Transient Temperature Profile of a Hot Wall Due to an Impinging Liquid Droplet

1978 ◽  
Vol 100 (1) ◽  
pp. 167-169 ◽  
Author(s):  
M. Seki ◽  
H. Kawamura ◽  
K. Sanokawa
Keyword(s):  
Surface Temperature ◽  
Turning Point ◽  
Liquid Drop ◽  
Water Drop ◽  
Liquid Droplet ◽  
Contact Temperature ◽  
Transient Temperature ◽  
Initial Surface ◽  
Heater Surface ◽  
Hot Surface

An experiment was made to investigate the heat transfer to a liquid drop impinging on a hot surface. The transient temperature of the heater surface was measured by a thin-film thermometer. The surface temperature fell to a contact temperature immediately after contact with the drop. The contact temperature increased with increasing initial surface temperature T0. In the case of the water drop, however, it was approximately constant for 200°C ≲ T0 ≲ 300°C; and it increased again for T0 ≳ 300°C. The surface temperature at the turning point, i.e., T0 ∼ 300°C, roughly coincided with the Leidenfrost point.

scholarly journals Thermal Radiation from a Neutron Star in SN 1987A

1988 ◽  
Vol 108 ◽  
pp. 448-449
Author(s):  
Ken’ichi Nomoto ◽  
Sachiko Tsuruta
Keyword(s):  
Neutron Star ◽  
Surface Temperature ◽  
Large Magellanic Cloud ◽  
Initial Surface ◽  
X Rays ◽  
Surface Layers ◽  
Sn 1987A ◽  
Supernova 1987A ◽  
Hot Surface ◽  
Proto Neutron Star

The supernova 1987A in the Large Magellanic Cloud has provided a new opportunity to study the evolution of a young neutron star right after its birth. A proto-neutron star first cools down by emitting neutrinos that diffuse out of the interior within a minutes. After the neutron star becomes transparent to neutrinos, the neutron star core with > 1014 g cm−3 cools predominantly by Urca neutrino emission. However, the surface layers remain hot because it takes at least 100 years before the cooling waves from the central core reach the surface layers (Nomoto and Tsuruta 1981, 1986, 1987).From the hot surface, thermal X-rays are emitted. The detection limit for X- rays from SN 1987A by the Ginga satellite is 3 ×1036 erg s−1 (Makino 1987; Tanaka 1987). If the thermal X-rays are to be observed by Ginga, the surface temperature should continue to be as high as Ts > 8 ×106 (R/10km)−1/2 K until the ejecta becomes transparent. The exact value of the initial surface temperature depends on various factors during the violent stages of explosion, cooling stages of the proto-neutron star through diffusive neutrinos, and possible re-infalling of the ejected material. Therefore, until the surface layers become thermally relaxed Ts may satisfy the above condition.

Transient Cooling of Hot Porous and Nonporous Ceramic Solids by Droplet Evaporation

1994 ◽  
Vol 116 (3) ◽  
pp. 694-701 ◽  
Author(s):  
M. Abu-Zaid ◽  
A. Atreya
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Solid Surface ◽  
Boiling Point ◽  
Droplet Diameter ◽  
Droplet Evaporation ◽  
Porous Solid ◽  
Temperature Measurements ◽  
Transient Temperature ◽  
Initial Surface

This paper presents the results of an experimental investigation into transient cooling of low-thermal-conductivity porous and nonporous ceramic solids by individual water droplets. The initial surface temperature (Ts) of both solids ranged from 75 to 200°C. Both solids were instrumented with several surface and in-depth thermocouples and had the same thermal properties. This enabled investigation into the similarities and differences in the thermal behavior of porous and nonporous solids during droplet evaporation. The measured and theoretical contact temperatures, for both solids, were found to be in good agreement until they became equal to the boiling point of water (which occurs at an initial solid surface temperature of 164°C). Further increase in the initial solid surface temperature did not change the measured contact temperature. Instead, it became roughly constant at a value slightly greater than the boiling point of water. During the droplet evaporation process, surface and in-depth temperatures for the nonporous solid remain nearly constant, whereas for the porous solid there was a continuous decrease in these temperatures. A thermocouple in the porous matrix at the same location as that of the nonporous matrix cools faster under identical conditions, indicating an energy sink in the vicinity of the thermocouple. Also, evaporation time for the nonporous solid was found to be larger than that of the porous solid for the same droplet size and under the same conditions. These observations confirm that there is both in-depth and lateral penetration of water in the porous solid. The transient temperature measurements were used to determine the following quantities: (i) the recovery time (time required by the surface to recover to its initial temperature), and (ii) the size of surface and in-depth zones affected by the droplet. The instantaneous evaporation rate, and the instantaneous average evaporative heat flux for the nonporous solid, were also determined from video measurements of the droplet diameter on the solid surface and the transient temperature measurements. It was found that the average evaporative heat flux is higher for smaller droplets because of their smaller thickness on the hot surface.

scholarly journals Unifying the Controlling Mechanisms for the Critical Heat Flux and Quenching: The Ability of Liquid to Contact the Hot Surface

1992 ◽  
Vol 114 (4) ◽  
pp. 972-982 ◽  
Author(s):  
C. Unal ◽  
V. Daw ◽  
R. A. Nelson
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Transition Region ◽  
Critical Heat Flux ◽  
Homogeneous Nucleation ◽  
Contact Temperature ◽  
Solid Contact ◽  
Dry Patch ◽  
Hot Surface

We investigated the hypothesis that the critical heat flux (CHF) occurs when some point on a heated surface reaches a temperature high enough that liquid can no longer maintain contact at that point, resulting in a gradual but continuous increase in the overall surface temperature for most power-controlled systems. This hypothesis unifies the occurrence of the CHF with the quenching of hot surfaces by relating them to the same concept: the ability of a liquid to contact a hot surface, generally defined as some fraction of the liquid’s homogeneous nucleation temperature, depending on the contact angle. The proposed hypothesis about the occurrence of the CHF is investigated through a study of the boiling mechanism of the second transition region of nucleate pool boiling of water on copper. An idealized two-dimensional transient conduction heat transfer model was developed to investigate the heat transfer mechanism. The initial macrolayer thickness on the dry portion of the heater, in the second transition region, was found to be bounded between 0 and 11 μm. The radius of the dry patch varied from 15 to 23 mm (60 and 92 percent of the heater radius, respectively) for initial macrolayer thicknesses of 0 and 11 μm, respectively. The results indicated that the critical liquid-solid contact temperature at the onset of CHF (the surface temperature at the center of the dry patch) must be lower than the homogeneous nucleation temperature of the liquid for the pool boiling of water on a clean horizontal surface. The liquid-solid contact temperature was dependent on the initial dry patch liquid macrolayer thickness, varying from 180°C to 157°C for initial macrolayer thicknesses of 0 and 11 μm, respectively. Independent assessment of these values shows good agreement with extrapolated contact temperature data at the onset of film boiling. This indicates that the mechanism for the occurrence of the CHF could be similar to the mechanism generally accepted for the quenching of the hot surfaces. Further study of this mechanism to understand better the observed trends in other experimental results show qualitative agreement with those results. These include a significant decrease in the radius of the dry patch to 4 mm (16 percent of the heater radius) when the thermal conductivity of the heater was decreased to that corresponding to nickel. When the thickness of a copper heater was decreased from 10 mm (representing an infinitely thick medium) to 0.1 mm, a dry patch radius of 2.25 mm (9 percent of the heater radius) was found to be sufficient for the temperature at the center of the dry patch to reach the critical contact temperature. These comparisons are felt to provide some understanding as to why the second transition region has been observed only in limited cases.

scholarly journals Spreading and Drying Dynamics of Water Drop on Hot Surface of Superwicking Ti-6Al-4V Alloy Material Fabricated by Femtosecond Laser

Nanomaterials ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 899
Author(s):  
Ranran Fang ◽  
Zekai Li ◽  
Xianhang Zhang ◽  
Xiaohui Zhu ◽  
Hanlin Zhang ◽  
...  
Keyword(s):  
Femtosecond Laser ◽  
Surface Structure ◽  
Energy Savings ◽  
Water Drop ◽  
Capillary Surface ◽  
Water Flow Velocity ◽  
Capillary Action ◽  
Practical Applications ◽  
Alloy Material ◽  
Hot Surface

A superwicking Ti-6Al-4V alloy material with a hierarchical capillary surface structure was fabricated using femtosecond laser. The basic capillary surface structure is an array of micropillars/microholes. For enhancing its capillary action, the surface of the micropillars/microholes is additionally structured by regular fine microgrooves using a technique of laser-induced periodic surface structures (LIPSS), providing an extremely strong capillary action in a temperature range between 23 °C and 80 °C. Due to strong capillary action, a water drop quickly spreads in the wicking surface structure and forms a thin film over a large surface area, resulting in fast evaporation. The maximum water flow velocity after the acceleration stage is found to be 225–250 mm/s. In contrast to other metallic materials with surface capillarity produced by laser processing, the wicking performance of which quickly degrades with time, the wicking functionality of the material created here is long-lasting. Strong and long-lasting wicking properties make the created material suitable for a large variety of practical applications based on liquid-vapor phase change. Potential significant energy savings in air-conditioning and cooling data centers due to application of the material created here can contribute to mitigation of global warming.

Rewetting Analysis of Hot Moving Surface by Round Water Jet Impingement

2018 ◽  
Author(s):  
Avadhesh Kumar Sharma ◽  
Mayank Modak ◽  
Santosh K. Sahu
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Thermal Imaging ◽  
Transient Temperature ◽  
Axial Distance ◽  
Initial Surface ◽  
Moving Plate ◽  
Thermal Imaging Camera ◽  
Plate Velocity ◽  
Imaging Camera

Impinging jets are commonly utilized in the run-out table (ROT) cooling in the hot rolling process in steel manufacturing industries. The phenomenon of rapid cooling of a sufficiently hot surface is termed as the quenching. The present paper reports the rewetting behavior of 0.15 mm thick hot moving stainless steel foil (SS-304) by circular impinging jet from bottom side through experimental investigation. The transient temperature of the hot foil is recorded by using thermal imaging camera (A655sc, FLIR system). Tests are performed for a varied range of Reynolds number (Re = 2500–10000), nozzle to plate distance (z/d = 6), moving plate velocity (0–40 mm/s) and initial surface temperature 500±10 °C. Transient temperature obtained from thermal imaging camera is used to evaluate rewetting time and rewetting velocity. Based on the experimental investigation correlation has been proposed to predict non-dimensional rewetting velocity as a function of various parameters, namely, Reynolds number, non-dimensional axial distance and moving plate velocity.

The Influence of Micro Scale Ratchet Depth on the Motion of Leidenfrost Drop

2015 ◽  
Author(s):  
Jeong Tae Ok ◽  
Sunggook Park
Keyword(s):  
Surface Temperature ◽  
Water Drop ◽  
Lower Surface ◽  
Driving Mechanism ◽  
Entire Surface ◽  
Depth Effect ◽  
Micro Scale ◽  
Temperature Range ◽  
Aspect Ratios ◽  
Surface Temperatures

The influence of ratchet depth on the motion of Leidenfrost water drop was investigated as a continuous effort to reveal the driving mechanism. Continuous directional rebounding behavior of the drop was observed only at below 200°C on both micro ratchets with two different depth-to-period aspect ratios (1:5 and 1:10) and sharp ridges. Overall, the shallow ratchets generated more efficient drop mobility in the entire surface temperature range of 193–299°C due to the increased area between the bottom of the drop and the ratchet surface, caused by the geometrical benefit. However, the depth effect was only critical at relatively lower surface temperatures.

scholarly journals Development of hot surfaces generated by friction contacts of titanium alloy and steel

2020 ◽  
Vol 12 (9) ◽  
pp. 168781402095779
Author(s):  
He Pan ◽  
Yang Zhang
Keyword(s):  
Titanium Alloy ◽  
Friction Coefficient ◽  
Surface Temperature ◽  
Coal Mine ◽  
Ignition Source ◽  
Low Friction ◽  
Friction Process ◽  
Tc4 Titanium Alloy ◽  
Element Simulation ◽  
Hot Surface

When light alloys used in coal mine, the sparks generated by mechanical friction and impacts are the main effective ignition source. While the hot surfaces are concomitant in friction process, prior to the occurrence of mechanical sparks, whether the hot surfaces will be an effective ignition source. Then this paper focuses on the development of hot surfaces generated by TC4 titanium alloy at the low friction velocities. Experiments and finite element simulation methods were used together to describe the temperature field of TC4 titanium alloy. It was found that the temperature of hot surfaces increased with the load and increased much faster at higher relative speed. By means of regression analysis, the variation law of friction coefficient and contact pressure with loads and the variation law of hot surface temperature with friction coefficient and pressure were studied, then the fitting curve of hot surface temperature was obtained. The results of calculations and experiments indicate that hot surfaces generated by light alloy was possible to be an effective ignition source for methane air mixture in coal mine.

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.

scholarly journals Natural Convection Experiments in a Liquid-Saturated Porous Medium Bounded by Vertical Coaxial Cylinders

1983 ◽  
Vol 105 (4) ◽  
pp. 795-802 ◽  
Author(s):  
D. C. Reda
Keyword(s):  
Natural Convection ◽  
Surface Temperature ◽  
Fluid Layer ◽  
Saturated Porous Medium ◽  
Outer Cylinder ◽  
Power Input ◽  
Surface Boundary ◽  
Measured Temperature ◽  
Heater Surface ◽  
Numerical Predictions

An experimental effort is presently underway to investigate natural convection in liquid-saturated porous media utilizing a geometry and hydrodynamic/thermal boundary conditions relevant to the problem of nuclear-waste isolation in geologic repositories. During the first phase of this research program, detailed measurements were made of the steady-state thermal field throughout an annular test region bounded by a vertical, constant-heat-flux, inner cylinder and a concentrically placed, constant-temperature, outer cylinder. An overlying, constant-pressure fluid layer was utilized to supply a permeable upper surface boundary condition. Results showed the heater surface temperature to increase with increasing vertical distance due to the buoyantly driven upflow. The measured temperature difference (ΔT) between the average heater surface temperature and the constant outer-surface temperature was found to be progressively below the straight-line/conduction-only solution for ΔT versus power input, as the latter was systematically increased. Comparisons between measured results and numerical predictions obtained using the finite element code MARIAH showed very good agreement, thereby contributing to the qualification of this code for repository-design applications.

Thermal imaging of a shattering freezing water droplet

2020 ◽  
Author(s):  
Judith Kleinheins ◽  
Alexei Kiselev ◽  
Alice Keinert ◽  
Thomas Leisner
Keyword(s):  
Surface Temperature ◽  
High Speed ◽  
Water Drop ◽  
Liquid Core ◽  
Video Camera ◽  
Mixed Phase ◽  
Freezing Process ◽  
Underlying Mechanisms ◽  
Ice Production ◽  
Water Saturated

<p>The freezing of a supercooled water drop freely falling through a mixed-phase cloud is an ubiquitous natural process fundamental for the formation of precipitation in clouds. The freezing is known to proceed in two stages: first, a network of ice dendrites spreads across the volume of a supercooled droplet resulting in ultrafast release of latent heat and warming of the droplet up to the melting point of ice; during the second stage a solid ice shell grows from the outside into the droplet, leading to a pressure increase inside the liquid core. Once the pressure gets too high, either the shell cracks open or the droplet explodes. The resulting secondary ice fragments start growing in the water-saturated environment or cause the freezing of neighbouring droplets. This secondary ice production mechanism is important for the rapid glaciation of mixed-phase clouds, however, the details of the underlying mechanisms are poorly understood. To quantify this process of ice multiplication, the evolution of the droplet’s surface temperature during the second freezing stage was investigated with a high-resolution infrared thermography system (INFRATEC). Drops of about 300 µm diameter were levitated in an electrodynamic trap under controlled conditions with respect to temperature, humidity and ventilation. The surface temperature of the droplet was measured with the IR system while the freezing process and shattering of the freezing droplet was recorded by a high-speed video camera. Combining experimental results and comprehensive process modelling, we explore the thermodynamic conditions beneficial for secondary ice production upon freezing of freely falling drizzle droplets.</p>

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