Present technology has been shifting towards miniaturization of devices for energy production for portable electronics. Micro-combustors, when incorporated into a micro-power generation system, create the energy desired in the form of hot gases to power such technology. This creates the need for a design optimization of the micro-combustor in terms of geometry, fuel choice, and material selection. A total of five micro-combustor geometries, three fuels, and three materials were computationally simulated in different configurations in order to determine the optimal micro-combustor design for highest efficiency. Inlet velocity, equivalence ratio, and wall heat transfer coefficient were varied in order to test a comprehensive range of micro-combustor parameters. All simulations completed for the optimization study used ANSYS Fluent v16.1 and post-processing of the data was done in CFD Post v16.1. It was found that for lean, premixed fuel-air mixtures ( φ = 0.6 - 0.9) ethane (C 2H 6) provided the highest flame temperatures when ignited within the micro-combustor geometries. An aluminum oxide converging micro-combustor burning ethane and air at an equivalence ratio of 0.9, an inlet velocity of 0.5 m/s, and heat transfer coefficient of 5 W/m 2-K was found to produce the highest combustor efficiency, making it the optimal choice for a micro-combustor design. It is proposed that this geometry be experimentally and computationally investigated further in order to determine if additional optimization can be achieved.
Present technology has been shifting towards miniaturization of devices for energy production in order to be useful to the everyday person. This miniaturization has created the need for micro-combustion, the exothermic chemical reaction between a fuel and oxidizer yielding the production of heat at the micro level. Portable communication devices, personal computers, and even portable media players all depend on some sort of energy production, usually in the form of a battery. The inevitable bulk associated with conventional batteries has become cumbersome and a growing need for a smaller and lighter power supply has emerged [
Micro-combustors are miniaturized versions of macro scale combustors and are similar in function. The scale of such devices is separated into two groups: the microscale and the mesoscale. The microscale includes micro- combustor diameters up to 1 millimeter, while the mesoscale accounts for any other devices with a diameter larger than that. Several micro-combustor geometries have been tested and simulated in previous studies, including constant-diameter, backward-facing step, Swiss roll, and annular configurations [
In order to validate the solver preferences, boundary conditions, and other variables that were to be used in si- mulations for this study, the work of Murali [
The reacting flow was solved for by reaching a convergence of 1e−06 for the residuals. Most of these default parameters were described in the reference work of Murali [
It can be seen from the results that as the inlet velocity is increased, higher maximum temperatures inside the micro-combustor are reached, and the flame tends to shift more towards the outlet. In the region where the
maximum temperatures are located the flow becomes faster along the centerline of the geometry. The mass fraction profiles show the reaction region moving further downstream as inlet velocity is increased as well as complete combustion of the methane. Also, in terms of the flame’s shape, at higher velocities the flame becomes more pointed rather than rounded at lower velocities. All maximum temperatures calculated in ANSYS Fluent v16.1 were within 4% of the temperatures reported in the reference work of Murali [
Five micro-combustor geometries were considered for these simulations: a constant-diameter, diverging, converging, converging-diverging, and backward-facing step geometry. These geometries were chosen due to previous experiments and computational simulations focusing on micro-combustor performance. The most common geometry used for micro-combustor research is the constant-diameter micro-combustor. This geometry is very simple and is both easily manufactured for experiments and modeled for computational simulations. It has been shown that flames can easily stabilize inside this type of geometry, which is of particular interest in micro-combustor research. A diverging geometry was used in these micro-combustor simulations due to its ability to slow down the flow within it. It was thought that because of the decrease in the speed of the flow that the flame would be anchored closer to the channel inlet for higher velocities and thus more stable. The converging micro-combustor geometry was chosen for these simulations due to its ability to speed up the flow at the exit plane. A converging-diverging geometry was used in these simulations after review of the work of Yang et al. [
The selection of a fuel for combustion is very important. The fuel needs to have a high enough heat of combustion to produce the required energy output. Hydrocarbons as well as hydrogen are the most common fuels used in micro-combustion due to their cost, availability, and high volume to energy density ratio. The diatomic form of hydrogen (H2) is extremely combustible with air as an oxidizer at a wide range of concentrations (4% - 75% by volume). Hydrogen has one of the highest heats of combustion at about 140 MJ/kg and has a lower ignition energy compared to most hydrocarbons, making it a go-to fuel for any combustion application. Hydrocarbons, such as methane (CH4) and hexane (C6H14), are the main fuels used in combustion presently because of the energy (in the form of heat) produced in the reactions. Methane (CH4) has an adiabatic flame temperature of 2226 K and a heat of combustion (production of heat per mass unit) of 55.528 MJ/kg, which is the highest for any hydrocarbon fuel and makes it an ideal candidate for micro-combustion [
Material selection is important in micro-combustor design and performance. Not only must the material be able to withstand the high temperature flow caused by the combustion process, it must also protect against heat loss to the ambient surroundings. Three materials were chosen to be tested in these simulations: stainless steel, aluminum oxide, and crystal quartz. Several earlier researchers have used stainless steel as their material of choice when testing micro-combustors [
In order to determine what the best micro-combustor design is a variety of simulations were conducted using ANSYS Fluent v16.1 and analyzed using ANSYS CFD Post v16.1. The simulations were split up by the various design parameters studied: geometry, fuel, and material. Each of the five geometries was tested at an equivalence ratio of 0.6 and velocities of 1.0 and 0.5 m/s. The effect of changing the equivalence ratio for each fuel was tested with the 1 mm × 10 mm constant-diameter micro-combustor; lean equivalence ratios of 0.6, 0.7, 0.8, and 0.9 were used in these instances. Each of the three previously mentioned materials was tested for the micro- combustor with the 1 mm × 10 mm geometry for each fuel and with a range of heat transfer coefficients. The following simulations all have a fuel-air mixture entering the micro-combustor at the inlet at a temperature of 300 K with a prescribed uniform velocity. The mixture is ignited at 1,600 K, the pressure is set at atmospheric (101,325 Pa), and the ambient temperature of the surroundings is 300 K. These simulations are 2D axisymmetric with only half the geometry meshed and simulated. Residuals have converged to 1e−06 for all solutions. The results and discussion provided in this section all center on maximum temperatures calculated in the micro-com- bustor simulations due to the fact that micro-combustor efficiency and exit temperature are directly related: the higher the maximum temperature seen in the micro-combustor, the higher the exit temperature and consequently the efficiency will be. In addition, flame shape and stability are also commented on.
In order to determine which geometry is best for a micro-combustor design, the five previously mentioned geometries were simulated in ANSYS Fluent v16.1 using stainless steel as the device material with a thermal conductivity of 20 W/m-K, a heat transfer coefficient of 50 W/m2-K, and inlet velocities of 0.5 and 1.0 m/s for each fuel-air mixture. Temperature and velocity profiles for a methane-air reaction for each type of micro-combustor (constant-diameter, converging, diverging, converging-diverging, and backward-facing step) can be seen in
Also of note is the location of the flame for the backward-facing step micro-combustor: it has been anchored closer to the inlet plane than all other geometries for the same initial conditions. Lastly, it can be seen from the profiles that the temperatures measured at the micro-combustor outlet for the constant-diameter and diverging geometries are similar in magnitude. Although blowout of the flame has been sustained by the converging micro-combustor at this inlet velocity, it has produced the highest outlet temperature out of the five geometries. Simulations at an inlet velocity of 0.5 m/s were also completed to use as a comparison and are shown in
which suggests that for the fuel at an equivalence ratio of 0.6 inlet velocity has little impact on flame temperature for this type of micro-combustor. The increase in temperature experienced by the backward-facing step geometry for hydrogen and ethane for these inlet velocities suggests that the higher velocity provides enough
Fuel | u = 0.5 m/s | ||||
---|---|---|---|---|---|
Constant-Diameter | Converging | Diverging | Converging-Diverging | Backward-Facing Step | |
CH4 | 1838.99 K | 1810.46 K | 1842.53 K | 1713.58 K | 1790.82 K |
H2 | 1700.58 K | 1810.95 K | 1706.28 K | 1776.72 K | 1580.89 K |
C2H6 | 2207.27 K | 1865.19 K | 2161.66 K | 1886.39 K | 1806.77 K |
Fuel | u = 1.0 m/s | ||||
Constant-Diameter | Converging | Diverging | Converging-Diverging | Backward-Facing Step | |
CH4 | 1898.09 K | 1606.13 K | 1913.33 K | 1720.45 K | 1788.59 K |
H2 | 1924.59 K | 1859.78 K | 1923.20 K | 1779.36 K | 1887.91 K |
C2H6 | 2301.62 K | 1024.80 K | 2167.38 K | 1130.32 K | 2019.43 K |
circulation around the step geometry to create more complete combustion of the mixture. In comparison with the methane-air simulations at the same equivalence ratio it was found that hydrogen-air combustion has overall produced greater maximum temperatures for an inlet velocity of 1.0 m/s. However, for an inlet velocity of 0.5 m/s the methane-air mixture generated higher temperatures than hydrogen-air for the constant-diameter, diverging, and backward-facing step geometries. This result suggests that an inlet velocity of 0.5 m/s is not fast enough for the combustion to get started in the remaining geometries and create enough energy to raise the temperature of the products. For the ethane-air simulations at 1.0 m/s, the converging and converging-diverging geometries experienced blowout and subsequent quenching of the flame at this relatively low inlet velocity which shows how powerful the ethane-air reaction is at this scale. Similar to the results for methane and hydrogen, the diverging and backward-facing step geometries kept the flame produced anchored within the micro-combustor for the ethane-air mixture. These simulation results show that although the diverging micro-combustor geometry produces the highest maximum temperature for both inlet velocities for all fuel-air combinations in a stainless steel micro-combustor, the converging geometry yields higher exit temperatures than the other geometries tested.
The influence of geometry on results from simulations completed with methane, hydrogen, and ethane fuels were presented above. In this section, we explore the effect of the fuel-to-air ratio. In each of the three cases, lean mixtures were simulated, in order to determine the best fuel for micro-combustion and the equivalence ratios that give maximum temperatures and stable flames. Fuel-lean equivalence ratios of 0.6, 0.7, 0.8, and 0.9 for mixtures in stainless steel constant-diameter micro-combustors of diameter 1 mm and length 10 mm were chosen. Material properties for stainless steel used in the following simulations are as follows: k = 20 W/m-K, h = 50 W/m2-K, e = 0.2 for the conductivity, coefficient of convection, and emissivity; respectively.
stated range of equivalence ratios. It can be seen that as the equivalence ratio is increased from 0.6 to 0.9 the maximum temperature of the lean fuel-air mixture also increases. A difference of approximately 540 K is seen in the maximum temperature between simulations of φ = 0.6 and φ = 0.9. For the entire range of equivalence ratios the flames have become stable at about the same distance away from the micro-combustor inlet. This suggests that increasing equivalence ratio has no effect on flame location within the micro-combustor for premixed ethane and air combustion. All flames are stable within the 1 mm × 10 mm micro-combustor for the range of equivalence ratios, and flame shape is also not impacted by increasing fuel content of the mixture. Simulations of the methane-air combustion for equivalence ratios of 0.6, 0.7, 0.8, and 0.9 were conducted for an inlet velocity of 0.5 m/s and are shown in
As predicted, as the equivalence ratio for all three fuels is increased the maximum temperature calculated by the simulations also increases. A larger fuel content of the fuel-air mixture allows for more combustion and thus
higher measured temperatures. Since an equivalence ratio of 0.9 yields the largest maximum temperatures for each fuel, a qualitative comparison can be made from
Lastly, material selection was investigated for micro-combustion using three different materials: stainless steel, aluminum oxide, and quartz. For each material, a 1 mm × 10 mm constant-diameter micro-combustor with a 0.2 mm thick wall was simulated with methane-air, hydrogen-air, and ethane-air mixtures at an equivalence ratio of 0.6. The heat transfer coefficient was varied for each material-fuel combination in order to study the effect on maximum temperature, flame shape, and flame location. Values of 5, 10, and 50 W/m2-K were chosen in order
to quantitatively and qualitatively compare results for environments with light and strong forced convection, as well as natural convection. Early on in the simulations it was determined that the flame temperature increases with decreases in the heat transfer coefficient. Therefore natural convection was found to be the optimal operating condition for a micro-combustor: the lower the heat transfer coefficient the less heat loss experienced across the walls of the device. Temperature profiles of the stainless steel micro-combustor with a mixture of methane and air at φ = 0.6 and a heat transfer coefficient of 5 W/m2-K at several inlet velocities are presented in
In an effort to put a numerical value on what constitutes the best micro-combustor design, overall combustor efficiency was calculated using the following equation:
where
Velocity (m/s) | Maximum Temperatures | ||||
---|---|---|---|---|---|
Constant-Diameter | Converging | Diverging | Converging-Diverging | Backward-Facing Step | |
0.3 | 2528.23 K | 2492.65 K | 2530.21 K | 2345.23 K | 2517.95 K |
0.5 | 2602.59 K | 2453.30 K | 2611.45 K | 2383.48 K | 2501.01 K |
0.8 | 2662.69 K | 2320.57 K | 2661.62 K | 2151.85 K | 2516.17 K |
1.0 | 2697.60 K | 1886.25 K | 2680.72 K | 2073.02 K | 2527.29 K |
Velocity (m/s) | Efficiency | ||||
---|---|---|---|---|---|
Constant-Diameter | Converging | Diverging | Converging-Diverging | Backward-Facing Step | |
0.3 | 5.54% | 6.66% | 5.54% | 6.48% | 5.51% |
0.5 | 5.79% | 7.63% | 5.80% | 7.20% | 5.51% |
0.8 | 6.39% | 8.33% | 6.44% | 6.33% | 5.51% |
1.0 | 6.80% | 3.97% | 6.83% | 2.97% | 5.52% |
dependent on the exit temperature measured at the outlet. Blowout of the flame was experienced by several configurations, as indicated by the decrease in the maximum temperature as inlet velocity is increased. Below in
The efficiencies calculated for these simulations are fairly low for a micro-combustor: as previously mentioned, Waitz, Gauba, and Tzeng [
In order to determine the best design for a micro-combustor, several considerations were taken into account, such as the type of fuel used, geometry, and material selection. A series of CFD simulations were completed for different micro-combustor design configurations using ANSYS Fluent v16.1. From the analysis of results with methane-, hydrogen-, and ethane-air mixtures it was determined that ethane was the best fuel choice due to its high thermal energy output at lower inlet velocities. As far as micro-combustor geometry is concerned, the diverging combustor produced the highest maximum flame temperatures while the converging geometry gave the highest exit temperatures. Since combustor efficiency is directly related to the difference between inlet and outlet temperatures, the converging geometry proved to be the best choice. Finally, three combustor design materials were tested: stainless steel, aluminum oxide, and crystal quartz. Of these materials, aluminum oxide allowed less heat loss to the surrounding environment for the same initial conditions. It was determined that an aluminum oxide converging micro-combustor with premixed ethane and air at an equivalence ratio of 0.9, inlet velocity of 0.5 m/s, and heat transfer coefficient of 5 W/m2-K returned the highest combustor efficiency of cases considered. Although earlier experimental and computational research on the performance characteristics of converging micro-combustors is limited, our study suggests that further investigations should be undertaken in order to determine if this type of geometry is feasible for micro-combustion and micro-power generation.
Leigh T. Powell,Ralph C. Aldredge, (2016) Design Optimization of a Micro-Combustor for Lean, Premixed Fuel-Air Mixtures. Journal of Power and Energy Engineering,04,13-26. doi: 10.4236/jpee.2016.46003