Studies on pool boiling heat transfer (modification of correlation of macrolayer thickness and measurements of macrolayer thickness at low heat fluxes)

The macrolayer thickness at critical heat flux has been determined based on the energy balance relation q_CHF=ρ_lH_fgδ_l·f, with measurements of the critical heat flux and the detachment frequency of vapor masses (coalesced bubbles) for various liquids at pressures from 0.05 MPa to 0.35 MPa for upward and vertical 20 mm diameter disk heaters. The macrolayer thickness correlation proposed in the fourth report of this series by Kumada and Sakashita [Trans. JSME, 58 (552) (1992), 2505] was modified with the data obtained in the present report. Macrolayer thicknesses at low heat fluxes for water and ethanol under atmospheric pressure were also measured while changing the orientation of the heating surface from vertical to downward. The measured macrolayers at low heat fluxes were thinner than those obtained from existing data measured by a probe method in the nucleate boiling region and agreed fairly well with the proposed correlation. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25 (8): 522–536, 1996     

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     

Measurement of surface dryout near heating surface at high heat fluxes in subcooled pool boiling

The authors have conducted measurements of liquid–vapor behavior in the vicinity of a heating surface for saturated and subcooled pool boiling on an upward-facing copper surface by using a conductance probe method. A previous paper [A. Ono, H. Sakashita, Liquid–vapor structure near heating surface at high heat flux in subcooled pool boiling, Int. J. Heat Mass Transfer 50 (2007) 3481–3489] reported that thicknesses of a liquid rich layer (a so-called macrolayer) forming in subcooled boiling are comparable to or thicker than those formed near the critical heat flux (CHF) in saturated boiling. This paper examines the dryout behavior of the heating surface by utilizing the feature that a thin conductance probe placed very close to the heating surface can detect the formation and dryout of the macrolayer. It was found that the dryout of the macrolayer formed beneath a vapor mass occurs in the latter half of the hovering period of the vapor mass. Two-dimensional measurements conducted at 121 grid points in a 1-mm × 1-mm area at the center of the heating surface showed that the dryout commences at specific areas and spreads over the heating surface as the heat flux approaches the CHF. Furthermore, transient measurements of wall void fractions from nucleate boiling to transition boiling were conducted under the transient heating mode, showing that the wall void fraction has small values (<10%) in the nucleate boiling region, and then steeply increases in the transition boiling region. These findings strongly suggest that the macrolayer dryout model is the most appropriate model of the CHF for saturated and subcooled pool boiling of water on upward facing copper surfaces.     

Temperature measurements near a heating surface at high heat fluxes in subcooled pool boiling

In previous papers (Int J Heat Mass Transfer, 2008;50:3481–3489, 2009;52: 814–821), the authors conducted measurements of liquid–vapor structures in the vicinity of a heating surface for subcooled pool boiling on an upward‐facing copper surface by using a conducting probe method. We reported that the macrolayer dryout model is the most appropriate model of the CHF and that the reason why the CHF increases with increasing subcooling is most likely that a thick macrolayer is able to form beneath large vapor masses and the lowest heat flux of the vapor mass region shifts towards the higher heat flux. To develop a mechanistic model of the CHF for subcooled boiling, therefore, it is necessary to elucidate the effects of local subcooling on boiling behaviors in the vicinity of a heating surface. This paper measured local temperatures close to a heating surface using a micro‐thermocouple at high heat fluxes for water boiling on an upward‐facing surface in the 0 to 40 K range of subcooling. A value for the effective subcooling, defined as the local subcooling during the period while vapor masses are being formed was estimated from the detected bottom peaks of the temperature fluctuations. It was established that the effective subcooling adjacent to the surface remains at considerably lower values than the bulk liquid subcooling. This suggests that, from nucleation to coalescence, the subcooling of a bulk liquid has a smaller effect on the behavior of primary bubbles than the extent of the subcooling would appear to suggest. An empirical correlation of the effective subcooling is proposed to provide a step towards quantitative modeling of the CHF for subcooled boiling. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20277     

Evaporation of macrolayer in nucleate boiling near burnout

The mechanism of nucleate boiling heat transfer near burnout has been investigated by studying the evaporation of the liquid macrolayer underneath the vapor mass. It is proved that heat conduction across the macrolayer and evaporation at the free surface is not a mode efficient enough to account for the major portion of heat transfer. The alternative model proposed is a macrolayer primarily consumed through evaporation of much thinner microlayers at the bottom of vapor stems penetrating the macrolayer. Analysis based upon the proposed mechanism shows that the macrolayer does not totally dry out in a vapor-mass cycle before boiling crisis. Other mechanisms accountable for the consumption of the macrolayer are also discussed.     

Liquid–vapor structure near heating surface at high heat flux in subcooled pool boiling

Liquid–vapor behavior close to a heating surface was measured using two conductance probes with tip diameters smaller than 5 μm. Measurements were carried out for water boiling on an upward-facing copper surface under subcooling from 0 to 30 K. The probe signals and the void fraction distributions showed that there is little difference in the liquid–vapor structure beneath large vapor masses in saturated and subcooled boiling, that a macrolayer remains on the heating surface, and that in subcooled boiling it does not dry out even at heat fluxes far higher than CHF for saturated boiling. The thickness of the macrolayer forming beneath large vapor masses was determined from the location where the probe signals corresponding to the large vapor masses disappear. It was found that the thicknesses of the macrolayer formed in subcooled boiling are comparable to or thicker than those near the CHF in saturated boiling, and it is considered that this is most likely to be one of the causes why the CHF increases with the increasing subcooling.     

Micro‐bubble emission boiling from horizontal and vertical surfaces to subcooled parallel flow water

Heat removal of more than 10 MW/m² in heat flux has been required in high‐heat‐generation equipment in nuclear fusion reactors. In some conditions of water subcooling and velocity, there appears an extraordinary high heat flux boiling in the transition boiling region. This boiling regime is called micro‐bubble emission boiling (MEB) because many micro‐bubbles are spouted from the heat transfer surface accompanying a huge sound. The study intent is to obtain heat transfer performance of MEB in horizontal and vertical heated surfaces to parallel flow of subcooled water, comparing with CHF of this system. Three types of MEB with different heat transfer performance and bubble behavior are observed according to the flow velocity and liquid subcooling. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 130–140, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10077     

A new mechanistic model for pool boiling CHF on horizontal surfaces

This work proposes a new mechanistic model for predicting the critical heat flux (CHF) in horizontal pool boiling systems. It is postulated that when the vapor momentum flux is sufficient to lift the liquid macrolayer from the heating surface, wetting is no longer feasible, and a transition from nucleate to film boiling occurs. This is the same mechanism that has found success in predicting CHF in flow boiling systems. An experimental investigation of CHF with pentane, hexane, and FC-72 in saturated horizontal pool boiling with chamber pressures of 150, 300, and 450 kPa provides evidence that the new model captures the variation of CHF with pressure reasonably well compared with other well known models. The new model is also compared with existing data from the literature over a reduced pressure range of 2 × 10^?5–2 × 10^?1. The mean deviation between the predicted and measured CHF is typically within 20% over the parameter space covered.     

On the mechanism of macrolayer formation in nucleate pool boiling at high heat flux

In nucleate boiling at high heat flux, a liquid layer, known as the ‘macrolayer’, is trapped between the heating surface and the vapour masses. An analysis of the mechanism of formation of this macrolayer is presented. Based on the analysis, a theoretical expression has been derived for the initial thickness of the macrolayer. The agreement between the theoretical values of the initial macrolayer thickness and the experimental values published in the literature is reasonably good.     

Heat transfer and size distribution of active nucleation sites in boiling propane outside a tube

Size distributions of active nucleation sites in pool boiling of propane on the outside of single horizontal copper and steel tubes with different surface finishes (emery ground and fine sandblasted) have been calculated from heat transfer measurements over wide ranges of heat flux and pressure. The model assumptions of Schömann for the heat transfer near growing and departing bubbles, which were applied in the calculations, have been modified and the calculated results have been compared to experimental investigations by high-speed video techniques. New measurements of the departure diameter d_A and frequency f of the bubbles at the same conditions as the heat transfer measurements show that d_A and f are influenced by the azimuthal position of the active nucleation sites on the tube and that the calculated results react sensitively to the input data for d_A and f . A modification of the model is proposed.     

Model for boiling explosion during rapid liquid heating

The process of rapid liquid heating with a linearly increasing boundary temperature condition has been simulated by applying the analytical solution of 1D semi-infinite heat conduction in association with the molecular theory of homogeneous nucleation boiling. A control volume having the size of a characteristic critical cluster at the liquid boundary is considered, and the corresponding energy balance equation is obtained by considering two parallel competing processes that take place inside the control volume, namely, transient external energy deposition and internal energy consumption due to bubble nucleation and subsequent growth. Depending on the instantaneous rate of external energy deposition and boiling heat consumption within the control volume, a particular state is defined as the boiling explosion condition in which bubble generation and growth cause the liquid sensible energy to decrease. The obtained results are presented in terms of the average liquid temperature rise within the control volume, maximum attainable liquid temperature before boiling explosion and the time required to achieve the condition of boiling explosion. The model is applied for the case of water heating at atmospheric pressure with initial and boundary conditions identical to those reported in the literature. Model predictions concerning boiling explosion are found to be in good agreement with the experimental observations. The boiling explosion condition as predicted by the present model is verified by comparing the heat flux across the liquid–vapor interface with the corresponding limit of maximum possible heat flux, q_max,max, at the time of boiling explosion. A comparative study between the actual heat flux and the limit of maximum heat flux, q_max,max, at the time of boiling explosion for different rates of boundary heating indicates that, with much higher boundary heating rates, it is possible to heat the liquid to a much higher temperature before theoretical instantaneous boiling explosion occurs.     

Photographic study of high-flux subcooled flow boiling and critical heat flux

This study examines both high-flux flow boiling and critical heat flux (CHF) under highly subcooled conditions using FC-72 as working fluid. Experiments were performed in a horizontal flow channel that was heated along its bottom wall. High-speed video imaging and photomicrographic techniques were used to capture interfacial features and reveal the sequence of events leading to CHF. At about 80% of CHF, bubbles coalesced into oblong vapor patches while sliding along the heated wall. These patches grew in size with increasing heat flux, eventually evolving into a fairly continuous vapor layer that permitted liquid contact with the wall only in the wave troughs between vapor patches. CHF was triggered when this liquid contact was finally halted. These findings prove that the CHF mechanism for subcooled flow boiling is consistent with the interfacial lift-off mechanism proposed previously for saturated flow boiling.     

Characteristics of boiling curve in transition region between nucleate boiling and film boiling

An experimental study has been carried out for estimating surface temperature and heat flux during both a transient heating process from nucleate boiling to film boiling and a cooling process in the reverse direction. Experiments were at atmospheric pressure, and calculations used a newly developed inverse solution. Three different materials, gold, copper, and brass, were employed to make clear the effect of thermal properties on the boiling curves in the transient region including the maximum and minimum heat fluxes. It was determined that the histories of surface temperature and heat flux for the transition boiling region during either heating or cooling process can be tracked well. The experiment shows that hysteresis exists in the heating and cooling processes for the transition region while no hysteresis exists in the nucleate boiling region, except that the maximum heat fluxes reached during the heating and cooling processes are much different. It was found that the characteristics for the heating process are minimally influenced by thermal properties, while characteristics of the cooling process are greatly affected. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 20–34, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20097     

Experimental investigation of bubble behaviors in subcooled flow boiling

The experimental investigation on vapor bubble growth is performed for analyzing subcooled boiling in a vertical annular channel with inner heating surface and upward water flow under atmospheric pressure. Bulk liquid mass flux ranges from 79 kg/m2s to 316 kg/m2s, and subcooling is from 40 K to 60 K. The bubble behaviors from inception to collapse are captured by High-speed photography. The performance of bubble growth recorded by the high-speed photography is given in this paper. The bubble behaviors, effect of the bubble slippage on the heat transfer, and various forces acting on the bubble are discussed.     

Film boiling heat transfer from a horizontal circular plate facing downward

The two‐dimensional, steady, pool film boiling heat transfer from a horizontal circular plate facing downward to a stagnant saturated liquid is studied theoretically. It is assumed that the vapor‐liquid interface is smooth and that radiation can be disregarded. The relevant governing equations for the vapor film are solved for saturated water at atmospheric pressure using an improved two‐equation boundary‐layer integral method. It is shown that the dimensionless temperature profile is affected by the wall superheat ΔT_sat and that the ratio of Nu to X^0.2 is an increasing function of ΔT_sat. Here, Nu represents the mean Nusselt number and X the film‐boiling Rayleigh number. In addition, it is revealed that the one‐equation boundary‐layer integral method developed by Nishio and colleagues is fairly accurate in predicting the film thickness, the representative radial velocity, and the mean Nusselt number. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 72–84, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10071     

Enhanced boiling heat transfer in mesochannels

Heat transfer characteristics are studied for a hybrid boiling case that combine features of spray cooling and flow boiling. In such a hybrid system, a liquid is atomized and the surrounding vapor is entrained into the droplet cone to provide an initial quality for enhanced boiling. An in-house experimental setup was developed to obtain surface temperature and heat flux measurements in a series of converged mesochannels for hybrid boiling. To compare the heat transfer performance of this hybrid technique, a flow boiling module was also developed using the same series of converged mesochannels. The inlet and exit hydraulic diameter of the mesochannels was 1.55 and 1.17 mm, respectively. The heat flux was in the range of 15–45 kW/m² and the estimated mass flux varied from 45 kg/m²s at the channel inlet to 110 kg/m²s at the channel outlet. Moreover, a model was presented to predict surface temperatures and heat transfer coefficients for flow boiling and hybrid boiling in mesochannels. This model was developed based on Chen’s formulation (1966) [21] but with two essential modifications. First, the laminar entry length effect was taken into consideration for heat transfer coefficient calculation. Second, the boiling enhancement factor was calculated based on the fluid properties. The model was compared to the experimental data and several other correlations for both cases. This model shows good agreement with the experimental data (mean deviations on the order of 12–16%).     

Critical Heat Flux of Boiling Heat Transfer in a Moderate Narrow Space

INTanDUCTI0NBoilingheattransferandcriticalheatflux(CHF)inaconfinednarrowspacehavebeenstudiedexperi-melltallybyanumberofinvestigatorsinthepastfewdecades.However,thereisnoanypopularlyacceptedmodelintheheattransferinnarrowspaceboiling,althoughsomepopularknowledgeabouttheboilingheattransferinthenarrowspacehavebeenacceptedbymanyresearchers.Theknowledgecanbecon-cludedasthatthenucleateboilingheattransferisenhancedatlowheatfluxregionanddeterioratedathighheatfiuxregi0nespeciallyatCHF.Theenhanceme…     

Pool boiling on a superhydrophilic surface

Titanium Dioxide, TiO₂, is a photocatalyst with a unique characteristic. A surface coated with TiO₂ exhibits an extremely high affinity for water when exposed to UV light and the contact angle decreases nearly to zero. Inversely, the contact angle increases when the surface is shielded from UV. This superhydrophilic nature gives a self-cleaning effect to the coated surface and has already been applied to some construction materials, car coatings and so on. We applied this property to the enhancement of boiling heat transfer. An experiment involving the pool boiling of pure water has been performed to make clear the effect of high wettability on heat transfer characteristics. The heat transfer surface is a vertical copper cylinder of 17 mm in diameter and the measurement has been done at saturated temperature and in a steady state. Both TiO₂-coated and non-coated surfaces were used for comparison. In the case of the TiO₂-coated surface, it is exposed to UV light for a few hours before experiment and it is found that the maximum heat flux (CHF) is about two times larger than that of the uncoated surface. The temperature at minimum heat flux (MHF) for the superhydrophilic surface is higher by 100 K than that for the normal one. The superhydrophilic surface can be an ideal heat transfer surface. Copyright © 2002 John Wiley & Sons, Ltd.     

Zero‐flux zone model analysis of a single bubble in boiling liquid

We start from a bubble, the basic unit of boiling, to explore liquid boiling. Research indicates that the heat and mass transfer between liquid and vapor is the determinant factor of boiling heat transfer. We have analyzed interfacial vaporization and condensation of a single bubble in boiling liquid based on a zero‐flux zone model. We have deduced the expression of zero‐flux angle and discussed the relationship between zero‐flux zone and the other parameters in order to comprehend the mechanism of boiling heat transfer. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 249–256, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10026     

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