| The uneven distribution of flow field in drying chamber will result in different final water content of Tibetan medicine after drying and affect the quality.At the same time,the mismatch between the resistance of fan and drying room system will cause the wind speed not meeting the requirements of Tibetan medicine drying process and the efficiency of fan is low.The flow field simulation and analysis of drying room based on computational fluid dynamics（CFD）is an advanced method in drying room design.However,it is difficult to effectively and accurately simulate the flow field effect of drying room driven by multi-fan with limited computer resources.In this paper,the 3D fan zone model is proposed to simplify the circulating fan to reduce the number of cells and to achieve the calculation accuracy.The model parameters applicable to the 3D fan zone model are obtained by establishing the fan aerodynamic performance numerical calculation model.The validity of the method and the reliability of the model are verified by theoretical and experimental results.The structure of drying oven was improved considering the complex flow field effect of multiple circulating fans affected by system resistance.On this basis,a method for calculating system resistance based on 3D fan zone model and porous media model is proposed,and the resistance reduction method of drying room and the matching of circulating fan and system resistance are discussed.The main research results of this paper are as follows:（1）The drying test of Tibetan medicine Radix Aucklandiae shows that there is a strong correlation between wind speed and drying rate.The different wind speed above the material layer results in different drying rate.High resistance of drying room system and mismatch between circulating fan and system resistance are the main reasons that wind speed can not meet the requirements of drying process.（2）The results of 3D fan zone model simulation are consistent with MRF model.and the number of grids of the new model is only 1.12% of MRF model.In addition,the simulated wind speed and pressure values in the drying room are in agreement with the experimental results,and the flow field characteristics of the drying room driven by multi-fan are simulated.The absolute average relative errors of wind speed and pressure are 6% and 2.75% respectively.（3）Flow field simulation results show that the wind speed above the material tray near the fan outlet is rather high,and the distribution is uneven,and eddy current occurs at the clapboard opening of drying room.Too much air flows through the uppermost zone of the drying room,Unfavorable structure results in uneven distribution of air velocity field in drying room with uniformity of only 0.73.（4）The size parameters of the wind deflector are optimized by numerical calculation to obtain the best separation size of the air wind board.After optimization,the standard deviation of the outlet air flow rate of the diversion cover is 0.001,which can better achieve the purpose of air equalization.By reducing the distance between the top surface of the drying room and the uppermost material tray,changing the opening of the partition to a push-pull door and adding an air guide cover at each air inlet,the air velocity distribution between the material trays can be more uniform.After the optimization,the wind speed uniformity index is about 0.81,which is 8%higher than that of the original structure.（5）The resistance of condenser accounts for 59% of the system resistance.The resistance of the condenser accounts for 59% of the system resistance.Reducing the number of tube row and increasing the windward area of the condenser were proposed to reduce the resistance of the drying room.According to the fan similarity law,the matching between the circulating fan and the system resistance is discussed.When the condenser has two rows of pipes and the fan speed is 2900r/min,the average wind speed on the material tray is 2.01m/s,which meets the requirements of Tibetan medicine drying process,and the working efficiency of the fan is 85%,compared with the four row tube condenser,the efficiency increased by 15%.The research done in this paper can provide technical guidance for numerical simulation modeling of multi-fan heat pump drying room,design of drying tank structure,calculation and reduction of resistance of drying room system and selection of circulating fan. |
