Critical Heat Flux Phenomenon Research Articles

Data-driven dimensional analysis of critical heat flux in subcooled vertical flow: A two-stage machine learning approach

This study presents a novel two-stage machine learning algorithm that identifies dominant dimensionless numbers for critical heat flux (CHF) prediction, circumventing the limitations of traditional dimensional analysis by integrating the Buckingham Pi theorem with advanced computational methods, including the active subspace method and machine learning. When applied to extensive datasets, including 1438 data points from subcooled departure from nucleate boiling (DNB) conditions and validated against a non-overlapping 24,284-point lookup table dataset, our algorithm effectively discerns and validates the dimensionless numbers critical to the CHF phenomena. Notably, it identifies a significant negative correlation between a newly discovered dominant dimensionless number and the boiling number Bochf = q″w,chf / (G hfg), and introduces a new simplified scaling law for water under earth's gravitational conditions, outperforming existing models with substantial predictive accuracy. This scaling law and the identified dimensionless numbers, derived from an initial dataset of 1438 data points, exhibit exceptional generalization capabilities when validated against a significantly larger dataset of 24,284 points. These findings also emphasize the nonlinear effects of inlet quality on the boiling number Bochf. The contribution of this research lies in its potential to deepen the understanding of CHF based on data-driven fashion, with the potential to enhance thermal management and design optimization across various applications.

A review: Critical Heat Flux (CHF) during pool boiling

A Pool Boiling occurs when a heated surface comes into touch with a liquid pool. The liquid is heated until it vaporizes and rises to the surface in the form of bubbles. A liquid is heated to the point of Pool Boiling, where vapor nucleation causes bubbles to form. Heat transmission is increased as these bubbles become larger and detach from the surface, allowing the cold liquid to come into contact with them and transmit heat. A review was conducted on the significance of nucleation site stability in the Pool Boiling thermal transfer method, which finds widespread applicability in diverse industrial settings. The efficiency of heat transfer and the boiling process are dependent on the stability of nucleation sites. The maximum heat flux that may be transported prior to the onset of film boiling is known as the critical heat flux (CHF), and it is an important consideration in the design of thermal transfer systems. Reducing contaminants and dissolved gasses in the liquid as well as altering surface characteristics including microstructures, coatings, and roughness can all increase the stability of nucleation sites. Although nucleation site stability requires a complex mathematical computation, it may be described by a few general equations and relationships, such as the crucial bubble size and heat flux. The critical heat flux (CHF) phenomena, its significance for Pool Boiling thermal transfer, and its use in a range of industrial and engineering contexts are covered in this page. The significance of fathoming the components driving CHF and the factors impacting it is stressed in the article for of making exact prescient models and compelling heat exchange frameworks. The CHF component, the few hypothetical and observational models that have been put on a mission to make sense of CHF, and the factors that impact it —, for example, liquid boundaries, surface elements, framework strain, and stream rate — are undeniably canvassed in this article. The improvement of novel surface modifications and strategies to help CHF and support the heat exchange effectiveness of Pool Boiling frameworks was the primary focal point of ongoing examination. The article's decision is that CHF is a convoluted peculiarity that keeps on collecting a ton of interest from specialists and presents practical open doors for raising the viability and guaranteeing the protected sending of heat exchange frameworks in different settings.

Sensitivity Analysis of Transient Critical Heat Flux by RIA Under High-Pressure Flow Boiling Conditions in TRTL

A reactivity-initiated accident (RIA) is a design-basis accident under which critical heat flux (CHF) is likely to be exceeded. The operational margin for RIAs is currently determined using steady-state CHF lookup tables, which provide conservative estimates relative to transient CHF phenomena. The Transient Reactor Test Loop (TRTL) facility at Oregon State University is capable of performing out-of-pile rapid heating experiments representative of a RIA at conditions representative of a pressurized water reactor (PWR). To further our understanding of and ability to predict transient CHF under PWR conditions, we performed a sensitivity analysis on a RELAP5-3D model of the TRTL facility coupled to the RAVEN code framework to define a proposed experimental test matrix to be performed at the TRTL facility. We then implemented a flow boiling CHF correlation into RELAP5-3D and performed a secondary sensitivity analysis inspecting the impact of the built-in RELAP5-3D CHF and heat transfer multipliers on both the prediction of CHF and key safety parameters, such as peak cladding temperature and heat flux. The results show that the multiplier with the highest influence toward the prediction of CHF occurrence and the safety parameters is the transient CHF multiplier. Operational performance envelopes have been developed for each of the test matrix cases and will be used for validation once the experiments are performed. The TRTL facility is currently performing shakedown testing to verify system performance prior to proceeding with the experimental campaign. Restart testing results include pump curve restart testing, pressure tests, and heater rod thermocouple transients.

Critical heat flux of water in vertical annular channel with a short heated rod under low pressure and low flow conditions

An experimental study on critical heat flux (CHF) has been conducted in a vertical annulus channel with a short heated rod at low pressure and low flow conditions. The heated rod of the test section was made of Inconel-625 with a diameter of 37.2 mm and a heated length of 390 mm. The pressure, inlet temperature and mass flux were varied in the range of 103–170 kPa, 63 −90℃and 59.6–131.9 kg m2 s−1. The CHF phenomenon was observed through the transparent quartz glass. It was found that the CHF positions firstly appeared in a small zone on the heated rod wall and the dry-out area would further expand if the power was not cut off immediately. The experimental results showed that most of CHF data were firstly detected at the upper thermocouple position near the outlet, while CHFs data were observed at lower positions with low mass flux. And based on Mishima and Ishii’s flow regime map, the analysis of experimental flow regime indicated that most of the CHF occurred in slug-churn and churn-annular flow regimes. Finally, the measured CHF data were compared with the results predicted by four commonly used correlations and the method of CHF look-up table, and a new correlation was developed. It was found that the new correlation can be better predicting experimental data with an error within ± 15%.

Experimental study on subcooled pool boiling heat transfer of sodium dodecyl sulfate surfactant solution

The pool boiling heat transfer coefficient (HTC) and critical heat flux (CHF) of sodium dodecyl sulfate (SDS) surfactant solution under different concentrations and subcooling conditions were experimentally studied. The concentration of SDS was in the range of 0 to 3000 ppm, and the subcooling degree was in the range of 2 to 10 °C. The heat transfer coefficient under different subcooling conditions was theoretically analyzed, and the real-time fluctuation of the wall temperature is discussed in detail. It is found that the subcooling has little effect on the natural convection of pool boiling but significantly impacts the nucleate boiling and that the phenomenon of temperature overshoot (TOS) is stronger in the SDS solution than that in de-ionized water. Furthermore, the origin and development process of TOS were deeply analyzed by showing the development process diagram. Meanwhile, the CHF phenomena were visualized and traced experimentally. The influence of different subcooling degrees and SDS concentrations on the CHF was deeply explored, and the development process diagram of the vapor film under CHF conditions was plotted. The results provide detailed theoretical support for the HTC and CHF of SDS solution under subcooling conditions.

Thermo-hydraulic performance of heated vertical flows of supercritical CO2

The thermo-hydraulic characteristics of heated supercritical CO2 (SCO2) flows are investigated numerically in a vertical pipe from first- and second-law perspectives, and the influence of the flow direction, mass flux and heat flux (both distribution and average value) are evaluated. Two mass flux (254 kg/(m2∙s) and 400 kg/(m2∙s)) and three average heat flux (30 kW/m2, 50 kW/m2 and 70 kW/m2) conditions are simulated at an inlet temperature of 288 K and a pressure of 8.0 MPa (corresponding pseudo-critical temperature of 308 K) in a 4-mm diameter pipe. The simulation results reveal that the heat transfer is enhanced and the irreversibility is reduced in downward flows relative to flows without gravity, whereas the heat transfer deteriorates and the irreversibility is increased in upward flows. Both higher heat fluxes and lower mass fluxes also further hinder heat transfer in the upward flows, and multiple peaks are observed in the axial wall temperature profile. Moreover, it is found that the heat-flux distribution has a significant effect on the heat transfer performance of upward flows; the heat transfer further deteriorates and the irreversibility is further increased when a linearly decreasing heat-flux distribution is applied to the wall, while the heat transfer deterioration is alleviated when a linearly increasing heat-flux distribution is used. An analysis of the heat transfer mechanism indicates that the turbulence production in the core region of the supercritical flow is suppressed, and the accumulation of gas-like fluid in the near-wall region is promoted by the buoyancy effect in upward flows, leading to severe heat transfer deterioration and a sharp increase in the wall temperature, which is similar to the critical heat-flux phenomenon in subcritical boiling. The present study provides insights into the heat transfer characteristics of SCO2 flows, as well as practical guidance on the design and optimisation of relevant components and equipment.

Critical heat flux analysis of divertor cooling flow channel in fusion reactor with CFD method

Situated at the bottom of the vacuum vessel, the divertor extracts heat and ash produced by the fusion reaction, minimizes plasma contamination, and protects the surrounding walls from thermal and neutronic loads. The vertical targets of divertor are designed to be able for up to 20 MW/m2 high heat flux. It is a great ordeal for both the material performance and the cooling ability. Critical heat flux margin is very crucial during the design of divertor. The ANSYS FLUENT is used in this paper to predict the critical heat flux on a monoblock structure with a twisted tape inside the tube. Numerical results are validated with the corresponding sets of experimental results. In this paper, CFD method used to predict critical heat flux of divertor cooling channel was first introduced. On the other hand, influence of inlet subcooling on critical heat flux is studied in detail. The inlet subcooling affect the critical heat flux much complicated for the single-side heated and swirl flow channel. Whether the influencing trend or the locations of critical heat flux occurrence are different under different inlet sub-cooling. The derivations between the simulation and experimental results were no more than 32%. This study proves the CFD tools can provide efficient help on the understanding of the critical heat flux phenomenon of complex construction.

Sensitivity analysis of in-pile critical heat flux experiments in TREAT for characterization of RIA power-transient effects

A reactivity-initiated accident (RIA) is one type of postulated design basis accident (DBA) that can cause a departure from nucleate boiling (DNB) event in pressurized-water reactors (PWRs). A DNB occurrence and its consequences depend on the thermophysical properties of the fuel components and coolant, characteristics of the transient energy insertion into the fuel rod, and the onset of the critical heat flux (CHF) phenomenon. To leverage the restart of the Transient Reactor Test (TREAT) Facility, an effort is currently underway to better understand the cladding-to-coolant heat transfer mechanisms and the CHF phenomenon under fast-transient irradiation conditions. This paper characterizes the impact of power transients on the thermal-hydraulic behavior of a TREAT Facility reactor heater rodlet CHF experiment to provide the priority of parameters that need to be investigated for an improved CHF model. Sobol sensitivity analysis methods and the Reactor Excursion and Leak Analysis Program (RELAP5-3D) code were used to identify key input parameters on the uncertainty in the prediction of peak outer- and inner-surface temperatures of the heater tube, as well as the time of the DNB event. A series of sensitivity analyses revealed the total energy deposition on the tube and the transient effects of power pulse had large impacts on the maximum temperatures. The CHF multiplier had the largest impact on the time occurrence of CHF. The overall results show the energy deposition rate in the tube is the most influencing factor to the manifestation of CHF and the resulting thermal-hydraulic behaviors of the tube. The multiplier for the CHF, which is interpreted as the predicted CHF value, has the largest Sobol indices for the time of the CHF in all cases, since it directly determines the occurrence of CHF. It is inferred that the uncertainties in the thermal-hydraulic behaviors of fuels increase with respect to the key parameters as the power pulse becomes broader, and an accurate estimation of the energy deposition rate is required to reduce the uncertainty in the evaluation of the integrity of fuel if the CHF is expected to occur near the peak power. The outputs are expected to provide rigorous interpretation of ongoing in-pile CHF experiments in the TREAT Facility reactor regarding thermal-hydraulic behavior of the fuel system aiming for a new transient in-pile CHF model.

Review on Development of Critical Heat Flux Mechanistic Model

In order to clarify the development of critical heat flux (CHF) mechanistic model and promote the CHF experimental and theoretical research, the existing achievements and progress of CHF mechanistic model are systematically reviewed and the basic assumptions and detailed modeling process of each model are analyzed in this study. In addition, the existing problems and corresponding possible solutions of each model are demonstrated. This study can provide a basis for current understanding and experimental approaches of CHF phenomenon.

Understanding triggering mechanisms for critical heat flux in pool boiling based on direct numerical simulations

Boiling is a ubiquitous process in many applications including power generation, desalination, and high-heat flux electronic cooling. At the same time, boiling is a complicated physical process involving hydrodynamics and interfacial heat and mass transfer on multiple scales. One of the key limiting factors of boiling is the critical heat flux (CHF), beyond which a vapor blanket forms on the heating surface and catastrophic device burnout occurs. Yet, detailed understanding of the mechanism that triggers CHF remains elusive. In this paper, we elucidate the CHF mechanism by studying the evolution of wet/dry region on the heater surface using lattice Boltzmann simulations. We incorporate the equation of state for real gases in the liquid-vapor phase change model for direct numerical simulations of the CHF phenomenon. The results of this framework clarify the difference between the triggering mechanism of CHF and film boiling by analyzing the pool boiling curve. We demonstrate that the heat flux of the wet region on the heater surface increases while the wet area fraction decreases with superheat, leading to the CHF. We show that a vapor recoil force due to the interfacial heat and mass transfer plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF. Compared with previous CHF models which treat CHF as an isolated point on the boiling curve, this work elucidates the triggering mechanism of CHF from a perspective of the dynamic evolution of the wet/dry region with increasing superheat, which could potentially serve as a guideline for future CHF enhancement designs.

Sensitivity study of momentum and turbulence models for subcooled boiling phenomena simulation

This sensitivity analysis of the momentum and turbulence equations uses the Eulerian two-phase approach and intends to achieve a validated modeling for subcooled boiling flows. The studied regime is relevant to many industry applications and to the critical heat flux phenomenon, an important process for design and safety analyses of water-cooled nuclear power plants. Ascending water flows in heated circular pipes are modeled under subcooled boiling condition and applying a combination of computational multi-fluid dynamics (CMFD) models, each one for the estimation of specific terms of the governing equations. Turbulence closure, drag, lift, turbulent interaction, turbulent dispersion, wall lubrication and interfacial area models are investigated through variations around a reference model set defined based on the literature review, changing each model one at a time to study their impacts on the simulation results. Key parameters, such as void fraction and temperature profiles, are chosen for the evaluation of the results. Despite the difficulty of tracking the contributions of simultaneously applied models, the Legendre–Magnaudet lift force and the Burns et al. turbulent dispersion force models show good agreement with the experimental data of a flow at 4.5 MPa used for the quality verification of the simulations. These models are also applied for flows with pressures ranging from 1.5 to 15 MPa, and they lead to promising results. This study gives interesting insights on the influence of the distinct models composing momentum and turbulence calculations, contributing to the validation effort of the CMFD approach for subcooled boiling flows.

Experimental correlation for critical heat flux in helical coils

The present work experimentally investigated the Critical Heat Flux (CHF) phenomena in helical coils. Literature on the CHF in helical coils is scarce. The centrifugal force and hence secondary flows of fluid is generated inside the coil due to its curvature. The Secondary flows of fluids may affect the heat transfer mechanism and hence, CHF inside helical coils. In present work, the experimental CHF data is compared with correlation available in literature. Fifteen empirical CHF correlations, that include correlation developed for straight horizontal tubes and helical coils, are compared with the experimental CHF databank. The CHF databank contains 413 CHF data of different helical coils. The data includes two fluids namely refrigerant R123 and water. Data covers the parameter range of tube diameter 5.5–10 mm, coil to tube diameter ratio 14–58, mass flux 92–2062 kg/m2s, heat flux 37–2984 kW/m2, density ratio 31–1352 and exit quality −0.1 to 1.2. The data mainly covers CHF in redeposition zone. The helical coil correlations are working reasonably well for long length helical coils but not working for short length helical coils. The straight tubes correlations are working reasonably well for all helical coils data. A generalised correlation is suggested for CHF in helical coils. The experimental data of CHF is published to contribute to the databank for helical coils.

Prediction of “critical heat flux” for supercritical water and CO2 flowing upward in vertical heated tubes

The prediction method for the occurrence of supercritical critical heat flux (CHF) is investigated both theoretically and statistically. The supercritical CHF is defined as the lowest heat flux that causes the wall temperature (Tw) peak to occur. The new definition is helpful to avoid the confusion caused by different definitions of the heat transfer deterioration (HTD) in supercritical flow. A new theory to explain supercritical CHF mechanisms is proposed, which considers the rupture of flow stability between the liquid-like and vapor-like layers of the supercritical flow in a heated tube as the main reason for supercritical CHF phenomena. From this theory, a general mathematical model for supercritical CHF identification for upward vertical flow is derived. To substantiate the general mathematical model, two databases for supercritical CHF of water and CO2 flowing upward in vertical circular tubes are compiled from the available literature, based on which the general model is reduced to a practical correlation applicable to both supercritical water and supercritical CO2 for CHF occurring before the pseudo-critical point. The new correlation differentiates the CHF cases from the non-CHF ones very well, and it has much higher prediction accuracy than any existing counterpart.

Numerical investigation of the effect of bump and indent inside a vertical tube on the subcooled flow boiling and critical heat flux

One of the essential industry problems is the critical heat flux (CHF) phenomenon in the flow boiling regime which leads to the temperature jumping and damaging to the systems. Increasing the vapour volume fraction decreases the heat transfer coefficient, and finally, temperature jump will occur. Also, the existence of the bumps and indent in the flow domain changes the flow pattern. In this study, by considering bumps and indent in the tube, the boiling of fluid flow in the vertical tube is discussed. For modelling and simulating the problems, the Euler-Euler model for studying the interaction of the liquid-vapour phases was used. Some models and material specifications are declared using the user-defined function (UDF) codes to the ANSYS Fluent program. The results show that the existence of bumps and indent inside the tube causes the flow of liquid phase to be less redirected in comparison to vapour phase flow due to having more momentum; therefore, at the end of the bumps in the tube, the amount of vapour volume fraction near the wall rises sharply. By increasing the flow mass flux, the vapour volume fraction at the end of bumps increases which lead to decreasing CHF value. It has also observed that if there are bumps and indents inside the tube, there will be no significant change in the liquid flow and vapour volume fraction in the other parts of the tube, as compared to the regular tube.

Experimental study for critical heat flux in 2x2 rod bundles at high pressure conditions

Critical heat flux (CHF) phenomena has been a topic of study for over fifty years yet remains a serious concern across industries and especially in water-cooled nuclear reactor design. CHF data in the high-pressure range is limited, and the effects of bundle geometry and non-uniform heat flux profile are challenging to quantify. In this study, CHF experiments were performed in upward flowing water in a 2 × 2 rod bundle with a cosine profile heat flux. Data were collected between 16.5 and 18 MPa at two mass flux conditions with three inlet subcooling conditions. Under these conditions, average CHF was shown to decrease with increasing pressure. However, pressure had less of an effect than decreasing the inlet subcooling or the mass flux, both of which reduce the CHF value considerably. Interestingly, correlations that have been developed for lower pressure continued to predict CHF occurrences with moderate accuracy outside their ranges of validity. Nearly all predicted values were within 20% of experimental values.

Semi-Analytical Modeling of Critical Heat Flux in Convective Boiling of Nanofluids

Using two-phase flows because of their high thermal conductivity coefficients is favorable in various industries such as oil, petro chemistry, and power plants that require significant thermal conductivity coefficients in heat exchangers. On the other hand, the limiting factor in benefiting from these flows is the critical heat flux phenomenon, the onset of which must be delayed by various means. One of the methods to achieve this goal is using nanofluids. Nanofluids, due to their higher thermal conductivity in comparison with base fluids, have demonstrated the capability to delay the onset of critical heat flux phenomenon. In the present Paper, silver, copper, alumina and nickel water-based nanofluidic flows in heat exchangers are analytically studied, and the critical heat flux is assessed under different mass flux and volume fraction conditions. Reports of Look Up Table are employed for calculation of critical heat flux in the present Paper. All four mechanisms of mass transfer are simulated, and pressure variations in both dimensions are considered. Effects of mass transfer on vapor core pressure are comprehensively assessed. The results obtained indicate a directly proportional relation between mass velocity and critical heat flux. In addition, increasing the concentration of nanoparticles also increases the critical heat flux. Of the studied nanofluids, the highest critical heat flux increase was observed for a 5 vol % water–silver nanofluid with a 81.6% increase in the critical heat flux relative to pure water.

Role of nanoparticles on critical heat flux in convective boiling of nanofluids: Nanoparticle sedimentation and Brownian motion

This study presents simulation of heat and mass transfer in two-phase flows of various nanofluids at various volume fractions and mass flow velocities. In contrary to most of research in the available literature, nanoparticles sedimentation along with Brownian motion of nanoparticles are comprehensively modeled and effects of these two important parameters on heat transfer and critical heat flux are evaluated. The aim of this modeling is to investigate the effect of nanoparticles and their sedimentation on the critical heat flux and mass transfer phenomena. All four mechanisms of mass transfer are simulated. The studied nanofluids are water-silver, water-copper, water-nickel and water-alumina with volume fractions of 1 to 5% and their results are compared against saturated water. Simulation of sedimentation together with Brownian motion of nanoparticles considerably improves the model precision. Nanofluids are seen to result in increased critical heat flux, heat transfer coefficient and net mass transfer. Moreover, the heat transfer coefficient and the critical heat flux are increased with the volume fraction of nanoparticles. Increase in mass flow velocity leads to rise of critical heat flux. In the event of nanoparticle sedimentation during the boiling process, the critical heat flux and heat transfer coefficient are seen to decrease.

Simulation of Critical Heat Flux Phenomenon Using a Non-heating Hydrogen Evolving System

Pool boiling critical heat flux (CHF) at saturated condition was simulated using the hydrogen at the copper cathode in the aqueous solution of sulfuric-acid. The critical current density (CCD) was observed at the cell potential-current density curve, which is analogous to the CHF at the boiling curve. The CHF values calculated by the gas generation rate at the CCD condition showed large discrepancies with the existing CHF values of water boiling system. Meanwhile, the superficial mechanism triggering the CHF and the CCD showed similar characteristics except the size of bubbles. The variation of bubble departure volume with respect to the current density was similar to that of the water vapor but the bubble volume of the present system was 440 times smaller at CCD condition. The critical heat flux value using the present non-heating experiment was corrected accounting for the bubble volumes and the discrepancy with the water CHF was less than 10%. It is concluded that the water CHF condition can be simulated by the non-heating hydrogen evolving system.

Научная методика создания капиллярно-пористых систем охлаждения для элементов теплоэнергооборудования электростанций

We investigated critical heat flux phenomena in metallic and porous structures characterised by low thermal conductivity. These structures are used to cool various thermal power plant equipment; their operation involves both gravity and capillary forces. The paper describes the failure mechanism in metallic steam generator surfaces and poorly thermally conductive low-porosity coatings made of natural mineral media (such as granite). We determined how heat flows depend on their duration and penetration depth of thermal disturbance. Tubular porous systems are less bulky and feature high intensity, higher thermal conductivity and reliability. We show that for granite coatings the maximum thickness of the particles detached due to compression forces is (0.25...0.3)·10--2 m. The compression curve sections that govern detachment of particles larger than 0.3·10--2 m are shadowed by the melt curve for high heat flows and short exposure times and by the tension curve in the case of low heat flows and short periods of time. The investigation should help us to design porous coatings usable in cooling systems.

NEW PARADIGMS OF CRITICAL HEAT FLUX PHENOMENON

Previous chapters dealt with experimental and analytical investigations of the critical heat flux (CHF) phenomenon under geometrical and operating conditions that are very common in the industry like flow boiling in vertical or horizontal tubes, boiling above the heated surface, and steady state flow. However, there are specific cases where the coolant flow may be oscillating during operating transients or where boiling takes place underneath the heated surface. Similarly, the heated surface may not be flat, but may have curvature of a vessel under consideration. Additionally, the heater may be hollow, having heat flux and coolant flow inside and outside as well as cases on an annular fuel. These conditions need different approaches for the evaluation of CHF owing to the different underlying phenomena. This chapter briefly describes such scenarios requiring CHF evaluation and the methodology followed for evaluating CHF under these specific conditions.