Enhanced film condensation of steam on spirally fluted tubes

摘要

Film condensation of stagnant water vapor on both vertical and horizontal spirally fluted tubes was studied theoretically. A sine flute was taken as a typical example. In the analysis two regions of the flute, namely the valley and the crest, are treated separately. The condensation process takes place on the crest of the flute and runs down to the valley by surface tension and gravity. In the valley region no heat transfer occurs, and the condensate moves down stream by gravity to be drained. The equation of motion and energy balance are used to conclude relations between condensate flow rate and film thickness for both regions. By equating the condensation rate in the crest region to the change of flow rate in the valley region, the film thickness and the local heat transfer coefficient are calculated using numerical integration. The average heat transfer coefficient and Nusselt numbers are calculated and compared to that of smooth tubes. The results show that the enhancement due to fluting may reach five times in the case of horizontal tubes, while for vertical tubes it is much lower. The theoretical results are compared to the available experimental results of film condensation on horizontal finned tubes and twisted vertical tubes, which are very similar to spirally fluted tubes, and they show good agreement.