Development of Micro Combustion Systems: Insights through Computations and Experiments

Transcription

1 Development of Micro Combustion Systems: Insights through Computations and Experiments Sudarshan Kumar 1 Abstract This paper reports the experimental and numerical investigations on the performance of micro combustion systems. Various backward facing multistep microcombustors configurations are considered. The investigations show that the flame stability limits at low and high flow rates are significantly enhanced as the number of backward steps are increased. In a these microcombustors, a normal flame propagation mode is observed to exist for low and moderate flow rate conditions. For higher flow rate conditions, X-shaped spinning flames are observed to exist. These spinning flames result in more uniform wall temperature profile as compared to normal flames and reduced CO emissions. Keywords: flame stability, microcombustion, laminar flames, heat transfer. 1 Introduction Various developments towards miniaturization of micro devices have led to the need of micro power generators with low-weight, long life and low recharge times [Fernandezpello (2002)]. High power density of the combustion based devices is expected to result in increased lifetime and reduced weight of these micro electronic and mechanical systems (MEMS). Lower pollutant emissions, especially NOx, due to lower operating temperatures of these systems as compared to the conventional systems and higher heat and mass transfer coefficients are some of the advantages associated with these systems [Fernandez-pello (2002)]. A micro gas turbine engine was proposed by Epstein and Senturia (1997) to power such small scale systems. Since then, substantial amount of work has been carried out towards understanding flame propagation [Maruta, Kataoka, Kim, Minaev and Fursenko (2005); Kumar, Maruta, Minaev and Fursenko (2008); Fan, Minaev, Kumar, Liu and Maruta (2008)] at small scales and development of small scale combustion based devices [Kuo and Ronney (2007); Kim, Kato, Kataoka, Yokomori, Maruyama, Fujimori and Maruta (2005); Kumar, Maruta and Minaev (2007); Khandelwal, Sahota and Kumar (2010); Sahota, Khandelwal and Kumar (2011)]. Maruta, Kataoka, Kim, Minaev and Fursenko (2005) have experimentally investigated the flame propagation characteristics of premixed methane-air mixtures in a 2.0 mm diameter heated channel with a positive temperature gradient along the direction of fluid flow. They observed the formation of stable flames at very low and high flow rates and unstable pulsating flames at moderate flow rates. Similar formation of unsteady flame propagation modes was observed in radial channels by various researchers. On application side, Kuo and Ronney (2007), Kim, Kato, Kataoka, Yokomori, Maruyama, Fujimori and Maruta (2005) have experimentally studied various configurations of Swissroll combustors with propane-air mixtures and different wall heat transfer conditions. They observed that flames can be successfully stabilized for a wide range of mixture equivalence ratios and flow rates by recirculating heat from burned gases to preheat the incoming fresh mixture through solid walls. Similar studies on flame stabilization behavior in radial microcombustors and backward step microcombustors were reported by Kumar, Maruta and Minaev (2007), Khandelwal, Sahota and Kumar (2010) and Sahota, Khadelwal and Kumar (2011). The objective of present work is to explore the possibility of altering the flow field by introducing a sudden flow expansion through a backward facing step and positively utilize the same to enhance the flame stability limits. To investigate the role of a backward facing step in stabilizing a flame at such small scales, a 2.0 mm diameter inlet is chosen. Multiple backward facing steps are incorporated into the combustor with a maximum diameter of 6.0 mm and a total length of 30 mm for the base configuration. Methane-air mixture is considered in the present work because it is extensively used by combustion researchers for understanding the combustion phenomena and methane is commercially available as compressed natural gas for various applications. Present study will help in improving the design of the microcombustors particularly by employing rearrangement of flow field through backward facing step configuration. 2 Experimental setup details The dimensional details of the microcombustors employed for the experiments during the course of the present work are 1 Department of Aerospace Engineering, Indian Institute of Technology Bombay Powai Mumbai India sudar@aero.iitb.ac.in

2 (a) (b) (c) simplified two-step reaction mechanism to understand the formation of other pollutants such as CO, CO 2 and H 2 O from the combustion zone. Grid adaption was employed to appropriately resolve the reaction zone structure within the flame front and results obtained are grid independent as the minimum grid size maintained within the reaction zone of the propagating flame front was as small as m. 3 Results and Discussion 3.1 Preliminary Observations Figure 1 Dimensional details of the backward facing microcombustors with (a) For two step combustor L = 30 mm, X = 12 mm, Y = 8 mm, D = 2 mm, A = 4 mm and B = 6 mm (c) For 3 step combustor with L = 30 mm, X = 10 mm, Y = 13 mm, Z = 7 mm, D = 2 mm, A = 3 mm, B = 4 mm and C = 6 mm shown in Fig. 1. The details of the setup are shown in Khandelwal, Sahota and Kumar (2010). It consists of methane and air feed systems, electric mass flow controllers (accuracy ±1% of the full scale) and backward facing step microcombustors. Two high pressure tanks containing methane and air were used to supply fuel and air to the combustor at ambient conditions of 1 atm pressure and 300 K temperature and the gases were thoroughly mixed before introducing into the combustor. The mixture equivalence ratio was varied from Ф 0.6 to 1.4. K-type thermocouples were used for measuring the outer wall temperature at different locations (accuracy ±5 K). The compositions of CO, CO 2, O 2, NO and NO 2 were obtained from the flue gas analyzer (KM9106 Flue Gas Analyzer) with an accuracy of ± 3% for CO 2 and ± 5% for CO. The mixture was ignited at the exit of the combustor and with a decrease in the mixture velocity, the flame moved inside the microcombustor and stabilized at station S 2. Further decrease in the mixture velocity led to flame stabilization near station S 1. All the measurements were carried out after a steady-state was reached, indicated by a constant flame position and constant wall temperature profile. Preliminary numerical simulations were carried out using a general purpose CFD code Fluent and the results obtained were analyzed along with the experimental results for understanding the flame stabilization behavior in these microcombustors. Two dimensional Navier-Stokes equations were solved in cylindrical coordinates along with energy and species conservation equations. Laminar flow calculations were carried out as the flow Reynolds number varies in the range of Heat losses from solid walls were considered and assumed to be 10 W/m 2 K and thermal radiation was neglected. Velocity inflow condition and pressure outlet conditions were employed at the inlet and exit of the combustor. Chemical reaction was modeled with a Figure 2 Various flame propagation modes in the microcombustors The preliminary experiments were carried out to observe the flame stabilization behavior in these microcombustors. Figure 2 shows the photographs of a stabilized flame in a microcombustor for a range of operating conditions. For lean mixtures and very small flow rates a typical curved flame is observed to exist in the microcombustor. An increase in the flow rate and mixture equivalence ratio leads to the formation of a typical X-shaped flame as shown in figure. At times, this X-shaped flame is stabilized both inside and outside the last step of the combustor. This X-shaped flame is actually a spinning flame which is rotating at a very high frequency in the combustor. The flame gets stabilized between two conditions of flow rates and mixture equivalence ratio as discussed in the next section. 3.2 Flame stability limits Fig. 3 shows the flame stability limits for a two and three step combustor. It is to be noted that for the same combustor length and similar dimensions, as the number of steps is increased to three, the lower flame stability limit increases further and helps in stabilizing the flame even at much lower flow rates. This can be observed from the curves LL (case 2) and LL (case 1). Similarly, the upper flame stability limits are also enhanced significantly for a three step case as compare to a two step combustor. The effect is more dominant at higher flow rates. For instance at Ф = 0.9, the upper flame stability limit is enhanced from 3.6 m/s to 5.3 m/s as the number of steps is increased from two to three. This indicates that by increasing the number of steps, the flame stability limits can effectively be increased. The lowest thermal input for the two step combustors ~ 5.5 W and it further reduces to ~ 4.5 W for the case of three step microcombustors. Similarly the series of out for combustors with varying dimensions and various fabrication materials

3 show that a stable flame can be achieved in a combustor volume of ~ 0.2 cm 3 and stainless steel material. The flame can be easily stabilized in quartz, stainless steel and brass combustors and it is very difficult to stabilize a flame in copper combustors [Khandelwal, Sahota and Kumar (2010)]. through an outer cup which redirects the flow of hot combustion products onto the outer wall of the combustor and transfers a part of heat from hot combustion products to the cold reactants through the solid wall of the combustor. It is clear that without any heat recirculation (dotted line), the flame is initially stabilized in the second step and it quickly moves downstream with an increase in the flow velocity. Once the flame is attached at the third junction (J3), the increase in the flow velocity affects the flame position by a small amount and then flame moves to the exit of the combustor. Similarly for the case of heat recirculation, the upper flame stability limit is increased from 2.8 m/s to 4.6 m/s due to heat recirculation. When flame is stabilized at junctions J2 and J3, the flow velocity has very little effect of flame position as shown by the bold line in Fig Flow field details Figure 3 Flame stability limits for two (case 1) and three (case2) step microcombustors 3.3 Effect of backward step on flame position Fig. 5 Numerical predictions of the recirculation zone formation and flame stabilization in a two step microcombustor (a) reaction rate contours (top) and flow streamlines (bottom) (b) CO contours (top) and temperature contours (bottom). [Khandelwal, Sahota and Kumar (2010)] Figure 4 Variation in the flame position with and without heat recirculation through an external cup Figure 4 shows the variation of the flame position with mixture velocity at a given mixture equivalence ratio of Ф = 0.8 for two conditions (a) with heat recirculation and (b) without heat recirculation. Heat recirculation is incorporated Fig. 5 shows the detailed distribution of flow streamlines, reaction rate contours, temperature and CO contours obtained from numerical modeling for a wall heat transfer coefficient of 10 W/m 2 K. It is clear from Fig. 5a that a recirculation zone is formed at the first and second step of the combustor. The formation of recirculation zone rearranges the flow velocity profile as shown by curves corresponding to an axial position of 9.0 and 10.5 mm in Fig. 6. Negative velocities are seen near the walls. Reaction rate contours show that flame is stabilized in the downstream of the first step. The formation of the recirculation zone helps in flame stabilization by altering the velocity profile. Top part of Fig. 5b shows the CO mass fraction contours. It shows that substantial amount of CO is formed in the reaction zone and it gets reconverted to CO 2 in downstream direction. The average mass fraction of CO at the combustion exit is very small, ~ Bottom

4 part of the Fig. 5b shows the temperature contours in the microcombustor. A peak temperature of ~ 1700 K is obtained in the combustor. The temperature contours further show that incoming reactants are preheated due to heat recirculation through solid walls. High temperature zone is limited to very small volume because of higher heat losses from stabilized flame to surroundings due to large surface area to volume ratio. temperature remains almost constant and overall variation is very small. These X-shaped flames are much longer in length and hence provide a better and more uniform heating as compared to the normal flame mode. Figure 7 Wall temperature profile along the length of a micro combustor 3.6 Emission measurements Figure 6 Normalized axial velocity distribution at different axial locations [Khandelwal, Sahota and Kumar (2010)] Fig. 6 shows the axial velocity profiles in radial direction at different axial locations, upstream of the flame position. The first step starts at 10 mm and the velocity profile is fully developed in the upstream, at 9 mm position. At an axial location of 10.5 mm, negative velocity can be seen for r/r = indicating the existence of recirculation zones immediately behind the step. Subsequently, at 12, 14 and 16 mm axial positions, flow tries to redevelop. It is to be noted that axial velocity near the wall region is very small for 10.5, 12 and 14 mm. At 16 mm, just upstream of the flame stabilization point, the velocity profile appears to have acquired a flatter profile and flow conditions are perhaps favorable for flame anchoring at that point. Hence flame gets stabilized at this position due to a balance between local flame velocity and flow velocity profile at that position [Glassman and Yetter (2008)]. 3.5 Wall temperature profiles The variation of wall temperature profile for a range of flow rates and a mixture equivalence ratio of Ф = 0.8 is shown in Fig. 7. Initially for very low flow rates, a normal flame is stabilized in the combustor and the average wall temperature is ~ 425 K. This temperature increases to ~ 550 K at a flow velocity of 1.5 m/s. For higher mixture velocities, a flame transition occurs and it leads to the formation of an X-shaped flame as shown in Fig. 2. This X-shaped flame continues to exist for a large range of flow velocities varying from 1.5 m/s to 4.5 m/s. During this operational mode, the average wall Figure 8 Variation of CO emission factor of planar and rotating flames in a 2-step profile channel with methane-air mixtures Figure 8 shows the variation of CO emissions with methaneair mixtures for a two step combustor. In a two step combustor, the X-shaped spinning flames are formed for a very small flow range. Therefore, flow velocities of 1.3 m/s and 1.5 m/s are considered for emission measurements and their comparison purpose. It is clear that for a flow velocity of 1.3 m/s a normal flame is stabilized and the emissions are relatively higher than as compared to the case of 1.5 m/s. Typical peak CO emission factor is ~ 55 g/kg of fuel for mixture equivalence ratios of Ф = 0.8 and 0.9. For higher

5 flow velocities, flame is stabilized at the exit of the combustor and CO emissions drop to very low values immediately. It is interesting to note that for same mixture equivalence ratios and a higher flow velocity of 1.5 m/s, a change in the flame propagation mode occurs from a stable flame mode to X-shaped spinning flame mode. Correspondingly, the CO emissions are reduced from 55 g/kg of fuel to 23 and 38 g/kg of the fuel respectively. This indicates that the X-shaped flame propagation mode can effectively help in reducing the emissions from such small scale combustion devices. 4 Conclusions The present paper reports the development of backward facing multistep microcombustors for various applications. It has been observed that the backward steps help in enhancing the flame stability limits in due to a change in the velocity profile and formation of recirculation zone at the step. A combustor with a typical thermal input of ~ 4.5 W and a volume of ~ 0.2 cm 3 was designed, fabricated and successfully tested during this work. Various flame propagation modes such as normal flame propagation modes are observed at low and moderate flow velocities. At higher flow velocities, X-shaped flame propagation modes are observed. The X-shaped modes results in more uniform temperature distribution and reduced CO emissions. Such systems are expected to find application in various heating micro power generation applications. [6] Kuo C H, Ronney P D 2007 Numerical modeling of non-adiabatic heat-recirculating combustors Proc. Comb. Inst., [7] Kim N I, Kato S, Kataoka T, Yokomori T, Maruyama S, Fujimori T and Maruta K 2005 Flame stabilization and emission of small Swiss-roll combustors as heaters Combustion and Flame [8] Kumar S, Maruta K, Minaev S 2007 Experimental investigations on the combustion behavior of methane-air mixtures in a micro scale radial combustor configuration Journal of Micromechanics and Microengineering [9] Khandelwal B, Sahota G P S, Kumar S, Investigations into the flame stability limits in a backward step micro scale combustor with premixed methane-air mixtures. Journal of Micromechanics and Microengineering 2010; 20: [10] Sahota G P S, Khandelwal B, Kumar S, Experimental investigations on a new active swirl based microcombustor for an integrated microreformer system. Energy Conversion and Management 2011; 52: [11] I Glassman, Yetter R A 2008 Combustion Academic Press, San Diego USA. Acknowledgement: The support from Department of Science and Technology is duly acknowledged through the fast track scheme for young scientists with a reference no. SR/FTP/ETA-56/2007. References [1] Fernandez-pello A C 2002 Micro-power generation using combustion: Issues and approaches Proc. Combust. Inst [2] Epstein A H, Senturia S D 1997 Macro Power from Micro Machinery Science [3] Maruta K, Kataoka T, Kim N I, Minaev S and Fursenko R 2005 Characteristics of combustion in a narrow channel with a temperature gradient Proc. Combust. Inst [4] Kumar S, Maruta K, Minaev S and Fursesnko R 2008 Appearance of target pattern and spiral flames in radial microchannels with CH 4 -air mixtures Physics of Fluids [5] Fan A W, Minaev S, Kumar S, Liu W and Maruta K 2008 Regime diagrams and characteristics of flame patterns in radial microchannels Combustion and Flame