8th Asia-Pacific International Symposium on Combustion and Energy Utilization October 10-12, 2006, Sochi, Russian Federation

Experimental and Numerical Analyses of Convective Ignition of PMMA in a Sudden-Expansion Channel

F. C. Hsiao,1 C. H. Chen,2 Y. C. Lin,3 and J. T. Yang*

Department of Power Mechanical Engineering, National Tsing Hua University Hsinchu, Taiwan, R.O.C.

1 Ph.D. student; 2Ph.D. candidate; 3graduate student *the corresponding author, Professor, e-mail: jtyang@pme.nthu.edu.tw

Convective ignition and flame spread of a solid fuel slab (PMMA) in a sudden-expansion chamber were investigated at specific inlet conditions u0 = 30 m/s and T0 = 1110K. Motion pictures of the ignition transient were acquired via a high-speed CMOS camera (1,280×1,024 pixels at 1,000 fps). Spatial and temporal distributions of temperature, velocity, and mixture fractions were numerically analyzed by large-eddy simulation (LES) to reveal the progress of ignition transient and flame spread. The ignition transient is featured by local periodic ignition-extinction mechanism. Main factors determining the success of local ignition are temperature, mixture ratio, and flow stream lashing. For the temperature highest at downstream the fuel slab and lowest in the step corner region, ignition starts from the downstream of fuel slab and gradually to the upstream. The ignition of recirculation zone is retarded by the strong extinction effects at reattachment zone during the ignition transient progress. Once ignition flame overcome reattachment zone, the recirculation zone is ignited instantly and become a powerful and stable heat source. The redeveloping boundary layer and recirculation zone are the two heat sources during the ignition transient. The region above the reattachment zone is the last region to be ignited during ignition transient. The flapping motion of recirculation zone is relevant to the development of the flame zone downstream, and thus affects the ignition. Through numerical and experimental results, transient features of the mechanisms in ignition and flame spread were revealed. This study provides a further insight to the traditional unrecognized ignition and flame spread mechanism.

1. INTRODUCTION Sudden-expansion combustors are combustion chambers utilizing backward-facing steps as the flame holders. Flame holders are required to ensure that the flame is held stably and that combustors function well. The recirculation zone behind a backward-facing step provides just the place where flow velocity is much lower than that in the free stream. Besides, the strong turbulence diffusion within shear layer acts as a “mixer” to improve mixing between the fuel and oxidizer. Characterized by its relatively simple geometry and reliable flame-holding capability, the sudden-expansion combustor is extensively installed in practical applications. Several ignition criteria can be referred to [1]. Similar to actual solid fuel ramjet engines, most of the research platforms simulating solid fuel ramjet combustor utilized external igniters to initiate solid fuel combustion [2]. One alternative approach to investigate the elementary solid fuel ignition problem was to employ a shock tube which provided uniform high-temperature environment within a very short period of time [3]. Literature focusing on the solid fuel ignition phenomena within a sudden-expansion chamber, or a solid fuel ramjet combustor, was rare. Except the research on control mechanisms of solid fuel ignition by Yang et al. [4, 5], which was essentially part of the work in [6], most of the other related studies dealt with effects on the ignition limit of combustor geometry (step height) and fuel composition. Wooldridge et al. [7] found that reducing step height resulted in the oscillatory manner (extinction and re-ignition) of flame in the recirculation zone until its complete flameout. Ghoniem et al. [8] investigate the fluid dynamic mechanism that drives heat release rate fluctuations, and how it couples with the acoustic field. After the solid fuel is ignited, the subsequent phenomenon is spread or propagation of the flame kernels. For the solid fuel installed horizontally, two types of flame spread are categorized. If directions of the flame spread and air stream are the same, the flame is said to be spread in a “concurrent” manner; otherwise the flame is spread under the “opposed” mode. Different controlling mechanisms were found to be dominant for the two modes of flame spread [9]. While the gas-phase chemical kinetics were critical to the concurrent flame spread, the flame-to-solid heat transfer was considered to be sufficient for opposed flame spread predictions. In numerical simulation, it can be found that the models were transformed from the work by Harsha et al. [10], which looked upon recirculation zone as a well-stirred reactor, into the large-eddy simulation (LES) approach [11], which captured most of the physics by modeling only subgrid eddies.

According to the reviewed literature concerning flame spread over the solid fuels, one common point was that either the investigated concurrent or opposed oxidizing flow speed was mostly less than 1 m/s, which was much lower than the test conditions of this study. Most of the research reviewed considered laminar diffusion flame only; the apparent turbulence behavior and vortex-shedding effects encountered in actual sudden-expansion combustion chambers have not been addressed on the flame spread issue. Thus fundamental mechanisms claimed in the literature have to be carefully inspected. In this research, the solid fuel slab is ignited via the

8th Asia-Pacific International Symposium on Combustion and Energy Utilization October 10-12, 2006, Sochi, Russian Federation pyrogen approach since the objective is aimed at investigating the ignition transient and flame spread phenomena.

2. EXPERIMENTAL DESIGN

2.1. Apparatus The wind tunnel system consists of a Roots blower, a vitiator which heats the air, a stream settling section (including a diffuser, a settling chamber, and a contraction cone), and a test section. A schematic diagram and the actual appearance of the overall wind tunnel construction are shown in Fig. 1. The heated oxidizing stream required by the test conditions is generated by the vitiator burning air-LPG (liquefied petroleum gas) mixture. The test section is a sudden-expansion chamber as in Fig. 1. The dimensions of the upstream cross-section of the step (inlet) are set to be 200 mm × 50 mm, and the length of the bottom wall is 400 mm. The original step height without fuel slab insertion is 50 mm, and in this research the step height (h) is set to be 25 mm. Refractory quartz glasses are adopted at the three sides other than the bottom wall of the test section for optical access. The ports for thermocouple installation are located 140 mm (upper wall) upstream of the step. The PMMA (polymethylmethacrylate) slab is adopted to simulate the fuel in the SFRJ combustor, and its thickness is 12 mm. The IDT High-Speed CMOS Digital Camera XS-5 is utilized. The analyzer is capable of capturing up to 10,000 frames per second. The CMOS camera is equipped with a standard CCTV lens to access complete view of the test section. Devices capable of measuring the temperature (T) of the flow are also necessary. A 125-µm K-type thermocouple produced by Omega Engineering, Inc. is used for temperature measurement. Outputs from the thermocouple are connected to the Advantech PCLD-8115 wiring terminal board; the signals are then captured by the combination of the Advantech PCL-818HG data acquisition system (ISA interface card) and the Genie program installed in a personal computer. The inlet oxidizing stream is preheated to about 810℃ and then the PMMA slab is pulled into the test section through the removable gate.

Fig 1. Schematic Diagram of the Wind Tunnel; Down: (Left) Test Section Draw; (Right) Picture of Test Section.

2.2. Numerical Simulation Concerning the unsteady flow, the numerical simulation was to solve Navier-Stokes Equations by LES.

The inlet conditions were T0 = 810℃and u0 = 30 m/s. The features of 3D flame motion in low March number flow with heat transfer was simulated and based on Kevin [12]. Velocity and pressure were calculated through the numerical method of Predictor-Corrector [13] and FFT [14].Cs = 0.24 in LES was used as Turbulence model to solve the transient boundary conditions. The calculation domain is 1000 mm(L)×200 mm(W)×100 mm(H), and the step height is also 25 mm. The governing equations are as follows.

a. Conservation of Mass

0=•∇+∂∂ uρρ

t (1)

8th Asia-Pacific International Symposium on Combustion and Energy Utilization October 10-12, 2006, Sochi, Russian Federation b. Conservation of Species

•

+∇•∇=•∇+∂∂

iiiii WYDYYt

ρρρ u)( (2)

c. Conservation of Momentum

τρρ •∇+=−∇+∇•+∂∂ fguuu ~~)( p

t (3)

d. Conservation of Energy

∑ •∇−∇•∇+∇•∇+′′′+=•∇+∂∂

iriii YDhTkq

dtdp

hht

qu ρρρ &0)( (4)

e. Equation of State

∑ℜ=i i

i

MYTtP ρ)(0 (5)

3. RESULTS and DISCUSSION

3.1. Ignition Progress Because the fuel slab was set under the complex high temperature flow field for the time of its ignition

delay, the fuel slab had been pyrolyzed for a period of time and a lot of fuel vaporization was in the combustion. Thus, it is possible to ignite any region in the combustion. The flame spread after ignition is not as from a fixed point and continuous as those most studied in papers we familiar, but it is from dynamic points and discontinuous. Kumar and Kuo [15] stated that the ignition of a solid propellant rocket can be divided to three states: (1) the heat reaction period before initial flame occurs; (2) flame spread period; (3) pressure-filling period, and the discussion in this research lies in period (1) and (2).

Fig. 2 is the ignition process captured by high speed camera with 1,000 fps. In this research, we define “initial ignition” as the first flame was observed on the surface of the fuel slab, and “complete ignition” as finally the combustion flame was filled with the combustor. From the images captured, as the beginning, some tiny flame kernels were formed at downstream of the fuel slab. They have to constantly overcome the heat loss resulting from the flow blowing. As those small flame developed for some period of time, the flame zone occupied the redeveloping boundary layer was formed. Being a new heat source, the flame zone was able to ignite its upstream region nearby. This upstream ignition process was obstructed when the flame passed through the reattachment zone, because the high strain rate and turbulence were intense to retard the ignition. While the upstream ignition kept trying to cross this zone, the flame zone formed at downstream had grew stronger, and it was able to continuously supply the heat to ignition the reattachment zone. Finally, once the flame ignited across the reattachment zone, the recirculation zone was ignited instantly and then become another powerful and stable heat source. This is because the recirculation zone itself has the characteristics of flame-holding. The flame then propagated into the shear layer through the vortical mixing, and soon spread along the downstream vortex shedding to ignite the region above reattachment zone.

Fig. 3 shows the measured time history of temperature at different locations on the surface of the fuel slab. The averaged data is the dynamic averaging of each value at each instant with its former nine values, which is used to filter the noise from the measurement. As the oxidizing stream was heated to about 810℃,the PMMA slab was pulled into the test section. Results show that the temperature at the downstream location of the fuel slab was the largest, and that near the step corner region was the lowest. Till 221s, except x/h = 0, the temperature at the other locations (x/h = 3.52, x/h = 7.04, x/h = 10.56, x/h = 12.32, x/h = 16) all increased, which means the ignition started. It can be noted that curves for non-averaged value of four points at downstream (x/h = 7.04, x/h = 10.56, x/h = 12.32, x/h = 16) raised intensely, and x/h = 3.52 raised some gradually. This means at the beginning of ignition, a flame zone located at downstream of the reattachment zone was formed first. Then, a further upstream ignition was preceded. The step corner region, x/h=0, is not ignited as no obvious increasing to its temperature, as can be found in Fig. 2.

8th Asia-Pacific International Symposium on Combustion and Energy Utilization October 10-12, 2006, Sochi, Russian Federation

Fig 2. Ignition Process by High Speed Camera (Xr ≈ 4.9h).

Fig 3. Temperature Evolution of Ignition from Measurement.

8th Asia-Pacific International Symposium on Combustion and Energy Utilization October 10-12, 2006, Sochi, Russian Federation

3.2. Flame Spread during Ignition Transient The process of ignition transient recorded by high speed camera is concluded to five stages. The first stage is

characteristic of the observation of the initial tiny flame on the surface of the solid fuel slab. The flame occurred downstream and formed as a single kernel as shown in Fig. 4. It showed up and was blown off by the stream with a period of 2 ms. The duration of this stage was about 376 ms and ended at the beginning of a more obvious smoke-like observed.

Fig 4. The First Stage of Ignition Transient.

The second stage features a clearer flame zone formed, as shown in Fig. 5. The flame began to shape a flame zone but not as the tiny spot flame in the former stage, and the flame in this stage was weak. Its development is similar to that in the first stage, with the flame shaped and then blown off by the stream for a period of cycle of 37 ms. The duration of this stage in this case was about 154 ms.

Fig 5. The Second Stage of Ignition Transient.

It goes to the third stage characteristic of more apparent flame zone formed as shown in Fig. 6. At this stage, more clear flame was observed and its range expanded to the further upstream. The development of flame zone still followed the ignition-extinction cycle, but its ignition was more powerful and rapid. The period of this cyclic development decreased from 16 ms to 4 ms as it proceeded, which indicates that the ignition and flame spread rate was increased due to the rising temperature by the flame. That is, as the ignition-extinction cycle proceeds, the surface region of fuel slab was heated more, and thus accelerated the cyclic development speed as the conclusion by Loh and Ferandez-Pello [16]. The duration of this stage in this case was about 578 ms.

The forth stage started at the time the flame ignited the reattachment zone. It was not easy to ignite the reattachment region because of the high strain rate and intense turbulence from the shear layer. The flame, as occurred right after ignition, was instantly broken by the vortex impinging in this region. The scattering broken flames can be observed in the left in Fig. 7. Some portion of the scattering flames spread upstream passing through the side wall direction. They did not propagate into the recirculation zone, because the broken flames become weaker when they arrived there and the temperature was low in the step region, as shown in the left in Fig. 7. Though the flame was continuously broken in reattachment zone, this region was unceasingly heated by these broken flames at the same time. Meanwhile, a portion of the broken flames spread downstream to heat this region, and thus the flame zone at downstream region (redeveloping boundary layer) grew stronger as in the right in Fig. 7. The variation of the length of the flame zone downstream was obvious as shown in the right in Fig. 7. The varied length was found to correspond to the flapping motion of the recirculation zone. The longer length of flame zone downstream corresponded to shorter recirculation length, and the shorter length to the long one. The period of this cycle was about 50 ms, and the duration of this stage in this case lasted at least 4 s.

The fifth stage is the time once the flame successfully spread into the recirculation zone and ignition occurred. The recirculation zone is characteristic of its well function of flame-holding because of relative low velocity and well mixing there. The ignition-extinction cycle had heated the reattachment zone, and then a portion of reversed flow with raising temperature flew back into the recirculation zone. Thus, together with the broken flames, the recirculation zone was ignited. The recirculation zone is much easier to be ignited by its feature of flame-holding. After ignited, recirculation zone become the second heat source and ignited shear layer instantly.

8th Asia-Pacific International Symposium on Combustion and Energy Utilization October 10-12, 2006, Sochi, Russian Federation

(Left) Ignition Further Upstream. (Right) Ignition-Extinction Period Accelerated.

Fig 6. The Third Stage of Ignition Transient.

(Left) Broken Flames at Reattachment. (Right) Variation of Length of Flame Zone.

Fig 7. The Forth Stage of Ignition Transient.

3.3. Variation of Reattachmet Length Fig. 8 is the time history of mixture fraction, temperature, and U-velocity at four locations from numerical

simulation. The ignition occurred at x/h = 5, because U-velocity at locations upstream this point (x/h = 3.26) decelerated and those at downstream (x/h = 5.86 and x/h = 9.78) accelerated. Besides, it can be noted that the reattachment length changed before and after the ignition. The reattachment point was at x/h = 9.78 and x/h=5.86 respectively before and after ignition.

8th Asia-Pacific International Symposium on Combustion and Energy Utilization October 10-12, 2006, Sochi, Russian Federation

(1) x/h = 3.26 (2) x/h = 5

(3) x/h = 5.86 (4) x/h = 9.78

Fig 8. Time History of Mixture Fraction, Temperature, and U-velocity.

3.4. Periodical Extinction Phenomena Fig. 9 and Fig. 10 represent the time history of temperature and mixture fraction in one second respectively

from numerical simulation. It is shown that the flame began to ignite into the recirculation zone at about 186.4 s, because the curve of x/h = 3.53 began to rise. But the fluctuation on this rising curve is smaller than those of the other points downstream. This feature in the curve of mixture fraction was also observed. The phenomenon indicated that the development of the flame zone at downstream was some kind of periodic motion because of the fluctuation of the curve. As the recirculation was ignited, the flame spread instantly in this region without no more periodic motion, and its curve raised smoothly and rapidly.

It was also noted that just at the time the curve of temperature of x/h = 3.53 started to rise, the fluctuation of the curve of x/h = 7.02 also began to expand. Additionally, curves of more downstream points (x/h = 10.56, 12.32, 16) have already been fluctuated since the time at about 186 s. This feature can also be observed on the mixture fraction curve. This phenomenon indicates that the periodic motion for the flame development downstream the reattachment is the competition among temperature, mixture fraction, and thus the heat flow. As at certain location at downstream, if the mixture fraction is sufficient for ignition and the temperature is high enough, the flame occurs. Then the fuel vaporization is consumed rapidly by the flame and results in the diminishing to the mixture fraction at that location. At this time, the flame is blown out because of the decreasing mixture fraction. Then the fuel vaporization is again able to occupy that region, resulting in the mixture fraction to rise again. This mechanism repeated and strengthened the flame zone at the downstream, and then it supplied continuous heat to the flame for further igniting its upstream region.

4. CONCLUSIONS Effects of ignition to PMMA in a sudden expansion flow and its transient mechanism were investigated.

The ignition transient is featured by local periodic ignition-extinction mechanism. Main factors determining the success of local ignition are temperature, mixture ratio, and flow stream lashing. For the temperature

8th Asia-Pacific International Symposium on Combustion and Energy Utilization October 10-12, 2006, Sochi, Russian Federation highest at downstream the fuel slab and lowest in the step corner region, ignition starts from the redeveloping boundary layer at downstream and gradually to the recirculation zone. The ignition of recirculation zone is retarded by the strong extinction effects at reattachment zone during the ignition transient progress; once ignition flame overcome reattachment zone, the recirculation zone is ignited instantly and become a powerful and stable heat source. The redeveloping boundary layer and recirculation zone are the two heat sources during the ignition transient. The region above the reattachment zone is the last region to be ignited during ignition transient. The flapping motion of recirculation zone is thought to be relevant to the development of the flame zone downstream, and thus affects the ignition.

Fig 9. Temperature Evolution of Ignition in 1s. Fig 10. Mixture Fraction Evolution of Ignition in 1s.

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