Studies on pool boiling heat transfer: Macrolayer thickness in transition boiling

Macrolayer thicknesses in transition boiling were determined from the energy balance relation q_tr = ρ_lH_fgδ_l·f , based on measurements of q_tr (the time-averaged heat flux in transition boiling) and f (the detachment frequency of vapor masses) for water and ethanol boiling on vertical and horizontal 15-mm-diameter surfaces under atmospheric pressure. The macrolayer thickness for the vertical surface, designed to prevent liquid contact with the periphery of the surface during the vapor mass hovering, agreed well with the correlation proposed previously by the present authors, when the heat flux at macrolayer formation is obtained from a nucleate boiling curve extrapolated to the superheat of transition boiling. The macrolayer on the horizontal surface was apparently thickened due to the inflow of bulk liquid beneath the growing vapor masses. © 1999 Scripta Technica, Heat Trans Jpn Res, 27(8): 568–583, 1998     

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     

Role of macrolayer evaporation in pool boiling at high heat flux

Analytical expressions for macrolayer thickness and the rate of heat transfer through a macrolayer in a high heat flux region near the critical value were reported in previous papers by the authors. The results of an experimental investigation into the liquid macrolayer formation are being reported in this paper. The initial thickness of this liquid layer formed between the heated surface and the vapour mass and the frequency of the vapour mass as a function of impressed heat flux have been measured. Using these data, the contribution of macrolayer evaporation to the heat flow from heated surface to bulk has been estimated. Experimental results of macrolayer thickness and frequency of vapour mass have been found to compare well with analytically predicted values. Contribution of heat conduction through the macrolayer has also been found to account for a considerable portion of wall heat flux.     

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.     

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     

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.     

Non-heating simulation of pool-boiling critical heat flux

This paper presents a non-heating experimental method that simulates the critical heat flux (CHF) phenomenon in pool boiling. In the experiments, with providing controlled air flow through the holes on a plate submerged in a pool of water, the liquid sublayer (macrolayer) thickness and bubble departure frequency have been successfully measured by a conductance probe. The CHF is reasonably predicted by applying the measured parameters to a liquid macrolayer dryout model. The measured trends of the macrolayer thickness and bubble departure frequency with air mass flux are also consistent with the present understanding. As a result of this experimental study, it is expected that the non-heating method would be useful to investigate the various parametric effects on pool and flow boiling CHF, with avoiding the difficulty in heating and large electric power requirement even for complex geometries.     

Boiling behaviors and critical heat flux on a horizontal plate in saturated pool boiling of water at high pressures

Observations of boiling behaviors and measurements of critical heat flux (CHF) were carried out for saturated water boiling on a horizontal, upward-facing plate at pressures from atmospheric to 7 MPa. The primary bubbles diminish in size almost in inverse proportion to pressure and commence to coalesce in the very low heat flux region. The diameter of detached coalesced bubbles increases with increases in the heat flux and reaches about 10 mm even at a pressure of 5 MPa. Detachment frequency of the coalesced bubbles was unaffected by the heat flux and pressure. The CHF predicted based on the macrolayer dryout model agrees well with the measured data.     

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     

Experimental study on saturated flow boiling heat transfer of R290/R152a binary mixtures in a horizontal tube

An experimental study on the saturated flow boiling heat transfer for a binary mixture of R290/R152a at various compositions is conducted at pressures ranging from 0.2 to 0.4 MPa. The heat transfer coefficients are experimentally measured over mass fluxes ranging from 74.1 to 146.5 kg/(m²·s) and heat fluxes ranging from 13.1 to 65.5 kW/m². The influences of different parameters such as quality, saturation pressure, heat flux, and mass flux on the local heat transfer coefficient are discussed. Existing correlations are analyzed. The Gungor-Winterton correlation shows the best fit among experimental data for the two pure refrigerants. A modified correlation for the binary mixture is proposed based on the authors’ previous work on pool boiling heat transfer and the database obtained from this study. The result shows that the total mean deviation is 10.41% for R290/R152a mixtures, with 97.6% of the predictions falling within ±30%.     

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     

Impact of volume fraction and fluid layer thickness on heat transfer effectiveness of water-based nanofluids

The heat transfer effectiveness of nanofluids is adversely affected by the delay in convection onset. The lesser effectiveness, when compared to that of base fluid, is observed in a range of nanofluid layer thickness. The heat transfer coefficient of water–Al₂O₃ nanofluid can be enhanced by sustaining the equilibrium between Rayleigh number, temperature, particle volume fraction, and enclosure aspect ratio. In this paper, the specific correlation of fluid layer thickness and the onset of convection, which can significantly dominate the heat transfer characteristics of nanofluids are investigated using the concept of critical Rayleigh number. The water layer thickness for convection onset is first experimentally assessed for different real-life heat flux densities. It is then performed for Al₂O₃–water nanofluid for varying volume fractions. With the increase in volume fraction even though thermal conductivity increases, the overall heat transfer enhancement of the nanofluid is reduced. Temperature involved (heat flux density), the volume fraction of the nanofluid used, nanofluid layer thickness (space availability for the cooling system), and mass of the nanoparticle influence heat transfer enhancement. A higher volume fraction may not always result in enhancement of heat transfer as far as nanofluids are concerned.     

Characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film

An experimental study has been conducted to elucidate characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film. The experiment was done at atmospheric pressure for the following conditions: an initial surface temperature from 200 to 400^°C, a subcooling of 20– 80 K, average velocity of 0.52– 1.24 m/s, and the block material is copper and carbon steel. The surface temperature and heat flux are estimated from the measured temperatures in the block during the quench by a two‐dimensional inverse solution. It follows that as the position of wetting advances downward, the position at which the heat flux becomes a maximum also advances downward. The time at which the position of maximum heat flux begins to move is one of the most important parameters and can be predicted by a proposed correlation. In addition, it is revealed that the maximum heat flux for copper depends on the length to which it occurs from the leading edge. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 345– 360, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20167     

Effect of channel length on critical heat flux under conditions of high subcooling and high velocity in short heated channels

The characteristics of critical heat flux (CHF) in existing experiments under high subcooling and high velocity in short heated channels have, for the first time, been systematically and quantitatively investigated to provide a CHF correlation that can properly predict the effect of channel length, especially when the channel length-to-channel diameter ratio L/D is less than about 20. The major test conditions of existing CHF experiments investigated in this study were channel diameter 1 to 4 mm, L/D 1 to 25, 0.1 to 1.2 MPa pressure, 34 to 117°C inlet water subcooling and 500 to 40 700 kg/(m² · s) mass flux in circular channels, and 3 to 20 mm gap size, 6 to 40 L/D_e, 0.1 to 3.1 MPa pressure, 4 to 166°C inlet water subcooling, and 940 to 27,000 kg/(m² · s) mass flux in rectangular channels. The effect of L/D on CHF was evaluated referring to the analytical solution of CHF, which was previously derived by the author for the channel flow at high subcooling and high velocity. As a result, the effect of L/D was quantitatively clarified as an effect of magnitude in heat transfer of single-phase forced-convection flow, giving a larger CHF with a smaller L/D in the case of L/D less than about 20. The proposed correlation predicts CHF to within a ±35 percent error margin. ©1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 509–521, 1998     

Critical heat flux of R22 and R142b mixtures flowing in a vertical tube

An experimental study on the critical heat flux (CHF) in forced convective boiling of R142b/R22 mixtures was conducted with an electrically heated vertical test tube with 17 mm inner diameter, in which the inner surface temperature was controlled and the heated length varied from 0.058 to 0.661 m. Experiments were carried out under the following conditions: constant pressures of 1.3 and 2.0 MPa, constant mass fluxes of 1000 and 2000 kg/(m²s), and constant outlet qualities of −0.2, 0.0, and 0.1. Whole boiling curves were obtained and the effects of heated length, local quality, and mole fraction on the CHF were investigated. The CHF of mixtures was higher than that of pure fluids and showed a maximum in a CHF versus mole fraction plot at a certain mole fraction. However, the maximum was less significant for the longer test section and the higher outlet quality condition. © 1998 Scripta Technica. Heat Trans Jpn. Res., 26(5): 292–305, 1997     

Pool boiling heat transfer in binary mixtures of ammonia/water: Effect of heat of dilution and dissolution on heat transfer coefficient

Nucleate boiling heat transfer coefficients were measured on a horizontal heated wire during the pool boiling of non‐azeotropic mixtures of ammonia/water. The experiment was carried out at pressures of 0.4 and 0.7 MPa, at heat fluxes below 2.0 × 10⁶ W/m², and over a range of mass fraction. The heat transfer coefficients in the mixtures were smaller than those in single‐component substances. No existing correlation is found to predict boiling heat transfer coefficients over the range of mass fraction of interest. In the mixtures of the ammonia/water, the heats of dilution and dissolution were generated near a liquid surface while vapor with a rich concentration of ammonia was condensed and then was diffused into the bulk liquid; while in most other mixtures, little heat was generated during any dilution and dissolution. In relation to the heat generated, the effect of the heats of dilution and dissolution on pressure and temperature in a system (pressure vessel) is shown herein. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 272–283, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10034     

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.     

A new critical heat flux correlation for vertical water flow through multiple thin rectangular channels

A critical heat flux (CHF) study of the vertical up-flow of water through multiple thin rectangular channels was conducted. Pressures varied from 89.8 to 115 kPa, inlet temperatures from 291 to 306 K, and mass fluxes from 9.5 to 39 kg m^?2 s^?1. Electrical resistance heaters embedded in aluminum provided a uniform heat flux. A more universal and robust CHF correlation based on the geometry of the Advanced Test Reactor at Idaho National Laboratory was developed. This new CHF correlation predicts 126 data points from this and three previous studies within an error of ±8.5% with a 95% confidence.     

Characteristics of heat transfer inside a tube during sodium‐water reaction in a FBR steam generator

Sodium reacts chemically with water in the case of an unexpected tube failure of a steam generator (SG) in a fast breeder reactor (FBR). In order to predict the event with high accuracy, it is very important to understand the characteristics of heat transfer inside the tube in detail during the tube failure due to the sodium–water reaction. Experiments were performed by using purified water under the following conditions: initial pressure of 11.2–13.4 MPa, initial water temperature of 200 °C, and water mass flux of 45.7 to 3630 kg/(m²s). The test tube was heated rapidly by high‐frequency induction current. The time averaged heat flux was estimated by using an inverse solution from the measured temperatures at two points on three different locations along the tube. It was confirmed that the derived values agreed with the measured heat fluxes on the outer surface within 20% accuracy. It was found that the characteristics of the heat transfer strongly depend on the flow rate. The heat transfer on the wall changed from nucleate boiling to transient‐film boiling during increasing the heat flux and returned to the nucleate boiling during decreasing the heat flux. A counterclockwise cycle always appeared in the transition boiling region, where the nucleate and film boiling coexisted and the area ratio of these varied with time. The adequacy of heat transfer correlations to evaluate tube overheating was confirmed. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20320     

Boiling heat transfer and dryout phenomenon of CO2 in a horizontal smooth tube

Evaporation heat transfer characteristics of carbon dioxide (CO₂) in a horizontal tube are experimentally investigated. The test tube has an inner diameter of 6.0 mm, a wall thickness of 1.0 mm, and a length of 1.4 m. Experiments are conducted at saturation temperatures of 5 and 10 °C, mass fluxes from 170 to 320 kg/m² s and heat fluxes from 10 to 20 kW/m². Partial dryout of CO₂ occurs at a lower quality as compared to the conventional refrigerants due to a higher bubble growth within the liquid film and a higher liquid droplet entrainment, resulting a rapid decrease of heat transfer coefficients. The effects of mass flux, heat flux, and evaporating temperature are explained by introducing unique properties of CO₂, flow patterns, and dryout phenomenon. In addition, the heat transfer coefficient of CO₂ is on average 47% higher than that of R134a at the same operating conditions. The Gungor and Winterton correlation shows poor prediction of the boiling heat transfer coefficient of CO₂ at low mass flux, while it yields good estimation at high mass flux.