Sweating-boosted air cooling with water dripping

In power plants, the condenser section plays an important role in total thermal efficiency. Extensive considerations by researchers are put toward increasing the heat dissipation efficiency while maintaining low cost and water consumption. One way to archive such situation is to consider replacing the water cooled condenser (WCC) with an air cooled condenser (ACC), but resulting in a significant increase in the condenser size due to the relatively low heat transfer coefficient (HTC = 20-50 W/m²K) of the air compared to that of the water. Inspired by the phase change heat transfer of water during the perspiration of mammals, a sweating boosted air cooling approach is proposed herein to dramatically increasing the HTC and minimizing the water consumption. In this experimental study, a wind tunnel and distilled water dripping system were used to examine the thermal performance of copper samples with a tested surface area of 2 in by 2 in. Two surface enhancement approaches were adapted herein. In the first approach, the testing samples were integrated with microchannels, copper woven meshes (200 meshes/in) that were sintered on top of the surface, and finally nanostructures that was synthesized by a hot alkaline oxidation process. In the second approach, the tested samples were coated with TiO2 via an atomic layer deposition (ALD) process. This method allows for rapidly spreading of the dripping water droplets over the whole tested surface. This rapid spreading behavior is due to two main reasons, first the low resistance flow of microchannel which delivered the water globally; second the high capillary pressure generated by the micro-/nano-structures which delivered the water locally. Three different flat-surface samples were developed in this study, as flat surface with sintered copper meshes (design A), grooved surface with sintered copper meshes (design B); and grooved surface with sintered copper meshes coated with ALD TiO2 film (design C). The performance of the surfaces of this approach was quantitatively characterized with the wick testing. The heat transfer performances for all samples were also examined. The experimental results showed that the convection heat transfer plays a limited role in the heat dissipation. In addition, HTC was enhanced by increasing the dripping water rate consumption until the surface reached the flooding condition. The results showed that the evaporation rate of water was augmented with the increase of Reynolds number. The maximum HTC was 182.45 W/m²K with a water dripping rate of 12 ml/h, resulting in an enhancement approximately 214.07% compared to the case without water dripping. Further research on a higher HTC requires an optimized combination of Reynolds number and water consumption.