Research On The Regulation And Control Mechanism Of The Coupling In Heat And Mass Transfer And Optimization Of Fresh-keeping Environment With Controlled Atmosphere For Litchi Fruit

China is the most producing area of litchi in the world,and cotributes more than65%of the yield of litchi all over the world.Litchi is harvested in hot summers,and loses water in the pericarp easily without any treatment,which accelerate browning of pericarp that the commodity value is seriously affected.However,litchi fruit is extremely difficult to maintain the fresh quality by storage,which is limited by the relative poor conditions of fresh-keeping technology and equipment for fruits and vegetables.According to the statistics,the loss rate of fruits and vegetables during circulation is 20³0%,which equals hundreds of billions RMB in China,while the loss rate is controlled to be under 5%in Europe and the United States and other developed countries.As the improvement of living standard of citizens,the requirement on the fresh and safe food is being improved day by day.Thus,there are growing concerns on developing the physical fresh-keeping technology for fruits and vegetables,which is friendly to our environment,low cost and without any drug residual.Improving the efficiency and stability of the regulation of fresh-keeping environment,maintaining the fresh quality of litchi fruit and reducing the cost can be achieved by studying the theories of environment regulation and the optimization of fresh-keeping equipments.Objects of this paper were mainly to investigate the physical fresh-keeping technologies and equipments for litchi fruit by theoretical modeling and experiments.The main work is shown as follows:?1?The effects of different physical fresh-keeping environment on maintaining the fresh quality of litchi fruit were investigated by using different storage environment,controlled atmosphere,controlled both temperature and humidity,and controlled temperature,on the fresh-keeping test bed with controlled atmosphere for fruits and vegetables.By which,the quality change of litchi fruit under different fresh-keeping environment was evaluated and the change law of pericarp brown index,color difference,fruit weight loss rate and other indexes were acquired;the relationship between the litchi fruit quality and different physical fresh-keeping environment was analyzed by principal component analysis;the relationship between color difference a*of litchi pericarp and other fresh qualities was built that pericarp color difference a*had a linear relation with pericarp water loss rate,brown index,anthocyanins content,fruit weight loss rate and sensory evaluation value,the average correlation coefficient was 0.8609.The test about the effects of different storage position on the fresh-keeping quality of litchi fruit was conducted,by which the distribution of temperature and humidity and the change of quality according to that during 35 d storage were obtained.?2?According to the regulation of fresh-keeping environment,the coupled model with heat and mass transfer litchi under physical fresh-keeping environment was built.The effects of enclosed structure,evaporator and other factors on cooling,humidification and liquid nitrogen injection were taken into account in the coupled models in which temperature,humidity and gas content in the physical fresh-keeping environment were determind,and a test was performed to verify the accuracy of such models;a mathematical model about the heat and mass transfer between litchi fruit and fresh-keeping environment during the pre-cooling process was established,which can predict the temperature and weight loss rate of litchi fruit during pre-cooling process.A small-scale test bed with cooling by semiconductor was built,by which the overall mass transfer coefficient was acquired and the accuracy of the model was improved.The calculated value and test value of litchi fruit during pre-cooling matched well with each other that the maxiume difference between them in fruit temperature was 2.8?,the mean difference rate and root-mean-square error was 13.1%and 16.4%,respectively,and the mean difference rate and root-mean-square error was 10.7%and 12.2%in weight loss rate,respectively;the mathematical model about the vapour condensation and evaporation on the surface of litchi fruit was built,which can be used to predict the vapour condensation and evaporation on litchi fruit that are just taken out from a low temperature environment.A test was conducted to verify the accuracy of the model.?3?The effects of different package methods on the fresh-keeping quality of litchi fruit were investigated on the fresh-keeping test bed with controlled atmosphere for fruits and vegetables.To study the effects of different package methods on litchi fruit,'Huaizhi'litchi fruit was chosen as the test materials and packaged with picking box,Poly Ethylene?PE?package with holes and microporous membrane package,respectively,by which the effects of different package methods on the temperature and humidity in the package and fruit fresh-keeping quality were obtained.A 2-D numerical model about the fresh-keeping test bed with controlled atmosphere was developped by using ICEM CFD software with meshing the grid.The airflow in box-package was analyzed to obtain a relative suitable air duct velocity of litchi fruit with box-package during pre-cooling.To reduce energy consumption,6 m/s is the high-priority ventilation air velocity.Improving the ventilation velocity can shorten the process of pre-cooling but will improve the variable coefficient of temperature among litchi fruits in the package;When the air duct velocity is bigger than 6m/s,the cooling rate trends to be steady along with the growing of air duct velocity,the variable coefficient was steady at roughly 0.25.?4?To investigate airflow field characteristics in the container and change law of litchi fruit temperature under different fresh-keeping environment,CFD method was used to build the regulation models about the physical fresh-keeping environment for litchi fruit.The performance of the fresh-keeping transportation truck with controlled atmosphere for fruits and vegetables with litchi products was studied by building a 2-D numerical model considening air turbulence.To solve the model,the CFD software FLUENT and the SIMPLE algorithm were used,and a porous model was adopted to represent the influence of the products.By such methods,the effects of exhaust valve location,vehicle speed,valve diameter and other factors on oxygen exchange process were determined.A total 12cases in different parameters were made and solved,after which the oxygen exchange rules and characteristics of the fresh-keeping transportation container with controlled atmosphere for fruits and vegetables were obtained.A 3-D model and a test were conducted to verify the accuracy of the models that the prediction error was 3.6%in air temperature and 3.3%in O₂ volume fraction during oxygen exchange.The results of this work above provide a certain reference value for improving the efficiency,maintaining the fresh quality and guaranteeing the commodity value of litchi fruit,so that the technologies of fresh-keeping can be improved so as to promote the circulation of litchi fruit.