The role of insert devices on enhancing heat transfer in a flat-plate solar water collector

Highlights: • The criteria to use twisted-tapes or wire-coils in solar collectors are given. • A detailed friction study was carried out for all the inserts: Re = [80–9000]. • The studied inserts decrease absorber temperature for all the working conditions. • There is a predictable link between insert geometry, flow pattern and heat transfer. • A moderate roughness wire-coil was the best tested insert for solar collector risers. - Abstract: This work presents a comparative experimental study of heat transfer enhancement in a flat-plate solar water collector using insert devices. Three wire-coils and three twisted-tapes were selected with representative geometrical characteristics typically employed in industrial applications. Isothermal pressure drop tests were carried out to obtain the fully-developed Fanning friction factor for a range of Reynolds numbers Re = [80–9000]. The increase in friction factor in comparison to smooth tube was computed for all the devices. Depending on Reynolds number and insert geometry f_i/f_s values ranged from 1.3 to 79.8. Furthermore, detailed temperature profiles were obtained for different sections along the absorber plate and the risers for five different mass flow rates covering the Reynolds range from [400–2500]. The increase of the inner heat transfer coefficient by the inserts caused an important decrease of the absorber temperature. At increasing mass flow rates (from Re ≈ 1000), all the inserts showed a very similar thermal performance which make them suitable for inserting within harp-type solar collectors, where pressure drop is not a constraint. The best inserts TT03, WC01 and WC02 gave at Re ≈ 1500 maximum absorber temperature decreases (insert vs smooth tube) of 5.05 °C, 5.40 °C and 5.34 °C. In serpentine-type solar collectors, due to pressure drop constraints, the wire coil WC01 with a moderate pitch to wire-diameter ratio (p/d = 1.5 and e/d = 0.07), is the best specimen to insert. WC01 presents a moderate pressure drop increase (f_i/f_s = 2.8 at Re ≈ 1000), an early promotion of turbulent flow (at Re ≈ 700), and a significant reduction of the absorber temperature (decreasing 4.84 °C vs smooth tube at Re ≈ 1000).