Heat Transfer Crisis Research Articles

Study of the heat-transfer crisis on heat-release surfaces of annular channels with swirl and transit flows

The results of the experimental study of the heat-transfer crisis on heat-release surfaces of annular channels with swirl and transit flow are presented. The experiments were carried out using electric heated annular channels with one and (or) two heat-release surfaces. For the organization of transit flow on a convex heat-release surface, four longitudinal ribs were installed uniformly at its perimeter. Swirl flow was realized using a capillary wound tightly (without gaps) on the ribs. The ratio between swirl and transit flows in the annular gap was varied by applying longitudinal ribs of different height. The experiments were carried out using a closed-type circulatory system. The experimental data were obtained in a wide range of regime parameters. Both water heated to the temperature less than the saturation temperature and water-steam mixture were fed at the inlet of the channels. For the measurement of the temperature of the heat-release surfaces, chromel-copel thermocouples were used. It was shown that the presence of swirl flow on a convex heatrelease surface led to a significant decrease in critical heat flows (CHF) compared to a smooth surface. To increase CHF, it was proposed to use the interaction of swirl flows of the heat carrier. The second swirl flow was transit flow, i.e., swirl flow with the step equal to infinity. It was shown that CHF values for a channel with swirl and transit flow in all the studied range of regime parameters was higher than CHF values for both a smooth annular channel and a channel with swirl. The empirical ratios describing the dependence of CHF on convex and concave heat-release surfaces of annular channels with swirl and transit flow on the geometrical characteristics of channels and the regime parameters were obtained. The experiments were carried out at the pressure p = 3.0–16.0 MPa and the mass velocity ρw = 250–3000 kg/(m2s).

Enhancement of Thermocapillary Effect in Heated Liquid Films for Large Waves at High Reynolds Numbers

The characteristics of the heated water film flowing down a vertical plate at Re = 150, 300, and 500 were studied. The fluorescence method was used for measuring the film thickness. The temperature field on the film surface was measured by an infrared scanner. The analysis of the temperature pulsations on the heated film surface was made. The high-frequency component of temperature pulsations faded at the bottom area of the heater. Part of the temperature perturbations (small waves) was removed from interrivulets regions (valleys) to the rivulets by transverse thermocapillary forces. At high heat flux, only largest waves with maximum ripple of temperature reached the lower edge of the heater. There is a decrease in the mean integral energy fluctuations of temperature in the interrivulets regions near the heater lower edge. In the heated regions between rivulets, the relative amplitude of large waves increases with decreasing average thickness (or local Reynolds number). The analysis of results obtained for large Reynolds numbers showed that the relative amplitudes of large waves in the regions between rivulets at high heat fluxes are much greater than those for small Reynolds numbers and in isothermal falling films. In the interrivulet zone, Marangoni number increases with a rise of the heat flux. The growth of relative amplitude of low-frequency waves in interrivulets regions helps prevent film rupture and crisis of heat transfer.

Wall effect on heat transfer crisis

Various evaporation regimes were studied experimentally in a wide range of droplet sizes and wall temperatures at a change in thermal–physical and geometrical characteristics of the wall. With the increasing diameter of the wetting spot of a water sample from 1 to 30mm, the width of the transition region of boiling crisis increases. Dynamics of large droplets boiling is determined not only by wall overheating, but also by the droplet shape. Evaporation of droplets on the surfaces, whose longitudinal dimensions are much larger than the droplet diameter, should take into account free convection of gas and thickness of diffusion boundary layer, which depends on the droplet diameter. Wall surface polishing causes a multiple increase in evaporation time within the transitional boiling region, in contrast to the rough wall. For We≈0 and polished surface of the wall, the time of evaporation and Leidenfrost temperature in the transitional boiling region increase many times, in contrast to the rough wall (for high We number the result is opposite). To assess the effect of wall thickness on the rate of evaporation, it is important to consider the ratio of the droplet diameter to the wall thickness, depth of solid wall cooling, ratio of wall thermal conductivity to conductivity of liquid, and values of dimensionless Bio (Bi) and Fourier (Fo) numbers.

Heat pipe based passive emergency core cooling system for safe shutdown of nuclear power reactor

On March 11th, 2011, a natural disaster created by earthquakes and Tsunami caused a serious potential of nuclear reactor meltdown in Fukushima due to the failure of Emergency Core Cooling System (ECCS) powered by diesel generators. In this paper, heat pipe based ECCS has been proposed for nuclear power plants. The designed loop type heat pipe ECCS is composed of cylindrical evaporator with 62 vertical tubes, each 150 mm diameter and 6 m length, mounted around the circumference of nuclear fuel assembly and 21 m × 10 m × 5 m naturally cooled finned condenser installed outside the primary containment. Heat pipe with overall thermal resistance of 1.44 × 10−5 °C/W will be able to reduce reactor temperature from initial working temperature of 282 °C to below 250 °C within 7 h. The overall ECCS also includes feed water flooding of the core using elevated water tank for initial 10 min which will accelerate cooling of the core, replenish core coolant during loss of coolant accident and avoids heat transfer crisis phenomena during heat pipe start-up process. The proposed heat pipe system will operate in fully passive mode with high runtime reliability and therefore provide safer environment to nuclear power plants.

Heat transfer and wave characteristics of a binary freon film flowing over a three-dimensional textured surface

The results of the study of heat transfer, crisis phenomena, and wave characteristics of laminar-wavy liquid films falling over a vertical three-dimensional texture surface have been considered. The R21/R114 freon mixture is served as a working fluid. Some peculiarities of heat transfer, hydrodynamics, and development of crisis phenomena in a binary liquid film falling over heat-releasing surfaces of different geometry have been revealed. The dependence of the heat-transfer coefficient on the flow rate for a three-dimensional texture surface in the regime of evaporation is shown to be similar to the dependence for a smooth surface with slightly intensified heat transfer in the region of inlet film Reynolds numbers of 100–500. The heat transfer coefficients of nucleate boiling on the studied types of structured surfaces are much lower than for a smooth tube. In the regime of undeveloped nucleate boiling, the critical heat flux for all surface types is deter-mined by the dependence obtained by considering the appearance of the heat transfer crisis in an evaporating wave liquid film.

Boiling crisis in droplets of ethanol water solution on the heating surface

Evaporation of droplets of ethanol water solution on a heated surface is studied experimentally at high heat fluxes. The behavior of water-alcohol mixtures was examined under the conditions of heat transfer crisis. Direct measurement of the current mass of evaporating droplets allows us to study the behavior of liquid batches in significantly nonstationary processes. An insignificant alcohol admixture to water increases significantly the transitional area of the crisis. The maximal length of transitional area corresponds to a mass concentration of alcohol of about 30%. For this concentration the heat transfer coefficient of the water-alcohol solution is maximal. It is shown that addition of highly volatile liquids to water allows efficient control of evaporation rate, which can be used for engineering processes.

Investigation of the third heat transfer crisis on a vertical surface

The process of development of the third heat transfer crisis for vertical orientation of the heating surface was studied experimentally. Experiments were carried out with acetone under the conditions of saturation for the pressures in the working volume from 20 to 28 kPa. In all experiments, the third heat transfer crisis was preceded by propagation of evaporation front along the heating surface. The threshold values of heat flux densities, above which a stable vapor film is formed on the whole heating surface, are lower for vertical orientation of this heating surface than for the horizontal one. Data on the threshold heat flux densities and overheating before boiling-up were obtained. Above these values, formation of evaporation fronts was observed. The range of operation parameters corresponding to formation of the sites of unstable film boiling on the heating surface after boiling-up was determined.

Simplified film model of a heat transfer crisis in tubes

A simplified model is proposed dealing with a heat transfer crisis during forced flow of water in tubes. The formula for the critical heat flow is obtained. The verification showed that this formula with a single empirical coefficient embraces 4745 experimental values of critical heat flows at a mean square error of 0.0691 in a wide range of parametric variations.

Heat transfer crisis on sintered porous surfaces – experimental investigations

There were presented the results of theoretical analysis of boiling heat transfer on heating surfaces covered with thin-layered capillary porous structures. The paper discussed the results of experimental investigations into intralayer boiling crisis and accompanying phenomena. It was observed that the structural parameters of the porous covering affected the course of the process. Hysteresis phenomenon manifested itself when the heat flux initiating intralayer heat crisis was reached. The crisis mechanism hypotheses, the description of which was available in literature, were discussed.

Simplified model of heat-transfer crisis in rod bundles

Simplified model of heat-transfer crisis in rod bundles

Cold-neutron tomography of annular flow and functional spacer performance in a model of a boiling water reactor fuel rod bundle

Dryout of the coolant liquid film at the upper part of the fuel pins of a boiling water reactor (BWR) core constitutes the type of heat transfer crisis relevant for the conditions of high void fractions. It is both a safety concern and a limiting factor in the thermal power and thus for the economy of BWRs. We have investigated adiabatic, air–water annular flows in a scaled-up model of two neighboring subchannels as found in BWR fuel assemblies using cold-neutron tomography. The imaging of the double suchannel has been performed at the ICON beamline at the neutron spallation source SINQ at the Paul Scherrer Institute, Switzerland. Cold-neutron tomography is shown here to be an excellent tool for investigating air–water annular flows and the influence of functional spacers of different geometries on such flows. The high-resolution, high-contrast measurements provide the spatial distributions of the coolant liquid film thickness on the fuel pin surfaces as well as on the surfaces of the spacer vanes. The axial variations of the cross-section averaged liquid hold-up and its fraction in the gas core shows the effect of the spacers on the redistribution of the two phases.

Cold neutron tomography of annular coolant flow in a double subchannel model of a boiling water reactor

Dryout of the liquid coolant film on fuel pins at the top of boiling water reactor (BWR) cores constitutes the type of heat transfer crisis relevant for the conditions of high void fractions. It is a limiting factor in the thermal power, and therefore the economy, of BWRs. Ongoing research on multiphase annular flow, specifically the liquid film thickness, is fundamental not only to nuclear reactor safety and operation but also to that of evaporators, condensers, and pipelines in a general industrial context. We have performed cold neutron tomography of adiabatic air water annular flow in a scaled up model of the subchannel geometry found in BWR fuel assemblies today. All imaging has been performed at the ICON beamline at the neutron spallation source SINQ at the Paul Scherrer Institut in Switzerland. Neutron tomography is shown to excel in investigating the interactions of air water two phase flows with spacer vanes of different geometry. The high resolution, high contrast measurements provide spatial distributions of the coolant on top of the surfaces of the spacer, including the vanes, and in the subchannel downstream of the spacers.

Modeling processes in the melt localization facility for aes-2006 with vver-1200 during an unanticipated accident

The results of a computational analysis of the behavior of melt in the facility used to localize melt for VVER-1200 (AES-2006 design), which were obtained using the GEFEST-ULR code, are presented. The main parameters of the melt are determined: the time variation of the component composition, the temperature and density of the oxide and metallic components and their relative arrangement. Using the KORSAR/GP thermohydraulic code, the minimum margin to crisis of heat transfer at the outer surface of the wall of the melt localization facility is analyzed. It is confirmed that heat transfer from the outer surface of the wall with normal operation of the cooling loop is reliable.

Steady Propagation of the Vaporization Front in Metastable Liquid

The boiling up of a metastable liquid when the vaporization fronts appear is considered theoretically and experimentally. Boiling up occurs usually on the surface of a heater. At the first stage, the growth of a spherical vapor bubble is observed. If the temperature of liquid exceeds the threshold value, the vaporization fronts develop near to the line of contact of a vapor bubble and heater. Fronts of vaporization extend along a heater with constant speed. It is a direct transition from one phase convection to film boiling. Such scenario of crisis of a convective heat transfer is also possible in the nuclear reactor equipment. The model of steady propagation of the vaporization front is developed. The temperature and velocity of propagation of the interface are determined from the balance equations for the mass, momentum, and energy in the neighborhood of the vaporization front and the condition of stability of motion of the interface. It is shown that a solution of these equations exists only if the liquid is heated above a threshold value. The velocity of propagation of the vaporization front also has a threshold value. The calculated velocity of the interface motion and the threshold value of temperature are in reasonable agreement with available experimental data for various liquids within wide ranges of saturation pressures and temperatures of the overheated liquid. The developed model adequately describes the experimental data for various substances in a wide range of temperature of an overheated fluid. In this model, the steady propagation of the vaporization front is possible only if the temperature of a metastable liquid exceeds some threshold value. The velocity of the vaporization front also has a threshold value.

Procedure of calculating liquid flowrates in wall film and heat transfer crisis in tubes

A brief review of investigations into liquid distribution between the flow core and wall film in tubes previously carried out is given. The relationship between mass transfer processes and heat transfer crisis is shown. The procedure of calculating heat transfer crisis in tubes with uniform heat input along the length based on experimental data from the databank of the Physics and Power Engineering Institute (FEI) is presented.

Third heat transfer crisis at subcooling

The effect of liquid subcooling below the saturation temperature on the third heat transfer crisis was studied experimentally at pool boiling. Experimental data on the threshold values of superheating and heat fluxes, above which the evaporation front and third heat transfer crisis for acetone at subcooling from 0.3 to 10 K are formed, were shown. Formation of evaporation fronts is the necessary, but not sufficient condition for the third heat transfer crisis at subcooling. It was found that formation of a stable vapor film after propagation of condensation fronts over the heater surface is possible at heat fluxes considerably lower than the first critical one.

Nonstationary heat transfer crisis in annular dispersed flows

A model of motion and heat transfer of annular dispersed two-phase flows is developed. The results of an investigation of the heat transfer crisis in a vertical steam-generating channel in nonstationary regimes with a decreasing heat transfer rate of the heater are presented

Spatial-temporal characteristics of the error in regulating the energy release distribution in RBMK-1000

The standard errors and time constants for the distribution of energy release and other parameters of RBMK-1000 in the first two power-generating units of the Kursk nuclear power plant are determined. The standard error is 2.4 and 1.8%, respectively, for the fuel-assembly powers and the coefficients of radial nonuniformity of the energy-release distribution. The time constant for the energy-release distribution for the fuel-assembly powers, the coefficient of axial nonuniformity, and the axial skewness is ∼1.5 h, and the time constant for the coefficients of radial nonuniformity and the minimum margin up to a crisis of heat-transfer is 0.7 h.

Rupture of a subcooled liquid film falling down a heated grooved surface

This paper presents an experimental study of rupture of a subcooled water film falling down an 1 m long heated copper plate with longitudinal grooves of 0.5×0.15 mm2 cross sectional area and 2 mm spacing. It was found that the threshold heat flux at which an initial stable dry patch forms on the grooved surface is about two times higher than that on a smooth surface. Furthermore, the grooves prevent dry patches from spreading over the total heated surface thus essentially delaying the onset of the heat transfer crisis. The main governing parameters of the experiment and their respective values are: initial film temperature (20–95°C), heat flux (0–1.26 W/cm2) and volumetric flow rate (11.1–38.2 l/h) (Re=56.2–653.2).

Interconnection between dynamics of liquid boiling-up and heat transfer crisis for nonstationary heat release

This paper deals with investigation results on crisis phenomena for nonstationary heat release under the conditions of free convection and in falling liquid films. It is shown that the character of the crisis development and the critical heat flux for nonstationary heat release significantly depend on the characteristics determining the temperature head of liquid boiling-up. According to experimental data with an arbitrary temporal function of heating, the character of the dependence between the critical heat flux and the heat release increasing rate is significantly effected by ready nucleation sites. It is found that a change in the nonstationary critical heat flux in the range of high times between impulses for periodic heat release is connected with deactivation of ready nucleation sites on the heat-releasing surface. According to new experimental results, in the studied range of irrigation degree alteration (Rein = 30–1660), parameters characterizing decay of the falling liquid film with stepped heat release (the distribution of the time of boiling-up expectation along the liquid film, the velocities of movable boundaries in the boiling-up and drying fronts, the intensity of liquid ejection from the heat-releasing surface) complexly depend on the Reynolds number, wave characteristics, and heat flux density.