Abstract
For phase-change cooling schemes for electronics, quick activation of nucleate boiling helps safeguard the electronics components from thermal shocks associated with undesired surface superheating at boiling incipience, which is of great importance to the long-term system stability and reliability. Previous experimental studies show that bubble nucleation can occur surprisingly early on mixed-wettability surfaces. In this paper, we report unambiguous evidence that such unusual bubble generation at extremely low temperatures—even below the boiling point—is induced by a significant presence of incondensable gas retained by the hydrophobic surface, which exhibits exceptional stability even surviving extensive boiling deaeration. By means of high-speed imaging, it is revealed that the consequently gassy boiling leads to unique bubble behaviour that stands in sharp contrast with that of pure vapour bubbles. Such findings agree qualitatively well with numerical simulations based on a diffuse-interface method. Moreover, the simulations further demonstrate strong thermocapillary flows accompanying growing bubbles with considerable gas contents, which is associated with heat transfer enhancement on the biphilic surface in the low-superheat region.
Highlights
Pool boiling, one of the most common and ubiquitous phase-change phenomena, finds extensive application in a wide range of energy solutions from air-conditioning and refrigeration to nuclear and fusion reactor cooling[1,2,3,4]
The range of effectiveness of boiling as a reliable heat transfer scheme is determined by the critical heat flux (CHF) and the onset of the nucleate boiling (ONB)
By chemically grafting SAM onto the surface, they showed a dramatic decline of the incipient boiling superheat with decreasing wettability. Hypotheses such as the presence of nanobubbles[26] and accelerated molecular mobility[27] on hydrophobic surfaces were advanced as alternative nucleation mechanisms that can possibly account for the apparent deviation from the classical nucleation theory
Summary
One of the most common and ubiquitous phase-change phenomena, finds extensive application in a wide range of energy solutions from air-conditioning and refrigeration to nuclear and fusion reactor cooling[1,2,3,4]. According to the prevailing heterogeneous nucleation model[9], bubbles tend to grow from pre-existing vapour embryos trapped in the defects of the boiling surface Both sufficient surface roughness and liquid superheating (relative to the saturation temperature) have long been considered necessary conditions for bubbles to overcome the interfacial energy barrier and emerge from the mouth of the crevice. By chemically grafting SAM (self-assembled layers) onto the surface, they showed a dramatic decline of the incipient boiling superheat with decreasing wettability Hypotheses such as the presence of nanobubbles[26] and accelerated molecular mobility (leading to greater chances of initiating spontaneous phase change)[27] on hydrophobic surfaces were advanced as alternative nucleation mechanisms that can possibly account for the apparent deviation from the classical nucleation theory. The simulations show that the high gas contents in the bubble give rise to a strong thermocapillary effect, which is deemed partially responsible for moderate heat transfer enhancement in the low-superheat region
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