1. Introduction
The ever-growing demand for batteries for small-scale portable electronic devices with features like larger energy density, shorter recharge time and better performance has paved the way for advanced micro-electro-mechanical systems (MEMSs) to be developed rapidly. Because conventional chemical batteries possess problems like low power density, long recharging time and environmental pollution, etc., combustion based micro-devices are one of the best alternatives because of their very high specific energy, which is several times higher than that of the most advanced chemical batteries [
1]. Combustion-based micro-power devices include micro-gas turbines, micro-heaters, micro-thermo-electric generators [
2], micro-thermo photo voltaic generators [
3] and micro-fuel cells. Although some of the devices mentioned above are very difficult fabricate and operate, thermo-electric generators have been found to be a feasible option. This is because they can convert a certain percentage of thermal energy received from the micro-combustor into electrical energy through the Seebeck effect. The characteristic scale of micro-combustors is typically two orders of magnitude smaller than that of conventional combustors; thus, the surface to volume ratio is greatly increased, which eventually leads to the short residence time of the gaseous mixture, a significant heat loss effect, and a radical quenching effect. Flame stability in micro-combustors faces severe challenges, especially for premixed combustion.
Numerous studies have been conducted on premixed combustion with different fuels such as H
2, CH
4, C
3H
8, NH
3, etc. and various geometrical configurations, both simple types (like planar [
4], rectangular [
5], and cylindrical) and modified types (with backward steps [
6], cavities [
7], and porous media [
8] as well as the Swiss-roll type, swirling [
9], splitter plate, convergent–divergent type, bluff body [
10], catalytic type, cross plate insert [
11] etc.). Cai et al. [
12] numerically investigated the relevance of a bluff body as well as the exergy efficiency of a hydrogen-fueled meso-combustor. It was observed that the magnitude and uniformity of the lower wall temperature was improved, and the exergy efficiency was also considerably improved at all velocities considered. Cai et al. [
13] also numerically studied the effect of a secondary fuel injection on the NOx emissions of an ammonia-fueled combustor; it was observed that the NO emissions were reduced by 28%. Singh et al. [
14] performed unsteady simulations for premixed CH
4–air and H
2–air mixtures in micro-tubes to analyze flame dynamics. Lamioni et al. [
15] investigated the effect of slit pattern on flame structure in perforated burners and observed distinguished shaped flames as the number of slits varied. Different boundary conditions were tried and for a specific BC, the normalized flame propagation speed was found not to depend on the type of fuel. However, a geometry modification was primarily implemented so that the flow recirculation and the mixture residence time could be increased.
Ma et al. [
16] studied a novel Swiss-roll micro-combustor with two combustion chambers, in which the effect of the material on the flame’s blow-off limit for CH
4–air was analyzed numerically. The study revealed that the flame could be effectively anchored due to flow re-circulation with backward steps, and CH
4 blow-off limits could be further extended owing to the better heat re-circulation in the long preheating channels. Sankar et al. [
17] studied the application of different techniques for implementing liquid fuels in different types of micro-combustors for power generation. Cai et al. [
18] introduced a novel micro-disc burner with an annular step by using H
2 as fuel. It was observed in the study that a smaller Re value and larger Φ are suitable for the combustor; moreover, with the increase in thermal conductivity λ, the uniformity of the outer wall temperature was also increased. Additionally, as the surface emissivity
value decreased, parameters such as the wall temperature level, the flame temperature, and the excess enthalpy zone increased; this was primarily due to smaller loss of heat to the environment, but the thermal output power of the combustor was considerably reduced. Pan et al. [
5] experimentally investigated the flammability limits of a rectangular catalytic micro-channel using a CH
4–air mixture, in which stable flames were observed with mixture velocity ranging from 0.022 to 0.072 m/s for an equivalence ratio of one (Φ = 1).
Tang et al. [
11] developed a micro-planar combustor with a cross plate insert using a C
3H
8–air mixture, and they experimentally and numerically investigated its combustion characteristics. For an equivalence ratio of one (Φ = 1), a stable flame was observed with mixture velocity ranging from 0.2 to 1.2 m/s. Cai et al. [
19] numerically studied the flame dynamics and stability of a CH
4–air mixture in a quartz planar micro-combustor, and it was observed that the flame was stabilized, with mixture inlet velocity ranging from 0.2 to 0.4 m/s for an equivalence ratio of one. Singh et al. [
20] investigated the effect of H
2 addition on the flame dynamics of premixed methane/air mixtures in a micro-tube of length 120 mm and diameter 2 mm. A FREI was studied, and it was observed that the FREI had a non-monotonic variation with the mixture. Different diameter tubes (1, 1.5, and 2 mm) were considered in order to study the effect of diameter on the FREI, and increased FREI frequency was found for the 1 mm rather than the 2 mm dia. tube. Peng et al. [
21] experimentally and numerically investigated the combustion and thermal phenomena of a H
2-C
3H
8–air mixture in a stepped cylindrical tube with partially filled porous media. The study revealed that for an equivalence ratio of one, the flame was stabilized, with mixture velocity ranging from 4 to 7 m/s. From all the above discussed studies, it can be concluded that the major drawback of premixed combustion is that its stability limits are too low, which makes it really difficult for the flame to be stabilized properly, owing to the narrow range of inlet velocities. Another drawback of premixed combustion is the high possibility of flashback.
For non-premixed combustion, also called diffusion combustion, flame stability is mainly affected by the mixing of fuel and oxidant, which results in peculiar flame behaviors. Liu et al. [
22] numerically investigated the mixing of methane and oxygen in Y-shaped meso-scale combustors with and without porous media and observed that for the case with porous media, as the mass dispersion effect was more significant, fairly good mixing could be achieved even in horizontal channels of shorter length. It was also found that the mixing phenomenon became worse when the included angle between two inlet channels decreased or the inlet velocity increased. Ning et al. [
23] experimentally investigated the impact of porous media on mixture ignitability, flame stability, and flammable range for diffusion methane–air flames in Y-shaped meso-scale combustors of three different diameters (i.e., d = 4, 5 and 6 mm), wherein it was observed that the ignition distance increased with increasing inlet velocity and decreased with a decreasing channel diameter. It was also observed that with the addition of porous media, the CH
4/air mixture was able to be ignited near the splitter (owing to enhanced mixing) and the flame propagation velocity was significantly decreased. Additionally, the flame stability was greatly improved and the flammable range of the combustor was enlarged. Muraleedharan et al. [
24] numerically investigated the effect of wall material and thickness on flame height and flame temperature for micro-jet flames with hydrogen as fuel. It was found that when the burner conductivity was low, flame temperature was high, whereas flame length increased with thermal conductivity.
Li et al. [
4] numerically studied the effects of inlet velocity and channel height on the mixing performance, flame stability limit, and combustion efficiency of H
2 and air in a 2D planar micro-combustor with a separating plate. The results revealed that improved mixing can be obtained with a decrease in inlet velocity and channel height; moreover, for identical inlet air velocities, the combustion efficiency increases with a reduction in combustor height, which indicates improved mixing in a narrower channel. Additionally, the flame blow-out limit also exhibited a non-monotonic trend with increasing combustor height, that is, a micro-combustor with a medium height could achieve the largest blow-out limit. Xiang et al. [
25] experimentally investigated the effects of total flow rate and fuel/air ratio on the propagation behaviors of diffusion flames, and noise emission was observed in flame propagation in the upstream direction under comparatively higher mixture velocities. Li et al. [
26] numerically studied the flame stability of a non-premixed H
2–air mixture in a micro-combustor with a slotted bluff body and found that for a medium blockage ratio of 0.55, the flame blow-off limit appeared to be the highest (34 m/s). It was also found that as the blockage ratio increased, a stretching effect occurred, which ultimately resulted in local extinction and flame splitting. As we have discussed in premixed combustion, plenty of research has been carried out on non-premixed combustion with various geometries and different fuels as well. Among these configurations, some prominent ones include central and bilateral bluff bodies [
27], a Swiss-roll combustor [
16], a swirling micro-combustor [
28], and a micro-disc-combustor with an annular step and radial preheated channel [
29].
The key takeaway Is that non-premixed combustion is the best alternative to premixed combustion as the former can overcome the major drawbacks of the latter, which are flame stability range and flashback. Plenty of research has been conducted on non-premixed combustion with several geometrical configurations as well as various fuels, but one research area which is yet to be explored completely is that of blended fuels, which has always been an area of interest; this is because the combustion phenomenon can be enhanced in multiple ways by blending hydrocarbon fuels such as methane, propane, etc. with other components such as hydrogen. Hydrogen was chosen as the blending component owing to its high laminar burning velocity and stability range. Zhou et al. [
30] studied flame shape transition and structure characteristics by adding H
2 to CH
4 to a planar micro-combustor (but for premixed combustion). The results showed immense improvement in flame blow-out limit, due to the higher laminar flame speed of hydrogen compared to methane. Balaji et al. [
31] suggested the manufacturing of hydrogen from biomass by anaerobic digestion. Another alternative is biogas dry reformation. It was observed that the process absorbs CO
2 rather than emitting it. Hydrogen was produced from CH
4, CO
2, and H
2O (at rates of 52%, 38% and 10%, respectively) at a temperature of 837.5 °C.
Hou et al. [
32] studied the effect of hydrogen addition (up to 25%) for methane meso- scale combustion and observed that the combustible range is significantly enhanced. Additionally, the combustion reaction was enhanced and the blow-off limit was extended due to the improved chemical effect. Guo et al. [
33] also experimentally and numerically investigated the effect of hydrogen addition to methane-air flames, and it was observed that FREI and global quenching show a monotonic decrease. It was also observed that heat loss to the combustor walls is very relevant in cases of flame stabilization, and the quenching distance of CH
4-H
2 flames decreases as more hydrogen is added. Wang et al. [
34] investigated the dynamics of FREI with the addition of hydrogen to methane–air micro-combustion and observed that as the quantity of hydrogen increased, wall temperature showed a decreasing tendency while the flame propagated faster. Raissi et al. [
35] explored the effect of adding hydrogen to a methane–air composition and found that it caused nonlinear changes in pressure, velocity, and temperature inside the combustor. Eckart et al. [
36] investigated the extinction strain rate of CH
4-H
2 diffusion flames for a counter-flow configuration by varying the oxygen content in the oxidizer, and the results showed that as the oxygen content reduced, ESR also reduced. The research also included a comparison of reaction mechanisms and proved that along with LBV, ESR can also be an optimization criterion. Rajamanickam et al. [
37] experimentally examined the effect of hydrogen addition to methane in a canonical combustor with a bluff body, and the study revealed that when methane was enriched with more than 30% hydrogen, local extinctions disappeared and vortex shredding modes were completely suppressed. Moreover, the study established the significance of adding hydrogen for the stabilization of turbulent flames. Shin et al. [
38] analyzed the effect of hydrogen addition on flame structure and stability to methane, propane, and nitrogen in a coaxial diffusion combustor. The analysis disclosed thinner flame length and width. Additionally, high OH intensity and expansion of the flame stability region were observed as hydrogen was added. Wei et al. [
39] experimentally investigated the combustion characteristics of a combustor with a block with premixed CH
4-H
2 blends for micro-TPV application. While attempting to enhance the performance of combustor, the dimensions of the block, CH
4 mixing ratio, and combustor length were varied; finally, the optimum performance was achieved with a combustor length of 29 mm, a block thickness 0.6 mm, and a CH
4 molar fraction of 10%. The maximum power output was 2.9 W with an efficiency of 2.2%.
Therefore, understanding the trend in flame stabilization when adding H2 to CH4 in different proportions as fuel is relevant in that it will contribute to the area of combustion research. So, we proposed to investigate the effect of the addition of H2 to CH4 by comparing different combustion parameters such as the flame location, flame temperature, combustion efficiency, etc. of pure methane to methane blended with hydrogen (40% CH4-60% H2 by volume). To further understand the transition of parameters, an intermediate case (60% CH4-40% H2) was also considered.
4. Conclusions
This investigation focused on the non-premixed combustion characteristics of methane–hydrogen blends in a planar micro-combustor equipped with a splitter. The primary findings regarding the impact of hydrogen blending, inlet velocity, and global equivalence ratio on flame location, flame temperature, combustion efficiency, and outer wall temperature are outlined below.
1. The stability limit witnessed a significant increase of 50% with the addition of hydrogen, i.e., extending from 1–2 m/s for MH0 to 1–3 m/s for MH40 and MH60 at a stoichiometric air–fuel ratio (Φg = 1). This increased stability limit enabled the combustor to be operated for a wide range of velocities without extinguishing the flame.
2. The heat recirculation analysis of the combustor wall revealed a monotonic increase in the ratio of Qre to Qloss with increasing inlet velocity for all compositions. Moreover, as more heat was recirculated from the wall into unburnt gas compared to the heat loss from the wall to its surroundings (Qre > Qloss), the flame started lifting off, which can be used as a salient criterion of the diffusion of flames for determining occurrence of the lift-off phenomenon. The lift-off phenomenon affects the flame structure as well as wall temperature distribution, which subsequently affects the performance and operational range of the combustor.
3. From the study of the effect of hydrogen blending from MH0, MH40, and MH60 at an inlet velocity of 2 m/s and a global equivalence ratio (Φg) of unity, the flame location showed significant enhancement, reducing from 3.6 mm (MH0) to 2.2 mm (MH60). Additionally, there was a significant increase of 100 K in the outer wall temperature for MH60 compared to MH0. As more hydrogen was added to the fuel, the flame was stabilized further upstream of the combustor, which established the possibility of applying higher inlet velocity, which ultimately increased the operational range of the combustor. This increase in outer wall temperature will be advantageous for power generation using thermo-photovoltaic cells.
4. The effect of the global equivalence ratio can be studied either by varying inlet fuel velocity and keeping inlet air velocity constant (or vice versa). To understand the difference in both, simulations were initially performed by varying the inlet fuel velocity and keeping the inlet air velocity constant (2 m/s) in order to produce lean and rich conditions (Φg = 0.8–1.2) for the fuel compositions MH0, MH40, and MH60; then, the fuel inlet velocity was kept constant (Vfuel = 0.1 m/s) and the air velocity was varied to achieve the desired global equivalence ratios. Although the trend in the results of both the simulation setups remained the same with a difference in the magnitudes of both flame location and flame temperature, the latter results are more accurate as the results became comparable among different fuel compositions (MH0, MH40, and MH60) without much variation in firing rate. While the flames failed to stabilize for lean air–fuel ratios of MH0 and MH40, a stable flame was achieved for MH60 at Φg = 0.9 and 0.8. MH60 in lean conditions ensured a cleaner combustion (without emission) by providing a stable flame by complete combustion. Flame location and flame temperature showed a consistent reduction with the increase in the amount of hydrogen in the fuel. Flame temperature increased as the global equivalence ratio increased for each composition. At Φg = 1.0, flame location decreased from 2.33 mm (MH0) to 2.2 mm (MH60), and flame temperature decreased from 2070 K(MH0) to 1800 K(MH60).
Moreover, combustor efficiency and combustion efficiency decreased as the global equivalence ratio increased for each composition. Due to complete combustion of the lean mixture, MH60 exhibited the maximum value for both combustion efficiency (100%) and combustor efficiency (44%) at Φg = 0.8 and then reduced as the mixture became rich. At a stoichiometric air–fuel ratio (Φg = 1.0), pure methane exhibited the highest values of 99% and 45% for combustion efficiency and combustor efficiency, respectively.
On the whole, the addition of hydrogen to the blend extended the stability thresholds for both the inlet velocity and global equivalence ratio of methane. The enhanced outer wall temperature suggests potential applications in micro-power generation utilizing thermo-photovoltaic cells. Moreover, potential improvements in stability limits can be explored further through minor modifications to the shape and dimensions of the splitter.