Effect of acoustic cavitation on boiling heat transfer

Boiling heat transfer on a horizontal circular copper tube in an acoustical field is investigated experimentally and the relation between the liquid cavitation, the boiling and the micro bubble radii are analyzed theoretically. The results show that cavitation bubbles have an important influence on the nucleation, growth and collapse of vapor embryo within cavities on the heat transfer surface and that the enhancement of boiling heat transfer by acoustic cavitation mainly depends on whether the vapor embryo is activated by the cavitation bubbles to initiate boiling.     

Gleichgewichtsradien von Dampfblasen und Flüssigkeitstropfen

The equations for equilibrium radii of vapour bubbles and liquid droplets are derived starting from the Thomson-Helmholtz-equation for the vapour pressure at curved interfaces, and the results are then compared with those from well-known equations of the technical literature. These equations usually make use of the vapour-pressure equations of a plane vapour-liquid interface. The differences in equilibrium radii of vapour bubbles proved to be negligible for high boiling temperatures, they become, however, important for low boiling temperatures. For liquid droplets the deviations are considerably higher and the use of the vapour pressure for a plane vapour-liquid interface is not correct.     

Transport-mechanisms in natural nucleate boiling in absence of external forces

In literature it is generally supposed that under terrestrial conditions the driving force in natural, nucleate boiling heat transfer is namely buoyancy caused by earth gravity, which is expressed in the empirical correlations for technical applications. However, experiments in microgravity performed during the past three decades demonstrate unanimously that up to a medium level heat flux the overall heat transfer in pool boiling is nearly independent from gravity. We refer and discuss in this paper on results of experiments performed with various liquids and liquid states and also using various heater geometries on mission platforms which provide low gravity for short and long periods. Beside the measurements of the experimental parameters to determine the heat transfer, we observed the macroscopic boiling process itself with movie films and videos in order to study the bubble dynamics. From these records we learned about the mechanisms of heat and vapour bubble transport, about the interaction between solid heater, superheated liquid, and vapour without gravity or other external force only generated by the bubbles themselves, and we observed significant details about the boiling process not recognized so far. These findings are essential for a better understanding of the complex physical process; and therefore they are important for the formulation of empirical correlations, and in future for numerical simulations to predict properly boiling heat transfer for technical applications.     

Electric field effects during nucleate boiling from an artificial nucleation site

An experimental study of saturated pool boiling from a single artificial nucleation site on a polished copper surface has been performed. Isolated bubbles grow and depart from the artificial cavity and the bubble dynamics are recorded with a high speed camera. Experimental results are obtained for bubble growth, departure and vertical rise both with and without the application of an electric field between an upper electrode and the boiling surface. As detailed in a previous paper from the same research group the high spatial and temporal resolution of the video sequences facilitated the development of a baseline experimental bubble growth law which predicts the bubble volumetric growth characteristics for a range of surface superheats at atmospheric pressure. The presence of an electric field has been found to positively augment the convective heat transfer over that of buoyant natural convection. Further to this, for high electric field strengths, the bubble shape, volumetric growth characteristics and bubble rise are different from that of the baseline cases. These results provide compelling evidence that electric fields can be implemented to alter the bubble dynamics and subsequent heat transfer rates during boiling of dielectric liquids.     

Simulation of void fraction profile evolution in subcooled nucleate boiling flow in a vertical annulus using a bubble-tracking approach

A three-dimensional bubble-tracking model of subcooled nucleate boiling flow in a vertical channel at low-pressure conditions is proposed with specific application to the case of boiling in an annulus with a central heating rod. Vapour is distributed in the liquid in the form of individually tracked bubbles. The overall behaviour of the liquid–vapour system results from motion, interaction, coalescence and boiling mechanisms prescribed mostly at the level of bubbles. Bubbles are nucleated at nucleation sites randomly distributed over the heated surface. After nucleation, bubbles slide on the heated wall, detach and then migrate into the lower-temperature region away from the heated surface, where they condense. The proposed model was applied to experiments on subcooled boiling from Purdue University (USA). Experimental and calculated void fraction radial profiles at different axial locations are compared.     

Shock waves in liquids containing gas bubbles

Results of theoretical and experimental studies on shock wave propagation in boiling and nonboiling liquids with gas bubbles are reviewed. The structure of shock waves, their reflection and attenuation in two-phase gas-liquid media are considered.     

Experimental study on the influence of SO2 gas injection to pure liquids on pool boiling heat transfer coefficients

SO₂ gas is injected into the different pure liquids using new innovative method via meshed tubes. Many experiments have been performed to investigate the influence of gas injection process on the pool boiling heat transfer coefficient of pure liquids around the horizontal cylinder at different heat fluxes up to 114 kW m^?2. Results demonstrate that presence of SO₂ gas into the vapor inside the bubbles creates a mass transfer driving force between the vapor phase inside the formed bubbles and liquid phase and also between the gas/liquid interfaces. Local turbulences and agitations due to the gas injection process around the nucleation sites leads the pool boiling heat transfer coefficient to be dramatically enhanced. Besides, some of earlier well-known correlations were unable to obtain the reasonable values for the pool boiling heat transfer coefficients in this particular case. Therefore, the most accurate correlation among the examined correlations was modified to estimate the pool boiling heat transfer coefficient of pure liquids. Experimental data were in a good agreement with those of obtained by the new modified correlation with absolute average deviation of 10 %.     

Acoustic waves in a liquid with a bubble screen

In this work, certain peculiarities of the dynamics of pressure waves in a liquid containing bubbles are studied. The specification of a model of bubbly liquids with regard to acoustic damping of the bubbles is considered. Our theoretical results are compared with experimental ones.Received: 30 June 2002, Accepted: 2 February 2003, Published online: 11 June 2003     

A model of critical boiling flow with account of wall boiling-up

A model for simulation of critical flows of boiling liquids is proposed, which takes into account the existence of two vapor phases: α-bubbles “attached” to the channel walls and β-bubbles moving in the flow. The model takes into account the possibility of breakdown of both groups of the bubbles due to both the Kelvin-Helmholtz instability caused by the flow around the bubbles and the Rayleigh-Taylor instability induced by the increase in the expansion rate of the vapor-water mixture. It is shown that the experiments on depressurization of high-pressure vessels can be explained by assuming that the initial boiling centers are located only on the walls, and in the flow core the bubbles appear only after the injection of the fragments of broken wall bubbles into the flow. The experimental oscillograms are compared with the calculated curves obtained using the model proposed and the models which take into account only one instability mode, i.e. the Kelvin-Helmholtz or Rayleigh-Taylor instability. The waves which accompany the vessel depressurization process are described.     

Current topics in the dynamics of gas and vapor bubbles

Summary A review of topics of current interest in the dynamics of gas and vapor bubbles in liquids under conditions of no relative motion is presented. The aspects of gas bubble dynamics considered include: radiation pressure (Bjerknes) forces, analytical and numerical studies of transient forced oscillations, connection of the subharmonic resonance with the onset of acoustic cavitation, thermal behaviour of the gas in small-amplitude and large-amplitude oscillations. For vapour bubbles a detailed discussion of the growth process is given. A scaled formulation of this process is also presented and phenomena connected to the forced oscillations are discussed. Some aspects of the dynamics of vapor bubbles in «cold» liquids (i.e. cavitation bubbles) are also included. Finally several phenomena encountered in the dynamics of non-spherical bubbles are reviewed, namely the effect of viscosity on the stability of the spherical shape, a variational formulation, and phase-change effects.

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Sommario Si presenta una rassegna di argomenti attualmente oggetto di ricerca nella dinamica di bolle di gas e di vapore nei liquidi. La discussione è limitata al caso in cui non esista moto di traslazione relativo fra le fasi. Gli aspetti considerati della dinamica di bolle di gas includono: forze di pressione di radiazione (o di Bjerknes), studi analitici e numerici dei transitori nelle oscillazioni forzate, rapporto fra la risonanza subarmonica e l'apparizione di fenomeni di cavitazione acustica, comportamento termico del gas contenuto nella bolla durante oscillazioni di piccola ampiezza e di grande ampiezza. Per le bolle di vapore si presenta una discussione dettagliata del processo di crescita, una formulazione di similarità di tale processo, e alcuni fenomeni connessi alle oscillazioni forzate. Vengono inoltre discussi alcuni aspetti della dinamica di bolle di vapore in liquidi «freddi» (cioè bolle di cavitazione). Infine si considerano alcune questioni connesse alla dinamica di bolle non sferiche, e cioè l'effetto della viscosità sulla stabilità della forma sferica, una formulazione variazionale, e fenomeni relativi al cambiamento di fase.

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This paper is based in part on an invited review lecture given at Euromech Colloquium 98 «Mechanics and Physics of Gas Bubbles in Fluids» held at Eindhoven, 22 – 24 November 1977. The study reported herein has been supported in part by Gruppo Nazionale di Fisica Matematica of C.N.R.     

Model of explosive pulsed vaporization of dielectric liquids by intense laser irradiation of their surfaces

The mechanism of explosive vaporization interaction of laser radiation with matter is studied theoretically. It is shown that, in dielectric liquids with a free surface, periodic explosive boiling is possible if the laser radiation intensity exceeds the rate of heat transfer from the region of laser radiation absorption. Analytical expressions are obtained to estimate the pulsating boiling period and the thickness of the surface liquid layer dispersed by fluctuation vapor bubbles during each boiling. The degree of absorption of laser radiation by the aerosol formed above the liquid surface is estimated. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 6, pp. 17–24, November–December, 2008.     

绕水翼空化流动多尺度数值研究

空化的多尺度效应是一种涉及连续介质尺度、微尺度空化泡以及不同尺度间相互转化的复杂水动力学现象, 跨尺度模型的构建是解析该多尺度现象的关键. 本文基于欧拉-拉格朗日联合算法, 通过界面捕捉法求解欧拉体系下大尺度空穴演化, 通过拉格朗日体系下离散空泡模型求解亚网格尺度离散空泡的运动及生长溃灭. 同时, 通过判断空泡与网格尺度间的关系判定不同尺度空化泡的求解模型. 基于建立的多尺度算法对绕NACA66水翼空化流动进行模拟, 将数值结果与实验进行对比, 验证了数值计算方法的准确性. 研究结果表明, 离散空泡数量与空化发展阶段密切相关, 在附着型片状空穴生长阶段, 离散空泡数量波动较小, 离散空泡主要分布在气液交界面位置; 在回射流发展阶段, 离散空泡逐渐增加并分布在回射流扰动区; 在云状空穴溃灭阶段, 离散空泡数量增多且主要分布在气液掺混剧烈的空化云团溃灭区. 在各空化发展阶段, 离散空泡直径概率密度函数均符合伽玛分布. 空化湍流流场特性对拉格朗日空泡空间分布具有重要影响, 离散空泡主要分布在强湍脉动区、旋涡及回射流发展区域.      

Experiments and mechanism analysis of pool boiling heat transfer enhancement with water-based magnetic fluid

A pool boiling heat transfer comparison among water-based magnetic fluids in the absence and presence of a magnetic field with its carrier liquid water was made. The experimental results show that the boiling heat transfer of magnetic fluid increased much in the absence of a magnetic field, and the applied magnetic field made the boiling heat transfer of magnetic fluid enhance further. The effect of a magnetic field on bubbles was analyzed. It was clarified that the nonuniform magnetic field changed the bubble departure diameter and shape during boiling.     

Maximum size of bubbles during nucleate boiling in an electric field

By taking account of the electric field effects on the shape of bubbles. Fritz's analysis on maximum bubble volume during boiling was extended to the boiling process in the presence of a uniform electric field. It was found that the maximum bubble volume decreases with increase in electric field strength and the dielectric constant of the boiling liquid. The decrease of bubble departure size with increase in electric field strength was confirmed by experimental observations.     

Internal characteristics of hydrogen boiling

Study of the internal characteristics of boiling is of great importance in understanding the physics of this complex process; data on internal characteristics may be used for derivation of formulas for integral boiling characteristics (heat-liberation coefficients, critical thermal flux densities). Only a limited number of works have been dedicated to study of boiling of cryogenic liquids over a wide saturation pressure range. In [1] the internal characteristics of nitrogen boiling were studied at pressures from 0.1 to 8 bar, while oxygen was studied from 0.22 to 2 bar. In [2] detachment radius and bubble-detachment frequency were determined for hydrogen boiling over the pressure range 1–11 bar. The present study is dedicated to an examination of internal characteristics of hydrogen boiling over the saturation pressure range from triple point pressure (0.072 bar) to 2.0 bar.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 103–108, July–August, 1976.     

A model of bubbles rising in a fluidized bed

A model for a single fully developed bubble moving in an unbounded fluidized bed is presented. The model allows bubble growth or shrinkage during the rise inside the bed, as well as dependence of the rise velocity upon specified bed parameters. Limiting cases of nearly spherical bubbles and of sufficiently large bubbles whose form resembles that of a spherical segment are considered in more detail. The form of bubbles rising in either fluidized beds or one-phase liquids, and its dependence on the effective “surface tension” acting on the bubble boundary are discussed.     

Film boiling of an electrically insulating fluid in the presence of an electric field

This paper deals with the experimental and theoretical investigation of the influence of an electric field on the heat transfer rate during stable film boiling of the electrically insulating fluid FC-72. In particular the case of stable saturated film boiling from a horizontal plate is studied. The experiments show that the heat transfer rate increases ±50 when an electric field of 27.3 kV/cm is applied. A new correlation for the heat transfer rate in the presence of an electric field based on the heat transfer model of Klimenko is derived in this paper. Therefore the behavior of the liquid-vapor interface is studied in more detail. This study shows that the electric field has a two fold effect on the interface. On the one hand the distance between adjacent bubbles decreases and on the other hand the bubbles elongate in the presence of the electric field. The new correlation is in good agreement with the experiments. Received on 20 October 1998     

Acoustic Waves of Various Geometry in Multi-Fraction Bubbly Liquids

The propagation of acoustic waves of various geometry in mixtures of a liquid and a disperse phase consisting of small bubbles which differ from one another by both the radii and the thermophysical properties is investigated. A systemof differential equations of motion of the mixture is written and the dispersion relation is derived. The dispersion curves are constructed and damping of the pressure pulses is compared for the plane, cylindrical, and spherical waves in the bubbly liquids considered. The theory is compared with the experimental data.     

Bubble dynamics: a review and some recent results

Several aspects of small-amplitude oscillations of bubbles containing gas, vapor, or a gas-vapor mixture are discussed. An application to pressure-wave propagation in a bubbly liquid is described. Nonlinear forced oscillations are considered in the light of recent research on forced oscillations of nonlinear systems. The growth of vapor bubbles, an extension of the Rayleigh-Plesset equation to non-Newtonian liquids and appreciable mass transfer at the interface, and a boundary integral numerical method for nonspherical cavitation bubble dynamics are also briefly discussed.