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.     

Marangoni heat transfer in subcooled nucleate pool boiling

The liquid motion induced by surface tension variation, termed the Marangoni effect, and its contribution to boiling heat transfer has been an issue of much controversy. Boiling heat transfer theory, although acknowledging its existence, considers its contribution to heat transfer to be insignificant in comparison with buoyancy induced convection. However, recent microgravity experiments have shown that although the boiling mechanism in a reduced gravity environment is different, the corresponding heat transfer rates are similar to those obtained under normal gravity conditions, raising questions about the validity of the assumption. An experimental investigation was performed in which distilled water was gradually heated to boiling conditions on a copper heater surface at four different levels of subcooling. Photographic investigation of the bubbles appearing on the surface was carried out in support of the measurements. The results obtained indicate that Marangoni convection associated with the bubbles formed by the air dissolved in the water which emerged from solution when the water was heated sufficiently, significantly influenced the heat transfer rate in subcooled nucleate pool boiling. A heat transfer model was developed in order to explain the phenomena observed.     

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.     

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.     

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.     

Effects of carbon nanotube arrays on nucleate pool boiling

Experiments were performed to assess the impact coating silicon and copper substrates with nanotubes (CNTs) have on pool boiling performance. Different CNT array densities and area coverages were tested on 1.27 × 1.27 mm² samples in FC-72. The CNT preparation techniques used provided strong adherence of CNTs to both substrate materials. Very small contact angle enabled deep penetration of FC-72 liquid inside surface cavities of smooth uncoated silicon surfaces, requiring unusually high surface superheat to initiate boiling. Fully coating the substrate surface with CNTs was highly effective at reducing the incipience superheat and greatly enhancing both the nucleate boiling heat transfer coefficient and critical heat flux (CHF). Efforts to further improve boiling performance by manipulating CNT area coverage of the substrate surface proved ineffective; best results were consistently realized with full surface coverage. Greater enhancement was achieved on silicon than on copper since, compared to uncoated copper surfaces, the uncoated silicon surfaces were very smooth and void of any sizeable nucleation sites to start with. This study is concluded with detailed metrics to assess the enhancement potential of the different CNT array densities and area coverages tested.     

Analysis on hysteresis in nucleate pool boiling heat transfer

Many experiments reported for nucleate pool boiling heat transfer from microelectronic assemblies or porous surfaces in dielectric fluids have shown the existence of hysteresis phenomenon, which is known as a temperature overshoot (TOS) in boiling incipience and constrains engineering applications. Here, theoretical analysis has been carried out. It confirms that hysteresis may occur during both the boiling incipience and the boiling development. Hence, a new type of hysteresis, termed Temperature Deviation (TD), is described. Using thermodynamic nucleation, the physical mechanisms for both TOS and TD are analyzed in detail. Three parameters for quantitatively understanding the hysteresis phenomenon and possible measures for remedying it are proposed and discussed.     

Experimental investigation and CFD modeling of the dynamics of bubbles in nanofluid pool boiling

This work investigated the dynamics of bubbles in pool boiling of nanofluid with coated and sodium dodecyl sulfate (SDS) solution with different nanoparticles. Also, computational fluid dynamics (CFD) module was used for mathematical modeling of bubbles in pure water boiling.Different macroscale parameters such as: shapes, numbers and contact angle of bubbles also were investigated experimentally and verified by CFD modeling results. Porous layers of nanoparticles on stainless steel substrate in conjunction with SDS additions were shown to modify formation of bubbles in comparison to reference condition. Improvement in surface hydrophilic conditions and boiling performance was observed by Multi-wall Carbon Nanotube (MWCNT) porous layers, in spite of coated surface by CuO and Al₂O₃ (γ) water based nanofluid. Growth time of bubbles also changes by the presence of porous layers and surfactant solution which resulted from change in surface tension force. Number of bubbles increased by MWCNT and SDS solution and decreased by CuO and Al₂O₃ nanofluid boiling. Results showed the comprehensive change in bubble dynamics and surface wettability by porous layer of nanoparticles and SDS solution.     

The effect of fouling on nucleate pool boiling of small wires

Pool boiling experiments with small diameter wires were conducted in earth gravity and microgravity conditions. Bare wire and fouled wire with a scale deposition of calcium carbonate was used. The wettability on the scale wire was higher than that on the bare wire. Though more vigorous bubbling was observed on the scale wire when compared to that on the bare wire at the same heat flux, the boiling curve for the scale wire was approximately the same as that on the bare wire. However, the critical heat flux (CHF) on the scale wire was higher than that of the bare wire. On the scale wire, the departure diameter of bubbles was relatively smaller than that on the bare wire. The smaller diameter of bubbles detaching from the scale wire is considered to be due to the high wettability and high nucleation site density. As the result, the coalescence of bubbles near the wire was prevented, and the CHF was delayed and increased on the scale wire when compared to that on the bare wire. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 316–329, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20016     

Effects of surface wettability on nucleate pool boiling heat transfer for surfactant solutions

Experiments for pool boiling of deionised water and acetone with different surfactant, 95% sodium dodecyl sulfate (SDS), Triton X-100 and octadecylamine, have been conducted under atmospheric pressure to investigate the effect of surface wettability. The boiling curves for different concentrations of surfactant solution on both smooth and roughened surfaces were obtained. The results show that the addition of surfactant can enhance the water boiling heat transfer, and the enhancement is more obvious for SDS solution; but has little influence on the acetone boiling curve. While the roughened surface enhanced the heat transfer for Triton X-100 solution, it also decreased the heat transfer coefficient for SDS solution. All these can be explained by including the changing of surface wettability, which has been neglected for a long time and should be an important parameter influencing boiling heat transfer. By incorporating such effects, the modified Mikic-Rohsenow pool boiling model, we proposed, can predict these experimental data well.     

Effect of surface particle interactions during pool boiling of nanofluids

The pool boiling characteristics of nanofluids is affected by the relative magnitudes of the average surface roughness and the average particle diameter. In the present work, an attempt has been made to study the interactions between the nanoparticles and the heater surface. The experimental methodology accounts for the transient nature of the boiling phenomena. The boiling curves of electro-stabilized Al₂O₃ water-based nanofluids at different concentrations on smooth and rough heaters and the burn-out heat flux have been obtained experimentally. Extensive surface profile characterization has been done using non-intrusive optical measurements and atomic force microscopy. A measure of the surface wettability has been obtained by determining the advancing contact angle. These results give an insight into the relative magnitudes of dominance of the prevalent mechanisms under different experimental conditions. Boiling on nanoparticle coated heaters has been investigated and presented as an effective solution to counter the disadvantageous transient boiling behavior of nanofluids.     

Single bubble dynamics and transient pressure during subcooled nucleate pool boiling

This article investigates the growth and collapse of a single vapor bubble during subcooled nucleate pool boiling of water at a vertical copper surface. Two high-speed cameras and a hydrophone were used for a synchronized measurement of the bubble life cycle and the pressure transient in the surrounding liquid at a system pressure of 25 kPa. A pressure transient with the basic form of one and a half sine was expected. This expectation was basically confirmed, but additional significant minima and maxima corresponding to a frequency of approximately 60 Hz were found in the pressure transient. The comparison of the bubble volume and the pressure transient leads to the conclusion, that heat transfer effects will have to be considered to explain the deviations from the sine. Pressure wave reflections inside the evaporator are of minor importance since their wave length is much larger than the extensions of the evaporator.     

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     

Critical heat flux enhancement in pool boiling using alumina nanofluids

The pool boiling characteristics of dilute dispersions of alumina nanoparticles in water were studied. Consistent with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). During experimentation and subsequent inspection, formation of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly changes surface texture of the heater wire surface which could be the reason for improvement in the CHF value. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20301     

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.     

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。