Research On Heat And Mass Transfer Mechanism Of Fruits And Vegetables In Cold Storage

The physiological process of fruits and vegetables do not stop after picking off,as the result,the fruits and vegetables are still carried out the role of respiration and evaporation after entering the cold storage.Because of its high temperature and water content,the fruits and vegetables will produce heat and mass transfer with the surrounding environment in cooling process,resulting in quality decline.Therefore,the research on heat and mass transfer mechanism of fruits and vegetables has certain academic significance and engineering reference value for fruits and vegetable storageIn this paper,the author chose a single apple as the research object.By establishing the physical and mathematical models of heat and mass transfer between the single apple and the surroundings,the finite difference method was used to study the internal temperature and the mass transfer of the apple during the cooling process.Also,compared and analyzed the factors influence the heat and mass transfer.Secondly,a three-dimensional unsteady computational fluid dynamics(CFD)model of a box of apples was established.By using the numerical simulation method,the author researched the fluid flow as well as heat and mass transfer of the apples during the natural cooling process.The main research work and conclusions are as follows:(1)During the cooling process of spherical fruits and vegetables,the epidermis responds faster to the outside temperature,and the temperature changes in the layers of the fruit show different degrees of hysteresis.The temperature of epidermis decreased quickly at beginning and then reduced gradually.The cooling rate of the internal part first increased gradually and then reduced during the whole cooling process.The cooling rate of each layer is different,and the closer to the center,the smaller the rate of cooling.The temperature tends to be consistent after a certain period of time.(2)By establishing a mathematical model of internal thermal conductivity of a single apple,used the finite difference method to solve the temperature change of the inner layer of the fruit during the cooling process,the effects of the fruit size as and the airflow velocity on the heat transfer were obtained.The research showed,fruit size had little effect on skin temperature changes,but had greater impact on the heart temperature.When the apple diameter increased from 0.08 m to 0.09 m,the maximum temperature difference of the internal part at the same time was 1.08?.The airflow velocity had a certain impact on the skin temperature of fruits and vegetables.Increased the airflow speed appropriately can reduce the time of cooling of the fruits and vegetables epidermal and improve the temperature gradient of the cooling.However,the velocity has little effect on the cooling process of the center of the fruit.(3)To solve the change curve of the moisture content over time of the fruit,the physical and mathematical models of the internal mass transfer of the apple and the surroundings were established.The influences of the velocity and the relative humidity of the wet air on the mass transfer process were also studied.Fruit moisture content is affected by skin temperature and velocity of airflow around.At the same skin temperature,the lager the air flow rate,the greater the amount of moisture.However,during the cooling process,the greater the speed of airflow,the faster the fruit skin cooling.As a result,the moisture content reduced.Considering these two factors,In the process of cooling,the fruit moisture content decreased when the airflow velocity increased,as well as the water loss rate.When the air velocity increased from 0.3m/s to 0.7m/s,the rate of water loss decreased 3.2%.The relative humidity has a greater effect on the amount of moisture,and the fruit water loss rate reduced by 75% when the relative humidity increased from 75% to 95%.(4)By simulating the natural cooling process of the whole box of apple,got that the rule of relative humidity changes of the cargo area.The research indicates that the airflow can replace the wet air with high moisture content in the cargo area quickly.But because of the lag behind of the internal layers during cooling process,the temperature of the cargo area did not reduced rapidly.Therefore,the relative humidity reduced to 66.01% rapidly.With the cooling going on,the relative humidity increased gradually because of the temperature drop of the cargo area.At the steady state,the relative humidity of the cargo area was 87.07%.