Characteristics Of Critical Heat Flux Research Articles

Numerical investigation on boiling crisis in a full-length 5 × 5 fuel assembly under typical pressurized water reactor conditions

Critical heat flux (CHF) presents a research priority in the Pressurized Water Reactor (PWR) safety, as it can lead to the severe accidents and release radioactive materials. In this study, a wall heat flux partition model, proposed by Demarly [1], was employed within the two-fluid model by Computational Fluid Dynamics (CFD) methodology to investigate the characteristics of CHF under typical PWR conditions. A multi-scale interface model was coupled to consider the interfacial transfers in bulk flow during flow regime transition. The models were validated by using experimental data on subcooled flow boiling, boiling crisis in vertical tubes, and rod bundles. The PWR sub-channel and bundle test (PSBT) benchmark, comprising a full-length 5 × 5 fuel assembly with three different types of spacers, was simulated in this study. The CHF value was evaluated, demonstrating an acceptable accuracy compared to the experimental data, with a deviation of less than 10 % in the selected conditions. The CHF location in the cross section was successfully predicted by CFD approach, while the axial location of CHF was underestimated than experimental measurement. Besides, the impacts of mixing vanes with different deflection angles on boiling heat transfer characteristics were analyzed, revealing the crucial role of mixing vanes in the distribution of vapor and dry-patch area fraction in both the circumferential and flow directions.

Critical heat flux and heat transfer behaviors on aged RPV outer wall surface in flow boiling under IVR conditions

Critical Heat Flux (CHF) plays an important role in the nuclear power plant safety strategy both during normal operation and under accident conditions. During normal operation, the outside wall is exposed to high-temperature environment. The surface is oxidized by air, which is so-called “aged surface”. To specify the characteristics of aged surface and the effect on CHF, the specimen aging experiments and flow boiling experiments under External Reactor Vessel Cooling (ERVC) were conducted. The results of specimen aging experiments show that surface characteristics get steady after 500 h aging and the oxide layer is the Fe-Fe2O3-Fe3O4 mixture. After aging, the surface gets rougher and has better hydrophily. In the flow boiling experiments, characteristics of heat transfer and CHF are studied. Heat transfer is enhanced by extended surface area and increased nucleation sites at lower angle. The influence of capillary suction is not obvious and the enhancement of CHF is limited. As the inclination angle increases, the departure of bubbles is more frequent and capillary suction is dominant to assist liquid supply into the liquid sublayer and delay the boiling process. At higher angle, CHF is more obviously enhanced but heat transfer gets worse. Overall, CHF of RPV outer wall is increased after surface aging, and it is beneficial to improving the safety margin of IVR mitigation measures.

Theoretical Study on the Characteristics of Critical Heat Flux in Rectangular Channel of Natural Circulation under Motion Conditions

With the wide application of sea-based reactors, the impact of ocean conditions on the safety performance of reactors has gradually attracted attention. In this paper, by establishing the thermal hydraulic transient analysis model and the critical heat flux (CHF) model of natural circulation system, the CHF characteristics in the rectangular channel of natural self-feedback conditions under ocean conditions are studied. The results show that the additional acceleration field generated by ocean conditions will affect the thermal hydraulic parameters of the natural circulation system, that is, the external macroscopic thermal hydraulic field. On the other hand, the boiling crisis mechanism will be affected, that is, the force on the bubble and the thickness of the liquid film. Within the parameters of the study, ocean conditions have a great impact on CHF of natural circulation, and the maximum degradation of CHF is about 45%. The obtained analysis results are significant to the improvement of design and safety operation of the reactor system.

Computational prediction of key heat transfer mechanisms and hydrodynamic characteristics of critical heat flux (CHF) in subcooled vertical upflow boiling

The present study investigates extensively the capability of computational fluid dynamics (CFD) to predict two-phase fluid flow and heat transfer characteristics of FC-72 flow boiling in vertical upflow. The computational method performs transient analysis to model the entire flow field and bubble behavior for highly subcooled flow boiling in a rectangular channel heated on two opposite walls at high heat flux conditions of about 81% of critical heat flux (CHF). Transient variations of local vapor fraction, thermal energy transfer and concentration, and wall temperature are estimated through the computational methodology based on the Volume of Fluid (VOF) approach combined with shear-lift modeling to overcome a fundamental weakness in modeling multiphase flows. Detailed information about bubble distribution in the vicinity of the heated surface, thermal conduction inside the heating wall, local heat fluxes passing through the solid-fluid interface, and velocity and temperature profiles, which are not easily observed or measured by experiments, is carefully evaluated. Predictions are compared to the experimental data for three sets of operating conditions. Overall, computationally predicted flow pattern, bubble behavior, and heat transfer parameters (such as wall temperature excursion and thermal energy concentration) clearly represent phenomena of premature CHF which take place slightly earlier than actual operating conditions. But, despite these slight differences, the present computational work does demonstrate the ability to effectively predict the severe degradation in heat transfer performance commonly encountered at heat fluxes nearing CHF.

Study of flow boiling heat transfer characteristics of critical heat flux using carbon nanotubes and water nanofluid

Flow boiling heat transfer characteristics using a new nanofluid, multi-walled carbon nanotubes (MWCNTs)/pure water were investigated in a horizontal tube under atmospheric pressure. In order to prepare nanofluid, gum Arabic (GA) as a surfactant and MWCNTs nanoparticles were used in the ratio 1:1 with concentrations of 0.001 and 0.01% by mass, and its stability was determined by measuring zeta potential. During the experiments, detailed observations of the boiling process showed physical differences between nuclear boiling of pure water and nanofluid especially after CHF phenomenon. These differences were mainly due to the deposition of carbon nanotube nanoparticles on the heat transfer surface and improvement in wettability and re-wetting characteristics. The experimental data showed that using nanofluid can improve the flow boiling characteristics of CHF. Based on these results, it was found that the nanofluids heat transfer coefficient was larger than that of pure water and it increased with increasing mass flux and also with increasing heat flux for both pure water and nanofluids. The enhancement of the maximum heat transfer coefficient of MWCNTs-GA/water nanofluid was approximately 4.3% at CHF and 0.005 volume fraction of nanoparticles. The results also showed that the pressure drop of nanofluid increases with increasing mass flux and the enhancement of maximum pressure drop was approximately 0.12 vol% at CHF and 0.005 vol% of nanoparticles.

Experimental study on the characteristics of CHF and pressure fluctuations of surfactant solution flow boiling

The critical heat flux (CHF) and presure fluctuation characteristics of surfactant solution flow boiling were studied experimentally and compared to that of pure water. The tested fluid was an aqueous solution of cetyltrimethyl ammonium chloride (CTAC) with addition of sodium salicylate (NaSal) at the same mass concentration. Tests were performed in a flow channel with 6.0×3.5mm cross-section over flow rate range of 192–406ml/min, at 80°C inlet temperature and 101.3kPa outlet pressure. The present experimental results show that, the CTAC/NaSal solutions behave an increased CHF: the value of CHF kept nearly constant at a level about 1.26 times of that of pure water when the solution concentration was beyond 100ppm (ppm: part per million). And the CTAC/NaSal solution had stronger pressure fluctuations than water. Visualization of the flow boiling process shows that bubbles in CTAC/NaSal solution have smaller diameters and larger population density than that in water, which may be the reason of higher CHF for surfactant solution case. And the bubbles in CTAC/NaSal solution existed for a longer time than in water, which was about ten times of the bubbles in water. These bubbles increased flow velocity and enhanced disturbances, which may be another reason of flow boiling heat transfer enhancement and stronger pressure fluctuations.

Characteristics of critical heat flux under rolling condition for flow boiling in vertical tube

This paper presents the characteristics of the critical heat flux (CHF) for the boiling of R-134a in vertical tube under rolling motion in a marine reactor. It is important to predict CHF of marine reactor under rolling motion in order to consider the safety margin of the reactor. MArine Reactor Moving Simulator (MARMS) test was conducted to measure the CHF of R-134a flowing upward in a uniformly heated vertical tube under rolling motion. A CHF loop mounted on rolling equipment, which can periodically roll from side to side through rotating by motor and mechanical power transmission gear. The CHF tests were performed in a 9.5mm I.D. test section with heated length of 1m. Mass flux ranges from 285kg/m2s to 1300kg/m2s, inlet subcoolings from 3 to 38°C and outlet pressures from 1.3 to 2.4bar, respectively. Amplitudes of rolling range from 15° to 40° and period from 6 to 12s. Fluid-to-fluid (FTF) scaling was applied to convert the test matrix of MARMS from water to R-134a equivalent conditions. CHF ratios (ratio of the CHF under rolling condition to the stationary CHF) as mass flux and pressure in rolling motion are quite different from those of other existing transient CHF experiments. For the mass fluxes below 500kg/m2s (region of relative low mass flux) at 13, 16bar, CHF ratios seem smaller than unit but in region (region of relative high mass flux) where mass fluxes are above 500kg/m2s, it was found that the ratios increased. Moreover, rolling CHFs tend to enhance compare with stationary CHFs in the entire mass flux region at 24bar. To understand the CHF mechanism under rolling motion, considering the flow regime in this study using a new approach was analyzed.

D124 臨界圧近傍における限界熱流束の特性(OS-15: 沸騰・凝縮伝熱および混相流の最近の進展(2))

Supercritical pressure water cooled reactor (SCWR), which is operated at supercritical pressure, is one of the next generation reactors for the purpose of improving economic efficiency and safety. In the SCWR, water pressure passes the critical pressure during startup, shutdown or in case of loss of coolant accident (LOCA). In the near-critical pressure region which the pressure slightly below the critical pressure, critical heat flux (CHF) condition tends to occur at relatively low heat flux and then there is a risk of serious damage of fuel rod due to abrupt rise of surface temperature. In this study, experiments on CHF at near-critical pressure in vertical upward flow inside a tube were conducted to clarify characteristics of CHF at near-critical pressure.

Theoretical study on the characteristics of critical heat flux in vertical narrow rectangular channels

Abstract A mathematical separated flow model of annular upward flow has been developed to predict the critical heat flux (CHF) in uniformly heated vertical narrow rectangular channels. The theoretical model is based on fundamental conservation principles: the mass, momentum, and energy conservation equation of the liquid film and the momentum conservation equation of the vapor core together with a set of closure relationships. The comparisons between the theoretically predicted results and the experimental data demonstrate that a good agreement between them has been achieved. With the applications of the present model, the influences of several parameters (such as system pressure, inlet mass flux and channel size) on the CHF in rectangular channels are analyzed, and the CHF is compared with that in annular channels.

An analytical study of critical heat flux in natural convective boiling in a vertical tube

An analytical study is performed for the prediction of Critical Heat Flux (CHF) during natural convective boiling in a vertical uniformly heated tube, submerged in a saturated liquid bath, by a model where vapor forms a paraboloid under co-current flow situation. Mass, momentum and energy balances for the two-phase flow in the vertical tube are used to construct the model together with the criterion that the CHF is considered to be taking place at the exit of the heated tube when mass flux is maximum. About 300 CHF data are predicted for saturated Water, R113 and R12 at different pressures of P = 0.1 to 3.0 MPa for different tube geometries (diameter, D = 1.12 to 18.4 mm, and heating length, Lhe = 23 to 960 mm). The characteristics of CHF predicted by this model are explained and compared with similar existing experimental works. The effects of working fluids, working pressure, and tube geometry are mainly explained and compared to verify the basic performance of the model.

Effect of nanoparticles on CHF enhancement in pool boiling of nano-fluids

To investigate the characteristics of CHF (Critical Heat Flux) enhancement using nano-fluids, pool boiling CHF experiments of two water-based nano-fluids with titania and alumina nanoparticles were performed using electrically heated metal wires under atmospheric pressure. The results showed that the water-based nano-fluids significantly enhanced CHF compared to that of pure water. SEM (Scanning Electron Microscopy) observation subsequent to the pool boiling experiments revealed that a lot of nanoparticles were deposited on heating surface during pool boiling of nano-fluids. In order to investigate the role of the nanoparticle surface coating on CHF enhancement of nano-fluids, pool boiling CHF of pure water was measured using a nanoparticle-coated heater prepared by pool boiling of nano-fluids on a bare heater. It was found that pool boiling of pure water on the naonoparticle-coated heater sufficiently achieved the CHF enhancement of nano-fluids. It is supposed that CHF enhancement in pool boiling of nano-fluids is mainly caused by the nanoparticle coating of the heating surface.

Mass velocity and cold-wall effects on critical heat flux in an advanced light water reactor

The characteristics of Critical Heat Flux (CHF) were investigated for a square array of rod bundles which could possibly be loaded into an integral-type advanced light water reactor. The parametric effects of the mass velocity and the unheated rod were examined by conducting CHF experiments with 5 × 5 test bundles in a Freon-loop. The influence of a cold wall on the CHF was interpreted by introducing a simple phenomenological model which accounts for the influence of a thermal mixing inside the boiling channel. A local parameter CHF correlation applicable to an integral-type reactor was developed from the CHF data base for square-arrayed rod bundles. The local thermal–hydraulic conditions calculated by the subchannel analysis code MATRA were used for the optimization of the correlation coefficients. Correction factors for the low mass velocity, spacer grids, and the non-uniform axial power shapes have been devised which reflected the results of the data assessment and the experimental observations. As a result of the thermal margin evaluation at steady state conditions, it was revealed that the integral-type reactor core has a greater DNBR margin than a typical 1000 MW e PWR core.

Critical Heat Flux in Non-uniformly Heated Tube under Low-Pressure and Low-Mass-Flux Condition

In an actual boiling channel, e.g. boililer water-tubes, the circumferential heat flux is not uniform. Thus, critical heat flux (CHF) of non-uniformly heated tube becomes an important design factor of conventional boilers, especially for the compact water-tube boiler with tube-nested combustor. The small compact boiler is operated at low-pressure and low-mass flux condition compared with the large scale boiler, thus the redistribution of liquid film will strongly affect on the characteristics of CHF. In this investigation, non-uniformly heat flux distribution along the circumferential direction was generated by using the Joule heating of SUS304 tubes which had the wall thickness distribution. The heated length of test-section was 900mm, inner diameter was 20mm and outer diameter was 24mm. The center of the inner tube surface was shifted by ε=0, 0.5, 1.0, 1.5mm from the center of the outer tube surface. The heat flux ratio between maximum and minimum heat fluxes of these tubes correspond to 1.0, 1.7, 3.0 and 7.0, respectively. Experimental conditions were as follows ; system pressures were 0.3 and 0.4 MPa, mass flux range were 10-100kg/m2s, inlet temperatures were 30 and 80 deg. C. The experimental results showed an increase in the critical heat flux by the existence of the redistribution of the flow. These characteristics have been well explained by using the similar concept of Butterworth's spreading model.

Critical heat flux in non-uniformly heated tube under low-pressure and low-mass-flux condition

In an actual boiling channel, e.g., a boiler water-tube, the circumferential heat flux is not uniform. Thus, the critical heat flux (CHF) of a non-uniformly heated tube becomes an important design factor for conventional boilers, especially for a compact water-tube boiler with a tube-nested combustor. A small compact boiler is operated under low-pressure and low-mass-flux conditions compared with a large-scale boiler, thus the redistribution of liquid film strongly affects the characteristics of CHF. In this investigation, non-uniform heat flux distribution along the circumferential direction was generated by using the Joule heating of SUS304 tubes with the wall thickness distribution. The heated length of test-section was 900 mm with an inner diameter of 20 mm and an outer diameter of 24 mm. The center of the inner tube surface was shifted by e=0, 0.5, 1.0, 1.5 mm from the center of the outer tube surface. The heat flux ratio between maximum and minimum heat flux of these tubes corresponded to 1.0, 1.7, 3.0, and 7.0, respectively. The experimental conditions were as follows: system pressure at 0.3 and 0.4 MPa, mass flux of 10–100kg/(m2s), inlet temperatures at 30° and 80°. The experimental results showed an increase in the critical heat flux substantiated by the existence of the redistribution of the flow. These characteristics are explained by using a concept similar to that of Butterworth's spreading model. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 47–60, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20095

Influence of two-phase flow characteristics on critical heat flux in low pressure

Abstract In the present study, the critical heat flux (CHF) was measured over wide quality range from subcooled boiling to annular-mist flow at the atmospheric pressure. At a low pressure around the atmospheric pressure, the two-phase flow pattern and void fraction changes easily along the heated length and the flow becomes more complex due to the low density of gas phase. By using Pyrex glass tube as a test channel installed with a heater rod at the centerline, the two-phase flow situation was observed visually. The flow pattern was kept nearly constant over the length of test section due to the low power input into the fluid. Therefore, the characteristics of CHF could be investigated at each flow patterns of bubbly, slug, annular and annular-mist flow. In the subcooled boiling region of bubbly flow, the CHF decreased with increase of quality and was less sensitive to flow rate. In the slug flow region, the CHF showed a minimum value. With more increase of quality in the annular flow, the CHF increased and reached a peak value at a certain quality depending on flow rate. The peak of CHF occurred almost at a constant vapor mass velocity. In the annular-mist flow region, the CHF decreased with increase of quality. In the region, the effect of heated length on the CHF was systematically measured and validity of an analytical model of CHF considering dryout of liquid was investigated.

Characteristics of Critical Heat Flux at High Pressure in an Inclined Tube.

Characteristics of Critical Heat Flux at High Pressure in an Inclined Tube.

Characteristics of critical heat flux in two phase thermosyphon - relationship between maximum falling liquid rate and instability on interface of falling liquid film

Critical heat flux in a two-phase thermosyphon is usually dealt with from two different ways in a limitation of liquid flow rate falling along a vertical tube: one is a maximum falling liquid rate due to the envelope method, and the other one due to instability of falling liquid film. The difference between the maximum and instability criteria is first made clear. The CHF in the thermosyphon is shown to be predicted well by the maximum liquid rate due to the maximum criterion better than due to the instability criterion. In addition, the comparison implies that the CHF phenomenon in the thermosyphon is considered to be caused when the falling liquid reaches the maximum value rather than when the instability of the falling liquid on the interface is brought about.

Effect of channel length on critical heat flux under conditions of high subcooling and high velocity in short heated channels

The characteristics of critical heat flux (CHF) in existing experiments under high subcooling and high velocity in short heated channels have, for the first time, been systematically and quantitatively investigated to provide a CHF correlation that can properly predict the effect of channel length, especially when the channel length-to-channel diameter ratio L/D is less than about 20. The major test conditions of existing CHF experiments investigated in this study were channel diameter 1 to 4 mm, L/D 1 to 25, 0.1 to 1.2 MPa pressure, 34 to 117°C inlet water subcooling and 500 to 40 700 kg/(m2 · s) mass flux in circular channels, and 3 to 20 mm gap size, 6 to 40 L/De, 0.1 to 3.1 MPa pressure, 4 to 166°C inlet water subcooling, and 940 to 27,000 kg/(m2 · s) mass flux in rectangular channels. The effect of L/D on CHF was evaluated referring to the analytical solution of CHF, which was previously derived by the author for the channel flow at high subcooling and high velocity. As a result, the effect of L/D was quantitatively clarified as an effect of magnitude in heat transfer of single-phase forced-convection flow, giving a larger CHF with a smaller L/D in the case of L/D less than about 20. The proposed correlation predicts CHF to within a ±35 percent error margin. ©1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 509–521, 1998

Effect of channel length on critical heat flux under conditions of high subcooling and high velocity in short heated channels

The characteristics of critical heat flux (CHF) in existing experiments under high subcooling and high velocity in short heated channels have, for the first time, been systematically and quantitatively investigated to provide a CHF correlation that can properly predict the effect of channel length, especially when the channel length-to-channel diameter ratio L/D is less than about 20. The major test conditions of existing CHF experiments investigated in this study were channel diameter 1 to 4 mm, L/D 1 to 25, 0.1 to 1.2 MPa pressure, 34 to 117°C inlet water subcooling and 500 to 40 700 kg/(m2 · s) mass flux in circular channels, and 3 to 20 mm gap size, 6 to 40 L/De, 0.1 to 3.1 MPa pressure, 4 to 166°C inlet water subcooling, and 940 to 27,000 kg/(m2 · s) mass flux in rectangular channels. The effect of L/D on CHF was evaluated referring to the analytical solution of CHF, which was previously derived by the author for the channel flow at high subcooling and high velocity. As a result, the effect of L/D was quantitatively clarified as an effect of magnitude in heat transfer of single-phase forced-convection flow, giving a larger CHF with a smaller L/D in the case of L/D less than about 20. The proposed correlation predicts CHF to within a ±35 percent error margin. ©1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 509–521, 1998

Critical Heat Flux in Vertical Two Phase Concentric Tube Thermosyphon. Enhancement of Critical Heat Flux.

An experimental study has been made to elucidate the characteristics of critical heat flux (CHF) in a concentric tube thermosyphon. The experiment has been done by using saturated water, ethanol and R 113 over the experimental range of configuration : inner diameter of heated tube Di=9-17 mm, outer diameter of inner tube do=3-15 mm and heated tube length L=100-1 000 mm. The experiment shows that the CHF is enhanced with an increase in the inner tube diameter, and then the CHF is reversely decreased beyond a certain diameter of inner tube for each heated tube diameter. The maximum CHF is about two or eight times as large as that without inner tube depending on the condition of the inner and outer tubes. For larger inner tube, the characteristic of CHF is similar to that of natural convective boiling in a vertical annular tube.