Mechanism Of Moisture And Heat Transfer During Hot Air Drying Of Walnuts And Study On Process Optimization

Xinjiang is the main production area of Walnut in China.Except a small part of fresh walnut harvested is directly used for fresh food,other walnuts must be dried.However,the traditional drying equipment and process have the problems of poor uniformity,low efficiency and high cost of walnut drying,which can not meet the requirements of walnut processing and sales today.At present,the research on hot-air drying of walnut only stays on the numerical simulation under no-load condition of walnut drying equipment,but does not study the heat-moisture coupling process of walnut itself.It is difficult to play a good supporting role in upgrading walnut hot-air drying equipment and optimizing drying process.This subject took Walnut in Yecheng City,Xinjiang as the research object,through the establishment of porous media model(walnut layer),explored the thermalwet coupling process of walnut under hot air drying,studied the law of temperature,humidity,pressure and wind speed in walnut layer changing with drying time,and then verified the numerical simulation results.Firstly,in order to explore the optimal drying process under the current hot air drying equipment,the walnut hot air drying experiment was carried out,and the process parameters of constant temperature and variable temperature hot air drying were compared.Then,the water content,drying rate,sensory quality and economic cost of walnut were evaluated.The experimental results show that the optimum drying process under the current drying equipment was [early 35?(10 h),middle 55?(10 h),late 45?(10 h)].At the same time,physical property parameters and air parameters of walnut needed for numerical simulation were experimented,deduced and calculated,and parameters were initialised through UDF.Secondly,in order to explore the laws of heat-moisture coupling transfer in different walnut layers,the mathematical model of heat-moisture coupling transfer in walnut layers was established.UDF for calculating energy and humidity source terms related to Walnut drying was compiled in C language.The thermal-moisture coupling process of porous media models in walnut layers of 80cm,100cm and 120cm was simulated by FLUENT software.The research shows that under the same setting of other simulation parameters,the speed and temperature in the drying room are not uniform locally.With the increase of walnut layer thickness,the pressure at the air inlet gradually increases and the pressure in walnut layer gradually decreases from bottom to top.On the contrary,the change of water content in walnut layer gradually increases from bottom to top.The pressure and temperature of walnut during drying process increase gradually.The water content shows obvious gradient change.Finally,in order to verify the heat-moisture coupling transfer law of walnut during numerical simulation,the walnut hot air drying experiment was carried out to verify.At the same time,according to the problems found during simulation and experiment,such as uneven distribution of air flow in walnut drying room,excessive local pressure at spoiler and uneven drying of walnut upper and lower layers in silo,the structure of air inlet,spoiler,screen surface and silo was optimized and its rationality was verified by simulation.Comprehensive experiments and numerical simulation show that the optimum drying process [early 35?(10 h),middle 55?(10 h),late 45?(7 h)] and improved drying room can further improve the uniformity of hot air drying of walnut and shorten the drying time of walnut.