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Experimental studies on flame stabilization in a three step rearward facing configuration based micro channel combustor
Applied Thermal Engineering 58 (2013) 363e368
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental studies on flame stabilization in a three step rearward facing configuration based micro channel combustor Bhupendra Khandelwal a, *, Anil A. Deshpande b, Sudarshan Kumar b a b
Department of Power and Propulsion, Cranfield University, Cranfield, Bedfordshire, UK, MK43 0AL, UK Department of Aerospace Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India
h i g h l i g h t s Three rearward step micro channel combustor with methane-air mixtures. Increasing the number of steps increases flame stability limits. The increase in first and second step length improves the lower and upper limits. Increase in third step length affects the flame stability limit at high flow rates. Power input for stable flame in combustor is lower than other combustors reported.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 December 2012 Accepted 28 April 2013 Available online 9 May 2013
In this study a detailed experimental analysis on the characterization of flame stability behaviour in a 2.0 mm support diameter micro-combustor with three rearward facing steps has been reported. Premixed mixture of methaneeair has been used as a fuel for present investigations. Maximum and minimum diameter in the micro-combustor was maintained at 2 mm and 6 mm respectively. The effect of change in number of steps, length of steps, mixture equivalence ratios (f) and flow rates on stability limits of flame and flame position has been investigated. It was observed that the zone of recirculation created due to the sudden expansion at the rearward step aids in stabilizing the flame inside the microcombustor and improves the limits of flame stability significantly. The increase in the first and second step length helps in improving the lower and upper flame stability limits. The increase in the third step length affects the flame stability limit at higher flow rates only. Pollutants measurement shows that no NOx emissions were produced and CO emissions increase as the equivalence ratio (f) increases. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Micro combustion Recirculation Backward step Micro power generation
1. Introduction Recently there has been considerable research and developments towards miniaturization of electronic devices due to increased demand of micro power production devices with long life, low recharge time, low weight and high power density as compared to electrochemical batteries [1e4]. Combustion-based device has high power density which results in reduced weight, increased lifetime of micro-scale electronic and mechanical systems (MEMS) [2]. Energy density of electricity producing devices based on hydrocarbon fuel is 20e50 times higher than the advanced Li-ion concept electrochemical batteries. Therefore, a miniaturized power-generating device even operating at very low
* Corresponding author. E-mail address: [email protected] (B. Khandelwal). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.04.058
efficiencies would provide a good alternate to conventional batteries. The increased amount of surface to volume ratio and radical quenching on wall makes it difficult to reach an efficient and steady combustion at small scales [5,6]. Radical quenching is due to adsorption of radicals on combustion chamber walls and recombination, which eventually leads to reduction in homogenous chemistry. In 1977 Epstein and Senturia [7] proposed a concept of micro heat engine. Since then, considerable amount of research has been done for the application and development of small devices to generate power based on the combustion of hydrocarbon fuels. Kim and Kwon [8] computationally and experimentally studied flame propagation characteristics in a two staged micro combustor with premixed propaneeair mixtures. They observed that recirculation plays an important part in stabilizing the flame, and aids in increasing the flame stability limits. In an experimental study on single step based micro combustor by Pan et al. [9], it was found that nozzle to combustor diameter ratio plays a vital role in
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stabilizing the flame. They studied effect of different parameters on flame stability limits including nozzle to combustor diameter ratio, wall thickness to combustor diameter ratio and hydrogen to oxygen mixing ratio. It was also observed that nozzle to combustor diameter ratio changes uniformity and magnitude of temperature distribution on wall. Flame structure and stability behaviour in a radial micro channel (externally heated) burning premixed methaneeair mixtures have been extensively studied by Kumar et al. [10e12]. It is also proposed that the configuration of combustor studied, could be used as a micro power generation system as in case of disk-type combustion chamber for micro gas turbines [13]. In different studies by Kumar et al. [10e12] various non-stationary flame structures were observed, which were in addition to the conventional circular stable flame structure. Non-stationary flame structures include travelling flames, rotating pelton-like flames, and so forth. Later, Khandelwal et al. [14e16] have also reported similar studies on the effect of heat and flow recirculation on flame dynamics, stability and structure. Different configurations of combustors were studied and reported include single step micro combustor [14], double step micro combustor [15] and diverging micro channel combustor [16]. These combustors can be used as small scale combustion chambers to be used in micro power production devices and systems. Zhang et al. [17] experimentally studied combustion of hydrogen assisted preliminary flame ignition and flame dynamics. It is observed that preliminary flame ignition assisted by hydrogen helps in expanding the flame stability limits for steady combustion of methaneeair mixtures. It is also observed that there is no substantial distinction between hydrogen combustion and hydrogen added to the hydrocarbon combustion in the configuration studied. The improvement in the performance is in fact due to presence of platinum metal inside the combustor which acts as a catalyst. Similar other studies of flame behaviour, stabilization and combustion characteristics in different small-scale systems are reported in literature, such as free piston knock engine [18], micro thermophoto voltaic system [9], radial channel combustors [10e12,19e21], miniaturized combustors [22e24], Swiss-roll combustor [4], cylindrical combustors [25,26] and staged combustors [8,14,15,27]. Presently there is no literature available on safety concern of combustion based batteries, but if manufactured appropriately such devices would pose no risk. The aim of the present study is to experimentally examine the effect of changing the flow field by abrupt flow expansion with help of a rearward facing step on flame stability limits and micro combustor performance. To examine the effect of a rearward facing step in improving the flame stability limits and stabilizing the flame at such minute scales, a 2.0 mm inlet diameter combustion chamber is selected. Several rearward facing steps have been integrated into the combustion chamber which is having a maximum diameter of 6.0 mm keeping the base configuration. Methaneeair mixture is selected in the present study due it is extensive use by combustion researchers around the world for understanding the combustion phenomena. Methane is also commercially available in form of compressed natural gas which is used for various industry applications. The present work would help in taking a step forward for improving the design and characteristics of the microcombustors, predominantly by employing a reorganization of the flow field by a sequence of rearward facing steps.
Fig. 1. Schematic of experimental set-up.
fuel to the combustion chamber. Combustor is kept under ambient conditions of 1 atm pressure and temperature of 300 2 K. Premixing of gases is done before introducing them into the micro combustor. Pre-calculated mass flow rates of air and methane were supplied by help of using electronic mass flow controllers. These electronic mass flow controllers were operated by using a personal computer and a command module. Command module is connected to personal computer and electronic mass flow controllers (AALBORG, GFC Model; for air, 0e5 SLM for high flow rates and 0e500 MLPM for low flow rates; for methane, 0e500 MLPM for high flow rates and 0e50 MLPM for low flow rates) through serial port. (AALBORG, Model GFC; for CH4, 0e50 MLPM at lower flow rates and 0e500 MLPM at higher flow rates and for air, 0e500 MLPM at lower flow rates and 0e5 SLM for higher flow rates). The accuracy of these electronic mass flow controllers is 1% of the full scale. The equivalence ratio of mixture have been varied from f ¼ 0.7e1.4. The section of micro channel combustor consists of triple cylindrical rearward facing step. The detailed dimensions of the combustor are shown in Fig. 2. The length of the first (A), second (B) and third (C) step has been changed to examine their effect on flammability limits, and related details are discussed in the following section. Wall thickness of micro combustors has been
2. Experimental set-up and procedure A schematic diagram of the experimental set-up is shown in Fig. 1. The schematic diagram consists of air and fuel feed system. Electric mass-flow controllers have been used to precise amount of fuel and air. Two pressurized tanks have been used to supply air and
Fig. 2. Dimensional details of rearward facing step micro-combustor.
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kept at 1 mm. Stainless steel (SS304) and quartz glass materials are used for fabricating the combustor. Stainless steel micro combustor has been manufactured by drilling and quartz micro combustor has been manufactured by melting quartz. Effect of internal surface finish has been ignored in this study. The air and fuel mixture velocity mentioned in this study is mixture flow velocity at the 2.0 mm section of the micro channel combustor. The premixed mixtures of airemethane of precalculated equivalence ratio have been supplied by the supply line. The direction of the flow is towards negative x direction. Mixture was ignited at the exit of the micro channel combustor. With the decrease mixture velocity, the flame moves inside the micro channel combustor and firstly stabilize at station S3. A further reduction in the mixture velocity eventually leads to flame stabilization close to station at S2 and S1. All the experimental readings have been taken after a steady state have been achieved. 3. Results and discussion Fig. 3. Flame stability limits of micro channel combustor.
3.1. Preliminary investigations Dimensional details of all the micro channel combustor tested in this study with their volumes have been listed with brief details Table 1. To check the effect of change in length, step lengths of first, second and third steps have been changed systematically. The thickness of the wall of micro channel combustors has been consistently maintained at 1 mm. The equivalence ratios of the mixture were systematically changed from 0.6 to 1.3 for the experiments on the different micro channel combustors listed in Table 1. 3.2. Flame stability limits Fig. 3 shows the flame stability limits of micro-combustor case 2. Equivalence ratio has been varied from 0.6 to 1.3. Lower limit is the minimum velocity limit below which flame quenches. Region 1 shows the range of velocities for which flame is stable between station S2 and S3. It was noticed that the flammability limits increase significantly with an increase in methane-air mixture equivalence ratios from 0.6 to 0.95. The flame is anchored near station S2 at lower limits. But, with the increase in equivalence ratio above 1.1, stabilization of flame occurs at the exit of micro channel combustor for even low flow rates. The flame looked similar to a rich pre-mixed flame at the exit plane. Higher flow velocity beyond region 1 resulted in the flame stabilization in region 2. Region 2 is the volume between stations S3 and exit plane of the combustor. A further increase in the velocity led to flame being stabilized at exit plane. The range of equivalence ratios and velocities for which flame is stable at exit plane is shown in region 3. It was observed that flame does not stabilize between section S1 and S2. There are two possible reasons due to which flame does not stabilize in between station S1 and S2. Firstly, the step size is very small as
diameter changes from 2 mm to 3 mm resulting in very small recirculation at station S1. Another possible reason is perhaps the channel size being 3 mm very small for stabilizing a flame for methaneeair mixture. 3.2.1. Effect of increase in the number of steps To investigate the effect of change in the number of steps on flame stability limits, a configuration with two steps (case 1) was chosen from previous work reported in Ref. [15] and compared with three step configuration. The length of 30 mm and exit diameter of 6 mm was maintained constant in a three step (case 2) configuration to be compared with 2 step combustor configuration (case 1). The results obtained from a two step configuration are compared with the flame stability limits of a three step base configuration. Fig. 4 shows the flammability limits for a three step (case 2) and two step (case 1) combustor configurations with a total length of 30 mm. It can be observed from Fig. 4 that there is a substantial decrease in the lower flame stability limits for the three step case (2) as compared to a two step base configuration (case 1). It is also
Table 1 Dimensional details of micro-combustors. No
A
B
C
Volume (cm3)
1 2 3 4 5 6 7 8
e 10 7 7 7 7 7 10
22 14 7 7 7 10 10 10
8 6 10 7 5 7 10 10
0.50 0.40 0.41 0.33 0.27 0.37 0.45 0.47
Fig. 4. Effect of increase in number of steps on flame stability limits. LL, ML and BL indicate the lower limit, intermediate limit where flame stabilizes at the exit plane and flame blow off limits respectively.
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observed that the mid limit (ML) and blowout limit (BL) also increases significantly as the number of steps is increased. It can be inferred from these experiments that the increase in the number of steps helps in altering the flow profile near the wall and enhancing the flame stability limits for a three step combustor. The minimum flow velocity for which the flame can be stabilized at f ¼ 0.9 is reduced from 0.8 m/s to 0.5 m/s due to increase in the steps from two to three. Minimum thermal input required for the 3 step microcombustor to stabilize the flame in the combustor is considerably less than two step configuration micro-combustor, for the range of equivalence ratio varying from 0.75 to 0.95. The thermal input required is w5 W for equivalence ratio ranging from f ¼ 0.75e0.95. It is to be noted that the level of thermal input in this study is similar to that of reported in Refs. [8] and [15] for two staged microcombustors. However, between f ¼ 0.75e0.95 thermal input required is considerably less than that of Ref. [15]. 3.2.2. Effect of change in length of the first step Flammability limits for case 7 and case 8 micro-combustors are plotted in Fig. 5. To explain the effect of the change in the length of first step on flammability limits, first step length (A) was changed systematically and length of second step (B) and third step (C) is maintained constant. The first step length was varied from 7 to 10 mm. It can be seen from the Fig. 5 that the lower limit of flame stability limits for all the cases (7 and 8) studied and tested here remains approximately same for a range of f ¼ 0.7e1.0. The first step length plays an important role in deciding the lower flame stability limits at slightly rich mixtures. The flame is stable within the micro channel combustor for step length of 10 mm (Case 8) at equivalence ratio of f ¼ 1.2. The flammability limits within the micro channel combustor reduces from f ¼ 1.2e1.15 when the length is decreased from 10 to 7 mm (case 7). Upper flammability limits increases in micro channel combustor with an increase in length of first step. It can be seen from Fig. 5 that the upper flammability limits (blowout) limits for case 8 (10 mm length) increase marginally when the step length is increased from 7 to 10 mm. 3.2.3. Effect of change in second step length on flammability limits Change in the flammability limits with methaneeair equivalence ratios by varying length of second step (B) is shown in Fig. 6. Two cases of micro-combustors have been studied by varying the
Fig. 5. Effect of the first step length change on flammability limits.
Fig. 6. Effect of the change in length of the second step on flame stability limits.
length of second step from 7 mm (Case 4) to 10 mm (Case 6). It can be observed from the figure that the lower limit for cases (Case 6 and Case 6) presented in Fig. 6 is almost same for a range of methane-air equivalence ratios from f ¼ 0.7e1.15. First step length plays an important role in deciding the lower limits for slightly rich mixtures as observed in other study [16]. For case 4, flame remains stable inside the micro-combustor till f ¼ 1.15, whereas, for case 6, flame remains stable inside the microcombustor till f ¼ 1.2. Significant changes in the upper flammability limit were observed when second step length is changed. With an increase in length of the step, upper limit also increases significantly. A decrease in the step length from 10 mm to 7 mm decreases the upper flammability limits from 2.8 m/s to 2.4 m/s. 3.2.4. Effect of change in third step length on flammability limits Changes in upper flammability limits due to variation in second step length (B) with methaneeair mixture equivalence ratios have been presented in Fig. 7. Third step length has been systematically varied as 10, 7 and 5 mm, length of first and second steps is kept constant. It has been noticed that the lower flammability limits for all the cases considered in this study remained almost similar for the range of methaneeair mixture equivalence ratios. With the
Fig. 7. Effect of the change in length of the third step on flame stability limits.
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variation in third step length, significant variation is observed in upper flammability limits. Upper flammability limits increases with an increase in third step length. This is apparent from the trends corresponding to cases 3, 4 and 5 in Fig. 7. For example, at f ¼ 1.2, the upper flammability limit for case 3 is 3.7 m/s, but for case 4 and case 5, the flame blows off at a flow velocity of 3.2 and 2.8 m/s respectively. 3.2.5. Variation of flame position with mixture velocity Fig. 8 shows actual pictures of the flame for case 2 micro channel combustor considered in this study. This figure shows a stabilized flame at different position with f ¼ 0.8. Mixture velocity is increased systematically to investigate the effect on flame position and pattern of the flame. It can be observed from Fig. 8 that flame is anchored inside the micro-combustor at x ¼ 9 mm for a mixture velocity of 0.8 m/s. Flame moves outside as the velocity is increased. Rearward facing steps play a vital role in anchoring the flame and flame remains stable for a wide range of velocities at step. The flame remains stabilized at station S3 from V ¼ 0.9 m/s to V ¼ 1.2 m/ s. This is due to the fact that local recirculation increases the flame stability limits. A spinning flame is observed between V ¼ 1.2 m/s and V ¼ 2.0 m/s. Spinning flame is observed at these velocities due to its tendency to stabilize outside and flow recirculation is trying
Fig. 9. Variation of flame position (case 2).
to keep the flame stabilized at the same position. For mixture velocities higher than 2.0 m/s, incoming velocity overcomes the resistance by recirculation and flame comes out of the microcombustor. For better understanding of flame behavior, flame position at different flow velocities has been extracted from Fig. 8 and plotted in Fig. 9. For a range of low and high velocities, flame moves linearly with the flow velocity. However, near the step flame remains stable for a large range of velocities. This is due to local recirculation near the step. This phenomenon is also observed by other researchers earlier [8,15,18]. Further work on performance of thermo photo voltaic cells is required to determine power required from micro combustion based device. 4. Conclusions Detailed experimental study on flame stabilization in a rearward facing three step micro channel combustor has been carried out. The effect of change in number of steps, length of steps, mixture equivalence ratio (f) and flow rate on stability limits of flame and flame position has been investigated in the present study. The study suggests that a stabilized flame can be achieved with improved flammability limits in a multi-step based micro channel combustor configuration over a range of mixture ratios and inlet flow conditions. A rearward step plays a significant function in stabilizing the flame in such minute scale combustors by changing the flow field by help of rapid expansion. It is observed in the study that as the number of steps in micro channel combustor is increased, flammability limits in a micro channel combustor increases significantly. The flame is stabilized in the downstream of the recirculation zone and flow recirculation helped in enhancing the flammability limits. The increase in the first and second step length helps in improving the lower and upper flame stability limits, this eventually widened the flammability limits. The increase in the third step length affects the flammability limit at higher flow rates only. The effect of the increase in the length of the third step is relatively insignificant for lower flame stability limits. Further study is required on safety assessment and practical implication of combustion based devices with thermo photo voltaic cells. References
Fig. 8. Flame pictures of case 2 at f ¼ 0.8.
[1] J. Vican, B.F. Gajdeczko, F.L. Dryer, F.L. Milius, A. Aksay, R.A. Yetter, Proc. Combust. Inst. 29 (2002) 909e916. [2] A.C. Fernandez-pello, Proc. Combust. Inst. 29 (2002) 883e899.
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[3] C.H. Kuo, P.D. Ronney, Proc. Combust. Inst. 32 (2007) 3277. [4] N.I. Kim, S. Kato, T. Kataoka, T. Yokomori, S. Maruyama, T. Fujimori, K. Maruta, Combust. Flame 141 (2005) 229e240. [5] T.T. Leach, C.P. Cadou, Proc. Combust. Inst. 30 (2005) 2437e2444. [6] S. Raimondeau, D. Norton, D.G. Vlachos, R.I. Masel, Proc. Combust. Inst. 29 (2002) 901e907. [7] A.H. Epstein, S.D. Senturia, Science 276 (1997) 1211. [8] K.B. Kim, O.C. Kwon, J. Power Sources 182 (2008) 609e615. [9] J.F. Pan, J. Huang, D.T. Li, W.M. Yang, W.X. Tang, H. Xue, Appl. Thermal Eng. 27 (2007) 1089e1095. [10] S. Kumar, K. Maruta, S. Minaev, Phys. Rev. E 75 (1) (2007) 016208. [11] S. Kumar, K. Maruta, S. Minaev, Proc. Combust. Inst. 31 (2007) 3261e3268. [12] S. Kumar, K. Maruta, S. Minaev, J. Micromech. Microeng 17 (4) (2007) 900e908. [13] M. Wu, J. Hua, K. Kumar, J. Micromech. Microeng. 15 (2005) 1817e1823. [14] B. Khandelwal, G.P.S. Sahota, S. Kumar, in: Proc.. of 21st National Conference on IC Engines and Combustion, BIET, Davangere, India, 2009, pp. 609e616. [15] B. Khandelwal, G.P.S. Sahota, S. Kumar, J. Micromech. Microeng 20 (9) (2010) 095030. [16] B. Khandelwal, S. Kumar, Appl. Thermal Eng. 30 (2010) 2718e2723. [17] Y. Zhang, J. Zhou, W. Yang, M. Liu, K. Cen, Int. J. Hydrogen Energy 32 (2007) 1286e1293.
[18] H.T. Aichlmayr, D.B. Kittelson, M.R. Zachariah, Combust. Flame 135 (2003) 227e248. [19] S. Kumar, K. Maruta, S. Minaev, R. Fursesnko, Phys. Fluids 20 (2008) 024101. [20] A.W. Fan, S. Minaev, S. Kumar, W. Liu, K. Maruta, Combust. Flame 153 (2008) 479e489. [21] A.W. Fan, S. Minaev, E. Sereshchenko, Y. Tsuboi, H. Oshibe, H. Nakamura, K. Maruta, Combust. Explosion Shock Waves 45 (2009) 245e250. [22] J. Guidez, P. Roux, N. Poirson, E. Jourdanneau, M. Orain, F. Grisch, Advances in Aerospace Sciencesdprogress in Propulsion Physics, , In: EUCASS Book Series, vol. 1, Torus Press, Moscow, 2009. [23] J. Guidez, C. Laurent, P. Roux and N. Poirson, ISABE 2007-1169 Beijing, China, 2007. [24] J. Guidez, S. Lahraichi, F.X. Nicoul, C. Laurent, N. Poirson, P. Roux, Micro Combustors for Micro Gas Turbine Engine (2008). OBE AAAF, Avignon 2008. [25] A. Minotti, C. Bruno, F. Cozzi, Numerical Simulations of a Micro Combustion Chamber. AIAA 2009-447 (2009). [26] S.O. David, Modeling of Pressure and Temperature Development in Microscale Combustion. AIAA 2005-170 (2005). [27] B. Khandelwal, G.P.S. Sahota, S. Kumar, Energy Conversion Manage. 52 (10) (2011) 3206e3213.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental studies on flame stabilization in a three step rearward facing configuration based micro channel combustor Bhupendra Khandelwal a, *, Anil A. Deshpande b, Sudarshan Kumar b a b
Department of Power and Propulsion, Cranfield University, Cranfield, Bedfordshire, UK, MK43 0AL, UK Department of Aerospace Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India
h i g h l i g h t s Three rearward step micro channel combustor with methane-air mixtures. Increasing the number of steps increases flame stability limits. The increase in first and second step length improves the lower and upper limits. Increase in third step length affects the flame stability limit at high flow rates. Power input for stable flame in combustor is lower than other combustors reported.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 December 2012 Accepted 28 April 2013 Available online 9 May 2013
In this study a detailed experimental analysis on the characterization of flame stability behaviour in a 2.0 mm support diameter micro-combustor with three rearward facing steps has been reported. Premixed mixture of methaneeair has been used as a fuel for present investigations. Maximum and minimum diameter in the micro-combustor was maintained at 2 mm and 6 mm respectively. The effect of change in number of steps, length of steps, mixture equivalence ratios (f) and flow rates on stability limits of flame and flame position has been investigated. It was observed that the zone of recirculation created due to the sudden expansion at the rearward step aids in stabilizing the flame inside the microcombustor and improves the limits of flame stability significantly. The increase in the first and second step length helps in improving the lower and upper flame stability limits. The increase in the third step length affects the flame stability limit at higher flow rates only. Pollutants measurement shows that no NOx emissions were produced and CO emissions increase as the equivalence ratio (f) increases. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Micro combustion Recirculation Backward step Micro power generation
1. Introduction Recently there has been considerable research and developments towards miniaturization of electronic devices due to increased demand of micro power production devices with long life, low recharge time, low weight and high power density as compared to electrochemical batteries [1e4]. Combustion-based device has high power density which results in reduced weight, increased lifetime of micro-scale electronic and mechanical systems (MEMS) [2]. Energy density of electricity producing devices based on hydrocarbon fuel is 20e50 times higher than the advanced Li-ion concept electrochemical batteries. Therefore, a miniaturized power-generating device even operating at very low
* Corresponding author. E-mail address: [email protected] (B. Khandelwal). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.04.058
efficiencies would provide a good alternate to conventional batteries. The increased amount of surface to volume ratio and radical quenching on wall makes it difficult to reach an efficient and steady combustion at small scales [5,6]. Radical quenching is due to adsorption of radicals on combustion chamber walls and recombination, which eventually leads to reduction in homogenous chemistry. In 1977 Epstein and Senturia [7] proposed a concept of micro heat engine. Since then, considerable amount of research has been done for the application and development of small devices to generate power based on the combustion of hydrocarbon fuels. Kim and Kwon [8] computationally and experimentally studied flame propagation characteristics in a two staged micro combustor with premixed propaneeair mixtures. They observed that recirculation plays an important part in stabilizing the flame, and aids in increasing the flame stability limits. In an experimental study on single step based micro combustor by Pan et al. [9], it was found that nozzle to combustor diameter ratio plays a vital role in
364
B. Khandelwal et al. / Applied Thermal Engineering 58 (2013) 363e368
stabilizing the flame. They studied effect of different parameters on flame stability limits including nozzle to combustor diameter ratio, wall thickness to combustor diameter ratio and hydrogen to oxygen mixing ratio. It was also observed that nozzle to combustor diameter ratio changes uniformity and magnitude of temperature distribution on wall. Flame structure and stability behaviour in a radial micro channel (externally heated) burning premixed methaneeair mixtures have been extensively studied by Kumar et al. [10e12]. It is also proposed that the configuration of combustor studied, could be used as a micro power generation system as in case of disk-type combustion chamber for micro gas turbines [13]. In different studies by Kumar et al. [10e12] various non-stationary flame structures were observed, which were in addition to the conventional circular stable flame structure. Non-stationary flame structures include travelling flames, rotating pelton-like flames, and so forth. Later, Khandelwal et al. [14e16] have also reported similar studies on the effect of heat and flow recirculation on flame dynamics, stability and structure. Different configurations of combustors were studied and reported include single step micro combustor [14], double step micro combustor [15] and diverging micro channel combustor [16]. These combustors can be used as small scale combustion chambers to be used in micro power production devices and systems. Zhang et al. [17] experimentally studied combustion of hydrogen assisted preliminary flame ignition and flame dynamics. It is observed that preliminary flame ignition assisted by hydrogen helps in expanding the flame stability limits for steady combustion of methaneeair mixtures. It is also observed that there is no substantial distinction between hydrogen combustion and hydrogen added to the hydrocarbon combustion in the configuration studied. The improvement in the performance is in fact due to presence of platinum metal inside the combustor which acts as a catalyst. Similar other studies of flame behaviour, stabilization and combustion characteristics in different small-scale systems are reported in literature, such as free piston knock engine [18], micro thermophoto voltaic system [9], radial channel combustors [10e12,19e21], miniaturized combustors [22e24], Swiss-roll combustor [4], cylindrical combustors [25,26] and staged combustors [8,14,15,27]. Presently there is no literature available on safety concern of combustion based batteries, but if manufactured appropriately such devices would pose no risk. The aim of the present study is to experimentally examine the effect of changing the flow field by abrupt flow expansion with help of a rearward facing step on flame stability limits and micro combustor performance. To examine the effect of a rearward facing step in improving the flame stability limits and stabilizing the flame at such minute scales, a 2.0 mm inlet diameter combustion chamber is selected. Several rearward facing steps have been integrated into the combustion chamber which is having a maximum diameter of 6.0 mm keeping the base configuration. Methaneeair mixture is selected in the present study due it is extensive use by combustion researchers around the world for understanding the combustion phenomena. Methane is also commercially available in form of compressed natural gas which is used for various industry applications. The present work would help in taking a step forward for improving the design and characteristics of the microcombustors, predominantly by employing a reorganization of the flow field by a sequence of rearward facing steps.
Fig. 1. Schematic of experimental set-up.
fuel to the combustion chamber. Combustor is kept under ambient conditions of 1 atm pressure and temperature of 300 2 K. Premixing of gases is done before introducing them into the micro combustor. Pre-calculated mass flow rates of air and methane were supplied by help of using electronic mass flow controllers. These electronic mass flow controllers were operated by using a personal computer and a command module. Command module is connected to personal computer and electronic mass flow controllers (AALBORG, GFC Model; for air, 0e5 SLM for high flow rates and 0e500 MLPM for low flow rates; for methane, 0e500 MLPM for high flow rates and 0e50 MLPM for low flow rates) through serial port. (AALBORG, Model GFC; for CH4, 0e50 MLPM at lower flow rates and 0e500 MLPM at higher flow rates and for air, 0e500 MLPM at lower flow rates and 0e5 SLM for higher flow rates). The accuracy of these electronic mass flow controllers is 1% of the full scale. The equivalence ratio of mixture have been varied from f ¼ 0.7e1.4. The section of micro channel combustor consists of triple cylindrical rearward facing step. The detailed dimensions of the combustor are shown in Fig. 2. The length of the first (A), second (B) and third (C) step has been changed to examine their effect on flammability limits, and related details are discussed in the following section. Wall thickness of micro combustors has been
2. Experimental set-up and procedure A schematic diagram of the experimental set-up is shown in Fig. 1. The schematic diagram consists of air and fuel feed system. Electric mass-flow controllers have been used to precise amount of fuel and air. Two pressurized tanks have been used to supply air and
Fig. 2. Dimensional details of rearward facing step micro-combustor.
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kept at 1 mm. Stainless steel (SS304) and quartz glass materials are used for fabricating the combustor. Stainless steel micro combustor has been manufactured by drilling and quartz micro combustor has been manufactured by melting quartz. Effect of internal surface finish has been ignored in this study. The air and fuel mixture velocity mentioned in this study is mixture flow velocity at the 2.0 mm section of the micro channel combustor. The premixed mixtures of airemethane of precalculated equivalence ratio have been supplied by the supply line. The direction of the flow is towards negative x direction. Mixture was ignited at the exit of the micro channel combustor. With the decrease mixture velocity, the flame moves inside the micro channel combustor and firstly stabilize at station S3. A further reduction in the mixture velocity eventually leads to flame stabilization close to station at S2 and S1. All the experimental readings have been taken after a steady state have been achieved. 3. Results and discussion Fig. 3. Flame stability limits of micro channel combustor.
3.1. Preliminary investigations Dimensional details of all the micro channel combustor tested in this study with their volumes have been listed with brief details Table 1. To check the effect of change in length, step lengths of first, second and third steps have been changed systematically. The thickness of the wall of micro channel combustors has been consistently maintained at 1 mm. The equivalence ratios of the mixture were systematically changed from 0.6 to 1.3 for the experiments on the different micro channel combustors listed in Table 1. 3.2. Flame stability limits Fig. 3 shows the flame stability limits of micro-combustor case 2. Equivalence ratio has been varied from 0.6 to 1.3. Lower limit is the minimum velocity limit below which flame quenches. Region 1 shows the range of velocities for which flame is stable between station S2 and S3. It was noticed that the flammability limits increase significantly with an increase in methane-air mixture equivalence ratios from 0.6 to 0.95. The flame is anchored near station S2 at lower limits. But, with the increase in equivalence ratio above 1.1, stabilization of flame occurs at the exit of micro channel combustor for even low flow rates. The flame looked similar to a rich pre-mixed flame at the exit plane. Higher flow velocity beyond region 1 resulted in the flame stabilization in region 2. Region 2 is the volume between stations S3 and exit plane of the combustor. A further increase in the velocity led to flame being stabilized at exit plane. The range of equivalence ratios and velocities for which flame is stable at exit plane is shown in region 3. It was observed that flame does not stabilize between section S1 and S2. There are two possible reasons due to which flame does not stabilize in between station S1 and S2. Firstly, the step size is very small as
diameter changes from 2 mm to 3 mm resulting in very small recirculation at station S1. Another possible reason is perhaps the channel size being 3 mm very small for stabilizing a flame for methaneeair mixture. 3.2.1. Effect of increase in the number of steps To investigate the effect of change in the number of steps on flame stability limits, a configuration with two steps (case 1) was chosen from previous work reported in Ref. [15] and compared with three step configuration. The length of 30 mm and exit diameter of 6 mm was maintained constant in a three step (case 2) configuration to be compared with 2 step combustor configuration (case 1). The results obtained from a two step configuration are compared with the flame stability limits of a three step base configuration. Fig. 4 shows the flammability limits for a three step (case 2) and two step (case 1) combustor configurations with a total length of 30 mm. It can be observed from Fig. 4 that there is a substantial decrease in the lower flame stability limits for the three step case (2) as compared to a two step base configuration (case 1). It is also
Table 1 Dimensional details of micro-combustors. No
A
B
C
Volume (cm3)
1 2 3 4 5 6 7 8
e 10 7 7 7 7 7 10
22 14 7 7 7 10 10 10
8 6 10 7 5 7 10 10
0.50 0.40 0.41 0.33 0.27 0.37 0.45 0.47
Fig. 4. Effect of increase in number of steps on flame stability limits. LL, ML and BL indicate the lower limit, intermediate limit where flame stabilizes at the exit plane and flame blow off limits respectively.
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observed that the mid limit (ML) and blowout limit (BL) also increases significantly as the number of steps is increased. It can be inferred from these experiments that the increase in the number of steps helps in altering the flow profile near the wall and enhancing the flame stability limits for a three step combustor. The minimum flow velocity for which the flame can be stabilized at f ¼ 0.9 is reduced from 0.8 m/s to 0.5 m/s due to increase in the steps from two to three. Minimum thermal input required for the 3 step microcombustor to stabilize the flame in the combustor is considerably less than two step configuration micro-combustor, for the range of equivalence ratio varying from 0.75 to 0.95. The thermal input required is w5 W for equivalence ratio ranging from f ¼ 0.75e0.95. It is to be noted that the level of thermal input in this study is similar to that of reported in Refs. [8] and [15] for two staged microcombustors. However, between f ¼ 0.75e0.95 thermal input required is considerably less than that of Ref. [15]. 3.2.2. Effect of change in length of the first step Flammability limits for case 7 and case 8 micro-combustors are plotted in Fig. 5. To explain the effect of the change in the length of first step on flammability limits, first step length (A) was changed systematically and length of second step (B) and third step (C) is maintained constant. The first step length was varied from 7 to 10 mm. It can be seen from the Fig. 5 that the lower limit of flame stability limits for all the cases (7 and 8) studied and tested here remains approximately same for a range of f ¼ 0.7e1.0. The first step length plays an important role in deciding the lower flame stability limits at slightly rich mixtures. The flame is stable within the micro channel combustor for step length of 10 mm (Case 8) at equivalence ratio of f ¼ 1.2. The flammability limits within the micro channel combustor reduces from f ¼ 1.2e1.15 when the length is decreased from 10 to 7 mm (case 7). Upper flammability limits increases in micro channel combustor with an increase in length of first step. It can be seen from Fig. 5 that the upper flammability limits (blowout) limits for case 8 (10 mm length) increase marginally when the step length is increased from 7 to 10 mm. 3.2.3. Effect of change in second step length on flammability limits Change in the flammability limits with methaneeair equivalence ratios by varying length of second step (B) is shown in Fig. 6. Two cases of micro-combustors have been studied by varying the
Fig. 5. Effect of the first step length change on flammability limits.
Fig. 6. Effect of the change in length of the second step on flame stability limits.
length of second step from 7 mm (Case 4) to 10 mm (Case 6). It can be observed from the figure that the lower limit for cases (Case 6 and Case 6) presented in Fig. 6 is almost same for a range of methane-air equivalence ratios from f ¼ 0.7e1.15. First step length plays an important role in deciding the lower limits for slightly rich mixtures as observed in other study [16]. For case 4, flame remains stable inside the micro-combustor till f ¼ 1.15, whereas, for case 6, flame remains stable inside the microcombustor till f ¼ 1.2. Significant changes in the upper flammability limit were observed when second step length is changed. With an increase in length of the step, upper limit also increases significantly. A decrease in the step length from 10 mm to 7 mm decreases the upper flammability limits from 2.8 m/s to 2.4 m/s. 3.2.4. Effect of change in third step length on flammability limits Changes in upper flammability limits due to variation in second step length (B) with methaneeair mixture equivalence ratios have been presented in Fig. 7. Third step length has been systematically varied as 10, 7 and 5 mm, length of first and second steps is kept constant. It has been noticed that the lower flammability limits for all the cases considered in this study remained almost similar for the range of methaneeair mixture equivalence ratios. With the
Fig. 7. Effect of the change in length of the third step on flame stability limits.
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variation in third step length, significant variation is observed in upper flammability limits. Upper flammability limits increases with an increase in third step length. This is apparent from the trends corresponding to cases 3, 4 and 5 in Fig. 7. For example, at f ¼ 1.2, the upper flammability limit for case 3 is 3.7 m/s, but for case 4 and case 5, the flame blows off at a flow velocity of 3.2 and 2.8 m/s respectively. 3.2.5. Variation of flame position with mixture velocity Fig. 8 shows actual pictures of the flame for case 2 micro channel combustor considered in this study. This figure shows a stabilized flame at different position with f ¼ 0.8. Mixture velocity is increased systematically to investigate the effect on flame position and pattern of the flame. It can be observed from Fig. 8 that flame is anchored inside the micro-combustor at x ¼ 9 mm for a mixture velocity of 0.8 m/s. Flame moves outside as the velocity is increased. Rearward facing steps play a vital role in anchoring the flame and flame remains stable for a wide range of velocities at step. The flame remains stabilized at station S3 from V ¼ 0.9 m/s to V ¼ 1.2 m/ s. This is due to the fact that local recirculation increases the flame stability limits. A spinning flame is observed between V ¼ 1.2 m/s and V ¼ 2.0 m/s. Spinning flame is observed at these velocities due to its tendency to stabilize outside and flow recirculation is trying
Fig. 9. Variation of flame position (case 2).
to keep the flame stabilized at the same position. For mixture velocities higher than 2.0 m/s, incoming velocity overcomes the resistance by recirculation and flame comes out of the microcombustor. For better understanding of flame behavior, flame position at different flow velocities has been extracted from Fig. 8 and plotted in Fig. 9. For a range of low and high velocities, flame moves linearly with the flow velocity. However, near the step flame remains stable for a large range of velocities. This is due to local recirculation near the step. This phenomenon is also observed by other researchers earlier [8,15,18]. Further work on performance of thermo photo voltaic cells is required to determine power required from micro combustion based device. 4. Conclusions Detailed experimental study on flame stabilization in a rearward facing three step micro channel combustor has been carried out. The effect of change in number of steps, length of steps, mixture equivalence ratio (f) and flow rate on stability limits of flame and flame position has been investigated in the present study. The study suggests that a stabilized flame can be achieved with improved flammability limits in a multi-step based micro channel combustor configuration over a range of mixture ratios and inlet flow conditions. A rearward step plays a significant function in stabilizing the flame in such minute scale combustors by changing the flow field by help of rapid expansion. It is observed in the study that as the number of steps in micro channel combustor is increased, flammability limits in a micro channel combustor increases significantly. The flame is stabilized in the downstream of the recirculation zone and flow recirculation helped in enhancing the flammability limits. The increase in the first and second step length helps in improving the lower and upper flame stability limits, this eventually widened the flammability limits. The increase in the third step length affects the flammability limit at higher flow rates only. The effect of the increase in the length of the third step is relatively insignificant for lower flame stability limits. Further study is required on safety assessment and practical implication of combustion based devices with thermo photo voltaic cells. References
Fig. 8. Flame pictures of case 2 at f ¼ 0.8.
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