Experimental and numerical evaluation of a solar dryer for drying banana (Musa spp)

Abstract

A forced convection indirect type solar dryer having a double pass flat collector that can be used for drying bananas was designed, constructed and tested experimentally and numerically evaluated. The study mainly tried to address the problem associated with losses of bananas due to the average distance from neighboring countries to the market and the failure of market to consume the product within its shelf life. Hence, preservation method of bananas through solar drying technology was evaluated in this study so that to extend their shelf life. The descriptions of the design methodology, development and operation of the newly constructed solar dryer are reported. The physical properties of banana fruits and relevant environmental factors of Botswana were considered in the development of the dryer. The dryer comprises of solar panel, flexible pipe, solar collector and drying chamber. The solar collector is made up of two glasses of 5 mm thickness and 1.6mm black painted aluminum absorber plate, all are enclosed in a casing made from plywood. The drying chamber is made from plywood with 20mm thickness. The dryer features a unique arrangement as the drying chamber is underneath the solar collector and the solar collector itself can be adjusted to an angle of 00 up to 350 at an interval of 50. The drying chamber is incorporated with a fan that sucks hot air from the collector on to the product for drying. To investigate its performance, the solar collector was tested and achieved an optimal peak outlet temperature of 345K with a maximum efficiency of 72.5%, whereas the maximum useful heat gained by the solar collector from the solar radiation was 828.21W. The minimum and maximum ambient temperature, relative humidity and solar insolation recorded were 306.8K and 313K, 13.6% and 44.5% and 529 W/m2 and 983W/m2 respectively. Approximately 3.69kg of banana pulps sliced to a thickness of 3 to 5mm with moisture content of up to 82% dried within 8-13hrs to the final moisture content of >20%. The geometry of the developed solar dryer was modeled using CFD software and 3D CFD simulations for the solar collector and drying chamber were performed using ANSYS 18.2. The fluid flow was characterized by RNG k-𝜀 turbulent viscous model and Discrete Transfer Radiation Model (DTRM) to allow radiation within the flow domain. A total number of 3,816,337 elements with nodes of 746,255 were generated in order to enhance smooth convergence of the simulation. The CFD results were compared with experimental results, and a good correlation was established between the two approaches particularly on airflow and the maximum temperatures of both the absorber plate and top glass. During the experiment, the maximum temperature of the absorber plate was 374K while that of CFD results was 384K. The maximum temperature reached on the top glass of the collector from the experimental result was 330K, while for CFD the result was approximately 325K-331K.