Relative Stability of Boiling of FC—72 and HFE—7100 with Applications to Electronic Device Cooling

IntroductionBoiling of different modes can coexist on the sameheating element under certain conditions. In pool boilingwith an electrically heated wire, steady-state andunsteady-state two-mode boiling (nucleate and filmboiling) has been studied in detailll-4]. There exists aunique "equilibrium line" along which the nucleate andfilm boiling modes could steadily coexist. Furthermore,the intersections between the equilibrium line and theboiling curves have separated respectively the nucleateand f…     

Features of Boiling on an Artificial Surface—Bubble Formation, Wall Temperature Fluctuation and Nucleation Site Interaction

The artificial surfaces are applied to study the pool boiling features, including the bubble behaviors, the surface temperature fluctuation, the heat transfer characteristics and nucleate site interaction. Three sets of experiments are carried out to investigate the influences of cavity shape, cavity size, cavity spacing on the boiling phenomena. Experimental results reveal that bubbling from the cylindrical as well as reentrant cavity is generally stable. The influence of cavity diameter on the bubble behaviors and the temperature fluctuation seems very weak while the effect of cavity depth cannot be neglected. As for the two cavity conditions, the bubble behaviors show the different features depending on the dimensionless cavity spacing. Three significant factors (thermal interaction, hydraulic interaction, bubble coalescence) control the nucleation site interaction, and the competition and dominance of the factors yield four interaction regimes.     

Experimental Study on Jet Impingement Boiling Heat Transfer in Brass Beads Packed Porous Layer

Jet impingement boiling has been widely used in industrial facilities as its higher heat transfer coefficient(HTC)and critical heat flux(CHF)can be achieved in comparison with the pool boiling.By covering beads packed porous layer on the heated wall surface,the enlarged heat transfer area and rise of nucleation sites for boiling occur,thus,the heat transfer performance of boiling can be enhanced.For the jet impingement boiling with brass bead packed porous layers,the heat transfer performance is crucially influenced by the characteristics of porous layer and working fluid flow,so the experiments were conducted to investigate the effects of the jet flow rate,fluid inlet subcooling,number of porous layer and brass bead diameter of porous layer.Comparison study shows that impingement boiling promotes the HTC and CHF as 1.5 times and 2.5 times respectively as pool boiling at similar conditions.Higher heat transfer performance can be obtained in the cases of a higher jet flow rate and a higher fluid inlet subcooling,and there exist the optimal layer number and bead diameter for heat transfer.Particularly,a double-layer porous layer results in an increase of 39%in heat flux at superheat of 30 K compared with a single-layer case;a single porous layer at d=8 mm brings an increase of 23%in heat flux at superheat of 30 K compared with that of bare plain surface.Besides,the actual scene of jet impingement boiling was recorded with a camera to investigate the behavior evolution of vapor bubbles which is highly correlated to the heat transfer process.     

Experimental Investigations on Boiling Heat Transfer Inside Miniature Circular Tubes Immersed in FC-72

To investigate the size effect on the characteristics of boiling heat transfer, boiling behavior of FC-72 in heated vertical miniature circular tubes immersed in a liquid pool was experimentally studied. Two AISI 304 stainless steel tubes with inner diameters of 1.10 mm and 1.55 mm correspondingly, were heated by swirled Ni-Cr wire heaters and sealed in Lucite blocks by silicon adhesive. Both the top and the bottom ends of the circular test sections were open to the liquid pool. The boiling curves and heat transfer coefficients were obtained experimentally. The boiling behaviors at the outlets of the miniature tubes were also visualized with a digital video camera. Experimental results show that the tube geometry has a significant effect on the boiling characteristics. Vapor blocking at the outlet of the smaller circular tube with a diameter of 1.10 mm caused severe boiling hysteresis phenomena. The CHF decreased with reducing in tube size.     

High-Speed Visualization of Bubble Behaviors for Pool Boiling of R-141b

A visualization study on the behavior of bubbles has been carried out for pool boiling of R141b on a horizontal transparent heater at pressure 0.1 MPa. The behaviors of bubbles were recorded by a high-speed camera placed beneath the heater surface. The departure diameter, departure time of bubbles and nucleation site density at different heat flux were obtained. The visualization results show that bubble departure diameter and departure time decrease , while the nucleation site density increases as the heat flux increases. It is also observed that there is no liquid recruited into the microlayer in the experiment. Based on the experimental results, boiling curve for R141b was predicted by using the dynamic microlayer model. As a result, the agreement between the predictive result based on the dynamic microlayer model and the experiment data for boiling curve of R141b is good at high heat flux.     

Analysis of Film-Boiling Heat Transfer on a High Temperature Sphere Immersed into Liquid Sodium

Film-boiling heat transfer is a key phenomenon governing severe accident sequence in a sodium-cooled fast reactor. Experimental the fuel-coolant interaction process which may occur during a and theoretical work on film-boiling heat transfer in sodium has hardly been carried out in the past. An experiment has been conducted in the early seventies to investigate sodium pool boiling. In this experiment, a hot tantalum sphere was immersed into subcooled liquid sodium. Film boiling was obtained for various sets of parameters： sodium subcooling from 4.1 K to 29. 1 K, initial sphere temperature ranging from 1,802.6 K to 2,633.7 K, sphere diameters of 1.27, 1.91 and 2.54 cm and sodium depths of 7.6 cm and 11.4 cm. In the present work, a simplified analysis based on the boundary layer theory is developed to describe pool film-boiling heat transfer on a hot sphere in liquid sodium. Two extreme cases are considered depending on sodium subcooling. In the case of high subcooling, most of the heat lost by the sphere is used to heat the sodium while for low subcooling, it is used to vaporize the liquid at the liquid-vapor interface. It will be shown that the scaling analysis predicts the heat fluxes within the order of magnitude when compared to the available experimental data. Besides, it allows an estimation of the contribution of these fluxes to the liquid heating and vaporization processes.     

Fundamental Research on Convective Heat Transfer in Electronic Cooling Technology

During the past six years comprehensive research programs have been conducted at the Beijing PolytechnicUniversity to provide a better understanding of heat transfer characteristics of existing and condidate cool-ing techniques for electronic and microelectronic devices.This paper provides a review and summary of theprograms with emphasis on direct liquid cooling.Included in this review are the heat transfer investigationsrelated to the following cooling modes:liquid free,mixed and forced convection,liquid jet impingement,flowingliquid film cooling,pool boiling,spray cooling,foreign gas jet impingement in liquid pool,and forced convectionair-cooling.     

Nucleate Pool Boiling Heat Transfer of Pure Refrigerants and Binary Mixtures

Heat transfer coefficients in nucleate boiling on a smooth flat surface were measured for pure fluids of R-134a, propane, isobutane and their binary mixtures at different pressure from 0.1 to 0.6 MPa. Series of experiments with different heat flux and mixture concentrations were carried out. The influences of pressure and heat flux on the heat transfer coefficient for different pure fluids were studied. Isobutane and propane were used to make up binary mixtures. Compared to the pure components, binary mixtures show lower heat transfer coefficients. This reduction was more pronounced as the heat flux increasing. Several heat transfer correlations are obtained for different pure refrigerants and their binary mixtures.     

Investigation on Active Thermal Control Method with Pool Boiling Heat Transfer at Low Pressure

In order to maintain a desirable temperature level of electronic equipment at low pressure, the thermal control performance with pool boiling heat transfer of water was examined based on experimental measurement. The total setup was designed and performed to accomplish the experiment with the pressure range from 4.5 kPa to 20 kPa and the heat flux between 6 kW/m~2 and 20 kW/m~2. The chosen material of the heat surface was aluminium alloy and the test cavity had the capability of varying the direction for the heat surface from vertical to horizontal directions. Through this study, the steady and transient temperature of the heat surface at different pressures and directions were obtained. Although the temperature non-uniformity of the heat surface from the centre to the edge could reach 10℃ for the aluminium alloy due to the varying pressures, the whole temperature results successfully satisfied with the thermal control requirements for electronic equipment, and the temperature control effect of the vertically oriented direction was better than that of the horizontally oriented direction. Moreover, the behaviour of bubbles generating and detaching from the heat surface was recorded by a high-resolution camera, so as to understand the pool boiling heat transfer mechanism at low-load heat flux. These pictures showed that the bubbles departure diameter becomes larger, and departure frequency was slower at low pressure, in contrast to 1.0 atm.     

The effect of heating direction on flow boiling heat transfer of R134a in micro-channels

This paper presents effects of heating directions on heat transfer performance of R134a flow boiling in micro-channel heat sink.The heat sink has 30 parallel rectangular channels with cross-sectional dimensions of 500μm width 500μm depth and 30mm length.The experimental operation condition ranges of the heat flux and the mass flux were 13.48 to 82.25 W/cm2 and 373.3 to 1244.4 kg/m2s respectively.The vapor quality ranged from 0.07 to 0.93.The heat transfer coefficients of top heating and bottom heating both were up to 25 kW/m2 K.Two dominate transfer mechanisms of nucleate boiling and convection boiling were observed according to boiling curves.The experimental results indicated that the heat transfer coefficient of bottom heating was 13.9％ higher than top heating in low heat flux,while in high heat flux,the heat transfer coefficient of bottom heating was 9.9％.higher than the top heating,because bubbles were harder to divorce the heating wall.And a modified correlation was provided to predict heat transfer of top heating.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures ：Part I—Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

A mechanism is proposed for nucleate pool boiling heat transfer along with a general model for both pure liquids and binary mixtures. A combined physical model of bubble growth is also proposed along with a corresponding bubble growth model for pure liquids on smooth tubes. Using the general model and the bubble growth model for pure liquids, an analytical model for nucleate pool boiling heat transfer of pure liquids on smooth tubes is developed.     

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

ＮｕｃｌｅａｔｅＰｏｏｌＢｏｉｌｉｎｇｏｆＰｕｒｅＬｉｑｕｉｄｓａｎｄＢｉｎａｒｙＭｉｘｔｕｒｅｓ：ＰａｒｔＩ－ＡｎａｌｙｔｉｃａｌＭｏｄｅｌｆｏｒＢｏｉｌｉｎｇＨｅａｔＴｒａｎｓｆｅｒｏｆＰｕｒｅＬｉｑｕｉｄｓｏｎＳｍｏｏｔｈＴｕｂｅｓＧｕｏｑｉｎ...     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：Part Ⅱ－Analytical Modelfor Boiling Heat Transfer of Binary Mixtureson Smooth Tubes and Comparison of Analytical Models for both Pure Liquids and Binary Mixture

ＮｕｃｌｅａｔｅＰｏｏｌＢｏｉｌｉｎｇｏｆＰｕｒｅＬｉｑｕｉｄｓａｎｄＢｉｎａｒｙＭｉｘｔｕｒｅｓ：ＰａｒｔＩＩ－ＡｎａｌｙｔｉｃａｌＭｏｄｅｌｆｏｒＢｏｉｌｉｎｇＨｅａｔＴｒａｎｓｆｅｒｏｆＢｉｎａｒｙＭｉｘｔｕｒｅｓｏｎＳｍｏｏｔｈＴｕｂｅｓａｎ...     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：part II—Analytical Model for Boiling Heat Transfer of Binary Mixtures on Smooth Tubes and Comparison of Analytical Models for both Pure Liqu

A combined physical model of bubble growth is proposed along with a corresponding bubble growth model for binary mixtures on smooth tubes. Using the general model of Wang et al.^[1] and the bubble growth model for binary mixtures, an analytical model for nucleate pool boiling heat transfer of binary mixtures on smooth tubes is developed. In addition, nucleate pool boiling heat transfer of pure liquids and binary mixtures on a horizontal smooth tube was studied experimentally. The pure liquids and binary mixtures included water, methanol, ethanol, and their binary mixtures. The analytical models for both pure liquids and binary mixtures are in good agreement with the experimental data.     

Nucleate Boiling Enhancements on Porous Graphite and Microporous and Macro–Finned Copper Surfaces

Enhancements in nucleate boiling heat removal with dielectric liquids, by increasing either the bubbles nucleation sites density and/or the wetted surface area, are desirable for immersion cooling of high-power computer chips. This article presents the results of recent investigations of nucleate boiling enhancement of FC-72, HFE-7100, and PF-5060 dielectric liquids on porous graphite, copper microporous surfaces, and copper surfaces with square corner pins, 3 mm × 3 mm in cross-section and 2, 3, and 5 mm tall. All surfaces have a footprint measuring 10 × 10 mm. These investigations examined the effects of liquid subcooling up to 30 K and surface inclination, from upward-facing to downward-facing, on nucleate boiling heat transfer coefficient and critical heat flux. Natural convection of dielectric liquids for cooling chips while in the stand-by mode, at a surface average heat flux <20 kW/m², is also investigated for the different surfaces.     

CaCO3垢的形成对池核沸腾传热的影响

在一水平圆形加热表面上通过实验考察了饱和池核沸腾和过冷池核沸腾时CaCO3垢的生成对传热的影响。结果表明，在饱和池核沸腾和过冷池核沸腾的初始阶段沸腾传热系数均呈先降低后升高、达到一个最大值后稳定降低的趋势，而且在初始阶段出现了负污垢热阻现象。在相同操作条件下，过冷池核沸腾传热系数明显低于饱和池核沸腾传热系数。在分析污垢的生成和生长影响表面活化中心的基础上，对污垢的形成对沸腾传热的影响进行了机理分析。     

水平窄空间沸腾传热的实验研究

通过对五种尺寸的窄空间试验元件分别以水和乙醇做工质进行实验。研究了窄空间间距、窄空间尺寸、不同工质及不同热流密度对窄空间沸腾性能的影响。结果表明：当窄空间尺寸与热流通等因素组合恰当时。其换热系数可比大空间池沸腾提高3～6倍；临界热流密度有所降低。     

Thermal distribution analysis of rectangular fin considering multiboiling heat transfer modes

In this investigation, a numerical method is used to compute the thermal distribution analysis of a rectangular fin with surface emissivity and internal heat generation. Here, the thermal conductivity, heat generation, emissivity at the surface, and coefficient of heat transfer depend on temperature linearly. The role of four distinct multiboiling heat transfer modes such as laminar film boiling (condensation), laminar convection, turbulent convection, and nucleate boiling are discussed in detail and the corresponding outcomes are displayed graphically. Isolated (insulated) and convective tip boundary conditions for the fin tip are employed in this study. The solution is obtained using shooting technique involving Runge Kutta Fehlberg method. It is emphasized that the thermal distribution shows a diminishing trend for the convective tip condition compared to the insulated tip. In addition to this, it is illustrated that laminar film boiling and laminar convection are two effective modes of heat transfer in comparison with turbulent convection and nucleate boiling for a finned surface in boiling liquids. The study on fin efficiency shows that fin efficiency increases with the increase in internal heat generation number.     

Prediction of critical heat flux on natural convective boiling in vertical short‐thick tube submerged in saturated liquid

An experimental and semitheoretical study was carried out for the critical heat flux (CHF) on natural convective boiling in uniformly heated vertical short‐thick tubes and vertical short‐thick annular tubes submerged in saturated liquids. By adapting a mathematical dealing method based on the theoretical formulas of CHF of both the natural convective boiling in vertical narrow‐long tubes and the pool boiling, a simple semitheoretical formula was derived. The new formula expands the prediction range of CHF from pool boiling of vertical plates to very long vertical tubes and agrees well with the data of the tubes, annular tubes submerged in water or other liquids under various pressure conditions. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 402–410, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10103     

Effect of pressure, subcooling, and dissolved gas on pool boiling heat transfer from microporous, square pin-finned surfaces in FC-72

The present research is an experimental study of the effects of pressure, subcooling, and non-condensable gas (air) on the pool nucleate boiling heat transfer performance of microporous enhanced finned surfaces. The test surfaces, solid copper blocks with 1-cm² bases and 5×5 square pin-fin arrays of 2, 4 and 8 mm fin lengths, were immersed in FC-72. The test conditions included an absolute pressure range of 30-150 kPa and a subcooling range of 0 (saturation) to 50 K. Effects of these parameters on nucleate boiling and critical heat flux (CHF) were investigated. In addition, differences between pure subcooled and gas-saturated conditions as well as horizontal and vertical base orientations were also investigated. Results showed that, in general, the effects of pressure and subcooling on both nucleate boiling and CHF were consistent with previously tested flat surface results, however, subcooling was found to significantly affect the high heat flux region of the microporous finned surfaces nucleate boiling curves. The relative enhancement of CHF from increased subcooling was greater for the microporous surface than the plain surface but less than a microporous flat surface. The horizontal orientation (horizontal base/vertical fins) was found to be slightly better than the vertical orientation (vertical base/horizontal fins). Correlations for both nucleate boiling and CHF for the microporous surfaces were also developed.