Experimental study of effect of Reynolds number and Nusselt Number on the performance of flat-plate solar Air Heater having artificial roughened (Rhombus shape) absorber plates

The use of artificial roughness on a surface is an effective technique to enhance heat transfer to fluid flowing in a duct. This roughness can be provided by sand blasting, fixing wires, wire mesh or by providing roughness in the form of ribs, dimples, protrusion etc. reported by Dippery and Sabersky (1963), Sheriff and Gumley (1966), Saini and Saini (1997), Saini and Verma (2008), Hans et al. (2009) and Bhushan and singh (2011). Several investigations have been carried out to study the effect of artificial roughness on heat transfer used in compact heat exchanger by Elyyan et al. (2008) and Webb (1994) and in solar air heater by Momin et al. (2000), Varun et al. (2008), Singh et al. (2011), Lanjewar et al. (2011), Jauker et al. (2006) and Layek et al. (2007).The roughness destroy the laminar sublayer and create turbulence in the flow. The turbulence leads to increase in pumping power which is required for flow of air in the duct. Therefore roughness is created in such a region which is near to the absorber plate i.e. laminar sublayer only. 

The roughness was first used in solar air heater and resulted in better heat transfer in comparison to that in conventional solar air heater by Prasad and Mullic (1985). Prasad and Saini (1988) studied the effect of roughness and flow parameters on heat transfer for transverse ribs. It was observed that maximum heat transfer occurred near to the reattachment points. The maximum enhancement in Nusselt number was reported to be 2.38 times over smooth duct.Verma and Prasad (2000) has been carried out experimental study for thermo hydraulic optimization of the roughness and flow parameters for Reynolds number (Re) range of 5000-20,000, relative roughness pitch (P/e) range of 10-40 and relative roughness height (e/Dh) range of 0.01-0.03. 

The optimal thermohydraulic performance was reported to be 71%. Karwa et al. (1999) has been experimentally investigated the effect of repeated rectangular cross-section ribs on heat transfer for duct aspect ratio (W/H) range of 7.19-7.75, P/e value of 10, e/Dh range of 0.0467-0.050 and Re range of 2800-15,000. The enhancement in the Stanton number was reported to be 65-90% Gupta et al. (1997) experimentally investigated the effect of e/Dh, inclination of rib with respect to flow direction and Reynolds number (Re) on the thermohydraulic performance of roughened solar air heater. The detailed studies on roughness geometries used in solar air heater ducts are also available in Varun et al. (2007), Hans et al. (2009) and Bhushan and Singh (2010). 

The application of artificial roughness in the form of rhombus shape on absorber plate is attractive roughness geometry for solar air heater due to its less complicated manufacturing process. In this paper experimental data has been collected by performing experiment to see the effect of roughness parameters (rhombus shape) on heat transfer.