CFD Modelling and performance evaluation of a forced convection mixed-mode solar grain dryer with a preheater.

Abstract

Solar drying of agricultural food products as an art of food preservation has been in existence since the 17th century. In most tropical and subtropical countries, the drying process of harvested agricultural products such as grains is mainly carried out using the method of open-air drying or sun drying to preserve the harvest. With the advances of technology over time, new solar drying methods such as indirect and mixed-mode solar drying are evolving. Mixed-mode solar dryers are among the most efficient solar drying methods for improving the harvest and storage of grains. One of the advances in the development of solar dryers is the use of computational fluid dynamics (CFD) and computer-aided design (CAD) codes to model, simulate, and analyze dryer systems' performance. This study was conducted in two phases. The first phase entailed the use of CAD and CFD codes to model and simulates a forced convection mixed-mode solar grain dryer integrated with a preheater. A 3D model was developed with great accuracy using SolidWorks code and, the CFD simulation was carried out using ANSYS Fluent code. In the second phase, an experiment was conducted using an existing indirect solar dryer which was modified and converted to a mixed-mode solar dryer suitable for the study. The modeling and simulation results were validated against experimental results to evaluate the dryer system' performance. The study was conducted at various airflow speed and preheater temperatures ranging from 0.5 m/s to 2 m/s and 30 ℃ to 40 ℃, respectively. The type of grains used in the experiment were corn grains whereby 72 freshly harvested maize ears/cobs were dried. The study was conducted under the weather conditions of Durban, South Africa, at the University of KwaZulu-Natal. This study aimed to investigate solar drying technologies towards performance enhancement of a forced convection mixed-mode solar grain dryer that incorporated a preheater through modeling and optimization. This approach was followed in order to develop a better understanding of the effects of forced convection and air preheating on airflow distribution and temperature distribution within a solar dryer. The results from both the CFD modeling and experiment were satisfactory, resulting in a correlation with a maximum relative error of 16.3 %. The dryer system's performance results indicated a maximum thermal efficiency of 58.8 % with a corresponding drying rate of 0.0438 kg/hr. The minimum thermal efficiency for the dryer system was 47.7 %, with a corresponding drying rate of 0.0356 kg/hr. The fastest drying time of maize ears was achieved in 4 hours and 34 minutes from an initial moisture content of 24.7 % wb to 12.5 % wb. At the same time, open-sun drying yielded the slowest drying time of 15 hours from an initial moisture content of 27.3 % wb to 12.7 % wb. There was a significant improvement in the dryer system's performance, whose initial efficiency was 36 % when operating as an indirect solar dryer. These results are a clear indication that using a solar dryer system in mixed-mode operation with forced convection and the assistance of a preheater or backup heater can significantly improve drying processes and increase food preservation. The study further presents design concepts of incorporating cost-effective solar thermal energy storage systems that can be implemented to optimize solar dryers. In this case, solar energy can be harvested and stored during peak sunshine hours and made available for usage during off-peak sunshine hours.