Rarefaction and scale effects on heat transfer characteristics for enclosed rectangular cavities heated from below

The fluid-flow and heat transfer in a buoyancy-driven microcavity heated from below are numerically investigated. In spite of the fact that microcavities are widely used in microelectro-mechanical systems, now a day, more interest in the evacuated cavity on Eqn. solar collectors are very common to reduce heat loss from the system. This paper provides a useful information for engineers to estimate heat transfer in low pressure cavities. The finite volume technique was used to solve the governing equations along with temperature jump and slip flow boundary conditions.The simulations are carried out for various cavity aspect ratios (H/L) and different Rayleigh number for both macroand micro-fluids. The effect of Knudsen number in the rarefied flow regime (microfluidic) has also been investigated. It is shown that for both cases the effect of aspect ratios on heat transfer becomes significant at high Rayleigh numbers and when the aspect ratio is below 5. It was also found that increasing Knudsen number reduces the heat transfer. The interaction between Nusselt, Rayleigh, Knudsen numbers, and the aspect ratio was investigated using the design of experiments, results show that no interaction between these parameters. To help engineers to estimate heat transfer in low pressure cavities, widely used in solar energy applications, a correlation for convection heat transfer coefficient is introduced.