The Condensation Phenomenon Under Synergetic Effect Of Finned Tube And Surface Wettability

The process of steam condensation is one of the most important unit processes in the real industrial production. People have researched on and paid close attention to the method that can enhance the condensation process in the long run. Currently, the technology of extended surface to enhance the condensation heat transfer process is widely used, for instance, the finned tube, corrugated tube, and spiral tube. Dropwise condensation has aroused great interest since last century because of its high heat transfer coefficient. Surface wettability has a great influence on the morphology of condensation. The mechanism of dropwise condensation is becoming more and more clearly, and fabrication of the surface where dropwise condensation can form is becoming more and more maturely.In this work, two important factors for condensation heat transfer were combined basing on the concept of compound enhancement. The two factors’re surface wettability and extended surface. The surface wettability of copper finned tube was changed to investigate the phenomenon and mechanisms of steam condensation without noncondensable gas and steam condensation with noncondensable gas, and analyzed the synergetic effect of this compound enhancement, aiming to provide a kind of new thought for the application of finned tube in the real industrial production.An experimental facility of vertical tube condensation was set up. Superhydrophobic surface and hydrophobic surface was prepared on the finned tube to do the steam condensation experiment without noncondensable gas. The behaviors of condensate was recorded by a high speed photography, the temperature of cooling water and steam was monitored by thermocouple, and the pressure of steam was monitored by a high precision pressure gauge. The experiment result shows that under the condition of steam condensation without noncondesable gas, the condensation on the copper finned tube is filmwise condensation; droplets are distributed on the fin and in the groove between two fins and there’s condensate in the groove between two fins after a droplet leaving the hydrophobic finned tube surface; and for the superhydrophobic finned tube, the groove between two fins is full of condensate and very small droplets are on the fin. The heat transfer performance is:hydrophobic finned tube> superhydrophobic finned tube> copper finned tube> copper flat tube. The experiment with noncondensation gas of7.5%,15%and25%was also conducted. The condensation characters of hydrophobic finned tube, superhydrophobic finned tube and copper finned tube were investigated. The renewal frequency of hydrophobic finned tube surface becomes slower and the contact angle hysteresis becomes bigger as the percentage of noncondensable gas becomes higher. When the percentage of noncondensable gas is7.5%, the groove between two fins of the superhydrophobic finned tube is no longer full of condensate, and the condensation characters become similar to the hydrophobic finned tube, that is, droplets are in the groove or on the fin and condensate remains in the groove after a droplet leaving the surface. When the percentage of noncondensable gas is up to25%,the condensate levels is close to sphere, and the phenomenon of droplet jumping and droplet leaving the tube surface during the process of scouring. When the percentage of noncondensable gas is7.5%, the heat transfer performance is:superhydrophobic finned tube hydrophobic finned tube>copper finned tube; the three kinds of tube nearly has same heat transfer performance when there is15%noncondensable gas since the resistance of gas phase boundary layer is the key factor for condensation heat transfer.