Numerical Simulation Research On Flows And Heat Transfer In Microchannels

As an important branch of MEMS, microfluid technology has achieved great development recently and been widely adopted in various fields including biology, medicine, aerospace, mechanical engineering and electronics. However, mature theories and dependable experiments still lack in the research of the flow and heat transfer inside the microfluid. Hence, the purpose of this paper is to exhibit the microfluid flow using numerical simulation, in order to make effective predictions about flow characteristics and propulsive performance of micronozzles and the heat-transfer property of microchannels, and to strengthen and widen the application of micro flow in the field of engineering.According to quasi-one-dimensional isentropic fluid theory in a nozzle, the influences of NPRs on micronozzle's flow structure are studied through continuum method, including the influences on Mach number, pressure, total pressure drop and density. It is discovered that within the range of designed NPR, the increase of pressuredrop-ratio could efficiently restrain boundary layer separation and improve micronozzle's efficiency. The"supersonic annular phenomenon"in the high-velocity flow field is found and its essence is"Mach disk"formed in an over-expanded micronozzle. The formation mechanism and corresponding conditions are analyzed in terms of shock wave theory. With a fixed exit pressure, the range is defined by NPRs, which supersonic annular phenomenon possibly exists in.The flow characteristics and propulsive performance in two-dimensional micronozzles are numerically simulated using continuum and DSMC method. It is found that the results from DSMC are more likely to be close to the reality. The influences of geometric structures and boundary conditions on flow characteristics and propulsive performance are studied. It is discovered that the propulsive performance of micronozzle with smooth-corner type throat is better than the ones with sharp-corner or flat-corner. What is more, the most suitable radius of smooth-corner is 0.686 times of the throat width. Lighter propellant and high-temperature propellant could both improve the specific impulse, but at the cost of sacrifice in propulsive efficiency. The suggestions about micronozzle's optimization are also given.In terms of the principle of heat sink, two types of micro heat exchangers are put forward: micro aircooled heat exchanger and micro liquid-cooling heat exchanger. Based on Visual Basic platform, a set of microchannel heat exchanger thermal designing software is developed and its computational result accords well with the statistics from reference literatures.