Critical Heat Flux Prediction Methods Research Articles

An evaluation of critical heat flux prediction methods for the upward flow in a vertical narrow rectangular channel

Narrow rectangular channels have widespread application in various domains owing to their significant enhancement in boiling heat transfer. In the present work, the CHF (critical heat flux) prediction method has been comprehensively evaluated and analyzed by the experimental data covering wide operating conditions in narrow rectangular channels. The Bubble Crowding Model can predict the high critical heat flux region. The prediction deviation of ANN (Artificial Neural Network) with two hidden layers and five neurons to predict CHF can reach 20 %, then the influences of thermal-hydraulic parameters on prediction of CHF were obtained based on this constructed ANN. Among the tens of CHF correlations, Tong's, Sudo's, and Mudawar's correlations were selected, and the results indicate that Sudo's correlation can forecast well in 1–4 MPa and Mudawar's correlation had relatively low prediction errors for a wide pressure range. The LUT (look-up table) needs proper correction factors to accurately predict the CHF in narrow rectangular channels.

Critical heat flux measurement and visualization in R-134a on a vertical single rod with and without mixing-vanes spacer: The characteristics and phenomenon of critical heat flux

In this study, a critical heat flux (CHF) experiment using R-134a as working fluid was carried out in a vertical single rod geometry. A unique feature of this experiment are high speed visualization studies obtained at a viewing port located at the CHF location. The 9.5 mm O.D. heated rod was placed in a 19 x19mm square channel to simulate the Pressurized Water Reactors (PWRs) fuel lattice. Test conditions were selected and scaled to cover a wide range of water conditions relevant to PWRs. The CHF database, experimental trends and comparisons to existing CHF prediction methods are presented. The visualization results of the flow boiling at the CHF location and the CHF mechanism are discussed with a particular emphasis on the impact of grid spacers on the boiling phenomena. Quantitative analysis of bubble layer thickness is carried out, including the relationship between bubble layer thickness and CHF.

Investigations of near-wall bubble behavior in wire heaters pool boiling

Current work establishes a pool boiling critical heat flux prediction method based on percolation theory. For the first time, we observe the experimental bubble footprint?s power-law distributions with almost the same exponent in wire heaters? water pool boiling crisis, which is borne out strongly that boiling crisis is a typical continuum percolative scale-free behavior, and its characteristics seems not to be influenced by the critical heat flux value. The proposed 1-D Monte-Carlo method successfully simulates the phase transition of interactive near-wall bubbles. This research enriches and extends applications of continuum percolation theory in boiling phenomena, and could be an instruction for the followed critical heat flux enhancement studies.

Program development and preliminary CHF characteristics analysis for natural circulation loop under moving condition

Critical heat flux (CHF) has traditionally been evaluated using look-up tables or empirical correlations for nuclear power plants. However, under complex moving condition, it is necessary to reconsider the CHF characteristics since the conventional CHF prediction methods would no longer be applicable. In this paper, the additional forces caused by motions have been added to the annular film dryout (AFD) mechanistic model to investigate the effect of moving condition on CHF. Moreover, a theoretical model of the natural circulation loop with additional forces is established to reflect the natural circulation characteristics of the loop system. By coupling the system loop with the AFD mechanistic model, a CHF prediction program called NACOM for natural circulation loop under moving condition is developed. The effects of three operating conditions, namely stationary, inclination and rolling, on the CHF of the loop are then analyzed. It can be clearly seen that the moving condition has an adverse effect on the CHF in the natural circulation system. For the calculation parameters in this paper, the CHF can be reduced by 25% compared with the static value, which indicates that it is important to consider the effects of moving condition to retain adequate safety margin in subsequent thermal-hydraulic designs.

The Study of Critical Heat Flux in Upflow Boiling Vertical Round Tube under High Pressure

The Critical Heat Flux (CHF) prediction under high pressure condition, even close to the vicinity of the critical pressure of water, is an important issue. Although there are many empirical CHF correlations, most of them have covered the pressure under 15MPa. In this study, based on the CHF experiment database of upflow boiling in vertical round tube from 15MPa to the vicinity of the critical pressure of water, the Katto, Bowring, Hall-Mudawar, Alekseev correlations, and Groeneveld LUT-2006 are comparatively studied. With an error analysis of the predicted CHF to the experiment database, the prediction capability and the applicability of these correlations are evaluated and the parametric trends of CHF varying with pressure from 15MPa to critical pressure are proposed. Simultaneously, according to the characteristics of Departure from Nucleate Boiling (DNB) type CHF under high pressure condition, the constitutive correlations of Weisman & Pei model are proposed. The prediction results of three entrainment and deposition correlations of Kataoka, Celata, and Hewitt corresponding to the Dry-Out (DO) type CHF are analyzed. Based on the two improved models above, a comprehensive CHF mechanistic model under high pressure condition combining the DNB and DO type CHF is established. The verification based on the experiment database of upflow boiling in vertical round tube and the parametric trends analysis of CHF varying with thermal-hydraulic and geometric parameters are carried out. Findings of this study have a positive effect on further development of CHF prediction method for universal CHF mechanism, especially under high pressure region.

An overview of measurements, data compilations and prediction methods for the critical heat flux in water-cooled tubes

This paper describes the history and state of the art of measuring and predicting the Critical Heat Flux (CHF) in water-cooled tubes. Over the past 70 years, over 40,000 tube-CHF measurements have been obtained in over 160 separate studies. About 17 separate compilations of CHF datasets in water-cooled tubes have been produced, many overlapping each other. In addition, hundreds of tube-based CHF prediction methods can be found in the literature. The pertinent experimental details and concerns for these datasets and the proliferation of CHF prediction methods are discussed. A graphical presentation of the ranges of conditions covered by these CHF datasets is included.

Prediction of the critical heat flux for saturated upward flow boiling water in vertical narrow rectangular channels

A study, on the theoretical method to predict the critical heat flux (CHF) of saturated upward flow boiling water in vertical narrow rectangular channels, has been conducted. For the assessment of this CHF prediction method, 608 experimental data were selected from the previous researches, in which the heated sections were uniformly heated from both wide surfaces under the high pressure condition over 41bar.For this purpose, representative previous liquid film dryout (LFD) models for circular channels were reviewed by using 6058 points from the KAIST CHF data bank. This shows that it is reasonable to define the initial condition of quality and entrainment fraction at onset of annular flow (OAF) as the transition to annular flow regime and the equilibrium value, respectively, and the prediction error of the LFD model is dependent on the accuracy of the constitutive equations of droplet deposition and entrainment. In the modified Levy model, the CHF data are predicted with standard deviation (SD) of 14.0% and root mean square error (RMSE) of 14.1%. Meanwhile, in the present LFD model, which is based on the constitutive equations developed by Okawa et al., the entire data are calculated with SD of 17.1% and RMSE of 17.3%. Because of its qualitative prediction trend and universal calculation convergence, the present model was finally selected as the best LFD model to predict the CHF for narrow rectangular channels.For the assessment of the present LFD model for narrow rectangular channels, effective 284 data were selected. By using the present LFD model, these data are predicted with RMSE of 22.9% with the dryout criterion of zero-liquid film flow, but RMSE of 18.7% with rivulet formation model. This shows that the prediction error of the present LFD model for narrow rectangular channels is similar with that for circular channels.

The Corrected Simulation Method of Critical Heat Flux Prediction for Water-Cooled Divertor Based on Euler Homogeneous Model**supported by the National Magnetic Confinement Fusion Science Program of China (No. 2010GB104005) and National Natural Science Foundation of China (No. 51406085)

An accurate critical heat flux (CHF) prediction method is the key factor for realizing the steady-state operation of a water-cooled divertor that works under one-sided high heating flux conditions. An improved CHF prediction method based on Euler's homogeneous model for flow boiling combined with realizable k-ϵ model for single-phase flow is adopted in this paper in which time relaxation coefficients are corrected by the Hertz-Knudsen formula in order to improve the calculation accuracy of vapor-liquid conversion efficiency under high heating flux conditions. Moreover, local large differences of liquid physical properties due to the extreme nonuniform heating flux on cooling wall along the circumference direction are revised by formula IAPWS-IF97. Therefore, this method can improve the calculation accuracy of heat and mass transfer between liquid phase and vapor phase in a CHF prediction simulation of water-cooled divertors under the one-sided high heating condition. An experimental example is simulated based on the improved and the uncorrected methods. The simulation results, such as temperature, void fraction and heat transfer coefficient, are analyzed to achieve the CHF prediction. The results show that the maximum error of CHF based on the improved method is 23.7%, while that of CHF based on uncorrected method is up to 188%, as compared with the experiment results of Ref. [12]. Finally, this method is verified by comparison with the experimental data obtained by International Thermonuclear Experimental Reactor (ITER), with a maximum error of 6% only. This method provides an efficient tool for the CHF prediction of water-cooled divertors.

Fundamental Issues of Critical Heat Flux Phenomena During Flow Boiling in Microscale-Channels and Nucleate Pool Boiling in Confined Spaces

This article presents a comprehensive literature review on the fundamental issues of critical heat flux (CHF) during flow boiling and nucleate pool boiling in microscale channels and confined spaces. First, distinction between macro- and micro-scale channels is discussed. Then the CHF mechanisms are discussed. Next, experimental and theoretical studies of subcooled flow boiling CHF in microscale channels together with the prediction methods are reviewed and analyzed. Following this, experimental and theoretical studies on saturated flow boiling CHF together with the prediction methods are summarized and discussed. Furthermore, experimental and theoretical studies on nucleate pool boiling CHF in confined spaces together with the relevant prediction methods are reviewed as well. So far, limited studies on CHF microscale channels and confined spaces are available in the literature. There are numerous discrepancies in the existing studies on CHF results, mechanisms, and prediction methods. Furthermore, there are no generalized prediction methods for CHF in microscale channels and confined spaces. According to this review, future research needs for the experiments, mechanisms, and prediction methods of CHF phenomena in microscale channels and confined spaces have been addressed.

Anomalies and other concerns related to the critical heat flux

This paper summarizes various unusual trends in the critical heat flux (CHF) that have been observed experimentally in tubes or bundle subassemblies. They include the following: • Occurrence of a minimum in the CHF vs. quality ( X) curve at high flows – leading to an initial upstream CHF occurrence in uniformly heated channels. This phenomenon has been observed at high flows in both water and Freon. • Occurrence of a limiting quality region on the CHF vs. X curve where the CHF drops by 30–90% for a nearly constant quality. This is thought to correspond to the boundary between the entrainment controlled and the deposition controlled region and causes problems for prediction methods of the form CHF = f( X). • Impact of flow obstructions on the occurrence of upstream CHF and the limiting quality region. The additional mixing by grid spacers or bundle appendages results in a more homogeneous phase distribution, and diminishes the effects of flow regime/heat transfer regime transitions responsible for some of the unusual CHF trends, and results in a more gradually decreasing CHF vs. X curve. • Absence of a CHF temperature excursion at high flows and high qualities – this is found to be caused by a change in slope of the transition boiling part of the boiling curve from a negative value (usual trend that results in a temperature excursion) to a positive slope. • Gradual disappearance of the sharp temperature excursion at CHF when increasing the pressure towards and beyond the critical pressure – no drastic change is observed in the axial temperature distribution of a heated tube experiencing CHF when, for constant mass flux and inlet temperature, the pressure is gradually increased from subcritical to supercritical. • CHF fluid-to-fluid modelling: differences in CHF trends at certain conditions between refrigerants and water at equivalent conditions. The mechanisms responsible for these trends and the implications for bundle geometries are discussed. Concerns regarding the reported uncertainty of predicted CHF values and the range of application of CHF prediction methods are also discussed.

Assessment of a DNB-type theoretical critical heat flux model for rod bundles with non-uniform axial power shapes

A general critical heat flux (CHF) prediction method with a wide applicable range and reasonable accuracy is essential to the thermal-hydraulic design and safety analysis at the conceptual design stage for a new pressurized water reactor (PWR). In this study, the Korea Advanced Institute of Science and Technology (KAIST) liquid sub-layer dryout CHF prediction model for Departure from Nucleate Boiling (DNB) region has been implemented in a sub-channel analysis code, and investigated for the method's possible use in a rod bundle environment with various non-uniform axial power shapes. The KAIST model showed comparable prediction capability to Lin's method for bottom-, center-, and top-peaked heat flux shapes. The KAIST model, without any correction factors or empirical constants, turned out to be suitable to fulfill the needs for a basis of a general CHF prediction method as compared to Lin's method and Westinghouse-3 (W-3) correlation.

Modeling capability of R134a for a critical heat flux of water in a vertical 5 × 5 rod bundle geometry

Critical heat flux (CHF) tests have been performed for a vertically upward R134a flow in a 5 × 5 rod bundle in the following parameter ranges: outlet pressure 1.5–3.0 MPa, mass flux 50–2500 kg m −2 s −1, inlet quality −0.08 to −0.75 and critical quality −0.17 to 1.58. Parametric trends of the CHF data agree well with the general understanding. Water equivalent CHF data is generated using a set of well-established modeling parameters, then compared with (a) the EPRI/Colombia University’s water CHF database for similar geometry and experimental conditions and (b) commonly used CHF prediction methods. The water equivalent CHF data generated from the present tests shows a good agreement with the actual water CHF data for both Katto’s and Ahmad’s modeling methods when considering the differences in the detailed geometric and flow conditions. They are also predicted well by the 1995 CHF look-up table. These results indicate that R134a can be a good modeling fluid for the CHF of water in rod bundles at least in the investigated parameter ranges.

Studies on critical heat flux in flow boiling at near critical pressures

Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length at different reduced pressures ranging from 0.24 to 0.99 with R-134a as the working fluid. The onset of CHF was determined by the sudden rise in the wall temperature of the electrically heated tube. Experiments were performed over a wide range of parameters: mass flux values from 200 to 2000 kg/m 2 s, pressure from 10 to 39.7 bars and heat flux from 2 to 80 kW/m 2 and exit quality from 0.17 to 0.94. The results show considerably lower critical heat flux at high pressures. Well known CHF prediction methods, such as the look-up table and correlations of earlier workers show poor agreement at high pressures. A new correlation has been proposed to estimate the CHF in uniformly heated vertical tubes up to the critical pressure and over a wide range of parameters.

Critical heat flux and turbulent mixing in hexagonal tight rod bundles

Experimental and theoretical investigations have been performed on critical heat flux ( CHF) and turbulent mixing in tight, hexagonal, 7-rod bundles. Freon-12 was used as working fluid due to its low latent heat, low critical pressure and well known properties. It has been found that the two-phase mixing coefficient depends mainly on mass flux. It increases with decreasing mass flux and ranges from 0.01 to 0.04 for the test conditions considered. More than 900 CHF data points have been obtained in a large range of parameters: pressure 1.0–3.0 MPa and mass flux 1.0–6.0 Mg/m 2s. The effect of different parameters on CHF has been analysed. It has been found that the effect of pressure, mass flux and vapour quality on CHF is similar to that observed in circular tubes. Nevertheless, the CHF in the tight rod bundle is much lower than that in a circular tube of the same equivalent hydraulic diameters. The effect of wire wraps on CHF is mainly dependent on local vapour qualities and subsequently on flow regimes. Based on subchannel flow conditions, the effect of radial power distribution on CHF is small. Comparison of the test results with CHF prediction methods underlines the need for further work.

Assessment of a tube-based bundle CHF prediction method using a subchannel code

At the conceptual design stage for advanced water-cooled reactors (AWCRs), a general critical heat flux (CHF) prediction method with a wide applicable range and reasonable accuracy is essential to the thermal-hydraulic design and safety analysis. A basic idea for general CHF prediction is to utilize the tube-based CHF models covering wide applicable ranges with the help of supplementary terms for bundle effects. In this study, feasibility assessments have been performed with the bundle CHF data relevant to pressurized water reactors. A subchannel analysis is adopted in order to be able to consider the geometrical variations properly even in untested fuel bundle geometries. As a result, a CHF look-up table method (i.e., the use of a round tube CHF table with appropriate bundle effect factors) turns out to be a promising way to fulfill the needs in many aspects among some selected correlations and theoretical models. Though improvements of the supplementary factors, especially for the cold wall and the bundle heated length effects, are desirable to make better predictions, the tube-based bundle CHF prediction method clearly shows a potential as a general CHF predictor.

A comparison of critical heat flux in tubes and bilaterally heated annuli

This paper examines the critical heat flux (CHF) behaviour for annular flow in bilaterally heated annuli and compares it with that in tubes and unilaterally heated annuli. It was found that the differences in CHF between bilaterally and unilaterally heated annuli or tubes strongly depend on pressure and quality. The CHF in bilaterally heated annuli can be predicted by tube CHF prediction methods for the simultaneous CHF occurrence at both surfaces, and the following flow conditions: pressure 7–10 MPa, mass flux 0.5–4.0 mg m −2 s −1 and critical quality 0.23–0.9. The effect on CHF of the outer-to-inner surface heat flux ratio, was also examined. The prediction of CHF for bilaterally heated annuli was based on the droplet-diffusion model proposed by Kirillov and Smogalev (1969, 1972). While their model refers only to CHF occurrence at the inner surface, we extended it to cases where CHF occurs at the outer surface, and simultaneously at both surfaces, thus covering all cases of CHF occurrence in bilaterally heated annuli. From the annuli CHF data of Becker and Letzter (1975), we derived empirical functions required by the model. The proposed equations provide good accuracy for the CHF data used in this study. Moreover, the equations can predict conditions at which CHF occurs simultaneously at both surfaces. Also, this method can be used for cases with only one heated surface.

A comparison of critical heat flux in tubes and annuli

This paper examines the differences in critical heat flux (CHF) between internally heated annuli and tubes, cooled by water, based on an 8 mm tube CHF look-up table as the reference. The quality, gap size and pressure were identified as the most important parameters to account for differences in CHF between concentric annuli and tubes. Eccentricity, void migration parameter γ and quality caused the main differences in CHF between eccentric and concentric annuli. These parameters were used to develop CHF correlations for both types of annuli. The proposed correlations predict the CHF with an r.m.s. error of 9.26% for concentric annuli (based on 1547 CHF data points), and 11.6% for eccentric annuli (based on 331 CHF data points). These correlations predict the correct parametric and asymptotic trends, and are more accurate than other CHF prediction methods for annuli.

Critical heat flux measurements in a round tube for CFCs and CFC alternatives

Critical heat flux (CHF) measurements have been conducted for HFC-134a, HCFC-123, HCFC-22, CFC-12 and CFC-11 in a multi-fluid loop. The test pressures are up to 2 MPa (corresponding to water equivalent pressure of 7–10 MPa) and mass fluxes from 1000 to 4000 kg m −2 s −1. The heated length is adjustable from 0.5 to 1 m, the I.D. of the tubular test section is 4.2 mm, and the critical quality varies from 0.07 to 0.6. The parametric trends of test results are examined and the test results show good agreement with existing CHF prediction methods. It also provides confirmation that CFC alternatives such as HFC-134a, HCFC-123 and HCFC-22 can be used with confidence as reliable CHF modelling fluids.