Critical Heat Flux Values Research Articles

Numerical investigation on the influence of porous media structural parameters on pool boiling heat transfer performance

This work investigates pool boiling heat transfer (BHT) and bubble dynamics from a porous medium. The influence of the porous media structural parameters, such as porosity, pore density, porous medium height, thermal conductivity, and wettability, are mainly investigated. The findings indicate that the presence of porous media can increase the critical heat flux (CHF) by an average of 3.75 times and the BHT coefficient by an average of 3.84 times when porosity varies between 57.5% and 98.0% as compared to the plain surface. It is also found that both the CHF and BHT coefficient increase as the porosity decreases if porosity ε≥71.4%. However, they drop with the porosity decreases if porosity ε≤71.4%. On the other hand, the number of nucleation sites, heat transfer area, and bubble escape resistance increase as pore density increases. In addition, increasing the porous media height may enhance BHT performance, but too high a porous media increases the bubble escape resistance and restricts the separation of bubbles. Moreover, the CHF value and the maximum BHT coefficient increase with the thermal conductivity of porous media linearly. Finally, the stronger the wettability, the faster the bubble detachment, and the stronger the BHT performance.

Dry spot propagation during boiling crisis on thin metal heaters

This work presents the results of an experimental investigation aimed at elucidating the role of heater-side parameters on the growth of an irreversible dry spot during the boiling crisis of FC-72 on thin metal foils. A two-sided funnel shape of the heater is adopted to induce the occurrence of the boiling crisis in the middle of the foil and capture the expansion of the critical dry patch through high-speed infrared thermometry. In agreement with pre-existing literature, critical heat flux values achieved during this experimental campaign are shown to increase with bulk subcooling, thermal effusivity, and thickness of the heater. The transient temperature field at the boiling surface is used to capture the spatiotemporal evolution of the dry spot and investigate the effect of heater’s parameter on dry spot propagation. Results collected in this work show that the growth of an irreversible dry patch is strongly affected by the heat diffusion process occurring within the thin heater. The dry spot propagation rate is found to be proportionally correlated with the thermal diffusivity of the heater and the volumetric heat dissipation rate achieved at the boiling crisis, the latter being inversely proportional to heater’s thickness. These results provide important insights into the dryout dynamics related to the boiling crisis in thin wall heat transfer devices. Nevertheless, they open questions about the role of fluid thermophysical properties, that still deserve to be studied in detail.

Inherent scatter in pool boiling critical heat flux on reference surfaces

Evaluating boiling heat transfer enhancement depends on reliable reference values in the form of boiling curves and critical heat flux (CHF) values. Typically, the evaluation is performed in pool boiling conditions with water at atmospheric pressure. Literature includes a wide scatter in reference values, prompting this study to comprehensively evaluate boiling performance and CHF on reference surfaces to investigate the scatter's origin and set a definitive reference value. The study recorded 125 boiling curves and CHF values on nominally identical bare copper surfaces, establishing an average boiling curve and mean CHF value. Despite consistent experimental conditions, the recorded CHF values displayed significant variability with a mean CHF of 1112 ± 102 kW m−2 and a scatter from 902 kW m−2 (−19 % of average CHF) to 1339 kW m−2 (+25 % of average CHF). Using Rohsenow's correlation on the average boiling curve, a Csf factor of 0.0151 was obtained. The acquired CHF data is proposed to serve as a foundational benchmark for future research in enhancing pool boiling heat transfer.

Pressure fluctuation analysis for identifying the CHF approaching condition during subcooled flow boiling through a mini-channel

This study presents a new method for predicting the Critical Heat Flux (CHF) approaching condition in subcooled flow boiling based on pressure fluctuation analysis. Through experimental investigations, water flow in an upward direction was conducted within a mini-channel with one side heated to analyze the sequential boiling phenomena, including ONB (Onset of Nucleate Boiling), OFI (Onset of Flow Instability), and CHF. While ONB was determined using the slope of the wall temperature–heat flux, OFI was determined by considering the pressure drop through the channel and outlet pressure fluctuation. By analyzing the inlet pressure fluctuations, a new condition called “the CHF approaching condition” was introduced, which serves as a CHF precursor. The CHF approaching condition was found to be approximately 90% of the CHF value across all experimental scenarios. The proposed method provides an effective tool for predicting and preventing CHF, thereby reducing the potential risk of damage to the heating surface.

Prediction of Critical Heat Flux in Tube Bundles With Crossflow

Abstract Evaporators and boilers with flow across horizontal tube bundles are widely used in industries including refrigeration and chemical processing. As yet there is a lack of a well-verified correlation for predicting local or average CHF (critical heat flux) in such bundles. The available data and correlations are reviewed. The available methods for predicting bundle mean CHF were found to be too conservative. More realistic values of bundle average CHF are proposed based on published test data. The author's well-verified correlations for CHF on a single tube with the subcooled and saturated flow at zero quality across it are compared with test data for tubes in tube bundles together with other correlations. The author's correlations agree well with all data while the others fail for data other than their own. The results of the comparison are presented and discussed. The needs for further research are identified.

Critical heat flux at high mass velocities in a microchannel and surface tension measurements for R514a

This paper presents pioneer experimental data on critical heat flux (CHF) during flow boiling of low-pressure fluid R514a in a microchannel. The authors conducted experiments in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm, a saturation temperature range of 56 to 122.7°C, and a mass velocity range of 1500 to 7500 kg/m2s. This research achieved high critical heat flux values, up to 1.85 MW/m2. The results were compared with the CHF of R123 under similar conditions and a higher CHF for R514a was observed. The performance of CHF predictive methods against the experimental data was performed. Since predictive methods for the CHF in microchannels need surface tension data, but there is a lack of experimental information about this property for R514a in the literature, this paper also presents experimental results for the surface tension of R514a and an expression for calculation of this property as a function of the temperature.

Corrosive effect on saturated pool boiling heat transfer characteristics of metallic surfaces with hierarchical micro/nano structures

Surface modification is of critical interest to enhance boiling heat transfer in terms of heat transfer coefficient or critical heat flux (CHF), which is significantly affected by distinct surface morphology and wettability and it can improve the efficiency and safety of equipment. Furthermore, actual service environment may cause severe corrosion to the processed structured surfaces while its consequence on boiling heat transfer is still obscure. In this article, comprehensive researches are conducted to unravel corrosive effect on metallic samples made of stainless steel (SS) and Inconel materials with microstructures. Different constructions (i.e., microgroove, microcavity and micropillar array) and characteristic dimensions (∼20, 50 μm) of microstructure, various duration time (up to 300 days) and pH values (∼7.0–8.5) of corrosive environment are compared thoroughly. Conclusions can be drawn that not all microstructures can enhance pool boiling heat transfer characteristics, especially in terms of CHF values. More importantly, CHF value of SS microgroove sample firstly increases from 60.94 to 94.09 W·cm−2 in 50 days, then decreases to 47.77 W·cm−2 in 300 days, which can be attributed to competition result between formation of hierarchical micro/nano structure with enhancing wicking capability and chemistry condition with increasing contact angle. In addition, distinct bubble dynamics during pool boiling is also analyzed. The insights obtained from this article can be used to guide surface modification method and to reveal evolvement rule of engineered metallic surface in highly corrosive and harsh boiling heating transfer environment.

The effect of solid nitrogen on nucleate boiling heat transfer in a slush nitrogen pool

The nucleate boiling heat transfer in a slush nitrogen pool has been studied across a heat-flux range from 5 to 400 kW/m2 and a system pressure range from 12.5 kPa (triple point pressure) to 101 kPa (atmospheric pressure). The effect of solid nitrogen on the heat transfer process was found to vary with the heat flux and the pressure. Experimental results reveal that the nucleate boiling heat transfer coefficient (HTC) of slush nitrogen is higher than that of subcooled liquid nitrogen at the low heat flux, which can be attributed to the fact that the solid nitrogen particles (with their latent heat of melting) absorb heat in the thermal boundary layer and form a sharper temperature gradient near the boiling surface. However, at the high heat flux, the HTC of slush nitrogen is lower than that of subcooled liquid nitrogen, which can be attributed to the invasion and inhibition occurring between the solid nitrogen particles and the bubbles during boiling, leading to the accumulation of vapor phase with low thermal conductivity on the boiling surface, thereby the heat transfer will be suppressed. As a result, a dual effect for the nucleate boiling heat transfer in slush nitrogen can be observed. Furthermore, the critical heat flux (CHF) of slush nitrogen is obtained and compared with the CHF of subcooled liquid nitrogen. According to the CHF model, the bubble period is the dominant parameter in determining the CHF. A calibrated CHF correlation for slush nitrogen is developed based on the hypothesis that the bubble period is prolonged as the effective viscosity of slush nitrogen increases with the solid volume fraction. The CHF values of slush nitrogen with the solid volume fraction of 10%, 20% and 30% are 0.96, 0.91 and 0.84 times those of subcooled liquid nitrogen, respectively, at the pressures higher than 50 kPa. Consequently, the solid volume fraction of slush nitrogen and the degree of subcooling of subcooled liquid nitrogen are found to have opposite effects on CHF.

Development of a new criterion for predicting critical heat flux in rod bundles with supercritical water flowing upward

Investigations of heat transfer characteristics in channels with supercritical fluids are crucial for power cycle applications, such as supercritical water-cooled reactors. To ensure the safety of the system, heat transfer deterioration (HTD) should be avoided. The criteria to identify the onset of HTD in heated rod bundles are still not determined. This study summarizes indicators for the onset of HTD in a rod bundle with supercritical water flowing upward based on observations from experimental data. Existing criteria for the tube flow are assessed by comparing predicted critical heat flux (CHF) values with experimentally identified values. Considering effects of different operating parameters, a new criterion for predicting HTD behavior in rod bundles is developed. The results indicate that the new criterion has a better performance than all existing criteria, with a mean average relative deviation of 0.26 %. Predicted CHF values are within ± 20 % of the respective experimentally identified CHF values.

Pool boiling heat transfer: Thermal performance for alternating and extended operational conditions

Oxide formation on metallic heat transfer surfaces poses a risk parameter in high-power thermal systems maintained by multi-phase cooling. In spite of the significance for practical applications, the potential change in chemistry and morphology on the surfaces exposed to continuous or alternating boiling process plays a vital role in heat transfer efficiency. In this comprehensive study, the effect of heat flux rate, boiling crisis, operation time, surface roughness, and pre-oxidation by the system calibration on oxide layer formation and its inevitable effect on boiling heat transfer (BHT) have been evaluated separately. A sort of experimental scenarios rather representing real operating conditions encountered in phase-change cooling apparatuses were carried out on smooth and rough copper substrates in saturated de-ionized water at atmospheric pressure. In order to practice the scenarios, a series of subsequent critical heat flux (CHF) runs and continuous boiling operations at certain heat flux levels ranging between 15 and 60 W/cm2 were performed within different numbers and sequences as a part of the Design of Experiments (DOE). Each operating scenario was analyzed and compared individually, while questions regarding the effect of the number of CHF runs, heat flux level during long-term operation, surface roughness, pre-oxidation, and rigorous long-term stability operation on boiling heat transfer were examined exhaustively. As a result, CHF and maximum heat transfer coefficient (HTC) of all surfaces exposed to boiling processes considerably enhanced at different rates corresponding to applied scenarios. The rate of heat flux level applied on heating surfaces was not determined to be a significant factor, whereas the HTC and CHF enhanced as the number of subsequent boiling crises (CHF runs) ultimately raised. On the other hand, HTC of the surface non-exposed to CHF runs demonstrated a drastic enhancement by 45% after 24 h of continuous boiling exposure at 45 W/cm2 compared to initial performance with 1.0 W/cm2K. The heat transfer from the surfaces exposed to extended long-term operations showed a final stable thermal behavior, while CHF values doubled from an initial value of approximately ∼73 W/cm² to 144.7 W/cm2 as exposure time increased. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and scanning electron microscope (SEM) analyses were executed to evaluate the influence of micro-nano-scale oxide particles/layers developed on the heater surfaces and to interpret the changes in heat transfer performance.

Ionic liquid as a cosurfactant for critical heat flux enhancement during boiling with aqueous surfactant solutions

Surfactants are often used to improve boiling heat transfer due to the ease of their implementation. While aqueous surfactant solution significantly increases the heat transfer coefficient (HTC), it is known to deteriorate the critical heat flux (CHF) relative to the base fluid water. The high foamability of most surfactant solutions causes vapor crowding on the heater surface, making surface modification techniques ineffective for CHF enhancement. Here, we address the issue of relative degradation of CHF due to surfactant by using an ionic liquid as a cosurfactant. We employ a unique boiling fluid, where 1-ethyl-3-methylimidazolium chloride ([C2mim][Cl]), a low-foaming ionic liquid, is introduced as a co-surfactant in aqueous sodium dodecyl sulfate (SDS) solution. Motivated by the synergistic increase in foamability of the aqueous surfactant solution due to the addition of ionic liquid, we examine the boiling performance at various combinations of concentrations of the two additives in the solution. The mixture consistently exhibits higher CHF and HTC than an SDS only solution, with the best enhancement observed for a mixture of ≈ 30 ppm SDS and ≈ 750 ppm [C2mim][Cl], resulting in ≈1.6× and ≈1.5× enhancements in CHF and HTC, respectively, compared to the SDS only solution (≈ 30 ppm). We next fine-tune the concentration of cosurfactant to demonstrate an exceptionally high HTC of ≈93 kW/(m2 – ℃) with a CHF value comparable to that of pure water. Enhanced foamability and in-situ improvement in the wettability of the heater surface enhance the HTC and the CHF, respectively. This facile technique for enhancing HTC with water as the base fluid, without compromising on CHF, holds promise for high heat flux thermal management applications.

Enhancing pool boiling in microgravity using micro-fins and an electrostatic field under different liquid subcooling

Despite boiling heat transfer is an effective way to remove heat and its performances comply with the high requirements of the electronics cooling and aerospace devices, in reduced gravity levels environments a decrease of the Critical Heat Flux (CHF) occurs. In the past decades, several techniques were developed to enhance the performances of a boiling system, such as external force fields and surface modification. Adding micro-structures on a boiling surface can delay the CHF occurrence, due to capillarity. As capillary forces are independent of gravity, this technique is potentially useful in microgravity. In the present study, micro-finned surfaces of new geometry were tested during parabolic flights in microgravity conditions under the action of an electric field and at different subcooling levels ranging from 5 to 35 K. The experimental results, reported here for the first time, showed that the micro-fins alone (whose characteristic length ranges from 30 to 120 µm) were not effective to increase CHF in microgravity, even if the heat transfer coefficients were higher with respect to the smooth surface tested for comparison. The addition of an external electric field enhanced the CHF due to its vapor removal capability and the bubble breaking effect. Moreover, the combined effect of micro-fins and electric field showed a synergic effect, increasing the CHF value up to 2.2 times. The CHF increased linearly with the subcooling level. The results indicate that micro-fins enhancing action can be severely reduced by the accumulation of vapor in the proximity of the surface, due to lack of buoyancy. However, if vapor is removed with an alternative action (e.g., electric forces) the fin enhancement is restored.

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.

Boiling mechanism of biphilic surfaces based on Helmholtz instability and Taylor instability

This study demonstrated that combination hydrophilic copper and hydrophobic nano-silver surfaces simultaneously enhanced the critical heat flux (CHF) and heat transfer coefficient (HTC) in pool boiling heat transfer (BHT) with deionized water as the employed working fluid. To further investigate the influence of geometric factors on heat transfer, this study designed three patterns denoted as squares/stripes/networks, ranging from 50 μm to 3000 μm. The effects of pattern size d, spacing p, and pitch ratio p/d on heat transfer results were obtained. Based on bubble visualization, the study further analyzed the reasons for inflection points in the boiling curve and the mechanisms caused by porous hydrophobic coating. Finally, the study proposed a numerical model suitable for predicting CHF of biphilic surfaces based on Helmholtz instability and Taylor instability. The model began with vapor columns escaping toward far-field and the fluid supplied to the heating surface. Concrete model derivation and adjustments are categorized according to the relationship between pattern size and bubble detachment diameter. For surfaces where the bubble detachment diameter exceeds the pattern size, this study originated from the perspective of Helmholtz instability on the vapor column surface. The resulting model is anticipated to quantitatively solve for the former and modulate the wavelength of Taylor instability on biphilic surfaces. Regarding surfaces with bubble detachment diameters smaller than the pattern size, this research begins with the existence time of the micro-liquid layer. The derived model validates the predominant role of p/d in the CHF values on micron-scale biphilic surfaces.

Analysis of heat transfer and solidification within CANDU corium

Severe nuclear accidents which result in substantial damage to the reactor core have the potential to release radiation into the environment if the integrity of the reactor vessel is not maintained. In the event of a full loss of core cooling, the expected failure mechanism for Canada Deuterium Uranium (CANDU) calandria vessels is thermal stresses, which are influenced by the heat transfer within the corium. The current study uses computational fluid dynamics (CFD) to simulate the fluid flow, heat transfer, and crust formation in the corium using of full-scale CANDU calandria vessel. Uncertainties in the boundary conditions are considered with a sensitivity analysis. Results indicate that about 90% of the corium volume is in the mushy and solid form. The heat flux over the majority of the vessel wall is below the critical heat flux values and reaches the maximum value at the edge of the step region.

Critical heat flux, heat transfer and pressure drop at high mass fluxes for R123 in a single microchannel

This paper presents new experimental data on pressure drop, heat transfer coefficient, and critical heat flux during flow boiling of low-pressure fluid R123 in microchannels. High mass velocities were used, above the values found in the literature in tests with this fluid. Experiments were conducted in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm. The mass velocities used were between 1500 and 7500 kg/m2s. Saturation temperature ranged from 40.1 to 85.1 oC. The outlet vapor quality varied between -0.54 and 0.53 for the CHF results, between 0 and 0.5 for the pressure drop results, and between 0 and 0.35 for the HTC results, respectively. Record critical heat flux values for flow boiling of R123 in smooth microchannels were achieved, reaching 1.44 MW/m2. An analysis was made of the influence of an upstream pressure drop in avoiding instabilities during critical heat flux experiments. The highest heat transfer coefficient values during convective boiling of R123 in smooth microchannels were also obtained, with values close to 43.3 kW/m2K. The saturation temperatures used were among the highest for flow boiling of R123 in smooth microchannels found in the literature. Correlations for frictional pressure drop, heat transfer coefficient, and critical heat flux were evaluated in predicting the experimental data. An analysis was made on the influence of the void fraction correlation when estimating both the accelerational and frictional pressure drop.

Predicting Critical Heat Flux in Rectangular Channels: Which Correlation to Trust?

The PALLAS-reactor is an advanced nuclear reactor dedicated to the production of medical isotopes and (medical) nuclear technological research currently under construction in The Netherlands RELAP5 is the primary tool used for modeling the accident scenarios relevant for licensing the PALLAS-reactor. The Groeneveld Look-Up Tables (LUT) are the main Critical Heat Flux (CHF) model of RELAP5. These have been extensively validated against experiments with round tubes. However, literature suggests that the LUT over-predict CHF inside the narrow, rectangular fuel channels of research reactors such as the PALLAS-reactor. As literature abounds with CHF correlations of limited applicability and low degrees of validation and confidence, correlations that can accurately predict CHF in rectangular channels are needed. The goal of this work is twofold. First, it aims at selecting a number of promising CHF correlations and to assess these against experiments representative of the PALLAS-reactor design. Secondly, this work ascertains whether the LUT are indeed optimistic with respect to CHF in rectangular channels. To this purpose, a literature review has short-listed the Sudo-Kaminaga (S-K), the Kim-modified S-K, and the Hall–Mudawar (H-M) correlations. These correlations are implemented in the SPECTRA code developed at NRG and assessed against two experiments in rectangular channels. The Kim-modified S-K correlation fits the experiments better than the original S-K scheme for high mass fluxes, while predicting higher CHF values at low flow rates. Interestingly, the H-M correlation predicts the experiments accurately, although the correlation was developed from round tubes data. At high mass fluxes, the LUT predictions converge towards the experiments, thereby suggesting that the effect of the channel geometry becomes less important at high flow rates. At low mass fluxes, on the other hand, the LUT markedly fail to match the experiments. This work contributes to accurately estimating CHF in narrow rectangular channels, which is crucial for licensing research reactors such as the PALLAS-reactor, thus assisting in the betterment of many people’s lives.

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.

A COMBINED ACTIVE (PIEZOS) AND PASSIVE (MICROSTRUCTURING) PARTIAL FLOW-BOILING APPROACH FOR STABLE HIGH HEAT-FLUX COOLING WITH DIELECTRIC FLUIDS

Controlled but explosive growth in vaporization rates is made feasible by ultrasonic acoustothermal heating of the microlayers associated with microscale nucleating bubbles within the microstructured boiling surface/region of a millimeter-scale heat exchanger (HX). The HX is 5 cm long and has a 1 cm × 5 mm rectangular cross section that uses saturated partial flow-boiling operations of HFE-7000. Experiments use layers of woven copper mesh to form a microstructured boiling surface/region and its nano/microscale amplitude ultrasonic (~1-6 MHz) and sonic (< 2 kHz, typically) vibrations induced by a pair of very thin ultrasonic piezoelectric-transducers (termed piezos) that are placed and actuated from outside the heat-sink. The ultrasonic frequencies are for substructural microvibrations whereas the lower sonic frequencies are for suitable resonant structural microvibrations that assist in bubble removal and liquid filling processes. The flow and the piezos' actuation control allow an approximately 5-fold increase in heat transfer coefficient value, going from about 9000 W/m<sup>2</sup>-°C associated with microstructured no-piezos cases to 50,000 W/ m<sup>2</sup>-°C at a representative heat flux of about 25 W/cm<sup>2</sup>. The partial boiling approach is enabled by one inlet and two exit ports. Further, significant increases to current critical heat flux values (~70 W/cm<sup>2</sup>) are possible and are being reported elsewhere. The electrical energy consumed (in W) for generating nano/micrometer amplitude vibrations is a small percentage (currently < 3%, eventually < 1% by design) of the total heat removed (in W), which is a heat removal rate of 125 W for the case reported here.

Critical heat flux for downward flows in vertical round pipes

Pressurized thermal shock (PTS) is one of the causes of pressure vessel failure during nuclear reactor accidents. Critical heat flux (CHF) for downward flow is an essential thermohydraulic parameter for predicting PTS using nuclear reactor system codes. While many experimental data have been accumulated and many correlations have been proposed for CHF for upward flow, only a few studies exist on downward flow. This study reviewed the literature that measured CHF for downward flow in round pipes and arranged the proposed correlations. Then, we extracted the parameter values that determine experimental conditions and CHF values from the literature and compared the CHF values predicted from the correlation in each study and the measured CHF values under those conditions across the literature. Each correlation shows relatively good prediction accuracy (average prediction accuracy of 10%–30%) for experimental data from their literature, but the accuracies sometimes decrease for experimental data from other literature. No correlation accurately predicts all the experimental data of the literature, indicating an issue in extrapolating existing correlations. Therefore, we developed a correlation that can accurately predict the experimental data of the collected literature. First, we used a neural network to select the essential dimensionless quantities that comprise the correlation. Then, we regarded the prediction accuracy when all candidate dimensionless quantities extracted from the literature were used for the input variables of the network as the achievable limit prediction accuracy and searched for the minimum combination of dimensionless quantities required to achieve it. The results showed that only the dimensionless mass flux (G*) and the ratio of the heating length to the channel diameter (L/D) are the essential parameters to achieve it. We developed a correlation equation using these two dimensionless quantities and achieved 17.6% of the average prediction accuracy. This result considerably improved existing correlation equations with 25%–40% average prediction accuracy for the same experimental data. Furthermore, unlike many existing correlations, it has excellent practical features, such as applicability over wide parameter ranges and no need to separate cases.