Recyclic double pass packed bed solar air heaters

In the present work, analytical models to predict the thermal and thermohydraulic efficiencies of two different designs of double pass packed bed solar air heater under external recycle have been proposed. It is predicted that Model A with the single air pass through the first channel as well as the recycle of the air exiting from the second channel through the third channel can enhance the heat transfer rate more as compared to Model B. The wire mesh screen as an absorbing media above the absorber plate is used in both of the solar air heater models. The recycle ratio and the mass flow rate are varied from 0.1 to 1 and 0.01-0.025 kg/s, respectively. The results of the study depict that the recycle ratio and the mass flow rate substantially increases the heaters efficiencies by increasing the fluid velocity. The maximum value of the thermal efficiency of Model A is found to be about 6.6% higher than that of Model B. The optimum values of the recycle ratio and the mass flow rate, at which the heaters yield maximum values of thermohydraulic efficiencies, are identified and presented. In addition, the effect of the channel depth ratio on the thermal performance improvement is also delineated. (C) 2014 Elsevier Masson SAS. All rights reserved.