Falling Water Film Research Articles

Experimental Study of Effects of Upward Steam Flow Rate on Quench Propagation by Falling Water Film

Emergency core cooling (ECC) mater is carried up to the upper plenum and falls down again into the core during the reflood phase in PWR-LOCA. Therefore the quench front also propagates downward from the top of the core. The effect of upward steam flow rate on the top-down quench propagation was experimentally investigated. It was found that top-down quench velocity was delayed by upward steam flow. This effect is more significant when rod surface temperature is low and the falling water flow rate is small. The effect of the flow rate and the rod temperature on the quench velocity was correlated based on the experimental results under the conditions of atmospheric pressure, saturation temperature for water and steam, rod surface temperature of 350–600°C, down-ward water velocity of 0.01–0.1 m/s and upward steam velocity of 0–20 m/s.

Falling water films on a vertical cylinder with a downward step

This study concerns falling water films on a vertical cylinder of 45 mm diameter with a downward step in the region where the film flow is not developed and has very weak waves. Varying the step height H from 1.5 to 5.0 mm and the film Reynolds number Re from 50 to 2000, film flows were observed and film thicknesses were measured by the needle contact method near the step. The effects of the steps on water films were compared with each other and discussed. The streamline shapes of free water films formed under the step were theoretically obtained when the ring-shaped air layer was observed. The results agree well with the experimental curves obtained from the photographs.

Experimental study of effects of upward steam flow rate on quench propagation by falling water film.

Emergency core cooling (ECC) mater is carried up to the upper plenum and falls down again into the core during the reflood phase in PWR-LOCA. Therefore the quench front also propagates downward from the top of the core. The effect of upward steam flow rate on the top-down quench propagation was experimentally investigated. It was found that top-down quench velocity was delayed by upward steam flow. This effect is more significant when rod surface temperature is low and the falling water flow rate is small. The effect of the flow rate and the rod temperature on the quench velocity was correlated based on the experimental results under the conditions of atmospheric pressure, saturation temperature for water and steam, rod surface temperature of 350–600°C, down-ward water velocity of 0.01–0.1 m/s and upward steam velocity of 0–20 m/s.

Falling water films on a vertical cylinder with a downward step.

Abstract This study concerns falling water films on a vertical cylinder of 45 mm diameter with a downward step in the region where the film flow is not developed and has very weak waves. Varying the step height H from 1.5 to 5.0 mm and the film Reynolds number Re from 50 to 2000, film flows were observed and film thicknesses were measured by the needle contact method near the step. The effects of the steps on water films were compared with each other and discussed. The streamline shapes of free water films formed under the step were theoretically obtained when the ring-shaped air layer was observed. The results agree well with the experimental curves obtained from the photographs.

Falling Water Films over a Ring Attached to a Vertical Cylinder : Wave Characteristics Measured by a Capacitance Method

This experimental study concerns falling water films on a vertical cylinder to which a ring with a square cross section of 1.5 mm×1.5 mm is attached. Using the electric capacitance technique, some characteristics of surface waves of water film were measured near the ring under the same conditions as in the previous experiment in which the film thicknesses were measured by a needle contact method. When the ring was in the non-equilibrium flow region, the predominant frequency of waves fmax increased just downstream of the ring and then gradually decreased. When the ring was in the equilibrium flow with disturbance waves, little difference was found in fmax upstream and downstream of the ring. Wave velocity was not considerably influenced by flowing over the ring. The effect of the probe diameter on accuracy of the capacitance technique was theoretically discussed by assuming two-dimensional sinusoidal waves.

The Performance of Various Types of Regenerators for Liquid Desiccants

A solar-assisted cooling system is very attractive because the available solar energy and the required cooling load are generally in phase. Of all the solar-assisted cooling methods developed to date, the absorption cooling system and the desiccant dehumidification/evaporative cooling system are considered most suitable for solar or other low-grade thermal energy applications. An efficient liquid desiccant cooling/dehumidification system has been proposed and analyzed by the authors. This system promises low operating parasitic power requirements, and it accepts thermal energy at a temperature as low as 60/sup 0/C, which can be efficiently obtained in a flat-plate solar collector. A schematic diagram of this system is shown. Briefly, this system operates by bringing outside air plus recycled air in contact with a liquid desiccant solution (triethylene glycol, in this case) in the absorber unit. A portion of the dried air is then cycled through the evaporator in contact with the falling water film, thus cooling the water. Cooled water from the evaporator is recycled to cool the absorber unit and hold down the desiccant temperature, and and thereby produce cool and dry air.

Falling Water Films over a Ring Attached to a Vertical Cylinder : Flow Configuration and Film Thickness

This experimental study concerns falling water films on a vertical cylinder to which a ring with a squared cross section of 1.5 mm×1.5 mm is attached. Film flows near the ring were observed while varying the longitudinal distance to the ring from the water distributor outlet from x0=100 to 1100 mm and with the film Reynolds number from Re=50 to 800. The flow configuration was remarkably changed with x0 and Re, and they were classified into several groups. Using the needle contact method, longitudinal distributions of film thicknesses were measured near the ring for two typical ring positions. When the ring was at x0=100 mm in the non-equilibrium flow region with very weak waves, the waves were amplified downstream of the ring and the flow attained an equilibrium not far from the ring. When the ring was at x0=500 mm, in the equilibrium flow with disturbance waves, the film flow disturbed by the ring seemed to be restored to the level of the upstream one.

Falling Water Films Flowing Over a Ring Attached to a Vertical Cylinder : Flow Configuration and Film Thickness

Falling Water Films Flowing Over a Ring Attached to a Vertical Cylinder : Flow Configuration and Film Thickness

Experimental study of upper core quench in PWR reflood phase.

The present study investigates experimentally the characteristics of quench phenomena, top quench and bottom quench, in the upper part of the core during reflood phase in a PWR-LOCA. The characteristics of quench phenomena in the upper part of the core have not been predicted well even by a best estimate safety analysis code such as TRAC. The following are revealed from the experiment. (1) Quench phenomena in the upper part of the core are classified into three types; 1 top quench of heated rods, 2 top quench of non-heated rods, and 3 bottom quench of heated rods, with respect to the relation of quench velocity vs. quench temperature. (2) Top quench of heated rods is influenced by the falling water film on the adjacent non-heated rod. (3) Top quench velocity of heated rod rises with an increase of quench temperature; this is a tendency opposite to the prediction by the existing theory. (4) Top quench of non-heated rods has the same tendency as the existing theory. The effects of system pressure and coolant flow rate are also investigated.

A new correlation for quench front velocity

The purpose of this paper is to present a new correlation for the prediction of the quench front velocity in the rewetting of hot dry surfaces by falling water films. This correlation is valid in the full range of the operating parameters where it proves very successful, providing a root mean square error of about 2.5%, with respect to accurate numerical solutions of the mathematical problem involved.

GAS ABSORPTION INTO WAVY AND TURBULENT FALLING LIQUID FILMS IN A WETTED-WALL COLUMN

Experiments are conducted for gas absorption in a long wetted-wall column. Liquid-side mass transfer coefficients are measured for absorption of CO2 and O2 into falling water films on the outside of a stainless steel pipe 2.72 cm OD and 183 cm absorption length. The liquid film Reynolds number ranges from 129 to 10500 which encompasses the wavy-laminar, wavy-transition and turbulent flow regimes. The experimental data are correlated by a dimensionless equation of the form kt = (ktD) (v2/g) 1/3 = a-Rep-Sc1/2. The correlation is well supported by a viscosity-damped turbulence model at the gas-liquid interface which tends to confirm that viscosity is probably the major mechanism causing eddy damping and not surface tension as proposed by Levich and Davies. The form of the above correlation also represents previous experimental work at different temperatures and for different gases quite well.

An experimental investigation of dryout heat flux on a horizontal nonisothermal tube during heat transfer

This paper presents an experimental investigation of film breakdown heat flux for water film flowing over a heat dissipator tube inside which hot fluid flows. Data for clean tubes and with continuously falling water for long period are reported. Influence of flow rate on dryout heat flux for both type of tubes and on heat transfer prior to dryout for tube with longer usuage is presented. The effect of temperature difference between hot fluid and falling water film on the film breakdown heat flux and heat transfer coefficients prior to film breakdown is also reported.

The measurement and prediction of temperatures at the free interface of falling water films under heat transfer

AbstractThis paper describes an experimental measurement of temperatures at the free liquid interface of distilled water films falling vertically downwards on the outside surface of a copper tube heated internally by a countercurrent circulating liquid. An infrared radiometer was used for the temperature measurements. A prediction for these temperatures is made by solving numerically the governing equations of momentum, continuity and energy. Experimentally estimated temperature profiles were employed as the wall temperature boundary conditions. Internal turbulence eddies and surface wave celerity were considered in the numerical solution by using the eddy diffusivity profile of Mills and Chung. Good agreement between experiment and theory was obtained

Heat Transfer to Falling Water Film on a Vertical Surface

An experimental study was performed on the heat transfer to a turbulent water film which is falling down the outside surface of a heated vertical tube. The test section was made of stainless steel tube 13 mm in outside diameter and 1,500 mm long. It was found that a nearly constant heat transfer coefficient was obtained for the lower half of the test section covering a length of about 1 m, which was direct-heated by electric current. The resulting values of the Nusselt number in this region were correlated to the Reynolds number, and the plots fell in a region intermediate between those given by Wilke and by McAdams. An analytical model taking account of the undulation on the film surface is proposed. The predictions from the model give fairly good agreement with the experimental data.

Frequency of Undulations on a Falling Water Film

An experimental study has been undertaken on the frequency of presented by the undulations occurring on the surface of a falling liquid film. The configuration of the film surface was observed by measuring the changes in the film thickness by means of a capacitometer. The flow regime was in the range of turbulent flow region, in which the undulations occur prominently and travel down along with the fluid film. The undulations proceed downward in an almost fixed pattern. The frequency of the passage of crests observed from a fixed point was evaluated from power spectral analysis to be about 20–30Hz, and this value changed very little with the Reynolds number and with heat flux applied to the falling liquid film. It is shown that this constancy of observed frequency is related to the fact that the large undulations move at the same velocity as the bulk of the fluid film, and that the speed of propagation is corresponding to a group velocity for ripples on the film surface.

Frequency of Undulations on a Falling Water Film

An experimental study has been undertaken on the frequency of presented by the undulations occurring on the surface of a falling liquid film. The configuration of the film surface was observed by measuring the changes in the film thickness by means of a capacitometer. The flow regime was in the range of turbulent flow region, in which the undulations occur prominently and travel down along with the fluid film. The undulations proceed downward in an almost fixed pattern. The frequency of the passage of crests observed from a fixed point was evaluated from power spectral analysis to be about 20–30Hz, and this value changed very little with the Reynolds number and with heat flux applied to the falling liquid film. It is shown that this constancy of observed frequency is related to the fact that the large undulations move at the same velocity as the bulk of the fluid film, and that the speed of propagation is corresponding to a group velocity for ripples on the film surface.

Heat Transfer to Falling Water Film on a Vertical Surface

An experimental study was performed on the heat transfer to a turbulent water film which is falling down the outside surface of a heated vertical tube. The test section was made of stainless steel tube 13 mm in outside diameter and 1,500 mm long. It was found that a nearly constant heat transfer coefficient was obtained for the lower half of the test section covering a length of about 1 m, which was direct-heated by electric current. The resulting values of the Nusselt number in this region were correlated to the Reynolds number, and the plots fell in a region intermediate between those given by Wilke and by McAdams. An analytical model taking account of the undulation on the film surface is proposed. The predictions from the model give fairly good agreement with the experimental data.

Effect of interfacial shear on gas absorption into a turbulent falling film with co-current gas flow

Experimental measurements of the liquid side mass transfer coefficient are reported for CO 2 absorption into a turbulent falling water film at 25°C with co-current gas flow. The liquid film Reynolds number was in the range 1830 to 8030. As the gas velocity was increased from 0 to 14 m/s there was a more than four fold increase in the mass transfer coefficient. The data is well correlated by ( K L K o L ) = 1+2.3( τ i τ o w ) . Data for the skin friction coefficient are also reported, showing a large increase above the dry wall value for high liquid and gas phase Reynolds numbers.

Heat and Mass Transfer in Countercurrent Flow of Air and Water Film in a Rectangular Vertical Duct

Experiments were carried out concerning heat and mass transfer between falling water film and upward air flow in a rectangular vertical duct with a large aspect ratio under the various conditions of rate of flow, water temperature and humidity. The behaviour of the falling water films, e.g., the film thickness and the wave inception, was also examined. The results represented in terms of teh Nusselt number and Sherwood number can be explained using the Reynolds Number based on the mean air velocity relative to the water surface regardless of the water flow rate. When the water temperature at the entrance is varied from 20 to 50°C, the Nusselt number increases, while the Sherwood number decreases in spite of an increases in the rate of evapotation. When saturated humid air is supplied, the Nusselt number is larger and the Sherwood number is slightly smaller compared with air having a relative humidity of 65 per cent.

The physics of rewetting in water reactor emergency core cooling

Surface rewetting is essential for the re-establishment of normal and safe temperature levels following dryout in rod clusters or boiler tubes, or following postulated loss-of-coolant accidents in water reactors. Rewetting experiments have been performed with tubes and rods with a wide range of materials and experimental conditions (surface temperatures 300–800°C, constant water flows 0.1–30 g s −1). The physical processes involved in the rewetting of high temperature surfaces have been shown to be identical for both falling water films and bottom flooding. The variation of rewetting velocity with mass flow has been determined, and shown to be independent of hydraulic diameter over the range 0.2–6 mm of practical interest. Data have also been obtained on the mass ‘carryover’ fraction. Theoretical solutions for the rewetting velocities have been obtained by analysis of thermal conduction in the surface. At low mass flows, effectively one-dimensional (axial) conduction cools the surface ahead of the rewetting front, and gives agreement with experiment. At higher mass flows the rewetting velocity is substantially independent of surface thickness and conductivity. The present data and the available world data for rewetting are shown to be in agreement with the theory.