Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

Confinement effects on nucleate boiling and critical heat flux in buoyancy-driven microchannels

Pool boiling and CHF experiments were performed for vertical, rectangular parallel-plate channels immersed in the dielectric liquid FC-72 at atmospheric pressure to elucidate the effects of geometrical confinement in immersion cooled electronics applications. Heat transfer enhancement in the low flux region of the nucleate boiling curve was observed for channel spacings near and below expected bubble departure diameters, but was widely different for two different heater materials. Relative degradation of CHF with decreasing channel spacing was found to be a strong function of channel aspect ratio and independent of surface material and finish.     

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     

Augmentation of nucleate boiling heat transfer and critical heat flux using nanoparticle thin-film coatings

Nanoparticle thin-film coatings applied to boiling surfaces using a layer-by-layer (LbL) assembly method demonstrated significant enhancement in the pool boiling critical heat flux (CHF) and nucleate boiling heat transfer coefficient. Up to 100% enhancement of the critical heat flux and over 100% enhancement of the heat transfer coefficient were observed for pool boiling of nickel wires coated with different thin-films of silica nanoparticles. Surface characterization revealed that the surface wettability changed drastically with the application of these coatings, while causing virtually no change in the surface roughness. It is concluded that the nanoporous structure coupled with the chemical constituency of these coatings leads to the enhanced boiling behavior.     

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.     

A theoretical model for the prediction of the critical heat flux in heated microchannels

A theoretical model for the prediction of the critical heat flux of refrigerants flowing in heated, round microchannels has been developed and presented here. The model is based on the two-phase conservation equations and includes the effect of the height of the interfacial waves of the annular film. Validation has been carried out by comparing the model, a numerical solution of a non-linear system of five differential equations, with a critical heat flux (CHF) database including three different refrigerants from two different laboratories. More than 96% of the data are predicted within a ±20% error band and a mean absolute error of 8%. Furthermore, it is also possible to predict CHF data from a third laboratory for water and R-113 flowing in rectangular (using the width of the channel as the characteristic dimension) and circular microchannel heat sinks with multiple channels. All together, 90% of the entire database, including four different fluids and different geometries, are predicted by the model within a ±20% error band and a mean absolute error of 9.3% for channels from 0.215 to 3.15 mm in size, mass fluxes from 29 to 1600 kg/m² s, heated lengths from 10 to 126 mm and subcoolings from 2 to 77 °C.     

Prediction of liquid hydrogen flow boiling critical heat flux condition under microgravity based on the wall heat flux partition model

Critical heat flux (CHF) of liquid hydrogen (LH2) flow boiling under microgravity is vital for designing space cryogenic propellant conveying pipe since the excursion of wall temperature may cause system failure. In this study, a two-dimensional axisymmetric model based on the wall heat flux partition (WHFP) model was proposed to predict the CHF condition under microgravity including the wall temperature and the CHF location. The proposed numerical model was validated to demonstrate a good agreement between the simulated and experimentally reported results. Then, the wall temperature distribution and the CHF location under different gravity conditions were compared. In addition, the WHFP and vapor-liquid distribution along the wall under microgravity were predicted and its difference with terrestrial gravity condition was also analysed and reported. Finally, the effects of flow velocity and inlet sub-cooling on the wall temperature distributions were analysed under microgravity and terrestrial gravity conditions, respectively. The results indicate that the CHF location moves upstream about 5.25 m from 1g to 10⁻⁴g since the void fraction near the wall reaches the breakpoint of CHF condition much earlier under the microgravity condition. Furthermore, the increase of the velocity and decrease of the sub-cooling have smaller effects on the CHF location during LH2 flow boiling under microgravity.     

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.     

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.     

Comparison study of liquid replenishing impacts on critical heat flux and heat transfer coefficient of nucleate pool boiling on multiscale modulated porous structures

The critical heat flux (CHF) and heat transfer coefficient of de-ionized (DI) water pool boiling have been experimentally studied on a plain surface, one uniform thick porous structure, two modulated porous structures and two hybrid modulated porous structures. The modulated porous structure design has a porous base of 0.55 mm thick with four 3 mm diameter porous pillars of 3.6 mm high on the top of the base. The microparticle size combinations of porous base and porous pillars are uniform 250 μm, uniform 400 μm, 250 μm for base and 400 μm for pillars, and 400 μm for base and 250 μm for pillars. Both the CHF and heat transfer coefficient are significantly improved by the modulated porous. The boiling curves for different kinds of porous structures and a plain surface are compared and analyzed. Hydrodynamic instability for the two-phase change heat transfer has been delayed by the porous pillars which dramatically enhances the CHF. The highest pool boiling heat flux occurring on the modulated porous structures has a value of 450 W/cm², over three times of the CHF on a plain surface. Additionally, the highest heat transfer coefficient also reaches a value of 20 W/cm² K, three times of that on a plain copper surface. The study also demonstrates that the horizontal liquid replenishing is equally important as the vertical liquid replenishing for the enhancement of heat transfer coefficient and CHF improvement in nucleate pool boiling.     

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.     

Time and space resolved wall temperature and heat flux measurements during nucleate boiling with constant heat flux boundary conditions

The lack of time and space resolved measurements under nucleating bubbles has complicated efforts to fully explain pool-boiling phenomena. In this work, time and space resolved temperature and heat flux distributions under nucleating bubbles on a constant heat flux surface were obtained using a 10 × 10 microheater array with 100 μm resolution along with high-speed images. A numerical simulation was used to compute the substrate conduction, which was then subtracted from the heater power to obtain the wall-to-liquid heat transfer. The data indicated that most of the energy required for bubble growth came from the superheated layer around the bubble. Microlayer evaporation and contact line heat transfer accounted for not more than 23% of the total heat transferred from the surface. The dominant heat transfer mechanism was transient conduction into the liquid during bubble departure. Bubble coalescence was not observed to transfer a significant amount of heat.     

The effect of heated wall thickness and materials on nucleate boiling at high heat flux

The present work is to numerically investigate the effect of heater side factors on the nucleate boiling at high heat flux, which is characterized by the existence of macrolayer. Two-region equations are proposed to study both thermo-capillary driven flow in the liquid layer and heat conduction in the solid wall. The numerical results indicate that the thermo-capillary driven flow in the macrolayer and evaporation at the vapor-liquid interface constitute a very efficient heat transfer mechanism to explain the high heat transfer coefficient of nucleate boiling heat transfer near CHF. For a very thin wall and/or wall with a poor thermal conductivity (heat side factors) are found to have significant effect on flow pattern in the liquid layer and the temperature distribution in the heated wall.     

Studies on critical heat flux in flow boiling at near critical pressures

Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length at different reduced pressures ranging from 0.24 to 0.99 with R-134a as the working fluid. The onset of CHF was determined by the sudden rise in the wall temperature of the electrically heated tube. Experiments were performed over a wide range of parameters: mass flux values from 200 to 2000 kg/m² s, pressure from 10 to 39.7 bars and heat flux from 2 to 80 kW/m² and exit quality from 0.17 to 0.94. The results show considerably lower critical heat flux at high pressures. Well known CHF prediction methods, such as the look-up table and correlations of earlier workers show poor agreement at high pressures. A new correlation has been proposed to estimate the CHF in uniformly heated vertical tubes up to the critical pressure and over a wide range of parameters.     

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.     

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.     

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.     

Surface wettability effects on critical heat flux of boiling heat transfer using nanoparticle coatings

This study investigates the effects of surface wettability on pool boiling heat transfer. Nano-silica particle coatings were used to vary the wettability of the copper surface from superhydrophilic to superhydrophobic by modifying surface topography and chemistry. Experimental results show that critical heat flux (CHF) values are higher in the hydrophilic region. Conversely, CHF values are lower in the hydrophobic region. The experimental CHF data of the modified surface do not fit the classical models. Therefore, this study proposes a simple model to build the nexus between the surface wettability and the growth of bubbles on the heating surface.