Numerical Simulation And Experimental Study Of Condensation Heat Transfer In Multi-channel Drying Cylinder Channel

At present,the development of China's paper industry is deeply constrained by energy.The energy consumption of paper industry is huge,accounting for about 2% of the total industrial energy consumption.The drying part of the paper machine is the most energy-consuming part in the paper-making process,and the drying cylinder is the core component of the drying part.The traditional steam dryer is difficult to discharge condensate water because of steam condensation.The condensate water will form a water ring,which not only affects the development of paper machine to high speed,but also increases the thermal resistance,which affects drying efficiency of dryer and paper quality.Despite a series of technical reforms such as siphon and spoiler rod,the water accumulation in dryer has not been effectively solved.The multi-channel drying cylinder is a new type of drying cylinder for solving the problem of water accumulation in the steam drying cylinder.The incoming steam is restricted to a small channel along the axis of the inner wall of the dryer to flow super condensation and heat release,forming a gas-liquid two-phase flow which is driven by the subsequent steam to discharge the dryer,and there is no water accumulation in the drying cylinder.The heat transfer coefficient of multi-channel dryer is influenced by the law of gas-liquid two-phase flow of steam condensation.Therefore,it is important to analyze the influencing factors of steam condensation in the channel for improving the condensation heat transfer of two-phase flow and drying performance of dryer.In order to deeply study the heat transfer characteristics of multi-channel dryer,the application of numerical simulation method in the research of multi-channel dryer was explored,and the reasonable structure parameters of multi-channel dryer were determined.In this paper,the gas-liquid two-phase flow of vapor condensation in a single channel is studied by means of numerical simulation and experimental study.The main research contents are as follows:(1)The numerical simulation of steam condensation in multi-channel dryer channel is carried out.Since the own model of Fluent software can't realize the numerical simulation of steam condensation well,this paper fits the variation curve of steam physical property parameters,establishes the source term according to the phase change model,and writes UDF program to access Fluent to supplement.It is concluded that the condensation heat transfer coefficient increases with increasing steam mass flow rate,saturation temperature and aspect ratio.increasing steam mass flow rate and aspect ratio will increase two-phase flow pressure drop.The flow pressure drop decreases with increasing saturation temperature.In a certain range,the numerical simulation can effectively predict the condensation characteristics of steam,but there are still deviations,which needs to be further improved.(2)The visualization of steam condensation flow in rectangular channel of multi-channel dryer cylinder was studied.Based on the experimental platform,while under the influence of mass flow rate,vapor quality and mass flow of cooling water,the condensation heat transfer of steam in the channel was measured experimentally.Six different flow patterns were observed during flow Condensing for vapor,which are the flow patterns of bubble flow,plug flow,stratified flow,wavy flow,slug flow and annular flow.At the same time,results show that when the mass flow of cooling water increased,the flow pattern change and the steam mass flow rate increase are opposite to the flow pattern change process.the vapor quality directly affects the proportion of gas-liquid condensation in the channel,and the influence on the flow pattern change is obvious.(3)The condensation characteristics of steam in rectangular channel are studied,and the experimental data are compared with the existing correlations.The results show that increasing steam mass flow rate and vapor quality will increase the shear force between gas and liquid phases,change the flow pattern to the direction of liquid film thinning,decrease the thermal resistance,increase the condensation heat transfer coefficient and two-phase flow pressure drop.Because the experiment is unilateral heat transfer,the change of cooling water mass flow rate has little effect on the condensation heat transfer coefficient,and the pressure drop of two-phase flow decreases with increasing cooling watermass flow rate.By comparing the experimental data with the existing correlations,it is found that Yan's correlations can predict the experimental results very well.(4)Aiming at the influence of channel shape on the condensation characteristics of steam,trapezoidal channels were designed and manufactured to compare the difference in condensation characteristics between the rectangular and trapezoidal channels under the same steam mass flow rate.Results show that under the influence of gravity,condensate accumulates obviously at the bottom of trapezoidal channel.Because of the upward flow of vapor condensate film around the trapezoidal channel,the disturbance of vapor flow along the horizontal axis results in a larger average pressure drop of two-phase flow in trapezoidal channel than in rectangular channel.The change of trapezoidal channel shape improves the condensation heat transfer coefficient,while in the whole process of vapor mass flow rate change,The average condensation heat transfer coefficient of rectangular channel is 2.8% higher than that of trapezoidal channel.Therefore,for multi-channel dryer design,it is more reasonable to choose rectangular channel structure.In this paper,numerical simulation and experimental study on condensation heat transfer of steam in multi-channel dryer channel are carried out.The results show that the two-phase flow characteristics of steam condensation are complex,but the heat transfer capacity of multi-channel dryer is improved.With the help of simulation software,the design and research of multi-channel dryer can be effectively assisted,and the rectangular channel is more in line with the design requirements of multi-channel dryer.