水平管を流下する液膜の蒸発熱伝達 : 第4報, 溝付管による伝熱促進

Enhancement of heat transfer in evaporation of falling film was studied on horizontal tubes with triangular grooves on the tube circumference. An analysis of laminar film in a vertical triangular groove revealed that grooves are effective in enhancing evaporation heat transfer. To verify the enhancement associated with grooved tubes, an experiment was performed in the range of film Reynolds number 100 to 1000 with saturated refrigerant R11 (CCl3F) at a pressure of 0.2 MPa used as the working fluid. Three grooved tubes were tested and enhanced heat transfer coefficient well beyond the geometrical surface increase, yielding larger enhancement than expected from the analytical prediction and about 4 to 8 times higher heat transfer coefficient than that for plain tube. The observed enhancement is probably attributable to an extension of falling film in the valley of the groove as predicted in the analysis. Moreover an intermittent wetting of the crest portion of grooves caused by wavy falling film and subsequent evaporation of thinner film left behind is contributed largely to the enhancement.