DSMC Investigation On Gaseous Mixing And Combustion At Microscale

Over the past century, the development of the human sociaty has been closely associated with fossil fuels. The consumption of fossil fuels provide energy and power for industry’s development, but at the same time, its pollutant emissions also brought huge challenges to the environment of human living. Due to the nonrenewability of fossil fuels and the global acceptance of the low carbon concept, researchers around the world pay more and more attentions on the study of improving fuel combustion efficiency, reducing emissions of pollutants and carbon dioxide, looking for new types of alternative energies, etc. Among these efforts to solve the problem of energy resources, the miniaturization of combustion technology, such as porous media combustion as well as micro-burner combustion etc. have got researchers’attention due to their high energy density, considerable area to volume ratio, outstanding ability of heat regeneration. Studying the mechanism of gas mixing and gaseous combustion at microscale in the miniaturization devices is not only beneficial to selecting appropriate operating parameters, designing and optimizing the related micro-burners and improving performance of these combustors, but also is of great significance for other related MEMS devices in the fields of medicine, chemical, biological aerospace and other branches. Hence, in this paper, the direct simulation Monte Carlo method (DSMC) was used to investigate the processes of gas mixing and combustion at microscale.For the simulation of gas mixing, the variable soft sphere (VSS) model which describes the process of molecular diffusion more accurately was used to simulate the gas mixing of CO and N2in parallel and Y-shaped micro-channels with a height of1μm. The influences of different wall accommodation coefficients and splitter plate thicknesses on the process of gas mixing were studied in the parallel microchannels and the influences of different wall temperatures, inflow rates and branch angles on the process of gas mixing were investigated in the Y-shape microchannels. The results show that in both cases of geometry exits the phenomenon that molecules of one component diffuse upstream to another component; the wall accommodation coefficients influence the process of gas mixing indirectly by influence the gas flow in the parallel microchannel, the splitter plate thicknesses influence the gas mixing by inducing small perturbations in the flow field near the end of the splitter plate and the mixing length decreases with the increase of the wall accommodation coefficient and splitter plate thickness; in the Y-shaped microchannel, the process of gas mixing mainly occurs in the junction area and the mixing length decreases monotonously with the increase of wall temperature and the decrease of the inflow rate; the influence of the branch angle on the process of gas mixing relates to the magnitude of its value, and smaller branch angle leads to more significant decrease in the mixing length.For the simulation of gas combustion, the non-premixed combustion of H2/O2in a Y-shaped microchannel with a height of10μm was numerically studied by employing the kinetic mechanism including6species and7reversible reactions. The influences of the entrance Knudsen number and wall surface conditions on the combustion characteristics are discussed in detail. The results show that no discernible flame phenomenon can be identified in the micro-channel and the wall heat flux is much higher than that at the conventional scale; the exothermic reaction mainly takes place in the junction area and first half part of the main channel; velocity profiles in the microchannel have a parabola form, and there exists typical velocity slip phenomenon at the wall surfaces, additionally, the maximum value of the velocity profiles increases with decreasing Knudsen number and increasing channel wall’s temperature; Knudsen number has significant influence on the combustion and a rise of Knudsen number can promote the process of gas mixing, but will lead to a decrease of reactants’ conversion rates in the main reaction region; the impacts of the wall conditions in different section are different, the branch channel walls mainly influence the reaction process in the branch channels, while the main channel walls have significant affect on components’ concentrations in the main channel.