Fundamental Numerical Analysis of a Porous Micro-Combustor Filled with Alumina Spheres: Pore-Scale vs. Volume-Averaged Models

Qingqing Li,Junrui Shi,Jun Li,Jiansheng Wang

doi:10.3390/app11167496

Qingqing Li, Junrui Shi + Show 2 more

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https://doi.org/10.3390/app11167496

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Abstract

Inserting porous media into the micro-scale combustor space could enhance heat recirculation from the flame zone, and could thus extend the flammability limits and improve flame stability. In the context of porous micro-combustors, the pore size is comparable to the combustor characteristic length. It is insufficient to treat the porous medium as a continuum with the volume-averaged model (VAM). Therefore, a pore-scale model (PSM) is developed to consider the detailed structure of the porous media to better understand the coupling among the gas mixture, the porous media and the combustor wall. The results are systematically compared to investigate the difference in combustion characteristics and flame stability limits. A quantified study is undertaken to examine heat recirculation, including preheating and heat loss, in the porous micro-combustor using the VAM and PSM, which are beneficial for understanding the modeled differences in temperature distribution. The numerical results indicate that PSM predicts a scattered flame zone in the pore areas and gives a larger flame stability range, a lower flame temperature and peak solid matrix temperature, a higher peak wall temperature and a larger Rp-hl than a VAM counterpart. A parametric study is subsequently carried out to examine the effects of solid matrix thermal conductivity (ks) on the PSM and VAM, and then the results are analyzed briefly. It is found that for the specific configurations of porous micro-combustor considered in the present study, the PSM porous micro-combustor is more suitable for simplifying to a VAM with a larger Φ and a smaller ks, and the methods can be applied to other configurations of porous micro-combustors.

Highlights

With the rapid development of micro-electro-mechanical systems (MEMS), the demand for miniaturized power devices becomes increasing urgent
This study develops a pore-scale model (PSM) of a structurally arranged porous media made of discrete alumina spheres to produce a quantitatively accurate porous micro-combustor
5, that is, the temperature difference between the and on the PSM calculation model decreases with the increase in Φ, which is consistent with decreases with the increase in the results shown in Figure 5, that is, the temperature difference between the PSM and volume-averaged model (VAM) decreases with the increase in Φ

Summary

Introduction

With the rapid development of micro-electro-mechanical systems (MEMS), the demand for miniaturized power devices becomes increasing urgent. The present main power sources for miniaturized electronics and micro-propulsion systems are conventional electrochemical batteries [1]. Owing to the higher energy densities of hydrogen and hydrocarbon fuels compared with batteries, micro-scale combustors were used as the heat source to provide power for miniaturized power systems, such as the micro-thermophotovoltaic (TPV) systems [3] or the thermoelectric (TE) systems [4]. Differing from macro-scale combustors, the combustor size is reduced to a millimeter and the increased surface-to-volume ratio intensifies the heat losses from the wall [9], making it difficult to sustain stabilized flames. It is crucial to develop effective flame stabilization technologies for small-scale combustors. Many useful strategies have been suggested, such as the backward-facing step in cylindrical tubes [10,11] to prolong the residence time and strengthen the mixing of the fuel mixture; catalytic combustion [12,13]

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