| It is of great practical significance to develop portable emergency or backup micro-energy systems driven by combustion.Micro-thermophotovoltaic and micro-thermoelectric systems have attracted the attention of many researchers due to the advantages of low operating noise,no mechanical friction loss,and simple and compact structure.Porous media combustion can significantly improve system efficiency due to its higher and more uniform wall temperature.Because of small characteristic length,short flow residence and chemical reaction time,there exsit wall temperature uniformity and flame stability problems.On the one hand,carbon peak and neutrality goals and energy structure require the reduction of nitrogen oxide emissions in micro-energy systems fueled by fossil fuels,and on the other hand,clean fuel utilization is suggested.In this paper,porous media burners with diverging channel and bluff-body flame holder fueled by hydrogen and low-emission burners with partially filled porous media fueled by methane are proposed.The effects of burner structure,porous media filling method,physical properties and operating coditions on the burner wall temperature and flame stability are studied numerically and experimentally.The findings could help to understand porous media combustion characteristics and guide the micro-energy system burner design.The main work of this paper is as follows:First,a comparative study of numerical modeling of porous media combustion is carried out.Numerical simulations are conducted on six typical empirical correlations of convective heat transfer coefficients,and their effects on temperature distribution and flame are studied.The best empirical correlation for steel mesh porous media is selected.By studying the effect of inlet velocity and porosity on porous media combustion,it is revealed that the change of the dominance of velocity and heat transfer effects determines the flame position.And flame anchoring and wall temperature uniformity problems at low porosity or high inlet velocity are explained.Secondly,a diverging porous media combustor is proposed to solve the problem of narrow stable combustion range of micro-scale porous media combustion.By numerical simulation,the effects of equivalence ratio,inlet velocity,wall thermal conductivity,solid thermal conductivity of porous medium matrix and channel diverging ratio on radiation efficiency and stable combustion range are studied.Suggestions for the burners design are provided:small wall thermal conductivity is preferred to improve radiation efficiency and stable combustion range while there is a trade-off between radiation efficiency and stable combustion range when choosing the thermal conductivity of porous medium matrix and channel diverging ratio.Compared with the straight channel,enhanced heat recirculation and flow filed characteristics improve the blowout limits of the diverging channel at equivalence ratios of 0.6 and 0.8 by 167%and 186%,respectively.Net heat recirculation efficiency is proposed to reveal the effect of porous media on temperature field and it is showed that the critical net heat recirculation efficiency for excess enthalpy combustion is 18.2%.Besides,wall temperature lag is elucidated in this study.Furthermore,a porous media combustor with a bluff-body flame holder is proposed to further increase wall temperature and blowout limit.Results show the improvement of blowout limits by 33%,19%,12%,and 20%with equivalence ratios of 0.6,0.8,1.0,and 1.2,respectively.And the performance is better under high inlet velocities and low equivalence ratios.Heat recirculation for different equivalence ratios and inlet velocities is analyzed by introducing wall to gas,solid porous media to gas and bluff-body flame holder to gas convection efficiency,as well as the mechanism of increased blowout limits.Finally,in order to reduce the lean-burn limit and NOx emissions and improve the power output range and system efficiency,in this paper,a micro-thermoelectric system based on porous media combustion is proposed and experimentally studied.The experiment obtains the lowest methane lean-burn limit of 0.46 in current literature and the critical Peclet number of 54 for submerging porous media combustion.Heat recirculation efficiency is used to explain the effects of equivalence ratio and inlet velocity on system efficiency.Besides,four partially filled porous media arrangements are designed.And it is found that wall temperature and system efficiency increase due to heat sink effect of the downstream porous media.Meanwhile,the system operating near the lean-burn limit reduces NOx emissions by 2/3 as compared to high equivalence ratio conditions. |
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