Numerical Analysis Of Condensation Heat Transfer Characteristics In A Single Tube Under Rolling Condition

For the secondary side passive residual heat removal system,when operating under marine conditions,the change of the relative position of cold and heat sources and the effect of rolling additional force field will affect the circulating driving force,resulting in the change of steam flow rate,condensate film flow and vapor-liquid interface,and then change the effective heat transfer performance of condensate heat exchanger.In this paper,the condensation characteristics of pure steam in a single tube under rolling conditions are numerically simulated by CFD software.Based on the reasonable verification of the applicability of condensation model and rolling model,the effects of the changes of inlet steam velocity,wall undercooling and other parameters on the condensation heat transfer characteristics of pure steam in rolling tube under different tilt angles,rolling angles and rolling cycles are analyzed.The calculation results show that due to the gravity under the inclined condition,the condensate in the pipe will flow downward and concentrate to a small part of the area at the bottom of the pipe,so that the accumulation of liquid film along the thickness of the pipe length in most areas around the pipe is not obvious,which significantly reduces the influence of the liquid film thermal resistance of the whole pipe on the condensation process,resulting in the increase of the average heat transfer coefficient of the pipe compared with that in the vertical state.And the heat transfer coefficient increases with the increase of inclination angle.Under the rolling condition,the condensation heat transfer coefficient fluctuates periodically due to the periodic change of the circumferential thickness of the condensate film.The increase of the total axial average condensation heat transfer coefficient relative to the average heat transfer coefficient in the vertical static state in a cycle during the rolling process is higher than 20% under various conditions.At the same time,due to the action of additional inertial force,the liquid film is hindered by additional pressure drop,and the circumferential movement in the pipeline is delayed compared with the rolling period.Therefore,there is a certain phase difference between the fluctuation period of instantaneous axial average condensation heat transfer coefficient and the rolling period,and the heat transfer coefficient curve is not completely symmetrical in the first half period and the second half period.In addition,the circumferential movement speed of the liquid film at different positions along the pipe length is also different,which is reflected in the rapid spreading and concentration speed of the liquid film at the thick liquid film at the bottom of the pipe.After the liquid film spreading at the bottom of the pipe is completed,the liquid film spreading at the top of the pipe has not been completed.Therefore,the minimum instantaneous axial average condensation heat transfer coefficient of the pipe is always higher than the average condensation heat transfer coefficient in the static vertical state,The maximum instantaneous axial average condensation heat transfer coefficient is also slightly higher than the static average condensation heat transfer coefficient at the maximum rolling angle.Under the same parameters,increasing the rolling period will reduce the additional pressure drop in the rolling process,make the liquid film concentration speed faster and the spreading speed slower,resulting in a small decrease in the total axial average condensation heat transfer coefficient.The increase of rolling angle will increase the concentration of liquid film at the maximum angle of pipeline movement,so as to reduce the circumferential liquid film of pipeline,increase the maximum value of instantaneous axial average condensation heat transfer coefficient compared with the minimum value,and increase the total axial average condensation heat transfer coefficient greatly.When the inlet steam flow rate increases,the liquid film will be thinned due to the increase of gas-liquid interface shear force,so as to improve the total average condensation heat transfer coefficient.The increase of wall undercooling will not only promote the condensation process by increasing the condensation driving force,but also lead to the thickening of liquid film,which will hinder the condensation process.Under the combined influence of the two,the heat transfer coefficient first increases and then decreases with the increase of wall undercooling.