Experimental investigation of filmwise and dropwise condensation inside transparent circular tubes

Abstract

In this study, we conducted air-cooled condensation experiments inside bare and tetrafluoroethylene (TFE)-coated Pyrex glass tubes. Using infrared thermometry and side-view visualization with a charge-coupled device (CCD) camera, high-resolution local wall temperature distributions on the tubes and the formation of filmwise and dropwise condensation inside the tubes could be determined. In bare-tube experiments, the condensation flow patterns belonged to a fully stratified flow regime. Dropwise condensation phenomena inside the TFE-coated tube were clearly observed at all locations in the condenser tube. Inside the TFE-coated tube were distributed several condensate droplets with sizes from 10μm to 1mm order. The local heat flux values in the TFE-coated tube were higher than those in the bare tube in the upstream region of the tube. In the downstream region, the local heat flux values on the TFE-coated surface were similar to or slightly lower than those of the bare tube. This may explain the distributions of condensation droplets in different local positions. The local temperatures on the outer wall of the TFE-coated tube were significantly higher than those of the bare tube. These results indicated that the local condensation heat transfer coefficients in dropwise mode were much larger than those in filmwise mode. As the vapor quality inside the TFE-coated tube decreased, the dropwise condensation heat transfer capability was degraded. This may be the result of the uniform distributions of relatively larger droplets and the reduced vapor shearing effect in the lower quality region.