Development And Application Of Numerical Simulation Technology For Hot-air Drying Of Wolfberry On Drying Oven Scale

In recent years,wolfberry is more and more popular as a medicinal material and food with rich nutritional value,but its moisture content is large and easy to spoilage.Hot-air drying technology is the main means to prevent the deterioration of wolfberry in practice.Compared with other drying methods,the energy consumption of hot-air drying is lower and the efficiency of hot-air drying is higher.The moisture in wolfberry can be discharged into the surrounding environment through hot air to achieve the purpose of maintaining quality and preservation of large quantities of wolfberry.With the improvement of computer performance and simulation software function,numerical simulation is more and more widely used in the study of hot-air drying for wolfberry on drying oven scale.At present,the actual moisture evaporation process of wolfberry has not been considered in the numerical simulation technology of hot-air drying of wolfberry on drying oven scale.Meanwhile,it is difficult to calculate the real drying process in the drying oven.To solve this problem,this paper intends to construct a drying oven scale hot-air drying numerical simulation technology that can truly reflect the moisture evaporation process of wolfberry.In order to realize the construction and application of the simulation technology,the research ideas of this paper are as follows:(1)The hot-air test of wolfberry at material level will be carried out.(2)The optimal drying kinetics model of hot-air drying for wolfberry will be selected.(3)The kinetics derivative model of hot-air drying for wolfberry will be constructed.(4)Numerical simulation technology for hot-air drying of wolfberry on drying oven scale will be developed.(5)The numerical simulation technology will be utilized to study the actual drying process in the dryer.Around the above ideas,the specific contents and elementary outcomes of this research are as follows :(1)An experimental study on hot-air drying of wolfberry was carried out on the material level.According to the theory of orthogonal experiment and the actual drying conditions of wolfberry,10 groups of orthogonal experiments were carried out considering three factors: hot-air temperature(30～70 °C),hot-air relative humidity(30%～60%),and hot-air velocity(1.2～3.6 m/s).The drying kinetics curves and drying rate curves under 10 groups of working conditions were established,and the hot-air drying characteristics of wolfberry were determined.(2)Based on the drying kinetics curves obtained from the hot-air drying tests of wolfberry at the material level,the Abbasi model was determined by using the self-developed drying kinetics model selection software.At the same time,the model coefficients were expressed by three commonly-used working conditions,namely,hot-air temperature,hot-air relative humidity and hot-air velocity.Then,based on Abbasi model,the kinetics derivative model of hot-air drying for wolfberry was constructed,which can be used to calculate the water evaporation rate of wolfberry.(3)Based on the moisture evaporation rate of wolfberry calculated by the derivative model,the numerical simulation technology of hot-air drying for wolfberry on drying oven scale that can truly reflect the moisture evaporation process of wolfberry was developed.The development process of this technology is as follows: Firstly,the physical model of the actual drying oven was established.Then,the drying mathematical model considering the heat transfer process,mass transfer process,flow process and shrinkage process in the drying oven was established,which coupled with the moisture evaporation rate calculated by the derivative model.Finally,the above mathematical model was solved based on the commercial software FLUENT.The evolution process of temperature,humidity and moisture ratio distribution with time in the dryer was obtained.(4)The actual drying process of wolfberry was simulated by hot-air drying numerical simulation technology.Firstly,the moisture ratio obtained by numerical simulation technology and experimental calculation was compared,and the relative error was 1.8%,which verified the accuracy of the numerical simulation technology.Then,the conditions of hot-air temperature 50 °C,hot-air relative humidity 50%and hot-air velocity 2.4 m/s were simulated.The results showed that the time difference between the fastest drying column and the slowest drying column was 3.4 h.Keeping the relative humidity and velocity of hot air unchanged,only the hot-air temperature was increased to 60 °C.The results showed that the drying time difference was 2.4 h,and the moisture ratio nonuniformity was reduced by 29.4%.Increasing the hot-air temperature would reduce the moisture ratio nonuniformity.Finally,the relative humidity and temperature of hot air were kept constant,and the hot-air velocity was reduced to 1.2 m/s.The results showed that the difference in drying time needed was 5.3 h,and the moisture ratio nonuniformity increased by 55.9%.Reducing the hot air velocity would increase the water ratio nonuniformity.The above research work provides a strong support for the study of hot-air drying of wolfberry on drying oven scale with certain reference significance for practical engineering.