Entropy generation and optimal analysis for laminar forced convection in curved rectangular ducts: A numerical study

Abstract

The present paper analyzes entropy generation induced by forced convection in a curved rectangular duct with external heating by numerical methods. The problem is assumed as steady, three-dimensional and laminar. The local entropy generation distributions as well as the overall entropy generation in the whole flow fields, including the entrance region and fully developed region, are analyzed. The effects of three important factors, including Dean number, external wall heat flux and cross-sectional aspect ratio, on entropy generated from frictional irreversibility and heat transfer irreversibility are investigated separately in detail. The results show that the major source of entropy generation in the flow fields with larger Dean number and smaller wall heat flux comes from frictional irreversibility; whereas for the flow fields with smaller Dean number and larger wall heat flux the entropy generation is dominated by heat transfer irreversibility. The competition between the entropy generation from the irreversibility caused by fluid friction and heat transfer is complicatedly related with the three factors, making the relationship between the resultant entropy generation and the three factors non-monotonous. Based on the minimal entropy generation principal, the optimal condition can induce the minimal entropy generation in the flow fields. Through the optimal analysis, the optimal aspect ratio is found to be dependent on heat flux and Dean number. For each case with specific aspect ratio and wall heat flux, there exists an optimal Dean number, and the optimal Dean number is found to increase with wall heat flux. The detailed optimal analysis is provided in the present paper, which is worthwhile for heat exchanger design from the second law of thermodynamics since the thermal system could have the least irreversibility and best exergy utilization if the optimal Dean number and aspect ratio can be selected according to the practical design conditions.