A New Paradigm for Understanding and Enhancing the Critical Heat Flux (CHF) Limit (original) (raw)
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Boiling and quenching heat transfer advancement by nanoscale surface modification
Scientific reports, 2017
All power production, refrigeration, and advanced electronic systems depend on efficient heat transfer mechanisms for achieving high power density and best system efficiency. Breakthrough advancement in boiling and quenching phase-change heat transfer processes by nanoscale surface texturing can lead to higher energy transfer efficiencies, substantial energy savings, and global reduction in greenhouse gas emissions. This paper reports breakthrough advancements on both fronts of boiling and quenching. The critical heat flux (CHF) in boiling and the Leidenfrost point temperature (LPT) in quenching are the bottlenecks to the heat transfer advancements. As compared to a conventional aluminum surface, the current research reports a substantial enhancement of the CHF by 112% and an increase of the LPT by 40 K using an aluminum surface with anodized aluminum oxide (AAO) nanoporous texture finish. These heat transfer enhancements imply that the power density would increase by more than 100%...
The Hoodoo: A New Surface Structure for Enhanced Boiling Heat Transfer
Journal of Thermal Science and Engineering Applications, 2013
The hoodoo is introduced as a beneficial surface structure for enhancing boiling heat transfer. A full parametric study was conducted to determine which attributes of the hoodoo structure promote boiling heat transfer enhancement. Hoodoo size and spacing were observed to have the most profound effect on boiling heat transfer, nucleation site activation, and critical heat flux (CHF). The CHF enhancement factor, defined as the ratio of CHF on the structured surface to that of a smooth surface, varies from 1.05 to 1.67 for FC-72 and hexane working fluids. Droplet spreading studies confirm the hemiwicking properties of the hoodoo surface, and it is postulated to be the primary mechanism for CHF enhancement. Measured wicking front speeds varied from 12 to 40 mm/s and were observed to obey a power-law dependence on time with an exponent of approximately 0.5. Plausible thermohydraulic mechanisms for CHF enhancement on the hoodoo surfaces are discussed. Downloaded From: http://thermalscienceapplication.asmedigitalcollection.asme.org/ on 01/17/2015 Terms of Use: http://asme.org/terms
2022
Nanofluid flow boiling and critical heat flux (CHF) are interesting topics for many researchers and have industrial application for preventing potential damages. On this basis, numerous experimental studies have been performed on the determination of CHF; however, due to the weakness of the existing correlations for this purpose, an accurate numerical study on CHF is yet needed to be reported. In the present study, the subcooled flow boiling of nanofluid has been simulated to predict CHF. For estimating the nucleation site density (NSD), a correlation is used, into which the liquid properties, flow characteristics and surface properties contribute. The studies were performed at different concentrations of alumina nanoparticles. The simulation results indicated error percentages below 10% in CHF estimation, confirming the reliability of the results. No significant difference was observed between the CHF values calculated in the simulation studies considering constant thermal properties for the nanoparticles, as compared to those obtained with variable thermal properties for the nanoparticles. Taking the tube in a horizontal position, rather than the vertical position, increased the CHF value. However, the increase in CHF was negligible.
CHF as a Non-Equilibrium Drying Transition
The Critical Heat Flux (CHF) phenomenon is the formation of a vapor film between the heater and the liquid when the heat supply exceeds a critical value, the critical heat flux value. We propose a physical explanation for the CHF that is based on the spreading of the dry spot under a vapor bubble. The spreading is initiated by the vapor recoil force, a force coming from the uncompensated mechanical momentum of the fluid molecules being evaporated into the bubble. Since the evaporation intensity increases sharply near the triple (gas-liquid-solid) contact line, the influence of the vapor recoil can be interpreted in terms of an increase of the apparent contact angle. As the vapor recoil force is always directed towards the liquid side, it increases the dry spot under the bubble. Therefore, for the usual case of complete wetting of the heating surface by the liquid, the CHF can be understood as an out of equilibrium drying transition from complete to partial wetting. The value of the CHF should be close to that defined by the equality of the contributions of the vapor recoil and the surface tension. We present the results of a 2D numerical simulation of the bubble growth at high system pressure when the bubble is assumed to grow slowly, its shape being defined by the surface tension and the vapor recoil force. The numerical results confirm this physical mechanism of the boiling crisis. Near the gas-liquid critical point of a given fluid, the bubble growth is very slow. The increase of the apparent contact angle was observed experimentally together with the growth of the dry spot, thus confirming the proposed explanation.
Boiling Heat Transfer: Convection Controlled by Nucleation
Heat Transfer - Models, Methods and Applications, 2018
Due to the peculiar way of evolution of boiling heat transfer research, a model "theater of director" (MTD), pumping effect of growing bubble (PEGB) and MTD-based universal correlation (UC) remain beyond the attention of researchers for more than half a century. In parallel, there are periodic fundamental events, demonstrating the irrationality of such indifference. Since the 1980s, not having found a way to enhance boiling heat transfer, other than that uncovered by the MTD-UC, high-performance boiling surfaces are being developed by artificially increasing effective radius (ER) of nucleation centers (bypassing the reference to the relevant theoretical basis). In 2009, an independent review declares transient conduction and microconvection as the dominant boiling heat transfer mechanism, not knowing that this is just the PEGB. In 2014-2017, the real versatility and accuracy of the UC is confirmed by independent studies, which involve extensive databases on the pool and flow boiling (with some interpretation problems). Assessing the current status of the study, the chapter emphasizes the complete fiasco of traditionally adopted approaches, models and theories, led to the dominance of purely empirical relationships written in a dimensionless form. Heat transfer research community is invited to gain will and rid of the heavy burden of the past.
International Journal of Heat and Mass Transfer, 2020
Boiling is a ubiquitous process in many applications including power generation, desalination, and high-heat flux electronic cooling. At the same time, boiling is a complicated physical process involving hydrodynamics and interfacial heat and mass transfer on multiple scales. One of the key limiting factors of boiling is the critical heat flux (CHF), beyond which a vapor blanket forms on the heating surface and catastrophic device burnout occurs. Yet, detailed understanding of the mechanism that triggers CHF remains elusive. In this paper, we elucidate the CHF mechanism by studying the evolution of wet/dry region on the heater surface using lattice Boltzmann simulations. We incorporate the equation of state for real gases in the liquid-vapor phase change model for direct numerical simulation of the CHF phenomenon. The results of this framework clarify the difference between the triggering mechanism of CHF and film boiling by analyzing the pool boiling curve. We demonstrate that the heat flux of the wet region on the heater surface increases while the wet area fraction decreases with superheat, leading to the CHF. We show that a vapor recoil force due to the interfacial heat and mass transfer plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF. Compared with previous CHF models which treat CHF as an isolated point on the boiling curve, this work elucidates the triggering mechanism of CHF from a perspective of the dynamic evolution of the wet/dry region with increasing superheat, which could potentially serve as a guideline for future CHF enhancement designs.
Scientific Reports, 2018
Boiling is a key heat transfer process for a variety of power generation and thermal management technologies. We show that nanopillar arrays fabricated on a substrate enhance both the critical heat flux (CHF) and the critical temperature at CHF of the substrate and thus, effectively increase the limit of boiling before the boiling crisis is triggered. We reveal that the enhancement in both the CHF and the critical temperature results from an intensified rewetting process which increases with the height of nanopillars. We develop a predictive model based on experimental measurements of rewetting velocity to predict the enhancement in CHF and critical temperature of the nanopillar substrates. This model is critical for understanding how to control boiling enhancement and designing various nanostructured surfaces into specific applications.
Critical heat flux (CHF) enhancement by surface modifications has been an extensively researched area in pool boiling heat transfer. Here we report a fundamental mechanism of CHF enhancement where nano/micro ridges are fabricated on surfaces to fragment and evaporate the metastable non-evaporating/adsorbed film present at the base of a bubble in the contact line region. CHF increase of 125125% is obtained with only 12540% increase in surface area. An analytical model is extended to explain the CHF enhancement and to determine the average non-evaporating film thickness, which serves as the critical height for nano/micro structures for pool boiling heat transfer enhancement. V C 2013 AIP Publishing LLC. [http://dx.