Effect of the Unmixedness of Unburned Gases on the Pressure

Energy & Fuels 2008, 22, 2221–2228

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Effect of the Unmixedness of Unburned Gases on the Pressure Fluctuations in a Dump Combustor Jung Goo Hong,† Kwang Chul Oh,‡ Uen Do Lee,§ and Hyun Dong Shin*,† Department of Mechanical Engineering, Korea AdVanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea, EnVironmental Parts R&D Center, Korea AutomotiVe Technology Institute, 74 Yongjung-ri, Pungse-myun, Chonan, Cheungnam, 330-912, Republic of Korea, and Combustion Research Facility, Sandia National Laboratories, LiVermore, California 94551 ReceiVed January 9, 2008. ReVised Manuscript ReceiVed March 29, 2008

Combustion instability is a serious obstacle for the lean premixed combustion of gas turbines and can even cause fatal damage to the combustor and the entire system. Thus, enhanced understanding of the mechanisms of combustion instability is necessary for designing and operating gas turbine combustors. In this study, in order to elucidate the instability phenomena, an experimental study was conducted in a rearward-step dump combustor with LPG and air. The fuel supply conditions and the mixing distances (Lfuel) between fuel and air are used as experimental parameters to examine the effects of fuel modulation and unmixedness. The fluctuations of pressure, heat release, and equivalence ratio were measured by a piezoelectric pressure sensor and a high speed intensified charge coupled device (ICCD) camera, respectively. The unmixedness was measured by acetone laser induced fluorescence (LIF) at nonreacting flow because of stratification of the fuel in air. Various combustion modes occurred in accordance with the equivalence ratio and the fuel supply conditions. In the case of the fully premixed condition, the spatial fuel distribution inside the combustion chamber exists in a homogeneous state compared with the partially premixed condition, which leads to instant heat release, with relatively greater intensity for the chemiluminescence of the flame and higher amplitude of pressure fluctuations. On the contrary, in the partially premixed condition, the spatial fuel distribution exists in a heterogeneous state from the combustion chamber entrance, and the flame’s burnout or reignition progresses in accordance with the stratified distribution of fuel. The unmixedness of fuel and air leads to a relatively smaller intensity for the chemiluminescence of the flame and reduced amplitude of pressure fluctuations.

1. Introduction Lean premixed combustion systems are very susceptible to dynamic instabilities. Generally, combustion instability causes severe problems such as shortening the lifetime of the overall system and unstable operation with harmful emissions. A number of diverse studies have been conducted on combustion instability in a turbulent lean premixed flame. Studies conducted to date can be classified as studies to identify the cause of combustion instability,1–3 theoretical approaches,4,5 experimental approaches,6–11 numerical simulation approaches as follow-up * Corresponding author. Tel.: 82-42-869-8821. Fax: 82-42-869-8820. E-mail: [email protected]. † Korea Advanced Institute of Science and Technology. ‡ Korea Automotive Technology Institute. § Sandia National Laboratories. (1) Ku¨lsheimer, C.; Bu¨chner, H. Combust. Flame 2002, 13, 70–84. (2) Schadow, K. C.; Gutmark, E.; Parr, T. P.; Parr, D. M.; Wilson, K. J.; Crump, J. E. Combust. Sci. Technol. 1989, 64, 167–186. (3) Lee, J. G.; Santavicca, D. A. J. Propulsion Power 2003, 19, 735– 750. (4) Vaezi, V.; Aldredge, R. C. Exp. Thermal Fluid Sci. 2000, 20, 162– 169. (5) Lieuwen, T. J. Sound Vib. 2001, 242, 893–905. (6) Bu¨chner, H.; Hirsch, C.; Leuckel, W. Combust. Sci. Technol. 1993, 94, 219–228. (7) Broda, J. G.; Seo, S.; Santoro, R. J.; Shirhattikar, G.; Yang, V. Proceedings of the 27th Symposium (International) on Combustion, 1998; pp 1849-1856. (8) Lee, S. Y.; Seo, S.; Broda, J. G.; Pal, S.; Santoro, R. J. Proc. Combust. Inst. 2000, 28, 775–782.

studies,12–14 studies on active and passive control of combustion oscillation once it is generated,15–19 and studies on the effects of factors that influence combustion instability for designing and operating systems.20 While various factors, involved in a complex manner, underlie combustion oscillation, some compelling studies have focused on the interaction between flame and vortex1,21 and others have shed light on the perspectives of

(9) Huang, Y.; Yang, V. Combust. Flame 2004, 136, 383–389. (10) Lee, J. G.; Kim, K. W.; Santavicca, D. A. Proc. Combust. Inst. 2000, 28, 415–421. (11) Shih, W. P.; Lee, J. G.; Santavicca, D. A. Proceedings of the 26th International Symposium on Combustion, 1996; pp 2771-2778. (12) Lieuwen, T. J. Propulsion Power 2003, 19, 765–781. (13) Huang, Y.; Sung, H. G.; Heieh, S. Y.; Yang, V. J. Propulsion Power 2003, 19, 782–794. (14) Hantschk, C. C.; Vortmeyer, D. Combust. Sci. Technol. 2002, 174, 189–204. (15) Cohen, J. M.; Stufflebeam, J. H.; Proscia, W. J. Eng. Gas Turbines Power, ASME 2001, 123, 537–542. (16) Kelsall, G.; Troger, C. Appl. Thermal Eng. 2004, 24, 1571–1582. (17) Hathout, J. P.; Fleifil, M.; Annaswamy, A. M.; Ghoniem, A. F. Proc. Combust. Inst. 2000, 28, 721–730. (18) Annaswamy,; A. M. Ghoniem; A. F. IEEE Control Systems Magazine 2002. (19) Paschereit, C. O.; Gutmark, E.; Weisenstein, W. Proceedngs of the 27th Symposium (international) on combustion, 1998; pp 1817-1824. (20) Venkataraman, K. K.; Preston, L. H.; Simsons, D. W.; Lee, B. J.; Lee, J. G.; Santavicca, D. A. J. Propulsion Power 1999, 15, 909–918. (21) Bu¨chner, H.; Bockhorn, H. Symposium on Energy Engineering in the 21st Century, 2000; 1573-1580.

10.1021/ef800018s CCC: $40.75  2008 American Chemical Society Published on Web 06/04/2008

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Figure 1. Schematic diagram of the experimental apparatus. Table 1. Experimental Conditions condition partially premixed condition items

fully premixed condition

device diameter of fuel holes (mm) number of fuel holes Lfuel (mm) mixing time (ms) equivalence ratio (Ø) mean velocity at the dump plane (m/s) flow Reynolds number dynamic range of PCB 106B sensitivity of PCB 106B resolution of PCB 106B rise time of PCB 106B linearity of PCB 106B sampling rate from the transducer A/D digitization resolution

mixing chamber (0.0166 m3)

equivalence ratio fluctuation and time scale that influence periodical pressure fluctuations.5,12,22,23 Ku¨lsheimer and Bu¨chner1 explained that unstable combustion induced by the periodic large vortex structure generated from the recirculating zone. This means that the vortex and flame are closely related in terms of time and space. In addition, the authors argued that the amplitudes of combustion-driven pressure oscillations are controlled by breaking the vortex through secondary air, thereby restoring stability.21 Lieuwen et al.22,23 argued that the main cause of combustion instability is the equivalence ratio fluctuation. The authors contend that the pressure fluctuations caused by periodic heat release propagate upstream. This in turn causes the equivalence ratio to change at the fuel mixing location. This change in the equivalence ratio leads to a change in heat release downstream, thereby causing combustion instability in a continuous feedback system coupling manner. Both insistences have a same idea of heat release fluctuation as the cause of instability, but the (22) Lieuwen, T.; Torres, H.; Johnson, C.; Zinn, B. T. J. Eng. Gas Turbines Power 2001, 123, 182–189. (23) Lieuwen, T.; Zinn, B. T. Proceedings of the 27th Symposium (international) on combustion, 1998; pp 1809-1816.

choked fuel flow 0.3

unchoked fuel flow

0.7 4 94, 158, 221, 285 4.48, 7.52, 10.52, 13.57

1.0-0.42 21 9130 0.001-8.3 psi 300 mV/psi (0.04 mV/Pa) 0.0001 psi (91 dB) e9 µs e1% FS 1000 Hz 12 bit ((5 V)

approaches to heat release fluctuation are slightly different. This is because there are differences in the mixing method between fuel and air and the consideration of unmixedness of fuel and air.24 And also they established the theory of time lag model which explained that combustion instability is affected by the mixing length of mixing zone and the acoustic boundary condition. As a survey of previous studies indicates, it is reported that large vortex structure, equivalence ratio fluctuation and unmixedness of fuel and air influence pressure fluctuations, but a relative relationship among each of the factors has not been deduced. Ducruix et al.25 showed the summary of driving and coupling processes; when interacting with the proper phase lag, driving and coupling mechanisms can lead to combustion instabilities. Driving processes are flame/boundary interactions and flame/vortex interactions. Coupling processes are flame response to upstream modulations, flame response to strain rate and flame response to composition inhomogeneities. Lee et. al10 showed that local flame structure variation due to spraying secondary fuel in different position or angle, rearranged heat (24) Seo, S. H. KSME Int. J. 2003, 17, 906–913. (25) Ducruix, S.; Schuller, T.; Duroxs, D.; Candel, S. J. Propulsion Power 2003, 19, 722–734.

Unmixedness of Unburned Gases in a Dump Combustor

Energy & Fuels, Vol. 22, No. 4, 2008 2223

Figure 2. Schematic diagram of OH LIF and acetone LIF measurements (acetone seeding in the fuel side for acetone LIF measurement).

Figure 3. Flame behavior of mode 1 in the case of unchoked fuel flow condition (Ø ) 0.64, Lfuel ) 285 mm): (a) pressure signal and FFT, (b) high speed intensified CCD images, (c) OH LIF images.

release and took an effect on reducing pressure fluctuation. Venkataraman et al.20 verified experimentally that pressure fluctuation has relation with mixing air velocity in combustor inlet, swirl intensity, centerbody recess, and mixedness, etc, in dump combustor. So this parametric research told us that there exist parameters with combustion oscillation and gave us verification of its effects, and took a contribution to a design of practical combustor and system. Therefore, for the present study, in order to elucidate the combustion-driven instabilities, an experimental study was conducted in a rearward-step dump combustor with LPG and air. An experimental device was implemented for two conditions: a fully premixed condition using a mixing chamber and a partially premixed condition that fuel is injected toward the air flow immediately before the combustion chamber. In the case of the partially premixed conditions, two fuel supply conditions were established: a choked fuel supply condition uninfluenced by pressure fluctuations at the down stream and an unchoked fuel flow condition influenced by pressure fluctuations. To investigate the effect of changes in the equivalence

ratio caused by the unmixedness of fuel and air as well as the modulation of fuel, we used the various lengths of mixing passage. 2. Experimental Setup and Method Figure 1 illustrates the experimental setup, which consists of three major elements: a combustor, mass flow control devices for fuel and air, and pressure measurement and flame imaging systems. The combustion chamber is made of a stainless steel tube for measurement of pressure fluctuation and a quartz tube for visualization of the flame and its overall length is 700 mm. The fuel supply system consists of concentric tubes having fuel supply holes. Dried air is introduced at the bottom of the tube and the fuel (LPG) is injected to the air stream through four fuel supply holes. The air and the fuel flow rates are controlled by the mass flow controller (MFC). Two different fuel supply conditions were adjusted by varying the diameter of the fuel holes: 0.7 mm diameter holes in the case of unchoked fuel and 0.3 mm diameter holes in the case of choked fuel, as listed in Table 1. The injected fuel mixes with the air as it passes through the mixing distance (Lfuel). The mixture passes through the swirler just prior to the dump plane and then finally


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Figure 4. Flame behavior of mode 2 in the case of unchoked fuel flow condition (Ø ) 0.55, Lfuel ) 285 mm): (a) pressure signal and FFT, (b) high speed intensified CCD images and OH LIF images in noisy (I) and silent (II) periods, respectively.

Figure 5. Flow patterns in a confined, premixed, swirl-stabilized combustor26,27 and OH LIF images.

reaches the combustion chamber. The swirler is located in the mixing section and consists of vanes positioned at an angle of 45°. For measurement of the dynamic pressure of the combustion chamber at the dump plane (P (5) in Figure 1), we used a piezoelectric pressure sensor, PCB Model 106B; the sensitivity of the 106B model is 44 mV/kPa, and its dynamic range is from 0.5 to 40 kHz in response time. Images of flame shapes were taken by a digital camera (Nikon, COOLPIX 995) and a camcorder (SONY, DCR-TRV 300). Phase resolved images were acquired using a high speed ICCD camera (Phantom V7.0) without any optical filters with

0.02 ms time resolution. Two kinds of laser diagnostics, OH and acetone laser induced fluorescence (LIF), were performed to examine the reaction zone and unmixedness of the mixture at the inlet of the combustion chamber, respectively. As shown in Figure 2, the laser pulse for LIF excitation was generated by a second harmonic Nd:YAG laser pumped dye laser with a frequency doubler. For OH radical measurements, the laser was tuned to 283.01 nm to excite the Q16 line of the A2Σ+ r X2Π+ transition with pulse energy of 12 mJ. The ICCD (512 × 512) camera (100 ns gate) was equipped with a UG-11 and a WG-305 filter. For



Figure 6. Flame shapes and pressure fluctuations signal of modes 3 and 4 in the case of unchoked fuel flow condition (Lfuel ) 285 mm).

Figure 7. Flame shapes and pressure fluctuations signal in time domain (Ø ) 0.52).

measurement of the unmixedness of the mixture at the inlet of the combustion chamber, we used acetone LIF. Acetone is known to be a good tracer of LPG fuel because its physical properties (diffusion coefficients) are similar to those of propane, a major constituent of LPG. The laser pulse for LIF excitation was generated by a second harmonic Nd:YAG laser, which pumped dye laser with a frequency doubler, as shown in Figure 2. The laser was tuned to 283 nm to excite acetone, and an ICCD (512 × 512) camera equipped with an interference filter (450 nm × 10 nm) was used to obtain the LIF images.

3. Results and Discussion 3.1. Characteristics of Flame Mode. By varying the equivalence ratio (Ø) from 1 to 0.42 and monitoring changes in the overall characteristics of flame including flame’s periodic behavior and reattachment process, we were able to observe four representative modes of flame behaviors. In mode 1, the flame periodically burns near the dump plane, and vortical motion of the flame could be clearly observed. Schadow et al.2

explained this phenomenon. It is shown that the flow structures, or vortices, are formed by interaction between the flow instabilities and the chamber acoustic resonance. When these vortices dominate the reacting flow, the combustion is confined initially to the circumference of their cores and further downstream proceeds into their core, leading to periodic heat release, which may result in the driving of high amplitude pressure oscillations. Flames in mode 2 show alternately unstable behavior. In mode 3, the flame is stable and the main reaction zone tends to be far from the dump plane. In mode 4, the flame lifts off and then extinguishes when the equivalence ratio is under 0.42. Four different flame modes occur in the unchoked condition. However, in the choked condition, we could not observe mode 2. Only modes 1, 3, and 4 were observed. Figure 3 shows the flame behavior of mode 1, which are sequential high speed intensified CCD images and OH LIF images with the dynamic pressure signal, in the unchoked fuel flow condition when the equivalence ratio is 0.64 and Lfuel is 285 mm. As shown in Figure 3a, pressure fluctuations occur at 200 Hz and the flame is observed within 80 mm of the dump plane of the combustion chamber for 5 ms, which is the duration of a single period, as shown in Figure 3b. As suggested by the high speed intensified CCD images of mode 1, the main reaction zone is located in the vicinity of the dump plane, and periodic flame ignition, growth, burnout, and ignition source entrainment into fresh mixture processes occur repetitively. Figure 4 shows the flame behavior of mode 2 in the unchoked condition, with Lfuel fixed at 285 mm and the equivalence ratio reduced to 0.55. As shown in Figure 4a, the pressure fluctuations in the combustion chamber can be divided into periods and I and II. In the noisy period of I, the maximum peak-to-peak pressure fluctuation stands at around 10 kPa, and a similar amplitude to the pressure fluctuations of mode 1, shown in Figure 3a. In the silent period of II, the pressure fluctuation is reduced to a very small value. The noisy period and the silent period occur sequentially at a frequency of about 10 Hz. As shown in Figure 4b-I, flame extinction and reignition take place for 5 ms of the noisy period within 80 mm of the dump plane of the combustion chamber, which is similar to mode 1. For 50 ms of the silent period, the flame burns extensively from the dump plane of the combustion chamber to more than 200 mm downstream, as shown in Figure 4b-II. Figure 5a outlines the flow patterns within a confined, premixed, swirl-stabilized combustor.25,26 Gupta et al.25 have reported that a central recirculation zone (CRZ) and a side (26) Gupta, A. K.; Lilley, D. G.; Syred, N. Swirl Flows; Abacus Press, 1994.


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Figure 8. Radial distributions of acetone LIF signal according to axial positions and the maximum intensity at x ) 2 mm according to Lfuel (mean velocity at the dump plane ) 21 m/s, the flow Reynolds number ) 9130).

recirculation zone (SRZ) exist in both the center and the side with a turbulent shear layer (illustrated in dotted lines in Figure 5a) in the combustor. Furthermore, because of these flow patterns, the flame does not exist in a dispersed manner across the combustion chamber but is attached close to the dump plane and burns actively. The shapes of CRZ and SRZ can be clearly identified through the OH LIF images presented in Figure 5b. It was observed that the flame behavior in the noisy period of Figures 3 and 4I is consistent with the flow patterns reported by Gupta et al., and pressure fluctuations at this point were quite dramatic. On the other hand, the silent period of Figure 4II mostly existed apart from the dump plane, rather than in CRZ or SRZ, and pressure fluctuations at this point were only minimal. From this, it was deduced that CRZ and SRZ of flow patterns which formed large structure are important factors that influence the amplitude of pressure fluctuations. Figure 6 shows the flame shapes and the dynamic pressure signal of mode 3 and 4 at equivalence ratios 0.50 and 0.44, respectively, in the unchoked fuel flow condition. The amplitude of pressure fluctuations over time is minimal, as shown in Figure 6c and d, and the flame shape remained consistent without periodic change, as illustrated in Figure 6a and b. In the case of mode 4, most of the flame is a lift-off flame that is located more than 250 mm away from the dump plane, whereas in the case of mode 3, the flame exists in closer proximity to the dump plane compared with mode 4, but it is clear that the flame does not exist in SRZ. 3.2. Effect of Spatial Unmixedness of Fuel and Air. Figure 7 illustrates flame shapes and the dynamic pressure fluctuations at the same equivalence ratio (0.52), for both the cases of (a) the fully premixed condition and (b) the partially premixed condition with Lfuel set at 94 mm. Although the input equivalence ratio is the same, the flame shape and pressure fluctuations show different characteristics. In the case of the fully premixed condition, the flame clearly exists in the SRZ as well as the CRZ, whereas in the case of Lfuel set at 94 mm and a partially (27) Hedman, P. O.; Murray, R. L.; Fletcher, T. H. Proceedings ASME GT-2002-30053, 2002; p 1-11.

premixed condition, the flame does not exist in the SRZ and the amplitude of pressure fluctuations was minimal compared with that in the case of fully premixed condition. There is large deviation in the equivalence ratio due to stratification of the fuel in air and the amount of the deviation can significantly change the burning rate of the flame to lead the change of flame stabilization characteristics. This is attributable to the difference in heat release due to stratification of the fuel in air. When it become sufficient time and distance for completely mixing fuel and air in spatially, the flame exists in SRZ as the flame shape in (a) the fully premixed condition. On the contrary, in partially premixed flame, when the time and the distance for mixing fuel and air become insufficient, it gives rise to time and distance for fuel and air when they are mixed in the combustion chamber and the flame does not exist in SRZ as the flame shape in (b) the partially premixed condition. Figure 8a and b illustrates the unmixedness of fuel and air with varying Lfuel in the unchoked condition. The results of Figure 8 are obtained under the nonreacting flow without acoustic and flame fluctuations. The images are averaged from 100. As illustrated in Figure 8c, in the case of the fully premixed condition, the radial and axial distributions of acetone intensity are homogeneous. But in the case of (a) Lfuel ) 94 mm, the acetone intensity has maximum value at the inlet of the combustion chamber (r/D ) (0.15, x ) 2 mm) and the intensity grew weaker as it goes downstream. Figure 8d shows the maximum intensity of acetone according to Lfuel at x ) 2 mm. As Lfuel increases from 94 mm to the fully premixed condition, the maximum intensity of acetone decreases, and when Lfuel is over 285 mm, the intensity shows almost no difference with that of the fully premixed condition. From this result, it is found that the minimum necessary length of mixing, in order to get a mixture close to the perfectly premixed case, is 285 mm. Figure 9 illustrates pressure fluctuations and the intensity of OH* chemiluminescence according to Lfuel in the choked condition (Ø ) 0.64). As shown in the images, the smaller Lfuel was, the closer the flame came to the diffusion flame (I) with the combustion chamber due to insufficient mixing distance, or



Figure 9. Pressure fluctuations and normalized OH* chemiluminescence in the choked fuel flow condition/the fully premixed condition (Ø ) 0.64).

mixing time, and the intensity of heat release became smaller (Figure 9b). On the contrary, the closer the flame reached the premixed flame with sufficient Lfuel for completely mixed fuel and air spatially, the higher the heat release and the intensity of pressure fluctuations became. However, when the side recirculation zone was removed under the fully premixed condition without the combustion chamber, the flame showed stable characteristics, as displayed in the image (II). Øunmixedness shows the heterogeneity of fuel and air spatially with varying Lfuel. Peak to peak pressure fluctuations due to Øunmixedness tend to increase with varying Lfuel. From this, it can be deduced that the amplitude of pressure fluctuations can be controlled by spatial unmixedness. 3.3. Effect of the Choked and Unchoked Fuel Flow. Figure 10a shows the amplitude of pressure fluctuations and flame images taken by the ICCD camera at the peak position of pressure signal with varying Lfuel in the unchoked condition (Ø ) 0.64). Figure 10b demonstrates the flame’s OH* chemiluminescence at the maximum pressure. Although the inlet

equivalence ratio applied to the combustion chamber had the same condition, the unmixedness of fuel and air was different depending on Lfuel as shown in the choked case. Hence, the chemiluminescence of the flame also shows a relative difference as seen in the flame images taken by the ICCD camera. In addition, the relative difference can be measured from the amplitude of pressure fluctuations based on varying Lfuel, which is attributable to the effect of the spatial unmixedness of fuel and air. Compared with the choked fuel flow condition, it was slightly less sensitive to the Lfuel variation, and the intensity of the pressure fluctuations was slightly larger. In Figure 11, the results of the aforementioned three different experiment conditions are compared. Although the inlet equivalence ratio was identical in all three cases, the absolute amplitude of pressure fluctuations was different depending on each of the conditions. In the case of the fully premixed condition, pressure fluctuation occurred due to periodic reignition of the flame and burnout in the large vortex structure of SRZ. In this case, as Lfuel is shortened, the spatial fuel distribution becomes heterogeneous,


Hong et al.

Figure 11. Pressure fluctuations with respect to Lfuel (Ø ) 0.64).

Figure 10. Pressure fluctuations and normalized OH* chemiluminescence in the unchoked fuel flow condition (Ø ) 0.64).

and the flame’s periodic motion (ignition, growth, burnout, and ignition source entrainment into fresh mixture) proceeds in accordance with the stratified fuel distribution. Because of this, instant heat release does not take place as in the case of the premixed flame. Therefore, as shown in Figure 11a, the amplitude of pressure fluctuations tends to decrease based on Lfuel. When the fuel was supplied under two different conditions (choked and unchoked), the unchoked condition resulted in modulating fuel flow depending on the pressure fluctuations within the combustion chamber and showed greater heat release fluctuation compared with the choked condition. Therefore, different fuel supply methods resulted in different pressure fluctuations, which correspond to the amplitude equivalent to the portion represented by virgules. 4. Conclusion In this study, in order to elucidate the combustion-driven instabilities, an experimental study was conducted in a rearwardstep dump combustor with LPG and air. Various types of pressure fluctuations were observed with respect to the equiva-

lence ratio and fuel supply conditions by means of simultaneous measurement techniques. The results of this study are as follows: (1) Four different flame modes occur in the unchoked condition. However, in the choked condition, we could not observe any flame in mode 2. Only modes 1, 3, and 4 were observed. (2) It can be confirmed that the side vortex flow pattern among large vortex structures occurring in the central recirculation zone and side recirculation zone is a necessary condition to trigger pressure fluctuations, as shown in the flame shapes and pressure fluctuations in the fully premixed condition using the mixing chamber and partially premixed condition (Lfuel ) 94 mm) at the same equivalence ratio (0.52). There is large deviation in the equivalence ratio due to stratification of the fuel in air (Øunmixedness) and the amount of the deviation can significantly change the burning rate of the flame to lead the change of flame stabilization characteristics. (3) In the case of the fully premixed condition using the mixing chamber, the spatial fuel distribution inside the combustion chamber exists in a homogeneous state compared with the partially premixed condition, which leads to instant heat release, with relatively greater intensity for the chemiluminescence of the flame and higher amplitude of pressure fluctuations. On the contrary, in the partially premixed condition, the spatial fuel distribution exists in a heterogeneous state from the combustion chamber entrance, and the flame’s behavior in accordance with the stratified distribution of fuel. This leads to a relatively smaller intensity for the chemiluminescence of the flame and reduced amplitude of pressure fluctuations. This indicates that the amplitude of pressure fluctuations can be controlled by spatial unmixedness. Acknowledgment. This work was supported by the Korea Science and Technology Foundation through the Combustion Engineering Research Center (CERC) at the Korea Advanced Institute of Science and Technology, as well as by Mitsubishi Heavy Industries, Ltd., Japan. EF800018S

Effect of the Unmixedness of Unburned Gases on the Pressure

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