Thermal Stability of Pool Boiling Heat Transfer on Vertical Nanowire Surfaces Under Heater Size Effect

Boiling heat transfer in the field of high-load heat dissipation is considered a promising method for managing an extreme load of heat with less superheat. The phase change heat transfer provides superb heat transfer coefficients by accompanying latent heat. Two performance factors in boiling heat transfer are critical heat flux (CHF) and heat transfer coefficient (HTC). CHF means maximum cooling capacity for boiling heat transfer just before the transition to film boiling regime. When nucleate boiling becomes vigorous and form merged bubbles on the heated surface, a small increase in heat flux makes a significant increase of temperature due to the vapor film formation which blocks liquid replenishment. Since CHF comes with the blocked liquid inflow, how long are the liquid path from the bulk fluid and the hot spot in the center plays a significant role in the level of CHF. Numbers of research have reported changes in CHF with varying heater sizes. However, less research attention focused on thermal stability and uniformity with differing heater sizes. In this study, heated surfaces’ spatial and temporal temperature deviation and fluctuation are investigated with different sizes of square heaters in the pool boiling environment. Lab-made RTD sensor with local Pt probes is employed for measuring inner temperature data. The heater layer of ITO (Indium tin oxide) have a response time of 2.84 ms since it is a thin film heater with Joule heating, the size of the ITO heater is determined as 5 mm, 10mm, and 20 mm square to evaluate the combined effect of heater size and the hydrophilicity of surfaces. And Pt serpentine probes measure temperature heater in situ, in multiple points. Nanowire surface is fabricated with MaCE (Metal-assisted Chemical etching), silicon wafer on the backside of the RTD sensor. The backside of the sensor chip was cleaned with acetone, ethyl alcohol, and piranha solution (H2O2/H2SO4 = 1:3 by volume) in series. Then the sensor chip is immersed in buffered oxide etchant (HF/H2O = 1:5 by volume) to remove the oxide layer. The etching process for nanowire fabrication follows. The sensor chip is chemically passivated with a polytetrafluoroethylene etching module, then immersed into 5mM AgNO3 and 4.8 M HF solution for 30 min. The redox chemical reaction then etches the silicon surface down, leaving vertical, freestanding nanowire with 7 um in height and 100-200 nm in diameter on the sensor chip. Nanowire and plain surfaces are then evaluated by a pool boiling setup. The deionized (DI) water is used as a working fluid, in the saturated temperature at atmospheric pressure conditions. The results showed that both CHF and HTC are improved with nanostructure surface, recording maximum CHF of 248 W/cm² and HTC of 62000 W/m/K, which is 167% and 182% enhanced records compared to the plain surface, respectively. And the wall superheat was significantly lessened by capillary inflow. The effect of heater size on CHF is analyzed with the heater size correlation by Lienhard and Dhir, and results on both surfaces, nanowire and plain surfaces, show a good agreement with previous research. While the temporal fluctuation and spatial deviation of surfaces differ. First, increases in heater length lead to higher wall superheat at equivalent heat fluxes. The maximum temporal fluctuation on the 5×5 mm² surface is 13.38 K, while the maximum fluctuation on the nanowire surface records 3.7 K. Minimized temperature fluctuation is owing to the wicking phenomena by capillary inflow on the nanowire surface. The spatial deviation is also decreased on the nanowire surface, 11 K to 2.9 K. This shows improved thermal stability could be obtained with enhanced wicking by introducing fresh working fluid to the heated surface even for an extended heated surface. This study will be helpful in designing surfaces and analyzing the thermal stress for boiling heat transfer systems with extreme cooling loads.