1. Introduction
Due to expanding interest in and development of cutting-edge, highly-effective miniaturized and portable power generation devices, such as unmanned aerial vehicles (UAVs), microsatellite thrusters, miniature reactors, and sensors, the demand exists for more advanced technologies with reduced weight and high durability. Further, the combination of heat and power microsystems and thermo-photovoltaic applications, in which micro-combustion systems play a significant role, is advancing [
1,
2]. Although new energy resources such as batteries are emerging rapidly, they still have considerably lower energy densities compared to combustion-based sources [
3]. In particular, the density of the energy stored in a typical hydrocarbon fuel is orders of magnitude higher than that in a typical lithium-ion battery [
4]. Therefore, combustion-based power generation cannot be phased out immediately, especially in aviation and aerospace applications, and other devices such as micro satellites, for which weight is a critical and deciding factor. Thus, because combustion has been and remains the most reliable energy source, due to the high energy density of fossil fuels [
5,
6], and due to the increasing need for clean, green, and sustainable (CGS) energy, a critical need exists for an efficient method to sustain such micro-combustion.
Regarding CGS energy and its compatibility with other power generation systems based on combustion, numerous studies have been conducted on the application of synthetic gas (syngas), and have proven that syngas is potentially more advantageous than conventional fuels [
7], with advantages ranging from cheaper production to lower emissions [
8]. Syngas is mainly produced through gasification of heavy hydrocarbons (C
xH
y), biomass, or coal-based feedstocks, with carbon dioxide (CO
2) eliminated from the environment. The composition of syngas also varies depending on the raw material and the production process [
9,
10,
11,
12]. For instance, it exhibits high adiabatic flame temperatures in the presence of hydrogen (H
2) and carbon monoxide (CO) [
13]. Furthermore, the combination of H
2 and CO provides a lower flammability limit than methane (CH
4) and a higher upper limit than other hydrocarbons, having a wider flammability range than conventional fuels such as oil or natural gas. Thus, the presence of H
2 and CO in syngas exhibits flame-retardant characteristics, whereas the presence of inert gases such as nitrogen (N
2) and CO
2 reduces the flammability limits of syngas.
Regarding the unstretched laminar burning velocity,
, using a detailed chemical kinetics mechanism, Pio et al. [
14] studied the effect of syngas composition on reactivity, adiabatic flame temperature,
, energy, and pollutant production rate. Comparing their work with the data from the literature, the authors of Ref. [
14] found that the volumetric increment of CO
2 and CO reduced
, in addition to which the kinetic effect of CO
2 led the maximum values of
and
to shift towards the stoichiometric compositions. Syngas explosion properties in closed vessels and the impacts of different operating conditions, such as the equivalence ratio and syngas composition, on the flame characteristics have also been the subject of many studies [
15,
16,
17]. The impact of H
2-addition to CH
4 on the properties of syngas combustion was studied by Mardani et al. [
18]. It was shown that H
2-enrichment led to higher volumes of the hot regions and faster upstream movement, in addition to an increase in the distance between the locus of the maximum temperature and that of the stoichiometric mixture fraction, so that the maximum temperature exceeded the adiabatic flame temperature. Zhang et al. [
19] studied the effect of fuel variability for high hydrogen-containing syngas and demonstrated a good performance of the combustion process for certain fractions of H
2 and CO. Further, the effects of CO
2 and N
2 at various equivalence ratios were compared by Dam et al. [
20]. Similarly, the effect of varying syngas composition (based on different production methods) at different pressures was studied by Monteiro et al. [
21]. Although all the studies listed above showed important contributions of different fuel components to the combustion properties, the systems of interest were very large compared to micro-combustors. Wei et al. [
22] identified the operating cost as the major issue for syngas production, which was then suggested to have significantly reduced unit costs for higher production when industrialized with better manufacturing and optimization techniques. Research in this area is still in its rudimentary phases as burning at microscales and macroscales shows different behaviors [
23,
24,
25]. Therefore, further research is critically needed to study syngas combustion at microscales.
In a microscale system, the surface area to the volume ratio is higher than that at larger scales, which promotes heat losses from the system [
25,
26,
27,
28], thereby reducing the efficiency of a combustor. Further, a smaller size of the system leads to a reduced characteristic length scale of the flow and, therefore, a lower Reynolds number. Consequently, the flow is laminar. Although experimental analysis of combustion at both conventional scales and microscales is essential to validate the computational or analytical models and results, numerical simulations allow researchers to minimize the investments in experimental trials; in particular, preliminary results from numerical simulations can help researchers be more objective with their experimental setup and analysis. In microscale and mesoscale combustors, the stabilizing and destabilizing effects of the flame stretch, due to the wall heat losses, further complicate the analysis.
Various studies have been conducted in the past to sustain the microscale combustion process by incorporating a bluff body and a cavity into a micro-combustor [
29], and some researchers focused on the impact of the adiabatic walls on the flame stability in micro-combustors [
30]. In particular, Pizza et al. [
31] conducted extensive numerical simulations to study the characteristics of H
2-air combustion in a two-dimensional (2D) channel, employing various channel heights and inlet velocities of a reactant. Further studies have been conducted for other fuels, such as methane (CH
4) [
26,
32,
33], propane (C
3H
8) [
34,
35,
36], and syngas [
24,
37,
38]. The numerical work of Kousheshi et al. [
39] also analyzed the effect of syngas composition on the engine exhaust, with the reduction of the ignition delay time, sharper heat release rate, more NO
x, and less soot, CO, and unburned hydrocarbon production obtained for an H
2-rich mixture. These computational findings on the flame instabilities have also been confirmed experimentally [
40]. Brambilla et al. [
24] studied lean-premixed, syngas-air combustion in a microchannel of 7 mm height, by varying the equivalence ratios in the range
, the volumetric CO:H
2 ratio from 1:1 to 20:1, and the wall temperatures from 550 to 1320 K. As a result, the steady V-shaped and asymmetric flames, with the stationary and oscillatory modes, were observed in this work. Further, the stable flames, in addition to the flames with repetitive extinction and ignition (FREI), were shown for CH
4-air burning in a microchannel of 2 mm diameter by Maruta et al. [
41]. This experimental work investigated how the flame behavior depends on the equivalence ratio, identifying the stable flames for
, and the FREI were observed for
. The flat and stationary flames, with the cyclic oscillatory and the FREI behavior, were also observed for CH
4-air and C
3H
8-air combustion in another study by Maruta et al. [
23], with a fixed temperature gradient maintained at the channel wall. The phenomenon of repetitive extinction/ignition has been also observed in several other experimental studies for various fuels such as CH
4 [
42,
43] and ethane (C
2H
6) [
44].
Although syngas combustion at microscales has been studied previously, comprehensive studies accounting for detailed chemistry are necessary at these scales to further scrutinize the combustion characteristics, and the impacts of the mixture composition and the flow rate on various properties of syngas combustion at such small scales.
The present work is a step in this direction. Specifically, stoichiometric premixed syngas combustion in a microscale 2D channel was simulated by means of a detailed San Diego mechanism [
45], aiming to scrutinize the flame behavior for various mixture compositions and inlet velocities. The remainder of the manuscript is organized as follows: the methodology is explained in
Section 2; various combustion phenomena, such as extinction and ignition, stabilization, and instabilities, are observed and scrutinized in
Section 3; and the conclusions are summarized in
Section 4.
4. Conclusions
In this work, microscale combustion of syngas mixtures was studied, computationally, with the use of detailed chemical kinetics. The key agent of conjugate wall heat loss, which is inherent to microscale combustion, was addressed by imposing a temperature gradient on the wall, which also preheated the fuel mixture. The flame dynamics and other combustion characteristics at microscales were studied for various inlet flow velocities (ranging from 0.1 to 3.0 m/s) and syngas compositions (mixture A: 30% CO, 5% CH4, and 65% H2; mixture B: 70% CO, 5% CH4, and 25% H2; and mixture C: 60% CO, 10% CH4, and 30% of H2). Two different phenomena—the stable flames and the FREI—were observed with respect to the inlet velocities, such that for high inlet velocities ranging from 0.5 to 3.0 m/s, the flame was stable, whereas for the velocity range from 0.1 to 0.2 m/s, the flame became unstable and the FREI was observed. The ignition time, ignition location, and ignition length were analyzed for each case, and the stabilization characteristics, such as the stabilized flame location for the stable flames, were scrutinized. The ignition time and location, in addition to the stabilized flame location, where maximum domain temperatures are expected, are significantly important factors in designing micro-combustion-related technologies with syngas as a fuel. In particular, conjugate heat transfer/heat losses play an important role in micro-combustion devices, in which the ratio of the surface area to the volume is relatively higher than that of the conventional combustion devices. Further, the FREI properties were calculated and compared for the unstable cases. Regarding the fuel mixture composition, in the stable cases, mixture A showed more variations in terms of the total heat release, stabilization time, flame location, and flame span compared to those in mixtures B and C. Similarly, for the unstable cases, the time difference between the consecutive ignitions varied for all of the cases. Mixture A, which had a higher percentage of H2, was found to have a higher total heat release in the domain, and significantly different characteristics compared to the other two mixtures. This corroborates the major role and dominance of an H2 fraction in syngas combustion. Similarly, the effect of CO can be projected in the FREI mode for mixtures B and C in terms of the FREI period (). From a practical perspective, because syngas is generated from a variety of sources and methodologies, and can consist of different species of a wide range of concentrations, the findings of this work are critically important in selecting the appropriate composition of syngas depending on the application.
This research can be further extended in the following directions:
- (a)
to conduct in situ experiments for the operating conditions used in this work;
- (b)
to study the impact of the equivalence ratio on the flame behavior and instabilities;
- (c)
to consider 3D geometries and compare the results with those from 2D simulations; and
- (d)
to study the geometrical impacts on syngas combustion at microscales, while undertaking both experimental and numerical analyses over different geometries.