Effect of Compound Injection Angle on Liquid Film Cooling Effectiveness

contents:

1 abstract

2 Introduction

3 Experimental set up & Measurement plan

4 Conclusion

5 References

1 Abstract

Liquid film cooling is an important method to maintain structural integrity of rocket combustor walls. The effects of the changes of a liquid film cooling mass flow rate and operating conditions on wall heat flux characteristics of a liquid rocket engine combustion chamber were investigated by experiment. The study was conducted using a cylindrical test section, used as a thrust chamber water as coolant and on the inner surface of the test section containing flowing hot air. Coolant gas injections of two different configurations, viz., tangential angle of 30o and compound angles of 30o10o were used for the experiments. The hot gas used at two different temperatures of 404 and 383 K with main stream flow rates of 0.13 m3s-1 for both the temperatures and the coolant water applied at 303 K with four different flow rates of 2.63E-06, 2.15E-06, 1.52E-06 and 1.08E-06 m3s-1. Initially, the observations of surface temperature of test section were made with the help of thermocouples without applying the coolant and then it is noted with coolant applied for four different coolant injections. The data that are available from the experiments is used to evaluate the film cooling length and film cooling effectiveness. Also it is used to compare the above evaluations for all configurations and hence to evaluate the effect of liquid film cooling. It was found from the preliminary experiments that the liquid film was established. And the film cooling length is higher in the case of increasing the flow rate of the coolant by increasing the pressure. And the effect of compound angle injection configurations on liquid film cooling is on.

Keywords- Liquid film cooling, Film cooling effectiveness

2 INTRODUCTION:Rocket combustors are subjected to high heat transfer from the flowing hot gases and typically require active cooling. High thrust high specific impulse rocket engines, which are being developed as the propulsion devices for heavier pay load capacity use liquid propellants instead of solid fuel. The large values of heat fluxes at the high pressures and temperatures will have larger implications on the safety of the equipment, human beings and its launch pad installations. The high heat flux cause the wall temperatures to go beyond the melting point of the material itself leading to burn through and explosions. Inadequate measures to manage these adverse pressure and temperature conditions had led to significant number of engine thrust chamber failures during the development phase of several high thrust engines.There are many cooling techniques applied to solve this problem in which Regenerative cooling is the most widely used method of cooling a thrust chamber and is accomplished by flowing high-velocity coolant over the back side of the chamber hot gas wall to convectively cool the hot gas liner. The coolant with the heat input from cooling the liner is then discharged into the injector and utilized as a propellant. Dump cooling, which is similar to regenerative cooling because the coolant flows through small passages over the back side of the thrust chamber wall. The difference, however, is that after cooling the thrust chamber, the coolant is discharged overboard through openings at the aft end of the divergent nozzle. This method has limited application because of the performance loss resulting from dumping the coolant overboard. To date, dump cooling has not been used in an actual application.Film cooling provides protection from excessive heat by introducing a thin film of coolant or propellant through orifices around the injector periphery or through manifolded orifices in the chamber wall near the injector or chamber throat region. This method is typically used in high heat flux regions and in combination with regenerative cooling. Transpiration cooling provides coolant (either gaseous or liquid propellant) through a porous chamber wall at a rate sufficient to maintain the chamber hot gas wall to the desired temperature. The technique is really a special case of film cooling. With ablative cooling, combustion gas-side wall material is sacrificed by melting, vaporization and chemical changes to dissipate heat. As a result, relatively cool gases flow over the wall surface, thus lowering the boundary-layer temperature and assisting the cooling process. With radiation cooling, heat is radiated from the outer surface of the combustion chamber or nozzle extension wall. Radiation cooling is typically used for small thrust chambers with a high-temperature wall material (refractory) and in low-heat flux regions, such as a nozzle extension.In film cooling exposed chamber wall surfaces of rocket engine combustor are protected from excessive heat with a thin film of coolant or propellant, which is introduced through manifold orifices in the chamber wall near the injector. An important advantage of film cooling is the fact that it reduces heat transfer through the walls .For reducing the heat transfer to the wall, film cooling would be more effective with coolant injected as a liquid rather than a gas. When the coolant film is liquid it should essentially behave as an isothermal heat sink, as it evaporates and diffuses in the free stream. However this results in a two-phase flow consisting of annular liquid coolant film and a combustion gas core. Film cooling And the coolant injection configuration have some relation on this film cooling effectiveness, film stability, film uniformity & film cooling length. Most of the research in the field of film-cooling during the last decades dealt with the determination of the film-cooling effectiveness. Kinney et al[1] here investigated the effectiveness of liquid cooling films on the inner surface of tubes containing flowing hot air. Experiments were made in 2 and 4 inch diameter straight metal tubes with air flows at temperatures from 600 to 2000o F and diameter Reynolds numbers from 2.2 to 14 x 105; with water as the film coolant. Visual observations of liquid film flows were made in transparent tubes at flow conditions similar to those of the film cooling experiments with the help of a stroboscopic light and the air flows at temperatures of 80o and 800o F and diameter Reynolds numbers from 4.1 to 29 x 104. Flows of water, water-detergent solutions and aqueous ethylene glycerol solutions were investigated. Liquid-coolant films were established and maintained around and along the tube wall in co-current flow with the hot air. the results indicated that, in order to film cool a given surface area with a little coolant flow as possible, it may be necessary to limit the flow of coolant introduced at any angle axial position and to introduce it at several axial positions. Knuth [2] studied about the mechanism of film cooling under the influence of high velocity, turbulent gas stream. From that he developed correlations between pressure drops and liquid film coolant thickness, determined temperature of laminar mass diffusion sublayer, rate of mass addition at the boundary and predicts rate of evaporation using axially spaced thermocouples around circumference on the chamber wall. The coolants used are Water, (aq) Sucros solution, (aq) Zinc Chloride solution, Carbon Tetrachloride & Core gas was Union Oil Company No. 1 Thinner / Air Flat plate gaseous film cooling studies with different holes geometries are carried out by many researches [3-8] and the hole mouth have large cross section area gives more cooling effect.

H. W. Zhang et al[6] performed a numerical study to investigate the liquid film cooling in a rocket combustion chamber. Mass, momentum and heat transfer characteristics through the interface are considered in detail, and by solving the respective governing equations for the liquid film and the gas stream couple through the interfacial matching conditions. Heat transfer at the gas - liquid interface in a rocket combustion chamber with insulated wall is mainly dominated by convection of the free stream and transport of latent heat associated with the evaporation of the liquid film. When the wall is cooled by an external coolant, however, the sensible heat transfer become significant, and accordingly the convective transfer increases and latent heat flux decreases, leading to the elongation of the liquid film length. The interfacial temperature increases quickly in the entrance region and soon carries at the saturate temperature of the liquid film. The wall temperature is very low in the liquid film cooling region and increases sharply just beyond the point of the dry-out. The liquid film length decreases with the increase of the gas stream Reynolds number for the condition with an invariable coolant mass flux. The liquid film length increases with the increase of the external cooling.

DEFINITION Liquid film cooling thickness

The present work deals with the effect of liquid film injection configuration used to inject liquid coolant into cylindrical chamber. In which ordinary water used as coolant and hot air generated from heater which simulate the hot combustion products. The study uses, typical combinations of the axial and tangential angles 300-100 & axial angle 300

3. Experimental Setup And Measurement PlanA schematic of experimental setup is shown in Fig.1. It consist of three parts, (i) a hot air source with a temperature range of 300 to 800K, (ii) a test section and (iii) an exhaust systemFig. 1. Schematic of the Experimental setup

Atmospheric air drawn in by 10kW blower and is passed through an electric heater of 100kW, which heat up the air to a free-stream temperature that can be set from 300 to 500 K. A flow meter which control valve is connected in the line to control the core gas flow. The nichrome heater coil radiatively heat up the metal tubing through which the ambient air flow. There are a number of temperature sensors within the furnace for the safety of the heating coil. The temperature of the hot gas is controlled using a feedback loop with a temperature controller. The maximum flow Reynolds number that can be obtained is of order of 6x104. The whole of the heater assembly and blower is controlled through a control panel. The measurement panel board attached to the heater system works as a feedback loop to vary the main stream temperature. K-type thermocouples are used to measure the temperature inside the furnace. The hot air from the flow straightner enters the test section chamber through a calming section. The experiments are going to be done with hot air which simulate the hot combustion products and water is used as coolant.

Fig. 2. Injector heads

The coolant injection system consist of water supply tank and gaseous nitrogen supply cylinder for pressurising the water with pressure regulator and coolant injection manifold. The injector is made of two parts for easy fabrication and welded together. It consist of an elliptical grove as reservoir and 50 orifices of 0.55mm diameter are EDM drilled, as in Figure.2 from manifold. Injection orifices of tangential angle 300 and compound angle of 300-100 are drilled for coolant injection.The test section made of rolled copper tube of 120mm inside diameter, 2 mm thick and 770 mm long, which is instrumented with T-type thermocouple to measure the surface temperature as well as the free stream hot air temperature as shown in Figure.3. All thermocouple beads are fabricated and welded to the surface using a discharge type welder and held tightly using low conducting nylon band. Thermocouples are attaching the circumferentially on the surface of the test section with an equal interval of 30 mm along the length of the test section up to the length of 480 mm. Six such row of thermocouples are fixed circumferentially in an equal angular displacement of 60 degree on the test section. A total of 96 thermocouples are located on the test section circumferentially and two thermocouples are inserted into the cylinder to measure the inside main stream hot air temperature at its centre. All thermocouple outputs are fed to Keithley Multi-meter model 2750 capable of measuring 120 channels

Figure 3. Copper test section

Liquid film cooling experiments were done with main stream hot gas entry temperature of 404K and the coolant water temperature is at 303K and the flow rates corresponds to pressure 0.25,0.5,1 and 1.5 bar were investigated with tangential injection angle of 300 and the liquid film was established.

Fig. 4. Wall temperature variation

When film cooling is introduced the wall temperature near to the coolant injection decreases as film of coolant protect the wall from the hot air. The coolant mixes with mainstream hot air in the down stream leading to gradual temperature rise on the surface of the test section.the measured variation along the length of the test section are shown on the Fig.4.

Fig. 5. Wall temperature variation

From the graph it is clear that there is a decrease in wall temperature corresponding to the increase in coolant flow rate, also there is a slight increase in liquid film cooling length as the increase in coolant flow rate. In all the cases the wall temperatures were found almost constant for the initial 150mm and after that the effect of flow rate was observed.In Figures.6-9 shows the variation of wall temperature above and below the horizontal axis of the test section. It shows that the film cooling is higher in the lower part of the test section. The wall temperature is higher in the upper part of the horizontal axis is due to the down fall of liquid film due to the gravitational effect. On increasing the pressure of the coolant injection (coolant flow rate) the length of cooling effect on the upper and lower part of the horizontal axis part also increased.In figure 6 the larger variation on film cooling on upper and lower portion of the horizontal axis of the test section start at 90mm on wards. It may due to the gravitational effect on water jet and water jet strength at that flow rate.

Fig.6.Wall temperature variation above and below of testsection at 0.25 bar

In figure 7 the larger variation on film cooling on upper and lower portion of the horizontal axis of the test section start at 120mm on wards which is slight higher than 0.25 bar. It may due water jet strength at that flow rate and that may cause to increase in film adherence

Fig.7.Wall temperature variation above and below of testsection at 0.5 bar

In figure 8 the larger variation on film cooling on upper and lower portion of the horizontal axis of the test section start at 150 mm on wards which is slight higher than 0.5 bar. It may due increase in water jet strength due increases coolant flow rate and that may cause to increase in film adherence

Fig.8.Wall temperature variation above and below of testsection at 1 bar

In figure 9 the larger variation on film cooling on upper and lower portion of the horizontal axis of the test section start at 180 mm on wards which is slight higher than 1 bar. It may due increase in water jet strength due increases coolant flow rate and that may cause to increase in film adherenceFrom this there is gradual increase in flow rate the uniformity of liquid film on the test section on upper and lower side of the horizontal axis increase. And the increase in flow rate also increase the film adherence to the wall. Fig.9.Wall temperature variation above and below of testsection at 1.5 bar

CONCLUSIONS An experimental setup was made to study the liquid film cooling. Detailed instrumentation for the measurement of wall temperature are made. Experiments were conducted with core gas at 404K and water as coolant for different coolant flow rates by using tangential injector configuration of 300From the preliminary measurement it shows that the liquid film was established. There is a decrease in wall temperature corresponding to the increase in coolant flow rate, also there is a slight increase in liquid film cooling length as the increase in coolant flow rate. The film cooling length were increasing with corresponding increase in coolant water flow rateAnd the film liquid film cooling on compound injection orifice 300-100 is on

REFERENCES

[1]George R Kinney, Andrew E Abramson, John Sloop, Internal Liquid Film Cooling Experiment with Air Stream Temperatures To 20000F In 2 and 4 Inch Diameter Horizontal tubes , - 1952[2]Eldon L Kunth , The Mechanics Of Film Cooling, Part 1 & 2, 1954[3]C.S. Yang, C.L. Lin, C. Gau, Film cooling performance and heat transfer over an inclined film-cooled surface at different convergent angles with respect to highly turbulent mainstream, , Experimental Thermal and Fluid Science 32 (2008) 13131321, Applied Thermal Engineering 29 (2009) 167177[4]Brice Michel, Pierre Gajan, Alain Strzelecki, Nicolas Savary, Azeddine Kourta, Henri-Claude Boisson, Full coverage film cooling using compound angle, C. R. Mecanique 337 (2009) 562572[5]Hasan Nasir, Sumanta Acharya, Srinath Ekkad, Improved film cooling from cylindrical angled holes with triangular tabs: effect of tab orientations, International Journal of Heat and Fluid Flow 24 (2003) 657668[6]D. Lakehal, G.S. Theodoridis, W. Rodi, Computation of fillm cooling of a flat plate by lateral injection from a row of holes, International Journal of Heat and Fluid Flow 19 (1998) 418-430[7]Li Guangchao, Zhu Huiren, Fan Huiming, Influences of Hole Shape on Film Cooling Characteristics with CO2 Injection, Chinese Journal of Aeronautics 21(2008) 393-401[8]Jr-Ming Miao, Chen-Yuan Wu, Numerical approach to hole shape effect on film cooling effectiveness over flat plate including internal impingement cooling chamber, International Journal of Heat and Mass Transfer 49 (2006) 919938[9]H.W. Zhang, W.Q. Tao , Y.L. He, W. Zhang, Numerical study of liquid film cooling in a rocket combustion chamber, International Journal of Heat and Mass Transfer 49 (2006) 349358