Research On Heat Transfer Characteristics And Numerical Simulation Of Multi-Channel Cylinder Dryer Of Paper Machine

The energy consumption of drying section accounts for more than 50%of that in papermaking process.The improvement of the efficiency of the cylinder dryer,which is the major equipment of dryer section,is the way to solve the above issue.However,the impeded drainage of the condensate in traditional dryer depresses the efficiency of the cylinder dryer,and all methods to reduce the thickness of the condensate layer,it still hardly solve the problem.Based on this,an innovative multi-channel cylinder dryer system,in which the steam flows through the longitudinally oriented mini-channel,is proposed by our research group.It could definitely decrease the thickness of the condensate layer and greatly improve the dryer efficiency.The heat transfer characteristics of the multi-channel cylinder dryer were studied from the experiment and numerical simulation in order to determine reasonable multi-channel dryer structure parameters and steam flow patterns.This paper is supported by the National Natural Science Foundation of China（NO.51375286）.The main content and results are as follows:（1）The visualization for steam condensation in multi-channel cylinder dryer was studied.The annular flow,annular-wave flow,wave flow,slug flow,plug flow,bubbly flow and stratified flow are observed.The vapor saturation temperature shows greater influence on flow pattern transition compared with the mass flow rate.When the mass flow rates increase from 25 to 45 kg?m^-2?s^-1,the area occupied by the annular flow only increases 1.3times,but the decrease rate for saturation temperature increases from 110?C to 130?C and channel aspect ratio increases from1:5 to 1:2 are 2 and 2.4 times,respectively.The occupied area at different vapor mass flow rates decrease.The result indicates that the Tandon flow regime agrees with the experimental data well.（2）The characteristics of heat transfer for steam condensation in multi-channel cylinder dryer were studied.The shear force increases with the vapor quality,the steam condensation rate decreases with the steam saturation temperature,these facts could lower the main thermal resistance—the thickness of the condensate,and the condensing heat transfer coefficient rises accordingly.Owning to the low mass flow rate(less than 50kg?m^-2?s^-1),the mass flow rate has no significant effect on the condensing heat transfer coefficient within a small range of the vapor quality.The condensing heat transfer coefficient rises with the increase of the channel aspect ratio,but the effect of channel aspect ratio is faded with the increase of vapor mass flow.A heat transfer correlation for one side heat transfer rectangular small horizontal channel（hydraulic diameter less than 9mm）,which could be applied in the design of the multi-channel dryer,is proposed and the deviation between experimental data and prediction is within 25%.（3）The characteristics of two-phase flow pressure drop for steam condensation in multi-channel cylinder dryer were studied.The two-phase flow pressure drop rises with the increase of the mass flow rate,and the decrease of the saturation temperature and the channel aspect ratio due to the increase of the shear force and the turbulence intensity.The pressure drop is almost identical in different channels when the mass flow rates is low,and the difference between channels rises with the mass flow rate.The correlation of two-phase flow model is revised based on the correlation proposed by Lockhart and Martinelli,and the deviation between experimental data and prediction is less than 30%,the result agrees with experimental data well.（4）The numerical simulation on the effect of the structured and flow parameters on the characteristics of heat transfer for steam condensation in multi-channel cylinder dryer were studied.In this paper,the optimal structural parameters for the selected dryer are 150²00 for channel number,1:3 for channel aspect ratio and 1:1¹:3 for insulation interval ratio.A 1-D distributed model is set up based on various flow patterns to investigate variation of the local temperature and local condensing heat transfer coefficient affected by the steam parameters.Because the distribution of the condensate is symmetrical in annular flow,the variation trend in annular flow is more gently than wave flow and stratified flow.The model has the same trend with the experimental result.In 3-D CFD model study,simulation of the vapor condensing process at various rotation speeds,including 0,23,32,39,45 and 50r/min were carried out,respectively.The condensing heat transfer coefficient and the pressure drop increase with the rotation speed,but the trend is getting smaller.