Airflow And Heat Transfer Characteristics In A Cold Storage For Fruits And Vegetables Based On Vertical Wall Attached Jet

Under new normal of China’s economy,the development of cold chain industry is receiving more and more attention along with the proposal of supply-side structural reform policy.As an important part of cold chain logistics system,a cold storage plays a crucial role in providing effective and reliable low temperature storage environment for perishable agricultural products.During the storage of fruits and vegetables,airflow pattern in a cold storage is the key factor affecting the storage environment of products,which directly determines the storage quality and commercial value of fruits and vegetables.The reasonable design of airflow distribution can effectively improve the chilled air circulation,increasing the uniformity of air distribution,maintaining the nutritional value of garden stuff and prolonging the postharvest storage period while reducing the energy consumption of the refrigerated warehouse.The vertical wall attached jet is new ventilation mode in air-conditioning system developed on the basis of traditional ventilation modes,combining the advantages of mixing ventilation and displacement ventilation.By Coanda Effect of vertical wall,the chilled air can be sent down to the floor area,forming the “air lake” phenomena over the floor similar to displacement ventilation,which can effectively overcome the disadvantage of displacement ventilation on low cooling capacity.This dissertation developed a three-dimensional computational fluid dynamics(CFD)model of airflow and heat transfer to calculate velocity and temperature distribution inside an existing empty and fully loaded apple refrigerated warehouse based on vertical wall attached jet in Xi’an,with the emphasis on the effects of the air supply parameters and the different stacking modes of product,and made a comparison with the traditional direct blowing air mode of cooling unit.The main research contents are as follows:Based on the architectural construction of existing refrigerated storage and related reference materials,a three-dimensional CFD model about airflow and heat transfer in an existing empty refrigerated warehouse was developed under vertical wall attached jet and used to predict the airflow pattern of storage room while both mesh independence study and verification of the numerical model were carried out,which provided a reference for the stacking of product.Airflow and heat transfer of the apple cold storage was investigated under steady state condition by the developed CFD model,which was a three-dimensional SST k-ω model in the fully loaded refrigerated warehouse based on porous medium approach.Taking into account airflow aerodynamic interactions around and inside porous boxes,a parametric analysis was performed with various values of air supply velocity and air supply temperature in order to evaluate the distribution and uniformity of airflow in the clear region and within porous boxes under different operating conditions.On the basis of analyzing the effects of the air supply parameters on the airflow pattern in the cold storage,the influence of the stacking mode of product on the distribution of the airflow was further studied with the vertical wall attached jet ventilation.The influence of stacking spacing on airflow pattern and cooling effectiveness of storing product is mainly considered with a view to optimize the airflow distribution and improve the heat transfer characteristics within the stack.The result shows that the maximum temperature,the average temperature and the standard deviation of temperature in the product region gradually decrease with the increase of stacking spacing.The average temperature in the product region decreases from 273.21 K to 273.03 K and the maximum temperature is reduced from 274.52 K to 273.74 K as the stacking spacing increases from 0.1m to 0.3m with the standard deviation reduced by 43.6%.Considering the volume utilization rate of refrigerated warehouse,stacking spacing is recommended to be controlled between 0.2m~0.3m.Under the condition of the same air supply volume flow and temperature,a comparative study was made on air distribution characteristics when the vertical wall attached jet ventilation and the traditional direct blowing air ventilation systems were applied in the refrigerated warehouse,the storage quantity of which is constant.It is found that both a better cooling utilization rate and uniformity of velocity and temperature within porous boxes could be achieved by the vertical wall attached jet mode with the energy utilization coefficient increased by 19.4%,the velocity non-uniformity coefficient of the product area decreased by 47%,and the temperature non-uniformity coefficient decreased by 31%.