Experimental Investigation of Heat Transfer Behavior of Fruits during Precooling

Abstract

Food products of perishable nature need to be preserved from spoilage using some precooling techniques. Precooling is the process of cooling fruits and vegetables after harvest and prior to transportation over long distances to cold storage for distribution. The present work is an attempt to investigate experimentally the heat transfer behavior during precooling of fruits and vegetables in a rectangular duct under forced convection. In the experimental study done on a rectangular duct, the items being investigated are apple and sapodilla. Forced air cooling is achieved by suspending the food inside a 4-meter long rectangular air duct of 0.3×3 meter section of galvanized iron sheet which is insulated with a 1cm thick puff sheet. The humidity inside the duct is to be maintained constant. The temperatures of the cold air outside the package and of the food product at different locations inside the package are to be measured at regular time intervals. The air is circulated through the duct by means of a blower powered by a 1.5 HP electric motor. The air is then made to pass through a coil. This coil is the evaporator coil of a 5-ton capacity vapour compression plant. The test programs were performed on two types of fruits, apple, and sapodilla. The air velocity, V = 2.5 m/s was selected to examine the effects of cool air velocity on the cooling process. The temperature was monitored at three locations (surface, middle, and centre) for apple and sapodilla. In the present study, a heat transfer coefficient has been obtained based on the transient temperature measurement techniques. Two instances were used for determining the surface heat transfer coefficient for low Biot numbers and large Biot numbers respectively. It has been shown that the dimensionless temperature of apple and sapodilla decreases exponentially with time. A high reduction in temperature happened during the first 30 minutes followed by a lower rate of temperature drop for the remaining time.