Performance Enhancement of a Curved Solar Air Heater using CFD
The conventional flat plate solar air heater (SAH) has been widely used worldwide for solar thermal conversion. A key factor to evaluate the performance of SAH is its thermal efficiency. Various research works have been going on to enhance its performance. Effect on SAH is shown when the smooth flat rectangular flow channel is transformed into a curved surface. A significant increase in the outlet air temperature and, hence thermal efficiency is observed when compared with flat plate SAH.
The model considered for the computational domain of curved SAH having rectangular cross-section throughout along the passage connects inlet and outlet respectively. The dimension of all types of SAH taken exactly same in terms of length, width and height are considered as 1600 mm, 800 mm and 40 mm, respectively and the curvature radius of absorber plate is 3020 mm for all geometries. The CFD analysis using numerical simulations carried out by considering only the core flow section that has been come under investigation i.e. flow passage comprises of absorber plate, bottom plate, air inlet and the air outlet in a single pass flow SAH.
In double pass flow SAH the flow passage has been considered same as in single pass flow and the air available at the outlet of single pass flow directed into the passage in between glass glazing and absorber plate to collect more heat by the air stream as comes in contact with hot absorber plate exposed to uniform heat flux (q) of range 800-1100 W/m2. CFD analysis has been carried out for different geometries of absorber plate of SAH were flat plate smooth single pass (FPSP), flat plate smooth double pass (FPDP), curved plate smooth single pass (CPSP), curved plate smooth double pass (CPDP), curved plate semicircular-corrugated single pass (CPSCSP), and curved plate V-corrugated single pass (CPVCSP) solar air heater (SAH), respectively shown in Fig. 2(i) and Fig. 2(ii). The range corresponds to different relative height and relative pitch ratios of the absorber plate corrugation chosen as 0.125-0.3 and 0.834-3, respectively. Simulations for all cases have been performed at three different values of Reynolds number (Re) 2209.11-6058.04 corresponding to a mass flow rate in the range of 0.0172-0.0472 kg/sm2, respectively. All ranges have been mentioned in Table 1.
The numerical simulations have been carried out for different geometries of curved absorber plate to study the dynamic thermal performance of smooth (single and double pass mode) and effect of different relative height and pitch ratios of semicircular groove and V-groove corrugation of the absorber plate of curved solar air heater. The effect of the various design of absorber plate on heat transfer characteristics i.e. Nusselt number, air outlet temperature and thermal efficiency were obtained. Correlations for the Nusselt number and friction factor are obtained using numerical data.
The key aspects drawn from the results obtained in the present study are mentioned below:
1. Curved geometry of the smooth flow convex rectangular passage of the curved solar air heater proved to be more efficient than the flat plate solar air heater. A considerable increase in the air outlet temperature and thermal efficiency have been seen for the curved plate smooth single pass and double pass solar air heater compare to the flat plate smooth single and double pass solar air heater, respectively.
2. Increase in the relative height and pitch ratios of the corrugation of the absorber plate of curved solar air heater enhances the formation of secondary vortices which imparts more turbulence in the air, results in intense mixing of air near to wall of the absorber plate. It provides the scope of transferring more heat to the flowing air at larger values of mass flow rates compared to curved plate smooth solar air heater without corrugated absorber plate.
3. The maximum thermal efficiency of 91.93% has been seen for the solar air heater with curved plate V-corrugated single pass having P/ev =0.834 and 32.05% percentage increase in the air outlet temperature with respect to curved plate smooth single pass solar air heater.