Stability limits and chemical quenching of methane–air flame in plane micro-channels with different walls

Abstract

In order to elucidate the effects of wall material on stability limits and chemical quenching behavior, plane micro-channels with different wall materials were evaluated for the premixed combustion of methane–air. Experiments and numerical simulations with detailed chemistry kinetics schemes were carried out to explore the combustion characteristics along with the interaction between homogeneous and surface reactions on platinum, quartz glass, alumina ceramic and copper, and to estimate the effect of initial sticking coefficients associated with radical adsorption on chemical quenching. The experimental results indicate that the stability limits decrease in the order of platinum > copper > quartz glass > alumina ceramic for the different wall materials. The lower thermal conductivity wall leads to higher reaction temperature, which enhances the robustness of micro-flame. The simulation results indicate that chemical effect plays a critical role in the distribution of OH* radical. Homogeneous reaction is significantly inhibited on the platinum surface because of the depletion of reactants rather than the radical adsorption. Radical quenching is the most inhibited on the surface of alumina ceramic. The wall chemical effect on flame becomes very important as micro-channel is smaller than 0.7 mm.

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1. Introduction

Over the past decade, an interesting trend has developed in the miniaturization of electronic-mechanical devices in the fields of power, chemistry, aerospace, communication, military, and biomedicine.^1,2 These devices require stable, efficient and compact energy supply systems. However, high energy density can not be supplied by the traditional batteries.^3,4 Hydrocarbon-based micro-combustors are an enticing prospective energy source for portable power applications, and have several advantages compared with the traditional batteries,⁵ namely, long continuous working time, high energy density and environmentally friendly, etc. Therefore, micro-combustion has attracted considerable attention in recent years. One concept for designing a micro-combustor is to downscale the conventional scale gaseous combustor. However, homogeneous flame is typically quenched when confined in spaces with dimensions below 1.0 mm (equivalent to the flame quenching distance) because of the thermal and radical quenching on the wall surfaces.² As the characteristic length scales of a combustor decrease, the surface-to-volume ratio increases, leading to enhanced mass and heat transfer rates between the fluid and surface. As a result, thermal losses to the wall as well as radical adsorption followed by recombination into stable molecules rise. Despite the aforementioned challenges, devices capable of self-sustaining homogeneous flames in micro-channels have been developed.^2,4 Many useful strategies have attempted to improve the energy utilization efficiency and flame stability, such as Heat-Recirculating^6–11 and Swiss-Roll^12–15 micro-combustors to reduce heat losses. Another concern for micro-combustion is chemical quenching on the wall surface.

Homogeneous flame is difficult to maintain at the micro-scale because of both wall thermal and chemical effects.¹⁶ Heat loss to the wall and radical adsorption on the wall surface are two major effects of the wall on the flame. When the heat loss to the wall is greater than the heat generated by the combustion, the flame will be quenched. This behavior is referred to as thermal quenching, and can be avoided with adequate insulation of the combustor. Another quenching behavior is due to the removal of the active radicals from the reaction zone by diffusion to the surface of the wall and called chemical quenching.¹⁷ The heterogeneous reaction including radical adsorption at the surface bare sites, recombination, and desorption of recombined molecules becomes prevalent near the wall. The effects of these wall phenomena are usually in the negligible order of magnitude compared with the homogeneous reaction that dominates combustion in the macro-scale device. When the removal of radicals is overly activated, flame is quenched even when the heat loss to the wall is insignificant. The chemical quenching is more pronounced at high wall temperature where surface heat loss is small, because the heterogeneous reaction becomes dominant at high temperature. Chemical quenching is also affected by the surface properties and treatment method for defect removal.^17,18

Compared to the thermal quenching, the chemical quenching has received significantly less attention and only a few studies have been conducted to assess the effect of radical quenching in micro-combustion. Pfefferle et al.^19,20 investigated the effect of catalytic activity on gas phase ignition in an ethane–air boundary layer by measuring OH concentration profiles over heated quartz and platinum surfaces. They found that catalytic surface (induced production of active intermediates) can promote gas phase ignition and offset the effect of reactant depletion near the surface. This study on the wall chemical effect is focused on the role of surface activity in homogeneous flame. Furthermore, the wall effects of the radical quenching and recombination on homogeneous flame are also being explored. Aghalayam et al.²¹ studied the role of the wall quenching of radicals in autothermal, ignition and extinction behaviors of premixed hydrogen–air flame impinging on a flat surface by means of using numerical bifurcation techniques, with detailed gas phase reaction kinetics and surface radical recombination reactions. The results showed that the quenching out of radicals retard the system at ignition due solely to the surface reaction kinetics, as well as the combined kinetics and thermal effects of wall radical quenching can significantly expand the autothermal regime. Raimondeau et al.²² studied the methane–air flame propagation in micro-channels by using two-dimensional parabolic simulations with detailed gas-phase chemistry, multicomponent transport, radical recombination on the wall surface, possible temperature discontinuity at the wall due to lack of thermal accommodation, and heat loss through the wall. They found that the wall radical quenching and near-entrance heat loss are the key issues in controlling flame propagation of methane in a micro-channel. Yang et al.²³ examined the effects of temperature and wall material on flame quenching distance between two parallel walls. They found that the flame quenching distance decreases with increasing the wall temperature or the concentration of chemisorbed oxygen. Bai et al.²⁴ performed theoretical analysis on flame propagation with both kinetic and thermal quenching mechanisms in a micro-tube. The results showed that the radical quenching effect becomes very significant at higher wall temperature. Wang and Law²⁵ studied the explosion limits of hydrogen–oxygen and found that the wall destruction of radical is very crucial to the Z-shaped explosion limits.

Miesse et al.²⁶ experimentally investigated the effect of surface composition and material properties on the flame quenching distance in two parallel plates, along with designed and optimized the devices burning methane–air and propane–air mixtures in a 0.75 mm slot to achieve high conversion. The results showed that the micro-combustor needs to be insulated well enough that the heat of combustion is sufficient to keep the reacting mixture hot enough to sustain significant combustion and prevent thermal quenching, and the wall need to be fabricated from materials that do not quench radicals so that the homogeneous reaction can occur unimpeded. Kim et al.²⁷ also experimentally investigated the effects of surface property and wall temperature on the flame quenching of methane–air in a micro-combustor to assess the relative roles of chemical and thermal quenching, and estimate the relative significance of heat loss and removal of active radicals on the wall surface, finally understand the physical mechanisms involving the recombination processes of heterogeneous radical. The results showed that the flame quenching distances of methane–oxygen are almost constant and independent of surface characteristics for low surface temperature ranging between 100 and 350 °C, and are strongly dependent on wall material and surface characteristics when surface temperature increases beyond 400 °C. They also found that the radical removal on the wall surface play an important role in the quenching process. Zhou et al.²⁸ employed experiment combined with CFD simulation to investigate hydrogen–air combustion in three catalytic micro-combustors with different materials (alumina ceramic, quartz glass, and copper) to assess the thermal effect of wall material on their performances, the results showed that the higher wall thermal conductivities make the heterogeneous reaction dominate and induce the temperature distribution and heat loss in a catalytic micro-combustor, which is opposite to the conventional non-catalytic situation. They also found that the reaction mode transfers from homogeneous reaction to heterogeneous reaction with decreasing the flow rate. Chen et al.²⁹ studied the thermal effect of wall material (platinum, silicon and alumina) on the distributions of temperature and species concentration in a catalytic micro-reactor, using CFD-ACE simulations. The results showed that the wall material with lower thermal conductivity will lead to higher temperature gradient along the axial direction of the interior wall, promote the homogeneous reaction shift upstream and have a wider temperature distribution. Lee et al.³⁰ experimentally investigated the effect of scale and material on flame characteristics in heat recirculation and counter-current micro-channels. They found that the variation on the wall material significantly affected the flame stabilization compared to the scale-down effect.

The purpose of this work is to explore the stability limits and chemical quenching behaviors of methane–air premixed combustion in plane micro-channels with different wall materials including platinum, quartz glass, alumina ceramic and copper, as well as to examine the interaction between homogeneous and surface reactions with detailed chemistry kinetics schemes. Numerical simulation is also carried out to estimate the effect of initial sticking coefficients associated with radical adsorption.

2. Experimental apparatus

Micro-combustion experiments were performed and tested to see whether stable combustion can be achieved. A schematic diagram of the experimental setup is shown in Fig. 1, consists of micro-combustor, measurement instruments and gas feed system. Methane and air were metered through mass flow controllers and mixed in the mixing chamber. Micro-combustion was established for values of the equivalence ratio (ϕ) ranging from 0.02 to 20.0 for methane–air mixtures and the inlet velocity (ν) ranging from 0.02 to 2.0 m s⁻¹. The flame image was measured by the high speed motion analyzer Phantom® M310 and recorded by personal computer. Measurements of species concentration were performed with an Agilent 7890A Gas Chromatograph (GC).

Fig. 1 Schematic diagram of the experimental apparatus.

The micro-combustor used in this study is two parallel plates of length L = 10.0 mm and wall thickness δ = 0.4 mm. The plates are separated by a gap size (height) of d = 2.0 mm. The third dimension (width) of the micro-channels is 40.0 mm, and much larger than the gap size. The wall materials of micro-channels are platinum, quartz glass, alumina ceramic and copper, respectively. Their properties are given in Table 1. The inlet mixture is at room temperature and at atmospheric pressure.

Table 1 The properties of four wall materials

Material	Density (kg m^−3)	Specific heat capacity (J kg^−1 K^−1)	Thermal conductivity (W m^−1 K^−1)	Thermal diffusion (m^2 s^−1)
Platinum	21 450	135	72	2.49 × 10^−5
Quartz glass	2500	890	0.7	3.15 × 10^−7
Alumina ceramic	3940	37	32	2.20 × 10^−4
Copper	8978	386	387	1.12 × 10^−4

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3. Numerical models and simulation approach

A schematic view of the plane micro-channel geometry is illustrated in Fig. 2. The nominal micro-channel geometry is the same as the one employed in previous experimental studies. FLUENT-Kinetics³¹ coupled with CHEMKIN^32–34 were used to simulate the fluid flow and the interaction between homogeneous and surface reactions in micro-channels with different wall materials. The micro-channels for the reactant intake, the product exhaust, and the actual combustion chamber constrain the flows in micro-devices to relatively small Reynolds number. For micro-devices with very small characteristic lengths and consequently small Reynolds and Peclet numbers, the flow is primarily laminar.³⁵ In this study, the Reynolds number of the flow was between 7 and 300 at the inlet and the reaction region, and therefore fluid flow is laminar. Uniform node spacing of 5 μm is utilized for all scenarios analyzed. Different mesh size has been tested to ensure grid-independence in the computation. In prior to the productive run, dependency of the grid is carefully evaluated and the present grid is found to be fine enough. For example, nearly-double coarser grid system gives about 0.2% difference in OH* peak value and 0.05% difference of reactants and products concentration comparing with the present simulation. The equations of continuity, species, momentum, and energy are discretized with the finite-difference method. The thermal properties are computed using the CHEMKIN thermodynamic database and the transport properties are computed using CHEMKIN gas-phase transport libraries.³⁴ A second-order upwind difference scheme is used to discretize the governing equations and the SIMPLE algorithm is used to deal with the pressure–velocity coupling. A 2D segregated solver with the under-relaxation method is employed to solve the conservation equations. The solver first solves the momentum equations, then solves the continuity equation, and updates the pressure and mass flow rate. The energy and species equations are subsequently solved. The simulation convergence is judged upon the residuals of the whole governing equations. The convergence criteria for the scaled residuals are set to be 1.0 × 10⁻⁵ for continuity, 1.0 × 10⁻⁶ for velocity, 1.0 × 10⁻⁷ for energy and 1.0 × 10⁻⁸ for species concentration. Hereafter, x and y denote the streamwise and the wall-normal directions, respectively.

Fig. 2 Schematic diagram of the plane micro-channel geometry.

The incoming methane–air flow had uniform inlet temperature of 300 K. Fixed pressure and velocity are specified at the inlet and outlet, respectively. The solid and fluid phases are linked with coupled interface. The convection and thermal radiation boundary conditions are employed at the outer wall surfaces. On the wall, the mass fluxes corresponding to the surface reaction are defined and no-slip boundary conditions are imposed.

Detailed chemistry kinetics schemes are employed in both homogenous and surface reactions of methane–air. The oxidation of methane on the platinum surface was described by the detailed scheme of Deutschmann et al.,³⁶ which consists of 11 surface species and 24 elementary reactions. For other wall materials including quartz glass, alumina ceramic and copper, the radical quenching mechanism (shown in Table 2) proposed by Raimondeau et al.²² is employed, consists of 5 surface species and 10 elementary reactions. In their radical quenching model, OH, O, H and CH₃ radicals play a significant role in ignition and extinction of methane–air,³⁷ and they adsorbed and desorbed on the surface and recombined to form the stable gas-phase species (H₂, O₂, H₂O, CH₄ and C₂H₆).

Table 2 Radical quenching mechanism along with kinetic parameters

Reactions	Pre-exponential (s^−1) or sticking coefficient
CH3 + M* → CH3* + M	0–1
H + M* → H* + M	0–1
OH + M* → OH* + M	0–1
O + M* → O* + M	0–1
2CH3* + 2 M → C2H6 + 2 M*	10^13
2H* + 2 M → H2 + 2 M*	10^13
2OH* + M → H2O + O* + M*	10^13
2O* + 2 M → O2 + 2 M*	10^13
CH3* + H* + 2 M → CH4 + 2 M*	10^13
OH* + H* + 2 M → H2O + 2 M*	10^13

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The rate of radical adsorption r_ad based on the Langmuir adsorption model (namely Langmuir equation or Langmuir isotherm) is expressed by:

where p, [s], M, R, θ and S₀ are partial pressure of the radical over the surface, concentration of free sites, molecular weight, ideal gas constant, fractional coverage of the surface and initial sticking coefficient of the radical, respectively. The concentrations of free sites are estimated from the distance between the neighboring molecules on the wall surface, and are 2.706 × 10⁻⁹ (platinum),³⁶ 1.314 × 10⁻⁹ (quartz glass), 1.360 × 10⁻⁹ (alumina ceramic) and 3.153 × 10⁻⁹ (copper) mol cm⁻², respectively.

4. Results and discussion

4.1. Stability limits

Direct photo of flames in the micro-combustor of quartz glass is depicted in Fig. 3. According to the experimental results, the stability limits of homogeneous reaction for different wall materials are plotted in Fig. 4. Homogeneous flame quenches once the equivalence ratio exceeds the stability limits. However, the stability limits for the platinum were not measured, because there are no obvious homogenous reaction.^38–40 It is inhibited by heterogeneous reaction in catalytic condition, and can be ignored in most cases.^41–45 The copper, of all of the transition elements, has the part catalytic effect from the viewpoint of quantum chemistry and DFT (Density Functional Theory).^46,47 Therefore, its stability limits are not given because of no obvious extinction behavior.

Fig. 3 Direct photo of flames in the micro-combustor of quartz glass.

Fig. 4 Stability limits of homogeneous reaction in the micro-combustors of alumina ceramic, quartz glass and copper wall materials.

The lower stability limits (black line) for quartz glass are close to equivalence ratio of 0.05, which indicate that it has higher stability in lean case than rich. This behavior is mainly attributed to the lower thermal diffusion and higher mass diffusion of lean mixture.^26,48 Heterogeneous reaction helps enhance the homogenous one, and its rate depends on the mass diffusion from gas to the surface of the wall.⁴⁸ In the lean cases, the diffusion of methane dominates, and it is about two times faster than air. Its slightly lower thermal diffusion also helps preserve heat. The lower stability limits (red line) for alumina ceramic are fixed around equivalence ratio of 0.15 as the inlet velocity is higher than 0.8. In contrast, the upper stability limits (green line) increase for values of the equivalence ratio ranging from 4.08 to 10.82 as the inlet velocity is increased from 0.05 to 2.0 m s⁻¹. Extinction occurs more easily at low inlet velocity. This behavior is mainly attributed to the ratio of heat loss and heat generation.^26,48 The heat generation increases proportionally with the inlet velocity, while the heat loss increases more slowly with the inlet velocity.⁴⁹ Therefore, their ratio at high inlet velocity is lower and results in preserving more heat and helping sustain the homogeneous reaction.

The stability limits for alumina ceramic are narrow compared with quartz glass. At the inlet velocity of 0.2 m s⁻¹, the upper limit (equivalence ratio) for alumina ceramic is 0.94 lower than quartz glass, and the difference increases to 2.08 at the inlet velocity of 2.0 m s⁻¹. The lean case shows the similar behavior. The average lower limit for quartz glass is 0.118, and it lower than the alumina ceramic of 0.187. The reason is that lower thermal conductivity for the quartz glass leads to higher reaction temperature,^27,50 which enhances the robustness of micro-combustor. In general, the stability limits in micro-combustors with different wall materials decrease in the order of platinum > copper > quartz glass > alumina ceramic.

4.2. Surface activity of platinum

In the present work, the wall temperature T_w is kept constant and the equivalence ratio of methane–air mixtures is 0.8. The contours of normalized OH* mole fraction by its maxima in platinum micro-combustor at different wall temperatures are shown in Fig. 5. As observed, the OH* concentration near the platinum surfaces is very low due to the thermal quenching effect, and increases with the wall temperature. The wall-normal distributions of normalized OH* mole fraction near the platinum surfaces are shown in Fig. 6. Note that the OH* concentration becomes maximum at these streamwise positions (1.2 and 1.0 mm). The wall effect on OH* distribution is negligibly small at y = 1.6 mm, but the OH* concentration is monotonically decreased toward the platinum wall at y = 1.6–2.0 mm. This behavior shows that the thermal effect on the wall can be well represented by using the present numerical model. Note that the simulation results are represented at these streamwise positions (1.2 and 1.0 mm) of maximum OH* concentration hereafter.

Fig. 5 The contours of normalized OH* mole fraction at different wall temperatures. The fuel is methane (ϕ = 0.8) and the wall material is platinum. The inlet temperature T_in and the inlet velocity V_in are 300 K and 2.0 m s⁻¹, respectively.

Fig. 6 Wall-normal distributions of normalized OH* mole fraction near the platinum surfaces at x = 1.2 mm (T_w = 800 K) and x = 1.0 mm (T_w = 1000 K).

The simulation results for the wall-normal distributions of normalized OH* mole fraction and heat release rate (HRR) on the platinum and inert surfaces at x = 1.0 mm (T_w = 1000 K) are shown in Fig. 7. With the surface reaction on platinum, both the OH* mole fraction x_OH and the heat release rate are decreased at y = 1.6–2.0 mm. Under this condition, the heat generation is decreased by around 2.0%.

Fig. 7 Wall-normal distributions of normalized OH* mole fraction and heat release rate (HRR) on the platinum and inert surfaces at x = 1.0 mm (T_w = 1000 K).

The streamwise distributions of methane and oxygen concentrations (on a molar basis) near the platinum and inert surfaces at y = 0.02 mm (T_w = 1000 K) are shown in Fig. 8. As observed, both methane and oxygen concentrations near the platinum surface are decreased dramatically compare with the inert surface. This behavior is because of the rapid depletion of methane and oxygen on platinum by the surface catalytic reaction, which results in the suppression of homogeneous flame near the platinum surface (y = 0.02 mm). This result is consistent with the conclusion from the previous work of Chen et al.²⁹ who used hydrogen as fuel, and proposed that the homogeneous reaction will be inhibited by platinum, especially in fuel-lean condition. In the present work, the effect of radical adsorption on homogeneous flame is also examined by using separate simulation coupling surface catalytic reaction mechanism³⁶ excluding the adsorption of OH*, O*, and H* radicals on the platinum surface. However, the simulation results indicate that the homogeneous flame structure is almost unchanged, which shows that the homogeneous reaction on platinum is significantly inhibited because of the depletion of reactants rather than the radical adsorption.

Fig. 8 Streamwise distributions of methane and oxygen concentrations near the platinum and inert surfaces at y = 0.02 mm (T_w = 1000 K).

4.3. Surface activity of quartz glass, alumina ceramic and copper

The normalized OH* mole fraction for different wall materials at T_w = 1000 K are shown in Fig. 9. As observed, chemical effect plays an important role in the distribution of OH* radical. It has the largest chemical effect on the platinum surface, and the OH* concentration near the platinum surface zone (y = 0–0.02 mm) is the lowest as observed in Fig. 9d. This behavior is because of the radical quenching including radical adsorption (removal of active radicals from the reaction zone by diffusion to the catalyst surface), recombination, and desorption of recombined molecules.

Fig. 9 The contours of normalized OH* mole fraction at T_w = 1000 K. The fuel is methane (ϕ = 0.8) and the wall materials are quartz glass, alumina ceramic, copper and platinum, respectively. The inlet temperature T_in and the inlet velocity V_in are 300 K and 2.0 m s⁻¹, respectively.

Near the surface of quartz glass (generally identified as inert surface) as observed in Fig. 9a, the OH* concentration is much increased. The distribution of OH* radical near the copper surface (Fig. 9c) is slightly lower than quartz glass. This behavior is because copper has the part catalytic effect^46,47 as mentioned above. However, the OH* concentration near the surface of alumina ceramic (Fig. 9b) is much higher than quartz glass, which indicate that radical quenching is the most inhibited on the surface of alumina ceramic.

The wall-normal distributions of normalized OH* mole fraction near the surfaces of quartz glass and alumina ceramic for different wall temperatures are shown in Fig. 10. Note that the OH* concentration becomes maximum at these streamwise positions (1.2, 1.0 and 0.8 mm). At lower wall temperature (800 K), the effect of wall material on distribution of OH* concentration is very small. However, the OH* concentration at higher wall temperature (1200 K) is strongly dependent on wall material. This behavior is qualitatively consistent with the previous work of quenching distance by Miesse et al.,²⁶ who used quartz, alumina, stainless steel, and cordierite (Mg₂Al₄Si₅O₁₈) as wall material, and found that the quenching distance did not dependent on wall material when the wall temperature is below 500 K, while strongly depended when it is near 1273 K.

Fig. 10 Wall-normal distributions of normalized OH* mole fraction near the surfaces of quartz glass and alumina ceramic at x = 1.2 mm (T_w = 800 K), x = 1.0 mm (T_w = 1000 K) and x = 0.8 mm (T_w = 1200 K).

4.4. Effect of initial sticking coefficient

Sticking coefficient S is the term used to describe the ratio of the number of adsorbate radicals (or molecules) that adsorb (or stick) to wall surface to the total number of radicals that impinge upon that wall surface during the same period of time. The value of sticking coefficient is between 0 (none of the atoms stick) and 1.0 (all impinging atoms stick). The coefficient is a function of surface temperature, surface coverage (θ) and structural details as well as the kinetic energy of the impinging particles. The initial sticking coefficient is crucial for the radical quenching behavior because the quenching process is considered to be adsorption-limited.^22,26 In the present work, the effect of initial sticking coefficient is evaluated through the profiles of surface site fraction and OH* concentration distribution.

On the surfaces of quartz glass and alumina ceramic, the fraction of surface site obtained with initial sticking coefficient S₀ = 1.0 (i.e. radical adsorption on the wall surface is the most enhanced) at T_w = 1000 K are shown in Fig. 11. As observed, the fractions of surface site for four surface species (OH(s), O(s), H(s), and CH₃(s)) are on the order of 10⁻⁴ although the sticking coefficient S₀ is 1.0. On the other hand, the remaining surface sites (bare sites) are unoccupied. It shows that the quenching process is limited by radical adsorption, and radical adsorption is dominant for chemical effect.

Fig. 11 The fraction of surface site obtained with initial sticking coefficient S₀ = 1.0 at T_w = 1000 K.

The wall-normal distributions of normalized OH* mole fraction near the wall surface (y = 0.02 mm) with initial sticking coefficient S₀ = 1.0, 0.1, 0.01 and 0 (inert surface) at T_w = 1000 K are shown in Fig. 12. For initial sticking coefficients S₀ = 1.0 and 0.1, OH* concentration in the vicinity of the wall surface becomes closer to 0 compare with S₀ = 0.01 and 0 because of large quenching affinities. Note that the discrepancy of OH* distribution for different initial sticking coefficients S₀ becomes significant within 0.35 mm from the wall surface. Therefore, the wall chemical effect on flame becomes very important as micro-channel is smaller than 0.7 mm.

Fig. 12 Wall-normal distributions of normalized OH* mole fraction near the wall surface (y = 0.02 mm) with initial sticking coefficient S₀ = 1.0, 0.1, 0.01 and 0 at T_w = 1000 K.

5. Conclusions

In this study, the stability limits and chemical quenching behaviors of methane–air flame in plane micro-channels with different wall materials were carried out to explore the combustion characteristics along with the interaction between homogeneous and surface reactions on the surfaces of platinum, quartz glass, alumina ceramic and copper. It was also performed to estimate the effect of initial sticking coefficients associated with radical adsorption on chemical quenching.

• In general, the stability limits of different wall materials decrease in the order of platinum > copper > quartz glass > alumina ceramic.

• The lower thermal conductivity wall such as quartz glass leads to higher reaction temperature, which enhances the robustness of micro-flame.

• Chemical effect plays an important role in the distribution of OH* radical at the wall temperature of 1000 K.

• On the platinum surface, homogeneous reaction is significantly inhibited because of the depletion of reactants rather than the radical adsorption.

• On the surface of alumina ceramic, radical quenching is the most inhibited.

• OH* distribution for different initial sticking coefficients becomes significant within 0.35 mm from the wall surface. Therefore, the wall chemical effect on flame becomes very important as micro-channel is smaller than 0.7 mm.

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Footnote

† Current address: School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P. R. China.

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