| In the 21st century,the exploration of space,the globalization and informatization of human society have prompted the emergence of various aerospace equipment.Moreover,with the rapid development of micro-processing technology and the demand for portable electronic equipment,various micro-electro mechanical systems(Micro Electro Mechanical System,MEMS)or other small high-tech devices are emerging,and are widely used in communications,aerospace,military,biomedical,etc.The urgent need for compact,reliable,light weight,instantly rechargeable,efficient and powerful energy source,has accelerated the development of micro-power devices in the past decade,such as micro-rotary engine,micro turbine engine and micro thermo photovoltaic systems.The hydrogen fueled combustion with high energy density and low pollution also accelerates the study of micro combustion.However,the chemical reaction time is limited by the small volume,and the combustion stabilization is challenged by the high surface-area–to–volume ratio and the large heat loss ratio under micro-scale conditions.Thus,combustion in micro combustor is still a challenging issue,due to the flame instabilities,narrow operational range,non-uniform wall temperature and low efficiency.In this thesis,the combustion characteristics and the improvement of stability and radiation efficiency,thermo photovoltaic system with photovoltaic(PV)cells are investigated and mainly concentrated on five aspects with the support of China Scholarship Council.The mainline and the main innovations are listed in following.(1)The numerical calculation model of GaSb PV cells and InGaAsSb PV cells is respectively proposed by polynomial fitting and iterative algorithm,which is based on the Planck’s law and the working principle of photovoltaic cells.Furthermore,an experimental platform for micro combustion is rebuilt and tested with Brooks Mass Flow Controllers and the RAYTEK Infrared thermometer’s accuracy in the Thermal Process Lab1 of the National University of Singapore.Then,the thermal performance of premixed hydrogen-air combustion in the combustor with backward-facing step is experimentally investigated.Moreover,the steady flames in 12 kinds of micro tubes are obtained under the condition of various mass flow rates and equivalence ratio.The effects of step length and wall thickness are also conducted to present the flame stabilization,flame location and the outer wall temperature distribution.The results show that the outer wall temperature can be improved by the reduction of wall thickness and modified through the relocation of the high temperature zone.With the decrease of the step length,the flame location is reduced in the combustion chamber,while the same location of peak temperature is obtained in the combustors with varied wall thicknesses.Furthermore,the electrical power output and efficiency of micro-TPV system with the GaSb PV cells and InGaAsSb PV cells are calculated and compared,while the working performance of the InGaAsSb PV cells is 1.5 times higher than that of GaSb PV cells.(2)A numerical simulation model of premixed hydrogen-air combustion with detailed reaction mechanism is adopted to investigate the combustion characteristics,which involves 9species and 21 reversible reactions.In addition,a novel micro combustor with a nozzle-inlet is proposed to enhance the combustion stabilization and improve thermal efficiency by optimizing flow field in the combustion chamber.It is based on the structure of conventional burner and designed four forms of micro tube with a nozzle-inlet and a backward-facing step.Efforts have been dedicated to enhance the combustion stabilization and improve thermal efficiency by optimizing flow field in the combustion chamber,however,the high heat loss from outer wall and outlet of combustor also imply that the micro combustor has great potential to improve the efficiency of the integrated system.Then,the effects of inlet shape on fundamental flame characteristics of premixed hydrogen-air combustion,that is,flow field,species distribution and thermal performance are investigated.Furthermore,in order to identify the causes of inefficiency and conceive the possibility of globally more effective systems.The entropy generation analysis of premixed hydrogen-air combustion in the micro tube is also adopted,which included the entropy generation caused by thermal conduction,mass diffusion and chemical reaction of the combustion in micro tubes.The effects of mass flow rate and equivalence ratio are also carefully compared and analyzed.Besides,the exergy destruction is investigated by the entropy generation analysis,to select the best combustor design with the highest mean wall temperature and the lowest exergy destruction,which are more appropriate for the application of the micro device/system.(3)In order to expand combustible limit and address the flame instability of micro combustion,the combustors with two rearward-steps and various inlet shapes are proposed.Firstly,the stable flames under different mass flow rate in the combustors with varied step length and diameter are experimentally observed and compared.Hence,the three-dimensional numerical model can be verified,which is carried out to assess the flame location and flame front shape,as well,the OH radical and gas temperature distributions of premixed hydrogen-air combustion in the micro combustors.Then,the effects of the step length and the inlet shape of the combustor on the combustion characteristics and thermal performance are investigated and analyzed.It is observed that the flame location and temperature filed are significantly affected by the design of the micro combustor due to stretching of the flame and preheating of the reactants.Moreover,the combustor with outer diameter 4 mm or 5 mm are investigated and compared to improve the radiation efficiency of the micro-TPV system.The results indicate that a properly stepped/flared inlet section contributes to the ignition and enhances the preheating of the unburned reactants.The heat recirculation zone behind the rearward-step provides a suitable location for flame anchoring.Furthermore,the small size combustor obtains a higher wall temperature,while it is more sensitive to the change of step length and inlet shape than the combustors with OD=5mm.However,the flame stability and working performance of the combustor with OD=5mm is better than that of the combustor with OD=4mm.In summary,two rearward-steps can remarkably improve the flame stability and radiation efficiency in the combustor at micro scale.(4)A 3D numerical simulation model is developed,which is combined the combustion characteristics in porous medium and the micro scale combustion.Then,the numerical model is verified by the experiments,moreover,this work is focused on the experimentally investigation of the combustion in micro scale combustor with inserted porous medium.In addition,the combustion characteristics of premixed hydrogen-air in the combustor with and without porous medium are compared and analyzed.The the flame stabilization is greatly enhanced in the combustor with porous medium,which also effectively enhances the heat transfer and produces a higher wall temperature.This is because of the solid matrix of porous medium offers a high heat capacity,high conductivity and high emissivity than gas,the combustion in porous medium can be significantly changed.Furthermore,the appropriate porosity of porous medium are selected for 9 combustors,which are varied with combustor length(17~27 mm)and diameter(5~9 mm),to achieve a better thermal performance and high radiation temperature.It is found that with the increase of combustor diameter,the limitation in terms of inlet velocity and flame blowout are expanded and the radiation surface area is also increased for a higher energy output.Hence,the narrow operation condition of micro combustion can be extend and the power output and efficiency of the micro-TPV system with InGaAsSb PV cells are improved.(5)In order to conduct a further investigation of the effects of porous medium properties and the propane addition on the premixed hydrogen-air combustion in the porous medium.Then,the experimental study of the combustion of premixed hydrogen-propane-air in a combustor inserted with varied porous medium has been carried out to fundamentally understand the effects brought about by ten kinds of porous medium(varied porosity,wire diameter and pores per inch).The results indicate that the flame stability and heat transfer of premixed hydrogen-propane-air combustion can be significantly enhanced in the combustor inserted with porous medium.The addition of propane to the hydrogen fueled combustion shifts the flame location and the position of peak wall temperature downstream from the inlet to the outlet,moreover,propane blended ratio alters the combustor temperature distribution.The effects of porosity,wire diameter and PPI(pores per inch)of porous medium on the flow filed,combustion characteristics and thermal performance are experimental and numerically investigated.It’s found that the combustor inserted with porous medium,which has an areal density of 0.0524~0.0551 g/cm~2 and a porosity of 0.9,achieves the highest mean wall temperature and provides a high and uniform wall temperature for the micro-TPV system with a high electrical power output and efficiency.It also expand the application of the micro-TPV system or other micro device and system with micro combustion. |
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