Influence on thermal performance of spiral solar air heater with longitudinal baffles

This paper aimed to analyze the effect of the number of longitudinal baffles on flow and heat transfer characteristics of spiral solar air heaters (SSAHs). According to the analysis of velocity contours, the results showed that the velocity gradients that appeared at the place of inlet and outlet were contributed to the size difference between inlet/outlet and cross-sectional area of the channel, and the velocity gradients that appeared at the turnings were caused by the phenomenon of boundary layer separation, all which were contributed to increasing the pressure loss of SSAHs. Furthermore, it could be obtained that the collection efficiency firstly increased with about 2 longitudinal baffles to a peak value , then reduced, which proved that the positive effect of the Q(ab) increased by placing more longitudinal baffles would not compensate the negative effect of increasing the Q(L) by placing more longitudinal baffles. In accordance with the analysis of the effective efficiency and economic benefits, the SSAH with 2 longitudinal baffles was an optimal structure. To better predict the pressure loss and collection efficiency of SSAHs with the different number of longitudinal baffles, the correlations of Nu and the pressure loss of SSAHs were obtained by the least square method.