高剪切力下气泡聚并与破碎特性的数值研究

了解气泡在剪切力场中的聚并与破碎机理及生长与运动特性,对于气液搅拌槽中桨叶的优化设计具有重要意义。将欧拉-欧拉模型与群体平衡模型(population balance model, PBM)进行耦合,对不同剪切力下的气液两相流场进行求解,研究了剪切力、高剪切力下进气速度、气泡塔高度对气泡聚并与破碎的影响,并结合气泡的聚并与破碎模型对高剪切力下气泡的聚并与破碎机理进行了探究。研究表明,剪切力主要影响气泡的破碎,当剪切力较小时其对气泡破碎的影响较小,随着剪切力的增大其对气泡破碎的影响逐渐显著,使小气泡的含量大幅增多;高剪切力下进气速度、气泡塔高度对气泡的聚并与破碎的影响不明显。     

Physical mechanisms of heat transfer during single bubble nucleate boiling of FC-72 under saturation conditions-I. Experimental investigation

This paper is the first of a two-part study concerning the dynamics of heat transfer during nucleation process of saturated FC-72 liquid. Experimental results discussed in this paper provide new physical insight on the nature of heat transfer events at the nucleation site during the nucleate boiling process. The thermal field underneath a bubble during the boiling of FC-72 was measured with a spatial resolution of 22--40 μm. The time period of activation, area of influence, and magnitude of three different mechanisms of heat transfer active at the nucleation site were determined. These mechanisms consisted of: (1) microlayer evaporation following the rapid bubble expansion, (2) transient conduction due to rewetting of the surface during bubble departure, and (3) microconvection in the region external to the bubble/surface contact area. The area of influence of the transient conduction mechanism was found to be limited to the bubble/surface contact area, with most of the heat transfer occurring prior to the bubble detachment from the surface. The microconvection heat transfer mechanism was localized primarily outside the contact area and was found to be steady in nature. All three mechanisms of heat transfer were found to make significant contributions to the total surface heat transfer. The second part of this study provides the theoretical analysis of the results.     

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.     

Dynamics of discrete bubble in nucleate pool boiling on thin wires in microgravity

A space experiment on bubble behavior and heat transfer in subcooled pool boiling phenomenon has been performed utilizing the temperature-controlled pool boiling (TCPB) device both in normal gravity in the laboratory and in microgravity aboard the 22^nd Chinese recoverable satellite. The fluid is degassed R113 at 0.1 MPa and subcooled by 26°C nominally. A thin platinum wire of 60 μm in diameter and 30 mm in length is simultaneously used as heater and thermometer. Only the dynamics of the vapor bubbles, particularly the lateral motion and the departure of discrete vapor bubbles in nucleate pool boiling are reported and analyzed in the present paper. It’s found that these distinct behaviors can be explained by the Marangoni convection in the liquid surrounding vapor bubbles. The origin of the Marangoni effect is also discussed.     

Evaporation of the microlayer in hemispherical bubble growth in nucleate boiling of liquid metals

In this paper, a thermal analysis is used to estimate the extent of evaporation of the microlayer in hemispherical bubble growth, in nucleate boiling of liquid metals on heated surfaces. As the bubble grows, evaporation of the microlayer produces a dry patch at its center, whose size depends on the thermal and physical properties of the system, the roughness of the heating surface, and the boiling pressure. It was found that the area of this patch relative to that of the microlayer (or bubble base) is typically very small for liquid metals, and can be neglected in most theoretical analyses of bubble growth. It was further found that the loss of liquid from the microlayer due to evaporation into the bubble is at most a few percent, in a typical case.Since both the calculational model and mathematical analysis involve a number of simplifying assumptions, the numerical results of this pioneering study should be considered approximate.     

Numerical simulation of nucleate boiling on a horizontal surface at high heat fluxes

Nucleate boiling at high heat fluxes has been studied numerically by solving the equations governing conservation of mass, momentum and energy in the liquid and vapor phases. The interface is captured by using the level set method based on a sharp-interface representation. The evaporative heat flux from the liquid microlayer is incorporated in the analysis. The effects of wall superheat, number density of nucleation sites and waiting period on the bubble dynamics and heat transfer in nucleate boiling are investigated. The heat fluxes obtained from the present numerical simulations are compared with the experimental data reported in the literature.     

Modeling and numerical investigation of hydrogen nucleate flow boiling heat transfer

Liquid hydrogen flow boiling heat transfer in tubes is of great importance in the hydrogen applications such as superconductor cooling, hydrogen fueling. In the present study, a numerical model for hydrogen nucleate flow boiling based on the wall partition heat flux model is established. The key parameters in the model such as active nucleation site density, bubble departure diameter and frequency are carefully discussed and determined to facilitate the modeling and simulation of hydrogen flow boiling. Simulation results of the numerical model show reasonably well agreement with experimental data from different research groups in a wide operation condition range with the means absolute error (MAE) of 10.6% for saturated and 5.3% for subcooled flow boiling. Based on the model, wall heat flux components and void fraction distribution of hydrogen flow boiling are studied. Effects of mass flow rate and wall heat flux on the flow boiling heat transfer performance are investigated. It is found that in the hydrogen nucleate flow boiling, the predominated factor is the Boiling number, rather than the vapor quality. A new simple correlation is proposed for predicting hydrogen saturated nucleate flow boiling Nusselt number. The MAE between the correlation predicted and experimentally measured Nusselt number is 13.6% for circular tubes and 12.5% for rectangular tubes. The new correlation is applicable in the range of channel diameter 4–6.35 mm, Reynolds number 64000–660,000, saturation temperature 22–29 K, Boiling number 8.37 × 10^?5–2.33 × 10^?3.     

Temperature fluctuation in bubble nucleate pool boiling: A proposed new factor of evaporation

The phase change rate and related physical constants of superheated water at the liquid surface can be determined by the mechanism of hydrogen bonding and by the Maxwell‐Boltzmann principle. In this paper, we introduce a physical constant and attempted to demonstrate the existence and the value of this physical constant. First, we measured temperature fluctuations under the bubbles in a pool of boiling water. Second, a sample analytical model was introduced, to determine the value of the physical constant. Comparisons were made between the experimental data and those of the calculations. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 578–594, 2002; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.10059     

Prediction of departure diameter and bubble frequency in nucleate boiling in uniformly superheated liquids

A theory is presented to describe the motion of vapour bubbles growing at a nucleation site in a uniformly superheated liquid. By incorporating the classical theory for spherical phase growth, the equations of translatory motion were solved, enabling the radius and position of the bubble to be calculated as a function of time.     

Experimental investigation of bubble behaviors in subcooled flow boiling

The experimental investigation on vapor bubble growth is performed for analyzing subcooled boiling in a vertical annular channel with inner heating surface and upward water flow under atmospheric pressure. Bulk liquid mass flux ranges from 79 kg/m2s to 316 kg/m2s, and subcooling is from 40 K to 60 K. The bubble behaviors from inception to collapse are captured by High-speed photography. The performance of bubble growth recorded by the high-speed photography is given in this paper. The bubble behaviors, effect of the bubble slippage on the heat transfer, and various forces acting on the bubble are discussed.     

Physical mechanisms of heat transfer during single bubble nucleate boiling of FC-72 under saturation conditions. II: Theoretical analysis

This paper is the second part of a two-part study concerning the dynamics of heat transfer during the nucleation process of FC-72 liquid. The experimental findings on the nature of different heat transfer mechanisms involved in the nucleation process were discussed in part I. In this paper, the experimental results are compared with the existing boiling models. The boiling models based on dominance of a single mechanism of heat transfer did not match the experimental results. However, the Rohsenow model was found to closely predict the heat transfer through the microconvection mechanism that is primarily active outside the bubble/surface contact area. An existing transient conduction model was modified to predict the surface heat transfer during the rewetting process (i.e. transient conduction mechanism). This model takes into account the gradual rewetting of the surface during the transient conduction process rather than a simple sudden surface coverage assumption commonly used in the boiling literature. The initial superheat energy of the microlayer (i.e. microlayer sensible energy) was accurately calculated and found to significantly contribute in microlayer evaporation. This even exceeded the direct wall heat transfer to microlayer at high surface superheat temperatures. A composite model was introduced that closely matches our experimental results. It incorporates models for three mechanisms of heat transfer including microlayer evaporation, transient conduction, microconvection, as well as their influence area and activation time. The significance of this development is that, for the first time, all submodels of the composite correlation were independently verified using experimental results.     

Modeling of nucleate boiling heat transfer under an impinging free jet

A model using an analytical/empirical approach has been developed to predict the rate of heat transfer in the stagnation region of a planar jet impinging on a horizontal flat surface. The model has been developed based on the hypothesis that bubble-induced mixing would result in enhanced or additional diffusivity. The additional diffusivity has been included in the diffusion term of the conservation equations. The value of the effective diffusivity has been correlated with jet parameters (velocity and temperature) and surface temperature using experimental data. The important aspects of the bubble dynamics (generation frequency and average bubble diameter) have been acquired using high-speed imaging and an intrusive optical probe. The applicability of the proposed model has been investigated under conditions of partial and fully-developed nucleate boiling. Experiments have been carried out using water at atmospheric pressure, mass flux in the range of 388–1649 kg/m² s, degree of sub-cooling in the range of 10–28 °C, and surface temperature in the range of 75–120 °C. Results showed that the proposed model is able to predict the surface heat flux with reasonable accuracy (+30% and ?15%).     

Numerical study of single bubbles with dynamic contact angle during nucleate pool boiling

Nucleate pool boiling is typically characterized by cyclic growth and departure of vapor bubbles from a heated wall. It has been experimentally observed that the contact angle at the bubble base varies during the ebullition cycle. In the present numerical study, a static contact angle model and dynamic contact angle models based on the contact line velocity and the sign of the contact line velocity have been used at the base of a vapor bubble growing on a heated wall. The complete Navier–Stokes equations are solved and the liquid–vapor interface is captured using the level-set technique. The effect of dynamic contact angle on bubble dynamics and vapor volume growth rate is compared with results obtained with the static contact angle model.     

An experimental investigation of bubble nucleation of a refrigerant in pressurized boiling flows

An experimental investigation is performed to determine the effect of system pressure and heat flux on flow boiling and associated bubble characteristics of a refrigerant in a narrow vertical duct. A high-pressure flow boiling test loop was built and TLC (thermo-chromic liquid crystal) was applied to the back of the heater foil for high resolution and accurate measurement of heater surface temperature. Refrigerant R-134a is used as the test fluid at different pressures ranging from 690 to 827 kPa and different heat fluxes to quantify their influence in bubble characteristics such as bubble nucleation, growth, departure, and coalescence. Two synchronized high resolution and high-speed cameras are used to simultaneously capture TLC images as well as bubbling activities at high frame rates. By varying flow rate and system pressure, TLC and bubble images were captured and analyzed. Results show that the bubble generation frequency and size increase with heat flux. An increase in pressure from 690 to 827 kPa increased the bubble frequency and size by about 32 Hz and 20 μm, respectively. Bubble coalescence was also observed after departure from the nucleation site.     

Spray cooling characteristics of water and R-134a. Part I: nucleate boiling

An experimental study was made in the first of two papers to determine the effect of liquid sprays used to cool a hot surface. Both pure water and R-134a were served as a working medium sprayed from a single circular nozzle onto a Cu (oxygen free) metal of an electrically heated surface which was heated to an initial temperature with a range of wall superheat for steady-state nucleate boiling experiments using thermocouples for heat transfer measurements. Cooling characteristics (boiling curves) were obtained over a range of spray mass flux, Weber number, wall superheat and degree of subcooling. Boiling visualization was also conducted with varied heat flux levels at a specified We for R-134a and water.     

Numerical investigation of boiling flow of nitrogen in a vertical tube using the two-fluid model

The two-fluid model has been extensively used in modeling boiling flow of water, however, there are few equivalent studies of boiling flow of cryogenic liquids. In the present study, the two-fluid model which is basically included in the CFX code was modified by incorporating new closure correlations, then boiling flow of liquid nitrogen in a vertical tube was numerically simulated using the basic model and the modified model, respectively. Comparison with experimental data shows that the modified model is satisfactorily improved in accuracy. This study demonstrated that the following parameters and models are important for accurate prediction: the lift force, the bubble diameter distribution and the active site density, among which, the active site density has the most significant effect.     

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.