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.