Transport And Reaction In Microchannel Recator

Transport and reaction in micro-chemical system were explored in this dissertation. Fluid flow and heat transfer in microchannel were analyzed theoretically and numerically. Two-dimensional compressible momentum equations are solved by perturbation analysis and PISO algorithm to investigate the effects of compressibility and rarefaction on the local flow and heat resistance of gas flow in micro tubes. The computations were performed for a wide range of Reynolds number and inlet Mach number. The results of present study reveal that the local Fanning friction factor and Nusselt number is a function of inlet Mach number, Reynolds number and the length-diameter ratio of channel. For larger Reynolds and inlet Mach number, the friction coefficient and Nusselt number is higher than the value for macro tube, the gas flow in micro channel is dominated only by compressibility, and for smaller Reynolds and inlet Mach number, the Fanning friction factor and Nusselt number of gas flow in micro scale channel is lower than in a circular tube with conventional scale due to slip flow at the wall, rarefaction has more significant effect on fluid flow characteristics.Numerical analysis of fully developed laminar slip flow and heat transfer in trapezoidal micro-channels has been studied with uniform wall temperature and uniform wall heat flux boundary conditions. Through coordinate transformation, the governing equations are transformed from physical plane to computational domain, and the resulting equations are solved by a finite-difference scheme. The influences of velocity slip and temperature jump on friction coefficient and Nusselt number are investigated in detail. The calculation also shows that the aspect ratio and base angle have significant effect on flow and heat transfer in trapezoidal micro-channel.Conjugated heat transfer performance in a countercurrent micro heat exchanger is analyzed and simulated using one-dimensional gas convection and wall axial heat conduction model. The effects of structural parameters, operating conditions and heat conductivity of wall material on the performance of heat transfer of this micro heat exchanger are investigated. The simulation results indicate that the properties ofmicrochannel heat exchanger have much different from that of conventional one due to the synergetic effect between radial and axial heat conduction in the micro heater. Low conductivity metal, such as stainless steel, is an excellent wall material and available for microchannel heat exchanger. There exists an optimal operation flux, which can be use as a standard flux for a given micro heat exchanger, and micro heat exchanger is not suitable when the operation flux is lower than the standard flux. The maximum heat transfer efficiency of micro heat exchanger decreases with increasing of operating flux. The calculation results also show that micro heat exchanger with high aspect ratio channel and optimal wall width has higher efficiency than other configurations.The influence of un-identical flow on conversion is insignificant when reaction order larger than zero. But for minus order reactions, un-identical of fluid flow have different effects on conversion according to the operation parameter. Especially, un-identical of fluid flow have significant effects on performance of gas purification systems.The catalytic combustion reaction of hydrogen/air mixture was studied in microchannel reactor. The effects of operation conditions on reactor behavior were investigated in detail using both mathematic modeling and experiment work. Explosive mixtures of hydrogen/air (5vol%~15vol %) were safely handled in this reactor. The results showed that catalytic combustion reaction can be operated at high space velocity, isothermal and kinetic controlled regimes. Under the conditions of inlet hydrogen concentration of 8vol%, reaction temperature of 150℃, space velocity of 10~6 h-1, hydrogen conversion was still higher than 90%.