Convective Heat Transfer in a Helical Coil Solar Thermal Collector

Three dimensional solar concentrators can achieve concentration ratios in excess of 100 and optical efficiencies in excess of 95% throughout the day without the need for tracking. A helical coil heat exchanger has been designed and investigated numerically as the receiver for this solar thermal application. A computational fluid dynamics (CFD) model of a laminar flow in the heat exchanger was developed in ANSYS CFX and a uniform wall flux boundary condition applied to the outer surface. Due to the curvature of the pipe, Dean Vortices were setup within the flow that substantially increased the overall heat transfer to the solar receiver without significantly increasing the pressure drop across the heat exchanger. A full parametric study is conducted to investigate the effects of geometric properties (dimensionless pitch, coil radius etc) and flow variables (Reynolds number, Dean number, Helical number). The variation of the circumferentially averaged heat transfer coefficient with distance along the heat exchanger is reported. It has been shown that the flow is fully developed after approximately 3.5 turns of the heat exchanger coil and will remain stable throughout the remainder of the heat exchanger.