Study On Condensation Process And Enhanced Heat Transfer Under Different Gravity Conditions

Condensation heat transfer is widely used in various industrial fields,such as refrigeration systems,air conditioning systems,electronic equipment cooling,dehumidification systems and power generation industries.During the past few decades,researchers focused on the study of condensation heat transfer under constant gravity conditions.In recent years,phase-change heat systems have been widely used in the aerospace industry.The study of condensation heat transfer in microgravity environment has been paid more and more attention by scholars at home and abroad.Whether it is filmwise condensation or dropwise condensation,gravity plays an important role.For example,the condensate film flows down and gradually accumulates in the direction of gravity;the droplets rolling and falling under the guidance of gravity.Therefore,the study of condensation heat transfer process and heat transfer enhancement technology under different gravity conditions not only has important engineering guiding significance for the research and optimization of two-phase flow equipment for spacecraft in China,but also plays a guiding role in improving the operation efficiency of space electronic equipment,saving heat transfer equipment and ensuring the effective implementation of equipment safe operation.At present,the research on microgravity condensation heat transfer is mainly focused on foundation research,and there is no related research on the enhancement of condensation heat transfer in microgravity environment.In view of the shortcomings of the above problems,the main research contents and conclusions of this paper are as follows:Firstly,the feasibility of ultrasonic accelerating condensate drop shedding under different gravity conditions is analyzed theoretically,and the experimental study on condensation heat transfer on vertical aluminum plate surface under ultrasonic action is carried out,and the effects of ultrasonic power on droplet distribution,surface coverage and condensation heat transfer are studied.The results show that when the condensate droplets grow to a certain size,they roll off the surface.Compared with no ultrasonic,ultrasonic accelerated the shedding of condensate droplets.With the increase of ultrasonic power from 180 W to 900 W,the first drop time of droplets comes earlier and the total number of droplets increases.The heat flux and condensation heat transfer coefficient increase with the increase of ultrasonic power.When the ultrasonic power is 900 W,the enhancement rate is more than 2 times.However,with the increase of power,the economic coefficient of the test system decreases.Secondly,a kind of elliptical pin-fin plate is proposed,and the condensation heat transfer on the surface of elliptical pin-fin plate with different inclination angles is studied experimentally.The effects of geometric parameters and inclination angles of the pin-fin plate on condensation heat transfer are analyzed.It is found that the pin-fins have a certain retention effect on condensate droplets.When the horizontal pin spacing is small and the pin height increases,the shedding period of droplets becomes smaller,on the contrary,the shedding period of droplets increases with the increase of pin height.The heat flux and condensation heat transfer coefficient of elliptical pin-fin plate are higher than those of flat plate.With the increase of horizontal pin spacing and pin height,the heat transfer performance of elliptical pin-fin plate is improved.When the inclination angle decreases from 90°to30°,the heat flux and condensation heat transfer coefficient decrease by 16%and 25%,respectively,and the enhancement rate also decreases.Thirdly,the condensation heat transfer on the pin-fin surface in a horizontal rectangular channel is studied experimentally.The effects of geometric parameters,thermal conductivity,steam mass flow rate and refrigeration power on condensation heat transfer are analyzed.The results show that the geometric parameters have little effect on condensation heat transfer.When the mass flow rate of steam increases from 0.17 kg/(m~2·s)to 1.67 kg/(m~2·s),the condensation heat transfer coefficient increases by only about 80%.When the refrigeration power increases from 56.4W to 94.2W,the condensation heat transfer coefficient increases by about 20%.There is no obvious difference in strengthening rate and pressure drop of pin-fin plate under different working conditions.Fourthly,the condensation experiments of FC-72 on the elliptical pin-fin plate under different gravity conditions were carried out,and the effects of microgravity on the distribution of liquid film,gas-liquid interface distribution,steam temperature,base temperature and condensation heat transfer were analyzed.The results show that,in microgravity,the condensate film flows in different directions,the gas-liquid interface fluctuates and the liquid climbs obviously,and the steam temperature increases significantly in the unsteady state.For quasi-steady state,the heat flux and condensation heat transfer coefficient decreased by 18%and 20%,respectively.Finally,the condensation process of vapor on vertical wall surface under different gravity conditions is simulated by the VOF method,and the influence of vapor velocity and gravity on condensation heat transfer is analyzed.The results show that a uniform liquid film is formed on the wall surface for normal gravity and degravity.However,for microgravity,the gas-liquid interface is wavy,and the fluctuations of the liquid phase become more and more obvious and spread in the vapor flow direction with time.When the gravity is perpendicular to the wall surface,the liquid film becomes thicker over time.The smaller the gravity,the more significant the improvement of vapor velocity to the condensation heat transfer coefficient.As the vapor velocity increases from 0.5 m/s to2 m/s,the condensation heat transfer coefficient in microgravity decreased by about 28%and 12%,respectively.