| China is the largest country in fruit and vegetable production and consumption.Precooling,as the primary link in the process of fruit and vegetable cold chain logistics,can greatly maintain the quality of fruits and vegetables.Differential precooling has the characteristics of convenient operation,high precooling efficiency and low energy consumption,but it also has the disadvantages of complex stacking and short circuit of air supply.Usually differential precooling with horizontal air supply,due to the fruit weight and handling,stacking process of shock problems,resulting in the top layer of fruit and cover between the "top gap",causing cold air supply short circuit and low utilization rate.At the same time,there is a big gap between the packing form in the academic research and the actual production.Therefore,this paper conducts theoretical and experimental research on the basis of the widely used packing box structure in practical production,explores the feasibility and optimal air supply conditions of vertical air supply differential pressure precooling,and provides theoretical support and practical guidance for the further improvement of precooling effect and energy utilization.As a condition of forced convection heat transfer,the air supply parameters and packing box structure directly affect the precooling time,precooling effect and system energy consumption.In this paper,the most widely used B4 type plastic packing with opening rate about 28% was taken into consideration both in theory research and experiment.Taking apple as the research object,precooling time and precooling performance,including evenness,windward side maximum temperature,the coefficient of cooling,pressure difference between inlet and outlet and fruit weight loss rate under different air supply parameters(supply air velocity and supply air temperature)and air supply mode(vertical air supply and level air supply)was experimentally studied.The superiority and feasibility of vertical air supply were explored,and the best parameters were put forward to carry out vertical supply air differential pressure precooling.Firstly,based on the unsteady cooling process of differential precooling,the energy conservation equation and the heat and mass transfer equation were used to establish the thermal and mass transfer models of single and laminated fruits in the vertical air supply differential precooling process.MATLAB was used to solve the differential equation of fruit heat transfer in the two-dimensional spherical coordinate system.COMSOL was used to solve the temperature distribution in the box at different wind speeds.The theoretical results show that the temperature drop curve is different at different positions in the pre-cooling process: the temperature drop near the surface is larger per unit time,while the temperature drop at the center of the fruit is slower.With the increase of wind speed,the temperature difference at each point of the fruit gradually reduced.With the increase of wind speed,the required precooling time decreases gradually;When the wind speed exceeds 2.5m/s-3m/s,the corresponding pre-cooling time and effect are slightly improved,but the extent is small,and the economic benefit may be lower than the energy consumption required by the system.The experimental study on the influence of vertical and horizontal air supply on the precooling effect was carried out after modifying the existing differential pressure precooling test bed.The experimental results were sorted and analyzed,and the following conclusions were drawn:(1)the optimal air supply speed and temperature exist in different air supply modes of differential precoolingWhen the relative humidity of cold air is 85-90%,the cooling trend of fruits under the conditions of vertical air supply and horizontal air supply is similar: the approximate exponential distribution decreases rapidly in the early stage,and the cooling rate decreases gradually later,showing a linear distribution,and gradually to the final temperature of precooling and the temperature of cold storage.At the same time,with the increase of precooling wind speed,the precooling time is continuously shortened,and the cooling rate and uniformity of precooling are improved,that is,the increase of precooling wind speed in the process of differential pressure precooling is conducive to the improvement of precooling effect.When the wind speed reaches 2.5m/s,its cooling coefficient and the required precooling time tend to be stable.Then the wind speed increased further,and the precooling effect did not significantly improve,that is,there was a critical wind speed in the process of differential pressure precooling,so that the precooling effect and system energy consumption reached the best match.It is suggested to choose 2.5m/s-3m/s for the precooling wind speed in the process of vertical air supply differential pressure precooling.Under the same air supply condition,with the decrease of air supply temperature,the time required to reach the pre-cooling end is significantly shortened,and the pre-cooling uniformity and weight loss rate in the packaging box are reduced.Different air supply temperature has no significant impact on the pressure difference at the inlet/outlet wind end.The reduction of precooling temperature is conducive to the increase of precooling speed,but the energy consumption of the system will also increase.The relationship between the balance of pre-cooling effect and energy consumption of the system should be reasonably selected in practical production.(2)The vertical air supply is better to shorten the precooling time and improve the precooling effect than horizontal air supply,Under the same conditions,compared with the horizontal air supply,the vertical air supply can effectively improve the precooling speed,whose precooling duration is only 63% to 75% of the horizontal air supply,and the precooling speed is increased by 24% to 36.8%.In the process of horizontal air supply,the maximum temperature difference between the first windward surface and the last windward surface is about 3?,while in the process of vertical air supply,it is only 1.3?.The uniformity of vertical air supply is effectively increased by 30%-43.9% compared with the pre-cooling uniformity of horizontal air supply.For the pressure difference between the inlet and outlet air surfaces,the horizontal air supply is 1.32 times of the vertical air supply,and the vertical air supply can effectively reduce the pressure difference between the inlet and outlet air surfaces of packing boxes by 17.6% to 32.3%.Meanwhile,the weight loss rate of fruits in the process of vertical air supply is about 0.9% to 1.5%,which can effectively reduce the weight loss rate by 24.7% to 25.9% compared with the horizontal air supply.The weightlessness rate of horizontal and vertical air supply is much lower than the maximum weight loss rate of 5%.Therefore,in the actual production process can be ignored in the differential pressure precooling process especially in the vertical air supply differential pressure precooling process of fruit weight loss phenomenon.In conclusion,the vertical air supply differential pressure precooling has a better precooling performance,which means it fits more for the actual needs of China. |
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