Experimental study on the effect of wire coil insert geometry on the heat transfer enhancement of a flat-plate solar collector

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.This work presents an experimental study of the heat transfer and pressure drop characteristics of a flat-plate solar collector with two different geometries of helical wire-coil inserts. Isothermal pressure drop tests are conducted in a horizontal array of tubes, to obtain the fully-developed Fanning friction factor for a range of Reynolds number Re = [80-8000]. This allows identifying the different flow regimes that each wire-coil promotes, and serves as a basis for the interpretation of the heat transfer results. Heat transfer tests are performed in the solar collector loop of the experimental facility at ambient temperature, Pr = [16-28], in a range of Reynolds number Re = [140-605]. The heat enters the tube through the weldingdefined generatrix, which constitutes a particular boundary condition that plays a fundamental role in the mixed convection mechanism which appears in the tubes. The interaction of the secondary swirl flow generated with the mixed convection recirculations generated by buoyancy forces is considered for the two geometries of inserts. A considerable decrease in the temperature of the absorber is observed when the appropriate wire coil is selected for given flow conditions, proving the beneficial effect of this type of insert device for the overall augmentation of the flat-plate solar collector thermal efficiency.