Condensing Heat Transfer Characteristics Of Multi-channel Cylinder Dryer Under Rotating Condition

The energy consumption of the drying section of China’s paper machine accounts for more than 50%of the total energy consumption of paper machine operation.The improvement of cylinder dryer efficiency is of great significance to energy saving of paper-making industry.Compared with the traditional cylinder dryer,there is a new structure of the multi-channel cylinder dryer,which can effectively solve the problem of condensate discharge of steam,and the potential heat transfer efficiency of it is high.However,the current research has not solved the influence mechanism of steam parameters,rotating speed,and structural parameters on the condensation heat transfer and flow resistance of two-phase flow in multi-channel cylinder dryer,as well as the effects of two-phase flow pattern on condensation heat transfer coefficient under rotating condition.It limits the improvement of heat transfer efficiency and application of multi-channel cylinder dryer.In this work,the effects of rotating speed,steam parameters,and structural parameters on the condensation heat transfer coefficient,flow pressure drop,and two-phase flow pattern in multi-channel cylinder dryer under rotating conditions were studied both theoretically and experimentally.The change law of condensation heat transfer characteristics in the channel of multi-channel cylinder dryer were revealed,and models of condensation heat transfer coefficient and flow pressure drop under rotating condition were established.The main research contents and results are shown as following:(1)A rotating multi-channel cylinder dryer experimental system was designed and built.The composition of the experimental system was clarified.The structural design and transmission calculation of the rotating device were carried out.The strength calibration and vibration analysis of the test bench were completed by finite element method.Then,the reliability of the designed experimental system was verified.The results showed that the maximum deformation of the rotating device was 0.4mm under the condition of maximum centrifugal force,which meets the requirement of stiffness.The inaccuracy of condensation heat transfer coefficient and flow pressure drop obtained from the experiments were within 20%,which means the experimental system meets the requirements of this study.(2)The change of the two-phase flow pattern in the rotating horizontal channel under the influence of multiple parameters was investigated experimentally,and the flow pattern of two-phase flow in the horizontal channel under the rotating condition was plotted.Compared with the two-phase flow pattern in the horizontal channel at static state,the stirred flow is unique in the rotating channel,but there is no stratified flow.In addition,the rotating speed affects the type of flow pattern.The increase of the rotating speed increases the flow pattern types and decreases the proportion of annular and wavy flow.While the increase of steam mass flow rate and steam saturation temperature causes the increase of the proportion of annular and wavy flow.The result indicates that the flow pattern is in better agreement with the Mandhane flow pattern,but the bubble and stirred flow had some variability with the classical flow pattern.The flow pattern of two-phase flow in the horizontal channel under rotating condition was established.(3)The effects of rotation Reynolds number,steam parameters,channel aspect ratio and two-phase flow pattern on the heat transfer coefficient of steam condensation in a horizontal channel under rotating conditions were investigated,and a prediction model applicable to the heat transfer coefficient of steam condensation in a rotating rectangular channel is proposed.It is found that the effect of rotating Reynolds number on the heat transfer coefficient of steam condensation is related to the Reynolds number.When the Reynolds number is smaller than 24000,the steam condensation heat transfer coefficient increases and then decreases with the increase of the rotating Reynolds number.While when the Reynolds number is large than 30000,the steam condensation heat transfer coefficient increases with the increase of the rotating Reynolds number.The condensing heat transfer coefficient of steam in the channel decreases as the saturation temperature of steam increases,but increase as the channel height and width ratio decreases.The best condensing heat transfer performance in a multichannel cylinder dryer is achieved when the two-phase flow pattern is annularwave flow.In addition,the Cavallini correlation was found to be in the best agreement with the experimental results when compared with the heat transfer correlation,but there was still a large error.Based on this,a prediction model for condensation heat transfer in the horizontal channel under rotating condition was established with an average deviation of 12.8%,which can be used to predicted the condensation heat transfer coefficient in the channel of the multi-channel cylinder dryer under rotating condition.(4)The variation of the flow pressure drop in the rotating horizontal channel under the influence of multiple parameters was studied experimentally,and a flow pressure drop prediction model applicable to the rotating horizontal channel was established.It was found that the flow pressure drop in the channel decreases with the increase of the rotation Reynolds number.And the increase of Reynolds number and steam saturation temperature,the flow pressure drop in the channel tends to increase.In addition,the flow pressure drop in the channel increases as the channel aspect ratio decreases.In addition,the KIM correlation was found to be in the best agreement with the experimental results when the heat transfer correlation was compared,but there was still a large error.In order to the prediction of the flow pressure drop in the channel of multi-channel cylinder dryer under the rotating condition,a prediction model of the flow pressure drop in the horizontal channel under the rotating condition is established with an average deviation of 17.5%,and the results predicted by the model correlates well with the experimental data.(5)A mathematical model of steam condensation heat transfer resistance was developed to study the effect of channel spacing distance on the heat transfer characteristics of the multi-channel cylinder dryer,and a new dimensionless parameter was proposed to study the comprehensive effect of channel structure parameters on the condensation heat transfer coefficient and flow pressure drop of multi-channel cylinder dryer.The results show that the total thermal resistance of the channels with 1:2 aspect ratio is much higher than that of the 1:3 and 1:4 aspect ratio when the distance between the channels is large.By dimensionless analysis,it was found that the comprehensive property of the channels with 1:3 aspect ratio was the best.In order to make up for the shortage of existing experiments study,the heat transfer characteristics of the multi-channel cylinder dryer at high rotating speeds was investigated by numerical simulations.The results show that the condensation heat transfer coefficient of steam is positively correlated with the channel speed,but the growth rate decreases continuously and finally stabilizes.This work investigated the effects of multi-parameter on the condensation heat transfer coefficients,flow pressure drops and two-phase flow pattern in multichannel cylinder dryer,and establishes new prediction models.This study lays a theoretical basis for the improvement of heat transfer efficiency of multi-channel cylinder dryer and the optimization of its structural parameters.And it is of great significance for the industrial application of multi-channel cylinder dryer.