The Effect of the Surface Inclination on the Hydrodynamics and Thermodynamics of Leidenfrost Droplets

2014 ◽  
Vol 30 (2) ◽  
pp. 145-151 ◽  
Author(s):  
P. Pournaderi ◽  
A. R. Pishevar
Keyword(s):  
Contact Time ◽  
Heat Removal ◽  
Total Heat ◽  
Vapor Layer ◽  
Accurate Calculation ◽  
Simulation Process ◽  
Mesh Resolution ◽  
Surface Inclination ◽  
Hot Surface ◽  
Maximum Spreading

ABSTRACTIn this research, the effect of the surface inclination on the hydrodynamics and heat transfer of droplets impinging on very hot surfaces is studied. The applied numerical algorithm is based on the accurate calculation of the vaporization rate in the simulation process using a combination of the level set and ghost fluid methods. Also a mesh clustering technique is utilized to create sufficient mesh resolution near the surface in order to take into account the effect of the thin vapor layer between droplet and very hot surface. The results are verified against available experiments. The effect of the surface inclination on the droplet maximum spreading radius, droplet contact time and total heat removal from the surface is considered. Results show that for the studied regime, the maximum spreading radius of the droplet is decreased with an increase in the surface inclination while the droplet contact time on the surface is independent from the surface inclination. For inclinations greater than 45°, the total heat removal is decreased considerably with an increase in the inclination angle. For smaller inclinations, the dependency of the total heat removal on the surface inclination is not strong.

Wind Chill*

1948 ◽  
Vol 29 (10) ◽  
pp. 487-493 ◽  
Author(s):  
Arnold Court
Keyword(s):  
Low Temperature ◽  
Heat Loss ◽  
Convective Heat ◽  
Human Body ◽  
Empirical Formula ◽  
Heat Removal ◽  
The Body ◽  
Total Heat ◽  
Wind Chill ◽  
Convective Heat Loss

The rate of heat removal from the human body by wind and low temperature was termed Wind Chill by Siple and expressed by an empirical formula. This paper discusses the formula critically, pointing out that this measure of the convective heat loss may be less than three-quarters of the total heat lost by the body. Siple's formula is compared with those of others, and the use of the formula is discussed.

Experimental Investigation of Rewetting During Quenching of Hot Surface by Round Jet Impingement Using Al2O3–Water Nanofluids

2016 ◽  
Author(s):  
Avadhesh K. Sharma ◽  
Mayank Modak ◽  
Santosh K. Sahu
Keyword(s):  
Experimental Investigation ◽  
Cooling System ◽  
Removal Rate ◽  
Pure Water ◽  
Heat Removal ◽  
Jet Impingement ◽  
Dimensionless Number ◽  
Surface Cooling ◽  
Fuel Rods ◽  
Hot Surface

Impinging jet surface cooling is being used in many industrial and engineering applications due to their higher heat removal rate. Jet impingement is one of the methods to cool hot surfaces, especially in textile, metal and electronic industries. Due to high heat removal rate the jet impingement cooling of the hot surfaces is being used in nuclear industries. During the loss of coolant accidents (LOCA) in nuclear power plant, an emergency core cooling system (ECCS) cool the cluster of clad tubes using consisting of fuel rods. The usual water flow within a reactor core is bottom to top, parallel to the fuel rods. When a hot surface quenched at very high temperature using a jet of cold fluid, during the quenching the initial heat transfer is limited by film boiling. The effective cooling takes place only after the surface temperature is below the leidenfrost temperature. In the present work an experimental investigation has been carried out to analyze the rewetting phenomenon of a hot vertical stainless steel foil by circular impinging jets of pure water and Al2O3–water nanofluids. The rewetting time and rewetting velocity in the form of dimensionless number (Peclet number) obtained from the thermal images obtained from infrared thermal imaging camera (A655sc, FLIR System). Experiments are performed for different Reynolds number (Re = 5000, 8000), and Al2O3–water nanofluids concentration (Φ = 0.15%, 0.6%)

Direct Numerical Simulation of Heat Transfer in Spray Cooling Through 3D Multiphase Flow Modeling Using Parallel Computing

2009 ◽  
Vol 131 (12) ◽  
Author(s):  
Suranjan Sarkar ◽  
R. Panneer Selvam
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Parallel Computing ◽  
Phase Change ◽  
Power Electronics ◽  
Vapor Bubble ◽  
Heat Removal ◽  
Spray Cooling ◽  
Hot Surface ◽  
Liquid Vapor

Thermal management issues have become a major bottleneck for further miniaturization and increased power consumption of electronics. Power electronics require more increasing use of high heat flux cooling technologies. Spray cooling with phase change has the advantage of large amount of heat transfer from the hot surface of many power electronics. Spray cooling is a complex phenomenon due to the interaction of liquid, vapor, and phase change at small length scale. A good understanding of the underlying physics and the heat removal process in spray cooling through numerical modeling is needed to design efficient spray cooling system. A computational fluid dynamics based 3D multiphase model for spray cooling is developed here in parallel computing environment using multigrid conjugate gradient solver. This model considers the effect of surface tension, gravity, phase change, and viscosity. The level set method is used to capture the movement of the liquid-vapor interface. The governing equations are solved using finite difference method. Spray cooling is studied using this model, where a vapor bubble is growing in a thin liquid film on a hot surface and a droplet is impacting on the thin film. The symmetry boundary condition considered on four sides of the domain effectively represents a large spray made up of multiple equally sized droplets and bubbles and their interaction. Studies have also been performed for different wall superheats in the absence of vapor bubble to compare the effect of two-phase heat transfer compared to single-phase in spray cooling. The computed interface, temperature, and heat flux distributions at different times over the domain are visualized for better understanding of the heat removal mechanism.

Springboard Droplet Bouncing on Flexible Superhydrophobic Substrates

2017 ◽  
Vol 139 (2) ◽  
Author(s):  
Patricia B. Weisensee ◽  
Junjiao Tian ◽  
Nenad Miljkovic ◽  
William P. King
Keyword(s):  
Natural Frequency ◽  
Contact Time ◽  
High Speed ◽  
Flexible Substrate ◽  
Superhydrophobic Surfaces ◽  
High Speed Imaging ◽  
Impact Speed ◽  
Lift Off ◽  
The Impact ◽  
Maximum Spreading

Droplet impact on rigid, superhydrophobic surfaces follows the well-known spreading, recoil, and lift-off behavior at lower impact speeds (a), and splashing at higher impact speeds (b). The contact time tc of these bouncing droplets is independent of the impact speed, and difficult to control. Using high speed imaging (9500 fps) of water droplets impacting superhydrophobic substrates with stiffness 0.5 to 7630 N/m (rigid), we were able to show that substrate flexibility can reduce contact times. Upon impact on a flexible substrate, the droplet excites the substrate to oscillate at the membrane or cantilever natural frequency (d). The oscillation accelerates the droplet upwards, initiating early droplet lift-off at the edges of the droplet close to the point of maximum spreading (c). Droplets fully lift off before fully recoiling, i.e. in a pancake shape. We call this phenomenon the springboard effect. Contact times are reduced by up to 50% compared to rigid substrates.

Visualization of the impact of water drops on a hot surface: effect of drop velocity and surface inclination

2006 ◽  
Vol 42 (10) ◽  
pp. 885-890 ◽  
Author(s):  
Gian Piero Celata ◽  
Maurizio Cumo ◽  
Andrea Mariani ◽  
Giuseppe Zummo
Keyword(s):  
Surface Effect ◽  
Drop Velocity ◽  
Water Drops ◽  
Surface Inclination ◽  
Hot Surface ◽  
The Impact

Leidenfrost Cart

2012 ◽  
Author(s):  
Ali Hashmi ◽  
Benjamin Coder ◽  
Gan Yu ◽  
Yuhao Xu ◽  
Jonathon Spafford ◽  
...  
Keyword(s):  
Gravitational Force ◽  
Consumption Rate ◽  
Flat Surface ◽  
Theoretical Calculations ◽  
Maximum Load ◽  
Vapor Layer ◽  
Leidenfrost Effect ◽  
Hot Surface ◽  
First Time ◽  
Measured Water

Friction is a major inhibitor in almost every mechanical system. Enlightened by the Leidenfrost effect — a droplet can be levitated by a vapor layer on a hot surface — we demonstrate for the first time that a small cart also can be levitated by Leidenfrost vapor provided that the surface temperature is above the Leidenfrost point. The levitated cart can carry certain amount of load and move frictionless on the hot surface. The maximum load that the cart can carry is experimentally tested over a range of surface temperatures. We show that the Leidenfrost levitated cart can not only be propelled by gravitational force on a slanted flat surface, but also can be self-propelled on a ratchet shaped horizontal surface. In the end, we experimentally measured water consumption rate for the Leidenfrost levitated cart, and compared the results to theoretical calculations. If perfected, this frictionless Leidenfrost cart could be used in numerous applications.

Electrostatic Suppression of the Leidenfrost State Using AC Voltages

2017 ◽  
Author(s):  
Onur Ozkan ◽  
Vaibhav Bahadur
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Evaporation Rate ◽  
Important Determinant ◽  
Electrostatic Force ◽  
Vapor Layer ◽  
Lower And Upper Bounds ◽  
Dc Voltage ◽  
Leidenfrost Effect ◽  
Hot Surface

The Leidenfrost effect is a well-known phenomenon in boiling, wherein a vapor layer forms between a hot surface and the liquid, thereby degrading heat transfer. Electrowetting (EW) can be used to fundamentally eliminate the Leidenfrost state by electrostatically attracting the liquid towards the surface; the resulting enhanced wetting substantially increases heat transfer. This work presents preliminary results of a study to understand the influence of AC voltages on Leidenfrost state suppression; prior studies have only utilized DC voltages. It is seen that the AC frequency is a very important determinant of the effectiveness of Leidenfrost state suppression. The electrostatic force which attracts the liquid to the surface decreases with increasing AC frequency; this reduces the extent of suppression. This effect is measured and studied by high speed visualization of suppression as well as measurements of the evaporation/boiling rate under AC EW conditions. It is observed that the instabilities (resulting in suppression) at the vapor-liquid interface reduce at higher frequency. The evaporation rate also reduces with AC frequency, as less heat is picked up by the droplet. It is noted that the evaporation rate has lower and upper bounds, which correspond to the evaporation rates without any EW and with DC voltage, respectively. Overall, this work highlights the importance of the AC frequency as a tool to control the extent of suppression and the boiling heat transfer rate.

The Leidenfrost Phenomenon on Silicon Nanowires

2016 ◽  
Author(s):  
Manuel Auliano ◽  
Maria Fernandino ◽  
Peng Zhang ◽  
Carlos Alberto Dorao
Keyword(s):  
Heat Transfer ◽  
Silicon Nanowires ◽  
Droplet Evaporation ◽  
Vapor Layer ◽  
Si Nanowires ◽  
Evaporation Time ◽  
Nanostructured Surfaces ◽  
Leidenfrost Temperature ◽  
Efficient Heat Transfer ◽  
Hot Surface

In this paper, the effect of Si nanowires on the Leidenfrost point on impacting water droplet is presented. In the Leidenfrost regime, the low thermal conductivity of the vapor layer hinders the heat transfer from the hot surface. Nanostructured surfaces can dramatically increase the Leidenfrost temperature improving heat transfer at high temperature. To determine the point of the minimum efficient heat transfer, the droplet lifetime method was employed for both the polished and processed surfaces. The cooling performance was discussed in terms of the droplet evaporation time. The surface with the tallest NWs structure yielded the highest shift in the Leideinfrost point, about 156 % higher than a plain Si surface.

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