Simulation Analysis And Optimization Research Of Refrigeration And Air-conditioning System For Fruit And Vegetable Controlled Atmosphere Storage

With the rapid development of our country's economy and the improvement of people's living standards,the fresh food industry has huge development potential.As one of the most advanced storage and preservation technologies in the world,controlled atmosphere(CA)storage has become an important means of fruit and vegetable storage.The key link to improve the safety and quality of fruits and vegetables in the storage process is to continuously transform and optimize the performance of the CA storage.Refrigeration and CA systems,as the important components of CA storage,are regarded as the reliable basis for fruit and vegetable storage and preservation technology.The optimizations of both refrigeration and air conditioning technology are very important for the preservation effect.A lot of researches and analyses are needed to obtain the most suitable technological parameters for fruit and vegetable storage.The favorable technological conditions can not only improve storage quality,but also reduce both energy consumption and economic costs.In this paper,the pre-cooling and equipment operation of the controlled atmosphere system are the main starting point,and then the change process and distribution law of the temperature,humidity and component concentration under different operating parameters of the equipment can be studied.In addition,the refrigeration system is optimized to achieve the best pre-cooling effect.Taking a certain CA storage as the research object.With regard to the pre-cooling and CA processes of CA storage,the computational fluid dynamics(CFD)methods are employed to establish the three-dimensional unsteady state models for air flow and heat and mass transfer.The internal flow field was studied and the optimization of the refrigeration system was analyzed based on the theory.The main work and the corresponding conclusions are as follows:(1)When the air supply parameters(velocity,temperature and humidity)of the air cooler are different,the distribution laws of both temperature and humidity as well as the cooling and humidification time are studied.Air supply velocity and temperature are the main influencing factors of temperature changes in the storage.The greater the air supply velocity,the faster the temperature in the storage will drop,and the temperature uniformity is the best at 7m/s.The increase in the enthalpy value of the humid air in the storage is small,so changes in relative humidity have little effect on temperature;changes in air supply velocity,temperature,and humidity will cause fluctuations in the humid environment in the storage,but in general,The humidity of the supply air is most sensitive,and the humidity in the storage increases sharply with the increase of the supply air humidity.the research results provide references for the selection of technological parameters and design optimization of the pre-cooling process of the CA storage.(2)The factors affecting the oxygen reduction process were analyzed,and the effects of the nitrogen purity,nitrogen flow rate and storage capacity of the nitrogen generator on both the component concentration and the oxygen reduction time were studied.The oxygen content of the nitrogen filling parameters of the nitrogen generator cannot be greater than the storage standard requirements.As the nitrogen flow and purity increase,when the oxygen reduction ends,the smaller the oxygen concentration difference between the gas zone and the fruit and vegetable zone,the more uniform the concentration distribution;When the nitrogen parameter is unchanged,the oxygen reduction time increases with the decrease of the storage capacity.When the storage capacity is 80%,the nitrogen purity is 99%,and the flow rate is 30m~3/h,the oxygen reduction time in the storage is the fastest case.The research results have important reference value for the selection and operating parameters of the nitrogen generator.(3)Optimize and analyze the refrigeration system.Based on the cooling capacity of the system,the thickness of the economical insulation layer of the envelope structure is analyzed,and the best insulation material in the operating life is 100 mm polyurethane foam board;the COPs of four different refrigerants are compared and analyzed,the most suitable type of refrigerants is R717 ammonia refrigerant while influence of the evaporation temperature on the COP is taken into consideration,the evaporation temperature is determined to be-8°C,and therefore the value of COP is 5.71.Finally,the total energy consumption of refrigeration is studied based on the analytical results of the air supply parameters of the air cooler.As the wind velocity and temperature increase,the total energy consumption of refrigeration increases.The different operating parameters of the air cooler are investigated,an overall consideration of temperature and humidity distributions as well as the system's energy consumption is adopted,the final selection of the air cooler's operating parameters include: the air supply velocity is 6m/s,the air temperature is 273 K and the air humidity of is 90%.The research content of this paper can provide reference for improving the process design,equipment operation management and energy-saving optimization of CA storage.