Study On Heat Transfer Mechanism Of Vacuum Helical Ribbon Dryer With Compound Heating

In such situation with global energy shortage,high efficiency and energy saving have become an important development direction in the drying industry.A new drying equipment with compound heating vacuum helical ribbon dryer is developed.During the drying process,the jacket and the screw can heat materials at the same time.The helical ribbon stirring device continuously stirs the material,which makes the heat transfer and material movement in dryer to be very complicated.The research on heat transfer mechanism of vacuum helical ribbon dryer at home and abroad is relatively small,while the relevant literature reports are mostly limited to the experience of equipment use and structural modification.At present,the understanding of the heat transfer process between the material and the heating wall in the vacuum helical ribbon dryer is basically still in the experiment or engineering application experience.The design of vacuum helical ribbon dryer is mainly based on experience and uncertainty of operation parameter selection,which leads to unsatisfactory drying effect and even difficulty in satisfying high drying requirements.Therefore,the deep research on the heat transfer mechanism in the vacuum helical ribbon dryer is greatly significant for the perfection of the theoretical system,the optimization of equipment design and the improvement of the equipment application level on the vacuum helical ribbon dryer.In this paper,the heat transfer mechanism of the composite heating vacuum helical ribbon dryer is studied by means of theoretical analysis,experimental research and CFD numerical calculation.The research contents and results are as follows:(1)Based on the heat and mass transfer of fixed bed and the heat transfer model of the moving heating plate powder particle,the mechanism of heat conduction between the heating wall and material layer in the dryer is analyzed,which lays a foundation for the study of heat and mass transfer of subsequent dryers.According to the complex structure and motion characteristics of vacuum helical ribbon dryer,a calculation method of the characteristic parameter Nmix is improved in the heat transfer model of the moving heating plate powder particle.Meanwhile the prediction model of temperature and drying rate is developed for the composite heating vacuum helical ribbon dryer.The computational program of the model is programmed to realize the interface of the computational model by MATLAB.The calculation program was used to predict the drying rate and temperature of ofloxacin materials.The results show that the predicted values are in agreement with the experimental values.(2)The drying experiment of wet PVC powder was carried out by the composite heating vacuum helical ribbon dryer,which provided reliable experimental data for the determination of the boundary conditions of CFD model and the verification of numerical results.The experimental results show that the drying time can be shortened by increasing the temperature difference between the heat transfer oil and the material during the drying process.High vacuum can improve the energy efficiency of vacuum helical ribbon dryer.At the same time,the effect of vacuum degree on drying rate is mainly in the first half of dry,when the moisture content of the material is lowered,the influence of vacuum degree is weakened after the temperature rises.The prediction model is basically consistent with the experimental results.The energy efficiency of each time period is obtained by the thermal calculation of the whole drying process.Although the energy balance of the time period is not conserved,the total energy is basically conserved and the energy efficiency is 69.54%.(3)The gas-solid two-phase flow in a composite heating vacuum helical ribbon dryer is numerically simulated,the flow state and temperature distribution of the materials in dryer are fully understood.Based on the change of heating temperature with time in the heating process of the vacuum helical ribbon dryer with compound heating,a piecewise curve fitting method is adopted to fit the temperature curve.Reasonably write dynamic temperature boundary condition UDF code.Combined with the conventional gas-solid two-phase flow numerical simulation method,the CFD model of vacuum helical ribbon dryer is established on the basis of Eular-Eular model.By CFD calculation,the law of velocity field and distribution of material concentration and temperature field change with time in dryer barrel were obtain.(4)Based on the CFD model(3)was established.the velocity field,material concentration distribution and temperature field of dryer barrel were revealed under different rotational speeds.The calculation results show that With the increase of rotational speed,the mixing efficiency of the internal materials of the screw is increased,the material speed increases,the material and dryer's heating wall contact time reduces,the material and the heating surface convective heat transfer is strengthened,thus causes the material average temperature rise rate also to increase;The vortex depth of the material stacking surface is affected by the rotational speed of the screw belt,the faster the spiral belt rotates,The degree of vortex depth,but the effect of vortex depth is gradually reduced.In this paper,through in-depth research on materials heat transfer within the compound heating vacuum helical ribbon dryer,enriches the theory of heat transfer system of composite ribbon heating vacuum dryer,provides a scientific basis for the design and optimization of dryer operation parameters selection.At the same time,the CFD model suitable for vacuum helical ribbon dryer is developed,which provides a convenient and effective method for further study of vacuum helical ribbon dryer.