Jet Subcooling Research Articles

Jet quenching phenomena during emergency cooling of high temperature solid surface

In the case of loss of coolant accident, LOCA in nuclear reactor, heat removal can be successful only when water rewets the surface. Jet impingement is a highly potential technique of heat removal from high temperature solid surface. After the impingement of liquid jet on hot solid surface it is not capable to wet the surface immediately, it takes time (wetting delay) to get a favorable condition by the liquid to wet and then propagate over the surface. Generation of vapor and its explosion creates the barrier for the liquid front to move. An experimental investigation has been conducted during sub-cooled water jet impingement quenching of three different cylindrical blocks of copper, brass and steel. The jet velocity varied from 3 to15 m/s, jet sub-cooling was 5-80 K and initial block temperature was 250 to 600 ºC. The study was conducted to determine the dominating parameters of maximum heat flux, wetting delay, quench front movement, boiling region size and finally developed a correlation of maximum heat flux for steel. After elapsing the wetting delay period, a visible smaller boiling region was observed to move radially from the impinged centre. The leading edge of this boiling region is designated as the quench front. The boiling region is a vital region as the maximum heat flux occurred at this region during cooling. The boiling region increases with the movement of the quench front and therefore the position of maximum heat flux moves accordingly. The boiling region width increases with material conductivity and decreases with liquid sub-cooling and liquid velocity. The maximum heat flux increases with jet velocity, liquid sub-cooling, solid material conductivity and it is almost independent of solid initial temperature. Wetting delay increases with block initial temperature and decreases with jet velocity and liquid sub-cooling.

Rewetting and boiling in jet impingement on high temperature steel surface

Interface topologies and boiling phenomena are observed within the water jet impingement zone during quenching of a high temperature (300 °C–900 °C) steel plate by direct optical observations. Stable film boiling may occur, but surface asperities may easily penetrate the vapor film and interact with the flowing liquid. By cooling down more rapidly than the remaining solid surface, such asperities act as a kind of micro-fin. Rather surprisingly, non-coalescing bubbles on top of the thin vapor film have been observed, probably formed on top of penetrating surface asperities. After establishing contact between water and solid, so after rewetting, an intense bubble activity is normally seen of bubbles that are footed on a dry plate area with a contact line reflecting the light-emitting diode lights and condensing at later times. These are vapor bubbles. A high surface roughness promotes rewetting that may be initiated at several places simultaneously. For an initial plate temperature of 300 °C and a total height roughness of 5 μm, rewetting took place without the occurrence of a vapor film; surfaces at or above 450 °C exhibited vapor film formation, even for a high jet subcooling of 80 °C. Temperature and time of rewetting are strongly affected by the initial surface temperature and jet subcooling and less by the jet velocity. New correlations are provided to predict these dependencies.

Wet front propagation for water jet impingement on flat surface

A casted leaded-bronze block was impinged by a circular water jet. A 30-fps digital camera was used to video record the wet front propagation process on the surface of the block. The video frames were analyzed to illustrate the effect of different key parameters on the wetting front propagation. Regression correlations were developed to describe the effect of the jet Reynolds number (Red), the ratio of the degree of jet subcooling to the initial temperature of the block ΔTsubTi and the nozzle-to-surface spacing (H) on the wetting front propagation. The correlation is applicable for the experimental ranges investigated; 12,155 ≤ Red ≤ 36,460, 0.085 ≤ TsubTi ≤ 0.107 and H ≥ 30 mm. Good agreement was found between the experimental results and the correlation predictions with a maximum deviation of 25%. At high Reynolds number, the wetting front propagation was faster due to the high velocity and flow inertia. As the ratio ΔTsubTi increased, the wetting front propagation was accelerated. The effect of the nozzle-to-surface height on the propagation speed was negligible for H ≥ 30 mm. The effect of the increased liquid head on the terminal velocity was weakened by the drag forces on the jet as well as the jet instabilities associated with the large spacing.

Numerical and analytical study of film boiling in a planar liquid jet

Direct numerical simulation of film boiling in a planar liquid jet is performed by solving the conservation equations of mass, momentum and energy in the liquid, vapor and air phases. The liquid–air and liquid–vapor interfaces are tracked by a sharp-interface level-set method, which is modified to include the effect of phase change at the liquid–vapor interface. An analytical model to predict the vapor film thickness and wall heat flux in the stagnation region is also developed by simplifying the momentum and energy equations in the liquid and vapor phases. The computational results show a stable vapor film formation on the wall. The effects of jet subcooling, jet velocity, and wall temperature on the vapor film thickness and boiling heat transfer are investigated.

On the size of the boiling region in jet impingement quenching

Experimental investigations of jet impingement quenching for three different cylindrical blocks made of copper, brass and steel have been conducted with block initial temperature from 250 to 600 °C. Visible observations during the quench show that the wetted area can be divided into two regions – a central region with no apparent boiling and the outer annular region where the liquid boils vigorously. The width of the boiling region is of interest since there is a coupling between high heat transfer rates and the observed boiling pattern. Boiling width increases with material conductivity and decreases with jet subcooling and velocity. Boiling width is also influenced by the initial temperature of the solid.

Two-phase structure above hot surfaces in jet impingement boiling

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found.

Maximum heat flux in relation to quenching of a high temperature surface with liquid jet impingement

Experimental investigation has been conducted for quenching of hot cylindrical blocks made of copper, brass and steel with initial block temperature 250–400 °C by a subcooled water jet of diameter of 2 mm. The subcooling was from 5 to 80 K and the jet velocity was from 3 to 15 m/s. After impingement, the jet stagnates for a certain period of time in a small region near the centre and then the wetting front starts moving outwards. During this movement, when the surface temperature at the wetting front drops to 120–200 °C, the surface heat flux reaches its maximum value due to forced convection nucleation boiling. The maximum heat flux is a strong function of the position on the hot surface, jet velocity, block material properties and jet subcooling. A new correlation for maximum heat flux is proposed.

Delay of wetting propagation during jet impingement quenching for a high temperature surface

Transient heat transfer has been investigated experimentally with a subcooled water jet during quenching of hot cylindrical blocks made of copper, brass and steel for initial surface temperatures from 250 to 400 °C. The jet velocity was from 3 to 15 m/s and jet subcooling from 5 to 80 K with a jet diameter of 2 mm. When the jet strikes the hot surface, the wetting front becomes stagnant for a certain period of time in a small central region before wetting the entire surface. This wetting delay may be described as the resident time which is a strong function of block material and jet subcooling and also a function of initial block temperature and jet velocity. New correlations for the resident time and the surface temperature at the resident time at the wetting front have been proposed.

Ultra High Critical Heat Flux During Forced Flow Boiling Heat Transfer With an Impinging Jet

Abstract An ultra high critical heat flux (CHF) was attempted using a highly subcooled liquid jet impinging on a small rectangular heated surface of length 5∼10mm and width 4 mm. Experiments were carried out at jet velocities of 5∼60m/s, a jet temperature of 20°C and system pressures of 0.1∼1.3MPa. The degree of subcooling was varied from 80 to 170 K with increasing system pressure. The general correlation for CHF is shown to be applicable for such a small heated surface under a certain range of conditions. The maximum CHF achieved in these experiments was 211.9 MW/m2, recorded at system pressure of 0.7 MPa, jet velocity of 35 m/s and jet subcooling of 151 K, and corresponds to 48% of the theoretical maximum heat flux proposed by Gambill and Lienhard.

Transient Cooling of Hot Metal Plate with Impinging Water Jet.

Transient boiling heat-transfer rate to a two-dimensional impinging water jet was measured throughout the surface of a 20×150 mm rectangle, during cooling from about 400°C to 100°C. Time-dependent surface temperature distribution and the progress of a solid-liquid contact front were obtained for a jet subcooling from 14 K to 50 K. Solid-liquid contact at the stagnation line starts from the moment of jet impingement, where violent boiling occurs and generated vapor splashes the water film, resulting in no water supply to the outer region of the surface. The temperature of the outer surface remains high up to the time when the solid-liquid contact front reaches it, and a large temperature gradient appears near the front. The surface temperature for the initiation of solid-liquid contact is higher at locations nearer the stagnation line and with higher jet subcooling.