Numerical Modeling of Gas Flows in Micronozzles

Since many microfluidic devices are operated in the continuum-transition regimes, the understanding of heat transfer behavior and fluid flow characteristics in this regime becomes important. The conventional Navier-Stokes model may become less accurate because of its first order constitutive relations. A direct simulation Monte Carlo method can be considered a more accurate numerical simulation for higher Knudsen numbers. However, the large number of required simulated molecules can easily make it prohibitively expensive in terms of computational time. Therefore, the Burnett equations, with their higher order constitutive relations for viscous stress and heat fluxes, are used in modeling flow characteristics in micronozzles. A two-dimensional computational code is developed for the augmented-Burnett model and BGK-Burnett model, using the flux-vector splitting scheme for inviscid fluxes and central differencing for the viscous fluxes. The first order Maxwell-Smoluchowski slip boundary conditions and the second order Beskok slip conditions are implemented to obtain the correct model for flow behavior on solid surface. This developed model is used to simulate microscale gas flows in different types of converging-diverging micronozzles. The Navier-Stokes simulations have also been included for comparison purposes.