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

Zusammenfassung

This work presents an experimental study of the heattransfer and pressure drop characteristics of a flat-plate solarcollector with two different geometries of helical wire-coilinserts. Isothermal pressure drop tests are conducted in ahorizontal array of tubes, to obtain the fully-developed Fanningfriction factor for a range of Reynolds number Re = [80-8000].This allows identifying the different flow regimes that eachwire-coil promotes, and serves as a basis for the interpretationof the heat transfer results. Heat transfer tests are performed inthe solar collector loop of the experimental facility at ambienttemperature, 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 boundarycondition that plays a fundamental role in the mixed convectionmechanism which appears in the tubes. The interaction of thesecondary swirl flow generated with the mixed convectionrecirculations generated by buoyancy forces is considered forthe two geometries of inserts. A considerable decrease in thetemperature of the absorber is observed when the appropriatewire coil is selected for given flow conditions, proving thebeneficial effect of this type of insert device for the overallaugmentation of the flat-plate solar collector thermal efficiency.