Effects of buoyancy in a laminar flow of supercritical carbon dioxide in a horizontal microchannel at different gravities

Abstract

The effects of buoyancy were generally neglected in convective heat transfer inside horizontal microchannels because gravitational effects diminish compared to inertial and viscous forces with diminishing length scale. However, buoyancy effects can be significant for supercritical carbon dioxide (sCO2) slightly above the critical condition in microchannels, and the heat transfer coefficient can peak because of abrupt changes in the fluid's thermophysical properties. Previous studies considered buoyancy in channels with hydraulic diameter above 500 μm at a constant surface heat flux boundary condition. These effects were not well understood for microchannels with a hydraulic diameter as small as 100 μm, particularly with constant wall temperature boundary condition, but they hold great potential in the design of micro heat exchangers.This study numerically investigated the effects of buoyancy inside a 100-μm hydraulic diameter square microchannel with a total length of 27 mm and a heated length of 25 mm for laminar flow of sCO2. A total of 180 cases were analyzed to predict the effects of surface temperature ranging from 310 to 350 K, pressure ranging from 8 to 10 MPa, Reynolds numbers from 100 to 500, and gravitational acceleration from 0 g to 3 g, on the heat transfer. It was revealed that gravity has a significant effect on the heat transfer coefficient and on the thermal entrance length. The average heat transfer coefficient was increased by up to 138% on the bottom surface of the channel for 3 g compared to 0 g. The thermal entrance length showed a mixed behavior, such as it was increased for certain conditions while decreased for other conditions. For some cases, it was also observed that the pressure drop increased slightly with increasing gravity, and it was directly proportional to the wall shear stress. Finally, it was concluded that the gravity had a significant effect on the flow and heat transfer characteristics but was negligible for Richardson number below 10−4.