Study on axial wetting length and evaporating heat transfer in rectangular microgrooves with superhydrophilic nano-textured surfaces for two-phase heat transfer devices

Highlights: • A hybrid wick combined microgroove with superhydrophilic nano surface is developed. • Dramatic improvements in axial wetting characteristics are achieved by the RMSNSs. • The RMSNSs exhibits significantly enhanced evaporating heat transfer performance. • The heat transfer enhancement depends on the highly increased thin liquid film area. • A theoretical model is presented to deeply understand the enhancement effects. -- Abstract: Advanced thermal management solutions for various applications in energy systems have promoted the developments of novel hybrid wicks for two-phase heat transfer devices (TPHTD). In this study, a novel micro-nano hybrid wick combined rectangular microgrooves with superhydrophilic copper hydroxide nano-textured surfaces is developed. Comparative experiments on the axial wetting lengths, evaporating flow characteristics and evaporating heat transfer performance are systematically conducted for different microgrooves-based wick samples, i.e., the pristine rectangular microgrooves (RMs) and rectangular microgrooves with superhydrophilic nano-textured surfaces (RMSNSs). The distilled water and ethanol are both used as working fluids. The experimental results indicate that the RMSNSs wick produces remarkable increases in axial wetting lengths compared to the pristine RMs wick. Under the unheated condition, the total wetting length (L_t) of water in RMSNSs is more than 4 times of that in RMs. Under the same condition of heat flux, the L_t of RMSNSs is also distinctly larger. The improved axial wetting characteristics can be attributed to the effect of surface wettability enhancement, which have a critical influence on the evaporating heat transfer. The sample temperature of RMSNSs is much lower at each given heat flux and the decrement is more than 15 °C. The maximum incremental ratio of heat transfer coefficient can approach 100%. The enhanced evaporating heat transfer highly depends on the significantly increased thin liquid film area. Furthermore, a theoretical model is presented to facilitate the understanding on axial wetting and heat transfer enhancements effect of RMSNSs. The proposed model is applicable and satisfactorily accurate to predict the axial wetting lengths for microgrooves-based wick sample under the evaporating heat transfer condition. This study provides an in-depth insight into the enhanced axial flow and heat transfer characteristics of micro-nano hybrid wick incorporating microgrooves and nanostructures, and consequently can facilitate the further optimization design of microgrooves-based wick for advanced TPHTD. It is believed that this superior hybrid wick with advantages of facile and rapid fabrication is highly promising in the applications of heat energy transfer, conversion and management.