Heat Transfer Crisis Research Articles

Numerical investigation on critical heat flux for 5 × 5 fuel assemblies with a wide range of working conditions

To determine the safe boundary of fuel assembly operation and improve the heat transfer performance of fuel assembly, it is necessary to accurately calculate the thermohydraulic characteristics and the critical heat flux (CHF) in rod bundles. The Eulerian two-phase model coupled with extended wall boiling model was used to numerically investigate the 5 × 5 fuel rod bundle flows with four sets of mixing vane spacer grids. A wide range of working conditions were analyzed to validate the numerical method. The results indicate that the current numerical model can not only accurately calculate the CHF values but also predict the location of heat transfer crisis. Based on CFD (Computational Fluid Dynamics) results, the detailed two-phase flow distributions were provided to analyze the CHF performance under special structures, such as bowed rod and spacer backward shift. The quantitative results were obtained on the analyses of spacer position influence on CHF and the penalty effect of bowed rod. Furthermore, the prediction of present numerical model under low inlet subcooling conditions and cold rod CHF were discussed. The research of this paper provides support for the design and optimization of fuel assemblies.

The effect of porous coatings obtained by various methods on heat transfer and crisis phenomena during nitrogen boiling including non-stationary heating and cooling

The paper presents the main results of experimental investigation of the influence of porous coatings obtained by various methods on heat transfer and crisis phenomena during nitrogen boiling, including unsteady heating and cooling regimes. Modification of heater surfaces was carried out by the methods of directed plasma spraying, additive 3D printing and micro-arc oxidation. For all types of porous coatings, a significant intensification of heat transfer at atmospheric pressure relative to smooth heaters was obtained (maximum degree of heat transfer increase up to 6 times). The mechanisms playing a determining role in the process of heat transfer coefficients increase are suggested. The essential influence of structured capillary-porous bronze coatings on the dynamics of transient processes and development of crisis phenomena of non-stationary processes is shown, leading to degeneration of the heat transfer crisis at pulse supply of heat load and significantly reducing the time of complete cooling of the heater in the process of rewetting.

Experimental Investigations of the Onset and Growth of a Heat-Transfer Crisis in a Full-Scale Cross-Sectional Model FA of the RITM-200 Reactor for Small NPP

Experimental Investigations of the Onset and Growth of a Heat-Transfer Crisis in a Full-Scale Cross-Sectional Model FA of the RITM-200 Reactor for Small NPP

Investigation on the DNB-type critical heat flux in a fuel assembly with 5 by 5 rods

In this paper, Eulerian two-phase model coupled with wall boiling model was employed to simulate the departure from nucleate boiling (DNB) in a 5 × 5 fuel assembly. The numerical simulation accurately predicted the critical heat flux (CHF) and also its location. Compared with the calculated results from W-3 formula and FLICA subchannel code, the CFD (Computational Fluid Dynamics) calculations showed good agreement against experimental measurements with the deviation of 5%, but presented more detailed flow distribution. Based on CFD results, the method of CHF prediction was discussed with an eye on wall superheat, void fraction and vapor convective heat flux in rod surfaces. The distribution of flow field and liquid–vapor fraction affected by mixing vane were further studied. Mixing vane can continually influence the downstream thermal hydraulic characteristics, forming the diagonal circulation and square circulation with the flow development. As a result, the bubble aggregation firstly occurred on the surface of corner hot rod instead of central hot rod, which induced the local heat transfer crisis. The CFD calculations presenting the evolution process of physical parameters can provide guidance for rod bundle CHF test and fuel assembly design.

Fundamental and applied investigations of the liquid-metal cooled fast reactor thermal hydraulics (achieved results and further investigation issues)

The results of experimental and computational-theoretical investigations in the area of hydrodynamics and heat transfer of fast reactors and accelerator-driven system with liquid metal coolants are presented, and the problems and tasks for further investigations are formulated. Physical phenomena, effects, laws and process characteristics occurring in reactor power plants, including a flow path and a fast reactor core: the velocity and temperature fields, the structure and characteristics of turbulent transfer of momentum and energy, the hydraulic resistance of channels and fuel rod assemblies, the collector system hydrodynamics, the vibroacoustics, the heat transfer in channels and fuel rod assemblies with liquid metal cooling and inter-channel exchange are considered and analyzed. The data of experimental investigations are presented on a single-tube model of a large-module steam generator and on a fragmentary thermohydraulic model of a steam generator of a reactor power plant with twisted steam-generating tubes operating at subcritical and supercritical water pressure. The results of investigations of temperature and velocity fields on a small-scale water model of a fast reactor vessel with an integral layout in nominal, transient and emergency operating regimes are demonstrated. The investigation results on model of a fast reactor vessel with an integral layout are demonstrated that the effect of thermogravitational forces leads to temperature stratification with stagnant and recirculating formations, internal waves appear at the stratified boundaries, temperature pulsations, thermal fatigue of structural materials and a decrease in the service life of the equipment. It is shown that the liquid metal boiling process in fuel rod assemblies is formed under the influence of various factors, has a complex structure, is characterized by both stable (nucleate, annular-dispersed) and pulsation (slug) regimes with significant fluctuations of technological parameters, which can cause a crisis of heat transfer. Heat transfer was studied, a cartogram two-phase flow was obtained for liquid metal boiling in fuel rod assemblies, the effect of the surface roughness of fuel rods on liquid metal heat transfer and boiling regimes in fuel rod assemblies was found. The possibility of long-term stable cooling of the core during sodium boiling was shown by using “sodium cavity” over the reactor core. The kinetic and mechanical characteristics of the degradation process of the simulated fuel assembly in fast reactor core during the thermal interaction of the uranium-containing simulators of fuel with static sodium and their dependence on the parameters and design system were determined.Information about the key problems of thermophysical investigations of a high-temperature fast reactor with a sodium coolant for the production of hydrogen and reactors cooled by water at supercritical pressure is given.

EFFECT OF CRITICAL HEAT FLUX ON THE EFFICIENCY OF EVAPORATORS

The problems covered by the theory of heat are examined either from a phenomenological or statistical point of view. The aim of the study is to investigate the effect of critical heat flux on the efficiency of evaporators. The study will be examined with reference to the theories of scientists. Therefore, the theory of heat is divided into two parts under the name of thermodynamics and statistical physics. The internal molecular-atomic structure of the object or system investigated in thermodynamics and the mechanism of the occurring micro-processes are not considered at an obvious rate. In solving the researched problems, the principles determined as a result of previous experiments, as well as the known properties of objects or systems, are used. In statistical physics, on the other hand, the molecular atomic structure of the investigated system and the mechanism of microprocessors are based on apparent speed, and probability theory is widely used. As in other fields of science, thermodynamics has historically been formed and developed as a result of the demands of life, and practice, especially hydromechanics (heat engineering). In the practice of designing boiling-type devices for nuclear power plants and several other power plants, there are cases when heat-stressed surfaces are made in the form of horizontal tube panels. During the flow of a vapor-liquid flow under the influence of gravity, there is often no phase separation, which affects the laws of exchange processes and, in particular, the occurrence of a heat transfer crisis. As a rule, the values of the critical heat flow in horizontal pipes are lower than those in vertical pipes. To increase the critical power of the channel, various types of intensifiers are used. The subject is the study of critical heat fluxes and hydraulic resistance in horizontal pipes with a capillary-porous wall coating. The presence of a network structure near the heating surface significantly changes the physical picture of the flow and heat transfer in the near-wall boundary layer. This can lead to a significant change in both the critical steam content and hydraulic resistance. Keywords: thermodynamic, critical heat flux, evaporators, method

The influence of microstructured surfaces and liquid pulsations on increasing heat transfer in a minichannel

The paper compares two methods for improving the intensification of heat transfer during boiling in a rectangular minichannel. Studies were carried out of the influence of additive structures on the crisis of heat transfer in a minichannel with a two-phase flow, as well as studies of the influence of high-frequency pulsations of liquid flow on the intensification of heat transfer processes during flow boiling.

Determination of the hysteresis of the contact angle of wetting of boiling surfaces

For a deeper understanding of the fluid dynamics and heat transfer in the transition layer, when studying the dynamics of a two-phase flow during boiling and a heat transfer crisis, i.e. in the region of the dynamic gas-liquid-solid contact line and the adjacent meniscus, characterized by an apparent contact angle, it is necessary to analyse the wettability, contact angle hysteresis, and surface morphology. In this paper, samples of a graphene-coated copper substrate, a surface with nanotubes, and an uncoated copper surface for comparison are considered.

Обоснование теплотехнической надежности ВВЭР-1200 с установленными в ТВС перемешивающими решетками

One of the most important issues when it comes to the safety of a nuclear power plant is the need to ensure it not only during operation, but also at the design stage. To accomplish this task, it is important to correctly organize and accurately establish the conditions of heat transfer. This is necessary for the performance of a nuclear power plant. The work was focused on substantiating the thermal reliability of the VVER-1200 core with improved fuel assemblies. In this assembly, the design solution was the inclusion of mixing grates (MR) of the «vortex» and «running» types. As a result, it was found that this design feature makes it possible to increase the amount of electricity generation by increasing the critical power of fuel assemblies for the VVER reactor. The subject of the study were: the phenomenon of the crisis of heat transfer of the first kind, as well as thermalhydraulic processes in the core. In accordance with the intended purpose of the study, a solution technique was determined, for which the whole work was divided into two stages. During the first stage, a thermalhydraulic calculation was carried out, the temperature and heat flux distributions were obtained in the cross section and along the height of the fuel element, in which it is maximally loaded, taking into account the surface boiling zone. As a result of the second stage, it was found: the safety factor before the heat transfer crisis. Thus, this technique made it possible to obtain a result on the assessment of the heat engineering reliability of the reactor plant, based on the criteria found. As a final result, the following factors were determined: the reserve before the heat transfer crisis, pressure losses in the reactor plant in the near-wall boiling zone, flow parameters in a two-phase state, and uneven heat flows. Based on the results, it was proved and shown that the improvement of the FA design by adding four «vortex» and «running» gratings made it possible to increase the thermal reliability of the reactor core. This, in turn, makes it possible to operate the power unit at an increased power level, which will lead to an increase in the energy efficiency of the NPP. The significance of this work is: determination of indicators of thermal reliability of the AZ, responsible for the safety of the reactor plant.

Дослідження спектральних образів нейтронного шуму реакторів типу ВВЕР у нерегламентних теплогідравлічних режимах активних зон

The information on the peculiarities of the manifestations of potentially dangerous thermohydraulic anomalies, that occur unpredictably during the operation of the WWER reactor core, is generalized. Considerable attention is paid to the threat of uncontrolled occurrence of latent pre-emergency and emergency operating states in fuel assemblies during the generation of the vapor phase on the surface of fuel elements, including the heat transfer crisis with subsequent boiling of the coolant. It is noted that under these conditions, heat transfer, which should occur exclusively in the convective mode, is also characterized by the uncontrolled occurrence of the boiling process, from the activation of the first centers of vaporization to the formation of a developed bubble structure of the coolant. The limited possibilities of modern technical means of monitoring the technical condition of WWER, which do not provide detection of these anomalies and preemergency heat transfer regimes, are considered. Given the data on neutron flux fluctuations, it is shown that the noise components at the output of standard electron emission neutron flux meters carry important diagnostic information on thermohydraulic processes in the volume of the core, which is currently lost. With this in mind, it is concluded that it is necessary to immediately develop a methodology for the operational identification of random spectral realizations of neutron noise, which is registered at the outputs of neutron detectors of existing in-reactor control systems. The nature of the necessary reliable data on the spectral structure of the main types of diagnostic images (signatures or patterns) of neutron noise, which is registered at the output of standard neutron flux detectors in accordance with the main types of thermohydraulic anomalies, is determined. The analysis of known data and performed with the participation of authors of reactor experiments on the basis of which it is proved that the spectral structure of auto- and coherent signal density at the output of standard neutron flux detectors contains comprehensive diagnostic information on the type and location of vapor-liquid structures of WWER reactor core. With this in mind, the information characteristics of individual spectral ranges of neutron noise are analyzed and conclusions are made about their direct correspondence to the characteristic thermohydraulic processes as objects of diagnostics.

Heat transfer crisis investigation in a microchannel with and without nanoparticles coating

Heat transfer crisis was investigated during water boiling in a microchannel without coating and with coating of aluminum oxide nanoparticles. Experiments were performed on an advanced setup with a horizontally located microchannel with dimensions of 12.5x3x0.2 mm. Calculation of the critical heat flux in the microchannel without coating and with a coating from nanoparticles, carried out by formulas for heat transfer crisis in a horizontal pipe with microporous coating and one-sided heating. For the microchannel coated with nanoparticles the critical heat flux was calculated for the conditions of no influence of liquid subcooling and initial section. The satisfactory agreement between the experimental and calculated data was obtained. dependences of the critical heat flux from the coating thickness and characteristic size of particles were also shown. However, the data array is limited and needs to be expanded what will improve the equations for critical heat flux in a microchannel with nanoparticles coating.

Heat and mass transfer crisis in a metal hydride reactor

We present a simple lumped mathematical model of hydrogen absorption in a metal hydride reactor with a constant flow rate (sub-critical regime) and use it to predict a heat and mass transfer crisis in the reactor. To verify the model we compare calculations with the experiment on hydrogen absorption in the reactor filled with 5 kg of LaNi4.8Mn0.3Fe0.1 alloy with a hydrogen flow rate of 60 stL/min. The analytical model predicts the heat and mass transfer crisis with good precision.

Heat and mass transfer crisis in a metal hydride accumulator

Heat ttransfer is the bottleneck in design of efficient metal hydride devices. We have experimentally demonstrated heat and mass transfer crisis at hydrogen absorption in a metal hydride reactor filled with 4.69 kg of Mm0.8La0.2Ni4.1Fe0.8Al0.1. The crisis results in overheating of the MH bed and considerable slowing of absorption rate. For the case of constant hydrogen flow we have derived a simple solution for energy balance inside the reactor, which predicts the crisis.

Critical heat flux calculation model for tubes with twisted tapes

A new method for calculating the heat transfer crisis in tubes with twisted tapes on the basis of the existing physical model of CHF for nucleate boiling is presented. The change of CHF in case of using twisted tapes is taken into account by introducing of correction factors to the existing model. Verification of the present model has been carried by checking out of existing experimental works with data on CHF with a total of 187 points. The model gives good agreement with the experiment in a wide range of parameters.

Bubble dynamics during flow boiling in a flat 0.6 mm high channel

The vapour bubble dynamics during subcooled flow boiling in a flat microchannel with the height of 0.6 mm was investigated using high-speed visualization. Nucleate flow boiling was maintained by localized heat source. The data on the bubble dynamics was compared with well-known classic growth correlation formulas. The obtained data have shown that the bubble size can reach the channel height in about 0.1 ms, which can lead to heat transfer crisis in channels with local heating.

On the Problem of Creating a Physically Consistent Model of a Liquid Pool Boiling Crisis (Comment on the Article of E.D. Fedorovich “On the Expediency of Developing a Two-Stage Model of Boiling Crisis of a Liquid Wetting a Heating Surface”)

This article lays out the author’s views on the evolution of approaches to describing the heat transfer crisis during boiling in a large volume. The preparation of this commentary is related to the publication of E.D. Fedorovich’s brief report “On the Feasibility of Developing a Two-Stage Model of the Boiling Crisis of a Liquid Wetting the Heating Surface,” which outlines the idea of developing a two-stage model of the emergence and development of a heat transfer crisis during boiling, in which one of the stages is the violation of the stability of the two-phase wall layer and the other is the appearance of and fusion of dry spots on the heating surface.

On the Question of the Model of Boiling Heat Transfer Crisis (Comment on the Article by E.D. Fedorovich “On the Expediency of Developing a Two-Stage Model of Boiling Crisis of a Liquid Wetting a Heating Surface”)

Based on a brief review of the basic principles of S.S. Kutateladze’s hydrodynamic model of a boiling crisis and V.V. Yagov’s crisis models through the development of dry spots, it is concluded that the model of dry spots is more versatile and promising.

Numerical investigation of flow boiling in manifold microchannel-based heat exchangers

Microchannel heat sinks have the potential to achieve the physical limit of phase change heat transfer. With advances in nano-/microscale fabrication techniques, important progress on microchannel heat exchangers has been made in recent years. However, the two-phase flow evolution and its influence on flow boiling at microscale remains ambiguous. Detailed information on transport process in microchannel and underlying mechanism are required for the fully understanding and further optimization of the microchannel heat sink. Here, we constructed a three-dimensional manifold microchannel-based heat exchanger model to study the two-phase flow pattern and heat transfer performance using the Phase-field method. The dynamics of vapor bubbles in microchannels of 80 μm, 150 μm, and 300 μm in depth was carefully investigated with respect to Reynolds number and thermal boundary conditions. Parameters and underlying mechanism that influence the two-phase flow evolution and heat transfer performance in the manifold microchannels are systematically analyzed. Finally, mechanism of heat transfer crisis and critical heat flux (CHF) in microchannel is studied, and strategies to optimize the flow field to delay the CHF are discussed. Our study provides detailed information on the transport process at microscale and offers opportunities for the design and fabrication of next generation microscale heat sinks.

ФУНДАМЕНТАЛЬНЫЕ ИССЛЕДОВАНИЯ В ОБЛАСТИ ТЕПЛОФИЗИКИ ЯЭУ: ДОСТИГНУТЫЕ РЕЗУЛЬТАТЫ И ПРОБЛЕМЫ ДАЛЬНЕЙШИХ ИССЛЕДОВАНИЙ

The results of experimental and theoretical calculations of fundamental research in the field of hydrodynamics, heat transfer, physical chemistry and nuclear power technology cooled by water and liquid metal coolants are analyzed, problems and tasks of further research are formulated. The effects and physical phenomena, laws and characteristics for a wide range of processes occurring in the first and second circuits of reactor facilities, including the flow part, the core, the heating equipment, the means of monitoring and cleaning coolants from impurities as applied to watercooled thermal neutron reactors, fast liquid metal reactors, fusion plants, etc. Particular attention in the field of hydrodynamics and heat transfer is given to studies of the heat transfer crisis in watercooled reactors, hydrodynamics of collector systems, stratification and mixing of jets, vibroacoustics, turbulent transfer in complex channels, heat transfer in the channels and assemblies of fast reactor fuel elements, contact thermal resistance, boiling in large volume and bundles of fuel rods, condensation of liquid metals. In the field of physical chemistry and the technology of liquid metal coolants, the types of impurities and the sources of their entry into the coolants for various nuclear power plants, the characteristics of the mass and heat transfer processes in the circuits and equipment of reactor facilities with liquid metal coolants (sodium, potassium, lithium, lead, bismuth lead), the efficiency of using various purification devices (cold and hot traps) and monitoring the state of impurities in coolants are analyzed.Problem questions and suggestions for further research are formulated. Information is given on the key thermophysical problems of basic research in relation to the development of innovative nuclear energy technologies: water-cooled reactors with supercritical water pressure, high-temperature fast reactors with sodium coolant for hydrogen production, thermonuclear installations, nuclear power plants with liquid-metal space coolants.

Heat transfer crisis and fluid distribution in bundles of rods in the area of dispersed-annular flow mode

Heat transfer crisis and fluid distribution in bundles of rods in the area of dispersed-annular flow mode