Fluid-thermal characteristics of solar-exhaust gas powered dryer: An experimental validation

Abstract

A new method of supplementing heat energy in a solar-exhaust gas greenhouse dryer has been experimentally evaluated in this paper. An internal combustion engine using diesel produced exhaust gas which was channeled to a hybrid recuperative heat exchanger (HRHE) for energy recovery. Fluid and thermal characteristics of the dryer were reported for three modes of drying: solar mode (SM), solar-exhaust gas mode (SEGM), and exhaust gas mode (EGM). Consequently, the dryer room air temperature were found as: 14.82–58.46 °C, 34.49–61.97 °C and 25.75–30.77 °C respectively. Moisture evaporated as a result of temperature variations were computed as: 0–20.8 g, 0–17.79 g and 0–22.33 g respectively. Fluid characteristics of exhaust gas included: average density of 0.7218 kg/m3, volumetric flow rates from 4.2×10−3 – 1.67×10−2 m3/s, maximum residence time of 418 s in HRHE, mass flow rates from 11.32 to 45.07 kg/h, velocity in connectors ranging from 2.14 to 8.52 m/s, velocity in tubes from 0.035 to 0.14 m/s, Reynolds number ranging from 2681 to 10674 in connectors and 344–1368 in tubes, Nusselt number ranging from 14 to 43 in connectors and constant at 3.66 in tubes. Available energy in exhaust gas was found in the range of 2082.32–16002.5 kJ/h, corresponding to temperatures of 197.19–359.82 °C as a result of engine speeds varied from 750 to 2500 rpm. Kinetic energy in exhaust gas increased with increased velocity for both tubes and connectors to a maximum of 39.79 kJ/h in connectors and 1.289×10−2 kJ/h in tubes. The significance of this study is in promotion of faster product drying in SEGM and slower drying for delicate products requiring low heat energy supply in EGM.