Design And Numerical Simulation Of Low Temperature Vacuum Drying Device For Fruits And Vegetables

As a large agricultural country,China’s crop output ranks in the forefront of the world.However,in the process of storage and transportation,due to the lack of advanced technology and perfect equipment,the freshness of fruits and vegetables can not be guaranteed,which leads to decay and deterioration,causing huge economic losses to all parties.Therefore,drying and processing of fresh fruits and vegetables can effectively prolong the storage life and facilitate transportation.However,at present,the quality of dry products produced by drying equipment is uneven,which is easy to cause the loss of nutrients in the drying process,and the product quality is difficult to guarantee.Therefore,it is particularly important to develop a drying device with less energy consumption and good product quality.In this paper,a low-temperature vacuum drying device suitable for fruit and vegetable drying is independently designed and manufactured.The three-dimensional drawing of the main structure of the vacuum device is carried out by using Solid Works software,and the stress of the vacuum chamber under the limit state is analyzed.It is found that the maximum stress of the vacuum chamber is 6.59 MPa and the maximum displacement is 0.147 mm,which is lower than the yield strength of the chamber material.Therefore,the structure of the low-temperature vacuum chamber meets the design strength requirements.By simplifying the three-dimensional structure diagram,the internal structure model of the drying cabin is established,the model is meshed by ICEM-CFD software,and the distribution of fluid pressure field and velocity field in the cabin during vacuum pumping is simulated by FLUENT software based on finite volume method.The results show that in the initial stage of the vacuum pumping process,the pressure in the cabin decreases rapidly,the gas flow velocity is large,and the area with large velocity is concentrated near the exhaust port.Due to the existence of heating shelf and material layer,the gas flow rate in the cabin presents a large flow rate around and a small flow rate in the middle in the horizontal direction;In the vertical direction,the upper velocity is large and the lower velocity is small.With the continuous process of vacuum pumping,the gas velocity in the vacuum chamber also decreases gradually,and pressure drop in the chamber decreases exponentially.On this basis,the fruits and vegetables were simplified into a porous medium model to study the heat and mass transfer process in the drying process.The effects of different heating temperature and working pressure on the temperature distribution and humidity distribution of the material layer and the fluid in the tank are compared.The results show that in the drying process,the temperature of the material layer first rises from the surface,and the temperature rise rate of the uppermost material is faster than that of the other two layers.In the initial stage of heating,the temperature rise rate of the fluid in the cabin is faster,and then the temperature rise rate decreases due to the diffusion of moisture in the material.With the continuous heating process,the temperature of fluid and materials in the cabin gradually rises and finally reaches the heating temperature.The fluid temperature distribution in the vacuum chamber is relatively uniform,and the temperature changes significantly only near the material layer and heating shelf.The mass transfer process of the material layer is similar to the heat transfer process.The moisture content also diffuses from the surface of the material to the outside.In the early stage of drying,the moisture content on the surface of the material diffuses rapidly.At 60 min,the moisture content in the uppermost material also began to diffuse outward,while the moisture content of the middle and lower materials changed little.Under the same working pressure,the higher the heating temperature,the greater the temperature rise of the material layer,the faster the heating rate of the material layer,and the greater the fluctuation of the heating rate.Under the three heating temperatures,the temperature change of the uppermost material is the most obvious,and the change of the middle layer is the lowest.The lower the heating temperature,the easier it is for the material layer temperature to reach the heating temperature.The trend of material humidity distribution is the same,which is that the humidity content of the upper material is the lowest,the middle level is the highest,and the humidity content of the lower layer is the highest.At the beginning of drying,the change range of water content is relatively large;When the moisture content decreases to the range of 0.1 ～ 0.5,the change rate of moisture content slows down gradually;When the moisture content is lower than 0.1,the removal of moisture in the material becomes more and more difficult.When the heating temperature rises from 10 ℃to 20 ℃,the drying time of the material is shortened by 15%;When the heating temperature rises from 10 ℃ to 30 ℃,the drying time of the material is shortened by 35%.Therefore,at this stage,the temperature of the heating plate can be appropriately increased,which can effectively improve the drying rate and speed up the drying process.Under the same heating temperature,the lower the working pressure is,the faster the temperature rise rate of material layer and fluid in the tank is.Under different working pressures,the temperature rise rate increases first,then decreases,and finally tends to be stable.The lower the working pressure,the longer the duration of the accelerated heating stage,and the earlier the material temperature reaches the heating temperature.Compared with the conditions of different temperatures,the working pressure has little effect on the uniformity of fluid temperature field in the cabin.It can be concluded that the heating temperature is the main factor affecting the uniformity of fluid temperature field in the cabin.Under different pressures,the trend of material humidity distribution is also the same.With the decrease of working pressure,the time from drying to the required moisture content is shortened.Through comparison,it can be found that when the working pressure decreases from 1000 Pa to 800 Pa,the drying time of the material is shortened by 20%;When the working pressure decreases from 1000 Pa to 620 Pa,the drying time of the material is shortened by 30%.It can be found that increasing the heating temperature has a greater impact on the drying time than reducing the working pressure,which shows that the drying temperature has a greater impact on the drying time than the working pressure.