Heat Transfer Investigation In Forced-Air Pre-cooling Of Stone Fruit

With the per capita living standards improving, people’s demands for the quality and quantity of fresh fruits and vegetables are also increasing. Forced air pre-cooling, as an economic and effective freshness preservation measures for fruit and vegetable, is widely applied. Due to the characteristics of the drupe, during the forced air pre-cooling, the uneven of internal heat transfer can be caused easily, for example, not completely cooled or chilling injured etc.. It seriously effects the storage time and quality of the fruits. Theoretical analyses and experiments on cooling air temperature, cooling air velocity, stacking modes and the opening rate of the pre-cooling boxes were done by researchers, but seldom with the characteristics of the drupe on the internal heat transfer.Peach Hanlumi, which is quite common in Shandong, is selected to represent the drupe. According to the principle of Parallel Hot Wire Technique, the thermal conductivity of the pulp and stone was determined by using thermal probe method. The thermal conductivity of the pulp is about0.4-0.6W/(m· K), and The thermal conductivity of the stone is about0.5-0.7W/(m· K). The model of the external and internal of the fruit in the forced air pre-cooling was built, and the analysis of the internal heat transfer characteristics in the forced air pre-cooling process was done on the base of model result.The research in this paper shows that the unique shape of features of drupe fruit is one of the most important reasons for the uneven of pre-cooling. Meanwhile, the difference in thermal properties of pulp and stone are also a reason for the uneven of heat transfer. Compare the rate of change of the average temperature of the fruits of a variety of monomer model,7/8cooling time, pre-cooling coefficient from the simulation results. The new model is more realistic, and for the same fruit, the windward side, cutting-edge, the stone part of the cooling rate is faster than other parts. Considered the characteristics of fruit and economy, give the suggestion to improve the pre-cooling velocity and uniformity by adjusting the angle of the fruit axis and the cooling air direction, or taking a new type of forced air pre-cooling equipment. By simulating the fruit at0.5m/s,1.0m/s,1.5m/s,2.0m/s,2.5m/s wind speed, as well as pre-cooling under0℃,3℃,5℃temperature, result shows that it achieve the preservation temperature in the medium-speed wind speed and lower temperatures.This paper provides the theoretical basis and numerical reference for the optimization design of the pressure cooling equipment and energy-saving of cold storage.