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Experimental Studies on Flame Stabilization in Backward Facing Step Micro-Combustors

Anil A.Deshpande,SudarshanKumar Department of Aerospace EngineeringIndian Institute of Technology Bombay, Powai, Mumbai-400076 India

Premixed fuel-air mixtures are introduced in backward step profile meso-scale quartz combustors. The study of flame stability limits and various shapes of the stabilized flame in micro-combustor were examined using two and three step quartz micro-combustors. Various fuel-air mixtures are used for the experiment. Methane with 99.9% purity and liquid petroleum gas (LPG) having approximately 60% butane and 40% propane were used as fuel. Stable flames were observed to exist for a range of flow rate and mixture equivalence ratio conditions. The flame stability limits were relatively narrow for a 2-step micro combustor as compared to a 3 step microcombustor. Lesser CO emissions were observed for a 3-step micro combustor as compared to a 2 step microcombustor because introduction of a backward facing step increases the local residence time of mixture.

I. Introduction Recent development in electro mechanical engineering devices especially for space and military application, led

to requirement of very small in size, light in weight and with high energy density electrical power sources, conceived the concept of micro or meso-scale combustors. Presently, the power sources used are batteries which have relatively smaller energy density as compare to hydrocarbon fuels and longer recharging times if used for power generation. Recent development in electro-mechanical engineering devices especially for space and military applications led to the need of micro propulsion applications such as MIT micro gas turbine engine and micro power generation with constraints on weight and ease of operation for longer duration as compared to present traditional sources available with limited life and smaller power to weight ratio [1-3]. Requirement of very small sized, smaller weight and high energy density electrical power sources conceived the development of micro/meso scale combustors. In future miniature or micro-combustors are expected to be major contributors for power generation and heat sources at smaller scales. Though the challenges are huge at every step in the development of micro-combustors, efforts invested by researcher’s world over are already taking leap towards understanding the flame stabilization process in such micro/meso scale systems and their development for practical applications [2].

The aim of present work is to understand the concept of flame stabilization in two and three backward facing step micro combustors, optimize their operational regime by experimenting with various fuels and also understand the role of variation of various step dimensions and fabrication material on flame stability limits.

II. Nature of flame in micro-combustor

In micro combustion very small dimensions affect the stabilization of the flame, shape of the flame and its location inside the combustor. Factors influencing the combustion are heat loss due to very high surface area to volume ratio, wall effects leading to radical quenching, mixture flow rates and mixture equivalence ratios. The effect of dimensions of the micro combustor on the transverse profile of velocity, temperature and volumetric heat loss were analysed by Li et al. [4]. Apart from conventional stationaty flames, other shapes of flames observed were cone shape at exit, curved shape flame and flame with repetitive extinction and ignition (FREI), also called oscillatory motion of flame with high frequency which was observed by Maruta et al. [5] and spinning flames by Christopher et al. [6] and Xu and Ju [7]. Transition of flame regimes was affected by the channel width, mixture flow rate and mixture equivalence ratio. As a result, the simultaneous change of combustor dimensions and flow rate are expected to significantly affect the heat loss and flame-wall coupling. Therefore, it is important to understand the effect of variation of combustor dimensions on the flame stabilization. Methane and propane–air premixed flames in a mesoscale divergent channel were experimentally investigated by Xu and Ju [7]. They observed the formation of

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various stationary and spinning flames for both lean and rich methane and propane–air mixtures. Kim et al. [8] have studied the Swiss-roll combustors of various designs to understand the flame stabilization in such micro combustor configurations. Similarly, the formation of various rotating flame patterns in a radial channels with a frequency range of 20-70 Hz has been reported by Kumar et al. [9] and Fan et al. [10]. The formation of these modes leads to substantial leakage of partially burnt or unburnt fuel resulting in excessive CO emissions and substantial drop in combustion efficiency [10].

III. Micro-combustor configurations

A cylindrical configuration was considered as the basic geometry of the micro-combustor due to simplicity of manufacturing and uniform thermal properties across the surface [11-14]. Use of different fabrication material is important aspect during optimization at initial phase of study[11]. As the diameter of the micro-combustor decreases, the heat losses from flame to wall increase due to an increased surface-to-volume ratio. Figure 1 and 2 show the micro-combustor configuration for typical 3-step and 2-step micro-combustors with major dimensions and type of material used for fabrication. The minimum diameter used in all combustors through which the mixture is supplied is 2 mm, which is slightly less than the quenching diameter for fuel used in the present experiments.

Fig.1 Configuration and dimensional details for 3-step backword facing microcombustor

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For very rich mixtures and low flow rates, it was observed that mixture cannot produce a self-sustaining flame

inside the combustor and the mixture burns at the exit as a partially premixed flame. For mixtures close to stoichiometry, the flames are relatively more stable inside the micro combustor. If the mixture flow rate is increased further, recirculation of the mixture at step junction helps in increasing the residence time of the fuel-air mixture and stabilizes the flame inside the combustor. At certain conditions of flow rate, an X-shaped flame or a spinning flame appears for different fuel-air mixtures as shown in Fig. 5. These flames are quite similar to those of Xu abd Ju in a divergent channel and appear for both methane-air and LPG-air mixtures. It is also observed that this phenomena is absent in the combustors of high thermal conductivity materials such as mild steel and stainless steel.

VI. Flammability limits

Various combustors were experimentally investigated to determine the operational limits in the mixture

equivalence ratio and flow velocity domain. Stable flames were observed to exist at different velocities and mixture equivalence ratios. Figure 6 (a) shows operational limits for a 3 step quartz micro-combustor using methane as a fuel, it is observed that stable flame is possible only above Φ = 0.6. During this process, it was observed that after flame is stabilized at junction of second and third step, a small increase in velocity does not affect the location of flame [11] and the shape of flame changes to X-shape, which is anchored at the junction of third step. A further increase in flow velocity increases the span of spinning flame in the third step and leads to upper limit of existence of X-shaped flame. X-shaped flame is formed only when combustor has enough temperature gradient. This may be obtained either through preheating the combustor or running this combustor for some duration. The X- shaped flame is observed for a range of equivalence ratios as shown in Fig. 6 This flame is observed to exist till Φ =1.05.

Fig. 6 Stabilized operational limits for 3-step micro-combustor with methane-air mixtures

Similarly with LPG-air mixtures, the same procedure was repeated and it was observed that stable flame was observed to exist for a mixture with equivalence ratio greater than 0.55 The lower flame stability limit remained almost constant for the given range of mixture equivalence ratios. It was further observed that X-shaped flame dominated at large range of equivalence ratios in LPG-air mixtures as compared to methane-air mixtures. The reason for this may be the higher effective Lewis number. Thermal wall coupling and flame bifurcation plays an important role for the formation of spinning flames.

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For a typical two step quartz micro-combustor, the same procedure was adopted using methane and LPG-air

mixtures. Figure 7 (a) shows operational regime for methane-air mixture for two step combustor. Operational limits of combustor are relatively very narrow till equivalence ratio Φ = 0.85 and X-shaped flame was observed at two equivalence ratios (Φ = 0.75 and 0.8) and at a mixture velocity of V=1.45m/s only.

Fig. 7 Stabilized operational limit for 2-step micro-combustor using Methane and LPG

Figure 7(b) shows operational regime for LPG-air mixture for two step combustor. It is observed that lower

operational limit is almost steady and upper limit gradually increases with an increase in the mixture equivalence ratio. X-shaped flame has moderate operational regime and upper limit is close to the overall upper limit of stabilized flame. A marginal increase in the inlet velocity above the upper limit of X-shaped flame blows it off and a diffusion flame is formed only if mixture is ignited again at same velocity.

VII. Flue gas analysis of micro-combustor

Though the micro-combustors are still in development phase, to become a successful contender considering

future applications, low combustion efficiency or high emission will restrict its application and especially if system is developed for indoor applications. Therefore, exhaust gas analysis was carried out at all operating regimes of flow rates and mixture equivalence ratio.

Analysis of the exhaust gas samples was carried out using Quintox (KM9106) flue gas analyzer and effort was made to collect the maximum possible byproduct after combustion. Measured emissions are accurate to ± 5 % of the CO reading. The emission samples are also analyzed using Nucon-5765 gas chromatograph and measured results have been found to be consistent with those of flue gas analyzer. When carbon from the fuel is fully oxidized during the combustion CO2 is a desirable byproduct and higher the percentage of CO2 would mean a better efficiency for the combustor. Other important byproduct is CO. The level of CO emissions should be minimum during combustion process. Excessive CO is formed due to incomplete combustion and from rich mixtures. Complete combustion in such micro combustors is a major challenge due to high heat losses and smaller residence time of the fuel-air mixture inside the micro combustor. The aim of flue gas analysis is to study the CO emission level for the given range of operating conditions of these micro-combustor.

Figure 8 shows the variation of the CO emission factor for a three step micro-combustor using methane-air mixtures. Combustor was operated at flow velocity V = 1 and 2 m/s for different equivalence ratios. CO and CO2 readings were noted for the stabilized flame. It was observed that with an increase in the mixture equivalence ratios at V = 1.0 m/s, emission factor increases till Ф = 0.8 and flame is positioned in second step. Further increase in the equivalence ratio leads to a drastic reduction in the CO emissions and flame is stabilized in the third step of combustor. It was observed that the position of the flame plays an important role in the complete combustion. Peak emission factor is observed when flame is stabilized at the junction of step.

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Fig. 8 Emission factor of stabilized and spinning flames for 3-step micro-combustor using methane-air

mixture

The same procedure was repeated for three step quartz combustor with LPG –air mixtures. Figure 9 shows the variation of the CO emission factor for the given range of mixture equivalence ratios at different flow velocities. At a velocity of V = 1.0 m/s, steady propagating flame at outer junction is observed at lower equivalence ratios with very high CO emissions. As equivalence ratio approaches Ф = 1.0, CO emissions reduce and a further increase in the mixture equivalence ratio leads to an increase in the emissions. At a flow velocity of V = 2.0 m/s and above it is observed that emissions are reduced significantly and operating regime was dominated by spinning flames for range of mixture equivalence ratios. For a velocity of V = 2.0 m/s, a peak emission was observed at Ф = 0.8 and it is reduced for higher and lower mixture equivalence ratios. At Ф = 0.8 and above as spinning flame span increase leading to an small increase in the CO emissions. LPG has high emissions for steadilyy propagating flames as compared to spinning flames. In the case of methane, combustor has limited range of velocity and equivalence ratios, where flame is inside the combustor and that too with high emission where as for LPG-air mixtures, flame is inside the combustor and forms a spinning flame with high combustion efficiency.

Fig. 9 Emission factor of stabilized and spinning flames for 3-step micro-combustor using LPG.

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VIII. Conclusions

Hydrocarbon fuels are a convenient and high-density energy source. Two types of fuel were used for the experiments in the present work. Methane with 99.9% purity and liquid petroleum gas (LPG) having approximately 60% butane and 40% propane. Flame shapes are largely affected by the inlet velocity of mixture, mixture equivalence ratio and also thermal conductivity of material used for micro combustor. Observations were made for different flow velocities and range of equivalence ratios.

Flames with different shapes were observed to exist inside the combustor. As methane is a lighter, less reactive fuel, with lower density and higher diffusivity than LPG, flames were light in color and were positioned at downstream as compare to LPG. The operational limits increase with an increase in the number of backward facing steps in the microcombustor. The increase in limits can be attributed to a change in the velocity profile inside the channel due a sudden increase in the crosssection area of the channel due to backward facing step. For methane and LPG, maximum emission is observed when flame is stabilized at junction of two steps. It is observed that, for all inlet flow velocities at low equivalence ratios, emissions are high for stabilized flames. When spinning flames are formed, emissions reduce.

References

1 A.C. Fernandez-Pello, Micropower Generation Using Combustion, Issues and Approaches, Proc. Combust. Instit. 29, 883-899 (2002).

2W.M. Yang, S.K. Chou, J. Li , Micro-thermal-photovoltaic power generator with high power density, Applied Thermal Engineering, 29, 3144-3148 (2009)

3H.L. Cao, J.L. Xu , Thermal Performance of a micro-combustor for micro gas Turbine system, Energy Conversion and Management 48, 1569–1578 (2007).

4Z.W. Li , S.K Chou, C. Shu , H. Xue, W.M. Yang, Characteristics of premixed flame in micro-combustors with different diameters, Applied Thermal Engineering, 25, 271-281 (2005).

5K. Maruta, T. Kataoka, N.I. Kim, S. Minaev, R.Fursenko, Characteristics of combustion in a narrow channel with a temperature gradient, Proc. Combust. Instit. 30 2429-36 (2005).

6J.Christopher, Evans, C. Dimitrios, Kyritsis, Operational regimes of rich methane and propane oxygen flames in mesoscale non-adiabatic ducts, Proc. Combust. Instit, 32, 3107–3114 (2009).

7B. Xu, Y. Ju , Experimental study of spinning combustion in a mesoscale divergent channel, Proc. Combust. Instit, 31, 3285-3292 (2007).

8N,Kim,S.Kato,T.Kataoka,T.Yokomori,S.Maruyama,T.Fujimori,K.Maruta, Flame stabilization and emission of swiss roll combustor, Combust Flame, 141, 229-240 (2005).

9S. Kumar, K. Maruta and S. Minaev, Pattern formation of flames in radial microchannels with lean methane-air mixture, Physical Review-E 75, 016208 (2007).

10A. Fan, S Minaev, S. Kumar, W. Liu and K. Maruta, Experimental study on flame pattern formation and combustion completeness in a radial microchannel, Journal of Micromechanics and Microengineering, 17, 2398-2406 (2007). 11B. Khandelwal, G. P. S. Sahota and S. Kumar, Flame stabilization studies in a backward facing step configuration based microcombustor with premixed methane-air mixtures, Journal of Micromechanics and Microengineering, 20, 095030 (2010). 12B. Khandelwal, S. Kumar, ''Experimental investigations on flame stabilization behavior in a diverging micro channel with premixed methane-air mixtures,'' Applied Thermal Engineering, Vol. 30, 2718-2723, (2010)

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13Mohammad Akram, S. Kumar, ''Experimental studies on dynamics of methane-air premixed flames in meso-scale diverging

channels,'' Combustion and Flame, Vol. 158, 915-924, (2011).

14G. P. S. Sahota, B. Khandelwal, S. Kumar, ''Experimental investigations on a new active swirl based micro-combustor for

an integrated micro-reformer system ,'' Energy Conversion and Management, Vol. 52, 3206-3211 (2011).

15 K. Maruta, Micro and meso scale combustion, Proc. Combust. Instit. 33, 125-150 (2011)