The Combustion Performance Of Micro-heat Recirculation Combustor Fuelled With Liquid Hydrocarbon Fuel

With the development of the economic,the MEMS system has made considerable progress and has higher requirements for the power system.The micro combustor use fuel as energy storage media which has a huge energy density.The energy/mass ratio of micro combustor is much higher than lithium-ion battery and it has a great development potential.Overall,there are several problems which micro combustion should be faced with:1,the huge energy loss caused by the big surface/volume ratio;2,the short residence time for the gas in the combustor;3,the unstable flame caused by the narrow channel.In this paper,we studied the micro recirculation combustor fueled with liquid hydrocarbon fuel?n-heptane?,using the experimental method,chemical reaction analysis and computational fluid dynamic method.This thesis mainly includes four parts:the experiment,the simulation and calculation method used,the chemical reaction analysis,the performance of combustion and gas flow.We built an experimental system for the micro recirculation combustor.The differences between the working conditions of combustor with and without a porous medium?PM?were analyzed;we measured the working boundary and the temperature distribution of different combustors;the heat loss condition and the heat transfer in the devices were calculated;and we got the combustion efficiency.The results showed that:in the combustor without porous medium,there was an obvious oscillation and the porous medium could weaken it.The heat recirculator could stable the flame in the inner tube and expand the combustion boundary.The inner pipe temperature of combustor with bilayer vacuumed recirculator was higher and the heat loss was cut.In general,the burning completeness of single pipe combustor was lower than recirculation combustor.The combustion efficiency of combustor with bilayer recirculator gently changed with equivalence ratio?ER?and that with single monolayer recirculator changed dramatically.Based on the experiment results of micro recirculation combustor,the combustor was analyzed with detailed chemical reaction mechanism.We got the 1-D flame structure of n-heptane,the elementary reactions which had huge influence on the combustion;and we got the temperature distribution,the combustion efficiency and the reaction path under various convective heat transfer coefficient.It showed that:the n-heptane was almost completely consumed before the preheat zone.The reaction H+O₂=O+OH had greatest impact on the combustion.Before the flame,the heat absorbed by the gas changed little in various working conditions.With the increasing of convective heat transfer coefficient on the pipe surface,the gas temperature decreased and the flame finally extinguished.We checked the reaction path and find that there was low temperature combustion when the convective heat transfer coefficient was huge;a substantial part of fuel converted to peroxide molecules instead of carbon monoxide.When the pressure was low,the main reactants were hydrocarbons and when the pressure was huge,the combustion was mainly affected by the hydrogen-oxygen reactions.We studied the combustion and flow performance in the micro recirculation combustor by using the computational fluid dynamics method.Firstly,we studied the combustion characteristics of discrete phase n-heptane in the micro combustor with porous medium.The evaporation of discrete phase in the porous medium was studied.The conversion of main reactant,the temperature distribution on various sections and the flow charactieristic in the device were gotten.The influence of combustor structure on the gas heat loss was also studied.Secondly,the skeleton reaction mechanism was employed to analyze the reaction in the micro reacirculation combustor.The flame shape and the 2-D flame structure,the mixing of fuel/air were studied.And also we studied the main intermediate products,the combustion efficiency,the heat transfer in the device,the influence of convective heat transfer on the combustion and the influence of fuel injection on the combustion state were studied.It showed that,when the fuel situates in the inner pipe and near to the exit of pipe,the preheating for unburnt gas was weak.The discrete phase fuel evaporated in the porous medium.The evaporation of discrete phase absorbed heat and the temperature near the fuel was low.When the inlet was constant,the increasing of inner pipe radius caused a more serious heat loss;the increasing of outer pipe radius also caused heat loss which was weaker than inner pipe radius.When reaction zone was close to the wall?inner tube wall and regenerator wall?,the mass fraction of intermediate decreased sharply under the influence of wall;and gas temperature also significantly reduced due to the heat transfer of the solid wall.When the equivalent ratio was relatively small,larger velocity gradient contributed to a better mixing of fuel and air.When the flame was on the surface of the porous medium,the fuel mixed with air and the combustion accurs simultaneously.When the equivalence ratio was small,the flame stretch changes intensely.When equivalent ratio was relatively large,variation of flame stretch flattens gradually with increasing ER.The recirculation heat of device declined with the increasing of ER.With the increasing of convective heat transfer coefficient,the surface temperature of micro combustion decreases and the preheating for unburnt gas was also cut;but the combustion efficiency was bigger.As the injection point of fuel moved upstream,the mixing of fuel/air was better.