Numerical investigation and entropy generation for flow boiling evaporation in U-bend tube heat exchanger with elliptical and circular cross-sections

Abstract

• Entropy generation for flow boiling in a U-bend tube heat exchanger with the elliptical and circular cross-section. • Effect of mass velocities on entropy generation for flow boiling a in a U-bend tube heat exchanger. • Local entropy generation for flow boiling along the Length of the U-bend tube heat exchanger. • Effect of the bend on the flow pattern in the U-bend tube heat exchanger with an elliptical and circular cross-section. This paper presents the numerical analysis of entropy generation of two-phase flow boiling in a U-bend tube heat exchanger with the elliptical and circular cross-sections of the same hydraulic diameter for oriented downward flow. The entropy generation criterion due to pressure drop and heat transfer irreversibilities was utilized to measure the quality of energy transfer in the heat exchanger. For the purpose of comparison, the same geometrical parameters were used. The computations were implemented using computational fluid dynamics code for the operating parameters such as mass velocities ranging from 100 to 500 kg/m 2 s with respect to vapour qualities from 0.1 to 0.9 and a constant uniform heat flux of 10 kW/m 2 imposed on the tube’s outer surface. The results reveal that the U-bend tube heat exchanger with the elliptical cross-section performs better than the circular cross-section due to the lower entropy generations obtained in the elliptical cross-section compared to the circular cross-section. The effect of bend on liquid and vapour phase distribution in circular and elliptical cross-sections was illustrated by the flow pattern, just before and after the bend. The flow patterns formed after the bend were quite different from the ones formed before the bend. For the elliptical and circular cross-sections, the flow patterns were observed to be almost the same with the fluids taking the shape of the cross-section. The numerical calculations were validated and found to be consistent with what was found in the open literature.