Methodologies for Determination of Forced Convective Heat Transfer Coefficient for Indirect and Mixed Mode Solar Dryers

The indirect and mixed mode solar dryers with forced air circulation have been popular on commercial scale applications in India and abroad for past few decades. These dryers have been subjected for studies concerning computer simulation as well as experimental investigation with an aim to improve their thermal performance. The convective heat transfer coefficient from absorber plate to fluid (air) hcpf is one of the critical parameters that affect the thermal evaluation of any dryer. Since the design of these dryers is based on different modes of heat utilisation by food product in drying chamber, separate methods of hcpf determination are proposed. The laboratory models of these dryers were therefore, designed and constructed for experimentation under forced mode operation. Steady state no-load experiments on these dryers were performed with inlet-outlet vents open under indoor simulated condition. The experimental data for temperatures of various components of each dryer with different absorbed thermal energy and air flow was employed to determine hcpf. The absorbed thermal energy and air flow rate were varied from 300-800 W/m2 and 1 to 3 m/s respectively. Results of the experimental investigation indicate that for both dryers, the value of hcpf increases with temperature difference between absorber plate and air as well as air flow rate. However, the comparative study on dryers reveals that the increase in hcpf in mixed mode dryer is found to be higher than that of indirect dryer for the operating conditions investigated.