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The suppression effect of easy-to-activate nucleation sites on the critical heat flux in pool boiling
(2018-07-01)
Liu, Yi
;
Lu, Ming-Chang
;
Xu, Dongyan
;
機械工程學系
;
Department of Mechanical Engineering
We report experimental studies of the role of easy-to-activate nucleation sites in pool boiling on hybrid hydrophobic-hydrophilic surfaces. Microstructured surfaces with controlled wettability and nucleation sites are designed and fabricated to investigate their individual and collective effects on the boiling heat transfer performance. Our results reveal that excellent critical heat flux (CHF) up to 231.5 W/cm(2) is attainable only when the surface has good wettability but is free of easy-to-activate nucleation sites. However, with the presence of easy-to-activate nucleation sites on the boiling surface, only limited CHFs can be achieved irrespective of the surface wettability, suggesting a suppression effect of nucleation sites on the CHF. Based on the previous experimental observation of the dynamic network-like wetting pattern at high heat fluxes, a wetting-deterioration mechanism is proposed to explain the suppression effect of easy-to-activate nucleation sites on the CHF. Easy-to activate nucleation sites continuously generate bubbles that pose a large flow resistance for the liquid to wet the hot surface and thus lead to early dry-out.
Enhanced immersion cooling using two-tier micro- and nano-structures
(2018-02-25)
Hsu, Ya-Tzu
;
Li, Jia-Xiong
;
Lu, Ming-Chang
;
機械工程學系
;
Department of Mechanical Engineering
Continual increases in the functionality and miniaturization of electronic devices have resulted in a rapid increase in the power density of such devices. Thus, an efficient cooling technology is required to maximize heat dissipation and prevent electronic failure. Immersion cooling is a promising technique for the thermal management of high-power-density electronics. However, common working fluids in immersion cooling have high global warming potential, and the heat transfer performance of immersion cooling requires improvement to achieve efficient cooling of state-of-the-art high-power-density electronics. In this study, Novec 649, which has low global warming potential and a low boiling point, was applied as a working fluid for immersion cooling. A Si nanowire (SiNW) array, Si micropillar (SiMP) array, and Si nanowires on a Si micropillar (SiNW/MP) two-tier structure were employed to enhance boiling performance. The SiMP surface exhibited the highest critical heat flux (CHF) of 23.5 +/- 1.3 W/cm(2), whereas the SiNW surface exhibited the lowest CHF but a relatively high heat transfer coefficient (HTC). The SiNW/MP surface exhibited the highest HTC of 23611.7 +/- 1586.2 W/m(2) K and a relatively large CHF of 17.4 +/- 1.2 W/cm(2). Compared with a plain SiO2 surface, the CHF and HTC of the SiNW/MP two-tier structure could be enhanced by 30% and 455%, respectively. These results suggest that the SiNW/MP surface is effective for enhancing immersion cooling. (C) 2017 Elsevier Ltd. All rights reserved.
Transient oscillatory saturated flow boiling heat transfer and associated bubble characteristics of FC-72 over a small heated plate due to heat flux oscillation
(2012-01-31)
Wang, S. L.
;
Chen, C. A.
;
Lin, Y. L.
;
Lin, T. F.
;
機械工程學系
An experiment is carried out here to investigate the transient oscillatory boiling heat transfer and associated bubble characteristics of FC-72 flow over a small circular plate subject to a time varying heat flux with the plate flush mounted on the bottom of a horizontal rectangular channel. At the inlet the flow is maintained at saturated liquid state with zero vapor quality. The imposed heat flux oscillates periodically with time in the form of rectangular waves. In the experiment the FC-72 mass flux G varies from 300 to 400 kg/m(2)s, mean imposed heat flux (q) over bar ranges from 0 to 10 W/cm(2) and the amplitude of the heat flux oscillation Delta q is fixed at 10-50% of (q) over bar with the period of the heat flux oscillation varied from 10 to 30 s. The experimental results show that the time-averaging FC-72 oscillatory boiling heat transfer characteristics resemble that for stable flow boiling. However, the imposed heat flux oscillation causes significant temporal oscillations in the heated plate temperature, boiling heat transfer coefficient, bubble departure diameter and frequency, and active nucleation site density. These physical quantities oscillate at the same frequency as the heat flux oscillation and at a higher (q) over bar, a larger Delta q/(q) over bar, and a longer t(p) they exhibit stronger oscillations. Besides, a slight time lag in T-w oscillation is seen. Moreover, the size of departing bubbles, active nucleation site density and bubble departure frequency decrease as the heat flux is reduced to the low level of (q) over bar - Delta q. The opposite processes take place for the heat flux raised to the high level of (q) over bar + Delta q. Furthermore, at the mean imposed heat flux close to that for the ONB in the stable boiling we observe intermittent boiling in the flow. A regime map is provided to delineate the boundaries among single-phase liquid flow, intermittent boiling and persistent boiling. (C) 2011 Elsevier Ltd. All rights reserved.

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