A nucleate boiling limitation model for the prediction of pool boiling CHF

A nucleate boiling limitation model which is applicable to the heat transfer prediction in the nucleate boiling region and the CHF was proposed for a pool boiling. The present model was developed based on the direct observations of the physical boiling phenomena. The predicted boiling curves for the nucleate boiling region agree well with both the vertical and the horizontal surface data for all the contact angles. The predicted CHF for the vertical surface also agrees well with the experimental data, but the present model underpredicts the CHF by about 30% for the horizontal surface data.     

Two-tier model for nucleate pool boiling on microconfifured composite surfaces

A new model is developed to describe the heat transfer mechanism in nucleate pool boiling on a microconfigured composite surface. Both the microlayer and macrolayer thickness are determined from the model. This model can be extended to explain the nucleate boiling on plain surfaces. The enhancement mechanisms of heat transfer for the nucleate boiling on the microconfigured surface are analyzed.     

池沸腾临界热流密度及临界波长的实验研究

分别在光滑及波形结构的铜表面上对水和乙醇进行饱和池沸腾实验,观测了临界热流密度(CHF)下临界波长的变化趋势,并分析了表面结构对沸腾传热系数及CHF的影响。实验验证了光滑表面上,临界波长随工质的不同而变化,继而影响CHF,其实验值与经典的临界波长及临界热流密度理论一致。而粗糙表面上的乙醇沸腾实验进一步发现,波形结构可以减小临界波长,从而有效提高CHF,其影响规律与相关文献的理论模型较为符合。     

A new comprehensive model for nucleate pool boiling heat transfer of pure liquid at low to high heat fluxes including CHF

Based on the fractal distribution of nucleation sites present on heating surfaces, a new comprehensive model is developed for the nucleate pool boiling of pure liquid at low to high heat fluxes including the critical heat flux (CHF). The proposed model is expressed as a function of total number, minimum and maximum sizes of active nucleation sites, fractal dimension, superheat temperature, and properties of fluids. No additional empirical constant is introduced in the proposed model. This fractal model contains less empirical constants than the conventional models. The model predictions are in good agreement with the available experimental data.     

Effect of nanoparticles on CHF enhancement in pool boiling of nano-fluids

To investigate the characteristics of CHF (Critical Heat Flux) enhancement using nano-fluids, pool boiling CHF experiments of two water-based nano-fluids with titania and alumina nanoparticles were performed using electrically heated metal wires under atmospheric pressure. The results showed that the water-based nano-fluids significantly enhanced CHF compared to that of pure water. SEM (Scanning Electron Microscopy) observation subsequent to the pool boiling experiments revealed that a lot of nanoparticles were deposited on heating surface during pool boiling of nano-fluids. In order to investigate the role of the nanoparticle surface coating on CHF enhancement of nano-fluids, pool boiling CHF of pure water was measured using a nanoparticle-coated heater prepared by pool boiling of nano-fluids on a bare heater. It was found that pool boiling of pure water on the naonoparticle-coated heater sufficiently achieved the CHF enhancement of nano-fluids. It is supposed that CHF enhancement in pool boiling of nano-fluids is mainly caused by the nanoparticle coating of the heating surface.     

Boiling time effect on CHF enhancement in pool boiling of nanofluids

Using TiO₂–water nanofluids as the test liquid, pool boiling experiments were carried out to investigate the dependence of the nucleate boiling heat transfer, surface wettability and critical heat flux (CHF) on the boiling time in nanofluids. In the experiments performed at sufficiently high nanoparticle concentrations, the boiling heat transfer first degraded, then improved, and finally reached an equilibrium state. It was hence supposed that the present nanofluids had competing effects to deteriorate and enhance the nucleate boiling heat transfer. As for the surface wettability and CHF, the static contact angle asymptotically decreased whilst the CHF asymptotically increased with an increase in the boiling time. The maximum CHF enhancement measured in the present experiments was 91%, and strong correlation was found between the contact angle and the CHF. Although the boiling time needed to achieve the maximum CHF enhancement was less than a minute at high particle concentrations, a longer time of the order of 1 h was necessary at the lowest particle concentration tested in this work. This experimental result indicated that sufficient attention should be paid to the boiling time effect particularly in industrial applications of nanofluids to emergency cooling.     

Experimental study on the pool boiling CHF enhancement using magnetite-water nanofluids

This paper described the effects of a magnetite-water nanofluid (MWNF) on the critical heat flux (CHF) enhancement using an Ni–Cr wire in pool boiling. All experiments were performed at a saturated condition under atmospheric pressure. The CHF values between the MWNF and the other nanofluids with several volume concentrations were compared to evaluate the effect of the MWNF on the CHF enhancement. The CHF values of the MWNF were enhanced from approximately 170% to 240% of pure water as the nanoparticle concentration increased. In addition, the CHF for the MWNF showed the highest value among the evaluated nanofluids. In this paper, three methods were introduced to elucidate the mechanism underlying CHF enhancement. First, scanning electron microscope (SEM) images were obtained to explain the CHF enhancement mechanism due to the deposited nanoparticles, which is related to the surface wettability of the heating surface during the pool boiling. Second, bubble formation in pool boiling was analyzed using image processing to demonstrate the relationship between bubble dynamics and CHF enhancement. Finally, the magnetic field was analytically calculated using the Biot–Savart law to evaluate the effects of the magnetic field on the CHF.     

Photographic study of pool boiling CHF from a downward-facing convex surface

Heat transfer measurements and photographic studies are performed to capture the detailed evolution of the liquid–vapor interface near critical heat flux (CHF) for a 90-degree downward-facing convex surface. The test surface, with a width of 3.2 mm and a 102.6-mm radius, consists of a series of nine heaters that dissipate equal power. Instrumentation within each heater facilitates localized heat flux and temperature measurements along the convex surface, and transparent front and back windows enable optical access to a fairly two-dimensional liquid–vapor interface. Near CHF, vapor behavior along the convex surface is cyclical, repeatedly forming a stratified vapor layer at the bottom of the convex surface, which stretches as more vapor is generated, and then flows upwards along from the surface. Subsequently, heaters at the bottom of the convex surface, followed by the other heaters, are wetted with liquid before the nucleation/coalescence/stratification/release process is repeated. This study proves that despite the pronounced thickening of the vapor layer as it propagates upwards along the convex surface, CHF always commences on the bottom of the surface.     

A fractal model for critical heat flux in pool boiling

In this paper, a fractal model for the high heat flux nucleate boiling region and for the critical heat flux (CHF) is proposed. The expression for the critical heat flux (CHF) is derived based on the fractal distribution of nucleation sites on boiling surfaces. The proposed fractal model for CHF is found to be a function of wall superheat, the contact angle and physical properties of fluid. The relation between CHF and the number of active nucleation sites is obtained from the fractal distribution of active nucleation sites on boiling surfaces. The contact angle and the physical properties of fluid have the important effects on CHF. The predicted CHF from a boiling surface based on the proposed fractal model is compared with the existing experimental data. An excellent agreement between the proposed model predictions and experimental data is found.     

Experiments on pool boiling of a dielectric fluid on extended surfaces

An experimental study on pool boiling heat transfer from finned copper surfaces immersed in a saturated dielectric liquid (Galden HT-55) is presented. Two extended surfaces of different dimensions were tested using vertical and horizontal orientations. The effects of nonboiling waiting period, pressure and spine dimensions on boiling behavior were examined.A marked enhancement of heat transfer performance was observed on passing from plane to extended surfaces. Increasing the pressure improved the heat transfer coefficient and critical heat flux. The nonboiling period, orientation and pressure significantly influenced the boiling incipience and the hysteresis phenomenon that accompanies increasing and decreasing heat fluxes. Experimental data are expressed in terms of enhancement ratios of extended surfaces as a function of base surface superheat and pressure.     

Dry-out CHF correlation for R134a flow boiling in a horizontal helically-coiled tube

An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental parameters are the outlet pressures of 0.30–0.95 MPa, mass fluxes of 60–500 kg m^?2 s^?1, inlet qualities of ?0.36–0.35 and heat fluxes of 7.0 × 10³–5.0 × 10⁴ W m^?2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2 mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270° and 90°) of the outlet cross-section. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations.     

A numerical simulation of pool boiling using CAS model

This paper presents a new numerical model, called the CAS model, for boiling heat transfer. The CAS model is based on the cellular automata (CA) technique that is integrated into the popular SIMPLER algorithm for CFD problems. In the model, the CA technique deals with the microscopic nonlinear dynamic interactions of bubbles while the traditional CFD algorithm is used to determine macroscopic system parameters such as pressure and temperature. The popular SIMPLER algorithm is employed for the CFD treatment. The model is then employed to simulate a pool boiling process. The computational results show that the CAS model can reproduce most of the basic features of boiling and capture the fundamental characteristics of boiling phenomena. The heat transfer coefficient predicted by the CAS model is in excellent agreement with the experimental data and existing empirical correlations.     

Performance of different structured surfaces in nucleate pool boiling

Several structured surfaces have been developed in-house for the augmentation of boiling heat transfer using distilled water as test fluid under atmospheric pressure. These surfaces have either a number of parallel tunnels or orthogonally intersecting tunnels. Effect of design parameters like tunnel inclination and different cavity structure at the tunnel base on the boiling heat transfer has been investigated. Three different structures namely circular groove, rectangular groove and rounded base have been used at the end of the tunnels. Heat flux is varied in the range of 0–250 kW/m². Experimental results showed tunnels inclined at an angle 60° with the horizontal provide better augmentation compared to straight vertical tunnels. Amongst different base geometry the circular pocket produced most conducive condition for the boiling heat transfer. The use of tunnels also increases the degree of augmentation. The highest augmentation was obtained from the surface having intersecting inclined tunnels with a circular base.     

CHF model for subcooled flow boiling in Earth gravity and microgravity

This study is the first attempt at extending the Interfacial Lift-off CHF Model to subcooled flow boiling conditions. A new CHF database was generated for FC-72 from ground tests as well as from microgravity tests that were performed in parabolic flight trajectory. These tests also included high-speed video imaging and analysis of the liquid–vapor interface during the CHF transient. Both the CHF data and the video records played a vital role in constructing and validating the extended CHF model. The fundamental difference between the original Interfacial Lift-off Model, which was developed for saturated flow boiling, and the newly extended model is the partitioning of wall energy between sensible and latent heat for subcooled flow boiling. This partitioning is modeled with the aid of a new “heat utility ratio”. Using this ratio, the extended Interfacial Lift-off Model is shown to effectively predict both saturated and subcooled flow boiling CHF in Earth gravity and in microgravity.     

Effusivity-based correlation of surface property effects in pool boiling CHF of dielectric liquids

Although the effects of fluid properties, pressure, and subcooling, as well as heater geometry, on the pool boiling critical heat flux, or CHF, are relatively well established, explanations for the surface property effects remain controversial. Proposed formulations, embodying the dependence of CHF on the product of the heater thermal effusivity and thickness are described and compared with available data. A composite correlation for pool boiling CHF, accounting for the conduction and hydrodynamic limits, as well as the effects of pressure, subcooling, and length, is proposed. This effusivity-based correlation is found to predict a broad range of pool boiling CHF data for dielectric liquids, for thermal effusivity values between 0.2 and 120, pressure from 100 to 450 kPa, and subcoolings from 0 to 75 K, with a standard deviation of 12.5%.     

A transient model for heterogeneous nucleation under pulse heating in pool boiling

A transient analysis on heterogeneous nucleation under pulse heating in pool boiling is developed based on changes in Gibbs free energy and availability, with temperature distributions determined from one-dimensional transient heat conduction theory. Numerical solutions are obtained for critical radius and nucleation heat fluxes under various pulse widths in different working fluids. It is found that both the critical radius and nucleation work increase with pulse width while the nucleation heat flux decreases with pulse width. To verify the proposed theory, experiments are carried out for onset of nucleation in water, alcohol and R113 under pulse heating at different pulse widths. It is found that the experimental data for nucleation heat flux at short pulse widths agree well with the proposed transient model, and those at long pulse widths approach the previous steady model. Predicted results of critical radius, nucleation heat flux and the change in availability at various pulse widths for water, alcohol and R113 are presented and compared.     

Experimental research of pool boiling heat transfer in horizontal narrow spaces

Much progress has been made in high‐performance electronic chips, the miniaturization of electronic circuits and other compact systems recently, which brings about a great demand for developing efficient heat removal techniques to accommodate these high heat fluxes. With this objective in mind, experiments were carried out on five kinds of test elements with distilled water and ethanol as working liquids. The test elements used in these experiments consisted of five parallel discs with diameters varying from 5 mm to 40 mm. The experiments were performed with the discs oriented horizontally and uniform heat fluxes applied at the bottom surfaces. The influence of narrow spacing, space size, working liquid property, and heat flux on boiling heat transfer performance in narrow spaces has been investigated. Experimental results showed that the boiling heat transfer coefficient of a narrow space was 3 to 6 times higher than that of pool boiling when the narrow space size and heat flux combine adequately, but the critical heat flux was lower than that of pool boiling. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 307–315, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20017     

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。     

Theoretical studies on transient pool boiling based on microlayer model

An analytical model for transient pool boiling heat transfer was developed in this study. The boiling curves of the transient boiling were obtained based on the microlayer model proposed by the authors and the mechanism of transition from the non-boiling regime to film boiling, i.e., direct transition was theoretically examined. Since the nucleate boiling heat flux is mainly due to the evaporation of the microlayer and its initial thickness decreases rapidly with increasing superheat, the duration of nucleate boiling is markedly decreased as the incipient boiling superheat is increased. It is found that the direct transition is closely connected to the rapid dryout of the microlayer which occupies almost the whole surface at high wall superheat.