Abstract
Nearly a century of research on enhancing critical heat flux (CHF) has focused on altering the boiling surface properties such as its nucleation site density, wettability, wickability and heat transfer area. But, a mechanism to manipulate dynamics of the vapor and liquid interactions above the boiling surface as a means of enhancing CHF has not been proposed. Here, a new approach is implemented to limit the vapor phase lateral expansion over the heat transfer surface and actively control the surface wetted area fraction, known to decline monotonically with increasing heat flux. This new degree of freedom has enabled reaching unprecedented CHF levels and revealed new details about the physics of CHF. The impact of wickability, effective heat transfer area, and liquid pressure on CHF is precisely quantified. Test results show that, when rewetting is facilitated, the CHF increases linearly with the effective surface heat transfer area. A maximum CHF of 1.8 kW/cm2 was achieved on a copper structure with the highest surface area among all tested surfaces. A model developed based on the experimental data suggests that the thermal conductivity of the surface structures ultimately limits the CHF; and a maximum CHF of 7–8 kW/cm2 may be achieved using diamond surface structures.
Highlights
Boiling is a ubiquitous mechanism of heat transfer with numerous applications ranging from small-scale HVAC and refrigeration systems used in most buildings to large boilers in energy and process industries
If the bubble growth timescale becomes shorter than the rewetting timescale, the liquid can no longer rewet the surface. This limit may have been reached at heat flux values of 600– 700 W/cm[2] that are approximately 3 times the highest critical heat flux (CHF) values reported in prior pool boiling studies[20, 24,25,26]
The results clearly show that CHF increases with Ar, and a heat flux of ~1.8 kW/cm[2] is reached on structure #5 with the highest Ar
Summary
A century of research on enhancing critical heat flux (CHF) has focused on altering the boiling surface properties such as its nucleation site density, wettability, wickability and heat transfer area. In a recent comprehensive study, Rahman et al.[25] carefully engineered nearly forty surfaces with different wickability levels and clearly demonstrated that CHF increases linearly with the surface wickability They reported a maximum CHF of 260 W/cm[2] on a hierarchical structure with the highest wickability. Forces generated as a result of this phenomenon pull the bubble away from the heated surface This new degree of freedom in manipulating hydrodynamics of the vapor and liquid flow above the heated surface allows controlling the surface wetted area fraction that has been observed to decline monotonically with increasing heat flux[40,41,42,43]. This model is used to predict the highest achievable CHF
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