Oscillation of a liquid-vapor interface: experimental observations

The effects of liquid subcooling, velocity, and vapor superheat on the wavy nature of film boiling from a sphere in Freon-113 were studied. Experiments for both pool and flow film boiling were performed for a heater surface superheat of around 100 to 300°C, liquid velocity from 0- to 2.10 m/s, and Freon-113 subcooling from 0 to 25°C. Photographs taken in the film boiling regime show that the nature of the liquid-vapor interface is a function of the above-mentioned parameters. For low liquid subcooling ripples were present on the liquid-vapor interface. At greater subcooling these ripples disappear. For very large vapor superheat and liquid velocity ripples are always present on the liquid-vapor interface.     

Numerical Analysis of Film Boiling in Liquid Jet Impingement

A numerical approach is presented for computing film boiling in liquid jet impingement on a high-temperature plate. The conservation equations of mass, momentum and energy are numerically solved in the liquid, vapor, and air phases. The sharp-interface level-set formulation is employed to track the liquid-air interface, as well as the liquid-vapor interface with phase change. A simplified analytical model for a thin vapor film, whose thickness is several orders of magnitude smaller than the liquid layer, is incorporated into the level-set formulation. The multiscale approach is tested through the computations of film boiling in a circular water jet.     

Analysis of binary film boilingAnalyse de l'ebullition en film binaireUntersuchung des filmsiedens binärer gemischeAнaлиз кипeния Бинapнoй плeнки

Binary film boiling has been analyzed on the basis of the conservation laws and other principles. It has been shown that the liquid-vapor interface temperature depends on diffusion phenomena within the liquid mixture. The omission of the diffusion processes in an analysis might lead to erroneous heat-transfer results.     

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.     

Contact angles and interface behavior during rapid evaporation of liquid on a heated surface

Photographic observations of the boiling phenomena have played an important role in gaining insight into the boiling mechanism. This paper presents a brief historical review of the available literature on the photographic studies in pool and flow boiling. This is followed by the results of the photographic studies conducted in the authors' laboratory on liquid droplets impinging on a heated surface. Liquid-vapor interface and contact line movements are observed through a high speed camera at high resolution. The effect of surface roughness and surface temperature on dynamic advancing and receding contact angles has been studied. In addition, the effects of rapid evaporation on advancing and receding contact angles, liquid-vapor interface motion, and the dryout front propagation have been investigated.     

Numerical simulation of film boiling on a sphere with a volume of fluid interface tracking method

This paper presents a numerical method for the simulation of boiling flows on non-orthogonal body-fitted coordinates. The volume-of-fluid (VOF) method based on piecewise linear interface construction (PLIC) is used to track liquid–vapor interface and is extended to body-fitted coordinates. Some special treatment is taken to deal with the discontinuous velocity field due to phase change at the interface. A double staggered grid with the SIMPLE method is adopted to solve the flow field. This method is used to simulate natural convection film boiling and forced convection film boiling on a sphere at saturated conditions. The simulation results are compared with analytical correlations and experimental data.     

Numerical Investigation of Saturated Flow Boiling in Microchannels by the Lattice Boltzmann Method

Bubble formation in saturated flow boiling in 2D microchannels, generated from a microheater under constant wall heat flux or constant wall temperature conditions, is studied numerically based on a newly developed lattice Boltzmann model for liquid-vapor phase change. Simulations are carried out to study effects of inlet velocity, contact angle, and heater size on saturated flow boiling of water under constant wall heat flux conditions. Important information, such as effects of static contact angle on nucleation time and nucleation temperature, which was unable to be obtained by other numerical simulation methods, is obtained. Furthermore, effects of inlet velocity, contact angle, and superheat on nucleate boiling heat transfer in steady flow boiling of water under constant wall temperature conditions are also presented. It is found that the nucleate boiling heat transfer at the microheater is higher if the heater surface is more hydrophilic, because the superheated vapor at the hydrophilic wall has a thinner thermal boundary layer and a larger thermal conductivity.     

Forced Convection Film Boiling Heat Transfer Over a Vertical Cylinder

The knowledge of subcooled film boiling heat transfer is important as the basis of understanding the reflooding phenomenon during emergency cooling in a nuclear reactor under a loss-of-coolant accident. In this study, forced convection film boiling heat transfer from a vertical cylinder in Freon-113 flowing upward along the cylinder was measured for the flow velocities ranging from 0 to 1.3 m/s, and liquid subcoolings ranging from 0 to 20 K at pressures near atmospheric. A platinum heater with a diameter of 3 mm was heated by electric current. The heat transfer coefficients obtained are almost independent of vertical positions on the cylinder. The heat transfer coefficients are almost independent of velocity for the velocities lower than about 1 m/s and become higher for the velocities higher than 1 m/s. The heat transfer coefficients at each velocity are higher for higher liquid subcoolings. Improvement of film boiling heat transfer from the vertical cylinder with the increase in flow velocity is much less than that of horizontal cylinder in cross flow previously reported by the authors. This is mainly due to the difference of heat transfer enhancement mechanism; the former is the drag force on vapor flow acted by a liquid flow, and the latter is the pressure gradient near the front stagnation point caused by external potential flow.     

The effect of liquid film evaporation on flow boiling heat transfer in a micro tube

Flow boiling in micro channels is attracting large attention since it leads to large heat transfer area per unit volume. Generated vapor bubbles in micro channels are elongated due to the restriction of channel wall, and thus slug flow becomes one of the main flow regimes. In slug flow, sequential bubbles are confined by the liquid slugs, and thin liquid film is formed between tube wall and bubble. Liquid film evaporation is one of the main heat transfer mechanisms in micro channels and liquid film thickness is a very important parameter which determines heat transfer coefficient. In the present study, liquid film thickness is measured by laser focus displacement meter under flow boiling condition and compared with the correlation proposed for an adiabatic flow. The relationship between liquid film thickness and heat transfer coefficient is also investigated. Initial liquid film thickness under flow boiling condition can be predicted well by the correlation proposed under adiabatic condition. Under flow boiling condition, liquid film surface fluctuates due to high vapor velocity and shows periodic pattern against time. Frequency of periodic pattern increases with heat flux. At low quality, heat transfer coefficients calculated from measured liquid film thickness show good accordance with heat transfer coefficients obtained directly from wall temperature measurements.     

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.     

Investigation of heat transfer and bubble dynamics in a boiling thin liquid film flowing over a rotating disk

Nucleate boiling heat transfer and bubble dynamics in a thin liquid film on a horizontal rotating disk were studied. A series of experiments were conducted to determine the heat transfer coefficient on the disk. At low rotation and flow rates, vigorous boiling increased the heat transfer coefficients above those without boiling. Higher rotational speeds and higher flow rates increased the heat transfer coefficient and suppressed boiling by decreasing the superheat in the liquid film. The flow field on the disk, which included supercritical (thin film) flow upstream of a hydraulic jump, and subcritical (thick film) flow downstream of a hydraulic jump, affected the type of bubble growth. Three types of bubble growth were identified. Vigorous boiling with large, stationary bubbles were observed in the subcritical flow. Supercritical flow produced small bubbles that remained attached to the disk and acted as local obstacles to the flow. At low rotational rates, the hydraulic jump that separated the supercritical and subcritical regions produced hemispherical bubbles that protruded out of the water film surface and detached from the disk, allowing them to slide radially outward. A model of the velocity and temperature of the microlayer of water underneath these sliding bubbles indicated that the microlayer thickness was approximately 1/25th of that of the surrounding water film. This microlayer is believed to greatly enhance the heat transfer rate underneath the sliding bubbles.     

Characteristics of liquid ethanol diffusion flames from mini tube nozzles

A series of experiments was conducted to explore the combustion characteristics of a diffusion flames from mini tubes fueled by liquid ethanol with visual observations of the flame shape, the dynamic liquid-vapor interface during phase change inside the capillary tubes and the tube outer surface temperature using CCD and IR cameras. As the fuel supply rate increased, the interface location rose to the tube exit and the temperature gradient on the outer tube surface increased, consequently the evaporating became much stronger and the interface tended to be unstable. The combustion characteristics are closely related to the rapid phase change and violent evaporation and interfacial dynamics, with the violent evaporation, actually explosive boiling, inducing an explosive flame. The intensity of the explosive flame became stronger as the flowrate increased with the maximum flame height, interface location movement, and sound intensity all significantly increasing. The periodicity of the explosive flame was directly proportional to the interface moving distance and inversely proportional to the fuel flow rate.     

Flow and heat transfer characteristics of the evaporating extended meniscus in a micro-capillary channel

A mathematical model is developed to predict the transport phenomena during evaporation in the extended meniscus region of a micro-capillary channel. In this model, the vapor pressure variation and the disjoining pressure effect are included and the friction force at the liquid-vapor interface is considered as well. The results show that the local heat transfer coefficient has an extremely large value in the thin film region. The heat transfer rate, however, is larger for the meniscus than for the thin film region. The maximum liquid velocity appears at approximately 40% of the extended meniscus region and the variation of the heat flux has a negligible effect on the maximum liquid velocity. It is also found that the length of the extended meniscus region is affected by the heat flux, the channel height and the dispersion constant.     

Film condensation on a vertical microchannel

In this paper, a two dimensional laminar liquid film which condenses on a vertical microchannel is investigated analytically. A liquid film thickness, condensation mass flux flow and the variation of the velocity through the liquid thickness were determined by modified Navier–Stockes and energy equations. The effect of some parameters on the liquid film thickness, condensation mass flow rate and velocity are investigated. These parameters include slipping in temperature, β, and velocity, α, due to microscale interaction. It was found that, the liquid film thickness, δ, decreases as the slipping factors increases, and diminishes as a value of slipping factors (α and β) become more than or equal to 0.1. Increasing the slip in temperature due to microscale interaction causes the condensation mass flow rate to increase as the value of slip in velocity increases. Additionally, the slip value in the channel was found to increase as the slip value in velocity, α, increases.     

Turbulent film condensation on a half oval body

The paper is an investigation of turbulent film condensation on a half oval body. The high tangential velocity of the vapor flow at the boundary layer is determined from potential flow theory. The Colburn analogy is used to define the local liquid-vapor interfacial shear which occurs when the high velocity vapor flows across the body surface. The paper then presents a discussion of the results obtained for the local dimensionless film thickness and heat transfer characteristics. Furthermore, the present paper discusses the influence of Froude number, sub-cooling temperature and system pressure on mean Nusselt number.     

Flow boiling CHF in microgravity

Poor understanding of flow boiling in microgravity has recently emerged as a key obstacle to the development of many types of power generation and life support systems intended for space exploration. This study examines flow boiling CHF in microgravity that was achieved in parabolic flight experiments with FC-72 onboard NASA’s KC-135 turbojet. At high heat fluxes, bubbles quickly coalesced into fairly large vapor patches along the heated wall. As CHF was approached, these patches grew in length and formed a wavy vapor layer that propagated along the wall, permitting liquid access only in the wave troughs. CHF was triggered by separation of the liquid-vapor interface from the wall due to intense vapor effusion in the troughs. This behavior is consistent with, and accurately predicted by the Interfacial Lift-off CHF Model. It is shown that at low velocities CHF in microgravity is significantly smaller than in horizontal flow on earth. CHF differences between the two environments decreased with increasing velocity, culminating in virtual convergence at about 1.5 m/s. This proves it is possible to design inertia-dominated systems by maintaining flow velocities above the convergence limit. Such systems allow data, correlations, and/or models developed on earth to be safely implemented in space systems.     

A Level Set Method for Analysis of Film Boiling on an Immersed Solid Surface

A numerical method is presented for simulating film boiling on an immersed (or irregularly shaped) solid surface. The level set formulation for tracking the phase interfaces is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip condition at the fluid–solid interface. The boundary or matching conditions at the phase interfaces are accurately imposed by incorporating the ghost fluid approach based on a sharp-interface representation. The numerical method is tested through computations of bubble rise in a stationary liquid, single-phase fluid flow past a circular cylinder, and film boiling on a horizontal cylinder.     

The Cooling of PEFC with Pentane Boiling in Minichannels: A Study of Flow Instabilities Using Neutron Radiography Visualization

Polymer electrolyte fuel cells (PEFC) operate best at a steady temperature of about 80°C and have a very low heat flux compared to other heat transfer applications. Two-phase pentane cooling of bipolar plates is studied in order to optimize fuel cell cooling in transport applications. High-speed visualizations of boiling pentane in a circular steel tube (D _i = 1.1 mm, D _o = 2 mm) have been performed in a Neutrograph instrument at the Institut Laue-Langevin in Grenoble, France. The heat and mass flux were both very low and appropriate for cooling of PEFC. The spatial resolution of the images is approximately 0.15 mm and the maximum frequency is 154 Hz. In the images, the liquid-vapor differentiation is clearly visible. Time resolved measurements of the outer pipe wall temperature, synchronized with the images, show that at low mass flow rates, the pipe wall is high above the saturation temperature and the pipe filled with vapor and liquid slugs. At higher flow rates, the wall is superheated when filled with liquid, and at saturation temperature during boiling when exposed to a liquid-vapor mixture. An irregular switch between these two states was observed. The superheated wall is shown to be consistent with the superheated liquid in the pipe in both stable and time-dependent states. Unfortunately, the strong γ-radiation produced by the neutrons has a substantial effect on the onset of boiling, which is why comparisons with non-irradiated systems might be difficult. Simplified steady and time-dependent models are proposed to explain the measured wall temperature instabilities and superheat.     

A Novel Approach for Modeling Mixed Convection Film Boiling for a Vertical Flat Plate

A numerical model is developed to study mixed convection film boiling over a vertical flat plate. The integral form of conservation equations for each phase along with the appropriate interface conditions due to phase change is transformed into ordinary differential equation (ODE)-form. The length scale used in the model is based on Rayleigh–Taylor instability wave at the liquid–vapor interface. The heat transfer associated in the process is assessed and results are validated successfully for different available experimental results for natural convection and mixed convection film boiling. The mixed convection film boiling is characterized in terms of relevant nondimensional parameters for each phase.