Experimental study on flow boiling heat transfer performance of nanowire-printed substrates with porous-like structures

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In this study, ZnO nanowires (NWs) were demonstrated to have considerable potential for heat transfer enhancement in flow boiling. Unlike the conventional NW fabrication method using metal-assisted chemical etching, we use the microcontact printing (µCP) method comprising solution-based growing methodologies to create ZnO NWs on different substrates, including bare Si-, Cu-, and Al-coated surfaces. This ZnO NW fabrication approach successfully overcomes the substrate limitation of typical NWs, such as Si- and Cu-NWs, which can be only formed on its intrinsic material following the high-cost fabrication approaches required by the metal catalyst. Additionally, a high pore-connected structure was observed on the interfacial area of the ZnO NWs, with the nanocore distribution area considerably enhanced by lengthening the NW length, beneficial to bubble nucleation and surface coolant supply during nucleate boiling. The length of the ZnO NW substrates is controllable by varying the growing period during the hydrothermal growth procedure. Further, the ZnO NW substrates reveal a remarkable enhancement in the flow boiling heat transfer regardless of the substrate variation, thereby exhibiting high variability in the substrate selection and potential of being used as the cooling approach for the thermal management of the high heat flux applications. We conducted a series of experiments related to the surface wettability, wicking, and bubble characteristics for different test substrates under saturated flow boiling conditions to explicitly determine the underlying mechanism in the heat transfer enhancement of the ZnO NWs.