Numerical Simulation Of Heat And Mass Transfer In Hawthorn Slice Heat Pump Drying And Optimization Of Drying Chamber Structure

As a major agricultural country,China has a huge output of agricultural products,so the issue of long-term storage and quality assurance of domestic agricultural products has always been a focus of attention.At present,drying agricultural products is an important method to ensure long-term storage.The use of air source heat pumps for hot air drying of products greatly reduces labor and is more energy-efficient and environmentally friendly compared to other drying machinery.In the design process of air source heat pump systems and the study of drying processes and quality,it is particularly important to master the heat and mass transfer characteristics of agricultural products.This will make the optimization of drying equipment or processes more precise and effective.At present,research on the hot air drying process of hawthorn slices at home and abroad mostly focuses on the flow field analysis at the velocity level,and there is little research on heat and mass transfer characteristics.Among them,there is no research on the convective heat transfer effect during the drying process based on the principle of local non thermal equilibrium.This article took the heat pump drying process of hawthorn slices as the research object,established a mathematical model for heat and mass transfer in hawthorn slice hot air drying,used C language to write UDF program,and combined Ansys Fluent to simulate and verified the drying process of hawthorn slices in the air source heat pump drying system.Based on this,the structure of the existing heat pump drying room has been optimized.The main research work is as follows:（1）Utilized the intelligent air source heat pump fruit and vegetable dryer developed by the key laboratory conducted hot air drying experiments and analyzed the drying characteristics of hawthorn slices under different working conditions.The results showed that the hot air drying process of hawthorn slices was a typical slow drying process.In addition,eight common drying models were used to perform nonlinear fitting on the experimental results,and the results showed that the Asymptotic asymptotic model had the best fitting effect.Multiple nonlinear fitting of temperature and wind speed was performed on the parameters of the Asymptotic model,resulting in a complete model formula for the Fluent numerical model.Then,by comparing the moisture content change data obtained from two additional sets of drying experiments with their corresponding model predictions,the average relative errors of the two sets of data were 10.1%and5.23%,verifying the applicability of the model.The mechanical theory of water migration during the heat pump drying process of hawthorn slices was analyzed,and the experimental data was non-linear fitted using Fick’s second law to obtain an effective water diffusion coefficient D_eff range of 3.3879×10^-9 m²/s to 9.0176×10^-9 m²/s at drying temperatures ranging from 40℃to 70℃and wind speeds ranging from 0.5m/s to 1.9m/s in the drying section.The drying activation energy was fitted using the Arrhenius equation,and the range of activation energy was obtained from 13.10 k J/mol to 24.89 k J/mol.（2）The flow control equations and momentum equations in the fluid domain and porous medium domain during the drying process were derived and explained.The source term of the water vapor transport equation and the formula for calculating the moisture content of the hawthorn slice stacking layer were derived using the above drying model and combined with the deep bed drying theory.Based on the principle of local non thermal balance,the key parameter of energy equation coupling-convective heat transfer coefficient was introduced,and the range of factors affecting the convective heat transfer coefficient was determined through 11 sets of single factor experiments.Then 17 sets of Box Behnken experiments were designed based on the results of the single factor experiments,and variance analysis and response surface analysis were carried out on the experimental results.Finally,a mathematical regression model for the convective heat transfer coefficient was obtained,with the model fitting degree of 0.954.By comparing the measured values of the experiments with the predicted values of the model,the mean relative error（MRE）of the two was 7.33%,proving that the model is feasible,thus completing the coupling of the fluid solid energy equation.（3）Based on the determined heat and mass transfer model,a UDF program was developed and Fluent numerical simulation was conducted.The results were verified through drying experiments of hawthorn slices with different thickness layers,and it was proved that the established numerical model was suitable for the hot air drying process of hawthorn slices.The simulation results were analyzed from multiple perspectives,including velocity field,temperature field,moisture content,etc.It can be seen that this numerical model effectively achieved the coupling effect of multiple physical fields,simulated the distribution law of heat and humidity in the drying room,and revealed the problems of uneven drying inside the accumulation layer and low hot air flow rate in the existing drying process.（4）Based on the defects of the drying chamber revealed by the above simulation results,targeted optimization of the internal structure of the drying chamber was proposed using guide vane pipes and variable aperture uniform air plates.The best structural parameters were selected,and the optimization effect of the fully loaded drying chamber was analyzed through simulation.The optimization effects were as follows:the flow rate of hot air inside the stacking layer has been increased,and the average uniformity coefficient has been increased from 0.8477 to 0.9168,improving the problem of uneven drying inside the stacking layer of hawthorn slices.Moreover,the average temperature rise rate of hawthorn slices has been accelerated,and the heat and mass transfer effect has been enhanced.The time for the moisture content of hawthorn slices in the drying room to drop below safe storage conditions has been shortened by 17.2%at inlet temperatures of 50℃,60℃,and 70℃,respectively 15.4%、13%.Subsequently,to further demonstrate the effectiveness of the optimization,hot air drying experiments were conducted on hawthorn slices with different thickness layers in the drying chamber before and after the optimization.By comparing the change curves of moisture content and average temperature of hawthorn slices,it was proved that the optimized drying chamber significantly improved the drying efficiency.In summary,this article provides basic data and theoretical methods for in-depth research on the heat and mass transfer characteristics of hawthorn slices or similar agricultural products,and provides theoretical and technical references for optimizing the drying process and drying chamber structure of hawthorn slices.