Experimental Critical Heat Flux Data Research Articles

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.

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.

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.

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.

Developing a robust and flexible smart tool to predict a full range Critical Heat Flux (CHF) in different LWRs by using deep learning Artificial Neural Networks (ANN) via parallel multi-processing

In this paper, a practical smart tool has been developed to predict CHF in LWRs. The tool is based on the strengthen cascade feed forward ANNs in addition of some of the deep learning features. The most complete experimental CHF data available for LWRs are chosen as the training and test data sets. Input layer can get different thermodynamic qualities, mass flow rates, and pressures to predict the CHF. Then it gains a full range CHF simulator for LWRs fortunately covering both anticipated and unknown transients. Although training such a huge ANN structure and the huge training data set is time consuming, but after the training it acts fast especially via a multi-processing platform. Results are quite satisfactory due to the very good network performance and regressions. Briefly, this is a fast, flexible, and robust tool to predict the CHF deep inside the physics of the most broadening CHF data set ever published for the LWRs.

Investigation of the critical heat flux in small-diameter channels

The paper describes experimental studies into the hydrodynamics and heat exchange in a forced water flow in small-diameter channels at low pressures. The timeliness of the studies has been defined by the growing interest in small-size heat exchangers. Small-diameter channels are actively used in components of compact heat exchangers for present-day engineering development applications. The major difficulty involved in investigation of heat-transfer processes in small-diameter channels consists in the absence of common methodologies to calculate coefficients of hydraulic resistance and heat transfer in a two-phase flow. The channel size influences the heat exchange and hydrodynamics of a two-phase flow as one of the determining parameters since the existing internal scales (vapor bubble size, liquid droplet diameter, film thickness) may become commensurable with the channel diameter, this leading potentially to different flow conditions. It is evident that one cannot justifiably expect a change in the momentum and energy transfer regularities in single-phase flows as the channel size is reduced for as long as the continuum approximation remains valid. The authors have analyzed the experiments undertaken by Russian scientists to investigate the distribution of thermal-hydraulic parameters in channels with a small cross-section in the entire variation range of the flow parameters in the channel up to the critical heat flux conditions when the wall temperature increases sharply as the thermal load grows slowly. The experimental critical heat flux data obtained by Russian and foreign authors has been compared.

Critical Heat Flux (CHF) Correlations for Subcooled Water Flow Boiling at High Pressure and High Heat Flux

The subcooled water flow boiling is beneficial for removing the high heat flux from the divertor in the fusion reactor, for which an accurate critical heat flux (CHF) correlation is necessary. Up to now, there are many CHF correlations mentioned for subcooled water flow boiling in the open literatures. However, the CHF correlations’ accuracies for the prediction of subcooled water flow boiling are not satisfactory at high heat flux and high pressure for reactor divertor. The present paper compiled 1356 CHF experimental data points from 15 independent open literatures and evaluated 10 existing CHF correlations in subcooled water flow boiling. From the evaluation, the W-2 CHF correlation performs best for the experimental CHF data in all existing critical heat flux correlations. However, the predicted mean absolute error (MAE) of the W-2 correlation is not very ideal for all database and the MAE of the W-2 correlation is from 30% to 50% for some database. In order to enhance the CHF prediction accuracy in subcooled water flow boiling at high heat flux and high pressure, the present paper developed a new CHF correlation. Compared with other existing CHF correlations, the new CHF correlation greatly enhances the prediction accuracy over a broad range of pressures and heat fluxes which are desired in the cooling of high heat flux devices, such as those in the fusion reactor divertor. The validation results show that the new correlation has a MAE of 10.05% and a root mean squared error (RMSE) of 16.61%, predicting 68.1% of the entire database within ±10% and 81.5% within ±15%. The MAE of the new CHF correlation is 7.4% less than that of the best existing one (W-2 correlation), further confirming its superior prediction accuracy and reliability. Besides, the new CHF correlation works well not only for a uniform power profile but also for a non-uniform power profile in subcooled water flow boiling at high pressure and high heat flux.

Universal Critical Heat Flux (CHF) Correlations for Cryogenic Flow Boiling in Uniformly Heated Tubes

Experiments to determine critical heat flux (CHF) for cryogen flow in uniformly heated, round tubes have been performed throughout the globe during the past sixty years. However, experimental CHF data for cryogens are rarely published, remaining in the archives of authors, or in obscure technical reports of an organization or other inaccessible sources. In the present study, the Purdue University-Boiling and Two-Phase Flow Laboratory (PU-BTPFL) Cryogen Flow Boiling CHF Database is consolidated from world literature dating back to 1959. With 2312 data points for LH2, LHe, LN2 and LCH4, it represents the largest cryogen CHF database assembled to date. The database encompasses diameters from 0.5 to 14.1 mm, critical length-to-diameter ratios from 2.5 to 230.8 (term ‘critical’ refers to axial location where CHF is detected), system pressures from 0.01 to 4.07 MPa, reduced pressures from 0.1 to 0.93, mass velocities from 2.2 to 8203.9 kg m−2 s−1, inlet subcoolings from 0 to 78.9 K, inlet qualities from -2.06 to 0.95, critical subcoolings from 0 to 48.5 K, critical qualities from -1.23 to 1.00, critical void fractions from 0 to 1, and CHF values from 0.05 to 8203.9 kW m−2. The consolidated database represents an invaluable tool for development of CHF correlations – a primary goal for the present study – as well as future analytic and computational models. Using this database, new universal CHF correlations are constructed for two distinct CHF mechanisms, Departure from Nucleate Boiling (DNB) and Dryout, after careful physics-based segregation of the data. With mean absolute errors below 30%, the new CHF correlations are shown to provide good predictive agreement with the database.

Experimental correlation for critical heat flux in helical coils

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

Analysis of the enhancing mechanism in pool boiling heat transfer through wetting speed for rough aluminum surfaces with FC-72

This study focused on the critical heat flux (CHF) characteristics in pool boiling heat transfer for various rough surfaces with FC-72. In particular, the enhancing effect of the CHF according to the direction of roughness was verified. Random rough surfaces with various roughness range were fabricated by the polishing process, and a unidirectional rough surface (URS) with hairlines on the surface was prepared for comparison. We proposed a CHF prediction model to analyze the enhancement of CHF on rough surfaces. The proportional relationship between the increase in CHF and the difference between the areal wetting speed on a rough surface and on a smooth surface was verified. The experimental CHF data were located in the median range of the proposed model, which was defined using the range of the Taylor instability wavelength in the Zuber model. In addition, the proposed model can predict the CHF for URS sample using the average of the areal wetting speed in both parallel and normal directions.

Experimental characterization and modeling of critical heat flux with subcooled foaming solution

Boiling relies on the latent heat of vaporization to dissipate large heat fluxes within small temperature budgets, i.e., demonstrates high heat transfer coefficient (HTC). Addition of surfactants in water is a commonly used HTC enhancement technique during boiling. However, the vapor-crowding of the heater surface due to the foaming nature of solution induces a premature boiling crisis. Vapor-crowding also renders typical wettability improvement techniques futile for critical heat flux (CHF) enhancement. Resulting low CHF values limit the use of surfactants to low heat flux applications. Here we perform experiments to show that subcooling induced condensation of rising bubbles can be used to suppress vapor-crowding and enhance the CHF in comparison to saturated boiling with foaming solutions. We include the contribution of effective vapor-removal due to the condensation of rising bubbles in the model for saturated boiling to successfully predict the experimental CHF data over a wide range of subcooling. An enhancement of ≈3.5× in CHF in comparison to saturated condition was observed at 40°C subcooling. We further show that wettability improvement techniques such as nanostructuring are ineffective for CHF enhancement even under subcooled conditions. However, at any subcooling, preferential nucleation within microchannels followed by forced coalescence of otherwise non-coalescing bubbles forms separate vapor-removal and liquid-supply pathways to enhance the CHF (≈15−35%) in comparison to the baseline surface without microchannels. Furthermore, we use these new physical insights to revisit pool boiling with pure and subcooled water to propose an accurate mechanistic model for CHF data in literature. We believe that the physical insights presented in this work can be used to optimize the design of heater surfaces to further improve CHF with foaming solutions.

A coupled wicking and evaporation model for prediction of pool boiling critical heat flux on structured surfaces

Boiling is an effective heat transfer mechanism that is central to a variety of industrial processes including electronic systems, power plants, and nuclear reactors. Micro-/nano-structured surfaces have been demonstrated to significantly enhance the critical heat flux (CHF) during pool boiling, but there is no consensus on how to predict the structure-induced CHF enhancement. In this study, we develop an analytical model that takes into consideration key mechanisms that govern CHF during pool boiling on structured surfaces, namely, capillary wicking and evaporation of the liquid layer underneath the bubble. The model extends existing wicking-based CHF theories by introducing the competing evaporation mechanism. The model reveals a wicking-limited regime where CHF increases monotonically with the wicking flux, and an evaporation-limited regime where additional increases in the wicking flux do not significantly affect CHF. The model predictions are shown to agree with experimental CHF data from the literature for boiling of water on surfaces structured with square micropillar arrays. A parametric study is performed for such micropillared surfaces and has identified the optimal structure based on the competition between wicking and evaporation, capillary pressure and viscous resistance, and conduction and liquid-vapor interfacial resistance.

Experimental Investigation on Critical Heat Flux for Water Flowing in a Vertical Uniformly Heated Rifled Tube under Near-critical Pressures

This study experimentally investigated the critical heat flux (CHF) of departure from nucleate boiling (DNB) of water flowing under near-critical pressures in a 2 m-long vertical upward rifled tube with the size of ϕ35 × 5.67 mm. Operating conditions included pressures of 18–21 MPa, mass fluxes of 475–1000 kg·m−2·s−1, inlet subcooling temperatures of 3–5°C, and wall heat fluxes of 40–960 kW·m−2. Tube wall temperature distribution and heat transfer performance in different test conditions were obtained. The effects of the operating parameters on CHF were analyzed. Four heat transfer coefficient correlations were evaluated against our experimental data for further investigation of the two-phase heat transfer characteristics. A heat transfer correlation based on Martinelli number utilized in two-phase region and two empirical correlations used to predict the CHF in rifled tube at near-critical pressures were proposed. Meanwhile, experimental CHF data in rifled tube were compared with six widely used correlations and a CHF look-up table. The CHF enhancement effect in rifled tube is obvious as compared with the CHF data in smooth tube. Results show that DNB occurs at low vapor quality and subcooled region in the rifled tube at near-critical pressures. The increase in pressure leads to the early occurrence of DNB and the decrease in CHF, whereas the increase in mass flux delays the occurrence of DNB and results in the increase in CHF. DNB presents a tendency to move toward the inlet of the rifled tube. At individual operating conditions, DNB and dryout coexist at different sections of the rifled tube.

Corrections for the Hydrodynamic Instability-Based Critical Heat Flux Models in Pool Boiling—Effects of Viscosity and Heating Surface Size

This paper presents corrections for existing hydrodynamic instability-based critical heat flux (CHF) models in pool boiling by taking into account the effect of the viscosity, geometry and size of the liquid–vapor interface. Based on the existing literature, the Kelvin–Helmholtz (KH) theory, used by the most commonly adopted CHF models, can lead to noticeable errors when predicting the instability conditions. The errors are mainly due to the inaccuracy of the inviscid flow assumptions and the oversimplification of the interface geometry. In addition, the literature suggests the most unstable condition predicted by the viscous correction for viscous potential flow (VCVPF) theory for the cylindrical interfaces best match the observed air column breakup conditions in water. In this paper, the most unstable instability conditions predicted by the VCVPF theory are used to correct the existing CHF models. The comparison between the existing and corrected CHF models suggests that the corrected models always predict a higher CHF value. In addition, the corrected Zuber model predicts similar CHF value to the Lienhard and Dhir model. The comparison with experimental data suggests that the correction to the Zuber model can increase its prediction accuracy in most cases, but not necessary for the Lienhard and Dhir model. When compared to experimental CHF data for boiling cryogens at different pressures, the corrected CHF models are consistently more accurate than the original CHF models.

Presentation and comparison of experimental critical heat flux data at conditions prototypical of light water small modular reactors

The critical heat flux (CHF) is a two-phase flow phenomenon which rapidly decreases the efficiency of the heat transfer performance at a heated surface. This phenomenon is one of the limiting criteria in the design and operation of light water reactors. Deviations of operating parameters greatly alters the CHF condition and must be experimentally determined for any new parameters such as those proposed in small modular reactors (SMR) (e.g. moderate to high pressure and low mass fluxes). Current open literature provides too little data for functional use at the proposed conditions of prototypical SMRs.This paper presents a brief summary of CHF data acquired from an experimental facility at the University of Wisconsin-Madison designed and built to study CHF at high pressure and low mass flux ranges in a 2×2 chopped cosine rod bundle prototypical of conceptual SMR designs. The experimental CHF test inlet conditions range from pressures of 8–16MPa, mass fluxes of 500–1600kg/m2s, and inlet water subcooling from 250 to 650kJ/kg. The experimental data is also compared against several accepted prediction methods whose application ranges are most similar to the test conditions.

Development of a dry patch model for critical heat flux prediction

The concern regarding passive safety systems and corium-cooling related to severe accidents has been raised following the Fukushima accident. As a critical heat flux (CHF) is a predominant restricting factor of the heat removal capacities of both types of safety systems, a number of researches have been performed to understand and improve the CHF. Through recent experimental observations, it is known that a dry patch plays an important role in CHF initiation. We developed a dry patch model based on these observations. There exist quenchable and unquenchable dry patches at high wall heat fluxes. Experimental observation shows that the formation of unquenchable dry patches is the main source of CHF initiation. In the dry patch model, we proposed thermal and hydraulic criteria for the onset of the unquenchable dry patch at a high heat flux. An unquenchable dry patch with a critical size can be generated when the following two criteria are satisfied. As a hydraulic criterion, we assume that the coalescence of the whole dry spots occurs, generating a dry patch if all of them are in contact. As a thermal criterion, we consider the temperature of the dry patch. An unquenchable dry patch will be formed if its peripheral temperature reaches the Leidenfrost temperature, such that it may not be rewetted even with bubble detachment. The critical size of the dry patch is obtained by CFD simulation such that its peripheral temperature is equivalent to the Leidenfrost temperature. The wall dry area fraction can be obtained by calculating the probability of the formation of the unquenchable dry patches satisfying both criteria for its critical size. We demonstrated that the dry patch model successfully predicted the experimental CHF data obtained in pool boiling and forced convective flow boiling of water.

Modeling of Dry-Out Incipience for Flow Boiling in a Circular Microchannel at a Uniform Heat Flux

Dry-out is an essential phenomenon that has been observed experimentally in both slug and annular flow regimes for flow boiling in mini and microchannels. The dry-out leads to a drastic drop in heat transfer coefficient, reversible flow and may cause a serious damage to the microchannel. Consequently, the study and prediction of this phenomenon is an essential objective for flow boiling in microchannels. The aim of this work is to develop an analytical model to predict the critical heat flux (CHF) based on the prediction of liquid film variation in annular flow regime for flow boiling in a horizontal uniformly heated circular microtube. The model is developed by applying one-dimensional (1D) separated flow model for a control volume in annular flow regime for steady, and sable saturated flow boiling. The influence of interfacial shear and inertia force on the liquid film thickness is taken into account. The effects of operating conditions, channel sizes, and working fluids on the CHF have been investigated. The model was compared with 110 CHF data points for flow boiling of various working fluids, (water, LN2, FC-72, and R134a) in single and multiple micro/minichannels with diameter ranges of (0.38≤Dh≤3.04 mm) and heated-length to diameter ratios in the range of (117.7 (117.7≤Lh/D≤470)470). Additionally, three CHF correlations developed for saturated flow boiling in a single microtube have been employed for the model validation. The model showed a good agreement with the experimental CHF data with mean absolute error (MAE) = 19.81%.

Critical heat flux of cyclohexane in uniformly heated minichannels with high inlet subcooling

The critical heat flux (CHF) for flow boiling of cyclohexane was experimentally investigated in electrically heated minichannels with inside diameters of (1 and 2)mm, and heated lengths of (360 and 710)mm at mass velocities ranging from (318 to 1274)kg/m2s, heat fluxes from (50 to 1000)kW/m2, and pressures of (1.0, 2.0, 3.0)MPa. Cyclohexane entered the test channel at ambient temperature of about 20°C, equivalent to high subcooling of (162.1, 206.4 and 236.3)°C at (1.0, 2.0 and 3.0)MPa respectively. The critical vapor qualities ranged from −1.35 to 0.83. Results indicated that CHF of cyclohexane increased with increasing mass velocity and decreasing heated length. Reducing channel diameter at the same length-to-diameter ratio was proved to have no effects on CHF values of cyclohexane at the evaluated conditions. CHF mildly increased with an increase in pressure at higher mass velocities, but critical vapor quality always decreased with the increasing pressure. Experimental CHF data were compared with the Katto–Ohno (1984) correlation and Shah (1987) correlation. Good agreements were achieved between the experimental data and those predicted with most deviations not exceeding 30%. A corrected Katto–Ohno CHF correlation is developed to fit the current experimental data. Excellent agreement was obtained with all the deviations (whatever of saturation CHF or subcooled CHF) in the range of ±10% with RMS error of 5.35%.

The effect of aspect ratio on flow boiling heat transfer of HFE-7100 in a microchannel heat sink

Heat transfer with phase change in microchannels may support applications with very high heat fluxes such as the thermal management of high-performance electronic devices. For electronics cooling application, refrigerating fluid HFE-7100 is one of the suitable candidates with appropriate properties and also considered as an environmental-friendly coolant. The objective of this study is to investigate the flow boiling heat transfer of dielectric liquid HFE-7100 in a copper-based microchannel heat sink. In particular, the effect of aspect ratio of a channel with approximately the same hydraulic diameter (about 1.12mm) on flow boiling heat transfer and critical heat flux (CHF) are thoroughly studied. Flow boiling in a diverging microchannel heat sink with the mean aspect ratio, depth-to-width ratio, of 0.83, 0.99, 1.65, 2.47, 4.23, and 6.06 are investigated experimentally. The experimental results show that at a given wall superheat the base and wall heat fluxes increase with increasing the mass flux. Moreover, the critical heat flux increases linearly with increasing the mass flux. The results further demonstrate that the aspect ratio of microchannels affects boiling heat transfer significantly. The wall heat flux at a given wall superheat and wall critical heat flux demonstrate the peak value at the aspect ratio of 0.99 possibly due to copious existence of the liquid film around the corner. The experimental CHF results are further compared with existing CHF correlations of flow boiling in a microchannel, and a new empirical CHF correlation incorporated the aspect ratio is proposed. The predictions of the proposed correlation demonstrate a consistent trend and excellent agreement with present experimental results. The overall mean absolute error of the proposed correlation is about 3.02%, and all of the experimental CHF data are predicted within a ±10% error band. Under the condition of the same mass flux, the base critical heat flux increases monotonously with increasing the aspect ratio mainly due to the bigger heat transfer area. In the present study, the highest base critical heat flux reached is 1140kW/m2 for the mass flux of 180kg/m2s in the present diverging microchannel. From these results, we conclude that the diverging design of the microchannel with a large aspect ratio and high mass flux is recommended to develop an electronic cooling system for the need of very high heat dissipation rate.

The effect of water absorption on critical heat flux enhancement during pool boiling

A pool-boiling critical heat flux (CHF) correlation that can reflect the effect of capillary wicking on micro- and nano-structured surfaces was developed based on Kandlikar’s CHF correlation dealing with wettability. Capillary wicking tests were performed to examine the significant pool-boiling CHF enhancement on a zirconium surface due to anodic oxidation modification. The amount of water absorption was measured using a capillary tube. Compared with a bare or wettable surface, the micro- and nano-structured surfaces showed much higher absorption ability. The behavior was different for each surface. The amount of water absorption indicated the effective capillary wicking on the surface and was used to explain the CHF enhancement provided by the zirconium surfaces. The tendency toward water absorption, depending on the surface, agreed with the experimental CHF data. A simple and additional contribution to Kandlikar’s CHF correlation was proposed to account for the effect of capillary wicking. This modified correlation was in good agreement with CHF results from experiments.