Flow and heat transfer characteristics of subcooled flow boiling on nanowires surfaces in a narrow microchannel

Nowadays, heat dissipation methods of single-phase air cooling or liquid cooling, such as natural convection and forced convection, are unable to meet the high heat flux density dissipated by the heat exchange components of the thermal management system in many relevant fields, such as energy, electronic, and chemical engineering industries. With the recent increase in the power density consumption and miniaturization trend of various equipments, the compactness and efficiency of heat exchangers are becoming increasingly demanding. Compared to conventional channels, heat sinks fabricated using micro-scale channels have a compact structure, higher surface area-to-volume ratio, and heat transfer coefficient, together with less demand for the working fluid and the pumping power supply, which have been gradually paid attention to as remarkable cooling scheme candidates for applications in modern-day and next-generation electronic elements. Recently, the rapid development of the surface modification technology has prompted the development of heat transfer surfaces with micro- or nano-scale structures ranging from a few micrometers to tens of nanometers, which can enhance the boiling heat transfer coefficient and critical heat flux. Therefore, the combination of structured surface and microchannel flow boiling is a highly promoted method of achieving a higher heat transfer performance. Nanowire is one of the micro-structures that can enhance boiling heat transfer. Previous experimental results showed that a nanowire-structured surface could increase the nucleate site density and enhance the capillary wicking process, thereby achieving a higher heat transfer coefficient and a critical heat flux. Herein, TiC nanowires were synthesized on a titanium wafer surface through the electro-deposition method. The diameters of the two types of nanowires were approximately 230 and 535 nm, and their heights were 4 and 12 mu m, respectively. The nanowires were staggered and closely arranged on the surface, forming micro-/nano-scaled gaps. The static contact angles were 150 degrees and 154 degrees, respectively. An experiment of subcooled vertical flow boiling on nanowire surfaces in a narrow rectangular microchannel was conducted using a high-speed camera. The working fluid was deionized water. The channel cross-section was 0.5 mm x 5 mm. The mass fluxes of the subcooled flow boiling were 200 and 300 kg/(m(2) s). The heat flux was in the 20-200 kW/m(2) range. The experiment was conducted under atmospheric pressure, with a subcooled degree of 10 K. The slopes of the boiling curves on the two surfaces were both negative near the ONB points and changed to positive with the increasing heat flux. The single-phase heat transfer performance on the 12-mu m-height nanowire surface was better than that on the 4-mu m-height nanowire surface. The 4-mu m-height nanowire surface in the two-phase condition had higher heat transfer coefficients. However, the heat transfer deterioration only occurred on the 4-mu m-height nanowire surface at a high heat flux. Thus, the heat transfer performance on the 12-mu m-height nanowire surface was better at a high heat flux. The two-phase periodic flow pattern on the 4-mu m-height nanowire surface was "elongated bubble-expanding to upstream and downstream-local dryout-rewetting". The main flow pattern on the 12-mu m-height nanowire surface was bubble coalescence and elongating to downstream.