Numerical simulation of boiling enhancement on a microstructured surface

Significant efforts have been made to augment nucleate boiling by surface modification with micro-machined structures, but a general predictive approach for heat transfer enhancement has not yet been developed. In this work, complete numerical simulations are performed for boiling enhancement on a microstructured surface by employing the sharp-interface level-set method, which is modified to handle the contact angle and the evaporative heat flux from the liquid microlayer on an immersed solid surface. The effects of cavity diameter and surface modification such as concentric grooves and multi-step cavities on bubble growth and boiling heat transfer are investigated.     

Models for enhanced boiling heat transfer by unusual Marangoni effects under microgravity conditions

A new approach is suggested to enhance boiling heat transfer through introduction of unusual surface tension effects. The surface tension of aqueous solutions of alcohols with a chain length longer than four carbon atoms offers a positive gradient with temperature when the temperature exceeds a certain value. Moreover, the positive gradient near the boiling point has a very large value. This will generate a considerable driving force for bubble departure. As a result, in the nucleate boiling of these solutions, the Marangoni effect around the bubble surface will not impede the bubble detachment from the heater surface but rather will provide an additional driving force for the bubble departure. This effect combines with the buoyancy under normal gravity and acts as a main driving force of bubble departure in microgravity. Models for predictions of the bubble detachment diameters, the nucleate boiling heat transfer coefficient, and the critical heat flux are developed.     

Numerical study of bubble growth and boiling heat transfer on a microfinned surface

The bubble growth and boiling heat transfer on a microfinned surface are studied numerically by solving the conservation equations of mass, momentum and energy. The bubble shape is tracked by a sharp-interface level-set method, which is modified to include the effect of phase change and to treat the contact angle and microlayer heat flux on an immersed solid surface. The present computation demonstrates that the microfinned surface enhances boiling heat transfer significantly compared to a plain surface. The effects of fin spacing and height on the bubble growth and heat transfer are investigated to find the optimal conditions for boiling enhancement.     

Surface wettability control by nanocoating: The effects on pool boiling heat transfer and nucleation mechanism

Experiments were performed to highlight the influence of surface wettability on nucleate boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20° to 110° by modifying nanoscale surface topography and chemistry. The bubble growth was recorded by a high speed video camera to enable a better understanding of the surface wettability effects on nucleation mechanism. For hydrophilic (wetted) surfaces, it was found that a greater surface wettability increases the vapour bubble departure radius and reduces the bubble emission frequency. Moreover, lower superheat is required for the initial growth of bubbles on hydrophobic (unwetted) surfaces. However, the bubble in contact with the hydrophobic surface cannot detach from the wall and have a curvature radius increasing with time. At higher heat flux, the bubble spreads over the surface and coalesces with bubbles formed at other sites, causing a large area of the surface to become vapour blanketed. The best heat transfer coefficient is obtained with the surface which had a water contact angle close to either 0° or 90°. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer.     

Confinement effects on nucleate boiling and critical heat flux in buoyancy-driven microchannels

Pool boiling and CHF experiments were performed for vertical, rectangular parallel-plate channels immersed in the dielectric liquid FC-72 at atmospheric pressure to elucidate the effects of geometrical confinement in immersion cooled electronics applications. Heat transfer enhancement in the low flux region of the nucleate boiling curve was observed for channel spacings near and below expected bubble departure diameters, but was widely different for two different heater materials. Relative degradation of CHF with decreasing channel spacing was found to be a strong function of channel aspect ratio and independent of surface material and finish.     

A Level Set Method for Analysis of Film Boiling on an Immersed Solid Surface

A numerical method is presented for simulating film boiling on an immersed (or irregularly shaped) solid surface. The level set formulation for tracking the phase interfaces is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip condition at the fluid–solid interface. The boundary or matching conditions at the phase interfaces are accurately imposed by incorporating the ghost fluid approach based on a sharp-interface representation. The numerical method is tested through computations of bubble rise in a stationary liquid, single-phase fluid flow past a circular cylinder, and film boiling on a horizontal cylinder.     

核态池沸腾中气泡生长和脱离的动力学特征_气泡的脱离直径与脱离频率

在前人工作的基础上提出了表征核态池沸腾中气泡脱离和生长过程的特征时间和特征尺度,并进而得到了气泡生长时间和气泡脱离直径的通用关系式,应用传热学类比方法建立了计算气泡脱离直径的一般公式。本文的研究结果与前人的实验结果甚为相符。     

Numerical and experimental investigation of heat transfer on heating surface during subcooled boiling flow of liquid nitrogen

Closure correlations describing bubble nucleation and departure on the heating surface is indispensable when modeling subcooled boiling flow using a two-fluid model. Due to the small contact angle and surface tension, nucleation and departure of nitrogen vapor bubble has different characteristics to those of high-boiling liquids. For the purpose of accurate two-fluid model formulation, these factors have to be taken into consideration. In this study, some closure correlations of the bubble departure diameter, active site density and bubble waiting time were tested in the frame of the two-fluid model and the CFX code. Benchmark experiments were then performed to evaluate the correlations. Comparison of the numerical results against the experimental data demonstrates that the surface tension is crucial to modeling the bubble departure diameter and the active site density. The bubble waiting time correlation formulated according to bubble growth is expected to be used as a criterion of judging the transition from subcooled to saturated boiling.     

LDV assisted bubble dynamic parameter measurements from two enhanced tubes boiling in saturated R-134a

LDV measurements and heat transfer experiments for nucleate pool boiling from two horizontal enhanced tubes with porous copper on copper surfaces immersed in saturated R-134a were conducted. The influence of tube position (alignment) and tube pitch on bubble dynamics and boiling characteristics were studied. Photographs indicate that the average number of bubbles increases with heat fluxes, which is the same as those in previous studies of single tube. However, the bubble departure diameters of the upper tube show an opposite trend with an increase as compared to previous single-tube studies. LDV measurements show that the present tube arrangement has significant influence on local velocity in both magnitude and trend. The heat transfer mechanism and modeling for the upper tube were studied and developed.     

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

Surfaces with spatial wettability patterns have been proven to enhance heat transfer coefficient and critical heat flux in pool boiling. To understand the physical mechanism behind this phenomenon and obtain the correlation among some critical parameters (bubble departure frequency, bubble size, nucleation site density, surface tension), pool boiling experiments were conducted. A Pyrex glass with a layer of indium-tin-oxide was used as the substrate. Hydrophobic patterns will serve as nucleation sites. Experiments were conducted in deionized water under atmospheric pressure at a relatively low heat flux. The processes of nucleation, growth, and departure of individual bubbles were visualized by using a high speed camera through the bottom of the heater surface. It has been found that the patterned surface performed the best in heat transfer for subcooled pool boiling when compared with hydrophilic and hydrophobic surfaces. The nucleation site density of the biphilic surface was much higher, when compared with that of the homogeneous surface. The individual bubbles always nucleate on the edge of the hydrophobic and hydrophilic area, and then move onto the hydrophobic pattern. Most of the individual bubbles detach from the wettability patterned surface in the diameter range from 300 µm to 450 µm (around 77.3%). The bubble departure periods scatter in the range from 80 ms to 1500 ms.     

Numerical Simulation of Bubble Growth and Heat Transfer During Flow Boiling in a Surface-Modified Microchannel

Flow boiling in a microchannel without or with surface modifications, such as fins, grooves, and cavities, has received significant attention as an effective cooling method for high-power microelectronic devices. However, a general predictive approach for the boiling process has not yet been developed because of its complexity involving the bubble dynamics coupled with boiling heat transfer in a microscale channel. In this study, direct numerical simulations for flow boiling in a surface-modified microchannel are performed by solving the conservation equations of mass, momentum, and energy in the liquid and vapor phases. The bubble surfaces are determined by a sharp-interface level-set method, which is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip and contact-angle conditions on immersed solid surface of microstructures. This computation demonstrates that the surface-modified microchannel enhances boiling heat transfer significantly compared to a plain microchannel. The effects of various surface modifications on the bubble growth and heat transfer are investigated to find better conditions for boiling enhancement.     

Direct numerical simulation of flow boiling in a finned microchannel

Direct numerical simulations of bubble growth and heat transfer associated with flow boiling in a finned microchannel are performed by solving the conservation equations of mass, momentum and energy in the liquid and vapor phases. The phase interfaces are determined by a sharp-interface level-set method which is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip and contact-angle conditions on the immersed solid surface of fins. The effects of fin height, spacing, and length on the flow boiling in a microchannel are investigated to find the better conditions for heat transfer enhancement.     

Effect of nucleation site spacing on the pool boiling characteristics of a structured surface

Pool boiling characteristics of pyramidal shaped re-entrant cavities (characteristic size 40 μm) etched in silicon were evaluated in this study. A test surface was fabricated to totally eliminate back heat loss and minimize spreading in the substrate. The effect of inter-cavity spacing and convection plumes from a heat source located below the test surface on nucleate boiling parameters is documented. High speed photography was used to record and quantify the bubble departure frequency, the departure diameter, the active site density and to observe the effect of interaction between neighboring nucleation sites. Experiments were conducted in saturated FC 72 at atmospheric pressure.     

Heat transfer during pool boiling based on evaporation from micro and macrolayer

An analytical model of heat transfer based on evaporation from the micro and macrolayers to the vapor bubble during pool boiling is developed. Evaporation of microlayer and macrolayer during the growth of individual bubbles is taken care of by using temporal and spatial variation of temperature in the liquid layer. Change of bubble shape during the entire cycle of bubble growth and departure is meticulously considered to find out the rate of heat transfer from the solid surface to the boiling liquid. Continuous boiling curve is developed by considering the bubble dynamics and decreasing thickness of liquid layer along with the increase of dry spot radius. Transient variation of macrolayer and microlayer thickness is predicted along with their effect on CHF. Present model exhibits a good agreement with reported experimental data as well as theories.     

Nucleation site interaction between artificial cavities during nucleate pool boiling on silicon with integrated micro-heater and temperature micro-sensors

Nucleate boiling is commonly characterised as a very complex and elusive process. Many involved mechanisms are still not fully understood and more detailed consideration is needed. In this study, bubble growth from micro-fabricated artificial cavities with varied spacing on a horizontal 380 μm thick silicon wafer was investigated. The horizontally oriented boiling surface was heated by a thin resistance heater integrated on the rear of the silicon test section. The temperature was measured using 16 integrated micro-sensors situated on the boiling surface, each with an artificial cavity located in its geometrical centre. Experiments with three different spacings 1.5, 1.2 and 0.84 mm in between cavities with a nominal mouth diameter of 10 μm and a depth of 80 μm were undertaken. To conduct pool boiling experiments, the test section was mounted inside a closed stainless steel boiling chamber with optical access and completely immersed in degassed fluorinert FC-72. Bubble nucleation, growth and detachment at 0.5 and 1 bar absolute pressure were investigated using high-speed imaging. The effect of decreasing inter-site distance on bubble nucleation frequency, bubble departure frequency and diameter with increasing wall superheat is presented. Furthermore, the frequency of horizontal bubble coalescence was determined. The regions of influence on the measured frequencies and bubble departure diameter were compared with recently published findings.     

Effects of heater size and working fluids on nucleate boiling heat transfer

The present study is an experimental investigation of nucleate boiling heat transfer mechanism in pool boiling from wire heaters immersed in saturated FC-72 coolant and water. The vapor volume flow rate departing from a wire during nucleate boiling was determined by measuring the volume of bubbles from the wire utilizing the consecutive-photo method. The effects of the wire size on heat transfer mechanism during a nucleate boiling were investigated, varying 25 μm, 75 μm, and 390 μm, by measuring vapor volume flow rate and the frequency of bubbles departing from a wire immersed in saturated FC-72. One wire diameter of 390 μm was selected and tested in saturated water to investigate the fluid effect on the nucleate boiling heat transfer mechanism. Results of the study showed that an increase in nucleate boiling heat transfer coefficients with reductions in wire diameter was related to the decreased latent heat contribution. The latent heat contribution of boiling heat transfer for the water test was found to be higher than that of FC-72. The frequency of departing bubbles was correlated as a function of bubble diameters.     

Temperature fluctuation at twin cavity in nucleate boiling—wavelet analysis and modelling

In the paper the wavelet analysis of heating surface temperature fluctuation at the artificial cavities in nucleate boiling has been carried out. It has been shown that depending on distance between cavities the process of bubbles departure from twin cavity can synchronize and create the unified conditions for heat transfer around the cavities. It has been found that synchronization of interacting cavities leads to decrease in mean frequency of bubbles departure. The new concept of promotion or inhibition of interacting cavities has been defined based on the wavelet analysis. Two-dimensional boiling field was approximated and modeled using CML method. Two mechanisms of interaction between bubble columns have been considered: the thermal and hydrodynamic ones. The simulation shows that thermal interaction between two cavities and bubble columns leads to deactivation of neighboring cavity whereas the hydrodynamic interaction sustains the activity of neighboring cavity.     

Quasi-homogeneous boiling nucleation on a small spherical heater in microgravity

Pool boiling experiments were conducted in the European Space Agency (ESA) multi-user facility, the bubble, drop, particle unit (BDPU) in the microgravity environment of space. A part of the study involved the heating of a small sphere immersed in R-123 to the onset of nucleate boiling. An analysis of the nucleation process is presented, based on a prior work for so-called quasi-homogeneous nucleation with a flat heater surface in microgravity. Reasonably good qualitative agreement exists between the analysis and measurements.     

Bubble Dynamics,Flow, and Heat Transfer during Flow Boiling in Parallel Microchannels

Significant efforts have recently been made to investigate flow boiling in microchannels, which is considered an effective cooling method for high-power microelectronic devices. However, a fundamental understanding of the bubble motion and flow reversal observed during flow boiling in parallel microchannels is lacking in the literature. In this study, complete numerical simulations are performed to further clarify the boiling process by using the level-set method for tracking the liquid–vapor interface which is modified to treat an immersed solid surface. The effects of contact angle, wall superheat, and the number of channels on the bubble growth, reverse flow, and heat transfer are analyzed.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：PartI－Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

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