may have been introduced too far into the combustion chamber for adequate mixing of fuel/air to take place before combustion. To attempt to solve this, a new fuel injector nozzle was machined as detailed in section 6.6.2. The new fuel injector would be threaded in place between the original injector nozzle and the propane hose and would move the point of injection forward into the intake diffuser. This would give the fuel a much longer time to mix with the air as it passed through the intake orifices and over the valve tips. The new injector nozzle proved to be very effective. The engine was never tested with the old injector nozzle after the valve frequency tuning had allowed the engine to operate correctly as the performance of the engine with the new fuel nozzle was significantly improved. There was no evidence of inadequate mixing with the new nozzle. Figure 7.2 Uneven Burning in the Combustion Chamber (left) & Burning With New Nozzle Fitted (right) 49 7.3. Valve Frequency Ratio Tuning The engine test using the 5kV power supply to provide spark resulted in ignition of the fuel/air mixture at a certain fuel pressure setting. The engine would not resonate and the sound of burning was very low. If gas flow was decreased, the burning would stop and if gas flow was increased, yellow flames would appear from the tailpipe. This led to the conclusion that the continuous spark had set up a standing flame front inside the engine which would only sustain at a certain fuel/air setting. The engine was acting as a simple propane burner. It was this conclusion that led to the desire to create the intermittent spark ignition system detailed in section 7.1. However, the new improved ignition system did not improve the quality of burning in the engine. Even with the sparking frequency turned down to under 0.5Hz, the engine would still ignite the fuel/air mixture in the same manner as before. With the sparking frequency that low, it ruled out that the problem was a standing flame front being set up in the engine. Further visual comparison of the movement of the petal valves before and after ignition concluded that the engine was taking in air by itself and was therefore attempting to resonate. Since the tailpipe had been left oversized, the excess length was trimmed back to the designed length of 1.1m and the test was run again. The change in length did not affect the quality of burning in the engine at all. When comparing the project engine to the previous engine which had been built in the university, it was noticed that the basic jet body dimensions were almost identical. The previous engine had achieved resonant combustion, albeit with an external supply of air. The only major difference in design was the valve plate and petal valves. This detail, coupled with the failed test following the length reduction, prompted an investigation into the vibration of the petal valves. 50 In Part II of The Propulsive Duct, C.E. Tharratt explains how a mechanical reed valve made up of two identical metal reeds sandwiched together provides added stiffness whilst retaining, as closely as possible, the response characteristics of a single metal reed. (Tharratt, 1965) In order to narrow down the problem, the engine was tested using a double set of 0.006 petal valves in place of one. This would increase static stiffness of the valves but keep the natural frequency of vibration as close as possible to that of a single petal valve. Using the double valve setup, the engine ran almost exactly the same way as it had in previous tests with a single 0.006 valve. The burning characteristics were very similar but the engine required a much higher air supply to be started and sustain burning. These results suggested that successful resonant combustion was reliant on the natural frequency of vibration of the petal valves. The theory necessary to calculate the natural frequency of the petal valves is detailed in section 5.2. An attempt was made to theoretically plot the response of a petal valve to the forcing frequency of the jet with varying frequency ratios. However, the analysis was regarded inconclusive due to the following reasons: The motion of the valve cannot be modelled as a simple spring/mass system without damping. This is due to the effect of the valve plate damping out one-half of the valves motion. This means that the momentum of the valve does not carry through from one cycle to the next and therefore renders conventional modelling inaccurate. Although the valve motion cannot be regarded as being damped, it cannot be modelled as a damped system either. Viscous damping and coulomb damping both restrict the motion of a spring system regardless of whether the amplitude is positive or negative. In a reed valve system, the valve is not restricted at all when the amplitude is positive but the valve plate does not allow the amplitude to become negative at any time. (Figure 7.3) Essentially the valve is returned to initial conditions [ x(u) = u ; x (u) = u] before the beginning of each negative pressure cycle. 51 Figure 7.3 Valve Motion Sign Convention (left) & Simplified Valve Motion Plot (right) These issues prevented an accurate theoretical solution for the response of the valve to be obtained without considerable further work. It was decided to carry out various tests, varying the natural frequency of the valves each time and observe the results. The next step in testing was to use a petal valve with a higher natural frequency of vibration than the original. Using the theory in section 5.2, the original 0.006 valve was calculated to have a natural frequency of 66Hz. If the thickness of the reed was increased to 0.010, the natural frequency would rise to 110Hz. Due to the unavailability of additional sheet spring steel in Ireland, it was decided to carry out testing using valves cut from shim steel. The shim steel valve would have the same vibrational characteristics as a spring steel valve of equal thickness but would be more prone to deformation. The shim steel valves would help determine whether or not the engine would resonate with different frequency ratios. A test was carried out using a 0.010 thick shim steel valve. The engine achieved resonance immediately but would only sustain for 15-20 seconds. Additionally, the engine would not sustain combustion without an external supply of air. Several 52 subsequent attempts were made to start the engine. Engine started each time but failed to sustain for more than 10 seconds. When the engine was disassembled following the test and the valves were examined, it was found that one petal had been bent so much that it no longer seated against the valve plate. (Figure 7.4) The sections of valves which covered the intake orifices were also visually deformed from the pressure of combustion. Figure 7.4 0.010" Deformed Shim Steel Valve Following Engine Run The audio was extracted from a video clip of the test and analysed using Audacity sound editor to determine the operating frequency of the engine. An operating frequency of ~150Hz was measured from the audio file. This is very close to the frequency of 142Hz which was estimated in section 5.1.1. Although the engine started with a valve frequency of 110Hz, the performance was not satisfactory. It was decided to try and run the engine with a valve frequency of roughly double that of the previous test. The simplest method of doing this was to insert a steel washer of a certain size behind the petal valve. This would shorten the effective length of the valve and thereby increase the natural frequency of vibration. It was calculated that a 47mm diameter washer would reduce the length of the valves by 9mm and increase natural frequency to ~250Hz. 53 The engine did not fire at all with this valve in place. It was determined that the static stiffness of the valve was too high to allow the air from the external supply to open the valves and create an air flow through the engine. It was calculated that using one of the 0.006 petal valves with the same diameter washer would result in a natural frequency of 150Hz but that the stiffness would be much lower. The valve was changed immediately and another test was run. The engine ran very erratically using this setup and was not able to sustain at a constant setting. It was determined that the frequency of the valves was too close to the forcing frequency of the engine for normal operation to be achieved. To solve this problem, it was decided to make a second washer which would increase the natural frequency of a 0.006 petal valve to 250Hz. It was calculated that a 53mm diameter washer was needed for this. Using this setup, the engine fired and sustained for over 1:30 minutes. The external air supply was shut off about five seconds after starting with no noticeable difference in running. When the engine cooled and the valves removed, visual inspection showed that the tips of many petals were broken and one petal had cracked along its line of flexure with the washer. (Figure 7.5) Figure 7.5 Impact and Fatigue Damage on a 0.006" Spring Steel Petal Valve 54 Due to time constraints with the project, it was decided to discontinue further valve frequency testing and use the remaining two valves to attempt to get pressure plots and inlet velocity data from the jet. 7.4. Valve Frequency High-Speed Camera Test To determine the accuracy of the theoretical valve vibration model, an experiment was conducted using a high-speed camera to measure the frequency of vibration. A 0.010 thick shim steel valve was used for the test. The valve was set up in front of the camera so that it was clamped between two pieces of steel at the root of the petal. (Figure 7.6) The valve was deflected by hand and released so that it vibrated naturally. The first recording of the vibration was taken at 200 fps (frames per second) but this was not high enough to accurately capture the vibration. A second recording was then taken at 600 fps. On playback, the time for five complete oscillations was noted. The period of one oscillation could then be determined and, hence, the natural frequency. The test recorded a frequency of 104Hz. The calculated frequency was 110Hz. The test was run again with the valve length shortened by 9mm. This was calculated to have a frequency of 250Hz. The vibration was first recorded at 600 fps but it was not high enough to accurately capture the motion. The recording speed was increased to 1500 fps and the natural frequency determined the same way as the first test. The recorded frequency was 227Hz. The calculated frequency was 250Hz. The experimental values of natural frequency corresponded relatively closely to the calculated values. This verified the theoretical model. 55 Figure 7.6 High-Speed Camera Experiment Setup 7.5. Pressure Cycle Data Collection To obtain pressure cycle plots within the engine, a high-temperature pressure transducer was fitted to the engine. The pressure transducer which was used was a Kistler 4045A5 with a cooling adapter. Two BSP sleeves were purchased to fit the transducer to the engine. These sleeves were welded to the engine at positions 55mm and 1075mm from the front of the engine. The transducer was connected to a laptop computer via a Handyscope to collect and store the data. The water for the cooling adapter was supplied by a 12V pump from a car windscreen wiper system. The pump was powered directly from the same battery which powered the ignition system. Since only one transducer was available for testing, the pressure readings from both points had to be taken from two separate tests. In order to keep the data as consistent as possible, the first test was started as normal but then the fuel was cut off via the main High-Speed Camera Petal Valve 56 valve on the propane tank. By leaving the setting on the regulator, a consistent fuel flow into the engine could be ensured. The tests were carried out using one 0.006 petal valve. The valve was one which had not been etched properly. When the engine did not sustain without air, it was put down to the defects in the valve. However, due to shortage of valves, the test was carried out anyway with the external air supply. The data from the transducer was recorded for the forward measurement position. The engine was then stopped and the transducer was moved to the rear measurement position. The engine was restarted and the transducer data recorded. Shortly after recording the last set of data, the valves failed and the engine ceased to run. Figure 7.7 shows the equipment set up for pressure data collection. Figure 7.7 Equipment Set Up For Pressure Data Collection Transducer Power Supply Transducer Cooling Pump Cooling Water Reservoir 57 8. Results This section displays the results of the experimental data acquired during testing of the engine. The following plot (figure 8.1) of pressure vs. time was obtained for the front end of the engine: Figure 8.1 55mm Position Pressure/Time Plot The plot displays the characteristic oscillating pressure cycles which occur at the front of a pulsejet engine. By analysing the plot, a burning frequency of 160Hz was calculated. A pressure/time plot was also obtained for a position just 25mm from the end of the tailpipe. This plot is displayed in figure 8.2. 0.10.40.91.41.92.40 0.01 0.02 0.03 0.04 0.05AbsPressure(Bar)Time(s)58 Figure 8.2 1075mm Position Pressure/Time Plot 00.20.40.60.811.21.40 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05AbsPressure(Bar)Time(s)59 9. Discussion 9.1. Jet Design This project has shown that a pulsejet engine can be designed using a set of simple equations. The equations derived by C.E. Tharratt do allow the basic dimensions of a pulsejet engine to be determined on the back of an envelope (Tharratt, 1965). However, very little emphasis is placed of the design of the intake valve system. Little or no research has been carried out to accurately determine the response of a mechanical spring valve to a forcing frequency. This response was determined to be a crucial aspect of the correct operation of a pulsejet engine. For an engine to achieve static thrust, the correct relationship between the frequency of operation of the engine and the natural frequency of the reed valves must be determined. The static stiffness of the valves must also be correct to allow the valves to open under static conditions. 9.2. Petal Valve Vibration Theory The theoretical model for determining the natural frequency of vibration of the petal valves was verified by capturing the vibration of the valve in front of a high-speed camera. However, as the length of the valve was reduced, the error in calculations appeared to increase. This may have been an error in the position where the valve was clamped. Section 5.2 explains how natural frequency is inversely proportional to L3. If the valve position in the clamp was even slightly off, it would result in a relatively large error in natural frequency reading. 9.3. Valve Life The operating life of the petal valves used in the project engine was extremely low. The longest continuous engine run lasted for only two minutes. After the engine runs the valves were found to have suffered severe impact damage and also fatigue damage at the valve root. 60 Fatigue cracks at the valve root can easily be eliminated by machining a new curved valve retainer plate with the correct root diameter. However, using the steel washer is the simplest and fastest way of varying natural frequency of vibration. The washers should be used to determine if a frequency ratio exists where valve impact damage is minimised. A new ret