Mathematical modelling and validation of the thermal buoyancy characteristics of a mixed mode natural convection solar crop dryer with back up heater

Abstract

This paper presents the modelling and subsequent validation of the thermal buoyancy characteristics for an experimental mixed-mode natural convection solar crop dryer (MNCSCDBH) for three plenum inlet opening to vent outlet opening ratios of 1:1, 1:1.3 and 1:1.5 selected for the purpose of this study. The simulation follows the authors’ experimental study on the effect of varying the plenum inlet width and vent outlet width on thermal buoyancy performance of the MNCSCDBH for three different heating scenarios namely; solar heating, back up heating and hybrid heating. Mathematical models, capable of predicting the thermal buoyancy performance of the MNCSCDBH for the three selected heating modes were formulated by coupling energy balance equations on the components of the air-heater with energy balance for the air. The models were based on a one-dimensional steady-state operation of the dryer. A MATLAB code was developed and used to predict the performance of the dryer under no load conditions. The temperature predictions were observed to be accurate within 0.03% to 9.3% for solar heating mode, 0.20% to 7.49% for backup heating mode, and 0.03% to 12.8% for hybrid heating modes, respectively. Even though some discrepancies were found between the experimental performance indicators and their corresponding values predicted by the model as presented in this study, the code can be used in predicting the ventilation performance of the dryer under no load conditions with a general prediction to within 0.03% to 12.8% relative difference of the observed outlet velocity and critical system temperature values. The validation results show that the simulation code can serve as an effective tool for comparing and refining the designs of the MNCSCDBH for optimum drying performance.