• Nuclear Engineering and Technology
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• v.52 no.3
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• pp.508-519
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• 2020

During transients or accidents, the reactor core is uncovered, and droplets entrained above the quench front collides with the uncovered fuel rod surface. Droplet impact cooling can reduce the peak cladding temperature. Besides zirconium-based cladding, versatile accidental tolerant fuel (ATF) claddings, including FeCrAl, have been proposed to increase the accident coping time. In order to investigate the effect of surface properties on droplet impact cooling of cladding surfaces, the droplet impact phenomena are photographed on the FeCrAl and zircaloy-4 (Zr-4) surfaces under different conditions. On the oxidized FeCrAl surface, the Leidenfrost phenomenon is not observed even when the surface temperature is as high as 550 ℃ with We > 30. Comparison of the impact behaviors observed on different materials shows that nucleate and transition boiling is more intensive on surfaces with larger thermal conductivity. The Leidenfrost point temperature (LPT) decreases with the solid thermal effusivity (${\sqrt{k{\rho}C_p}}$). However, the CHF temperature is relatively insensitive to the surface oxidation and Weber number. Droplet spreading diameter is analyzed quantitatively in the film boiling stage. Based on the energy balance a correlation is proposed for droplet maximum spreading factor. A mechanistic model is also developed for the LPT based on homogeneous nucleation theory.

• Journal of the Korean Society of Visualization
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• v.10 no.1
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• pp.21-26
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• 2012

Hydrophobic characteristics of high temperature metal surface were investigated by high-speed visualization of water droplet impact. An aluminum plate was used as the sample plate and the initial diameter of a water droplet was 2 mm. Transient behavior of a single droplet impinging on the surface with and without heating was captured by using a high speed camera running at 4,000 frames per second. The Leidenfrost phenomenon was demonstrated for the case of $300^{\circ}C$ surface temperature, however there was no rebounding of droplet on the cold plate due to hydrophilic nature. The experimental results show that the shape evolution of a droplet impinging on the surface varies with the Weber number, i.e. the ratio of impact inertia to capillary force. The overall water-repellent characteristics of the heated surface was very similar to that of the super hydrophobic surfaces.

• Journal of ILASS-Korea
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• v.25 no.2
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• pp.68-73
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• 2020

In the present study, experiments on the film boiling of liquid droplets on oxidized copper surface was conducted. The shape of pure water droplets was observed, and the evaporation rate of them was measured during the film boiling evaporation process. The droplet of initial volume 16 ~ 30 µl was applied onto the oxidized copper surface heated up to 300 ~ 500℃, then the shape of the droplet was analyzed during the film boiling evaporation. Experimental results showed that there was good correlation between dimensionless volume and dimensionless time. However, a significant difference in evaporation rate for small and large droplets discussed in previous study was not found.

• Journal of Advanced Marine Engineering and Technology
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• v.26 no.1
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• pp.132-137
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• 2002

Recently, impinging spray is used for atomization of diesel engine, but it bring on adhesion of fuel. Therefore, we studied about droplet behavior on high temperature plate changing the size of droplet, surface temperatures, and surface roughness of plate. In this study, We studied to confirm experimentally about mechanism of evaporation and ignition process of single fuel droplet. We observed evaporation time, evaporation appearance and ignition delay time by the photopraphs of 8mm video camera. Experimental results are summarized as follows: 1. The boiling point of fuel affect a evaporation and ignition process. 2. The surface roughness affect a evaporation time. 3. The ignition delay time relate to evaporation characteristic.

• Transactions of the Korean Society of Automotive Engineers
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• v.2 no.5
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• pp.85-92
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• 1994

Characteristics of breakup of a liquid droplet impinging on a hot surface has been investigated experimentally by using decane fuel. Factors influencing droplet breakup are surface temperature, impinging velocity, droplet diameter and incident angle. Droplets impinging on a hot surface begins to breakup at $220{\sim}235^{\circ}C$. This temperature varies with impinging Velocity, droplet diameter and incident angle. For wall temperature of $220{\sim}245^{\circ}C$ and above $270^{\circ}C$, breakup probability increases as impinging velocity increases showing S shape curve. For $245{\sim}265^{\circ}C$, a local minimum heat transfer rate occurs. In this temperature range, breakup probability shows nonmonotonous behavior as functions of impinging velocity. As droplet diameter decreases, impinging velocity required for droplet breakup increases. An optimum impinging angle for droplet breakup exists which are found to be about $75^{\circ}$.

• Nuclear Engineering and Technology
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• v.53 no.8
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• pp.2464-2476
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• 2021

Direct-contact heat transfer of a single saturated droplet upon colliding with a heated wall in the regime of film boiling was experimentally investigated using high-resolution infrared thermometry technique. This technique provides transient local wall heat flux distributions during the entire collision period. In addition, various physical parameters relevant to the mechanistic modelling of these phenomena can be measured. The obtained results show that when single droplets dynamically collide with a heated surface during film boiling above the Leidenfrost point temperature, typically determined by droplet collision dynamics without considering thermal interactions, small spots of high heat flux due to localized wetting during the collision appear as increasing We_n. A systematic comparison revealed that existing theoretical models do not consider these observed physical phenomena and have lacks in accurately predicting the amount of direct-contact heat transfer. The necessity of developing an improved model to account for the effects of local wetting during the direct-contact heat transfer process is emphasized.

• Journal of computational fluids engineering
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• v.21 no.4
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• pp.90-95
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• 2016

Leidenfrost drops with ellipsoidal shaping can control the bouncing height by adjusting the aspect ratio(AR) of the shape at the moment of impact. In this work, we focus on the effect of the AR and the impact Weber number(We) on the non-axisymmetrical spreading dynamics of the drop, which plays an important role in the control of bouncing. To understand the impact dynamics, the numerical simulation is conducted for the ellipsoidal drop impact upon the perfect non-wetting solid surface by using volume of fluid method, which shows the characteristics of the spreading behavior in each principal axis. As the AR increases, the drop has a high degree of the alignment into one principal axis, which leads to the consequent suppression of bouncing height with shape oscillation. As the We increases, the maximum spreading diameters in the principal axes both increase whereas the contact time on the solid surface rarely depends on the impact velocity at the same AR. The comprehensive understanding of the ellipsoidal drop impact upon non-wetting surface will provide the way to control of drop deposition in applications, such as surface cleaning and spray cooling.

• Nuclear Engineering and Technology
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• v.35 no.4
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• pp.274-285
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• 2003

Film boiling heat transfer coefficients for a downward-facing hemispherical surface are measured from the quenching tests in DELTA (Downward-boiling Experimental Loop for Transient Analysis). Two test sections are made of copper to maintain Bi below 0.1. The outer diameters of the hemispheres are 120 mm and 294 mm, respectively. The thickness of both the test sections is 30 mm. The effect of diameter on film boiling heat transfer is quantified utilizing results obtained from the two test sections. The measured heat transfer coefficients for the test section with diameter 120 mm lie within the bounding values from the laminar film boiling analysis, while those for diameter 294 mm are found to be greater than the numerical results on account of the Helmholtz instability. There is little difference observed between the film boiling heat transfer coefficients measured from the two test sections. In addition, the higher thermal conductivity of copper results in the higher minimum heat flux in the tests. For the test section of diameter 120 mm, the Leidenfrost point is lower than that for the test section of diameter 294 mm. Destabilization of film boiling propagates radially inward for the 294 mm test section versus radially outward for the 120 mm Test Section.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.5
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• pp.459-465
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• 2013

In the manufacturing process of steel plates, materials at high temperatures above $800^{\circ}C$ are rapidly cooled by using a circular impinging water jet to determine their strength and toughness. In this study, the basic heat and fluid flow is solved by using the existing numerical model for boiling heat transfer. Actually, steel undergoes a phase change from austenite to ferrite or bainite during the cooling process. The phase change induces changes in its thermal properties. Instead of directly solving the phase change and the material cooling together, we solve the heat transfer only by applying the thermal properties that vary with temperature, which is already known from other studies. The effects of the changes in the thermal properties on the cooling of steel and the necessity of calculating the phase change are discussed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.12
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• pp.1051-1056
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• 2014

After hot rolling, a high-temperature steel plate with a temperature higher than $800^{\circ}C$ is rapidly cooled by multiple circular water jets. In this cooling process, because the temperature of the steel plate is much higher than the boiling point of the cooling water, film-boiling heat transfer occurs and a very thin steam layer forms between the plate surface and the cooling water. The steam layer acts as a thermal resistance that prevents heat transfer between the cooling water and the steel plate. In addition to the film-boiling heat transfer, complex physical phenomena such as the free-surface flow of residual water that accumulated on the material and the material's high-speed motion also occur in the cooling process. In this study, the simultaneous cooling process of the upper and lower sides of a running hot steel strip is investigated using a three-dimensional numerical model and the cooling performances and characteristics of the upper-side cooling and lower-side cooling are compared.