scholarly journals The thermo-wetting instability driving Leidenfrost film collapse

2020 ◽  
Vol 117 (24) ◽  
pp. 13321-13328
Author(s):  
Tom Y. Zhao ◽  
Neelesh A. Patankar
Keyword(s):  
Liquid Droplet ◽  
Heated Surface ◽  
Thermal Control ◽  
Solid Substrate ◽  
Bulk Liquid ◽  
Dimensionless Number ◽  
Vapor Film ◽  
System Pressure ◽  
Film Collapse ◽  
Liquid Vapor

Above a critical temperature known as the Leidenfrost point (LFP), a heated surface can suspend a liquid droplet above a film of its own vapor. The insulating vapor film can be highly detrimental in metallurgical quenching and thermal control of electronic devices, but may also be harnessed to reduce drag and generate power. Manipulation of the LFP has occurred mostly through experiment, giving rise to a variety of semiempirical models that account for the Rayleigh–Taylor instability, nucleation rates, and superheat limits. However, formulating a truly comprehensive model has been difficult given that the LFP varies dramatically for different fluids and is affected by system pressure, surface roughness, and liquid wettability. Here, we investigate the vapor film instability for small length scales that ultimately sets the collapse condition at the Leidenfrost point. From a linear stability analysis, it is shown that the main film-stabilizing mechanisms are the liquid–vapor surface tension-driven transport of vapor mass and the evaporation at the liquid–vapor interface. Meanwhile, van der Waals interaction between the bulk liquid and the solid substrate across the vapor phase drives film collapse. This physical insight into vapor film dynamics allows us to derive an ab initio, mathematical expression for the Leidenfrost point of a fluid. The expression captures the experimental data on the LFP for different fluids under various surface wettabilities and ambient pressures. For fluids that wet the surface (small intrinsic contact angle), the expression can be simplified to a single, dimensionless number that encapsulates the wetting instability governing the LFP.

Experimental Study of the Vapor Film Behavior on a Highly Heated Surface Immersed Into Subcooled Water

2010 ◽  
Author(s):  
Kirill I. Belov ◽  
Yury P. Ivochkin ◽  
Konstantin G. Kubrikov ◽  
Alexandr A. Oksman ◽  
Sergei N. Vavilov ◽  
...  
Keyword(s):  
High Speed ◽  
Heated Surface ◽  
Time Lag ◽  
Experimental Studies ◽  
Saturation Temperature ◽  
Heating Surface ◽  
Nucleate Boiling ◽  
Vapor Film ◽  
Pressure Pulses ◽  
Film Collapse

Results of the experimental studies of the vapor film behavior on a highly heated hemispherical surface immersed into water subcooled relative to the saturation temperature are presented. Transition from film to nucleate boiling was studied with the use of high-speed videocamera. Temperature characteristics of the vapor-film collapse, pressure pulses, acoustic effects, and vapor-film thickness were measured, as well. The decisive influence of the material and condition of the heating surface and the degree of water subcooling on the mode of transition (explosion-like, quiet, or intermediate one) was confirmed. The explosion-like mode of the vapor-film collapse is accompanied by ejection of vapor jets (single or multiple) and pressure pulses with an amplitude of up to 1 MPa. A structure of the pulse packs under multiple jets ejection was investigated. Synchronous measurements of the pressure pulses and an area of the direct cold liquid contact with a hot surface made it possible to determine a time delay between an instant of the contact and the pressure pulse. Typical value of this time lag was several tens microseconds. The dependence of the pulse frequency and the number of pulses on the hemisphere temperature was obtained.

scholarly journals 610 Study of Micro-Mechanism of Vapor Film Collapse on the High Temperature Sphere Surface

2000 ◽  
Vol 2000 (0) ◽  
pp. 183-184
Author(s):  
Yutaka ABE ◽  
Hiroshi YANAGIDA ◽  
Hideki NARIAI ◽  
Miki YAGITA
Keyword(s):  
High Temperature ◽  
Vapor Film ◽  
Sphere Surface ◽  
Film Collapse

Experimental Investigation of Submerged Impinging Jet Using Cu-Water Nanofluid

2010 ◽  
Author(s):  
Qiang Li ◽  
Yimin Xuan ◽  
Feng Yu ◽  
Junjie Tan
Keyword(s):  
Heat Transfer ◽  
Experimental Investigation ◽  
Volume Fraction ◽  
Cooling System ◽  
Heated Surface ◽  
Particle Volume Fraction ◽  
Jet Impingement ◽  
Particle Volume ◽  
Impingement Cooling ◽  
System Pressure

An experimental investigation was performed to study the heat transfer and flow features of Cu-water nanofluids (Cu particles with 26 nm diameter) in a submerged jet impingement cooling system. Three particular nozzle-to-heated surface distances (2, 4 and 6 mm) and four particle volume fractions (1.5%, 2.0%, 2.5% and 3.0%) are involved in the experiment. The experimental results reveal that the suspended nanoparticles increase the heat transfer performance of the base liquid in the jet impingement cooling system. Within the range of experimental parameters considered, it has been found that highest surface heat transfer coefficients can be achieved using a nozzle-to-surface distance of 4 mm and the nanofluid with 3.0% particle volume fraction. In addition, the experiments show that the system pressure drop of the dilute nanofluids is almost equal to that of water under the same entrance velocity.

Numerical Simulation and Experimental Validation of the Dynamics of a Single Bubble During Pool Boiling Under Constant and Time-Varying Reduced Gravity Conditions

2003 ◽  
Author(s):  
H. S. Abarajith ◽  
D. M. Qiu ◽  
V. K. Dhir
Keyword(s):  
Numerical Simulation ◽  
Heated Surface ◽  
Bubble Diameter ◽  
Pool Boiling ◽  
Growth Period ◽  
Single Bubble ◽  
Time Varying ◽  
Reduced Gravity ◽  
System Pressure ◽  
Vapor Liquid

The numerical simulation and experimental validations of the growth and departure of a single bubble on a horizontal heated surface during pool boiling under reduced gravity conditions have been performed here. A finite difference scheme is used to solve the equations governing mass, momentum and energy in the vapor liquid phases. The vapor-liquid interface is captured by level set method, which is modified to include the influence of phase change at the liquid-vapor interface. The effects of reduced gravity conditions, wall superheat and liquid subcooling and system pressure on the bubble diameter and growth period have been studied. The simulations are also carried out under both constant and time-varying gravity conditions to benchmark the solution with the actual experimental conditions that existed during the parabolic flights of KC-135 aircraft. In the experiments, a single vapor bubble was produced on an artificial cavity, 10 μm in diameter microfabricated on the polished silicon wafer, the wafer was heated electrically from the back with miniature strain gage type heating elements in order to control the nucleation superheat. The bubble growth period and the bubble diameter predicted from the numerical simulations have been found to compare well with the data from experiments.

Numerical Investigation of a Liquid Droplet Impinging on a Heated Surface

2009 ◽  
Author(s):  
Andres Diaz ◽  
Alfonso Ortega ◽  
Ryan Anderson
Keyword(s):  
Heat Transfer ◽  
Numerical Investigation ◽  
Fundamental Problem ◽  
Water Drop ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Heat Removal ◽  
Droplet Spreading ◽  
Droplet Shape ◽  
Spreading Dynamics

Previous studies, most of them experimental, reveal that the cooling effectiveness of a water drop impinging on a heated surface depends on the wall temperature, droplet shape and velocity. All previous studies focus on the behavior of a droplet falling in a quiescent environment, such as still air. Evidence in the literature also shows that gas assisted droplet sprays, in which a gas phase propels the droplets, are more efficient in heat removal than sprays consisting of droplets alone. It is conjectured that this is due to an increase in the maximum droplet spreading diameter upon impact, a thinner film, and consequently an increase in the overall heat transfer coefficient. Recent experiments in the author’s group [1, 2] show that the carrier gas jet strongly influences droplet spreading dynamics by imposing normal and shear forces on the liquid surface. The heat transfer is greatly augmented in the process, compared to a free falling droplet. To date, there has been no fundamental investigation of the physics of gas assisted spray cooling. To begin to understand the complicated process, this paper reports on a fundamental problem of a single liquid droplet that impinges on a heated surface. This paper contributes a numerical investigation of the problem using the volume of fluid (VOF) technique to capture droplet spreading dynamics and heat transfer in a single drop event. The fluid mechanics is investigated and compared to the experimental data. The greatest uncertainty in the simulation is in the specification of the contact angle of the advancing or receding liquid front, and in capturing the onset of the three-dimensional fingering phenomena.

Visual Observation of Vapor Film Collapse Behavior During Vapor Explosion

2002 ◽  
Author(s):  
Yutaka Abe ◽  
Hideki Nariai
Keyword(s):  
High Temperature ◽  
Pressure Pulse ◽  
Film Surface ◽  
Vapor Film ◽  
Visual Data ◽  
Vapor Explosion ◽  
Auto Correlation ◽  
Piv Technique ◽  
Collapse Behavior ◽  
Film Collapse

During severe accident of a light water reactor, various thermal hydraulic phenomena including vapor explosion could threaten the integrity of the containment vessel. Thermal detonation model is proposed to describe the vapor explosion. According to the model, several processes should be sequentially satisfied for the trigger phenomena of the vapor explosion. One of the most important processes for the trigger phenomena is the vapor film collapse around high temperature molten material droplets. In the present study, the vapor film collapse behavior around high temperature solid particle submerged into water was experimentally investigated by attacking a pressure pulse to the vapor film on a high temperature sold particle. The interfacial phenomena between vapor and water were measure by using a high-speed video camera of the maximum speed of 40,500 fps. The visual data obtained were processed with visual data processing techniques. That is, the average vapor film thickness was estimated, dynamic behaviors of the interfaces were analyzed with PIV technique and the interface movement was estimated with the digital auto correlation techniques from the visual data obtained. Furthermore, the transients of the temperature and pressure were simultaneously measured. The interfacial temperatures between vapor and water, and between molted liquid and water are analytically estimated by solving the heat conduction equation with the data obtained as the boundary conditions. It is clarified that vapor collapse by pressure pulse occurs homogeneously around the vapor film surface on a high temperature particle. Microscopic information are obtained from the visual data by using visual data processing technique, PIV technique and digital auto-correlation technique. At the time the vapor film surface changes to white, the saturation temperature exceeds the interfacial temperature. The microscopic vapor film collapse behavior indicates the possibility of the phase change at the vapor film collapse.

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