Critical Heat Flux In Pool Research Articles

Numerical prediction of critical heat flux in pool boiling with the two-fluid model

Three-dimensional numerical simulations of the atmospheric saturated pool boiling are performed with the aim of predicting the critical heat flux. The two-phase mixture in pool boiling is described with the transient two-fluid model. The transient heat conduction in the horizontal heated wall is also solved. Dynamics of vapor generation on the heated wall is modeled through the density of nucleation sites and the bubble residence time on the wall. The heater’s surface is divided into zones, which number per unit area equals the density of nucleation sites, while the location of nucleation site within each zone is determined by a random function. The results show a replenishment of the heater’s surface with water and surface wetting for lower heat fluxes, while heater’s surface dry-out is predicted at critical heat flux values. Also, it is shown that the decrease of nucleation site density leads to the reduction of critical heat flux values. Obtained results of critical heat flux are in good agreement with available measured data. The presented approach is original regarding both the application of the two-fluid two-phase model for the prediction of boiling crisis in pool boiling and the defined boundary conditions at the heated wall surface.

Enhancement of the critical heat flux in saturated pool boiling using honeycomb porous media

Enhancement of the critical heat flux in pool boiling by the attachment of a honeycomb-structured porous plate on a heated surface is investigated experimentally using water under saturated boiling conditions. As the height of the honeycomb porous plate on the heated surface decreases, the CHF increases to 2.5 MW/m 2, which is approximately 2.5 times that of a plain surface (1.0 MW/m 2). Automatic liquid supply due to capillary action and reduction of the flow resistance for vapor escape due to the separation of liquid and vapor flow paths by the honeycomb-structure are verified to play an important role in the enhancement of the CHF. A simplified one-dimensional model for the capillary suction limit, in which the pressure drops due to liquid and vapor flow in the honeycomb porous plate balances the capillary force, is applied to predict the CHF. The calculated results are compared with the measured results.

Dispersion and Surface Characteristics of Nanosilica Suspensions

Nanofluids consisting of nanometer-sized particles dispersed in base liquids are known to be effective in extending the saturated boiling regime and critical heat flux in pool boiling. The heat transfer characteristics of nanosilica suspensions with particle sizes of 10 and 20 nm in pool boiling with a suspended heating Nichrome wire have been analyzed. The pH value of the nanosuspensions is important from the point of view that it determines the stability of the particles and their mutual interactions toward the suspended heated wire. When silica is suspended in water with no additives, the surface potential of the nanoparticles determines their movement toward the electrodes. Particles continuously deposit on the wire and extend the burnout heat flux, influenced by the chemical composition of the nanofluids. This agglomeration allows high heat transfer through interagglomerate pores, resulting in a nearly threefold increase in burnout heat flux. Particle size, zeta potential, and the burnout heat flux values under different volume concentrations are provided. The burnout heat flux of the wire does not increase monotonically with concentration, but depends on the agglomeration characteristics, particle shape, and the hydroxylated surface of the nanoparticles.

ハニカム状多孔質体装着による飽和プール沸騰限界熱流束の向上(<小特集>究極の除熱性能を目指して)

Abstract Enhancement of the critical heat flux in pool boiling by the attachment of a honeycomb-structured porous plate on a heated surface is investigated experimentally using water under saturated boiling conditions. As the height of the honeycomb porous plate on the heated surface decreases, the CHF increases to 2.5 MW/m 2 , which is approximately 2.5 times that of a plain surface (1.0 MW/m 2 ). Automatic liquid supply due to capillary action and reduction of the flow resistance for vapor escape due to the separation of liquid and vapor flow paths by the honeycomb-structure are verified to play an important role in the enhancement of the CHF. A simplified one-dimensional model for the capillary suction limit, in which the pressure drops due to liquid and vapor flow in the honeycomb porous plate balances the capillary force, is applied to predict the CHF. The calculated results are compared with the measured results.

Compositive effects of orientation and contact angle on critical heat flux in pool boiling of water

An experimental study was carried out to investigate the effects of heat transfer surface orientation and the solid–liquid contact angle on the boiling heat transfer and critical heat flux (CHF) in water pool boiling using a smooth heat-transfer surface under atmospheric pressure. The orientation angle was ranged from 0° (up-facing horizontal position) to 180° (down-facing horizontal position) with a pace of 45°. The three kinds of heat transfer surfaces having different solid–liquid contact angles were the normal surface with a contact angle of 55°, the hydrophilic surface with a contact angle of 30° and the superhydrophilic surface with a contact angle of 0°. The experimental results indicate that orientation and contact angle have complex, coupling effects on heat transfer and CHF. A predicting correlation for the CHF which takes the effects of both orientation and contact angle into account is established. The predicting correlation agrees reasonably well with the experimental data.

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.

Visualization of a principle mechanism of critical heat flux in pool boiling

A new experimental work was made to discover a principle mechanism of the burnout in pool boiling. Here, we directly observed a liquid layer structure under a massive vapor clot and the liquid layer-related burnout phenomenon. Based on the present observations, we have made a visual model for the formation and dryout of a liquid film under its vapor environment. At the formation process, liquid is trapped in interleaved space between growing bubbles and surface and the liquid trapping continues between coalesced bubbles and surface. In the dryout process, we especially observed vapor “holes” made by spontaneous breakup of discrete nucleating bubbles inside a vapor clot. The burnout can be triggered by the evaporation of the liquid film region expanded from rims of the holes.

A Study of Subcooled Pool Boiling of Water: Contact Area of Boiling Bubbles with a Heating Surface During a Heating Process

The contact area of bubbles with a transparent heating surface was optically measured during subcooled pool boiling of water on the ground. In the experiments, boiling bubbles were attached to the heating surface with a bubble holder and nearly reproduced the bubble behavior observed in low gravity. DC power was applied to the ITO heater and increased until the heater surface burned out. In quick heating, that is about 20 second until burnout and equal to the heating time during the low gravity period, the contact area was smaller than that for long time heating at the same heat flux. The experimental results suggest the reason why the critical heat flux in pool boiling is higher than the widely accepted predictions in microgravity. In a drop shaft experiment with constant heating, the contact area increased dramatically at the start of microgravity and became constant. Boiling bubbles coalesced and remained just over the heating surface.

Effects of Surface Coating on the Critical Heat Flux for Pool Boiling from a Downward Facing Surface

As part of a joint U.S.−Korean International Nuclear Engineering Research Initiative investigating methods to enhance external reactor vessel cooling (ERVC) under severe accident conditions, it was proposed that surface coating be used to enhance the critical heat flux (CHF) for downward facing boiling. Toward this end, metallic microporous coatings were selected, and different compositions of the coating material were evaluated, in order to obtain a mixture with desirable qualities. Durability and adhesion tests were also done to study the performance of each of the resulting coatings. Quenching of the candidate coatings was then conducted under downward facing boiling conditions, using hemispherical test vessels to obtain the local boiling curves. Compared to a plain vessel surface under identical experimental conditions, considerable enhancement was observed in the critical heat flux for vessels with surface coatings. Optical and SEM (Scanning Electron Microscope) photos showed that the candidate coating possessed the desired porous microstructure with interconnecting channels and pores, leading to appreciable increases in the local CHF limits.

A Theoretical Model to Predict Pool Boiling CHF Incorporating Effects of Contact Angle and Orientation

A theoretical model is developed to describe the hydrodynamic behavior of the vapor-liquid interface of a bubble at the heater surface leading to the initiation of critical heat flux (CHF) condition. The momentum flux resulting from evaporation at the bubble base is identified to be an important parameter. A model based on theoretical considerations is developed for upward-facing surfaces with orientations of 0 deg (horizontal) to 90 deg (vertical). It includes the surface-liquid interaction effects through the dynamic receding contact angle. The CHF in pool boiling for water, refrigerants and cryogenic liquids is correctly predicted by the model, and the parametric trends of CHF with dynamic receding contact angle and subcooling are also well represented.

Critical heat flux in pool boiling under the effect of adverse dielectrophoretic forces

We study the effects of externally applied electric fields on the critical heat flux in pool boiling for the case of dielectrophoretic forces over the bubbles pointing towards the heater. Experimental tests have been performed for cylindrical heaters with an axial wire as electrode. The results show substantial reductions in the value of the critical heat flux when the adverse dielectrophoretic forces over the bubbles are on the order of magnitude of the buoyancy forces. After that point, an increase in the strength of the applied field does not have an impact in the critical heat flux. In addition, the effects are smaller at saturation temperature than in subcooled conditions.

A dry-spot model of critical heat flux in pool and forced convection boiling

A new dry-spot model for high heat flux nucleate boiling region and critical heat flux (CHF) is proposed. The new concept of dry area formation based on Poisson distribution of active nucleation sites and the critical active site number is introduced. It is shown that CHF can be determined without any correction factor based on information on the boiling parameters such as active site density and bubble diameter, etc., in the nucleate boiling region. It turns out that the present model well explains the mechanism on how parameters such as flow modes and surface wettability influences CHF.

Critical heat flux in pool boiling on a vertical heater

Critical heat flux during pool boiling on a vertical heater of wire or plate has been measured employing water and R113. The experiment was made for a wire of 0.5 to 2 mm in diameter and for a plate of 5, 7 and 30 mm in width and from 20 to 300 mm in height. The pressure was 1 and 2 bar for water and 1, 2, 3 and 4 bar for R113. The experiment shows that for the case of both wire and plate of 5, 7 mm, a large coalesced bubble entirely surrounds the vertical heater and rises surrounding it, while for the case of w = 30 mm, a large bubble cannot surround and rises along its surface. The characteristic of CHF can be divided into two regimes depending on the flow condition when CHF takes place. Correlations are proposed for the CHF of the wire and the plate of w = 5, and 7 mm, yielding good accuracy. The CHF for the plate of w = 30 mm has a similar tendency to that in one side headed surface and can be predicted reasonably by existing correlation for one side heated surface.

Critical heat flux in pool boiling of aqueous surfactant solutions as determined by the quenching method

Pool boiling heat transfer of a copper microporous coating was experimentally studied in borated water with a concentration of boric acid from 0.0 to 5.0 vol percent (vol%) to determine the effect of boric acid on boiling heat transfer in water. A high-temperature, thermally conductive microporous coating (HTCMC) was created by sintering copper powder with an average particle size of 67 μm onto a 1 cm × 1 cm plain copper surface with a coating thickness of ~300 μm within a furnace in a vacuum environment. The tests showed that the nucleate boiling heat transfer coefficient (NBHT) of HTCMC became slightly less enhanced as the concentration of boric acid increased but the NBHT coefficient values were still significantly higher than those of the plain surface. The critical heat flux (CHF) values from 0 to 1.0 vol% were maintained at ~2,000 kW/m2, and then, they gradually decreased down to ~1,700 kW/m2 as the concentration increased further to 5.0 vol%. It is believed that the micro-scale pores of the HTCMC were partially blocked by the high boric acid concentration during the nucleate boiling such that the small bubbles were not effectively created using the HTCMC reentrant cavities as the boric acid concentration increased.

Critical heat flux in pool boiling of binary mixtures as determined by the quenching method

Critical heat flux in pool boiling of binary mixtures as determined by the quenching method

A New Model for CHF in Pool Boiling at Higher Pressure.

The concept of critical heat flux (CHF) is discussed based on the mechanism that the CHF is caused by the dryout of a liquid layer formed on a heating surface. It is suggested that a liquid macrolayer is formed due to the coalescence of bubbles for most boiling systems, and that the dryout of the macrolayer is controlled by the hydrodynamic behavior of coalesced bubbles on the macrolayer. Based on these considerations, a new CHF model is proposed for saturated pool boiling at higher pressures. The idea of this model comes from a close examination of the measured diameters of various bubbles and the photographic records obtained by Semeria (1963) for water boiling under higher pressures. In the model, a liquid macrolayer is formed due to coalescence of the secondary bubbles formed from the primary bubbles. The detachment of the tertiary bubbles formed from the secondary bubbles determines the frequency of the liquid macrolayer formation. The CHF occurs when the macrolayer is dried out before the departure of the tertiary bubbles from the heating surface. One of the formulations of the model gives the well-known Kutateladze or Zuber correlation for CHF in saturated pool boiling.

A physical approach to critical heat flux of subcooled flow boiling in round tubes

This paper presents an analysis of the critical heat flux (CHF) of subcooled flow boiling based on the liquid sublayer dryout mechanism, assuming that it is a similar phenomenon to CHF in pool boiling except for apparent differences between forced and natural convection. Employing the same formula of sublayer thickness as that derived for CHF in pool boiling, a physcial model of CHF is derived with an empirical coefficient relating to the velocity of a vapor blanket sliding on a thin liquid sublayer. Predicted CHF values are compared with experimental data for water, R-12, R-11, nitrogen, helium, and R-113, respectively, suggesting propriety of the present model.

Characteristics of pool boiling heat transfer in the vicinity of the critical heat flux point. (The relations between bubble motion and heat flux fluctuation)

In order to study the mechanism of critical heat flux of pool boiling, measurement of heat flux fluctuation and high speed photographic observation were carried out simultaneously on a constant temperature surface heated by electrically controlled Joule heating, in the vicinity of a critical heat flux point. They revealed the following : (1) when the surface is covered by a coalesced bubble, heat flux begins to increase because evaporation is enhanced ; (2) heat flux begins to decrease either while the surface is covered by a coalesced bubble due to dryout of the liquid on the surface, or just after nucleation begins under the rising bubble due to a relatively low heat transfer rate of nucleation ; and (3) heat applied from the time the surface is covered by a bubble until the time heat flux begins to ducrease does not vary widely and corresponds to latent heat of liquid about 11 -μm thick.

Hydrodynamic aspect of the effect of the thermophysical properties of the heater on the critical heat flux in bulk pool boiling of helium

The article examines the hydrodynamic aspect of the effect of the thermophysical properties of the heater on the critical heat flux in pool boiling of helium.

Acceleration and the Critical Heat Flux in Pool Boiling Heat Transfer

Many theories predicting the critical (or burnout) heat flux in saturation pool boiling have been proposed during the past decade. Although most are able to correlate existing experimental data, it is evident that the theoretical models upon which they are based are widely different in concept. Moreover, owing to lack of experimental evidence it is not clear which, if any, are based upon the more correct models. Certain of the theories implicitly contain the acceleration due to gravity raised to a different power; by performing a relatively simple experiment it may be possible to accept or reject certain of the theories. An experiment of this type is described in the present paper. On the basis of the results of this experiment it is possible to reject several of the above theories. The extent to which the acceleration of the boiling system is important as a parameter in determining the critical heat flux was ascertained experimentally. A simple centrifugal apparatus is described which was used for determining the critical heat flux in pool boiling to water at atmospheric pressure and accelerations in the range 1 < ( a/g) < 160. A two-variable non-linear correlation involving a logarithmic transformation was made upon the experimental results after allowing for the possible effect of pressure and potential sub-cooling by extrapolation to zero subcooling. The resulting equation is of the form: