Performances of solar air heater using polygonal and trapezoidal rib absorber plate for augmentation of heat transfer

.An experimental investigation carried out in a solar air heater (SAH) by implementing an artificial rough surface in the absorber plate to enhance the convective heat transfer coefficient by creating turbulence in the laminar sub-layer. Inclined and sharp edge shape of ribs like the polygonal and trapezoidal shape of ribs were employed in an absorber plate at a relative pitch distance of p/e = 5, 7.5, 10 with Reynolds number in the range of 2000 – 20,000. An empirical equation of Dittus Boelter and Blausis were taken to validate the performance of Nusselt Number (Nu) and friction factor (ƒ) characterization for a smooth and rough surface. The result reveals that strong turbulence that occurred in SAH due to roughness of the surface in the fluid flow direction leads to heat transfer enhancement and varying pitch distances produce persistent improvement. Significant enhancement found in polygonal rib absorber plate at relative pitch distances p/e = 7.5, has produced a higher Nu number than other shapes and it attained maximum thermo-hydraulic performances (THP) 2.95. An empirical correlation was developed for the Nu and ƒ of experimental values and found good agreement in the range of 8% with predicted values.