Numerical Prediction of Enhanced Heat Transfer Using Oval Tubes and Delta Winglets

Unsteady three-dimensional laminar flow and heat transfer in a channel with a built-in oval tube and delta winglets have been obtained through the solution of the complete Navier-Stokes and energy equations using a body-fitted grid and a finite-volume method. The geometrical configuration represents an element of a gas-liquid fin-tube cross flow heat exchanger. The air-cooled condensers of the geothermal power plants also use fin-tube heat exchangers. The size of such heat exchangers can be reduced through enhancement in transport coefficients on the air (gas) side, which are usually small compared to the liquid side. In a suggested strategy, oval tubes are used in place of circular tubes, and delta winglet type vortex generators in common-flow-down configuration are mounted on the fin-surface in front of the tubes, while another delta winglet pair in common-flow-up configuration is mounted downstream of the first set of winglets. An evaluation of this augmentation strategy is attempted in this investigation. The investigation was carried out for a winglet angle of attack of 40 degrees to the incoming flow. The structures of the velocity field, and the heat transfer characteristics have been presented. The results indicate that vortex generators in conjunction with the oval tube show definite promise for the improvement of fin-tube heat exchangers.