Research On Cold Shock Treatment System And Heat Transfer Process Of Fruits And Vegetables

China is rich in fruits and vegetables production,but the freshness preserving technique of fruits and vegetable is still lagging behind,and is lack of related equipment,which restrict the development of fruit and vegetable industry in China.In addition,with the issues related to human health and food safety being concerned,chemical preservative method is fully questionable;Exploring the green and environmental physical pretreatment method for fruits and vegetables and developing related equipment has become one of the important subjects in the field of preservation.The professed cold shock treatment equipment established based on the reverse Carnot cycle and heat transfer process of fruits and vegetables during cold shock treatment were researched,and the main contents are as follow:(1)In order to improve the superheat control precision of the cold shock treatment system,an intelligent superheat control strategy on the base of the traditional PID control strategy was proposed,with various influencing factors considered.The actual superheat control effect of the intelligent control strategy was checked in the cold shock treatment system of fruits and vegetables,and was compared with the control effect of thermostatic expansion valve.The experimental results show that: the intelligent control strategy gives a better performance such as faster dynamic response,lower overshoot,and shorter oscillation time compared with traditional PID control strategy.It is advantageous to superheat stability,if the compressor frequency changes as the interference factors are inputted in advance.The intelligent control strategy is better than the thermal expansion valve on the superheat control effect and heat transfer effect of evaporator,and the stability and energy efficiency of the system are enhanced.(2)The subcooler is extremely easy to freeze in the cold shock treatment system of fruits and vegetables in generating ice slurry,which could lead to ice blockage.To overcome the defects like high energy consumption and low efficiency of traditional de-icing technology,a novel de-icing method was investigated.The novel de-icing method was based on refrigerating medium bypass circulation and condensing heat of heat pump was used as heat source.Experiments on de-icing efficiency,operating performance and energy consumption of the novel de-icing method were conducted,and three practical de-icing methods with different heaters and air circulation modes were comparatively studied.The experimental results show that the novel de-icing method has an efficiency of 47.7%,which is the optimum de-icing efficiency among all three methods.Compared with the electric heat de-icing method by heating refrigerating medium of pipeline and the heat pump de-icing method by heating all refrigerating medium,the de-icing time of this novel method is shortened by 72.3% and 38.1% respectively,temperature recovery time is shortened by 19.2% and 61.8% respectively,water temperature fluctuation is decreased by 0.47? and 1.56? respectively,and de-icing energy consumption is reduced by 37.4% and 55.7% respectively.(3)A heat transfer model was established based on geometrical and physiological characteristic of the two-dimensional axial symmetric fruits and vegetables,and the numerical solution of the model was calculated through finite element-computer simulation program-COMSOL.The simulated results were validated with cucumbers and carrots as the experimental materials,and the effects of fruit size,water flow rate and cold shock temperature on the heat transfer process were analyzed.Through comparative analysis,it is shown that the simulated results agreed well with measured results,which indicates that the heat transfer model is of high reliability and accuracy.With regard to different types and sizes of fruits and vegetables,their internal heat transfer characteristics have obvious difference.When the flow rate increases from 0.5m/s to 2m/s,the heat transfer characteristic in the central tissues of cucumbers and carrots varies little,thus the flow rate of water pump can be reduced appropriately.Under the precondition that preservation quality of fruits and vegetables is guaranteed,the cold shock treatment technology with higher medium temperature and shorter treatment time was preferred.