Micro/nano structured phase change systems for thermal management applications
Phase change phenomena have been of interest mainly due to large latent heats associated with the phase transition and independency on external energy to drive the phase change process. When combined with micro/nano structures, phase change systems become a promising approach to address challenges in high heat flux thermal management. The objective of this thesis is to implement micro and nano structured surfaces for better understanding the underlying fundamentals of evaporation and boiling phase change heat transfer and enhancing the heat transfer performance. First, we study single bubble dynamics on superheated superhydrophobic (SHB) surfaces and the corresponding heat transfer mechanism of water pool boiling. Because of the large contact angle, such surfaces are ideal for correlating pool boiling with single bubble dynamics by accurately controlling the number of nucleation sites in a defined area. The fundamental parameters of single bubble dynamics are collected and put into the heat flux partitioning model. We find that latent heat transport and bulk liquid water convection contribute together to the heat removal on superhydrophobic surfaces. Next, we present a novel method to fabricate silicon nanowires by one-step metal assisted chemical etching (MACE) on micro-structured surfaces with desired morphologies. Patterned vertically aligned silicon nanowires are fabricated on dense cavity/pillar arrays due to trapped hydrogen bubbles serving as an etching mask. Uniformly grown silicon nanowires on structured surfaces can be fabricated if extra energy is introduced to remove the trapped bubbles. An enhanced pool boiling heat transfer performance on such structured surfaces is demonstrated. Finally, we study the ultimate limits of water evaporation in single 2D nanochannels and 1D nanopores. These ultimate transport limits are determined by the maximum evaporation fluxes that liquid/vapor interfaces can provide regardless of liquid supply or vapor removal rates. A hybrid nanochannel design is utilized to provide sufficient liquid supply to the evaporating meniscus and evaporated vapor is efficiently removed by air jet impingement or a vacuum pump. The effect of nanoscale confinement on evaporation flux has been investigated, with feature size ranging from 16 nm to 310 nm. An ultra-high heat flux of 8500 W/cm2 is demonstrated in a single 16-nm nanochannel at 40 °C.