Experimental investigation of enhancement of dropwise condensation heat transfer of steam-air mixture: Falling droplet effect

The presence of noncondensable gas (NCG) in dropwise condensation (DWC) makes a heat transfer mechanism different; in order to get a better understanding of the enhancement mechanism in dropwise condensation with noncondensable gas, the experiments on dropwise condensation, filmwise condensation (FWC), and dropwise-filmwise coexisting condensation (DFC) heat transfer with and without noncondensable gas were performed on specially designed vertical plates. The investigations were concerned with the dynamic behavior of the liquid condensate near the vapor-liquid interface. The experimental results indicated that the heat transfer characteristics of dropwise condensation of pure steam without noncondensable gas were similar to those of the dropwise-filmwise coexisting condensation surface. However, in the presence of noncondensable gas, without the effect of droplets falling onto the vapor-liquid interface, the heat transfer characteristics of the dropwise-filmwise coexisting condensation surface were parallel to those of the filmwise condensation surface, and indeed almost no heat transfer enhancement was found. Compared with the former two condensation modes, the heat-transfer coefficient of dropwise condensation with noncondensable gas was enhanced by 30-80% for the air mole concentration of 0.9% and 4.8%, respectively. The facts cannot be attributed to the effects of reduction of the thermal resistance of the condensate layer and changes in the shape of the condensation surface, which were mainly due to the departure behavior of the condensing droplets, but make a considerable contribution to the overall heat and mass transfer performance.