Investigation of the gravity effects on the mixed convection heat transfer in a microchannel using lattice Boltzmann method

This paper aims to study the gravity effects on the mixed convection heat transfer in a microchannel using lattice Boltzmann method. To include these effects, hydrodynamic boundary condition equations are modified. In this problem, cold fluid enters the microchannel and leaves it after cooling the hot walls. For a wide range of inlet Knudsen number (Kn), computations are performed, and for validation, appropriate comparisons between present and previous available results are made.As the results, stream lines, longitudinal variations of friction coefficient, Nusselt number, slip velocity and temperature jump, and velocity and temperature profiles in different cross sections are presented. The results show that lattice Boltzmann method can be used to simulate mixed convection in a microchannel, and the effects of buoyancy forces are important for Kn < 0.05, specially for hydrodynamic properties, and thus should be included. For Kn > 0.05, these effects can be ignored. In addition, it is observed that buoyancy forces generate a rotational cell in the microchannel flow, leading to the negative slip velocity at Kn = 0.005.

► For Kn < 0.05 the effects of gravity are important for hydrodynamic properties.
► The negative slip velocity was seen, for the first time in this work.
► Mixed convection causes large variations in slip velocity and friction coefficient.
► To use LBM, the hydrodynamic boundary conditions were modified.