Numerical investigation on combustion characteristics of premixed hydrogen/air in a swirl micro combustor with twisted vanes

The combustion characteristics of the swirl micro combustor with twisted vanes (Swirl-MC-TV) and the conventional micro combustor (Conventional-MC) are investigated and compared under different inlet velocities (8–40 m/s), wall materials (quartz, steel, and SiC), and equivalence ratios (0.6–1.4). The results show that the larger area of recirculation zones and the stronger recirculation intensity are the key factors for Swirl-MC-TV to stable combustion. When the inlet velocity is 40 m/s, compared with the Conventional-MC, the wall heat loss of the Swirl-MC-TV is reduced by 15.9%, and the reaction heat and combustion efficiency of the Swirl-MC-TV are increased by 17.5% and 5.9%, respectively. When the wall materials of steel and SiC, combustors have a better preheating effect and higher combustion intensity. When the equivalence ratio is greater than 0.6, the wall heat loss of Swirl-MC-TV is larger but the combustion efficiency and the reaction intensity are still higher than Conventional-MC.     

Analysis of turbulent combustion in inert porous media

The objective of this paper is to present an extension of a simplified reaction kinetics model that, combined with a thermo-mechanical closure, entails a full-generalized turbulent combustion model for flow in porous media. In this model, one explicitly considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. These mass fractions are double decomposed in time and space and, after applying simultaneous time-and-volume integration operations to them, distinct terms arise, which are here associated with the mechanisms of dispersion and turbulence. Modeling of these extra terms remains an open question and the derivations herein might motivate further development of models for turbulent combustion in porous media.     

Combustion characteristics and radiation performance of premixed hydrogen/air combustion in a mesoscale divergent porous media combustor

To improve flammability and radiation efficiency, a divergent porous media combustor is proposed and numerically studied. The local thermal non-equilibrium model is used to consider the temperature difference between gas and solid matrix. Effects of equivalence ratio, the wall thermal conductivity, solid matrix thermal conductivity, and divergent ratio on combustion characteristics, radiation efficiency, and flammability limits are studied. The results show that the divergent channel extends the blowout limit by 186% and obtains a maximum radiation efficiency of 29.3%, increased by 70% compared with the straight channel. A smaller wall thermal conductivity is recommended considering the flammability range and radiation efficiency. A careful choice of solid matrix thermal conductivity and the divergent ratio is suggested to balance their opposing effects on the radiation efficiency and the flammability.     

Effect of convex platform structure on hydrogen and oxygen combustion characteristics in micro combustor

The combustion characteristics of the micro combustor with a convex platform were simulated and the effects of the height of the convex platform and the inlet velocity on the combustion process were analyzed. The results show that the setting of convex platform can significantly increase the maximum velocity and reduce the outlet velocity. When the height of the boss continues to increase, the maximum velocity is more significant, but has little effect on the outlet velocity. At the same time, the increasing height of the convex platform increases, the turbulent kinetic energy and reduces the intensity of combustion on the axis. However further increase in the height does not reduce the effect significantly. The fuel conversion rate increases significantly, but the velocity decreases. In the micro combustor with a convex platform, increasing the inlet velocity increases the axial temperature, the fuel conversion rate decreases.     

Laminar flow with combustion in inert porous media

This work presents one-dimensional numerical results for combustion of an air/methane mixture in inert porous media using laminar and radiation models. Comparisons with experimental data are reported. The burner is composed by a preheating section followed by a combustion region. Macroscopic equations for mass, momentum and energy are obtained based on the volume average concept. Distinct energy equations are considered for the porous burner and the flowing gas. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to relax the entire equation set. Inlet velocity, excess air, porosity and solid-to-fluid thermal conductivity ratio were varied in order to investigate their effect on temperature profiles. Results indicate that higher inlet velocities result in higher gas temperatures, following a similar trend observed in the experimental data used for comparisons. Burning of mixtures close to the stoichiometric conditions also increased temperatures, as expected. Increasing the thermal conductivity of the preheating section reduced peak temperature in the combustion region. The use of porous material with very high thermal conductivity on the combustion region did not affect significantly temperature levels in the combustion section.     

Effect of multiple bluff bodies on hydrogen/air combustion characteristics and thermal properties in micro combustor

The bluff body is commonly used to improve micro combustion. The micro combustor with multiple rectangular bluff bodies in a single row was proposed. The effects of bluff bodies on H₂/air combustion characteristics were numerically studied. The temperature distributions, ignition position, combustion efficiency and blow-out limit were investigated via changing the total width and number of bluff bodies. The results show that the combined use of multiple bluff bodies can further expand the blow-out limit of H₂/Air. The effect of high temperature and viscous force on the flow velocity is main factors for the flame morphology. When the total width of bluff bodies is 2 mm, the blow-out limit decreases with the increase of bluff body number. When the total width of bluff bodies is 4 mm and 6 mm, the blow-out limit increases with the increase of the number of bluff bodies. With the increase of inlet velocity, the complete combustion efficiency decreases. The combustion efficiency in the combustor with wider blow-out limit decreases more slowly. It indicates that the combustor with multi-bluff bodies is more suitable for the operation conditions with high flow velocity.     

Numerical investigation of combustion characteristics for hydrogen mixed fuel in a can-type model of the gas turbine combustor

Numerical simulations are performed to analyze the combustion characteristics of propane fuel mixed with different amounts of hydrogen in a can-type combustor. The volume fraction of the hydrogen fuel varies from 0% to 100% in the fuel mixture. The results indicate that the hydrogen enrichment of the fuel significantly affects the flow structure, mixture fraction, and combustion characteristics. An increase in the volume fraction of hydrogen significantly affects the mean mixture fraction distribution, promotes combustion, and increases the flame temperature and the width of the flammable range within the combustor. Therefore, the degree of temperature uniformity at the outlet of the combustor increases with hydrogen enrichment, corresponding to an increase of 49.64% in the uniformity factor. The hydrogen enriched fuel can also reduce the emissions of CO and CO₂, owing to the reduced amount of carbonaceous fuel.     

Partial oxidation of methane in a reverse flow porous media reactor. Water admixing optimization

The influence of the addition of steam on methane–air partial oxidation in a reverse flow porous media reactor is investigated numerically. The model is validated via comparison with the experimental data obtained without steam addition. The model of chemical kinetics includes 6- component overall model and GRI 3.0 gas phase methane oxidation kinetics. It is shown that hydrogen concentration in the product gas may be increased by 0.5–1% and the methane-to-hydrogen conversion ratio by 10–15% by means of adding steam to a working mixture. The optimum equivalence ratio remains the same as in the water free case. Steam concentration which maximizes H₂ is in the range of 5–10%; steam concentration which maximizes the conversion ratio is in the range of 20–50%. The role of the thermal insulation of the reactor and of the working gas preheating in this aspect is shown quantitatively.     

Numerical and experimental study of the conversion of methane to hydrogen in a porous media reactor

In this paper, conversion of methane to hydrogen within a porous media reactor was investigated over the fuel-rich equivalence ratio range of 1.5 to 5. Experimental data were taken to validate the computational model and good agreement was established between the two. The characteristics of interest were wave velocity, peak combustion temperature, flame structure, volumetric heat release, wave thickness, and hydrogen yield. The parameters investigated that affect these characteristics included inlet velocity, equivalence ratio, and the thermal conductivity and the specific heat of the porous media. The computational model predicted a peak percentage conversion of methane to hydrogen of approximately 59% while experimental results show a peak of approximately 73%. The model also predicted the experimental trend that conversion efficiency increases as the inlet velocity of the initial fuel-air mixture increases. Species profiles obtained from the computational model showed the signature dual-reforming regimes known as partial oxidation and steam reforming inherent with fuel-rich filtration combustion. The main contribution of this paper is an understanding of the transient nature of the combustion wave for fuel-rich conditions and how the nature of the combustion wave influences conversion efficiency. As the combustion wave progresses, the steam-reforming zone thickness increases, resulting from the constant heat addition to the solid. A thick, high-temperature zone, which promotes steam reforming and is heavily dependent upon the specific heat of the porous media, is preferred to maximize conversion efficiency.     

Calculation of premixed combustion within inert porous media with model parametric uncertainty quantification

The present study focuses on uncertainties existing in porous media parameters and in the inlet reactant mixture conditions of solid oxide fuel cell off-gas combustion. Propagation of uncertainty from the model input parameters to the output stochastic variables is quantified using a non-intrusive spectral projection method based on polynomial chaos expansion. The non-intrusive nature of this method allows the solution of the stochastic problem to be obtained directly from the deterministic model without requiring modification of the governing equations. Quantification of uncertainty is investigated in a one-dimensional model for premixed combustion within inert porous media. The model includes detailed chemistry and solves the gas- and solid-phase energy balances coupled by convective heat exchange, including radiative heat transfer in the solid-phase. The results denote that the uncertainties in the porous media heat transfer parameters are relevant and originate a relatively high error bar on the CO emission and burning velocity. When the inlet reactant mixture uncertain conditions is also accounted for, it overcomes the influence of the other uncertain parameters on the gas- and solid-phase temperatures error bar. Both types of parametric uncertainty sources (inlet conditions and porous media parameters) are important in order to establish the error bar on the CO emission and burning velocity predictions.     

Numerical study of the effects of material properties on flame stabilization in a porous burner

Development of porous burners has been encouraged by lower emission standards as well as the advantages these burners offer; such as fuel flexibility, the ability to operate at low equivalence ratios, and effective flame speeds greater than the laminar flame speed. Although a burner may be constructed from a single section of porous media, a burner consisting of two sections with different characteristics has received significant attention in the last decade. Through proper selection of the properties of the two sections, the interface between the two sections serves as a flame holder preventing flashback for a range of conditions. In this paper, we present the results from a one-dimensional computational study on flame stabilization in a two section porous burner. The stable operating limits are predicted for a range of equivalence ratios and are compared to experimental values. A parametric study, in which the properties of the two sections are varied independently, is presented. The results indicate that matrix properties significantly affect the stable operating range. In addition, the upstream section acts primarily as a flashback arrestor and for the widest operating range, it should have a low conductivity, low volumetric heat transfer coefficient, and high radiative extinction coefficient. The downstream section acts primarily to recirculate heat through the matrix; it should have a high conductivity, high volumetric heat transfer coefficient, and an intermediate radiative extinction coefficient.     

Numerical study on premixed hydrogen/air combustion characteristics and heat transfer enhancement of micro-combustor embedded with pin fins

Aimed at improving the energy output performance of the Microthermal Photovoltaic (MTPV) system, it is necessary to optimize the structure of the micro combustor. In this paper, micro combustor with in-line pin fins arrays (MCIPF) and micro combustor with both end-line pin fins arrays (MCEPF) were presented to realize the efficient combustion and heat transfer enhancement, and the influence of inlet velocity, equivalent ratio, and materials on thermal performance was investigated. The results showed that pin fins embedding is beneficial to improving combustion, and the combustion efficiency of MCIPF and MCEPF reaches 98.5% and 98.7%, which is significantly higher than that of the conventional cylindrical combustor (MCC). However, with the increase of inlet velocity from 8 m/s to 14 m/s, MCIPF exhibits the highest external wall temperature with a range of (1302–1386 K), while MCEPF maintains the best temperature uniformity. As the inlet velocity increases to 10 m/s, the external wall temperature and temperature uniformity reach the optimum. Besides, under the conditions of different equivalence ratios, both external wall temperature and heat flux increases first and then decreases, meanwhile the temperature uniformity of MCEPF is significantly improved compared with that of MCIPF, they all exhibit the highest external wall temperature with an equivalence ratio of 1.1, and the thermal performance is greatly enhanced. By comparing the heat transfer performance of combustors with different materials based on MCEPF, it is interesting to find that the application of high thermal conductivity materials can not only increase the external wall temperature, but also improve the temperature uniformity. Therefore, materials with high thermal conductivity such as Aluminum, Red Copper and Silicon Carbide should be selected for application in micro combustors and their components. The current work provides a new design method for the enhanced heat transfer of the micro combustor.     

A numerical evaluation of combustion in porous media by EGM (Entropy Generation Minimization)

Combustion in Porous Media provides interesting advantages compared with the free flame combustion due to the higher burning rates, increased power dynamic range, the extension of lean flammability limits, and the low emissions of pollutants. A numerical code is developed in order to evaluate the effects of different parameters of combustion in porous media. The governing equations including Navier–Stokes, the solid and gas energy and the chemical species transport equations are solved using a multi-step reduced kinetic mechanism. Flame stabilization and the burner optimization are studied by EGM (Entropy Generation Minimization) method considering the effects of chemical affinities and reaction. It is found that the flames occurring at the upstream half of the porous layer are more stable and more efficient, producing less emissions than those occur at the downstream half of porous layer. Also at a specified equivalence ratio both the heat recirculation efficiency and the Merit number have similar trend by changing the flame location. For a FFL (Fixed Flame Location), there is an optimum value of equivalence ratio at which the burner efficiency is a maximum.     

A modification of a Nusselt number correlation for forced convection in porous media

We report numerical simulations of forced convection heat transfer rates of a steady laminar flow in a two-dimensional model of porous media to elucidate the differences observed between the numerical predictions of Kuwahara et al. (2001) [Int. J Heat Mass Trans. 44, 1153–1159] and Gamrat et al. (2008) [Int. J Heat Mass Trans. 51, 853–864]. A modification in the correlation given by Kuwahara et al. (2001) is proposed to make the results of the three numerical studies comparable and in agreement with the experimental data.     

Trends in modeling of porous media combustion

Porous media combustion (PMC) has interesting advantages compared with free flame combustion due to higher burning rates, increased power dynamic range, extension of the lean flammability limits, and low emissions of pollutants. Extensive experimental and numerical works were carried out and are still underway, to explore the feasibility of this interesting technology for practical applications. For this purpose, numerical modeling plays a crucial role in the design and development of promising PMC systems. This article provides an exhaustive review of the fundamental aspects and emerging trends in numerical modeling of gas combustion in porous media. The modeling works published to date are reviewed, classified according to their objectives and presented with general conclusions. Numerical modeling of liquid fuel combustion in porous media is excluded.     

Experimental and numerical study of syngas production during premixed and ultra-rich partial oxidation of methane in a porous reactor

The synthesis gas (syngas) production from the ultra-rich methane/oxygen mixtures via the thermal partial oxidation in an inert porous reactor was investigated numerically and experimentally. Thermodynamic analysis was firstly conducted based on Gibbs free energy minimization method to find the possible optimum routes of operation. Then, the experiments were performed on the constructed test-rig with a non-catalytic porous based reformer. The flame is stabilized within zirconia (ZrO₂) sponge, which has shown very high mechanical strength and thermal resistance. The main influencing parameters such as the equivalence ratio and thermal load have been investigated during different experiments. For this purpose, the reactor axial temperature profile and product compositions were determined experimentally. The obtained results reveal that the heat loss abatement; approaching to the adiabatic condition could effectively improve the amounts of syngas (H₂+CO) production. The maximum syngas production was obtained 69.5% of the exhaust gas at the equivalence ratio of 2.5 and thermal load of 8 kW. Moreover, the H₂/CO ratio was reported above 1.5, which can be suitable for feeding into other chemical processes. Finally, numerical simulation of the process was performed using the premixed and reactor network models. The contribution of heat loss from the reactor was also considered in the model due to its pivotal role observed in the experimental work. The average relative error of the reactor network model with respect to syngas generated from the reformer was found to be 6.72%. Therefore, the predictions obtained from this model are in fairly good agreement with the experimental data.     

多孔介质特性参数对燃烧温度及压力影响的数值模拟研究

考查了两段式多孔介质内预混气燃烧的温度与压力分布情况。建立了甲烷／空气预混气体在多孔介质内燃烧的二维数学模型,运用FLUENT软件求解瞬态控制方程的方法计算出燃烧稳定后多孔介质内的温度、与压力分布,并考查了不同当量比、多孔介质辐射衰减系数和导热系数对温度和压力分布的影响。结果表明,甲烷／空气预混气体在多孔介质中燃烧,当量比越大温度峰值越高,压力梯度越大;小孔介质辐射衰减系数的改变对温度分布和压力分布没有明显的影响,而大孔介质辐射衰减系数对温度分布和压力分布有较大的影响;增加多孔介质的导热系数,会使固相与气相温度均有所升高,燃烧区域压力降低。