Author(s)

T. M. Jaballa

Abstract

Knowledge of the heat transfer coefficient is believed to be of great contribution in the course of a full design of a solar dryer. To find an expression for the heat transfer coefficient, the general logarithmic velocity profile for turbulent flow is used for the analysis. It is assumed that the fluid is air at atmospheric pressure, and that the thin wall laminar sublayer has a small or negligible effect on the value of the fluid bulk temperature. The final form of the correlation equation gives a relation between Nusselt number and Reynolds’ number, and is given in a form which is easy to remember by the solar dryer designers. Comparison of the predicted values indicates that the correlation can predict values to within ± 6% of experimental values reported in literature. Expected errors of nearly ± 25% were observed when the correlation predictions compared with well known correlations found in literature and in text books. Keywords: heat transfer coefficient, crops solar dryers, turbulent velocity profile, turbulent thermal diffusivity. 1 Introduction Drying crops such as hay, corn, beans, tomatoes, onions, etc., has been known to man for thousands of years. The drying method used then was purely natural; by simply distribute the crop on a flat ground, and let nature dry it. Very little of industrial pollutants those days were existed, and the insects effect was not of any concern. Today this same method is still used locally, in the presence of industrial and growing natural pollution. Dates, tomatoes, unions, apricots, and even meat and fish are dried in open areas. Many of the diseases in the African countries are related to foods dried this way. A cleaner and a more healthy

Keywords

heat transfer coefficient, crops solar dryers, turbulent velocity profile, turbulent thermal diffusivity