Study On Heat Transfer Performance Of Pre-freezing Process And Vacuum Freeze-drying Process Optimization Of Banana

China is a big agricultural country,variety of agricultural products,the total output of fruits ranks first in the world,the shelf life of bananas is short,perishable and not easy to transport,a lot of waste every year due to improper preservation.Drying is considered a good way to extend the storage time of bananas.Early use of natural energy(solar or wind energy,etc.)to naturally dry banana chips,lower energy consumption,but the production efficiency and product quality are poor,subject to seasonal and climatic conditions,however,artificial drying technology is not limited by climate,and the drying time is short.Currently,commonly used banana drying methods are: deep fry,hot air drying,microwave drying.But there is a poor product shape,quality problems of poor flavor and serious loss of nutrients.In the field of fruit and vegetable drying,vacuum freeze-drying(referred to as freeze-drying)is considered to be the best way to obtain high-quality dry products,but there are shortcomings of high energy consumption.Therefore,it is of great significance to carry out the research on the reduction of the energy consumption and quality of freeze-dried banana.In order to improve the efficiency of vacuum freeze-drying of bananas,shorten freeze-drying time,measure its physical properties,and on this basis,using FLUENT numerical simulation software to study the heat transfer characteristics of banana pre-freezing process,provide a reference for accurately predicting the pre-freezing time;simultaneously,analysis of vacuum freeze-drying mechanism,the effect of vacuum freeze-drying process parameters on drying time and product quality was studied,short drying time obtained through single factor test and orthogonal test,low energy consumption,the best combination of process parameters with excellent quality.The main research work and research results of this article are as follows:(1)Through empirical formula calculation and experimental research,the thermal properties of bananas are determined,provide data support for the numerical simulation parameter setting of the pre-freezing process and the value of the freeze-drying process parameters.In this paper,the differential scanning calorimetry(DSC)was used to test the change law of banana specific heat capacity with time,eutectic point temperature,melting point temperature,latent heating value of phase change,freezing temperature.Using the transient hot-wire method to measure the thermal conductivity of banana,the empirical formula was used to determine the surface heat transfer coefficient of banana during pre-freezing,the initial moisture content of bananas was determined by vacuum drying method,drainage method to determine its density at room temperature,and use the principle of superposition to calculate the density after freezing;(2)Based on the above research,the phase change heat transfer principle and simulation method of banana pre-freezing process were studied.Based on the mathematical model describing the banana pre-freezing process,using Visul C++ to write a temperature control program to define the wall temperature of the shelf,using FLUENT software to analyze the temperature field during the freezing,three-dimensional unsteady numerical simulation of dynamic advancement of the frozen phase change interface.Predict the completion time of pre-freezing,and conduct experimental verification.Comparing the simulation values with the experimental values to verify the accuracy of the model(3)Based on the analysis of heat and mass transfer theory in the freeze-drying process,obtain the factors that affect the drying time.On the basis of,through single factor control variable method,the slice thickness,drying room pressure,shelf temperature during sublimation,analyze the influence of shelf temperature on drying time and sensory quality in the drying stage,and determined the best value range of the four factors;through orthogonal experiments,the significance of four factors on drying time and quality is obtained.Finally,considering the energy consumption and quality of banana freeze-drying,the optimal level combination parameters of vacuum freeze-drying are: Slice thickness 5mm,the drying chamber pressure is 30 Pa,the shelf temperature in the sublimation drying stage is 10?,analyze the shelf temperature at 40? in the drying stage.With the best conditions obtained,doing 3 parallel experiments,the result shows: the average drying time is 9.85 h,sensory quality 8.7 points,the accuracy of the orthogonal experiment is verified.