Determination of convective heat transfer coefficient of agricultural products in a greenhouse type dryer

The convective heat transfer coefficient is an important parameter in drying rate simulation, since the temperature difference between the air and product varies with this coefficient. In this study, a small scale greenhouse type dryer was designed and manufactured. Convective heat transfer coefficient of eggplant, pepper, apple, carrot, grape, fig has been determined under open sun at natural convection mode and in greenhouse drying at natural and forced convection mode. Convective heat transfer coefficients were determined by linear regression analysis from experimental data. The average convective heat transfer coefficient values of products were found in the range of 1.114-3.074 W/m²°C for open sun drying under natural convection mode, 1.315-2.592 W/m²°C for greenhouse drying under natural convection mode, 0.829-2.724 W/m²°C for greenhouse drying under forced convection mode, respectively. It was determined that the large variation in convective heat transfer coefficient is due to shape and size of the crop, porosity, moisture content, thermophysical properties and, drying conditions, experimental set-up and the climatic conditions.