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.

scholarly journals Final fate of a Leidenfrost droplet: Explosion or takeoff

2019 ◽  
Vol 5 (5) ◽  
pp. eaav8081 ◽  
Author(s):  
Sijia Lyu ◽  
Varghese Mathai ◽  
Yujie Wang ◽  
Benjamin Sobac ◽  
Pierre Colinet ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Evaporation Rate ◽  
Liquid Droplet ◽  
Scaling Law ◽  
Vapor Layer ◽  
Air Interface ◽  
Leidenfrost Effect

When a liquid droplet is placed on a very hot solid, it levitates on its own vapor layer, a phenomenon called the Leidenfrost effect. Although the mechanisms governing the droplet’s levitation have been explored, not much is known about the fate of the Leidenfrost droplet. Here we report on the final stages of evaporation of Leidenfrost droplets. While initially small droplets tend to take off, unexpectedly, the initially large ones explode with a crack sound. We interpret these in the context of unavoidable droplet contaminants, which accumulate at the droplet-air interface, resulting in reduced evaporation rate, and contact with the substrate. We validate this hypothesis by introducing controlled amounts of microparticles and reveal a universal 1/3-scaling law for the dimensionless explosion radius versus contaminant fraction. Our findings open up new opportunities for controlling the duration and rate of Leidenfrost heat transfer and propulsion by tuning the droplet’s size and contamination.

Study of Electrostatic-Induced Jumping Droplets on Superhydrophobic Surfaces

2017 ◽  
Author(s):  
B. Traipattanakul ◽  
C. Y. Tso ◽  
Christopher Y. H. Chao
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Electrostatic Force ◽  
Droplet Radius ◽  
Inertia Force ◽  
Resistance Force ◽  
Average Charge ◽  
Electrical Voltage ◽  
Fluid Systems ◽  
Set Up

Condensation of water vapor is an important process utilized in energy/thermal/fluid systems. When droplets coalesce on the non-wetting surface, excess surface energy converts to kinetic energy leading to self-propelled jumping of merged droplets. This coalescing-jumping-droplet condensation can better enhance heat transfer compared to classical dropwise condensation and filmwise condensation. However, the resistance force can cause droplets to return to the surface. These returning droplets can either coalesce with neighboring droplets and jump again, or adhere to the surface. As time passes, these adhering droplets can become larger leading to progressive flooding on the surface, limiting heat transfer performance. However, an electric field is known to be one of the effective methods to prevent droplet return and to address the progressive flooding issue. Therefore, in this study, an experiment is set up to investigate the effects of applied electrical voltages between two parallel copper plates on the jumping height with respect to the droplet radius and to determine the average charge of coalescing-jumping-droplets. Moreover, the gravitational force, the drag force, the inertia force and the electrostatic force as a function of the droplet radius are also discussed. The gap width of 7.5 mm and the electrical voltages of 50 V, 100 V and 150 V are experimentally investigated. Droplet motions are captured with a high-speed camera and analyzed in sequential frames. The results of the study show that the applied electrical voltage between the two plates can reduce the resistance force due to the droplet’s inertia and can increase the effects of the electrostatic force. This results in greater jumping heights and the jumping phenomenon of some bigger-sized droplets. With the same droplet radius, the greater the applied electrical voltage, the higher the coalescing droplet can jump. This work can be utilized in several applications such as self-cleaning, thermal diodes, anti-icing and condensation heat transfer enhancement.

Electrical Suppression of Film Boiling During Quenching

2016 ◽  
Author(s):  
Arjang Shahriari ◽  
Mark Hermes ◽  
Vaibhav Bahadur
Keyword(s):  
Heat Transfer ◽  
Electric Fields ◽  
Boiling Heat Transfer ◽  
Cooling Curve ◽  
Vapor Layer ◽  
Steam Generation ◽  
Voltage Dependent ◽  
Order Of Magnitude ◽  
Leidenfrost Effect ◽  
Solid Liquid Interface

Boiling heat transfer impacts the performance of various industrial processes like quenching, desalination and steam generation. At high temperatures, boiling heat transfer is limited by the formation of a vapor layer at the solid-liquid interface (Leidenfrost effect), where the low thermal conductivity of the vapor layer inhibits heat transfer. Interfacial electrowetting (EW) fields can disrupt this vapor layer to promote liquid-surface wetting. This concept works for a variety of quenching media including water and organic solvents. We experimentally analyze EW-induced disruption of the vapor layer, and measure the resulting enhanced cooling during quenching. Imaging is employed to visualize the fluid-surface interactions and understand boiling patterns in the presence of an electrical voltage. It is seen that EW fundamentally changes the boiling pattern, wherein, a stable vapor layer is replaced by intermittent wetting of the surface. This switch in the heat transfer mode substantially reduces the cool down time. An order of magnitude increase in the cooling rate is observed. An analytical model is developed to extract instantaneous voltage dependent heat transfer rates from the cooling curve. The results show that electric fields can alter and tune the traditional cooling curve. Overall, this study presents a new concept to control the mechanical properties and metallurgy, by electrical control of the quench rate.

Modeling the Influence of Marangoni Flows on the Leidenfrost State on Solid and Liquid Substrates

2018 ◽  
Author(s):  
Arjang Shahriari ◽  
Palash V. Acharya ◽  
Vaibhav Bahadur
Keyword(s):  
Heat Transfer ◽  
Layer Thickness ◽  
Nucleate Boiling ◽  
Vapor Layer ◽  
Boiling Regime ◽  
First Order ◽  
Leidenfrost Effect ◽  
Solid Liquid Interface ◽  
Solid Liquid ◽  
Marangoni Flows

Boiling heat transfer affects various processes related to energy, water and manufacturing. In the film boiling regime, heat transfer is substantially lower than in the nucleate boiling regime, due to the formation of a vapor layer at the solid-liquid interface (Leidenfrost effect). In this work, we present analytical modeling of the Leidenfrost state of droplets on solid and liquid substrates. A key aspect of this study is the focus on surface tension gradients on the surface of a liquid (Leidenfrost droplet or liquid substrate), which actuate thermo-capillary driven Marangoni flows. It is noted that this work develops a first-order simplified model, which assumes a uniform vapor layer thickness. The presence of Marangoni flows has non-trivial implications on the resulting thickness of the Leidenfrost vapor layer. Our analysis shows that the pumping effect generated in the vapor layer due to Marangoni flows can significantly reduce the Leidenfrost vapor layer thickness.

Droplet Impingement and Vapor Layer Formation on Hot Hydrophobic Surfaces

2014 ◽  
Vol 136 (9) ◽  
Author(s):  
Ji Yong Park ◽  
Andrew Gardner ◽  
William P. King ◽  
David G. Cahill
Keyword(s):  
Heat Transfer ◽  
Residence Time ◽  
High Speed ◽  
Hydrophobic Surface ◽  
Vapor Layer ◽  
Si Substrate ◽  
Hydrophobic Surfaces ◽  
High Speed Imaging ◽  
Temperature Range ◽  
The Impact

We use pump–probe thermal transport measurements and high speed imaging to study the residence time and heat transfer of small (360 μm diameter) water droplets that bounce from hydrophobic surfaces whose temperature exceeds the boiling point. The structure of the hydrophobic surface is a 10 nm thick fluorocarbon coating on a Si substrate; the Si substrate is also patterned with micron-scale ridges using photolithography to further increase the contact angle. The residence time determined by high-speed imaging is constant at ≈1 ms over the temperature range of our study, 110 < T < 210 °C. Measurements of the thermal conductance of the interface show that the time of intimate contact between liquid water and the hydrophobic surface is reduced by the rapid formation of a vapor layer and reaches a minimum value of ≈0.025 ms at T > 190 °C. We tentatively associate this time-scale with a ∼1 m s − 1 velocity of the liquid/vapor/solid contact line. The amount of heat transferred during the impact, normalized by the droplet volume, ranges from 0.028 J mm − 3 to 0.048 J mm − 3 in the temperature range 110 < T < 210 °C. This amount of heat transfer is ≈1–2% of the latent heat of evaporation.

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.

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.

Evaporation of Liquid Marbles on a Heated Surface: Coating-Assisted Leidenfrost Phenomenon

2011 ◽  
Author(s):  
Cedric Aberle ◽  
Mark Lewis ◽  
Gan Yu ◽  
Nan Lei ◽  
Jie Xu
Keyword(s):  
Heat Transfer ◽  
Liquid Core ◽  
Film Boiling ◽  
Vapor Film ◽  
Water Droplets ◽  
Evaporation Time ◽  
Liquid Marbles ◽  
Leidenfrost Effect ◽  
Surface Liquid ◽  
Hot Surface

The Leidenfrost effect is a well-known heat transfer phenomenon, which predicts that liquid droplets will show prolonged evaporation time when they are placed on a hot surface with a temperature higher than a critical value. This effect is due to film boiling, where a vapor film helps insulate the drop from the hot surface. In this paper, we show that specially engineered droplets — liquid marbles — can exhibit Leifenfrost effect at any temperature above the boiling point without experiencing any transition. Liquid marbles are spheres with a liquid core that are coated with hydrophobic particles. When brought into contact with a solid surface, liquid marbles are completely nonwetting due to the fact that the hydrophobic powder is in between the liquid and solid surface. Liquid marbles may be used as excellent microreservoirs for biosample handling and chemical reagent manipulation. In our study, liquid marbles are synthesized by coating water droplets with graphite particles. We investigate the thermal evaporation of the fabricated graphite liquid marbles on a hot substrate at prescribed temperatures, and compare the results with pure water droplets. The evaporation time of both liquid marbles and water droplets are recorded at various temperatures. If the temperature is above the Leidenfrost point, the evaporation of both liquid marbles and water droplets are prolonged with similar amount of time (about 100s), which indicates that similar physics might at play in both cases: heat transfer is impeded by a thin layer of vapor. If the temperature is below the Leidenfrost point, water droplets evaporate a hundred times faster. This is because the vapor film cannot self-sustain and levitate the droplet anymore. On the other hand, liquid marbles still evaporate slowly with the same level of time as Leidenfrost evaporation times, which indicates that the Leidenfrost effect still takes effect for liquid marbles even below the critical temperature. This might be due to the fact that the coating of the liquid marble helps levitate the liquid core, maintaining a layer of insulating vapor. In the end, we report detailed deformation of liquid marbles during evaporation. This coating-assisted Leidenfrost phenomenon could be useful in many applications where film boiling is desired. The strong thermal robustness of graphite liquid marbles over a wide temperature range, together with the inert reactivity, electrical conductivity and superior lubrication properties of graphite, make graphite liquid marbles potentially useful in a wealth of applications in microfluidics and lab on a chip devices.

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