Critical Heat Flux Data Research Articles

An experimental study of the CHF in a square channel with non-uniformly heated rod under low pressure and low flow conditions

Experimental research on critical heat flux (CHF) was carried out in a vertical square channel with non-uniformly heated rod at low pressure and low flow conditions. The influence of inlet subcooling on CHF was also investigated. The results showed that the CHF position of the non-uniformly heat flux distribution mainly appeared in the middle and lower part of the heater rod. More significantly, it was found that CHF was enhanced with the rise of the inlet subcooling. The CHF data measured in this experiment were also assessed with the commonly used CHF empirical correlations suitable for low pressure and low flow. The results indicated that the general correlations predicting for CHF of uniform heat flux distribution were not applicable for prediction CHF of non-uniform heat flux distribution and these correlations usually overestimated the experimental values. In addition, The F-factor approach proposed by Tong was used to predict the experimental data and the error predicted by F-factor were mostly between 30% and 50%. Finally, F-factor was modified based on experimental data and the error between the predicted value and experimental results was between 0 and 30 %, and most of the data was between 0 and 20 %.

Saturated/subcooled flow boiling heat transfer characteristics for R134a, HFE-7100 and Deionized water inside micro/mini-channels: Part Ⅱ – New CHF prediction model establishment

A new flow pattern map has been developed and presented in Part Ⅰ of this paper for R134a, HFE-7100 and Deionized water inside micro/mini-channels. To further explain the phenomenon and understand the mechanism of triggering critical heat flux (CHF), the theoretical model to predict the CHF of saturated/subcooled flow boiling inside the micro/mini-channels is proposed based on the liquid layer dry-out and Helmholtz instability (intermittent and continuous vapor blanket) theories. Both the effect of the dry-out flows and the liquid sublayer under the vapor blanket were taken into consideration. For saturated and subcooled boiling, the two mechanisms of triggering the CHF were revealed based on different flow patterns and experimental phenomena, respectively. The boiling heat transfer performance has been experimentally investigated under different liquid subcoolings, mass fluxes and geometry parameters. By analyzing the previous models and establishing a database, the comparison of new model predictions with present experimental data shows good agreement within a standard deviation of ± 10 %. Besides, prior CHF data were used to verify this new model, which also presents varying degrees of success in predicting the CHF.

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.

Critical heat flux of liquid hydrogen, liquid methane, and liquid oxygen: a review of available data and predictive tools

Available experimental data dealing with critical heat flux (CHF) of liquid hydrogen (LH2), liquid methane (LCH4), and liquid oxygen (LO2) in pool and flow boiling are compiled. The compiled data are compared with widely used correlations. Experimental pool boiling CHF data for the aforementioned cryogens are scarce. Based on only 25 data points found in five independent sources, the correlation of Sun and Lienhard (1970) is recommended for predicting the pool CHF of LH2. Only two experiments with useful CHF data for the pool boiling of LCH4 could be found. Four different correlations including the correlation of Lurie and Noyes (1964) can predict the pool boiling CHF of LCH4 within a factor of two for more than 70% of the data. Furthermore, based on the 19 data points taken from only two available sources, the correlation of Sun and Lienhard (1970) is recommended for the prediction of pool CHF of LO2. Flow boiling CHF data for LH2 could be found in seven experimental studies, five of them from the same source. Based on the 91 data points, it is suggested that the correlation of Katto and Ohno (1984) be used to predict the flow CHF of LH2. No useful data could be found for flow boiling CHF of LCH4 or LO2. The available databases for flow boiling of LCH4 and LO2 are generally deficient in all boiling regimes. This deficiency is particularly serious with respect to flow boiling.

DNBR Limit Analysis for the Inner channel of Annular Fuel

The departure from nucleate boiling ratios (DNBR) limit for annular fuel might vary from that of conventional nuclear fuel rod assemblies due to the distinct features of annular fuel assemblies. Precise estimation of DNBR is crucial for ensuring the safe and optimal functioning of nuclear reactors equipped with annular fuel assemblies. This study examines the DNBR limit of inner channel for annular fuel, using the look up table-2006 (LUT-2006).A comprehensive database having data for both normal operation and accident situations in pressurized water reactors (PWRs) using annular fuel was built to analyze the DNBR limit of the inner channel of annular fuel. The critical heat flux (CHF) data in this database were categorized into two groups with the limiting quality as the boundary. The prediction accuracy of LUT-2006 and the DNBR limits were evaluated respectively. Given that the inner diameter of the inner channel of annular fuel in mainstream designs typically ranges from 8-9 mm, and considering the potential occurrence of the limiting quality phenomenon in the inner channel, it would be more suitable to utilize the LUT-2006 based on an 8 mm tube standard to establish the DNBR limit for the inner channel of annular fuel. A conservative DNBR limit of 1.308 is determined for the inner channel of an annular fuel using the LUT-2006. In future thermal-hydraulic optimization designs of annular fuel, it is advisable to utilize LUT-2006 for calculating the DNBR of the inner channel of the fuel.

Mechanistic model of critical heat flux in rod bundles based on a high-precision subchannel code

A mechanistic model for predicting critical heat flux (CHF) in rod bundles is developed based on the high-precision subchannel code ATHAS. To account for the non-uniform distribution of quality in subchannels caused by the presence of mixing vane grids (MVGs) and guide tubes (GTs), further subdivision of conventional subchannels and the incorporation of MVG crossflow models are implemented in ATHAS to provide a more detailed local flow field for the CHF model. The CHF mechanism model is developed based on the concepts of bubble crowding and assumption of local phenomenon. An equivalent-tube concept is proposed to convert the turbulent intensity distribution on the tube surface to that on the rod bundle surface. The present model is implemented in ATHAS and evaluated against CHF experimental data of rod bundles under the conditions of interest for pressurized water reactors (PWRs). The predicted results show good agreement with the experimental CHF data, with an average error and root mean square of 0.99% and 4.69%, respectively, for a total of 601 data points. Furthermore, owing to the use of the subchannel subdivision method, the present model does not require any cold wall effect correction and can accurately predict the radial position of CHF.

A mechanistic model to predict saturated pool boiling Critical Heat Flux (CHF) in a confined gap

Boiling enables high rates of heat transfer from a surface made possible at a relatively low thermal resistance motivating the use of two-phase cooling for increasingly compact thermal management solutions. However, extreme geometrical confinement of the liquid above the boiling surfaces is known to have detrimental effects on maximum heat transfer rate by inducing premature onset of film boiling. Moreover, previously developed critical heat flux (CHF) models for confined geometries focused on triggering mechanisms associated with unconfined pool boiling and, thus, are not generalizable. This work proposes a new mechanistic model for predicting CHF during boiling within in narrow gap, specifically developed to account for confinement effects on the triggering mechanism. The model postulates that occurrence of CHF coincides with the irreversible growth of a dry spot on the boiling surface. Three competing forces govern the two-phase interface dynamics, namely vapor momentum, surface tension, and hydrostatic forces. Dryout is triggered when the vapor momentum force due to vaporization at the two-phase interface balances the combined surface tension and hydrostatic forces leading to irreversible growth of the dry spot. The present work offers a predictive confined CHF model that accounts for confined boiling surface shape, size, orientation, confinement gap spacing, and working fluid properties, with a single fluid-specific fitting coefficient that represents the ratio of vapor area to the confinement opening area near CHF conditions. Notably, the developed CHF model is also effective in predicting the threshold gap below which confinement reduces pool boiling CHF. The model is compared to 197 experimentally measured confined CHF data points available from 10 studies in the literature that represent 7 different working fluids and a range of boiling surface inclinations and shapes. The model predicts the confinement-reduced CHF values with a root mean square error of 21%, which is less than half of the error compared to all other available predictive models. This clarification of the triggering mechanism and improved prediction accuracy of CHF, as offered by the current study, will enable broader practical system implementation of compact two-phase cooling technologies.

Experimental Analysis of Bubble Behavior and Critical Heat Flux During Pool Boiling on Vertical Circular Tubes

Abstract The heat transfer during pool boiling incorporates a higher rate of heat dissipation capability at low-temperature differences. This technique is widely used in the nuclear industry for thermal management. In this study, the effect of tube diameter and length on critical heat flux (CHat atmospheric conditions in saturated water during pool boiling) is analyzed. The tubes of SS 304 are kept in the vertical orientation. The diameter of the tubes ranges from 1.2 mm to 9 mm. The experiments are performed with tubes having lengths varying from 50 mm to 1000 mm. It has been noted that tubes of smaller diameter show a greater magnitude of critical heat flux (CHF) for the given length. Compared to other tubes, the magnitude of CHF for a 1.2 mm diameter is higher. For a given diameter, a longer tube is found to have lower CHF than the ones having lesser length. The variation in the CHF magnitude is negligible for tubes with a diameter of more than 2.5 mm beyond a length of 200 mm. The location of occurrence of CHF is near the bottom end of the vertical tube. The study illustrates the behavior of bubble nucleation for various tube dimensions and heat fluxes. The inception and detachment of bubbles for different tubes are analyzed. The pool boiling regime is categorized and studied basing the behavior of the incepted and departed bubble while maintaining uniform heat flux. A mathematical relation that empirically accounts for the effect of tube dimensions , i.e., length and diameter on pool boiling CHF is proposed. The experimental CHF data obtained during pool boiling are tabulated toward contributing to the CHF databank.

Enhanced liquid replenishment for pool boiling heat transfer and its CHF mechanism on patterned freeze-casted surfaces

In the current investigation, the pool boiling heat transfer process of patterned porous surfaces, as well as a smooth surface, in ethanol was examined from an academic perspective. The freeze casting (also known as ice-templating) processes was used to fabricate the structured surfaces. It was discovered that freeze-casted coatings significantly improved the performance of boiling heat transfer in comparison to the smooth surface (SMS). The most remarkable increase in Critical Heat Flux (CHF) was achieved by checkerboard porous surface(CHECKERBOARD), with an improvement of 81.6% relative to that of SMS. The maximum Heat Transfer Coefficient (HTC) enhancement was obtained by full porous surface (FULLY-COVERED), with an HTC value 215.8% higher than that of the SMS. For the semicircular porous surface (SEMICIRCULAR), striped porous surface (STRIPED), annular porous surface (ANNULAR) and checkerboard porous surface(CHECKERBOARD) with 50% coating coverage, CHECKERBOARD demonstrated the best heat transfer performance, with CHF and HTC increased by 32.3% and 48.9%, respectively, compared to the SEMICIRCULAR surface. The mechanism of liquid supply at the CHF of the patterned porous surfaces was investigated through phenomenological observations of the boiling phenomena. A modified model, accounting for the coalesced bubble departure frequency and capillary wicking effects, was proposed for CHF prediction. It is noteworthy that the fluid replenishment considers both vertical and lateral replenishment of porous coatings. The CHF data from this study, along with existing literature, were utilized to validate the model, and the predicted results exhibited good agreement with the experimental data, with errors within ±8%.

Experimental investigation of helical fin and inclination effect on critical heat flux

Recently, diverse marine applications of nuclear power are being considered to meet the carbon reduction goals in the maritime industries. One of the distinctive characteristics of marine nuclear reactors is the limit in space and it requires high power density. To address this problem, tight lattice bundle with a helical shaped non-circular fuel rods can be employed. However, the critical heat flux (CHF) mechanisms for such alternative fuel rods have not been adequately investigated experimentally. In particular, no open-access CHF experiment under inclined and motion conditions in accordance with marine conditions has yet been reported. Thus, in this study, the CHF on a helical-finned heater was measured under static vertical and inclined conditions. Considering the reactor operating conditions, a high-pressure experiment was performed using refrigerant R134a. The fin and inclination effects were evaluated based on a comparison between the results of this study and the CHF data for a bare-rod under inclined condition obtained from a previous study. In addition, the inclination effect was analyzed phenomenologically based on the wall temperature behavior. The findings in this work can contribute to designing a marine nuclear reactor core by quantifying the effects of inclination on the CHF.

Data consolidation, correlations assessment, and new correlation development for pool boiling critical heat flux specific to cryogenic fluids

A common and crucial process for cryogenic fluid management is the need to fill the cryogenic fluids into warmer storage tanks efficiently while minimizing the evaporative and entrainment losses. However, no good predictive tool that can estimate the heat transfer behavior accurately during this fill process is available in the literature. In this study, we start with developing a new generalized correlation set for predicting critical heat flux (CHF), an important parameter on the boiling curve. First, a new pool boiling test facility is built to generate CHF data using a flat plate geometry. Based on our assessment of prior pool boiling data and correlations, 30 CHF datapoints in three different orientations with respect to gravity are obtained and validated against the existing CHF data from published literatures. Second, our experimental data is then combined with worldwide data for cryogenic fluids to generate a consolidate database that includes 2205 datapoints for 6 cryogenic fluids for 3 boiling surfaces (flat plate, cylinder and sphere). Third, a comprehensive assessment of prior pool boiling CHF correlations is conducted and it is observed that no one correlation or correlation form can capture all important parametric effects important to CHF. Fourth, a new correlation model for flat plate is setup that gave a mean absolute error (MAE) of 15.96% based on 1259 datapoints. In addition, multipliers to the flat plate correlation form are used to predict CHF for curved surfaces and gave MAE of 22.38% based on 355 datapoints and MAE of 17.16% based on 177 datapoints for cylinders and spheres, respectively. Values of CHF based on quenching is observed to be smaller than CHF based on boiling for the same test conditions regardless of the boiling surface material. Finally, a parameter to account for the thickness & thermal properties of the quenching surface is generated and added to the original correlation form and gave an MAE of 14.05% based on 245 datapoints.

Parametric experimental trends, interfacial behavior, correlation assessment, and interfacial lift-off model predictions of critical heat flux for microgravity flow boiling with subcooled inlet conditions – Experiments onboard the International Space Station

This study examines the critical heat flux data obtained during the “Flow Boiling and Condensation Experiment (FBCE)” which was launched to the International Space Station (ISS) in August 2021. The overarching goals of FBCE are to obtain flow boiling and flow condensation data in high quality, long-duration microgravity, and investigate gravitational effects on two-phase flow physics. The first stage of FBCE completed features the Flow Boiling Module (FBM), which collected flow boiling heat transfer data and flow visualization images in a highly accurate and steady microgravity environment from February 2022 until July 2022. Flow boiling experiments were performed with subcooled inlet of n-Perfluorohexane in a rectangular channel of dimensions 114.6-mm heated length, 2.5-mm heated width, and 5.0-mm height with either one or two, opposite, heated walls. The long-duration microgravity database encompasses a broad range of operating conditions: mass velocity of 199 – 3200 kg/m2s, inlet subcooling of 2.6 – 45.6°C, and inlet pressure of 124.8 – 176.7 kPa. Image sequences leading up to and at CHF are presented to both illustrate the physical mechanism triggering CHF and explain the experimental trends observed in microgravity. Experimental results show CHF is strongly dependent on mass velocity and higher degrees of inlet subcooling, but less dependent on inlet pressure in the tested ranges. Examination of the relationships and parametric trends between dimensionless groups governing CHF reinforces conventional trends and reveals, for subcooled CHF, a dependence of Boiling number at CHF on outlet thermodynamic equilibrium quality. Comparison of the new microgravity CHF data with Earth-gravity CHF data reveals, for single-sided heating, CHF in microgravity is degraded up to ∼38% at low mass velocity, with diminishing differences as mass velocity is increased, and for double-sided heating, less significant differences in CHF between the two gravitational environments. Experimental data are compared to predictions of various flow boiling CHF correlations which previously demonstrated their merit, and the most suitable one for the entire database is recommended. Flow visualization reveals a wavy vapor layer with wetting fronts described in the Interfacial Lift-off Model, which is used to predict CHF values with good accuracy.

A mechanistic model of critical heat flux for pool boiling based on supply failure mechanisms depending on the contact angle

Experimental and theoretical analyses are conducted to develop a mechanistic model of critical heat flux (CHF) for application to pool boiling of fluids with contact angles between 0° and 180°. To accurately predict the CHF over the entire range of the contact angle, it is postulated that an unquenchable dry patch occurs when liquid fails to be supplied to the dry patch; A supply failure mechanism is classified into two different mechanisms depending on the wettability of the boiling surface. The first is an evaporation mechanism which is dominant for non-wetting surfaces. This type occurs when the liquid supply rate in the liquid film due to the capillary force becomes lower than the evaporation rate. The second mechanism is a levitation mechanism which is dominant for wetting surfaces. This type occurs when the surface temperature under the dry patch exceeds the Leidenfrost point. To check the validity of the postulate, experiments involving the pool boiling of water and FC-72 are conducted on a titanium surface, which is a non-wetting surface for water but a wetting surface for FC-72. In the experiments, the temperature and heat flux distributions on the boiling surface are measured using infrared thermography for a quantitative characterization of boiling at the CHF. Experimental analyses confirm the postulate by showing that the CHF occurs due to the evaporation mechanism for the pool boiling of water, whereas the levitation mechanism for the pool boiling of FC-72. It is also observed that a large area of liquid film exists for the pool boiling of water, whereas this liquid film hardly exists for the pool boiling of FC-72. This observation indicates that the change in the dominant mechanism is due to the dependence of the liquid film area on the wettability of the boiling surface. After validating the postulate, a CHF correlation is developed that reflects the change of the supply failure mechanism depending on the contact angle. The developed correlation shows a good match to the extensive sets of CHF data previously obtained by various investigators to within 20%.

Prediction of Critical Heat Flux of Mixtures Flowing in Channels

Boiling of mixtures in channels is involved in many chemical processes, and refrigerant mixtures are finding a rapidly increasing use in the refrigeration industries. Hence, reliable prediction of CHF (critical heat flux) is needed. There is no published method that has been verified over a wide range of parameters. In this study, available data for CHF in tubes and annuli were compared to general correlations for tubes and annuli, which are well-verified for single-component fluids. This paper presents the results of this comparison. It was found that general correlations for single-component fluids were in satisfactory agreement with data for mixtures. The data included mixtures of refrigerants and chemicals, reduced pressure 0.02 to 0.8597, mass flux 200 to 3798 kgm−2s−1, temperature glide 2.1 to 21.9 K, and critical quality −0.46 to 0.99. The data for annuli were predicted with a MAD (mean absolute deviation) of 13.6%, and the data for tubes had a MAD of 13.9% when compared to the Shah correlations for single-component fluids. Based on the results of the present data analysis and results of research on pool boiling, a range is identified in which pure fluid correlations can be used for mixtures. This will be useful in the design of heat exchangers involving the boiling of mixtures in channels.

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%.

Critical heat flux correlations for tube and annulus geometries under inclination and rolling conditions

In floating nuclear reactors, critical heat flux (CHF) prediction is crucial because the CHF can be influenced under heaving, rolling, and inclination conditions, unlike in land-based reactors. Hence, the existing land-based empirical correlations of the CHF can be invalid under some conditions in marine reactors. Therefore, empirical correlations of the flow boiling CHF were developed in this study to predict the CHF under inclination and rolling motions in tubes and annuli. Available experimental CHF data (616 points) were collected including our previous experimental data (405 points) and their trends under marine conditions were analyzed. The CHF variation was expressed using two types of modified Froude numbers depending on the CHF regime—departure from nucleate boiling and dryout. These parameters were used in correlations for the CHF under inclination. CHF correlations under rolling conditions were developed. Rolling was found to attenuate the CHF variation through the inclination. This observation was reflected in the newly developed rolling CHF correlation. The proposed correlations adequately described the existing CHF experimental data under inclination and rolling within an uncertainty of 9.5%. The developed CHF correlations can be used for the safety analysis and reactor core thermal–hydraulic design of marine reactors.

On the Critical Heat Flux Assessment of Micro- and Nanoscale Roughened Surfaces.

The boiling crisis or critical heat flux (CHF) is a very critical constraint for any heat-flux-controlled boiling system. The existing methods (physical models and empirical correlations) offer a specific interpretation of the boiling phenomenon, as many of these correlations are considerably influenced by operational variables and surface morphologies. A generalized correlation is virtually unavailable. In this study, more physical mechanisms are incorporated to assess CHF of surfaces with micro- and nano-scale roughness subject to a wide range of operating conditions and working fluids. The CHF data is also correlated by using the Pearson, Kendal, and Spearman correlations to evaluate the association of various surface morphological features and thermophysical properties of the working fluid. Feature engineering is performed to better correlate the inputs with the desired output parameter. The random forest optimization (RF) is used to provide the optimal hyper-parameters to the proposed interpretable correlation and experimental data. Unlike the existing methods, the proposed method is able to incorporate more physical mechanisms and relevant parametric influences, thereby offering a more generalized and accurate prediction of CHF (R2 = 0.971, mean squared error = 0.0541, and mean absolute error = 0.185).

Experimental evaluations of the critical heat flux in terms of the heater dimensions, orientation, and surface morphology

Pool boiling experiments to evaluate the critical heat flux (CHF) have been conducted for decades. Although experimental results for the analyses of a myriad of parameters affecting the CHF have been reported steadily, the CHF results have a large discrepancy between them. This study attempts to acquire reliable CHF data by improving the methodology for pool boiling experiments in a direct heating method. In particular, contact resistance is carefully managed to attain the reliable data in every case of experiments. The experiments are performed with various parameters, namely, heater dimensions, orientation and surface morphology. The dimension effect on the CHF is evaluated with stainless steel heaters with widths of 10, 30, and 50 mm and lengths of 10, 30, and 100 mm. With the suggested size of the heaters for the practical application to real industry, the CHF is evaluated considering a change in the surface process: polishing with sandpaper, wire brushing, and buffing. As a result, unique CHF trends are observed, including CHF deterioration, and surface analyses are conducted using surface analysis instruments. Last, the effects of orientation (0° ~ 180°) on the CHF are evaluated, and the results are compared with the extant correlations and experimental data.

Machine learning prediction of critical heat flux on downward facing surfaces

Application of external reactor vessel cooling (ERVC) in-vessel retention (IVR) was an important measure to ensure the integrity of the lower head of a reactor pressure vessel (RPV). As a typical boiling heat transfer process, the critical heat flux (CHF) plays a crucial role in the safety margin of a RPV's IVR-ERVC strategy. Although there have been a lot of correlations and experiments about the CHF of pool boiling on downward facing surfaces, their application range was relatively limited. To further expand the usability, machine learning was introduced in this research after collecting most accessible CHF data of pool boiling on downward facing surfaces. Considering the small amount of these experimental data, some pseudo data obtained by fitting the existed experimental data were added. Three machine learning methods, the ε-support vector machine (ε-SVM), back propagation neural network (BPNN) and random forest were used to predict CHF. Among the three methods, ε-SVM provided the best accuracy in the prediction. The effects of orientation, surface dimensions, heating surface material and pressure on the downward facing surface boiling crisis were predicted by ε-SVM, the results showed that in general, the CHF increased with the increase of orientation angle, and increased with the pressure from 1 bar to 10 bar. Under atmospheric pressure, the CHF decreased with increasing the width of the heating surface, but there was a width value that could eliminate the influence of width. In addition, the CHF seemed to increase with the increase of thermal effusivity. However, there are still some inexplicable phenomena that need to be revealed by further research. Overall, this method is expected to be widely used in predicting the CHF of pool boiling on downward facing surfaces.

Experimental investigation on flow boiling characteristics of HFO-1234yf in a 0.5 mm microchannel

The flow boiling heat transfer of environmentally friendly refrigerant HFO-1234yf in a horizontal circular microchannel is experimentally investigated. The inner diameter and heating length of the test channel are 0.5 mm and 300 mm, respectively. Experiments are conducted at relatively high mass flux ranging from 750 to 1500 kg m−2s−1, heat flux from 25 to 85 kW m−2 and inlet pressure between 0.604 and 0.634 MPa. The heat flux is stepwise imposed to the test channel until dryout occurs. The flow boiling heat transfer characteristics are analyzed in detail. In the pre-dryout region, the heat transfer coefficient (HTC) is found to be mainly determined by the heat flux, while the effect of mass flux is weak. Two different HTC behaviors are observed at low and high mass fluxes, which can be attributed to the prominent effect of thin liquid film evaporation on heat transfer and the occurrence of partial dryout. Moreover, the oscillation of mass fluxes and wall temperatures are observed at high mass flux conditions, resulting in heat transfer deterioration. In the post-dryout region, the critical heat flux (CHF) is found to increase with mass flux, while the critical vapor quality would decrease. Finally, the experimental data of HTC and CHF are compared with correlations from the literature and good agreements are presented.