Using structured surfaces to enhance heat transfer in falling film flow

The heat transfer performance of an ordinary roughened (sand-blasted) surface and four commercially available structured surfaces were tested with falling film flow arrangement. The commercial surfaces were: High Flux (Union Carbide), Gewa-T (Wieland) and Thermoexcel-E and -EC (Hitachi). The test evaporator consisted of a jacket and a single, vertical 2 m long test tube. The evaporating liquid flowed down on the outside of the test tube. The tests were conducted with refrigerant R114 at evaporation temperature of 23 deg C. In the fully developed region (greater than 0.4 to 1.0 m), the flow was distorted due to strong nucleation and droplet entrainment. At higher evaporation rates (greater than 35 to 45%), the bottom parts of the test tube were not properly wetted and the heat transfer coefficient decreased although no permanent dry patches were yet seen. Nucleate boiling was dominant at all heat fluxes tested with the commercial surfaces. No boiling hysteresis was observed. They all showed excellent performance, the best one (Thermoexcel-E) having 12 times better heat transfer than the smooth tube. When heat flux was increased they all showed similar trend: the obtained enchancement compared to smooth tube was reduced. The sand-blasted tube could not compete with the structured surfaces. Its behavior was more like smooth tube.