Bubble Growth In Superheated Liquid Research Articles

A New Method for Estimating Bubble Diameter at Different Gravity Levels for Nucleate Pool Boiling

Abstract Nucleate boiling has significant applications in earth gravity (in industrial cooling applications) and microgravity conditions (in space exploration, specifically in making space applications more compact). However, the effect of gravity on the growth rate and bubble size is not yet well understood. We perform numerical simulations of nucleate boiling using an adaptive moment-of-fluid (MoF) method for a single vapor bubble (water or Perfluoro-n-hexane) in saturated liquid for different gravity levels. Results concerning the growth rate of the bubble, specifically the departure diameter and departure time, have been provided. The MoF method has been first validated by comparing results with a theoretical solution of vapor bubble growth in superheated liquid without any heat-transfer from the wall. Next, bubble growth rate, bubble shape, and heat transfer results under earth gravity, reduced gravity, and microgravity conditions are reported, and they are in good agreement with experiments. Finally, a new method is proposed for estimating the bubble diameter at different gravity levels. This method is based on an analysis of empirical data at different gravity values and using power-series curve fitting to obtain a generalized bubble growth curve irrespective of the gravity value. This method is shown to provide a good estimate of the bubble diameter for a specific gravity value and time.

Direct numerical simulation of evaporation and condensation with the geometric VOF method and a sharp-interface phase-change model

The paper describes a coupling of the geometric Volume-of-Fluid (VOF) method with a sharp-interface phase-change model for Cartesian and axisymmetric grids. Both the interfacial position and the temperature field are resolved with subgrid accuracy. Species transport with implicit diffusion is considered in the gas phase. The numerical implementation of the method developed here is described in detail, as well as its coupling with the incompressible Navier-Stokes solver PSI-BOIL, which features a hybrid, finite-volume/finite-difference approach based on a fixed, rectangular grid. Several verification cases have been undertaken to ensure correct implementation of the method in the code and to evaluate its performance. These include simulations of the Stefan problem, sucking problem and bubble growth in superheated liquid. In all cases, very good agreement with the analytical solution has been reached and grid convergence has been demonstrated. Simulations of evaporating and condensing rising bubbles, for which high-quality measured data are available, are also presented to serve as validation tests. Reasonable agreement of simulation results with experimental data has been recorded and applicability of the method to problems without inherent symmetry and featuring turbulence is shown. This work represents the first successful application of a geometric VOF method coupled with a sharp-interface phase-change model and species transport to non-trivial problems. The achieved performance of our algorithm in the verification and validation exercise represents an important step in the development of multiphase codes capable of accurately resolving complex three-dimensional multiphase flows.

New semi-analytical solution of the problem of vapor bubble growth in superheated liquid

This paper presents a mathematical model of the vapor bubble growth in an initially uniformly superheated liquid. This model takes into account simultaneously the dynamic and thermal effects and includes the well-known classical equations: the Rayleigh equation and the heat conductivity equation, written with consideration of specifics associated with the process of liquid evaporation. We have obtained a semi-analytical solution to the problem, which consists in reducing the initial boundary value problem with a moving boundary to a system of ordinary differential equations of the first order, valid in a wide range of operating parameters of the process at all its stages: from inertial to thermal, including the transitional one. It is shown that at large times this solution is consistent with the known solutions of other authors obtained in the framework of the energy thermal model, in particular, for the high Jacob numbers, it is consistent with the Plesset–Zwick solution.

An approximation approach for the simulation of vapor-liquid phase change by the volume-of-fluid method

An approximation approach for the implementation of the phase change model based on the volume-of-fluid method is proposed. In each interfacial cell, the interface normal direction is approximated to be parallel to one Cartesian axis line. The interface position is determined by the volume fraction value of the interfacial cell. The mass flux and heat flux at the interface are calculated in terms of temperature values of neighbor cells. The phase change model is easily coupled with ANSYS-Fluent by user defined functions. Film boiling on a flat plate, bubble growth in superheated liquid and bubble condensation in subcooled liquid are simulated. Simulation results of all three problems agree well with available analytical solutions and experimental data, which verify the accuracy of current approximation approach.

Viewing internal bubbling and microexplosions in combusting metal particles via x-ray phase contrast imaging

The combustion performance of metal particles for explosives, propellants, pyrotechnics, and bio-agent defeat can be optimized by controlling the chemistry, size, and coatings of the particles. Many pure and composite metal fuels used in these applications have been observed to microexplode during combustion. This causes particles to fragment, revealing fresh surfaces, which may enhance burn rates and efficiencies. Despite these potential benefits, few have attempted to control the bubbling and the microexplosion of metal powders as they combust in molten states. Here, we choose Al:Zr composite powders as a representative system to study bubbling and microexplosions and to identify active mechanisms. Using synchrotron x-rays and phase contrast imaging, we observe molten metal particles of various sizes as they burn in air at temperatures ranging from 2700 to 3500 K. We characterize bubble nucleation and growth in the interior of the particles during the combustion process, and we identify heterogeneous nucleation, slow growth and coalescence of bubbles, as well as rapid bubble growth leading to microexplosions. Bubble growth rates and Laplace pressures are calculated, and we find that during slow expansion growth rates are similar to classical predictions of bubble growth in superheated liquids. For rapid expansions we find that a critical growth rate of ∼0.5 m s−1, independent of the initial particle size, is necessary to enable microexplosions. We calculate the rate of gas generation that is needed to enable this growth and we conclude that nitrogen is the gas most likely driving rapid bubble growth and fragmentation. These results provide the first in situ observations of the mechanisms that control bubbling and microexplosions during the combustion of metal particles.

Evaluation of Interfacial Heat Transfer Models for Flashing Flow with Two-Fluid CFD

The complexity of flashing flows is increased vastly by the interphase heat transfer as well as its coupling with mass and momentum transfers. A reliable heat transfer coefficient is the key in the modelling of such kinds of flows with the two-fluid model. An extensive literature survey on computational modelling of flashing flows has been given in previous work. The present work is aimed at giving a brief review on available theories and correlations for the estimation of interphase heat transfer coefficient, and evaluating them quantitatively based on computational fluid dynamics simulations of bubble growth in superheated liquid. The comparison of predictions for bubble growth rate obtained by using different correlations with the experimental as well as direct numerical simulation data reveals that the performance of the correlations is dependent on the Jakob number and Reynolds number. No generally applicable correlations are available. Both conduction and convection are important in cases of bubble rising and translating in stagnant liquid at high Jakob numbers. The correlations combining the analytical solution for heat diffusion and the theoretical relation for potential flow give the best agreement.

Dynamic modeling of bubble growth in vapor-liquid phase change covering a wide range of superheats and pressures

Bubble growth in superheated liquid is a fundamental process in vapor-liquid phase change which occurs widely in thermal and chemical engineering. The strong coupling of heat, mass and momentum transfer at the interface brings difficulties to accurately predict the dynamics of bubble growth. At present, bubble growth under three extreme conditions, i.e. the very early growth stage, low superheats and low pressures, cannot be well described by traditional asymptotic solutions. In this work, a mathematical model was presented for better prediction of bubble growth in a superheated liquid. The model was derived from the equations of motion for a bubble and took account of the heat and mass balances at the interface. The model was validated with a series of experiments from the literature, covering a wide range of operating conditions. The newly proposed model can well predict the features of bubble growth at the very early stage (less than 10−6s), for superheats varying from 0.8K to 36K and for system pressures reduced from 1.0atm to 0.0124atm. Analyses on the thermodynamics and hydrodynamics manifested that the bubble growth was characterized by three typical stages, i.e. a thermal delay stage, a fast expansion stage and a steady growth stage. The time lengths of these stages were related to the levels of superheat or system pressure. Characteristics of these stages were further discussed and the roles of interfacial forces under the different operating conditions were demonstrated.

New insights into particle detection with superheated liquids

We report new results obtained from calibrations of superheated liquid droplet detectors used in dark matter searches with different radiation sources (n, α, γ). In particular, detectors were spiked with α-emitters located inside and outside the droplets. It is shown that the responses have different temperature thresholds, depending on whether α-particles or recoil nuclei create the signals. The measured temperature threshold for recoiling 210Pb nuclei from 214Poα-decays was found to be in agreement with test beam measurements using mono-energetic neutrons. A comparison of the threshold data with theoretical predictions shows deviations, especially at high temperatures. It is shown that signals produced simultaneously by recoil nuclei and α-particles have more acoustic energy than signals produced by one or the other separately. A model is presented that describes how the observed intensities of particle-induced acoustic signals can be related to the dynamics of bubble growth in superheated liquids. A growth scenario that is limited by the inertia of the surrounding liquid shows a trend that is supported by the data. An improved understanding of the bubble dynamics is an important first step in obtaining better discrimination between particle types interacting in detectors of this kind.

過熱液中における気泡成長のCIP法による数値解析(熱工学II)

過熱液中における気泡成長のCIP法による数値解析(熱工学II)

Multiphase Flow Dynamics, Volume 1: Fundamentals; Volume 2: Thermal and Mechanical Interactions

Multiphase Flow Dynamics, Volume 1: Fundamentals; Volume 2: Thermal and Mechanical Interactions

K-1722 曲率を用いた界面移動法による過熱液中の気泡成長の数値解析(S22-2 気液相変化に関する素過程と数値解析手法(2))(S22 気液相変化に関する素過程と数値解析手法)

K-1722 曲率を用いた界面移動法による過熱液中の気泡成長の数値解析(S22-2 気液相変化に関する素過程と数値解析手法(2))(S22 気液相変化に関する素過程と数値解析手法)

Bubble growth in superheated liquid based on nonequilibrium thermodynamic theory

A mathematical model describing the bubble growth in nucleate boiling is proposed based on non-equilibrium thermodynamics. Numerical simulation of the bubble growth process is performed. Some new arguments are suggested to explain the phenomenon which are distinct from the existing articles.

Mechanism of the initial stage of bubble growth in a liquid close to the superheat limit

The concept of the critical nucleus is the basis of the homogeneous nucleation theory which enables the upper limit of a superheat to be calculated. Only nucleus formation is considered. It is as if nuclei that reach the critical dimensions are removed from the system and replaced by an appropriate mass of liquid. The theory does not give any information concerning the dynamics of bubble growth either. Nevertheless, some researchers use the concept of the critical nucleus as the initial condition in a descritpion of the bubble growth in superheated liquid. A bubble begins to grow after the disturbance of the balance, when the pressure difference exceeds the surface tension. According to this scheme, the initial bubble growth results from the fact that the vapor pressure in the bubble is higher than that in the surrounding liquid.

Bubble growth in non-uniform temperature fields

A previous theoretical model for bubble growth in superheated liquids has been extended to non-uniform temperature fields. The growth rate has been assumed as controlled by heat transfer, surface tension, liquid inertia and non-equilibrium at the liquid—vapor interface. A comparison between the model predictions and some experimental growth data on heated walls is presented.

Generalized quantitative criteria for predicting the rate-controlling mechanism for vapor bubble growth in superheated liquids

Criteria are presented for predicting the relative importance of the inertia of the liquid and heat transfer in the liquid as controlling effects for spherically symmetric vapor bubble growth in liquids. The coupled governing equations including both of the above effects are first cast in appropriate dimensionless forms. Then numerical solutions of the coupled equations are compared to the limiting solutions for solely liquid inertia and solely heat transfer controlled growth processes, in order to determine the respective regions of importance of these two mechanisms. The cases of growth initiated by a step decrease and a linear decrease in system pressure are both considered. The criteria are developed for the case of an idealized fluid having a constant vapor density and a linear vapor pressure curve, and then shown to be approximately independent of the vapor density and pressure relations of the particular fluid. The applicability and usefulness of the criteria are supported and illustrated by comparison of predictions based on them with previous theoretical and experimental results taken from the literature.

Spherical vapour bubble growth in superheated liquids

Experimental data has been obtained on the growth of vapour bubbles in water at constant uniform superheats up to 20°C. The data is shown to agree within the limits of the experimental errors with the predictions of a simple theory derived assuming temperature equilibrium at the bubble wall. The theory is extended to predict the conditions for which differing fluids will exhibits similar growth rates, and it is shown for example that sodium vapour bubble growth may be modelled accurately in water.