Study Of Dropwise Condensation Characteristics Of Steam With Non-condensable Gas In Centrifugal Field

Dropwise condensation is a kind of gas-liquid phase change process with high heat transfer performance.It has been widely used in various industries such as aerospace,nuclear power engineering,petrochemical industry and so on.During the process of dropwise condensation,as the heat transfer unit,it is the key to control droplet's shape,size and dynamic characteristics for improving the condensation heat transfer.The above mentioned properties of droplet are affected by surface wettability,non-condensable gas content,surface subcooling degree,external field and so on.In this thesis centrifugal field is applied to condensation surface,which can enhance condensation process and heat transfer by regulate the dynamic characteristics of droplets.Hydrophobic and super-hydrophobic surface on copper tube were prepared to realize dropwise condensation;the centrifugal condensation experiment platform was designed and installed to drive vertical tube coaxial rotation for study of condensation in centrifugal field.The effect of non-condensable gas content and centrifugal field intensity on the mixed vapor dropwise condensation process were investigated experimentally.The movement process of the droplet on hydrophobic surface and super-hydrophobic surface during the mixed vapor condensation in centrifugal field was observed visually.The results showed that the droplet slide downwards,then slip off condensation surface,and the distance of moving decreases with the increase of the centrifugal intensity at the low centrifugal intensity.The droplet flies directly off surface at high centrifugal field intensity.At the moment of flying away from surface,the droplet becomes spindle shape,and the top head flied away from the surface,the tail contracted,returned the original position or left the surface as the companying droplet.When non-condensable gas content was 45%,droplets all bounce off the super-hydrophobic surface with the decreasing of surface subcooling degree,which was similar to the jumping droplet.When ratio of centrifugal force to gravity(centrifugal coefficient)was less than 5,the critical droplet radius was affected by the non-condensable content,the surface wettability and centrifugal field.At the same conditions,critical droplet radius on hydrophobic surface was less than that on the super-hydrophobic surface.When centrifugal coefficient was greater than five,the critical droplet radius was mainly controlled by centrifugal field,the radius was decreased with increasing of rotation speed.And radius on the hydrophobic surface was close to the super-hydrophobic surface with varying non-condensable content at the same centrifugal field.As for update frequency of condensation surface,sweeping showed a nonlinear relationship with centrifugal field.When non-condensable gas content was 45%,jumping droplet was found on the super-hydrophobic tube,and its initial horizontal velocity increased with the increase of the centrifugal field intensity,which was slightly larger than linear velocity of rotating tube.During process of centrifugal mixed vapor condensation,results showed that the non-condensable gas content dramatically influence on the heat transfer performance.With the increase of content,the heat transfer performance of hydrophobic and super-hydrophobic copper tubes significantly reduced.When centrifugal coefficient was more than five,the heat flux increased with the higher centrifugal coefficient,and the improvement was weakened by the increase of non-condensable content.