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.     

Boiling heat transfer on large diamond and SiC heaters: The influence of thermal wall properties

Pool boiling experiments were performed on large synthetic diamond and SiC substrates with integrated heating and temperature sensor elements. The boiling fluid was pure water at atmospheric pressure. The two heating substrates were identical with regard to geometry and design, and were mainly comparable in terms of surface roughness and static contact angle. This enabled the influence of the thermal heating wall properties on the resulting heat transfer to be investigated directly. The extraordinarily high value of the thermal conductivity of the synthetic diamond heater led systematically to higher mean heat transfer rates within the nucleate boiling regime compared to the SiC material. Strong thermal fluctuations due to the growth and detachment of vapor bubbles were recorded for both heaters. Indications for the existence of a local heat flux reversal from the liquid to the wall have been found by means of re-condensation phenomena and direct temperature measurements during boiling.     

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.     

Statistic analysis of the boiling curve for a droplet

Statistic analysis of the experimentally determined values of a water droplet's evaporation times was made. Measurements were taken from heating surface temperatures characteristic for liquid phase natural convection up to film boiling of a droplet. The results obtained confirm the hypothesis of two boiling curves in the region of transition boiling proposed by Witte and Lienhard. They also allow this concept on the region of nucleate boiling to be expanded.     

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.     

Numerical simulation of spray performance based on the Euler-Lagrange approach

Numerical simulations have been carried out to investigate the liquid atomization and spray process using the Discrete Phase Model of the commercial CFD code combined with the Wall-Film boundary conditions. The effects of spray parameters on droplets Sauter mean diameter (SMD), droplet collision speed, the thickness of liquid-film, the surface temperature and its uniformity were analyzed in the present study. The simulation results and the experimental data obtained in the available literature agree within 13.8%. The computational results show that the spray pressure is the main factor to realize the atomization. Increasing the mass flux and the spray pressure, the droplet collision speed increases while the corresponding maximum film thickness on the heated surface declines. The surface temperature changes indistinctively with the increase of the spray distance, but the temperature distribution tends to be uniform.     

Nucleate and film boiling on a microheater under pulse heating in a microchannel

Using MEMS technology, a Pt microheater (60 × 100 µm²) fabricated on a glass wafer is placed in a silicon-based microchannel of trapezoidal cross section. With the aid of a high-speed CCD and based on Pt's linear temperature-resistance characteristic, flow boiling phenomena and temperature response on the surface of the microheater in the microchannel under pulse heating are observed and recorded. At a given mass flux, nucleate boiling and film boiling begin to appear on the microheater with increasing heat flux. A flow boiling map, showing the effects of heat and mass flux on nucleate and film boiling regimes on the microheater at a pulse heating width of 2 ms, is presented. It is found that nucleate boiling is changed to film boiling as the heat flux supplied to the microheater is increased. Furthermore, increasing mass flux increases the heat flux required for the incipience of nucleate boiling and film boiling on the microheater in the microchannel.     

Pool Boiling on Modified Surfaces Using R-123

Saturated pool boiling of R-123 was investigated for five horizontal copper surfaces modified by different treatments, namely, an emery-polished surface, a fine sandblasted surface, a rough sandblasted surface, an electron beam-enhanced surface, and a sintered surface. Each 40-mm-diameter heating surface formed the upper face of an oxygen-free copper block, electrically heated by embedded cartridge heaters. The experiments were performed from the natural convection regime through nucleate boiling up to the critical heat flux, with both increasing and decreasing heat flux, at 1.01 bar, and additionally at 2 bar and 4 bar for the emery-polished surface. Significant enhancement of heat transfer with increasing surface modification was demonstrated, particularly for the electron beam-enhanced and sintered surfaces. The emery-polished and sandblasted surface results are compared with nucleate boiling correlations and other published data.     

Enhanced heat transfer performance of alumina sponge-like nano-porous structures through surface wettability control in nucleate pool boiling

For several decades, a porous surface has been recognized as an efficient medium to increase boiling performance in a nucleate boiling regime. Most feasible porous surfaces have been studied in millimeter and micron-sized domains. It has been believed that a higher wall superheat is required to commence incipient nucleate boiling under a submicron regime. In this study, we demonstrate that a significantly enhanced pool boiling heat transfer is observed in a submicron regime through three dimensionally interconnected hybrid pores: the Alumina sponge-like nano-porous structure (ASNPS). The structural uniqueness of the ASNPS leads to an enlarged surface area, increases the potential number of the active nucleation site density, and improves the vapor–liquid menisci through the reentrant pore. Simultaneously, by changing the surface wettability with a hydrophobic self-assembled monolayer (SAM) coating, the number of active nucleation site density is improved. Eventually, the combination of the ASNPS and hydrophobic SAM coating can achieve substantial heat transfer coefficient (HTC) enhancement in the nucleate boiling. Also, the thickness of the ASNPS is a critical issue to adequately augment the HTC in pool boiling. The thickness of the ASNPS is optimized by examining the boiling performance of the ASNPS fabricated in different amounts of anodizing times. A classical mechanistic model from literature was modified and compared with the experimentally obtained data. The modified mechanistic model – with the combination of forced-convection and thin liquid film evaporation – showed reasonable predictions.     

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     

A numerical investigation of the effect of heating methods on saturated nucleate pool boiling

Using a numerical model, the effect of heating methods on saturated nucleate pool boiling is investigated parametrically for smooth and rough nickel and copper heater plates. The boiling curve moved right with decreasing thickness for the smooth and rough nickel and copper heaters in the constant-heat-flux heating method. This trend was reversed in the constant-temperature heating method; the boiling curved shifted left with decreasing heater thickness. However, the later trend was not affected by the heater material and thickness and the surface roughness (mean cavity radius). The boiling curves were identical for the constant internal generation rate and the constant-heat-flux heating method. The use of ac instead of dc resistive heating caused the boiling curve generally to move left. This behavior was not linear with the heat flux, heater material, or surface conditions. No hysterisis was found when the heat flux was increased and then decreased gradually to original values.     

Effect of surface wettability and electric field on transition of film boiling to nucleate boiling

Abstract

The phenomena of liquid–solid contact during film boiling due to the effect of surface-wettability have been focused in the present study. The numerical simulations during film boiling exhibit the collapse of vapor layer when the surface-wettability is sufficiently high, that is, for the hydrophilic surface. Vapor film collapse results in contact of liquid with the heated surface, which transforms the boiling mode more toward the nucleate regime. The contact area of liquid increases with time. However, such transition is not observed in the case of hydrophobic surface or the surface with higher contact angles. When a sufficiently strong electric field is applied across the liquid-vapor interface, the vapor film collapses and results in similar transition from film boiling to nucleate boiling. The required intensity of electric field at which the vapor film collapses increases with the increase in surface-superheat.     

Subcooled flow boiling heat transfer of FC-72 from silicon chips fabricated with micro-pin-fins

Experiments were conducted to study the subcooled flow boiling heat transfer performance of FC-72 over silicon chips. For boiling heat transfer enhancement, two kinds of micro-pin-fins having fin thickness of 50 μm and fin heights of 60 and 120 μm, respectively, were fabricated on the silicon chip surface with the dry etching technique. The fin pitch was twice the fin thickness. The experiments were conducted at the fluid velocities of 0.5, 1 and 2 m/s and the liquid subcoolings of 15, 25 and 35 K. The micro-pin-finned surfaces showed a sharp increase in heat flux with increasing wall superheat and a large heat transfer enhancement compared to a smooth surface. The nucleate flow boiling curves for the two micro-pin-finned surfaces collapsed to one line showing insensitivity to fluid velocity and subcooling, while the critical heat flux values increased with fluid velocity and subcooling. The micro-pin-finned surface with a larger fin height of 120 μm provided a better flow boiling heat transfer performance and a maximum critical heat flux of 145 W/cm². The wall temperature at the critical heat flux for the micro-pin-finned surfaces was less than 85 °C for the reliable operation of LSI chips.     

Flow Boiling Heat Transfer in A Minichannel with Enhanced Heating Surface

This paper presents the results of flow boiling in a 1.0-mm-deep minichannel with asymmetrical heating. The heating element for the working fluid (FC-72) is a single-sided enhanced alloy foil made from Haynes-230. Two types of enhanced heating surfaces, prepared by laser texturing and with microrecesses varied in terms of size, were used for investigations. The experimental research focused on the transition from single-phase forced convection to nucleate boiling, that is, the zone of boiling incipience and further development of boiling. Flow structure was observed through a glass pane. Owing to the liquid crystal layer placed on the opposite side of the enhanced foil surface, it was possible to observe the onset of flow boiling (as a “boiling front”) and to measure temperature distribution on the heating wall through another glass pane. The objective of the study is to determine void fractions for increasing heat fluxes supplied to the heating surface. The flow structure photos were processed in Corel graphics software and binarized. The analysis of phase volumes was developed in Techsystem Globe and NIS-Elements Advanced Research software. The results of experiments with both types of enhanced heating foil were compared.     

Forced convective boiling of subcooled water at high velocity

This paper deals with heat transfer and critical heat flux (CHF) in subcooled flow boiling offering a fundamental study aimed at high heat flux cooling. Experiments with water at 0.12 MPa were conducted in a mass velocity range from 500 kg/m²s to 15,000 kg/m²s (velocity from 0.5 m/s to 15 m/s) and subcooling from 20 K to 60 K. A sheet of stainless steel (80 mm in heated length, 10 mm wide, and 0.2 mm thick) was mounted flush with a sidewall of a vertical rectangular channel (cross-section 20 mm by 30 mm) and heated directly using direct current. It was found that mass velocity and subcooling strongly affect CHF and heat transfer in non-boiling convection and partial nucleate boiling regimes. These two parameters have no appreciable influence in the fully developed nucleate boiling regime. In the parameter range used, CHF reached 15 MW/m². Boiling bubble behavior just prior to reaching CHF was found to vary depending on mass velocity and subcooling. 1998 Scripta Technica, Heat Trans Jpn Res, 27(5): 376–389, 1998     

Characterisation of the spray cooling heat transfer involved in a high pressure die casting process

A systematic experimental study was conducted to examine the heat transfer characteristics from the hot die surface to the water spray involved in high pressure die casting processes. Temperature and heat flux measurements were made locally in the spray field using a heater made from die material H-13 steel and with a surface diameter of 10 mm. The spray cooling curve was determined in the nucleate boiling, critical heat flux, as well as the transition boiling regimes. The hydrodynamic parameters of the spray such as droplet diameters, droplet velocities, and volumetric spray flux were also measured at the position in the spray field identical to that of the test piece. Droplet size and velocity distribution were measured using a PDA system. A new empirical correlation was developed to relate the spray cooling heat flux to the spray hydrodynamic parameters such as liquid volumetric flux, droplet size, and droplet velocity in all heat transfer regimes. The agreement between experimental data and predicted results is satisfactorily good.     

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 Study of Boiling Heat Transfer on Multiple and Single Nucleation Sites Using a Boiling-Meter

The general objective of this study is to contribute to a better understanding of heat transfer in a nucleate boiling regime. The aim is to determine the heat transfer characteristics under controlled operating conditions (thermodynamics of the fluid, noncondensable gas, surface state). Experimental investigations have been carried out in natural convection and nucleate boiling regimes. An experimental device was realized to perform boiling experiments using a boiling-meter, allowing investigations for different orientations of the wall. The boiling-meter is designed to investigate boiling for single and multiple nucleation sites. The purpose of this paper is to detail the experimental setup as well as the boiling-meter. This device allows the determination of the temporal heat transfer characteristics evolutions. In particular, this new device allows bringing to light the phenomenon of nucleation, growth, and detachment of generated vapor bubbles on a single artificial nucleate site, as well as for multiple natural nucleation sites. First results of the influence of the orientation of the heating wall for multiple and single nucleation sites on heat transfer are presented and analyzed.     

Analytical Model for Describing Experimental Results on Parameters Influencing Heat Transfer in Film Boiling with Spray Quenching

An analytically solvable mathematical model is developed to estimate heat transfer quantities in the film boiling region of metal quenching with water sprays. The model is based on the hydrodynamic of a single droplet which is separated from the metal by a vapor film. The temperature profile within the droplet is calculated as semi-infinite body because of the short contact time. It is validated with own experimental results and those from the literature. The influence of size and velocity of the droplet, spray flux, surface temperature, temperature of the cooling water and the salinity level are discussed. The droplet size and velocity play a less significant influence on the heat transfer. The heat transfer coefficient is found to increase linearly with the spray flux. The heat flux is proportional to the difference of boiling and water temperature. With the model it is shown, that even for the high impingement densities the droplet covered area is very small.     

Leidenfrost boiling of methanol droplets on hot porous/ceramic surfaces

An experimental and analytical study of film boiling methanol droplets on a porous/ceramic surface is reported. Droplet evaporation times in the wetting and film boiling regimes were measured on a polished stainless-steel surface and three ceramic/alumina surfaces of 10%, 25% and 40% porosity. It was found that the Leidenfrost temperatures increased as surface porosity increased. The Leidenfrost point of the 10% and 25% porous surfaces were nearly 100 K higher and 200 K higher, respectively, than that of the polished stainless-steel surface; methanol droplets could not be levitated on the 40% porous surface at surface temperatures as high as 620 K, which was the maximum surface temperature which could be imposed on this particular material with our apparatus. The evaporation time of liquid deposited on this surface was thus almost two orders of magnitude lower than for levitated droplets on the three other surfaces tested at the same temperature. In the Leidenfrost regime droplets evaporated faster on the porous surfaces than on the stainless-steel surface, and the evaporation time decreased with increasing surface porosity at the same surface temperature. The reduced evaporation times were thought to have their origin in a decrease of the vapor film thickness separating the droplet from the ceramic surface due to vapor absorption and flow within the ceramic material. An analysis of flow in a horizontal channel bounded by an impermeable ẇall above and a permeable wall of finite thickness below was used to model the film boiling process. The results provided a basis for correlating our evaporation time measurements.