CFD Based Heat And Mass Transfer Of Green Sichuan Pepper During Heat Pump Drying And Optimization Of Drying Room

As a characteristic agricultural product in China,green Sichuan pepper has played an important role in promoting the prosperity of industries and the prosperity of farmers’lives.In recent years,as the country vigorously promotes the development of special agriculture,the production and demand of green Sichuan pepper has further increased.Drying is the main origin processing method of green Sichuan pepper,and about 80%of green Sichuan pepper needs drying treatment.At present,heat pump drying technology has been widely used in the drying of green Sichuan pepper,but most of the research is limited to the traditional experimental exploration section,which is not only time-consuming and labor-intensive,but also difficult to comprehensively analyze the temperature and humidity changes in the drying process of green Sichuan pepper,which is less significant for the guidance of actual production.With the improvement of computer performance and simulation software capabilities,numerical simulation techniques are increasingly being used in the drying field.At present,the investigation of heat and mass transfer of green Sichuan pepper heat pump drying and the optimization of its drying room structure using numerical simulation technology are almost absent.Based on this,the following work was done in this paper:the basic parameters of green Sichuan pepper and air were measured;the optimal process parameters and drying constants of green Sichuan pepper heat pump drying were investigated through experiments;the heat and mass transfer phenomena in the drying process of green Sichuan pepper heat pump were investigated by using computational fluid dynamics(CFD)technology,and the structure of the drying room was optimized and designed,and the main research contents and conclusions were as follows:The true density and bulk density of green Sichuan pepper were measured based on the fine particle filling method as 961.59 kg/m~3 and 641.79 kg/m~3,respectively,the porosity of green Sichuan pepper was measured based on the density method as 0.33,the equivalent diameter of green Sichuan pepper particles was measured based on the equal volume substitution method as 0.0055 m,and the initial moisture content of green Sichuan pepper was measured using a halogen moisture tester as 2.08kg/kg(d.b.),and the basic parameters of air were also explored for the numerical simulation study in the drying process of green Sichuan pepper heat pump.The heat pump drying test of green Sichuan pepper was carried out with drying temperature,laying thickness and wind speed as test factors,and color difference value and effective moisture diffusion coefficient as evaluation indexes.The results of single-factor test showed that the color difference value of green Sichuan pepper decreased firstly and then increased with the increase of drying temperature and wind speed,and gradually increased with the increase of laying thickness.The results of the orthogonal test showed that the order of influence of each factor was:drying temperature>laying thickness>wind speed for color difference value;the order of influence of each factor was:drying temperature>wind speed>laying thickness for effective moisture diffusion coefficient.Based on the results of the orthogonal test,by giving the same weight value to the two evaluation indexes of color difference value and effective moisture diffusion coefficient,the optimal drying process of green Sichuan pepper heat pump was obtained as drying temperature 45℃,laying thickness 15mm and wind speed 5m/s.Five kinetic models were used to fit the drying process of green Sichuan pepper heat pump,and based on the fitting degree of the drying constant,the Page model was determined as the optimal model,and the The values of regression coefficients A and B in the expression of drying constant were 1.87 and 2175.01,respectively.Based on the heat and mass transfer theory of porous medium,a heat and mass transfer model of green Sichuan pepper drying by heat pump was established,and a user-defined function(UDF)was written by C Language,which was combined with ANSYS FLUENT software to numerically simulate the temperature and moisture distribution law of green Sichuan pepper drying process and verify the accuracy of the model.The results showed that the temperature and moisture content of green Sichuan pepper changed at different rates in different drying time periods,and the temperature of green Sichuan pepper rose faster in the pre-drying period and slowed in the middle and later periods,and the moisture content decreased slowly in the late drying period and faster in the front and middle periods.As the number of material layers increased,the temperature uniformity of green Sichuan pepper gradually increased and the moisture content gradually increased,and compared with the middle position,the temperature at the two ends of the material layer was higher,the moisture content was lower,and the moisture ratio inhomogeneity coefficient was larger.For the inhomogeneity of the flow field in the drying room during the drying process of green Sichuan pepper heat pump,this paper optimized the drying room using CFD.The single-factor test combined with the BBD(Box-Behnken design)test was used to investigate the effects of the additional deflector and the size of the air inlet and outlet on the flow field in the drying room,and the quadratic regression equations of the temperature inhomogeneity coefficient and the percentage decrease in moisture content were obtained;then the obtained quadratic regression equations were input into the multi-objective optimization by MATLAB software.Finally,the optimal structural parameters of the drying room were determined by the Pareto front solution set calculated by the multi-objective genetic algorithm,and the temperature inhomogeneity coefficient and moisture content rate of the optimized drying room were significantly reduced compared with the original structure.