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In this paper transient heat transfer in a solar food drier is modeled by using both computational fluid dynamics (CFD) and lumped capacitance modeling techniques. The CFD model is used to simulate the air flow and transient heat transfer within the dryer. A lumped capacitance model, developed from energy exchanges between various components of the drying chamber, is extensively compared with the results of the CFD model. The two models predict both a 40 ℃ rise in steady-state temperature within the drying chamber. Before reaching this temperature, there is about 8 ℃ RMS deviation between the transient temperature prediction of the two models. This deviation is mainly caused by the heat transfer coefficient around the food rack shelfs. Fitting these coefficients give rise to a minimal RMS deviation between the two models of 1.8℃ and 0.9℃ on the lower and upper rack shelfs respectively. The transient heat transfer phenomenon from CFD modeling also showed that the spatial distribution of temperature after 30 minutes is uniform thus validating the very assumption of lumped capacitance modeling. This demonstrates that the lumped capacitance model can supplant the computationally demanding CFD modeling technique in predicting transient heat transfer phenomenon in solar food dryers.

The moisture sorption isotherm at three maturity levels of the M a reko Fana chili pepper variety (red, brown and green) has been studied in this paper. The sorption isotherm was determined based on the standard static gravimetric method using a glycerol-water mixture in a relative humidity range of 10–92% at three temperature levels and nonlinear regression analysis was used to select suitable sorption models. The Clasius - Clapeyron equation was implemented to determine the isosteric heat of sorption of the chili pepper using the experimental equilibrium moisture content at different sorption temperature levels. The results showed that the GAB model was well fitted for green chili pepper, while the OSWIN model described well the brown and red chili variant. There was a difference in net isosteric heat between the adsorption and desorption isotherm of chili pepper maturity. For green chili, the maximum value of the net isosteric heat was 18 kJ mol −1 and 20 kJ mol −1 for adsorption and desorption isotherms, respectively and it decreased exponentially as moisture content increased. The desorption heat was higher than the adsorption heat for each maturity of chili pepper which indicated the existence of hysteresis in the sorption process. In comparison to literature data reported for different chili varieties, M a reko Fana has a lower heat of sorption and monolayer moisture content.

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