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In the most challenging nuclear power plant accidents, transient critical heat flux (CHF) is a primary phenomenon that drives peak cladding temperature, and ultimately, fuel failure. It has not yet been determined whether the use of steady-state CHF methods can accurately predict transient CHF under the conditions of a blowdown due to a loss-of-coolant accident. There are limited comprehensive experiments at prototypic conditions. To address this deficiency, a quality separate-effects test facility was built to simulate an electrically heated rod under blowdown conditions. Testing reached full pressurized water reactor thermal-hydraulic conditions. With scaled break sizes as large as a double-end cold leg break, CHF was repeatedly measured with depressurization rates ranging from 7 to 17 MPa s−1. These measurements at prototypic conditions acquired in a controlled methodology are novel to the body of knowledge. Several steady-state CHF methods and heater models were evaluated using RELAP5-3D simulations and the Dakota framework. The results showed that many steady-state CHF methods performed inadequately, but that recently developed wide-ranged, look-up table methods had the most acceptable results. Additionally, the results showed no significant correlation between prediction accuracy and the depressurization rates tested. [ABSTRACT FROM AUTHOR]

High heat dissipation is required for miniaturization and increasing the power of electronic systems. Pool boiling is a promising option for achieving efficient heat dissipation at low wall superheat without the need for moving parts. Many studies have focused on improving heat transfer efficiency during boiling by modifying the surface of the heating element. This paper presents an experimental investigation on improving pool boiling heat transfer using an open microchannel. The primary goal of this work is to investigate the impact of the channel geometry characteristics on boiling heat transfer. Initially, rectangular microchannels were prepared on a circular copper test piece with a diameter of 20 mm. Then, the boiling characteristics of these microchannels were compared with those of a smooth surface under saturated conditions using deionized water. In this investigation, a wire-cutting electrical discharge machine (EDM) machine was used to produce parallel microchannels with channel widths of 0.2, 0.4, and 0.8 mm. The fin thicknesses were 0.2, 0.4, and 0.6 mm, while the channel depth remained constant at 0.4 mm. The results manifested that the surface featuring narrower fins and broader channels achieved superior performance. The heat transfer coefficient (HTC) was enhanced by a maximum of 248%, and the critical heat flux (CHF) was enhanced by a maximum of 101% compared to a plain surface. Eventually, the obtained results were compared with previous research and elucidated a good agreement. [ABSTRACT FROM AUTHOR]

AbstractPressurized water reactors (PWRs) are prevalent in nuclear power plants worldwide, contributing most of the electricity produced from nuclear energy. Modern PWRs allow subcooled flow boiling under their normal operating conditions to enhance the heat transfer rate. However, subcooled flow boiling might also suffer from critical heat flux (CHF) under accidental conditions, called departure from nucleate boiling, a major safety concern for the reactor’s design and operations. Thus, thermal-hydraulic analysis (THA) of subcooled flow boiling in PWRs is crucial in optimizing fuel assembly design to ensure efficient and safe operations. However, performing a THA in PWRs is a very complicated task because of the complexities involved in the operating conditions of the reactor, the physics of subcooled flow boiling, and subchannel geometry due to the presence of mixing vane grids (MVGs). In recent years, computational fluid dynamics (CFD) has emerged as a powerful tool for predicting subcooled flow boiling in various geometries, including PWR subchannels. The current study reviews CFD approaches for analyzing subcooled flow boiling in typical subchannels of a PWR under its prototypical operating condition or close to it. The impacts of various factors such as MVGs and their components (dimples, springs, and mixing vanes), cold walls, wall heat flux distributions, bowed rods, etc. on parameters such as lateral velocity, coolant pressure and temperature, void fraction, heat transfer coefficient, Nusselt number, wall heat flux partitions, CHF, etc. required to describe the behavior of coolant flow and heat transfer in subchannels, are presented. Finally, a summary of key conclusions and the scope for further research is presented. [ABSTRACT FROM AUTHOR]

This study investigated the rolling motion effect on the location of critical heat flux (CHF) for subcooled flow boiling within a mini-channel that is heated from one side. The study employed 15° and 25° as rolling amplitudes and 10 and 30 seconds as rolling periods across two different flow rate conditions. The CHF values observed under static conditions and those under rolling motion were subsequently compared. A key finding was that CHF consistently occurred near the highest positions of the rolling platform, where reduced buoyancy force, due to the absence of centrifugal acceleration, combined with only the radial buoyancy component acting in the flow direction, resulted in a slower bubble rise, facilitating easier coalescence of bubbles into a vapor film, leading to CHF. [ABSTRACT FROM AUTHOR]

The results of the investigation into heat-transfer enhancement at increasing critical heat flux due to modification of a wall's inner surface are presented. The greater need for new, compact, and energy-efficient heat exchangers on the basis of minichannels for high-tech industries makes this investigation urgent. The potential for application of small diameter channels in systems where various dielectric liquids or freons at moderate and high reduced pressures can be used as a coolant is being actively investigated today. The experiments were performed in a heated vertical minichannel. The wall was modified by the rolling method, which has not yet been used in small diameter channels. The experiments were performed with a forced flow of R125 refrigerant at high reduced pressures of 0.43 and 0.56 in the range of mass flowrates from 200 to 1200 kg/(m2 s), which is the most applicable range for minichannel heat exchangers. Heat transfer during forced convection and flow boiling was studied. The experimental setup and the minichannel inner wall modification method are described. Experimental data on forced convection and flow boiling heat-transfer coefficients, critical heat fluxes, and pressure drops are presented. The heat-transfer data were compared with the results obtained previously with the inner surface modified by the action of laser pulses on the outer wall. The convective heat-transfer coefficient in a minichannel with the inner surface modified by rolling was found to be much greater than that in a smooth channel. The obtained convective heat-transfer coefficients are compared with the predictions by empirical formulas derived for large-diameter pipes with the wall surface modified by rolling. [ABSTRACT FROM AUTHOR]

AbstractThis study experimentally investigated the flow boiling critical heat flux (CHF) under heaving conditions pertinent to floating nuclear power plants (FNPPs). Experiments were conducted using R134a to simulate the operational pressure of pressurized water reactors, extending the CHF database to a pressure range of 5 to 18 MPa. The tested heaving motion reached up to 0.6 g of maximum acceleration, with a period that varied from 3 to 5.3 s. A parametric trend involving mass flux, critical quality, and motion conditions was analyzed through extensive tests encompassing static and heaving conditions.The results showed that CHF reduction was observed due to the effect of heaving acceleration, even in the absence of oscillations in other thermal-hydraulic factors. The CHF reduction was up to 9% in the present experimental conditions. The heaving motion effect was prominent under two specific thermal-hydraulic conditions: when the critical quality was close to zero and when it exceeded the value of 0.6. Furthermore, an examination of wall temperature responses suggested that a longer period of heaving motion can lead to an earlier occurrence of CHF. This fundamental effort was conducted to enhance the understanding of the CHF mechanism under motion conditions, with the aim of contributing to the safe and efficient design of FNPPs. [ABSTRACT FROM AUTHOR]

Copyright of FME Transactions is the property of University of Belgrade, Faculty of Mechanical Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Critical heat flux (CHF) is one of the most concerned thermal hydraulic phenomena in reactor safety analysis. It involves complex two-phase flow heat transfer mechanism, and has not been fully understood, so the prediction of critical heat flux mainly depends on CHF correlations obtained under limited experimental conditions. At present, CHF correlations are generally developed with pressure, mass flux and quality as key independent variables. And correspondingly, the test matrix of a CHF test consists of the above parameters. However, it is impossible to perform CHF tests accurately according to the predetermined quality. In CIAE, a CHF experimental research of a 5 χ 5 uniformly heated rod bundle has been carried out. In the experiment, the inlet temperature of the test section was directly taken as a parameter in the test matrix. The CHF data were achieved by stepwise increasing the heating power. The test conditions covered the pressure of 2.8-15.5 MPa, the mass flux of 845-3533 kg/(m2-s), and the inlet temperature of 100°C-300°C. The test data have been analyzed to obtain the thermal-hydraulic parameter influences on CHF by taking the inlet temperature as a variable. The results indicated that, within the test condition range, under the same inlet temperatures, CHF was hardly affected by pressure, and linearly increased with the increasing mass flux. With the increase of inlet temperature, the enhancement of CHF with the increasing mass flux gradually weakens. And CHF was linearly decreased with the increasing inlet temperature under the same mass flux. By contrast, the parameter influences on CHF were more complex by taking the local quality as a variable. According to the research, it can be concluded that, it has an advantage of simplifying the CHF correlation form to take the inlet temperature of the test section as a variable parameter. The research can provide new ideas for CHF experiment, data analysis and correlation development. [ABSTRACT FROM AUTHOR]

AbstractA unifying and accurate model to predict critical heat flux (CHF) over a wide range of conditions has been elusive since wall boiling research emerged. With the release of the experimental data utilized in the development of the 2006 Groeneveld CHF lookup table (LUT), by far the most extensive public CHF database available to date (nearly 25 000 data points), the development of data-driven prediction models over a large parameter space in simple geometry (vertical, uniformly heated round tubes) can be achieved. Furthermore, the popularization of machine learning techniques to solve regression problems has led to more advanced tools for analyzing large and complex databases.This work compares three machine learning algorithms to predict the CHF database. For each selected regression algorithm (ν-Support vector, Gaussian process, and neural network), a set of optimized hyperparameters is applied. Among the investigated algorithms, the neural network emerges as the most effective, achieving a CHF predicted/measured factor standard deviation of 12.3%, three times less than that of the LUT. In comparison, the Gaussian process regression and the ν-support vector regression achieve a standard deviation of 17.7%, about two times lower than the LUT. Hence, all considered algorithms significantly outperform the LUT prediction performance. The neural network model and training methodology are designed to prevent overfitting, which is confirmed by data analyses of the CHF predictions. Finally, future development directions (including data coverage, transfer learning, and uncertainty quantification) are discussed. [ABSTRACT FROM AUTHOR]

Nanofluids have gained considerable attention as potential alternatives to pure fluids in cooling systems, particularly in boiling flow systems operating in two-phase vapor–liquid conditions. While nanofluids enhance heat transfer in cooling systems, a significant challenge associated with their implementation is the issue of nanoparticle deposition and sedimentation. Numerical analyses often neglect the deposition layer formed on the walls of pipes and channels. Disregarding this layer in simulations can lead to significant errors in estimating fluid flow and heat transfer parameters. This study investigates the boiling flow of four different nanofluids (nickel–water, gold–water, aluminum–water, and titanium oxide–water) with a volume fraction of 0.03% and pure water in a vertical tube with a diameter of 1.45 mm using a two-dimensional simulation in MATLAB software. In the simulation, five mass transfer mechanisms including evaporation, condensation, deposition, entrainment, and fluid entry into the sedimentary area are considered. To simulate the sedimentary layer, the rules of the porous media have been used. In addition, Brownian motion of particles and Kapitza resistance are also considered in the calculations. The primary objective is to demonstrate the impact of deposition layer simulation on the heat transfer coefficient (HTC) and critical heat flux (CHF) in various nanofluids. The results indicate that the deposition layer causes an average reduction of 31% in the HTC and 16% in the CHF. The findings emphasize the importance of incorporating deposition layer simulation in analyzing boiling nanofluids. Ignoring sediment phenomenon can result in substantial errors in predicting heat transfer and CHF, with significant implications for the design and optimization of nanofluid-based cooling systems. [ABSTRACT FROM AUTHOR]

To study the effect of micro-structured surface with wedge-shaped channel on pool boiling heat transfer performance of FC-72, four kinds of mixed wettability surfaces with area ratio of the micro-pillar region to the smooth channel region of approximately 1:1 were fabricated in this study (the surfaces were denoted as the Multi tip surface, Multi star surface, Less tip surface and Less star surface). The experimental results indicated that the CHF increases with the increase of liquid subcooling. The structural surface parameters will affect the bubble dynamics behavior and thus affect CHF. The effect of capillary wick suction on the mixed wetting surface first increases and then decreases. The capillary wick suction plays a significant role in the increase of CHF, and the capillary wick force on the Less tip surface with the best heat transfer performance is the largest. The Zuber model is modified by combining three factors to propose a critical heat flux model suitable for mixed wetting surfaces. With the increase of heat flux, the bubble detachment frequency decreases, the bubble detachment diameter increases and the nucleation site density basically shows exponential growth. Bubbles in the micro-pillar array region will be driven to slip onto the smooth channel due to energy difference and the bubbles in smooth channels will also migrate in the direction of wider smooth channels under the action of Laplace force. [ABSTRACT FROM AUTHOR]

A notable distinction in this research is the utilization of a new method for calculating critical heat flux (CHF) based on a Look‐Up Table. The present study comprehensively investigates the effects of hybrid nanofluid, a type of passive heat transfer enhancement technique, on convection heat transfer coefficients and CHF. The study covers five different climates representing significant climate conditions in Iran, namely Bandar Abbas, Esfahan, Shiraz, Tehran, and Yazd, each with different solar irradiations. The nanoparticles considered in this study include silver, nickel, and aluminum, as well as Ag‐Au hybrid nanofluid with volumetric concentrations of 0.1%, 0.3%, 0.5%, 1%, and 2%. The modeling results reveal that the heat transfer coefficient increases with the volumetric concentration of nanoparticles. According to the results, at the CHF point for 2 vol% Ag–Au hybrid nanofluid and Ag, Ni, and Al nanoparticles, the heat transfer coefficient shows an increase of 28%, 11.5%, 10.6%, and 4.9%, respectively, compared to the results for pure water in Shiraz. Despite the acceptable results and effective performance of 2 vol% Ag–Au hybrid nanofluid for a linear Fresnel reflector, economically, 2 vol% nickel nanoparticles are identified as the most suitable choice. [ABSTRACT FROM AUTHOR]

AbstractIn this study, the heat transfer performance of γ-Fe2O3 nanofluid is investigated. The particle size used in the experiment was about 20 nm. It was found by X-ray diffraction that it was consistent with the characteristic peak and no other impurities. Nanofluids with different concentrations were configured through a two-step method. Since the γ-Fe2O3 nanoparticles are not easily dispersed, the ultrasonic time was relatively long. After a series of experiments and data processing, we could see that nanofluids have the best heat transfer performance at 0.07 g/L.Compared to a reverse-osmosis (R·O) water case, the enhancement of critical heat flux (CHF) was about 34.09%, and the heat transfer coefficient enhancement was about 49.32%. The movement of bubbles during the experiment was recorded and analyzed. Compared with the R·O water case, the bubbles were larger and fewer in the nanofluid case, and what is more, the bubble movement was relatively intense.The heating surface was characterized after the experiment, and it was found that the wettability of the heating surface was changed, and the roughness of the heating surface decreased. Scanning electron microscopy showed that the deposition of the nanoparticles on the heating surface was the main cause of CHF enhancement. When the concentration was 0.08 g/L, CHF decreased, mainly because the excessive deposition of the nanoparticles increased the thermal resistance of the heating surface and led to the deterioration of heat transfer. [ABSTRACT FROM AUTHOR]

AbstractIn case of a severe accident in a light water reactor, a debris bed may form in the core and possibly melt, as happened in Three Mile Island Unit 2. Knowledge about the coolability of such a molten pool surrounded by debris is crucial to investigate the possibility of stabilizing a part of the fuel inside the vessel. In particular, it is of primary interest to determine the maximum size of a molten pool surrounded by debris that may be stabilized under water. A key parameter is the maximum heat flux [critical heat flux (CHF)] that may be extracted from the pool boundary by water flowing within the debris bed. A facility was built at IRSN to determine the CHF under various conditions. A heated copper surface simulates the boundary of the pool and is placed in contact with a debris bed (monodisperse steel balls), under water. In this article, we first complete previous experimental work with steam and liquid flow rates, ball diameter, tilting angle of the heated surface, and pressure up to 2.5 bar absolute. A general CHF correlation depending on the tilting angle and the steam flow rate is derived, and an example of the use of this correlation to evaluate the maximum mass of the corium pool that can be stabilized under water is given, for some typical reactor conditions. In the last part of the article, the accuracy of the ASTEC code is evaluated based on first calculations intending to reproduce the experimental impact of liquid and steam flow rates. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Refrigeration is the property of Journal of Refrigeration Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In an effort to reduce the charging time of electric vehicles (EVs), the thermal management of the charging system has emerged as one of the most urgent issues. The charging rate has been restricted to avoid thermal failure especially in charging cable. Recently, a direct contact cooling technique that utilizes subcooled flow boiling for charging cables has been proposed and has shown promising thermal management performance. However, there has been a lack of clear explanation regarding its thermal characteristics due to its intrinsically complicated flow features. This study investigates the thermal characteristics of subcooled flow boiling in a horizontally aligned concentric annular tube intended to simulate the direct contact cooling technique employed for charging cables. For the experimental investigation, the wall temperature measurements and high-speed flow visualization images acquired from the transparent test module annulus, with inner and outer diameters of 6.35 mm and 22.0 mm, were utilized. Three key axial thermal characteristics of subcooled flow boiling in annulus have been analyzed, and they are flow regime transition, variation of heat transfer mechanisms, and occurrence of critical heat flux (CHF). The transition of flow regimes and the variation of dominant heat transfer mechanisms in the axial direction are mutually influencing due to the coupled effects between the hydrodynamic and thermal characteristics of simultaneously developing flows. The flow regime changes from partially developed boiling (PDB) to fully developed boiling (FDB) as the intensities of nucleate boiling and bubble recondensation vary in the axial direction, while the dominant heat transfer mechanism shifts from single-phase convection to nucleate boiling, corresponding to the flow regime transition from PDB to FDB. The departure from nucleate boiling (DNB) type CHF is observed, manifested by the wavy interface propagating from the far downstream to the upstream region, and the intensities of nucleate boiling and bubble recondensation are also identified as the dominant parameters in determining the occurrence of CHF. Charging time estimation reveals that the proposed method can achieve an 80 % charge for 100-kWh EV batteries in 98 s, while maintaining the cable wire temperature below the safety limit of 80 °C. This strongly supports its potential as a promising thermal management technique for future ultra-fast charging systems.

Graphene materials have great application prospects in the field of advanced heat transfer equipment for nuclear reactors due to their excellent thermal conductivity and chemical inertia. In order to explore the enhanced heat transfer performance of graphene materials, boiling deposition method to prepare graphene oxide nanocoatings was used in this paper. The working fluid concentrations of graphene oxide nanofluids are 0.001, 0.003 and 0.005 mg/mL, and the heating time is controlled at 1 hours and 2 hours. Six types of graphene oxide coatings were prepared on exposed copper surfaces through boiling deposition. The coating surface was observed through optical photography and scanning electron microscopy (SEM). By comparing and analyzing the influence of graphite oxide coating on surface morphology characteristics, it is found through optical photos that the deposition of graphene oxide on the copper surface shows an uneven spot like distribution. With the increase of nanofluid concentration and preparation time, the coverage gradually increases. When the preparation time is 2 hours, the graphene oxide coating completely covers the substrate copper surface. The deposition of graphene oxide on the copper surface exhibits an uneven spotted distribution. Through scanning electron microscopy analysis, it is found that the uneven spots are the vaporization core during boiling. By using X-ray energy dispersive spectroscopy (EDS) to scan the element content distribution in the spot area along a straight line, it is found that the carbon element content in the vaporized core is the lowest, and the carbon element content around it first increases and then decreases. These indicate that the deposition of graphene oxide nanofluid occurs through the phase transition at the bottom of the bubble, which is the reason for the non-uniform distribution of the deposition layer spots. Using the above graphene oxide coating for pool boiling experiments, it is found that when the coverage rate of graphene oxide coating is low, its impact on pool boiling heat transfer characteristics is minimal. When the coverage rate of the nano coating is high, it can significantly increase the critical heat flux, but the subsequent thickening of the coating has a smaller effect on improving the critical heat flux. Within the experimental range, the critical heat flux increases by 40% to 55% compared to the bare surface. Mechanism analysis finds that the sedimentary layer improves wettability and thermal conductivity, both of which jointly suppress the irreversible expansion of the evaporation drying area at the bottom of the bubble under high heat flux conditions, significantly increasing the critical heat flux density. Due to the similarity between the surface morphology of the coating and the exposed copper surface, the effect on the density of the vaporized core during pool boiling is relatively small, so the effect on the pool boiling heat transfer coefficient is relatively small.

Understanding the nanofluid flow boiling phenomena is essential for many engineering applications including atomic reactors, cooling of electronic devices, synthetic buildings and spacecraft. Hybrid nanofluids, which are commonly defined as mixtures of two or more nanoparticles, are yet unknown to have improved features. When the coupled nanoparticles are suitably coated with one another, one on top of the other, this would be a successful technique as the usage of such nanofluids is hardly reported. The current research aims to investigate the effects of various novel hybrid ZnO coated Cu (Cu@ZnO) nanofluid concentrations on the critical heat flux (CHF) and heat transfer coefficient (HTC) for their potential real-life application. The Cu@ZnO nanoparticles with hybrid properties were selected because Cu increased the conductivity of heat whereas ZnO improved chemical durability. Efficient spark discharge in liquid nitrogen was employed in three stages to produce robust Cu@ZnO nanoparticles with hybrid properties. In order to create durable nanofluids, the nanoparticles with mixed properties were dispersed using an ultrasonication process at four different volumetric concentrations: 0.025%, 0.050%, 0.075% and 0.1%. At 45°C, the nanofluids' thermal conduction was 31% higher than that of base fluid. The flow boiling (subcooled) tests are carried out in minichannel with novel nanofluids. When the flow boiling investigation was being conducted, a greater concentration improved the HTC and CHF. In comparison to water, 0.1% of nanofluid caused the maximum rises in CHF and HTC, which were found to be 168.09% and 186.9%, respectively. It is possible for the fluid to utilize latent heat, create bubbles, distinct and delay the commencement of CHF because the thick Cu@ZnO-coated surface possesses strong thermal conductivity. [ABSTRACT FROM AUTHOR]

Studies of the heat transfer coefficient and thermal resistance for a loop thermosyphon with a high heat flux density in the transport zone, as well as studies of the critical heat flux using a traditional thermosyphon, have been carried out. Measurements were carried out for pure water, water with porous coatings of the vaporization surface and nanofluid, all other things being equal, using technically polished copper surfaces as the boiling surface. The mechanism of formation of nanorelief on the vapor-generating surfaces of thermosyphons and an increase in the heat transfer coefficient and critical heat flux due to this is revealed. A dynamic model of the formation of this nanorelief is proposed. Accelerated and long-term life-test of a loop thermosyphon filled with water with iron oxide and copper oxide nanoparticles were carried out, and its stable performance was shown. [ABSTRACT FROM AUTHOR]

High heat transfer coefficient (HTC) and critical heat flux (CHF) are achieved in liquid film boiling by coupling vibrant vapor bubbles with a capillary liquid film, which has thus received increased interest for thermal management of high-power electronics. Although some experimental progress has been made, a high-fidelity heat transfer model for liquid film boiling is lacking. This work develops a thermal-hydrodynamic model by considering both evaporation atop the wick and nucleate boiling inside the wick to simultaneously predict the HTC and CHF. Nucleate boiling is modeled with microlayer evaporation theory, where a unified scaling factor is defined to characterize the change of microlayer area with heat flux. The scaling factor η is found to be independent of wicking structure and can be determined from a few measurements. This makes our model universal to predict the liquid film boiling heat transfer for various micro-structured surfaces including micropillar, micropowder, and micromesh. This work not only sheds light on understanding fundamental mechanisms of phase-change heat transfer, but also provides a tool for designing micro-structured surfaces in thermal management. [ABSTRACT FROM AUTHOR]

Copyright of Atomic Energy Science & Technology is the property of Editorial Board of Atomic Energy Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The rapid progress of electronic devices has necessitated efficient heat dissipation within boiling cooling systems, underscoring the need for improvements in boiling heat transfer coefficient (HTC) and critical heat flux (CHF). While different approaches for micropillar fabrication on copper or silicon substrates have been developed and have shown significant boiling performance improvements, such enhancement approaches on aluminum surfaces are not broadly investigated, despite their industrial applicability. This study introduces a scalable approach to engineering hierarchical micro-nano structures on aluminum surfaces, aiming to simultaneously increase HTC and CHF. One set of samples was produced using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while another set underwent an additional laser texturing step. Three distinct micropillar patterns were tested under saturated pool boiling conditions using water at atmospheric pressure. Our findings reveal that microcavities created atop pillars successfully facilitate nucleation and micropillars representing nucleation site areas on a microscale, leading to an enhanced HTC up to 242 kW m−2 K−1. At the same time, the combination of the surrounding hydrophilic porous area enables increased wicking and pillar patterning, defining the vapor–liquid pathways on a macroscale, which leads to an increase in CHF of up to 2609 kW m−2. [ABSTRACT FROM AUTHOR]

AbstractIn-vessel retention through external reactor vessel cooling (IVR-ERVC) is a strategy used to respond to nuclear reactor accidents. One of the key performance indicators determining its feasibility is critical heat flux (CHF). Our focus is on simulating real-world scenarios through surface pool boiling to improve the implementation of the IVR-ERVC strategy with hybrid nanofluids. Two groups of TiO2/COOH-CNTs hybrid nanofluids were prepared: group 1 with different concentrations at the same proportion and group 2 with different proportions at the same total concentration.Researchers compared the improvement of the two groups’ CHF and heat transfer coefficient (HTC), and analyzed the potential mechanism of heat transfer enhancement through roughness of surface, hydrophilicity, and scanning electron microscopy observations. The results showed that a mass concentration of 8 mg:8 mg per liter exhibited the best heat transfer performance, with a CHF enhancement up to 28.21% and an improvement in HTC as well. Meanwhile, correlations between alterations in surface roughness, hydrophilicity, and enhancements in CHF were observed. Finally, by detecting the deposition surface, the possible mechanism of TiO2/COOH-CNTs hybrid nanofluids in enhancing heat transfer was inferred. [ABSTRACT FROM AUTHOR]

The heat transfer measurements were conducted during pool boiling of water on surfaces with microchannels. Parallel grooves were made on a copper surface with widths ranging from 0.2 mm to 0.5 mm at intervals of 0.1 mm. The inclination angle of the grooves to the horizontal was set at 30° and 60°, and the depth of the microchannel grooves was 0.3 mm. The achieved heat flux ranged from 25 kW/m² to 1730 kW/m², and the heat transfer coefficients ranged from 12 kW/(m²K) to 475 kW/(m²K). The influence of geometric parameters such as width, inclination angle of the microchannel, surface extension, and Bond number on heat exchange efficiency was examined. A nearly sixfold increase in a (heat transfer coefficient) and a twofold increase in critical heat flux were observed compared to a smooth surface. [ABSTRACT FROM AUTHOR]

The recent advances in the growth of heat dissipation from microelectronic devices have led to the two-phase heat transfer method via nucleate boiling for better thermal management. In this study, the effect of surface wettability on the saturated pool boiling heat transfer performance is examined with deionized water. Three types of wettability surfaces are compared, i.e., superhydrophilic (SHPi), hydrophilic (HPi) and hydrophobic (HPo) surfaces. The SHPi surface is prepared by anodic oxidation of the copper surface, while the HPi and HPo surface is prepared by coating Cu–TiO2 and Cu–MWCNTs, respectively, on the copper surface using the electrochemical deposition method. The earliest incipience of nucleate boiling was observed with the HPo surface, while a most delayed onset of nucleation was obtained for the SHPi surface. The critical heat flux is found to be 1012 kW·m−2, 1251 kW·m−2, 1490 kW·m−2 and 1610 kW·m−2 corresponding to the plane copper, HPo, HPi and SHPi surfaces following the ascending order. The improved rewetting of the arid area underneath the formed vapour bubble caused a delay in the dry-out occurrence and resulted in a maximum critical heat flux for the SHPi surface. The maximum heat transfer coefficient of 88.42 kW·m−2·K−1, 64.7 kW·m−2·K−1 and 59.19 kW·m−2·K−1 have been observed for the HPi, HPo and SHPi surfaces, respectively, which translates to an increment of 60.2 %, 17.23 % and 7.25 %, respectively, as compared to plain surface. The SHPi surface induces the rightward shifting of the boiling curve as compared to the plane surface, which gives a lower heat transfer coefficient for a particular heat flux. [ABSTRACT FROM AUTHOR]

Utilizing pool boiling as a cooling method holds significant importance within power plant industries due to its ability to effectively manage temperature differentials amidst high heat flux conditions. This study delves into the impact of surface modifications on the pool boiling process by conducting experiments on four distinct boiling surfaces under various conditions. An experimental setup tailored for this investigation is meticulously designed and implemented. The primary objective is to discern the optimal surface configuration capable of efficiently absorbing maximum heat flux while minimizing temperature differentials. In addition, this study scrutinizes bubble dynamics, pivotal in nucleation processes. Notably, surfaces polished unidirectionally (ROD), exhibiting lower roughness, demonstrate superior performance in critical heat flux (CHF) compared to surfaces with circular roughness (RCD). Moreover, the integration of bubble liquid separation methodology along with the introduction of a bubble micro-layer yields a microchannel surface. Remarkably, this modification results in a noteworthy enhancement of 131% in CHF and a substantial 211% increase in the heat transfer coefficient (HTC) without resorting to particle incorporation onto the surface. This indicates promising avenues for enhancing cooling efficiency through surface engineering without additional additives. [ABSTRACT FROM AUTHOR]

Boiling with downflow in vertical channels is involved in many applications such as boilers, nuclear reactors, chemical processing, etc. Accurate prediction of CHF (Critical Heat Flux) is important to ensure their safe design. While numerous experimental studies have been done on CHF during upflow and reliable methods for predicting it have been developed, there have been only a few experimental studies on CHF during downflow. Some researchers have reported no difference in CHF between up- and downflow, while some have reported that CHF in downflow is lower or higher than that in upflow. Only a few correlations have been published that are stated to be applicable to CHF during downflow. No comprehensive comparison of correlations with test data has been published. In the present research, literature on CHF during downflow in fully heated channels was reviewed. A database for CHF in downflow was compiled. The data included round tubes and rectangular channels, hydraulic diameters 2.4 mm to 15.9 mm, reduced pressure 0.0045 to 0.6251, flow rates from 15 to 21,761 kg/m2s, and several fluids with diverse properties (water, nitrogen, refrigerants). This database was compared to a number of correlations for upflow and downflow CHF. The results of this comparison are presented and discussed. Design recommendations are provided. [ABSTRACT FROM AUTHOR]

Boiling is a high-performance heat dissipation process that is central to electronics cooling and power generation. However, there exists a practical limit of boiling heat transfer known as the critical heat flux (CHF), beyond which significant performance degradation is observed. Understanding the physical mechanism that triggers CHF is essential to meet the increasing cooling demands driven by power densification and device miniaturization. However, the high dimensionality, stochasticity, and dynamicity of the boiling process have led to strong challenges in the experimental characterization and modeling of boiling CHF. As such, high-frame rate, high-resolution, multi-physics boiling datasets are critical to advancing the fundamental understanding of boiling heat transfer. To this end, this paper presents a multimodal boiling dataset consisting of synchronized thermal, acoustic, and optical signals collected from five different heater surfaces under two distinct heat load conditions. With its high sampling frequency, diverse signal types, large data volume, and detailed recorded information, this dataset provides valuable ''data blood'' for the field of thermal crisis monitoring. This dataset will not only promote fundamental research on bubble dynamics during boiling but also support the implementation of advanced monitoring technologies in industrial applications such as power electronics, motors, data centers, and power plants.

Critical heat flux (CHF) is one of the most concerned thermal hydraulic phenomena in reactor safety analysis. It involves complex two-phase flow heat transfer mechanism, and has not been fully understood, so the prediction of critical heat flux mainly depends on CHF correlations obtained under limited experimental conditions. At present, CHF correlations are generally developed with pressure, mass flux and quality as key independent variables. And correspondingly, the test matrix of a CHF test consists of the above parameters. However, it is impossible to perform CHF tests accurately according to the predetermined quality. In CIAE, a CHF experimental research of a 5 × 5 uniformly heated rod bundle has been carried out. In the experiment, the inlet temperature of the test section was directly taken as a parameter in the test matrix. The CHF data were achieved by stepwise increasing the heating power. The test conditions covered the pressure of 2.8–15.5 MPa, the mass flux of 845–3533 kg/(m2·s), and the inlet temperature of 100°C–300°C. The test data have been analyzed to obtain the thermal-hydraulic parameter influences on CHF by taking the inlet temperature as a variable. The results indicated that, within the test condition range, under the same inlet temperatures, CHF was hardly affected by pressure, and linearly increased with the increasing mass flux. With the increase of inlet temperature, the enhancement of CHF with the increasing mass flux gradually weakens. And CHF was linearly decreased with the increasing inlet temperature under the same mass flux. By contrast, the parameter influences on CHF were more complex by taking the local quality as a variable. According to the research, it can be concluded that, it has an advantage of simplifying the CHF correlation form to take the inlet temperature of the test section as a variable parameter. The research can provide new ideas for CHF experiment, data analysis and correlation development.

Since boiling is a phenomenon with high heat transfer performance, it is expected to be used in various industrial fields such as emergency cooling in nuclear power plants, power device cooling for EVs and next-generation CPU cooling in data centers. So far, various research on boiling have been carried out in the background of these trends. In particular, many studies have been conducted to improve the critical heat flux (qCHF) which is the limit of boiling heat transfer. However, since the mechanism of qCHF is a complex phenomenon, the methods of qCHF improvement are still unconstructed. Here we demonstrate the high-performance heat transfer surface using electrodeposition which fabricates copper porous plate. We found that this copper porous plate has higher qCHF (3.7 MW/m2) than the plain surface (1.3 MW/m2) in saturated pool boiling of water at atmospheric pressure. The structure of copper porous plates is investigated by X-ray CT and SEM. The copper porous plates have a honeycomb porous structure consisting of porous area and vapor escape channels extending vertically from the substrate. In addition, the diameter of the vapor escape channels increases from the substrate to the top surface. This surface structure can supply the coolant to the heat transfer surface with the reduction of pressure drop. Our results show that the mechanism of qCHF improvement using porous plate depends on the wicking phenomenon and structure. We expect that these results will help in the development of high-performance heat transfer surfaces for various industrial applications.

Integrating microchannel flow boiling heat transfer with nanofluids is an effective method for cooling electronic devices. However, traditional methods have significant limitations in achieving facile synthesis of nanofluids with desirable properties. In this study, we propose a straightforward synthesis strategy using microfluidic reactors, which enables continuous and high-throughout synthesis of Al2O3 nanofluids with high purity and long-term stability. These nanofluids were found to enhance boiling heat transfer performance by delaying bubbles coalescence, increasing surface nucleation sites, improving the nucleation rate, and enhancing wettability by observation of the bubble behavior. Specifically, the critical heat flux (CHF) and corresponding heat transfer coefficient (HTC) are increased by a maximum of 32 % and 26 %, respectively, with negligible change in pressure drop. Moreover, the heat transfer performance deteriorates with increasing concentration of alumina nanofluids. These findings not only provide guidance of microchannel flow boiling with nanofluids, but also present valuable insights for the application of nanofluids in two-phase cooling systems for electronic devices.

External reactor vessel cooling (ERVC) is a key technology in the severe accident mitigation strategy of third generation pressurized water reactor nuclear power plants, and the critical heat flux (CHF) determines the pressure vessel integrity. Studies on ERVC boiling heat transfer enhanced by coating show that coating surface can significantly increase CHF, in order to investigate the aging effect of the real material of nuclear reactor pressure vessel SA508 steel surface on CHF characteristics of coating surfaces, experimental studies on boiling heat transfer characteristics of SA508 steel bare surface and TC4 titanium alloy porous coating surface based on SA508 steel under different aging conditions were carried out. The substrate of SA508 steel bare surface was made of copper block and 3 mm thick SA508 steel by explosive welding. The TC4 titanium alloy porous coating surface was prepared by spraying particles on the bare surface of SA508 steel by supersonic cold gas spray technology. The micro-scale (the diameter of particles is 10-50 μm) TC4 titanium alloy particles were accelerated to a high speed of 400-500 m/s by the propulsion from a stream of high-pressure inert gases with a much lower temperature than thermal spraying. Four kinds of aging experiments were carried out on the SA508 steel bare surface and three kinds of aging experiments on the TC4 titanium alloy porous coating surface at the inclination angle of 5°. The results show that the SA508 steel bare surface oxidizes rapidly in water, and the CHF tends to increase with the deepening of aging, within 20%. The scanning electron microscopy and EDS characterization were used to analyze the surface characteristic. The CHF enhancement is due to the generation of Fe3O4 magnetic micro-nano particle layers that improve surface wettability and added the nucleation sites by oxidation of SA508 steel. Nevertheless, the CHF increase due to oxidation is limited. Compared with the SA508 steel bare surface, the CHF of TC4 coating surface without oxidation increases more than 36.9%. The TC4 coating surface changes from super-hydrophilic to hydrophobic with the deepening of oxidation, the oxide generated by substrate SA508 steel is easy to clog the coating pores, and the CHF shows a deterioration trend. The application of porous coating technology to enhance the performance of ERVC needs to consider adding oxidation resistance layer between the SA508 steel substrate and the porous coating surface, and the problem of bonding strength and alternating stress of multilayer coating should be solved. This study helps to deepen the understanding of enhanced heat transfer technology for ERVC and lays the foundation for subsequent research.

Boiling is an effective energy-transfer process with substantial utility in energy applications. Boiling performance is described mainly by the heat-transfer coefficient (HTC) and critical heat flux (CHF). Recent efforts for the simultaneous enhancement of HTC and CHF have been limited by an intrinsic trade-off between them-HTC enhancement requires high nucleation-site density, which can increase bubble coalescence resulting in limited CHF enhancement. In this work, this trade-off is overcome by designing three-tier hierarchical structures. The bubble coalescence is minimized to enhance the CHF by defining nucleation sites with microcavities interspersed within hemi-wicking structures. Meanwhile, the reduced nucleation-site density is compensated for by incorporating nanostructures that promote evaporation for HTC enhancement. The hierarchical structures demonstrate the simultaneous enhancement of HTC and CHF up to 389% and 138%, respectively, compared to a smooth surface. This extreme boiling performance can lead to significant energy savings in a variety of boiling applications.

The effects of the anisotropic properties (wettability and roughness) of microgrooved surfaces on heat transfer were experimentally investigated during pool boiling using Novec-7100 as a working fluid. The idea for introducing the concept of anisotropic wettability in boiling experiments draws inspiration from biphilic surfaces. The investigation is also motivated by two-phase immersion cooling, which involves phase-change heat transfer, using a dielectric liquid as a working fluid. Very few studies have focused on the effects of surfaces with anisotropic properties on boiling performance. Thus, this study aims to examine the pool-boiling heat transfer performance on surfaces with microgroove-induced anisotropic properties under the saturation condition. A femtosecond-laser texturing method was employed to create microgrooved surfaces with different groove spacings. The results indicated that anisotropic properties affected the heat transfer coefficient and critical heat flux. Relative to the plain surface, microgrooved surfaces enhanced the heat transfer performance due to the increased number of bubble nucleation sites and higher bubble detachment frequency. An analysis of bubble dynamics under different surface conditions was conducted with the assistance of high-speed images. The microgrooved surface with a groove spacing of 100 μm maximally increased the BHTC by 37% compared with that of the plain surface. Finally, the CHF results derived from experiments were compared with related empirical correlations. Good agreement was achieved between the results and the prediction correlation. [ABSTRACT FROM AUTHOR]

External reactor vessel cooling (ERVC) is a key technology in the severe accident mitigation strategy of third generation pressurized water reactor nuclear power plants, and the critical heat flux (CHF) determines the pressure vessel integrity. Studies on ERVC boiling heat transfer enhanced by coating show that coating surface can significantly increase CHF, in order to investigate the aging effect of the real material of nuclear reactor pressure vessel SA508 steel surface on CHF characteristics of coating surfaces, experimental studies on boiling heat transfer characteristics of SA508 steel bare surface and TC4 titanium alloy porous coating surface based on SA508 steel under different aging conditions were carried out. The substrate of SA508 steel bare surface was made of copper block and 3 mm thick SA508 steel by explosive welding. The TC4 titanium alloy porous coating surface was prepared by spraying particles on the bare surface of SA508 steel by supersonic cold gas spray technology. The micro-scale (the diameter of particles is 10-50 μm) TC4 titanium alloy particles were accelerated to a high speed of 400-500 m/s by the propulsion from a stream of high-pressure inert gases with a much lower temperature than thermal spraying. Four kinds of aging experiments were carried out on the SA508 steel bare surface and three kinds of aging experiments on the TC4 titanium alloy porous coating surface at the inclination angle of 5°. The results show that the SA508 steel bare surface oxidizes rapidly in water, and the CHF tends to increase with the deepening of aging, within 20%. The scanning electron microscopy and EDS characterization were used to analyze the surface characteristic. The CHF enhancement is due to the generation of Fe3O4 magnetic micro-nano particle layers that improve surface wettability and added the nucleation sites by oxidation of SA508 steel. Nevertheless, the CHF increase due to oxidation is limited. Compared with the SA508 steel bare surface, the CHF of TC4 coating surface without oxidation increases more than 36.9%. The TC4 coating surface changes from super-hydrophilic to hydrophobic with the deepening of oxidation, the oxide generated by substrate SA508 steel is easy to clog the coating pores, and the CHF shows a deterioration trend. The application of porous coating technology to enhance the performance of ERVC needs to consider adding oxidation resistance layer between the SA508 steel substrate and the porous coating surface, and the problem of bonding strength and alternating stress of multilayer coating should be solved. This study helps to deepen the understanding of enhanced heat transfer technology for ERVC and lays the foundation for subsequent research. [ABSTRACT FROM AUTHOR]

The paper is concerned with an experimental study of heat transfer and boiling crisis development on a biphilic silicon surface made using a set of methods, including chemical vapor deposition and laser texturing. It is shown that the use of a biphilic surface with the proposed configuration of hydrophobic zones on a superhydrophilic base leads simultaneously to an increase in heat transfer by 60% and an increase in the critical heat flux by 76% as compared to an unmodified surface. [ABSTRACT FROM AUTHOR]

The article considers a study of the possibility to increase the boiling critical heat flux qcr through the use of surfaces consisting of areas with different heat conductivity. The results of experiments on studying pool boiling heat transfer for saturated dielectric fluid methoxynonafluorobutane (Novec 7100) on bimetallic surfaces are presented. The studies were carried out for bimetallic samples and also for samples made of copper and grade 08Kh18N10T stainless steel in the pressure range 0.1–0.4 MPa. A description of the experimental setup and the procedures used is given. The boiling curves for each sample in the entire presented range of fluid pressures with a step of 0.1 MPa are obtained, and the tables of critical heat-flux values are given. The effect that the liquid pressure has on the relative increase of qcr for bimetallic samples is shown. The values of qcr obtained on all samples are compared with one another, and the increase of qcr on bimetallic surfaces by up to 20% is shown. The previously performed studies are briefly reviewed, and the experimental data obtained by other researchers on boiling heat transfer on surfaces with modulated heat conductivity and for boiling of Novec 7100 fluid are presented, including that on samples with a modified heat-transfer surface. The obtained results are compared with rather few data of other researchers. The temperature field in a bimetallic sample is numerically simulated, and the temperature distribution over the heat-transfer surface is presented. The growth of qcr is due to nonisothermal properties of the heat-transfer surface, which causes the boiling to become regularized. [ABSTRACT FROM AUTHOR]

AbstractIn this work, an improved wall boiling model for nanofluids was proposed, taking into account the effect of bubble slip on the wall heat flux density, as well as the effects of nanofluid thermophysical properties and nanoparticle deposition on the density of nucleation sites and bubble departure diameter. The nucleation site density and bubble departure diameter were calculated and compared with the experimental data, the deviations were not more than 7.80% and 9.81%, respectively. The downward-facing surface’s critical heat flux (CHF) was numerically simulated using Al2O3-H2O nanofluids. The simulation results of CHF were compared with the experimental data, and the maximum deviation did not exceed ±16.9%. Compared to pure water, the average CHF enhancement of 0.001–0.01vol% Al2O3-H2O nanofluid was 65.4%. The impacts of thermophysical properties, contact angle, and surface roughness were analyzed separately using the control variable method. The results showed that the wall temperature and void fraction were mostly not affected by thermophysical properties, and the variation in contact angle and surface roughness had a substantial impact. For the CHF improvement of 0.001vol% Al2O3–H2O nanofluids, the proportions of contact angle, surface roughness, and thermophysical properties on the CHF enhancement are 57%, 8%, and 1%. CHF enhancement with nanofluid was strongly related to the changes in contact angle and surface roughness. [ABSTRACT FROM AUTHOR]

The experimental investigation of the critical heat flux (CHF) of saturated nucleate pool boiling of pure water and water-based Al2O3 nanofluids on the platinum wire with a diameter of 50 μm was conducted under earth gravity and hypergravity. The gravity level ranges from 1 to 3 g, the saturation pressures range from 0.1 to 0.6 MPa, and the Al2O3 concentration in the nanofluids ranges from 0.001wt% to 0.015wt%. The experimental results show that both pressure and gravity are vital factors enhancing the CHF, with the effect of pressure more pronounced. For nanofluids with concentration C > 0.005wt %, CHF initially increased with the increase in gravity. When the gravity is greater than 2 g, CHF does not increase further with the increase in gravity. Increasing nanoparticle concentration significantly enhances the CHF for low nanoparticle concentrations less than 0.005 wt%, and the CHFs change little for further increasing the concentration. Nanofluid has a stronger enhancement to the pool boiling CHF than the combination of the heating surface coated with the same kind of nanoparticles and the base fluid. With the increase of particles concentration, Surface modification gradually becomes dominant mechanism for CHF enhancement. [ABSTRACT FROM AUTHOR]

This study explores the nuanced interplay between surface wettability and morphology in nucleate boiling heat transfer. Surfaces with identical microstructures but varying wettability are created on copper substrates using laser texturing and a hydrophobic agent. The spacing between laser-etched microchannels is adjusted to achieve different contact angles and percentages of laser-textured areas. Boiling performance is assessed with variations in channel spacing and surface coverage, revealing significant differences between hydrophilic and hydrophobic surfaces. The critical heat flux (CHF) enhancement mechanisms are identified, emphasizing either enhanced liquid replenishment or separate boiling and liquid-supply regions. A critical laser line spacing of 100 μm is determined, where hydrophobic and hydrophilic variants exhibit similar CHF. Interestingly, hydrophilic surfaces show a weak correlation between the heat transfer coefficient (HTC) and contact angle, while hydrophobic surfaces demonstrate a strong correlation. This separation of surface morphology and wettability confirms that lower wettability promotes early nucleate boiling and high HTC, whereas hydrophilic variants contribute to enhanced CHF with lower HTC. These distinctions are most prominent at high coverage with laser-textured areas and diminish as coverage decreases. The findings provide valuable insights into optimizing surface properties for efficient boiling heat transfer applications.

Pool boiling represents a potentially efficient mechanism for thermal management of power electronic components. The present study investigates pool boiling heat transfer using Novec 649 on copper substrates with varying surfaces. Sandblasting, which is a relatively simple surface modification technique, has been used to obtain different copper surface roughness levels ranging from 0.1 µm to 9 µm arithmetic mean surface roughness values. The effects of surface modifications and orientation on boiling heat transfer coefficient (HTC), and critical heat flux (CHF), of Novec 649 have been experimentally investigated by performing pool boiling experiments at atmospheric pressure and saturation pool temperature. The experimental results showed that the size and orientation of the boiling surface do not have a significant impact on the CHF, whilst the HTC improves in the nucleate boiling regime with vertical orientation. In addition, for the range of surface roughness investigated improvements of ∼3X and ∼1.5X in HTC and CHF, respectively, were found relative to the original smooth surface. The HTC in the film boiling regime relative to the nucleate boiling regime was also quantified and found to be ∼20X for our geometries.

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 fuel cladding in nuclear reactors is subjected to long periods of high temperature and pressure operation, and a certain degree of reaction control is accomplished by adding chemicals such as boric acid to the coolant, which puts the fuel cladding in a chemically corrosive environment; in addition, in the pressurized water reactor first circuit, the cooling mass flows through the steam generator and stainless steel coolant piping, which are mainly made of nickel-based alloys, and even though these metal vessels or pipes have been passivated to produce a dense oxide film, which greatly reduces the rate of corrosion reactions, metal ions are still inevitably released into the coolant. Therefore, in practice, Chalk River unidentified deposits (CRUD) are naturally formed on the surface of the fuel cladding. It has been found that the CRUD layer is a porous hydrophilic layer with certain surface characteristics such as porosity, good wettability, as well as certain capillary characteristics. Considering the presence of CRUD layer on the fuel cladding, it is important to clarify the mechanism of the CRUD layer on the heat transfer characteristics of the flow in the reactor to guide the routine operation and maintenance of pressurized water reactors as well as the development of thermal safety design benchmarks. It is difficult to obtain CRUD layer directly. Some studies have shown that the SiO2 deposited layer has some similarity to the CRUD layer. Therefore, SiO2 deposited layer can be used to simulate CRUD layer. Based on the above analysis, zirconium-4 alloy was used in this paper as a substrate and deposited SiO2 deposition layers of different thicknesses on its surface by layer-by-layer deposition method to simulated CRUD through the SiO2 deposited layers. The surface morphological parameters of the SiO2 deposition layers, including contact angle, surface porosity and surface roughness, were subsequently measured. Experimental studies were carried out on a flow boiling experimental platform to compare and analyzed the effects of different thicknesses of SiO2 deposited layer on the flow heat transfer characteristics, and provided a reference for clarifying the heat transfer characteristics of the deposited layers on the fuel envelope surface. The SiO2 deposited surface shows better flow heat transfer characteristics and higher critical heat flux than the non-deposited surface, which is mainly due to the difference in surface wettability. The enhancement of heat transfer capability also differs between different thicknesses of SiO2 deposited surface, with the enhancement effect of 3 μm SiO2 deposited surface being more obvious compared to 1 μm SiO2 deposited surface, especially at the subcooling degree of 0 K, the critical heat flux increased by 77.4%.

A phenomenological study on CHF in a bilaterally heated annulus with equal heat flux on both sides was experimentally performed. The working fluid of the present test was R-134a. Variation characteristics of CHF and transition of CHF occurrence location were investigated under different pressure, mass flux and quality conditions. With the increase of critical thermodynamic quality, it was found that CHF first occurred on the outer surface of the annulus, then simultaneously occurred on both sides, and finally occurred on the inner surface at relatively high critical quality. After the CHF location transitioned to the inner rod, the sharp fall of CHF in the limiting critical quality region was observed. The critical quality corresponding to the CHF location transition decreased with the increase of mass flux and pressure. Besides, CHF in tube, internally heated, externally heated and bilaterally heated annuli were compared under the same hydraulic diameter conditions. The present study is conducive to improving the understanding of complicated CHF mechanism in bilaterally heated annulus, enriching the experimental database, and providing evidence for developing accurate CHF mechanism model for annuli.