Pre-cooling is the process that horticultural products are rapidly cooled to a target temperature after harvest.According to the national strategy of Emission peak and Carbon neutrality,new requirements are proposed for the low-carbon development of cold chain logistics.At present,there are many problems in pre-cooling in China,such as low efficiency,high energy consumption,low degree of automation.Currently research focused on pre-cooling energy efficiency is not systematic and the optimization of parameters lacks academic bases.The mechanism and theory of pre-cooling for spherical fruits and vegetables(F&V)are deeply studied in this paper.The pre-cooling time and comprehensive energy consumption models are constructed and verified.A systematic evaluation method of pre-cooling energy efficiency and optimization method of pre-cooling parameters are proposed to analyze the coupling effect of pre-cooling environmental factors and quality control,so as to realize the multi-objective optimization and evaluation of pre-cooling parameters based on efficiency,energy consumption and quality.The main work of this paper is as follows:(1)According to the pre-cooling scale coefficient,spherical F&V can be divided into ordinary scale and special scale.To solve the problem of poor accuracy in the process of numerical modeling of special scale F&V,a micells method is proposed in this paper.Compared with the porous media method and the equivalent sphere method,setting the number of individuals,the contact distance,and the distance between the micells reasonably,the actual pre-cooling process can be well matched by using the special scale F&V.Based on the macro quantities including pre-cooling time,uniformity,pressure loss and the microscopic quantities including heat transfer coefficient and surface friction coefficient,the effect of the micell structure parameters on the cooling performance is analyzed to find the optimal micell structure.Through simulation and experimental verification analysis under different conditions,The temperature relative deviation is less than1℃,and the pressure loss accuracy is higher than 85%.which lays a foundation for the establishment of sample database for pre-cooling.(2)In view of the problems of single factor and poor prediction accuracy of the existing semi-empirical semi-theoretical formula or theoretical formula of pre-cooling time.The high precision semi-theoretical formula of pre-cooling time is determined based on the theoretical analytical equation of spherical F&V cooling,the model accuracy reaches 95%and 93%,respectively.It can be seen from the formula that there is a certain functional relationship between the cooling coefficient C,the diameter of F&V,physical parameters and air supply parameters,and the single dimension and stacking dimension obtained by factor analysis and intermediate coefficient method.There have linear and exponential function relationship between the lag factor J and Biot number for single and stacked F&V,respectively.The accuracy of the total prediction model has a great relationship with F&V diameter.The prediction accuracy of special scale F&V is poor,and the time accuracy of general scale F&V is more than 90%.(3)For the prediction model of comprehensive energy consumption,the correlation of the refrigeration system energy consumption is obtained by analyzing the thermodynamic characteristics of the pre-cooling system.Then,the correlation of fan energy consumption is found based on analyzing the cold airflow resistance characteristics during the pre-cooling process.The comprehensive energy consumption of pre-cooling is proposed by combining the analysis of the refrigeration system energy consumption and the fan energy consumption.Partial derivative solution and squeeze theorem were used to analyze the influence of air temperature,air velocity and packing opening rate on pre-cooling time and energy consumption.By solving the optimal value of each working condition,the parameter optimization design scheme of high energy efficiency ratio of pre-cooling was determined.the dual-objective parameter optimization based on pre-cooling time and pre-cooling energy efficiency is realized.A package with trapezoidal concave opening and multi-rectangular openings was proposed to solve the problems of large pressure loss and high mechanical damage rate in the pre-cooling packaging of special scale F&V.Based on the existing General formula of cooling curve,the variable of supply air velocity is added by experimental analysis method.The results are verified by experiments and existing public literature,and the maximum error is less than 20%.In this experiment,a pre-cooling energy consumption model under special packaging was established.(4)Taking perishable blueberries as the research object,the concept of pre-cooling quality contribution rate is proposed to carry out the quality evaluation of blueberries in the cold chain cycle.Due to the improvement of pre-cooling time,the pre-cooling contribution rates of soluble solids,anthocyanin,texture,and commercialization rate of blueberries are 15.15%,19.48%and 4.34%,respectively.It can be found that the texture,color and nutrients of blueberries can be maintenance maximum when the pre-cooling time is shorter than 1h.Short pre-cooling time can start the synthesis of blueberry anthocyanin earlier,which helps to maintain the content of anthocyanin during the cold chain of blueberry.The strong pre-cooling stimuli such as high wind speed and low air temperature can lead to the increase of brittleness and decrease of elasticity of blueberries.Based on the analysis of the pre-cooling for blueberries,it can be found the operation strategies that sorting and packaging should be carried out before pre-cooling and that the flexible packages be used for direct transportation to avoid mechanical damage caused by brittleness.Furthermore,the optimal transportation moment should be determined according to the blueberry elasticity.In conclusion,the comprehensive energy consumption model and the multi-objective optimization of forced air pre-cooling for spherical fruits and vegetables have been analyzed to improve the pre-cooling efficiency and reduce the energy consumption and the quality loss,so as to improve the low carbon and digitization capacity of pre-cooling operation. |