Heat transfer augmentation in solar receiver heat exchanger with hole-punched wings

Thermal characteristics in a solar air heater duct with perforated/punched rectangular wing (P-RW) mounted on the absorber plate have been experimentally examined. Air as the test fluid flowed through the test duct having a constant wall heat-flux with Reynolds number (Re) based on the hydraulic duct diameter from 5300 to 22,600. Two parts on the P-RWs are offered herein. First, the characteristics of forward P-RWs in the tests were three porosity ratios (A(h)/A(w) = 0.12, 0.34 and 0.64) and three relative pitches (R-P = P/H = 1.0, 1.5 and 2.0) at a single attack angle (alpha = 60 degrees). The study revealed that the optimal thermal performance is found at Rp = 1.5 and A(h)/A(w) = 0.34. Second, the optimal P-RWs in the first part was further studied by varying the attack angles, alpha = 30 degrees, 45 degrees and 60 degrees, and by rearranging the P-RWs from the forward to the backward P-RWs. The results of the heat transfer rate and flow resistance inside the duct were displayed in terms of Nusselt number (Nu) and friction factor (f), respectively. The experimental results have showed that the P-RWs give the considerable increase in Nu over the flat-plate duct (smooth absorber plate) around 3.91-5.52 times while the increase in f is about 10.37-36.35 times. The highest thermal performance around 2.01 using the backward P-RWs is seen at A(h)/A(w )= 0.34, R-p = 1.5 and alpha = 45 degrees. Correlations for Nu and f have also been determined as a function of P-RW parameters.