On the Relation between Nucleation Site Density and Critical Heat Flux of Pool Boiling

ABSTRACT

It is traditionally accepted that the critical heat flux (CHF) decreases with increasing nucleation site density (NSD). However, such a CHF-NSD relation was no longer observed in the BETA-B experiment performed on nano-film heaters; instead the increase of NSD resulted in a gain in CHF. To address this seeming contradiction in the relation between critical heat flux and nucleation site density, the present work employed probabilistic analysis to reveal the different tendencies. A concept of effective NSD was proposed, which concerns the active nucleation sites appear within a bubble lifetime, and the resulting bubbles have the chance of direct interaction. We assumed that the boiling crisis on a heater surface is mainly induced by two mechanisms: dry spot expanding in isolated bubble regime for low-NSD surface, coalescence of dry spots under multiple bubbles in fully developed nucleate boiling regime for high-NSD surface, or a combination of the two in the transition regime for medium-NSD surface. Accordingly, we estimated the critical heat flux of each boiling regime at which the boiling crisis occurs. The result indicated that there is a threshold of nucleation site density below which the increase of NSD is contributing to CHF enhancement, while the trend is inverted beyond the threshold.     

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.     

Stochastic simulation of non-hydrodynamic burnout on conductively heated surface

This paper presents a stochastic simulation of boiling on a two-dimensional matrix, considering the interference of nearby bubbles and the lateral heat conduction within the heating element. Some salient dynamic features for boiling burnout have been properly reproduced. The effective critical heat flux (CHF) is shown to at the minimum of the hydrodynamic instability induced CHF and the local dryout-proporgation burnout. The stable and metastable regimes of nucleate boiling mode areidentified as well.     

Effects of Al2O3/R-123 nanofluids containing C19H40 core–shell phase change materials on critical heat flux

In this paper, the curiosity is coming from how to bring out the fluidic capability of nanofluids (fluid itself) for critical heat flux (CHF) enhancement away from surface deposition effects such as improved wettability. The pool boiling characteristics of dilute dispersions of alumina and the microencapsulated C₁₉H₄₀ phase change material (MPCM) in R-123 were studied. Whereas other nanofluid studies only reported that a significant enhancement of CHF was achieved by buildup of a porous layer of nanoparticles on the heater surface during nucleate boiling, it was found that the additional CHF enhancement of 24% occurred with the MPCM compared to alumina nanomaterials. With solid–liquid phase changes, PCMs in suspension delay the occurrence of CHF by absorbing heat around from the heater, nucleate bubbles and merged bubbles while PCM shells prevent leakage of molten cores and allows the return to solid with exchanges of heat at some distances. The present study found that PCMs could make fluidic effects of nanofluid not relying on the surface depositions.     

Heat transfer characteristics of submerged jet impingement boiling of saturated FC-72

Global heat transfer characteristics of submerged jet impingement boiling of a highly wetting dielectric fluid (FC-72) on a heated copper surface are presented. The effect of variation of the jet exit Reynolds number (Re) on boiling incipience, fully developed nucleate boiling, and critical heat flux (CHF) are documented. The jet exit Re is varied by variations of the jet exit velocity and the jet nozzle diameter for a fixed surface diameter. High-speed visualization is used to supplement trends observed in the heat transfer data. Scenarios of low and high incipience wall superheat are identified, corresponding to partially or fully developed nucleate boiling condition upon initiation of boiling. For the high incipience wall superheat scenario, the time of spread of boiling activity over the heated surface during temperature overshoot is found to be inversely proportional to the wall superheat temperature at boiling incipience. The incipient boiling wall superheat temperature is found to be uncorrelated with jet Re and jet diameter. A cumulative probability distribution function is used to characterize the onset of boiling with wall superheat temperature. At a fixed Re, CHF increases with increasing jet velocity and with decreasing jet diameter, indicating that the jet kinetic energy is a key parameter in CHF enhancement. The CHF data are compared with available jet impingement CHF correlations from literature on free surface and confined jets. The free surface jet CHF correlation by Monde and Katto (1978) [1] is seen to best capture the experimental data trends for Re greater than 4000.     

Heat Transfer and Crisis Phenomena at the Film Flows of Freon Mixture over Vertical Structured Surfaces

Results on experimental investigation of heat transfer in the liquid films dichlorofluoromethane R21 and dichlorotetrafluoroethane R114 Freon mixture over the vertical tubes are presented. We have studied the film flow over the outer surface of tubes with 50-mm diameter and different configurations: smooth surface, horizontal ribs, and diamond-shape knurling. Heat transfer coefficients were measured under the conditions of evaporation and nucleate boiling together with wave characteristics of the falling film, binary mixture composition, and critical heat fluxes corresponding to dry spots formation. The film Reynolds number at the inlet to the test section was varied from 15 to 250. At evaporation regime the heat transfer coefficient for a smooth surface decreases classically with an increase of Reynolds number. Dependence of heat transfer coefficient on irrigation density for the surface with diamond-shape knurling is similar to dependence for the smooth surface with insignificant heat transfer intensification. The heat transfer coefficients at nucleate boiling for the studied structured surfaces are close to those obtained for the smooth tube. Development of critical phenomena is determined by regularities of dry spots formation typical for evaporation of the wavy liquid film.     

Visualization of boiling structures in high heat-flux pool-boiling

The present paper presents experimental results of observation of liquid-solid contact and bubble behaviors around the critical heat flux of saturated and subcooled pool boiling on a plate of single crystal sapphire. The observation was conducted from the backside of a rectangular boiling surface and also from the side and backside of a boiling surface with a narrow width. The main results obtained are summarized as follows. The bubble base area is almost dry and lateral coalescence of bubbles forms coalescent dry areas. As the wall superheat increases, liquid-solid contact becomes like a canal meandering through dry areas. The dependency on the surface superheat of the contact-line length density (CLLD) which is defined as the total length of the boundary between wetted and dry areas in a unit area is almost the same as the boiling curve, and the value of CLLD at CHF is not strongly dependent on boiling liquid and subcooling. The relation of the number density of dry areas and their equivalent diameter in the large dry area region at CHF is not dependent on boiling liquid and subcooling, and it is similar to that of dropwise condensation.     

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     

Unified theoretical prediction of fully developed nucleate boiling and critical heat flux based on a dynamic microlayer model

A new dynamic microlayer model has been proposed to predict theoretically the heat flux in fully developed nucleate boiling regions including critical heat flux (CHF). In this model, the heat transfer with boiling is mainly attributed to the evaporation of the microlayers which are periodically formed while the individual bubbles are forming. Since the initial microlayer thickness becomes thinner with the increase of wall superheat, both the local evaporation and the partial dryout speed of the microlayer increase. As a result, the time-averaged heat flux during the period of individual bubble has a maximum point, the CHF, at the predicted continuous boiling curve.     

Effect of pressure, subcooling, and dissolved gas on pool boiling heat transfer from microporous, square pin-finned surfaces in FC-72

The present research is an experimental study of the effects of pressure, subcooling, and non-condensable gas (air) on the pool nucleate boiling heat transfer performance of microporous enhanced finned surfaces. The test surfaces, solid copper blocks with 1-cm² bases and 5×5 square pin-fin arrays of 2, 4 and 8 mm fin lengths, were immersed in FC-72. The test conditions included an absolute pressure range of 30-150 kPa and a subcooling range of 0 (saturation) to 50 K. Effects of these parameters on nucleate boiling and critical heat flux (CHF) were investigated. In addition, differences between pure subcooled and gas-saturated conditions as well as horizontal and vertical base orientations were also investigated. Results showed that, in general, the effects of pressure and subcooling on both nucleate boiling and CHF were consistent with previously tested flat surface results, however, subcooling was found to significantly affect the high heat flux region of the microporous finned surfaces nucleate boiling curves. The relative enhancement of CHF from increased subcooling was greater for the microporous surface than the plain surface but less than a microporous flat surface. The horizontal orientation (horizontal base/vertical fins) was found to be slightly better than the vertical orientation (vertical base/horizontal fins). Correlations for both nucleate boiling and CHF for the microporous surfaces were also developed.     

Nucleate boiling on the superhydrophilic surface with a small water impingement jet

In this study, the coating process on the copper surface with titanium dioxide (TiO₂) has been introduced. The coated surface exhibits extremely high affinity for water and the solid–liquid contact angle decreases nearly to zero by exposing the surface to ultra-violet light. This superhydrophilic characteristic was applied to nucleate boiling heat transfer of water jet impingement on a flat heated plate. By making use of this special heat transfer surface, the nucleate boiling heat transfer and the critical heat flux (CHF) of a bar water jet impingement on a large flat superhydrophilic surface was experimentally investigated. The experimental data were measured in a steady state. The purified water was employed as the working liquid. Three main influencing factors, i.e., subcooling, impact velocity and the surface coating condition, were changed and their effects on the nucleate boiling heat transfer and the CHF were investigated. The empirical correlations were obtained for predicting the CHF of steady boiling for a small round water jet impingement on a large flat superhydrophilic surface. The experimental results show that the CHF on the superhydrophilic surface is about 30% higher than that on conventional copper surface by decreasing the solid–liquid contact angle.     

Direct numerical simulations of pool boiling curves including heater's thermal responses and the effect of vapor phase's thermal conductivity

Effects of heater's thermal properties and vapor phase's thermal conductivity on saturated pool boiling above a large horizontal heater are simulated numerically based on an improved pseudo-potential liquid-vapor phase change lattice Boltzmann model. A transient conjugate heat transfer problem is under consideration, where the conjugate thermal boundary condition is imposed and heater's thermal responses during boiling processes are investigated. Saturated pool boiling curves from onset of nucleate boiling to critical heat flux (CHF), to transition boiling regime to stable film boiling regime are obtained numerically. It is found that the simulated critical heat flux (CHF) agrees reasonably well with existing analytical models. Also, the simulated boiling heat fluxes in stable film boiling regime are shown to be in good agreement with the existing analytical solution. Thus, this improved pseudo-potential liquid-vapor phase change lattice Boltzmann model is quantitatively validated. Simulation results demonstrate that there is significant maldistribution in temperature distribution near the top of heater surface in nucleate boiling regime, CHF point and transition boiling regime. As a result, two-dimensional heat conduction can not be ignored when evaluating heat flux closely beneath the heater's top surface. It is also shown that both heater's thermal conductivity and thermal mass (the product of density and specific heat at constant pressure) have no effect on CHF value as well as the boiling curve in nucleate boiling regime and film boiling regime for a thick heater. However, the transition boiling regime of the boiling curve moves to the left with the increasing heater thermal conductivity and heater thermal mass for a thick heater. Increasing the vapor theraml conductivity has no effect on CHF but would increase boiling heat flux in film boiling regime, and hence shortening the transition boiling regime.     

Non-linear dynamical analyses of transient surface temperature fluctuations during subcooled pool boiling on a horizontal disk

The class of dynamics in pool boiling on a large-size heater is assessed under subcooled pool boiling conditions. Transient surface temperature measurements are obtained using surface micro-machined K-type thin film thermocouples (TFT) in 10 °C subcooled pool boiling experiments on a 62.23 mm diameter silicon wafer using PF-5060 as the test liquid. Surface temperature data is obtained at each steady state condition to generate the boiling curve. The fraction of false-nearest neighbors, recurrence plots and space–time separation plots are obtained using the TISEAN package. The correlation dimension is then estimated from the re-constructed phase space data using a naïve algorithm. The correlation dimension varies from ～11.2 to 11.5 near onset of nucleate boiling (ONB), to ～7–10 in fully developed nucleate boiling (FDNB) ～7–9 near critical heat flux (CHF) condition, and from ～6.6 to 7.7 in film boiling. False-nearest neighbor estimates and recurrence plots show that nucleate boiling may be dominated by statistical processes near ONB and in partial nucleate boiling (PNB). In contrast, FDNB, CHF and film boiling seem chaotic and governed by deterministic processes.     

Enhanced boiling of HFE-7100 dielectric liquid on porous graphite

Enhanced boiling of HFE-7100 dielectric liquid on porous graphite measuring 10 mm × 10 mm is investigated, and results are compared with those for smooth copper (Cu) of the same dimensions. Although liquid is out-gassed for hours before performing the pool boiling experiments, air entrapped in re-entrant type cavities, ranging in size from tens to hundreds of microns, not only enhanced the nucleate boiling heat transfer and the critical heat flux (CHF), but also, the mixing by the released tiny air bubbles from the porous graphite prior to boiling incipience enhanced the natural convection heat transfer by ∼19%. No temperature excursion is associated with the nucleate boiling on porous graphite, which ensues at very low surface superheat of 0.5–0.8 K. Conversely, the temperature overshoot at incipient boiling on Cu is as much as 39.2, 36.6, 34.1 and 32.8 K in 0 (saturation), 10, 20 and 30 K subcooled boiling, respectively. Nucleate boiling ensues on Cu at a surface superheat of 11.9, 10.9, 9.5 and 7.5 K in 0 (saturation), 10, 20 and 30 K subcooled boiling, respectively. The saturation nucleate boiling heat flux on porous graphite is 1700% higher than that on Cu at a surface superheat of ∼10 K and decreases exponentially with increased superheat to ∼60% higher near CHF. The CHF values of HFE-7100 on porous graphite of 31.8, 45.1, 55.9 and 66.4 W/cm² in 0 (saturation), 10, 20 and 30 K subcooled boiling, are 60% higher and the corresponding superheats are 25% lower than those on Cu. In addition, the rate of increase in CHF with increased liquid subcooling is 50% higher than that on Cu.     

An Experimental Investigation of Nucleate Pool Boiling Heat Transfer of Nanofluids From a Hemispherical Surface

An experimental investigation of nucleate pool boiling heat transfer is carried out using SiO₂ nanofluid in atmospheric pressure and saturated conditions. The results show that the nucleate boiling heat transfer coefficient (HTC) of the nanofluids is lower than that of deionized water, especially in high heat fluxes. In addition, the experimental results indicate that the critical heat flux (CHF) improves up to 45% with the increase of the nanoparticle volume concentration. Atomic force microscopy images from the boiling surface reveal that the nanoparticles are deposited on the heating surface during the nanofluid pool boiling experiments. It is found that the boiling HTC deteriorates as a result of the reduction in active nucleation sites and the formation of extra thermal resistance due to blocked vapor in the porous structures near the heating surface. Furthermore, the improvement of the surface wettability causes an increase in CHF. Based on the experimental investigations, it can be concluded that the changes in the properties of the boiling surface are mainly responsible for the variations in nanofluids boiling performance.     

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.     

Bubble sweeping and jet flows during nucleate boiling of subcooled liquids

An experimental investigation was conducted to investigate nucleate boiling on a very fine heating wire. By using zoom routine and CCD camera system, the dynamical process of nucleate boiling was visually observed. Sweeping bubbles and several modes of jet flows were described and discussed. For some cases, big bubbles, small bubbles, sweeping bubbles and jet flows coexisted in boiling system, and greatly enhanced heat transfer. These phenomena are quite different from usual observation of nucleate boiling. In this paper, the process of bubble sweeping phenomenon is described in detail and the effect induced by sweeping bubbles is argued. And also, several jet flows are illustrated and discussed, as well as the interaction between bubble sweeping and jet flows.     

Measurement of a Heated Surface Temperature Using a High-Speed Infrared Camera During Critical Heat Flux Enhancement by a Honeycomb Porous Plate in a Saturated Pool Boiling of a Nanofluid

Abstract

This article presents an experimental study to investigate the critical heat flux (CHF) enhancement mechanism using honeycomb porous plate (HPP). The CHF enhanced significantly with combination of the HPP and nanofluid, up to 3.2?MW/m² at maximum compared to a plain surface, 1.0?MW/m². The mechanism by which the CHF is improved in this system was elucidated by measuring the temperature of the heated surface using an indium tin oxide (ITO) heater and a high-speed infrared camera. The pool boiling experiment of water and nanofluid is performed under saturated temperature and atmospheric pressure conditions. The CHF values obtained using ITO heater is in good agreement with a conventional CHF pool boiling experiment with HPP attachment. High-speed infrared camera is analyzed to understand the behavior of local temperature at various locations over time. It is observed at the burnout condition, the highest average temperature is occurred at the intersection of HPP wall. Moreover, the reversible dry spots were initiated in the cell part of the HPP, and small dry spots coalesced into a growth of large irreversible dryout that leads to burnout. Further CHF enhancement could be realized if the initiation of the dryout region could be suppressed.     

Contribution of thin-film evaporation during flow boiling inside microchannels

Flow boiling through microchannels is characterized by nucleation and growth of vapor bubbles that fill the entire channel cross-sectional area. As the bubbles nucleate and grow inside the microchannel, a thin film of liquid or a microlayer gets trapped between the bubbles and the channel walls. The heat transfer mechanism present at the channel walls during flow boiling is studied numerically. It is then compared to the heat transfer mechanisms present during nucleate pool boiling and in a moving evaporating meniscus. Increasing contact angle improved wall heat transfer in case of nucleate boiling and moving evaporating meniscus but not in the case of flow boiling inside a microchannel. It is shown that the thermal and the flow fields present inside the microchannel around a bubble are fundamentally different as compared to nucleate pool boiling or in a moving evaporating meniscus. It is explained why thin-film evaporation is the dominant heat transfer mechanism and is responsible for creating an apparent nucleate boiling effect inside a microchannel.     

An experimental investigation of pool boiling characteristics of alumina-water nanofluid over micro-/nanostructured surfaces

Abstract

Experiments were conducted to investigate the nucleate pool boiling heat transfer of pure water and alumina/water nanofluids on different micro- and nanostructured surfaces prepared via the thermal spray coating method. Results indicate that nanofluids boiling on all the test surfaces led to critical heat flux (CHF) values greater than that obtained for the base fluid (i.e., water). Higher roughness value, however, led to higher CHF values in boiling over the surfaces. Another finding of this study indicated that CHF values obtained with boiling on Cu-coated micro- and nanosurfaces were identical although the heat transfer coefficient (HTC) values obtained for boiling on the micro-structure surface were higher than those obtained for a nanostructured surface with almost the same roughness. A series of consecutive nanofluid boiling cycles were also performed on the aluminum-coated nanostructured surface. The CHF value obtained for water boiling on the surface undergoing repeated nanofluid boiling cycles was by 27% higher than that obtained for a clean surface although the relevant HTC values were nearly identical.