Experimental study of thermal performance in a closed loop pulsating heat pipe with alternating superhydrophobic channels

A pulsating heat pipe (PHP) with alternating hydrophilic/superhydrophobic channels was tested at vertical position, bottom heat mode and different heat power inputs. The device consists in a copper tube (internal/external diameters of 3.18/4.76 mm), bent into a planar serpentine of ten channels and five U-turns. The tube is partially functionalized with a superhydrophobic coating, in such a way to create an alternation of hydrophilic and superhydrophobic surfaces on the straight tubes along the loop, in the condenser zone. The aim is to investigate how the alternated wettability affects the start-up, the fluid motion along the tubes and the overall thermal performance of the device, which is compared to another PHP, having the same geometry and under the same working conditions, but completely hydrophilic. Distilled water, at 50% filling ratio is the working fluid. Power inputs varying from 20 to up to 350 W, in a stepwise increasing and decreasing fashion, are applied to the PHP. The condenser temperature is kept constant, at 20 °C. The device is monitored by sixteen thermocouples and one pressure transducer, mounted in contact to the fluid in the condenser region. Data analysis shows that the alternating wettability of tube sections strongly affects the flow motion, the start-up and the overall performance. In general, the alternating PHP presents a worse overall thermal performance: the thermal resistance is always higher and the start-up is achieved at higher heating power levels. Temperature at superhydrophobic surfaces exhibit a flat trend, suggesting that the flow was blocked in the functionalized area surface, while, for the hydrophilic inserts, more pronounced temperature fluctuations are observed. It is believed that the superhydrophobic coating hinders the liquid film formation, decreasing locally the flow motion. On the other hand, the enhancement of the inner wettability improves the flow motion, as the liquid film that covers the inner surface acts as lubricant.