Macrolayer formation and mechanisms of nucleate boiling,critical heat flux,and transition boiling

The drying process of a macrolayer on a 15 mm diameter boiling surface was observed with high speed video in the region of nucleate and of transition boiling close to the critical heat flux (CHF). It was found that the macrolayer rests beneath a large vapor mass. It partially dries in nucleate boiling and completely dries in transition boiling at the detachment of the vapor mass. The macrolayer thickness at CHF and in transition boiling was determined on the basis of the energy balance relation proposed by Katto and Yokoya. The macrolayer thickness at low heat flux was obtained by decreasing CHF with downward-facing heating surfaces and agreed well with the correlation proposed previously by the present authors. The macrolayer thickness in transition boiling with a vertical surface also agrees fairly well with the correlation, when the heat flux at macrolayer formation, given on the nucleate boiling curve, is extrapolated to surface superheat of transition boiling and when the surface temperature at macrolayer formation is equal to a time-averaged value. © 1998 Scripta Technical, Heat Trans Jpn Res, 27(2): 155–168, 1998     

Enhancement of nucleate boiling on composite surfaces

A composite heating surface composed of materials with different thermal conductivities can be expected to enhance heat transfer in nucleate boiling. This is because the end surface, with higher conductivity, will attain a higher temperature and as a result will serve to provide preferential nucleation sites. To confirm this idea, several composite surfaces were fabricated by uniaxially imbedding thin copper cylinders in the heat flow direction on a stainless steel circular plate 30 mm in diameter and 5 mm thick. The imbedded copper cylinders ranged from 1 mm to 4 mm in diameter and one to 77 in number. The heat transfer performance of these composite surfaces was investigated for pool boiling of saturated water at atmospheric pressure. It was confirmed that the copper cylinder surfaces exposed to water functioned as local hot spots to initiate preferential nucleate boiling, leading to higher boiling heat transfer coefficients than those on a homogeneous stainless steel surface. The measured void fraction above the heating surface verified intensive bubble generation on the surface of the copper cylinders. This situation continued up to a certain heat flux level and was then followed by nucleation on the mother surface of stainless steel around the copper cylinders. A numerical analysis of heat conduction within a composite wall simulated the temperature distribution within the wall and the variation in surface heat flux at the time of boiling incipience. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 216–228, 1998     

Reexamination of Correlations for Nucleate SiteDistribution on Boiling Surface by Fractal Theory

INTRODUCTIONThenumberandspatia1distributionofnucleatesiteplaysaparticu1arimportantroleinmodelingorpred-icatingofnucleateboilingprocess.Thenucleatesitedistributiondensityappearsasanimportantterminalmostallthemechanisticnucleateboilingmodels[1].Inpastdecades,manyresearchersproposedvariousexperimentalcorrelations.Forexample,MikicandRohsenowl2],Biereta1.[3],WangandDhirl4]devel-opedsomeempiricalcorrelationsbetweenthecumula-tiveactivenucleatesitenumberandthecharacteristicsizesuchasthemouthdia…     

Investigation of Enhanced Boiling Heat Transfer from Porous Surfaces

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Shape of “True” transition boiling curve and two‐mode boiling

Two‐mode boiling could occur on the same heating element, which provided information on the criterion separating the stable and metastable boiling regimes. The “equal‐area” criterion based on the complete boiling curve interpreted the criterion of coexistence of the two boiling modes. However, the transition boiling curve could not be constructed by the pool boiling tests. Literature works adopted various correlations of transition boiling curves for analysis, but failed to recognize the possible errors incorporated in such an approximation. This paper demonstrated the relative errors embedded in calculating the equal‐area criterion by assuming various shapes of the transition boiling curve. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(7): 593–601, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10113     

Study on the onset of nucleate boiling in narrow annular channel by genetic neural network

A study on ONB is performed by GNN in vertical narrow annuli with bilateral heating. Based on GA and ANN, a GNN model predicting q_ONB has been developed. The q_ONB prediction is investigated by GNN with distilled water flowing upward through narrow annuli with 0.95 mm, 1.5 mm and 2.0 mm gaps, respectively. The GNN prediction results have a good agreement with experimental data. A parametric study has been conducted to investigate the effects of system pressure, mass flux, wall superheat and geometrical structure on q_ONB by applying GNN. Simulation and analysis results show that GNN can effectually predict q_ONB.     

Theoretical Aspects of Nucleate Pool Boiling with Dielectric Liquids

Direct cooling with inert,dielectric liquids may well become the technique of choice for the thermal manage-ment of future electronic systems.Due to the efficiency of phase-change processes and the simplicity of naturalcirculation,nucleate pool boiling is of great interest for this application.This paper examines the characteristicsof vapor bubbles and nucleate pool boiling of the dielectric liquids.The results provide a theoretical foundationfor understanding and interpreting the often complex empirical results reported in the literature.     

Characteristics of heat transfer coefficients during nucleate pool boiling of binary mixtures

Experimental studies were made on heat transfer on a horizontal platinum wire during nucleate pool boiling in nonazeotropic refrigerant binary mixtures at pressures of 0.25 to 0.7 MPa and at heat fluxes up to CHF. The boiling features of the mixtures and the single-component substances were observed by photography. The relationship between the boiling behavior and the reduction of heat transfer coefficients in binary mixtures is discussed in order to propose a correlation useful for predicting the present experimental data over a wide range of low to high heat fluxes. It is shown that the correlation is applicable to alcoholic mixtures. The physical meaning of k, which was introduced to evaluate the effect of heat flux on the reduction of a heat transfer coefficient, is clarified based on measured nucleate pool boiling heat transfer data and visual observations of the boiling features. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 535–549, 1998     

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。     

冷却水中重悬浮颗粒影响沸腾传热的试验研究

试验研究了冷却水中添加氧化铁(Fe3O4/Fe2O3)颗粒在高温钢坯冷却过程中影响沸腾传热的性质。系统分析了冷却水(悬浮液)喷淋初速度、加入重颗粒的粒径、质量分数等因素对热钢坯温度下降强度、传热热流密度、沸腾传热系数、过冷沸腾长度等参数的影响,总结了冷却水中引入重悬浮颗粒用以强化沸腾传热的优化方案。     

Heat transfer correlation of film boiling from a downward-facing horizontal circular plate

Correlation equations for saturated and subcooled film boiling heat transfer from a downward-facing horizontal circular plate are proposed based on a theoretical study using an integral method. For the case of subcooled liquids, a dimensionless heat transfer parameter is introduced to effectively correlate within ±15% the experimental data obtained under both quenching and steady-state conditions. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(7): 459–468, 1997     

狭缝中流动沸腾传热过冷沸腾起始点的实验研究

以间隙为1．0mm和1．5mm的环形狭缝通道中流动沸腾传热的实验数据为基础，分析了影响过冷沸腾起始点热负荷的主要因素，给出了计算环形狭缝通道中流动沸腾传热过冷沸腾起始点的经验关联式，并将计算结果与实验值进行了比较。该关联式可以用来预测实验范围内的过冷沸腾起始点的热负荷。     

Theoretical analysis of the stability of vapor film in subcooled film boiling on a horizontal wire

Linear stability analysis of a thin vapor film in subcooled film boiling on a horizontal cylinder is reported. The effects of liquid inertia, vapor viscosity and compressibility, and heat transfer were taken into account. Theoretical predictions of the heat transfer coefficient at the neutral stability point were compared with experimental data at the minimum-heat-flux point that was obtained during rapid quenching of thin horizontal wires in water and ethanol. At high liquid subcooling, the experimental value was 60% of the theoretical prediction irrespective of the wire diameter and quenching liquid. This difference was considered to be due to the nonuniformity of the vapor film which was neglected in the theoretical analysis. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(4): 219–235, 1997     

Study of nucleate boiling heat transfer to slush hydrogen and slush nitrogen

Slush hydrogen is a mixture of liquid hydrogen and solid hydrogen particles, and is being considered as a spaceplane fuel or as a means of transport for hydrogen used as a source of clean energy. This paper describes nucleate boiling heat transfer characteristics of slush hydrogen and slush nitrogen. For the visual observation of heat transfer states, a heat transfer unit was placed in a glass Dewar designed to minimize the heat loss from an atmospheric environment. The heat transfer unit used was a circular flat plate 0.025 m in diameter made of electrolytic tough pitch copper. During testing, three different orientations of the heat transfer surface were used: horizontal facing up, vertical, and horizontal facing down. Heat transfer data for the normal boiling point (NBP) of liquid hydrogen, the triple point (TP) of liquid hydrogen, the NBP of liquid nitrogen, and the TP of liquid nitrogen were obtained up to the critical heat flux (burnout). These data for slush hydrogen and nitrogen, including the results of observation of the heat transfer surface were compared. This clarified the nucleate boiling heat transfer characteristics of slush hydrogen and slush nitrogen, which have rarely been investigated. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 13–28, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10068     

Effect of surface wettability and electric field on transition of film boiling to nucleate boiling

Abstract

The phenomena of liquid–solid contact during film boiling due to the effect of surface-wettability have been focused in the present study. The numerical simulations during film boiling exhibit the collapse of vapor layer when the surface-wettability is sufficiently high, that is, for the hydrophilic surface. Vapor film collapse results in contact of liquid with the heated surface, which transforms the boiling mode more toward the nucleate regime. The contact area of liquid increases with time. However, such transition is not observed in the case of hydrophobic surface or the surface with higher contact angles. When a sufficiently strong electric field is applied across the liquid-vapor interface, the vapor film collapses and results in similar transition from film boiling to nucleate boiling. The required intensity of electric field at which the vapor film collapses increases with the increase in surface-superheat.     

Effect of pore size on the nucleate pool boiling of structured enhanced tubes

In this study, pool boiling test results are provided for the structured enhanced tubes having pores with connecting gaps. The surface geometry of the present tube is similar to that of Turbo-B. Three tubes with different pore size (0.20 mm, 0.23 mm and 0.27 mm) were manufactured and tested using R-11, R-123 and R-134a. The pore size which yields the maximum heat transfer coefficient varied depending on the refrigerant. For R-134a, the maximum heat transfer coefficient was obtained for the tube having 0.27 mm pore size. For R-11 and R-123, the optimum pore size was 0.23 mm. One novel feature of the present tubes is that their boiling curves do not show a ‘cross-over’ characteristic, which existing pored tubes do. The connecting gaps of the present tube are believed to serve an additional route for the liquid supply and delay the dry-out of the tunnel. The present tubes yield the heat transfer coefficients approximately equal to those of the existing pored enhanced tubes. At the heat flux 40 kW/m² and saturation temperature 4.4° C, the heat transfer coefficients of the present tubes are 6.5 times larger for R-11, 6.0 times larger for R-123 and 5.0 times larger for R-134a than that of the smooth tube     

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

Convection film boiling on horizontal cylinders

Free, mixed and forced convection film boiling on a horizontal cylinder in a saturated or subcooled liquid is studied theoretically using a single model based on a two-phase laminar boundary layer integral method. The vapour flow is described accurately by including the inertia and convection terms in the momentum and energy equations, in order to study convection film boiling in the cases of very high superheat. Different film boiling cases are then analysed with this model. The case of high superheat and low subcooling was first analysed by comparing the model with an experiment consisting in the quenching of wires with very high superheat: the model was able to predict the measured heat transfer from the cylinder with errors less than 30%, performing better than previous models or correlations. Additional calculations in other high superheat conditions have also been performed and compared with a model which does not include the inertia and convection terms in order to have a more quantitative idea of their effects on the heat transfers. The case of low superheat and high subcooling is then analysed by comparing the model with other forced convection experiments with cylinders at lower temperatures. By analysing different experiments, it is found that there are in fact two different forced convection film boiling sub-regimes characterised by relatively “low” or “high” heat transfers, and that the existence of these sub-regimes is probably linked with the stability of the vapour film during film boiling. The model results compare quite well with the experimental data which belong to the “stable” sub-regime but, on the other hand, the model largely underestimates the heat transfer for experimental data which belongs to the “unstable” sub-regime. Finally, the model is compared to some free convection experimental data. The model was able to predict the measured heat transfers from the cylinder with errors less than 30% both in saturated and subcooled cases.     

Effects of molecule number on nucleate boiling in a system

Canonical molecular distribution was introduced to analyze characteristics of nucleate boiling when the liquid molecular number is very small. The minimum bulk phase volume in which phase change was able to occur was determined from the thermodynamic theory of bubble formation in a superheated liquid and from the energy distribution of the molecules in the bulk phase at a given temperature and pressure. The energy level of active molecules nearly independently distributing in systems was determined from conservation of mass. The maximum superheat temperature necessary for boiling nucleation was found to relate to the bulk phase volume with the temperature increasing as the volume decreased. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 258–264, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20061     

Premature transition to film boiling with stepwise heat generation 2nd report: Effect of wall material and surface condition

The effects of wall material and surface condition on the behavior of an initial boiling bubble of R113 subjected to transient heating were investigated using a heater with a large heat capacity. The behavior of the initial bubble is closely related to premature transition to film boiling of liquids with high wettability. An initial bubble, which is peculiarly shaped like a “straw hat” and leads to premature transition in saturated liquid nitrogen (as reported in a previous paper), also appears on a heated wall with large heat capacity and grows rapidly to cover the entire wall surface. From the observations using a high-speed video camera, the initial bubble is found to be a coalesced bubble into which small bubbles activated in succession along the heated surface are rolled. The growth rate of the initial bubble along the heated surface is not greatly affected by the thermal conductivity of the wall material but is affected markedly by the surface roughness. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res. 25(1): 51–63, 1996