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Sidy Ndao - One of the best experts on this subject based on the ideXlab platform.

  • Effects of Droplet Diameter and Fluid Properties on the Leidenfrost Temperature of Polished and Micro/Nanostructured Surfaces
    Journal of Heat Transfer-transactions of The Asme, 2016
    Co-Authors: Anton Hassebrook, Corey Kruse, Troy Anderson, Dennis R Alexander, George Gogos, Chris Wilson, Craig Zuhlke, Sidy Ndao
    Abstract:

    An experimental investigation of the effects of droplet diameters and fluid properties on the Leidenfrost Temperature of polished and nano/microstructured surfaces has been carried out. Leidenfrost experiments were conducted on a stainless steel 304 polished surface and a stainless steel surface which was processed by a femtosecond laser to form above surface growth (ASG) nano/microstructures. Surface preparation resulted in a root mean square roughness (Rrms) of 4.8 μm and 0.04 μm on the laser processed and polished surfaces, respectively. To determine the Leidenfrost Temperatures, the droplet lifetime method was employed using deionized (DI) water and HFE 7300DL. A precision dropper was used to vary the size of DI water droplets from 1.5 to 4 mm. The Leidenfrost Temperature was shown to display increases as high as 100 °C on the processed surface over the range of droplet sizes, as opposed to a 40 °C increase on the polished surface. Average increases of the Leidenfrost Temperature between polished and processed samples were as high as 200 °C. The experiment was repeated with HFE 7300DL; however, with no noticeable changes of the Leidenfrost Temperatures with droplet size whether on the polished or the processed surface. The difference in the Leidenfrost behavior between DI water and HFE 7300DL and among the various droplet sizes can be attributed to the nature of the force balance and flow hydrodynamics at a Temperature slightly below the Leidenfrost point (LFP).

  • Effects of droplet diameter on the Leidenfrost Temperature of laser processed multiscale structured surfaces
    Fourteenth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2014
    Co-Authors: Anton Hassebrook, Corey Kruse, Troy Anderson, George Gogos, Chris Wilson, Craig Zuhlke, Dennis Alexander, Sidy Ndao
    Abstract:

    In this paper, an experimental investigation of the effects of droplet diameters on the Leidenfrost Temperature and its shifts has been carried out. Tests were conducted on a 304 stainless steel polished surface and a stainless steel surface which was processed by a femtosecond laser to form Above Surface Growth (ASG) nano/microstructures. To determine the Leidenfrost Temperatures, the droplet lifetime method was employed for both the polished and processed surfaces. A precision dropper was used to vary the size of droplets from 1.5 to 4 millimeters. The Leidenfrost Temperature was shown to display shifts as high as 85 °C on the processed surface over the range of droplet sizes, as opposed to a 45 °C shift on the polished surface. The difference between the shifts was attributed to the nature of the force balance between dynamic pressure of droplets and vapor pressure of the insulating vapor layer.

  • extraordinary shifts of the Leidenfrost Temperature from multiscale micro nanostructured surfaces
    Langmuir, 2013
    Co-Authors: Corey Kruse, Troy Anderson, Dennis R Alexander, George Gogos, Chris Wilson, Craig Zuhlke, Sidy Ndao
    Abstract:

    In the present work, the effects of surface chemistry and micro/nanostructuring on the Leidenfrost Temperature are experimentally investigated. The functional surfaces were fabricated on a 304 stainless steel surface via femtosecond laser surface processing (FLSP). The droplet lifetime experimental method was employed to determine the Leidenfrost Temperature for both machine-polished and textured surfaces. A precision dropper was used to control the droplet size to 4.2 μL and surface Temperatures were measured by means of an embedded thermocouple. Extraordinary shifts in the Leidenfrost Temperatures, as high as 175 °C relative to the polished surface, were observed with the laser-processed surfaces. These extraordinary shifts were attributed to nanoporosity, reduction in contact angle, intermittent liquid/solid contacts, and capillary wicking actions resulting from the presence of self-assembled nanoparticles formed on the surfaces. In addition to the shift in the Leidenfrost Temperature, significant enha...

  • Extraordinary shifts of the Leidenfrost Temperature from multiscale micro/nanostructured surfaces.
    Langmuir, 2013
    Co-Authors: Corey Kruse, Troy Anderson, Dennis R Alexander, George Gogos, Chris Wilson, Craig Zuhlke, Sidy Ndao
    Abstract:

    In the present work, the effects of surface chemistry and micro/nanostructuring on the Leidenfrost Temperature are experimentally investigated. The functional surfaces were fabricated on a 304 stainless steel surface via femtosecond laser surface processing (FLSP). The droplet lifetime experimental method was employed to determine the Leidenfrost Temperature for both machine-polished and textured surfaces. A precision dropper was used to control the droplet size to 4.2 μL and surface Temperatures were measured by means of an embedded thermocouple. Extraordinary shifts in the Leidenfrost Temperatures, as high as 175 °C relative to the polished surface, were observed with the laser-processed surfaces. These extraordinary shifts were attributed to nanoporosity, reduction in contact angle, intermittent liquid/solid contacts, and capillary wicking actions resulting from the presence of self-assembled nanoparticles formed on the surfaces. In addition to the shift in the Leidenfrost Temperature, significant enha...

  • controlling the Leidenfrost Temperature through laser assisted surface micro nano texturing
    International Conference on Fuel Cell Science Engineering and Technology FUELCELL Collocated with ASME International Conference on Energy Sustainabili, 2013
    Co-Authors: Corey Kruse, Troy Anderson, Dennis R Alexander, George Gogos, Sidy Ndao
    Abstract:

    In the present work, the effect of surface features and wettability on the Leidenfrost Temperature are experimentally investigated. The surface features were fabricated on a 304 stainless steel surface using a femtosecond laser. This technique allows for a wide variety of surface microstructures (spikes, mounds, holes, and pyramids) to be created, ranging in size, shape, and spacing. Changing the fluence and shots of the laser produce different micro/nano textured surfaces. A smooth surface sample was fabricated as a reference surface with a measured Leidenfrost Temperature as a benchmark. The droplet lifetime experimental method was employed to determine the Leidenfrost Temperature for both the smooth and the textured surfaces. A precision dropper was used to control the droplet size to 4.2 microliters (diameter of 2.0mm) while surface Temperatures were measured by means of an embedded thermocouple. In comparison to the smooth stainless steel surface, a shift in the Leidenfrost Temperature, as high as 55 °C, was observed with the textured surface. The textured surface hasa high emissivity, compared to the smooth surface. As a result, in addition to the shift in the Leidenfrost Temperature, significant enhancement of the film boiling heat transfer coefficients were also observed.Copyright © 2013 by ASME

Detlef Lohse - One of the best experts on this subject based on the ideXlab platform.

  • Boiling regimes of impacting drops on a heated substrate under reduced pressure
    arXiv: Fluid Dynamics, 2018
    Co-Authors: Michiel A. J. Van Limbeek, Paul B. J. Hoefnagels, Minori Shirota, Detlef Lohse
    Abstract:

    We experimentally investigate the boiling behavior of impacting ethanol drops on a heated smooth sapphire substrate at pressures ranging from P=0.13 bar to atmospheric pressure. We employ frustrated total internal reflection imaging to study the wetting dynamics of the contact between the drop and the substrate. The spreading drop can be in full contact (contact boiling), it can partially touch (transition boiling), or the drop can be fully levitated (Leidenfrost boiling). We show that the Temperature of the boundary between contact and transition boiling shows at most a weak dependence on the impact velocity, but a significant decrease with decreasing ambient gas pressure. A striking correspondence is found between the Temperature of this boundary and the static Leidenfrost Temperature for all pressures. We therefore conclude that both phenomena share the same mechanism and are dominated by the dynamics taking place at the contact line. On the other hand, the boundary between transition boiling and Leidenfrost boiling, i.e., the dynamic Leidenfrost Temperature, increases for increasing impact velocity for all ambient gas pressures. Moreover, the dynamic Leidenfrost Temperature coincides for pressures between P=0.13 and 0.54 bar, whereas for atmospheric pressure the dynamic Leidenfrost Temperature is slightly elevated. This indicates that the dynamic Leidenfrost Temperature is at most weakly dependent on the enhanced evaporation by the lower saturation Temperature of the liquid.

  • vapour cooling of poorly conducting hot substrates increases the dynamic Leidenfrost Temperature
    International Journal of Heat and Mass Transfer, 2016
    Co-Authors: Michiel A. J. Van Limbeek, Minori Shirota, Andrea Prosperetti, Pascal Sleutel, Detlef Lohse
    Abstract:

    A drop impacting a smooth solid surface heated above the saturation Temperature can either touch it (contact boiling) or not (film boiling), depending on the surface Temperature. The heat transfer is greatly reduced in the latter case by the insulating vapour layer under the drop. In contrast to previous studies, here we use a relatively poor thermally conducting glass surface. Using a total internal reflection method, we visualise the wetting dynamics of the drop on the surface. We discover a new touch-down process, in which liquid–solid contact occurs a few hundred microseconds after the initial impact. This phenomenon is due to the cooling of the solid surface by the generation of vapour. We propose a model to account for this cooling effect, and validate it experimentally with our observations. The model leads to the determination of a thermal time scale (about 0.3 ms for glass) for the cooling of the solid. We conclude that when the impact time scale of the drop on the substrate (drop diameter/impact velocity) is of the order of the thermal time scale or larger, the cooling effect cannot be neglected and the drop will make contact in this manner. If the impact time scale however is much smaller than the thermal time scale, the surface remains essentially isothermal and the impact dynamics is not affected.

  • Dynamic Leidenfrost Effect: Relevant Time and Length Scales
    Physical Review Letters, 2016
    Co-Authors: Minori Shirota, Michiel A. J. Van Limbeek, Andrea Prosperetti, Detlef Lohse
    Abstract:

    When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid Temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost Temperature, or the dynamic Leidenfrost Temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid Temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost Temperature for droplet impact on an isothermal substrate.

  • the Leidenfrost Temperature increase for impacting droplets on carbon nanofiber surfaces
    Soft Matter, 2014
    Co-Authors: Detlef Lohse, Andrea Prosperetti, H Nair, Hendrik J J Staat, Tuan Tran, Arie Van Houselt
    Abstract:

    Droplets impacting on a superheated surface can either exhibit a contact boiling regime, in which they make direct contact with the surface and boil violently, or a film boiling regime, in which they remain separated from the surface by their own vapor. The transition from the contact to the film boiling regime depends not only on the Temperature of the surface and the kinetic energy of the droplet, but also on the size of the structures fabricated on the surface. Here we experimentally show that surfaces covered with carbon-nanofibers delay the transition to film boiling to much higher Temperatures compared to smooth surfaces. We present physical arguments showing that, because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, thus permitting contact boiling up to much higher Temperatures than on smooth surfaces. We also show that as long as the impact is in the film boiling regime, the spreading factor of impacting droplets is consistent with the We3/10 scaling (with We being the Weber number) as predicted for large We by a scaling analysis.

  • Leidenfrost Temperature increase for impacting droplets on carbon nanofiber surfaces
    arXiv: Fluid Dynamics, 2013
    Co-Authors: H Nair, Andrea Prosperetti, Hendrik J J Staat, Tuan Tran, Arie Van Houselt, Detlef Lohse
    Abstract:

    Droplets impacting on a superheated surface can either exhibit a contact boiling regime, in which they make direct contact with the surface and boil violently, or a film boiling regime, in which they remain separated from the surface by their own vapor. The transition from the contact to the film boiling regime depends not only on the Temperature of the surface and kinetic energy of the droplet, but also on the size of the structures fabricated on the surface. Here we experimentally show that surfaces covered with carbon-nanofibers delay the transition to film boiling to much higher Temperature compared to smooth surfaces. We present physical arguments showing that, because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, thus permitting contact boiling up to much higher Temperatures than on smooth surfaces. We also show that, as long as the impact is in the film boiling regime, the spreading factor of impacting droplets follows the same $\We^{3/10}$ scaling (with $\We$ the Weber number) found for smooth surfaces, which is caused by the vapor flow underneath the droplet.

Corey Kruse - One of the best experts on this subject based on the ideXlab platform.

  • Effects of Droplet Diameter and Fluid Properties on the Leidenfrost Temperature of Polished and Micro/Nanostructured Surfaces
    Journal of Heat Transfer-transactions of The Asme, 2016
    Co-Authors: Anton Hassebrook, Corey Kruse, Troy Anderson, Dennis R Alexander, George Gogos, Chris Wilson, Craig Zuhlke, Sidy Ndao
    Abstract:

    An experimental investigation of the effects of droplet diameters and fluid properties on the Leidenfrost Temperature of polished and nano/microstructured surfaces has been carried out. Leidenfrost experiments were conducted on a stainless steel 304 polished surface and a stainless steel surface which was processed by a femtosecond laser to form above surface growth (ASG) nano/microstructures. Surface preparation resulted in a root mean square roughness (Rrms) of 4.8 μm and 0.04 μm on the laser processed and polished surfaces, respectively. To determine the Leidenfrost Temperatures, the droplet lifetime method was employed using deionized (DI) water and HFE 7300DL. A precision dropper was used to vary the size of DI water droplets from 1.5 to 4 mm. The Leidenfrost Temperature was shown to display increases as high as 100 °C on the processed surface over the range of droplet sizes, as opposed to a 40 °C increase on the polished surface. Average increases of the Leidenfrost Temperature between polished and processed samples were as high as 200 °C. The experiment was repeated with HFE 7300DL; however, with no noticeable changes of the Leidenfrost Temperatures with droplet size whether on the polished or the processed surface. The difference in the Leidenfrost behavior between DI water and HFE 7300DL and among the various droplet sizes can be attributed to the nature of the force balance and flow hydrodynamics at a Temperature slightly below the Leidenfrost point (LFP).

  • Effects of droplet diameter on the Leidenfrost Temperature of laser processed multiscale structured surfaces
    Fourteenth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2014
    Co-Authors: Anton Hassebrook, Corey Kruse, Troy Anderson, George Gogos, Chris Wilson, Craig Zuhlke, Dennis Alexander, Sidy Ndao
    Abstract:

    In this paper, an experimental investigation of the effects of droplet diameters on the Leidenfrost Temperature and its shifts has been carried out. Tests were conducted on a 304 stainless steel polished surface and a stainless steel surface which was processed by a femtosecond laser to form Above Surface Growth (ASG) nano/microstructures. To determine the Leidenfrost Temperatures, the droplet lifetime method was employed for both the polished and processed surfaces. A precision dropper was used to vary the size of droplets from 1.5 to 4 millimeters. The Leidenfrost Temperature was shown to display shifts as high as 85 °C on the processed surface over the range of droplet sizes, as opposed to a 45 °C shift on the polished surface. The difference between the shifts was attributed to the nature of the force balance between dynamic pressure of droplets and vapor pressure of the insulating vapor layer.

  • extraordinary shifts of the Leidenfrost Temperature from multiscale micro nanostructured surfaces
    Langmuir, 2013
    Co-Authors: Corey Kruse, Troy Anderson, Dennis R Alexander, George Gogos, Chris Wilson, Craig Zuhlke, Sidy Ndao
    Abstract:

    In the present work, the effects of surface chemistry and micro/nanostructuring on the Leidenfrost Temperature are experimentally investigated. The functional surfaces were fabricated on a 304 stainless steel surface via femtosecond laser surface processing (FLSP). The droplet lifetime experimental method was employed to determine the Leidenfrost Temperature for both machine-polished and textured surfaces. A precision dropper was used to control the droplet size to 4.2 μL and surface Temperatures were measured by means of an embedded thermocouple. Extraordinary shifts in the Leidenfrost Temperatures, as high as 175 °C relative to the polished surface, were observed with the laser-processed surfaces. These extraordinary shifts were attributed to nanoporosity, reduction in contact angle, intermittent liquid/solid contacts, and capillary wicking actions resulting from the presence of self-assembled nanoparticles formed on the surfaces. In addition to the shift in the Leidenfrost Temperature, significant enha...

  • Extraordinary shifts of the Leidenfrost Temperature from multiscale micro/nanostructured surfaces.
    Langmuir, 2013
    Co-Authors: Corey Kruse, Troy Anderson, Dennis R Alexander, George Gogos, Chris Wilson, Craig Zuhlke, Sidy Ndao
    Abstract:

    In the present work, the effects of surface chemistry and micro/nanostructuring on the Leidenfrost Temperature are experimentally investigated. The functional surfaces were fabricated on a 304 stainless steel surface via femtosecond laser surface processing (FLSP). The droplet lifetime experimental method was employed to determine the Leidenfrost Temperature for both machine-polished and textured surfaces. A precision dropper was used to control the droplet size to 4.2 μL and surface Temperatures were measured by means of an embedded thermocouple. Extraordinary shifts in the Leidenfrost Temperatures, as high as 175 °C relative to the polished surface, were observed with the laser-processed surfaces. These extraordinary shifts were attributed to nanoporosity, reduction in contact angle, intermittent liquid/solid contacts, and capillary wicking actions resulting from the presence of self-assembled nanoparticles formed on the surfaces. In addition to the shift in the Leidenfrost Temperature, significant enha...

  • controlling the Leidenfrost Temperature through laser assisted surface micro nano texturing
    International Conference on Fuel Cell Science Engineering and Technology FUELCELL Collocated with ASME International Conference on Energy Sustainabili, 2013
    Co-Authors: Corey Kruse, Troy Anderson, Dennis R Alexander, George Gogos, Sidy Ndao
    Abstract:

    In the present work, the effect of surface features and wettability on the Leidenfrost Temperature are experimentally investigated. The surface features were fabricated on a 304 stainless steel surface using a femtosecond laser. This technique allows for a wide variety of surface microstructures (spikes, mounds, holes, and pyramids) to be created, ranging in size, shape, and spacing. Changing the fluence and shots of the laser produce different micro/nano textured surfaces. A smooth surface sample was fabricated as a reference surface with a measured Leidenfrost Temperature as a benchmark. The droplet lifetime experimental method was employed to determine the Leidenfrost Temperature for both the smooth and the textured surfaces. A precision dropper was used to control the droplet size to 4.2 microliters (diameter of 2.0mm) while surface Temperatures were measured by means of an embedded thermocouple. In comparison to the smooth stainless steel surface, a shift in the Leidenfrost Temperature, as high as 55 °C, was observed with the textured surface. The textured surface hasa high emissivity, compared to the smooth surface. As a result, in addition to the shift in the Leidenfrost Temperature, significant enhancement of the film boiling heat transfer coefficients were also observed.Copyright © 2013 by ASME

Junyoung Kang - One of the best experts on this subject based on the ideXlab platform.

  • Leidenfrost Temperature on porous wick surfaces decoupling the effects of the capillary wicking and thermal properties
    International Journal of Heat and Mass Transfer, 2019
    Co-Authors: Junyoung Kang, Hangjin Jo
    Abstract:

    Abstract The Leidenfrost Temperature TLFP of falling water droplet was studied on sintered porous wick surfaces. Various surface factors were analyzed to identify those that significantly contribute to increasing the TLFP. To decouple the effects of capillary wicking on porous wick surfaces, the results obtained using ethanol as a working fluid were compared to the results obtained using water. When ethanol was used, the capillary wicking did not differ significantly between the porous wick surfaces. The evaporation time of the droplets was measured at high Temperatures (100–600 °C) to evaluate the TLFP. The effect of surface permeability on the absorption of the vapor layer through a porous wick surface had a negligible influence on the TLFP. Within the range of low thermal effusivity of the heating surface, an analysis of the interface Temperature shows that in liquid ethanol, the thermal properties dominate the TLFP as well as the overall boiling regime. Similarly, in water (for which the capillary wicking effect cannot be ignored), the TLFP and film boiling regime were determined by the thermal effusivity. In both liquids, the thermal effusivity was the dominant determinant of the TLFP, regardless of the capillary wicking rate. However, the capillary wicking significantly affected the boiling heat transfer in water, until it reached the transition boiling regime.

  • induced liquid solid contact via micro nano multiscale texture on a surface and its effect on the Leidenfrost Temperature
    Experimental Thermal and Fluid Science, 2017
    Co-Authors: Junyoung Kang, Hyun Sun Park, Kiyofumi Moriyama
    Abstract:

    Abstract A significant increase in the Leidenfrost Temperature (LFT) was observed on a micro/nano multiscale textured surface (MTS) compared with a polished surface (PS) and a micro rough surface (MRS). MTS was fabricated by anodic oxidation and has nano-scaled needles with micro roughness. It showed improved surface wetting characteristics (0° contact angle with liquid spreading). On the other hand, MRS was fabricated by mechanical polishing and it only has micro roughness. LFT on MTS and MRS increased by approximately 150 °C and 30 °C, respectively, compared with one for PS. The textures on each surface influenced the water droplet dynamics. The relationship between LFT and the dynamics of water droplet were studied by high-speed photography. The key phenomenon determining LFT was the rebound process of the droplet during a few milliseconds. On MRS and MTS, the rebound phenomenon of the droplet was disturbed by the surface-texture-induced liquid-solid contact even when the surface was initially at a high Temperature over 300 °C. The precursor wetting front, observed only on MTS and the capillary wicking phenomenon are likely the responsible mechanisms that significantly increased LFT on MTS.

  • Induced liquid-solid contact via micro/nano multiscale texture on a surface and its effect on the Leidenfrost Temperature
    Experimental Thermal and Fluid Science, 2017
    Co-Authors: Junyoung Kang, Hyun Sun Park, Kiyofumi Moriyama
    Abstract:

    Abstract A significant increase in the Leidenfrost Temperature (LFT) was observed on a micro/nano multiscale textured surface (MTS) compared with a polished surface (PS) and a micro rough surface (MRS). MTS was fabricated by anodic oxidation and has nano-scaled needles with micro roughness. It showed improved surface wetting characteristics (0° contact angle with liquid spreading). On the other hand, MRS was fabricated by mechanical polishing and it only has micro roughness. LFT on MTS and MRS increased by approximately 150 °C and 30 °C, respectively, compared with one for PS. The textures on each surface influenced the water droplet dynamics. The relationship between LFT and the dynamics of water droplet were studied by high-speed photography. The key phenomenon determining LFT was the rebound process of the droplet during a few milliseconds. On MRS and MTS, the rebound phenomenon of the droplet was disturbed by the surface-texture-induced liquid-solid contact even when the surface was initially at a high Temperature over 300 °C. The precursor wetting front, observed only on MTS and the capillary wicking phenomenon are likely the responsible mechanisms that significantly increased LFT on MTS.

Miroslav Raudenský - One of the best experts on this subject based on the ideXlab platform.

  • INFLUENCE OF THE WATER Temperature ON THE COOLING INTENSITY OF MIST NOZZLES IN CONTINUOUS CASTING VPLIV Temperature VODE NA INTENZITETO OHLAJANJA Z MEGLI^NIMI (OBAMI PRI KONTINUIRNEM ULIVANJU
    2020
    Co-Authors: Miroslav Raudenský, M. Hnízdil, Jong Yeon Hwang
    Abstract:

    Small mist nozzles used in continuous casting were tested for heat-transfer intensity. These nozzles are used in the secondary cooling area of a steel slab casting machine. The impact pressure distribution was measured first. The laboratory measurements of the cooling intensity (the HTC distribution) were performed with a variable water Temperature. A Temperature range from 20 °C to 80 °C was used in the tests. Surprisingly, the water Temperature was found to have a strong influence. The most noticeable effect is a shift in the Leidenfrost Temperature to low Temperatures. Changing the water Temperature from 20 °C to 80 °C caused a change in the Leidenfrost Temperature of 130 °C. This can be significant and can change the cooling character of the continuous casting machine. It is interesting that with an increase in the cooling intensity, following an increase in the water Temperature in a high-Temperature region (above the Leidenfrost Temperature), there is a small difference of about 30 W/(m 2 K). Surprisingly, high differences in the Leidenfrost Temperature were found for an intensive cooling, where a difference of only 20 °C in the coolant Temperature makes a difference of about 100 °C in the Leidenfrost Temperature. The results of the experiments performed with an elevated water Temperature showed a high sensitivity of the cooling intensity to this parameter. The decreasing effect of the cooling intensity related to the water Temperature is more important for the spray cooling of high intensities.

  • effects of oxide layer on Leidenfrost Temperature during spray cooling of steel at high Temperatures
    International Journal of Heat and Mass Transfer, 2015
    Co-Authors: Monika Chabicovsky, M. Hnízdil, Ampere A Tseng, Miroslav Raudenský
    Abstract:

    Abstract Spray cooling is a common cooling method used in many high-Temperature metal processes. Using a combined numerical and experimental approach, the influence of the oxide layer on the Leidenfrost Temperature during spray cooling of surfaces at high Temperatures was investigated. The heat transfer from a metal surface covered by an oxide layer is described using the concept of the effective heat transfer coefficient and this concept is extended to the Leidenfrost Temperature. The effective Leidenfrost Temperature is introduced. The prediction of the effective Leidenfrost Temperature is compared with the numerical simulation and with the experiment, which was conducted on an austenitic stainless steel plate with varied oxide layer thicknesses. The test plate with the oxide layers was heated to 1000 °C and then cooled using flat jet nozzles. The present study confirms that the use of water in the spray cooling of hot surfaces can create a situation where the oxide layer not only serves as insulation but can also increase the cooling intensity for short time period, mainly by a shift of the Leidenfrost Temperature.

  • INFLUENCE OF WATER Temperature ON HEAT TRANSFER COEFFICIENT IN SPRAY COOLING OF STEEL SURFACES
    Engineering Mechanics, 2012
    Co-Authors: Monika Chabicovsky, Miroslav Raudenský, M. Hnízdil
    Abstract:

    Cooling of stainless steel surfaces with flat fan nozzles was studied experimentally. Several configurations of jets and pressures were tested. Tests were done with variable coolant (water) Temperatures (20 °C, 40 °C, 60 °C and 80 °C). The influence of coolant Temperature on the heat transfer coefficient was investigated. An increase in coolant Temperature caused a significant decrease of the Leidenfrost Temperature (Temperature at which the character of boiling is changed - the film boiling is changed into nucleate boiling). Changing the water emperature from 20 °C to 80 °C caused a change of the Leidenfrost Temperature of about 140 °C. Furthermore it was observed that in a high Temperature region (above Leidenfrost Temperature) the heat transfer coefficient has the highest value for the lowest water Temperature and for the high coolant Temperature (80 °C) the cooling intensity is the lowest.

  • secondary cooling in continuous casting and Leidenfrost Temperature effects
    Ironmaking & Steelmaking, 2005
    Co-Authors: Miroslav Raudenský, J Horsky
    Abstract:

    AbstractThe present study was motivated by problems at continuous casting plants where a variety of thermally induced defects were observed. The paper outlines an experimental method for the measurement of cooling intensity in the secondary cooling area where nozzles are applied. The precision of a variety of experimental methods is discussed. The nozzles have been investigated in terms of pressure setting, the influence of casting speed, and behaviour in the overlapping areas. The tests have provided information on heat transfer coefficient characteristics and heat flux distribution on the cooled steel surface. The paper presents new experimental findings regarding specification of the Leidenfrost Temperature, which is the point between high and low surface Temperature regions where a large difference in cooling intensity is observed. The paper also deals with the problems of homogeneity of cooling along the nozzle spray angle and in the overlapping area where thermal 'stripes' occur. The sensitivity of ...

  • Experimental Study of Leidenfrost Phenomena at Hot Sprayed Surface
    Heat Transfer: Volume 3, 2003
    Co-Authors: Miroslav Raudenský, J Horsky, V. Dumek, P. Kotrbacek
    Abstract:

    An experimental study was prepared to find the relationship between Leidenfrost Temperature and droplet size and velocity of impinging jets. The study is done for the case of steel surface cooling with two-phase nozzles. The sprayed surface moves under the spray at a velocity of 1 m/min. Cooling experiments were done for initial Temperature of 1250°C. Thermal experiments are transient: internal Temperature is measured and surface Temperature and heat transfer coefficient distribution is computed by the inverse task. Droplet size and velocity of the impinging jet was modified by setting water and air pressures at the input of the nozzle. Spray parameters for each pressure combination was measured using a laser-doppler anemometer. The paper shows the results of the thermal and fluid flow experiment and the correlation between Leidenfrost Temperature and flow parameters.The application of obtained results is expected for high Temperature cooling especially in continuous casting.Copyright © 2003 by ASME