Numerical Simulation of Rarefied Gas Flow in Straight Microchannel

Abstract

The rarefied gas flow in 2D rectangular straight and 90o bend micro-channel are investigated numerically by using a commercial CFD solver ANSYS FLUENT 17.1. The continuity, momentum and energy equation are solved by applying first order velocity slip boundary condition with constant wall temperature. Present simulation is carried out for Knudsen number and Reynolds number at the range of 0.001 to 0.1 and 0.045 to 1.87 respectively. The working fluid is taken as air and it is assumed as ideal gas for the simulation.The velocity slip boundary condition is implemented by selecting values of Tangential Momentum Accommodation Coefficient and Tangential Thermal Accommodation Coefficient. The numerical analysis of streamwise velocity and friction factor is also has been carried out at different AR and PR at fixed Knudsen number of 0.05. The numerical arrangement of the Navier– Stokes conditions with the Maxwell's slip boundary condition indicates great concurrence with analytical information and draws out the complex flow conduct all through the straight channel and near to the curve of L shape microchannel. The centreline pressure variation along the channel length and stream wise velocity variation along channel height is validated with analytical and previous numerical work. The results are found to be in good agreement. The pressure variation is found to be non linear and velocity distribution is the parabolic one. However the velocity at wall due to slip boundary condition increases with the increase in Knudsen number. With increase in the Knudsen number, the developing length of the flow, Nusselt number and wall shear stress reduces. The reduction up to the Knudsen number equals to 0.01 is sudden and than it reduces asymptotically. While changing the pressure ratio, it is being deducted that the magnitude of the velocity decreased. The length of the channel where the velocity is increasing drastically is also becomes small. The velocity decreases in a nonlinear manner while increasing the AR. The decrement of magnitude of velocity is higher at ARs 10 and 20 compare to the ARs 50 and 100. It is being observed that in 90o bend channel, The static pressure measurement demonstrates adverse pressure gradient near the bend along the microchannel at much lower value of Reynolds number as compared to conventional flow. The adverse pressure gradient, velocity profile, flow streamlines and velocity vectors in the bend of channel clearly indicates secondary flows near the bend at as low a Reynolds number. The flow acceleration and the presence of secondary flows near the bend causes a larger pressure drop as compared with a straight channel.