Thermodynamic analysis of optimal curvature ratio for fully developed laminar forced convection in a helical coiled tube with uniform heat flux

Abstract

In the present paper, the optimal curvature ratio for steady, laminar, fully developed forced convection in a helical coiled tube with constant wall heat flux was analyzed by thermodynamic second law based on minimal entropy generation principle. Two working fluids, including air and water, are considered. The entropy generation analysis covers a Reynolds number ( Re) range of 100 to 10 000, a coil curvature ratio ( δ ) range of 0.01 to 0.3, and two dimensionless duty parameters related with fluid properties, wall heat flux and mass flow rate, η 1 range of 0.1 to 3.0, and η 2 / 10 20 range of 0.01 to 1.0. The optimal δ for cases with various combinations of the design parameters is given in the present paper. In addition, a correlation equation for the optimal δ as a function of Re, η 1 and η 2 is proposed through a least-square-error analysis. For a thermal system composed of helical coiled tubes with fixed Re, wall heat flux and mass flow rate, the optimal δ should be selected so that the system could have the best exergy utilization and least irreversibility.