Experimental Investigation of the Thermal Buoyancy Characteristics of a Mixed Mode Natural Convection Solar Crop Dryer with Back Up Heater

Abstract

An experimental investigation into the thermal buoyancy characteristics of a mixed-mode natural convection solar crop dryer with back up heater is described. Twenty seven carefully planned tests were conducted under no load conditions for plenum inlet gap to vent outlet gap ratios of 1:1, 1:1.3 and 1:1.5, in order to determine the optimum thermal drive characteristics of the dryer. Air velocities, temperatures, ambient relative humidity, collector efficiency, exergy rate of change, thermal mass entropy change and air mass flow rate are presented. The effects of the various plenum inlet and vent outlet gap configurations described above on dryer thermal buoyancy are deduced and discussed. Results show that for all heating modes, an air inlet gap to vent gap ratio of 1.5:1 optimises the thermal performance of the dryer.

Keywords: Solar dryer, back up heater, exergy, plenum, thermal mass