Experimental study of condensation heat transfer on a horizontal aluminum tube with superhydrophobic characteristic

Abstract

In this study, we conducted a condensation heat transfer experiment for an observation of the condensation heat transfer performance from a superhydrophobic surface fabricated with the Self Assembled Monolayer (SAM) method. For the experiment of condensation heat transfer, we designed an experimental facility that could compare a superhydrophobic tube and a bare tube under the same conditions. The experiment was conducted according to a condition number defined by the coolant Reynolds number and the saturation pressure, and two repeat tests were carried out to analyze the attached condensation. Each condensation phenomenon was observed by a camera, and the droplet detaching frequency was measured in units of [drops/min]. As a result, the dropwise condensation was observed under the condition #1 and #2 in the superhydrophobic tube, and the performance was improved by about 105% over the bare tube with regard to the average overall heat transfer coefficient. Under the conditions #3 and #4, the flooded condensation was observed, and the performance improved by about 16%. By contrast, the attached condensation with a lower detaching frequency than the flooded condensation was observed under the condition #5, and a performance degradation of about 20% was observed. In addition, the condensation performance was analyzed with a ratio of the condensation thermal resistance to a total thermal resistance. As a result of the condensation thermal resistance, the ratio decreased greatly than that of the bare tube when the dropwise condensation occurred, and it was slightly decreased in the condition of the flooded condensation, and it increased in attached condensation. In the repeat tests, after passing the attached condensation, we observed that the performance of the condition #2 was further decreased compared with the situation after passing only the flooded condensation. From the result, attached and flooded condensation seemed to be related to the droplet pinning depth, and we predicted that the droplet pinning depth of attached condensation would be deeper than that of flooded condensation. In conclusion, attached condensation more seriously affects the condensation performance degradation than flooded condensation. Therefore, it is necessary to understand the condition of attached condensation and to prevent attached condensation phenomenon.