Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

Enhancement of the critical heat flux in saturated pool boiling using honeycomb porous media

Among the different ways of achieving heat transfer enhancement in pool boiling, modification of surface roughness and the associated change in the surface wettability are two widely studied methods. It is well established that microstructure can play an important role in varying surface roughness and improving interfacial wettability. In the present study, pool boiling heat transfer on a number of micro grooved and flat (brass and copper) surfaces with water as the working fluid is investigated experimentally for enhancement of critical heat flux. The micro structured sample surfaces feature parallel grooves having dimensions of tens of microns and are fabricated using a mechanical micromachining process. Classical pool boiling experiments are conducted for three different values of heat flux (75 kw/m2, 100 kw/m2, 125 kw/m2). The bubble nucleation process is visualized and the bubble departure diameter is measured with the help of a high speed camera. It is observed that the nucleation site density and departing bubble diameter increases, while the bubble departure frequency decreases with an increasing heat flux for both kind of surfaces and materials. Comparison of the results among the flat and micro-grooved surfaces reveals that the boiling on micro-grooved surfaces results in higher heat transfer coefficient, more nucleation sites, lower bubble departure frequency and larger bubble diameter under similar operating conditions.Among the different ways of achieving heat transfer enhancement in pool boiling, modification of surface roughness and the associated change in the surface wettability are two widely studied methods. It is well established that microstructure can play an important role in varying surface roughness and improving interfacial wettability. In the present study, pool boiling heat transfer on a number of micro grooved and flat (brass and copper) surfaces with water as the working fluid is investigated experimentally for enhancement of critical heat flux. The micro structured sample surfaces feature parallel grooves having dimensions of tens of microns and are fabricated using a mechanical micromachining process. Classical pool boiling experiments are conducted for three different values of heat flux (75 kw/m2, 100 kw/m2, 125 kw/m2). The bubble nucleation process is visualized and the bubble departure diameter is measured with the help of a high speed camera. It is observed that the nucleation site density and d...

Over the past decades, pool boiling on various wetting surfaces has been intensively investigated to enhance boiling heat transfer and critical heat flux. In this study, to enhance the two thermal performances simultaneously, we developed hydrophilic micro/nanotextured surfaces with hydrophobic patterns. Using a silicon substrate, well-arrayed microtextures and randomly arrayed nanotextures were fabricated hierarchically using micro/nanoelectromechanical system processes. The top of the microtextures was coated locally with hydrophobic characteristics using specific self-assembled monolayer coating methods. Based on experimental data, we postulate that the critical heat flux was enhanced by the capillary-induced flow between microtextures and that nanotextures with superhydrophilicity contribute to the delay of the critical heat flux by better wetting the dried area. Owing to the hydrophobicity at the top of the micropillars, the nucleate site density and boiling heat transfer increased.

Using deionized water as working medium, studies on visualization of the behavior of subcooled boiling bubbles in narrow rectangular channels have been conducted at atmospheric pressure, with the help of a high-speed digital camera. The effects of wall superheat, fluid subcooling and mass flux on the bubble dynamic model of nucleation site density, bubble departure frequency and bubble departure diameter were investigated. This paper analyzed the visual graphic results and obtained quantitative values about bubble nucleation site density, bubble departure frequency and bubble departure diameter under different conditions. Based on the results, wall nucleate boiling models in the conventional channel put forward by Nilanjana Basu have been corrected. The new correlation agrees reasonably well with existing experimental data in 2mm narrow rectangular channel.

The contact area of bubbles with a transparent heating surface was optically measured during subcooled pool boiling of water on the ground. In the experiments, boiling bubbles were attached to the heating surface with a bubble holder and nearly reproduced the bubble behavior observed in low gravity. DC power was applied to the ITO heater and increased until the heater surface burned out. In quick heating, that is about 20 second until burnout and equal to the heating time during the low gravity period, the contact area was smaller than that for long time heating at the same heat flux. The experimental results suggest the reason why the critical heat flux in pool boiling is higher than the widely accepted predictions in microgravity. In a drop shaft experiment with constant heating, the contact area increased dramatically at the start of microgravity and became constant. Boiling bubbles coalesced and remained just over the heating surface.

The numerical simulation of subcooled flow boiling of R-113 working fluid has been done for two different nanofluids (R-113/Al2O3, R-113/TiO2) under different volume concentrations (0.5%, 1%, and 3%). The numerical results were compared with experimental results obtained by previous researchers, and the comparison shows that the numerical results are in good accordance. Nucleation site density, bubble departure frequency, and bubble departure diameter, which are three key parameters, are investigated in this study. The results express that these three parameters have the highest variation at low Reynolds numbers. The influence of different nanoparticles concentrations on the variation of the heat transfer coefficient is studied. The results indicate there is an insignificant difference between the effect of 1% and 3% concentrations on the heat transfer coefficient that means an increase of nanoparticles more than 1% concentration cannot improve heat transfer. The effect of different non-drag forces such as lubrication force, turbulent dispersion force, and lift force is also studied. Two correlations are proposed for predicting the convective heat transfer coefficient.

Subcooled flow boiling is one of the major issues in the nuclear and power generation industries. If the fluid inlet temperature in the boiling area is less than the boiling temperature, the boiling process is called subcooled boiling. The symmetry of a physical system is a constant property of the system and is fixed by deformation. Using magnetohydrodynamic (MHD) forces and broken symmetry induced by nanosized particles, fluid and thermal systems can be more controlled. In this study, the effect of a magnetic field and nanoparticles on subcooled flow boiling in a vertical tube was investigated. For this purpose, a one-dimensional numerical code was used to simulate the flow and variations of various parameters that have been investigated and evaluated. The results showed that as the flow entered the heated area, the vapor volume fraction, Froude number, fluid cross-sectional area forces, mixture velocity, fluid velocity, bubble departure diameter, liquid and vapor Reynolds numbers, squared ratio of the Froude number to the Weber number, and fluid cross-sectional area forces coefficient increased. In the same region, the Eötvös number, root mean square (RMS) of the fluid cross-sectional area force, sound velocity, liquid superficial velocity, critical tube diameter, bubble departure frequency, and density of the active nucleation site were reduced. It was also observed that after the heated area and under the influence of the magnetic field and the nanoparticles, the values of the vapor volume fraction, Froude number, fluid cross-sectional area force, mixture velocity, fluid velocity, vapor, liquid Reynolds number, and squared ratio of the Froude number to the Weber number were decreased. Moreover, there was no significant effect on the Eötvös number, liquid superficial velocity, Taylor bubble Sauter mean diameter, bubble departure diameter, critical tube diameter, bubble departure frequency, or density of the active nucleation site.

Layer-by-layer carbon nanotube coatings for enhanced pool boiling heat transfer on metal surfaces

The evaporation and pool boiling on micro porous coated surfaces have been shown to provide among the highest heat transfer rates achievable from any type of surfaces. The heat transfer modes in these surfaces, present a number of interesting similarities and also, some fundamental differences, which are the result of the liquid supply methods to the heated surface. For the evaporation from porous coated surfaces, the liquid return to the heated surface is assisted by the capillary pressure at the liquid-vapor interface; while for pool boiling, gravity is the principal driving force that rewets the surface. In order to better understand the physical phenomena that governs the flow behavior of both the liquid and vapor phases, and the heat transfer process inside the porous media, comprehensive comparisons between these return mechanisms and their respective characteristics, and the performance and the critical heat flux (CHF) for each have been made, based on similar physical situations. These systematic comparisons illustrate that at a lower heat flux, the evaporation and pool boiling curves are almost identical due to the similar heat transfer modes, i.e., convection and nucleate boiling. While with further increases in heat flux, the heat transfer performance of the evaporation on micro porous media is generally superior to pool boiling on an identical surface. This shift is believed to be due to the fact that for evaporation on micro porous media, the heat transfer mode is dominated by the film evaporation, while in pool boiling, it is principally the result of fully developed nucleate boiling. It was also observed that the impact of the effective thermal conductivity of the porous coating on pool boiling performance is larger than for evaporation heat transfer on the identical micro porous coated surfaces. In general, the experimental data indicated that the CHF for evaporation heat transfer is much higher than for pool boiling on the same surfaces. The mechanism of CHF for evaporation on porous coated surfaces is believed to be the capillary limit; while for pool boiling the limit is the result of the hydrodynamic instabilities. This difference in mechanisms is clearly demonstrated by the experimental observations, where initially, the dry out process of the porous coated surfaces during evaporation is gradual, while for pool boiling; the entire surface reaches dry out in a very short time. In addition, the sensitivity of the CHF to the thickness of the porous coatings at a constant volumetric porosity and pore size, as well as the various optimal volumetric porosity of the CHF at a given thickness, are clearly the results of the differences induced by the various CHF mechanisms.

Analysis of vapor bubble diameter, departure frequency and dynamics in a single cavity

The trends of decrease in size and increase in power dissipation for micro-electronic systems present a significant challenge for thermal management of modern electronics. The preferable cooling solution could be micro heat exchangers based on forced flow boiling. Nanoparticle deposition can affect nucleate boiling heat transfer coefficient via alteration of surface thermal conductivity, roughness, capillary wicking, wettability, and nucleation site density. It can also affect heat transfer by changing bubble departure diameter, bubble departure frequency, and the evaporation of the micro and macrolayer beneath the growing bubbles. In this study, flow boiling was investigated for 0.001 vol% aluminum oxide nanofluids in a brass microchannel and compared to results for regular water. For the case of nanofluid flow boiling, high speed images were taken after boiling durations of 25, 75, 125, and 150 min. Bubble growth rates were measured and compared for each case. Flow regime oscillation was observed and regime duration was split into two periods: single-phase liquid and two-phase. The change in regime timing revealed the effect of nanoparticle suspension and deposition on the Onset of Nucelate Boiling (ONB) and the Onset of Bubble Elongation (OBE). The addition of nanoparticles was shown to stabilize bubble growth as well as the transition of flow regimes between liquid, two-phase, and vapor.

An experimental study of nanostructure modified nucleation site density and contact angle that significantly enhances the Heat Transfer Coefficient (HTC) and the Critical Heat Flux (CHF) in pool boiling heat transfer of water on copper surfaces has been conducted. The nanostructures on copper surfaces have been created by an electrodeposition technique. It has been found that the nanostructured copper surfaces show an increase in CHF of up to 142% and an increase in HTC of 33% over that of a mirror-finished plain copper surface. Calculations for nucleation site density and active nucleation site diameter reveal a direct correlation between these factors and the HTC, as well as the CHF. More interestingly, a contact angle study on the tested surfaces shows that there is a connection between the contact angle reduction and CHF enhancement, which confirms the contact angle mechanism of CHF with experimental evidence.

Saturated flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip

We develop a neural network model capable of predicting the margin to the boiling crisis (i.e., the departure from nucleate boiling ratio, DNBR) from high-resolution infrared measurements of the bubble dynamics on surfaces with different morphologies and wettability (or wickability). We use a feature ranking algorithm, i.e., minimum redundancy maximum relevance, to elucidate the importance of fundamental boiling parameters, i.e., nucleation site density, bubble departure frequency, growth time, and footprint radius, in predicting the boiling crisis. We conclude that these parameters are all necessary and equally important. This result has profound implications, as it undermines the general validity of many observations and mechanistic models that attempt to predict the critical heat flux (CHF) by describing how a single boiling parameter changes with the heat flux or from one surface to another. Notably, the neural network model can predict the DNBR on CHF-enhancing surfaces of different wickability without using any input information related to the surface properties. This result suggests that, at least on the considered surfaces, surface wickability enhances the CHF by modifying the bubble dynamics, i.e., the aforesaid boiling parameters, rather than acting as an additional heat removal mechanism.

This study fabricated the in-situ Cu(OH)2 hierarchical nanograss surface (HNS) via immersion method for simultaneous enhancement of the critical heat flux (CHF) and heat transfer coefficient (HTC) in diverse experimental conditions. Nanograss stripes were patterned, and then in-situ grown on the copper substrate through the chemical modification. The effect of the Cu(OH)2 HNS on the pool boiling heat transfer performance was systematically examined. Furthermore, the optimized HNS which can exhibit the highest CHF and HTC was identified based on comparative experiments. It is found that the numerous nanoscale cavities existing in the HNS can act as the active nucleation sites for facilitating the boiling process. Experimental results reveal that the HNS can reduce the independent bubble departure diameter, increase the bubble departure frequency and significantly delay the bubble mergence due to much improved capillary pumping and replenishment of cooling liquid. According to the experiments, the CHF on the optimized HNS sample, were improved by 97.8% compared with the smooth surface and the HTC is enhanced to 2.4 W/cm2K, which is equivalent to an augmentation of 25.7% compared to that on smooth surface. Meanwhile, the HNS can improve the heat transfer uniformity and stability with temporal temperature variations less than 1 K at CHF, which is pivotal to the efficient thermal management of miniaturized devices.