•Investigated experimentally are combines effects of surface roughness and inclination angle on saturation boiling of HFE-7000 liquid.•Correlated data for CHF, hMNB and corresponding surface ...superheats and ΔTexc as functions of Ra and surface inclination angle.•Compared present data and correlations with those reported in literature for other dielectric liquids.•Captured high speed images of nucleate boiling and near CHF to support interpretation of experimental data.
The saturation temperature of only 34 °C at atmospheric pressure, the high boiling figure-of-merit (BFOM), Critical Heat Flux (CHF) and maximum nucleate boiling heat transfer coefficient (hMNB) make HFE-7000 dielectric liquid a good choice for immersion cooling applications. Pool boiling experiments are performed to investigate the effects of inclination angle on saturation boiling of HFE-7000 dielectric liquid on 10 × 10 mm rough copper surfaces. Experiments varied surface average roughness, Ra, from 0.039 to 1.44 µm and inclination angle of the uniformly heated Cu surfaces, θ, from 0o (upward facing) to 180o (downward facing). Captured images of nucleate boiling in various regions of the boiling curve and near CHF using high-speed camera at 1,000 fps help the interpretation of the experimental results. The values of CHF, hMNB, and corresponding surface superheats as well as those of the surface temperature superheat prior to boiling incipience, ΔTexc, are correlated and compared to reported values for other dielectric liquids. The hMNB, which occurs at the end of the fully developed nucleate boiling region, is higher than the heat transfer coefficient at CHF and the corresponding surface superheat is lower. The CHF and hMNB, increase with increasing Ra and/or decreasing θ, while those of the corresponding surface superheats decrease with increasing θ and/or Ra. In the upward facing orientation (θ = 0o), increasing Ra from 0.039 µm (smooth polished) to 1.44 µm increased CHF by ~ 49%, from 20.7 to 30.85 W/cm2, and decreased the corresponding surface superheats by ~ 38.7% from 23.5 to 14.4 K. In this orientation, hMNB increased 250% from 0.88 to 2.2 W/cm2 K, with increased surface roughness from 0.039 to 1.44 mm and the corresponding surface superheat decreased ~ 41.4% form 22.7 K to 13.3K. The values of CHF and hMNB in the downward-facing orientation (θ = 180o) are ~ 27 - 33% and ~40%, respectively, of those in the upward facing orientation (θ = 0o). In the discrete bubbles’ region at low superheats, the nucleate boiling heat transfer coefficient increased, while that in the fully developed nucleate boiling region at high superheats, decreased with increased θ. Developed correlations of CHF, hMNB and the corresponding surface superheats as well as of ΔTexc are in good agreement with present experimental results and those reported for other dielectric liquids. Results confirmed the promising potential of HFE-7000 dielectric liquid for immersion cooling applications.
•Transition boiling and film boiling regimes, and the Leidenfrost point for spray cooling are reviewed.•Discussed are dominant mechanisms, data trends, and predictive correlations and models.•Shown ...is how to predict the quench curve using spray correlations as boundary conditions.•Also shown is how the spray system can be configured to optimize mechanical properties of a quenched alloy part.
This paper is the second part of a comprehensive two-part review of spray cooling. The first part addressed the mechanisms and predictive tools associated with the relatively low-temperature single-phase liquid cooling and nucleate boiling regimes, as well as critical heat flux (CHF). The present part is focused on the relatively high-temperature transition boiling and film boiling regimes, and the Leidenfrost point. Discussed are dominant mechanisms, data trends, and predictive correlations and models. This information is especially important to the quenching of metal alloy parts from high initial temperature during heat treating. It is shown how correlations for the different spray cooling regimes and transition points can be implemented into boundary conditions for heat diffusion models to predict the temperature-time (quench) curve everywhere within the quenched part. It is also shown how the quench curve can be combined with the alloy’s transformation kinetics to predict mechanical properties. By properly configuring the sprays used to quench complex-shaped parts, it is also possible to greatly enhance the mechanical properties while minimizing residual stresses.
•The heat transfer performance of two fin structures, namely regular and modified fins, was experimentally studied in a pool boiling facility.•An artificial nucleation site was fabricated on the ...heated surface to enhance the heat transfer performance.•The modified fin with the artificial nucleation site showed higher heat transfer performance than the regular fin.•A multilayer perceptron artificial neural network is applied for modeling the bubble departure diameter in boiling.
Boiling heat transfer associated with phase change is perhaps one of the most efficient cooling methodologies to manage extreme heat flux due to its large latent heat. Fin structures are used to further increase the magnitude of boiling heat transfer from the heated surface and have shown better performance than flat surface heat sinks. This work aims to experimentally investigate the heat transfer performance of two fin structures, namely regular and modified fins, in a pool boiling facility. The modified hollow fin structure is designed to enhance the regular fin's heat transfer performance by adding an artificial nucleation site. Heat transfer rates and heat transfer coefficients of the two fin structures are estimated in atmospheric pressure conditions using deionized water and compared with the literature. The results show that the regular fin heat sink shows a better heat transfer rate than the plane surface, while the modified fin structure shows higher heat transfer performance than the regular fin. This is attributed to the additional nucleation sites on the hollow fin, a better rewetting phenomenon, and therefore a favorable bubble growth and release mechanism. Also, a multilayer perceptron artificial neural network with a back-propagation training algorithm is applied for modeling the bubble departure diameter concerning wall superheat and subcooling level to predict the bubble behavior from the artificial nucleation site.
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Notwithstanding the gradual substitution of conventional fossil fuels with sustainable energies, the enduring relevance of nucleate boiling persists, owing to its remarkable prowess in heat transfer. ...This article undertakes a comprehensive review of the diverse applications of nucleate boiling within the renewable energy (concentrated solar thermal and geothermal energy) and unconventional thermal management (concentrator photovoltaic, wind turbine, electronic battery and fuel cell) as witnessed in recent years. These initial forays have incontrovertibly validated the vast potential of boiling across various nascent fields. Nevertheless, considerable scope remains for refining both the design of its energy conversion schemes and the methodologies employed in boiling modeling. Furthermore, the latest advancements in theoretical modeling of nucleate boiling are reviewed as well. In the future, greater emphasis will be placed on comprehending the intricacies of bubble dynamics, while adopting transient simulations and differentiation of individual bubbles. The objective of this article is to foster effective communication between researchers operating within the realm of renewable energy systems and unconventional thermal management and those engaged in the conventional study of boiling, thereby facilitating the wider application and optimal utilization of nucleate boiling within the domain of unconventional energy systems.
•Boiling research is still relevant and promising in the new energy era.•Boiling modeling of concentrated solar thermal and geothermal is rapidly improving.•Boiling mostly achieves overfulfilled cooling, but needs further optimization.•Refining bubble behavior is the core of improving nucleate boiling simulation.•Systematic boiling models coupling more bubble dynamic behaviors are trends.
•Copper surface is made hydrophobic through laser texturing.•Heat transfer is compared between a polished and textured surface.•Wettability is shown to have a significant influence on the flow ...boiling curve.•Existing modeling of critical heat flux has difficulty accounting for wettability.
The critical heat flux (CHF) marks the upper limit of safe operation of heat transfer systems that utilize two-phase boiling heat transfer. In a heat-flux-controlled system, exceeding the CHF results in rapid temperature excursions which can be catastrophic for system components. Recent studies have focused on the influence of surface wettability on the departure from nucleate boiling (DNB) through surface modifications and coatings, though many of these studies are limited to pool boiling systems. In this study, the surface wettability influence is studied on the boiling curves and specifically the point of DNB. A femtosecond laser is used to texture the surface to change the wettability from hydrophilic to hydrophobic. A parametric study is performed with mass flux, pressure, and inlet subcooling in a vertical rectangular channel that is heated from one side. CHF excursions are triggered under various system conditions and are compared with existing models. For the experimental conditions considered, the hydrophobic surface showed delayed onset of nucleate boiling compared to the hydrophilic surface, shifting the boiling curves to higher wall superheat. The hydrophobic surface also showed significantly lower CHF for the same system conditions and less sensitivity to changes in subcooling.
•Effect of multiple bubbles hydrodynamics on pool boiling heat transfer is studied.•CFD model taking into account phase and bubble shape changes is proposed.•Bubbles coalescence can reduce departure ...time.•Vortices enhance heat transfer performance and temperature uniformity.•Correlations for nucleation site density and liquid temperature are proposed.
In order to clarify the effect of the hydrodynamics of multiple bubbles on the pool boiling heat transfer process, artificial nucleation sites are manufactured on the substrate surface to generate isolated bubble or multiple bubbles. Then, thermodynamic, bubble dynamics and liquid velocity vector field are investigated, through experiment and Computational Fluid Dynamics (CFD) simulation. The interface capturing and energy equations with phase change model are implemented into the simulation, and a combination of thermal parameter measurements, charge coupled device (CCD) camera and particle image velocimetry (PIV) is adopted in the experiment. The results indicate that vapor temperature inside the bubble is significantly higher and the returning liquid around bubble has larger velocity. It will be seen from this that the bubble is an excellent heat carrier and convection flow promoter in pool boiling. Thus, the liquid temperature at the bubble passage is higher. The coalescence of adjacent bubbles can increase bubble diameter rapidly and reduce departure time, and this effect can be intensified through increasing heat flux or wall superheat. The vortices generated during bubble rise can enhance not only the boiling heat transfer performance but also the temperature uniformity. The correlations for nucleation site density and liquid temperature are proposed.
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•A state of art of boiling heat transfer on micro/nano structured surface was introduced.•The technique of micro/nano structures fabrication was introduced.•The enhanced boiling heat transfer was ...reviewed by the physical mechanism.•The enhanced critical heat flux was reviewed by the physical mechanism and model.•The further research and new insight was proposed.
In the recent decades, the rapid growth of surface modification and fabrication technologies has facilitated the achievement of boiling heat transfer enhancement on micro/nanostructured surfaces. In this paper, several researches on the micro/nanostructured surfaces that have been designed to enhance boiling heat transfer are introduced and closely reviewed. Firstly, theoretical and experimental researches on nucleate boiling heat transfer (NBHT) and critical heat flux (CHF) are introduced in the outline. The fabrication techniques for achieving these engineered surfaces, which are technically classified into machining, coating, chemical process, and micro/nanoelectromechanical systems, are described in detail in the paper. Explanations and analysis of the results of boiling heat transfer enhancement tests are presented in view of NBHT and CHF. Finally, the special features of the existing surfaces capable of enhancing boiling heat transfer are summarized, and the need for future research is also presented.
•The DNB-type CHF characteristics of subcooled flow boiling in compound heat transfer tubes have been investigated.•The combination of twisted tape inserts and internal thread can delay the ...occurrence of CHF.•The CHF was directly proportional to the pitch, mass flux and pressure and inversely proportional to the twist ratio.
In order to enhance the Departure from Nucleate Boiling (DNB) type critical heat flux (CHF) of the divertor in International Thermonuclear Experimental Reactor (ITER), the novel compound heat transfer tube (CHTT) was proposed as a compound CHF enhancement technique byinserting a twisted tape into an internalthreadtube (ITT). Firstly, the experiments were investigated for subcooled water flow boiling on four kinds of cooling tubes, including plain tube (PT), tube with twisted tape inserts (TWTT), ITT and CHTT, covering system pressures from 0.2 to 0.8 MPa, inlet temperatures from 25 to 65 ℃, mass fluxes from 543 to 2935 kg/m2s and heat fluxes from 1 to 17 MW/m2. The experimental results showed that the CHTT combined by using the twisted tape inserts and internal thread tube performed better in enhancing the subcooled CHF than the individual technique. The values of CHF in CHTT could be improved by 33.19%, 55.14% and 130.71%, respectively, compared with that of the TWTT, ITT and PT, respectively. Secondly, the effect of twist ratio, pitch, mass flux and pressure on CHF were also analyzed in CHTT. The CHF was directly proportional to the pitch, mass flux and pressure and inversely proportional to the twist ratio. Finally, the five existing subcooled CHF correlations were evaluated based on the present experimental data in CHTT. It was found that the Tong-75-Ⅱ correlation showed the best prediction accuracy with a mean absolute error (MAE) of 32.80% and a root mean square error (RMSE) of 33.72%.
•In this paper, the onset of nucleate boiling (ONB) of a swirl tube is studied.•It was found that ONB heat flux increased as sub-cooling and mass flow rate increased.•As the pressure increases, the ...ONB heat flux decreases due to a decrease in latent heat.•Existing ONB correlations show a high error rate in predicting ONB heat flux of swirl tube.•New ONB correlations are developed through Python code and dimensional analysis.
Future tokamak cooling systems should always be maintained in a single phase during operation to avoid potential hazards caused by vapor. To this end, a correlation to accurately predict sub-cooled flow boiling onset of nucleate boiling (ONB) under the one-side high heat load condition, which is the heat flux condition inside the tokamak, is required. According to this necessity, the ONB of a one-side heated swirl tube was experimentally explored in this study. The inserted swirl tape induces a secondary swirling flow, which causes ONB to occur at a higher heat flux condition than the smooth tube. As the twist ratio of the tape decreased, the ONB heat flux enhancement rate increased. Sub-cooling and mass flow rate have a proportional relationship with ONB heat flux, whereas pressure has an inverse relationship. The prediction accuracy of the existing ONB correlations was evaluated. However, most correlations tended to under-predict the experimental values. Because they were not developed for the purpose of predicting the ONB of the swirl tube under sub-cooled flow conditions, all system parameters and the effect of the swirl tape were not reflected. Therefore, we developed new correlations using Python combined with machine learning.
•DNB phenomena under different flow rates were observed for a 120 mm long ITO heater using the total reflection technique and IR thermometry.•The observation supported the dry match model.•The ...peripheral temperature of dry patches was 150°C, which was the Leidenfrost temperature for our test condition.•At higher flow rate, smaller vapor slugs and subsequent weak water slugs led to stable and gradual growth of dry patches.
Departure from nucleate boiling (DNB) is a limiting phenomenon in the safety of nuclear reactors. Several mechanistic models have been suggested for predicting the critical heat flux at the DNB. However, existing experiments could not fully reveal the underlying phenomena of mechanistic models. In this study, we closely examined the phenomenon of rewetting failure of a dry patch during DNB to provide a deeper understanding of the mechanism of DNB occurrence and to assist in subsequent mechanistic modeling. Specifically, we utilized the total reflection technique and IR thermometry to obtain the synchronized side-view, total reflection view, temperature, and heat flux profile images at the DNB. We established a water flow loop under atmospheric pressure with a vertical rectangular test section comprising an ITO heater coated on a sapphire substrate. The ITO heater was 10 mm wide and 120 mm long. The long length of the ITO heater enabled the development of elongated bubbles upstream of the DNB location. A series of tests were conducted under various flow rates. Slug flow was observed in all tests conducted. An alternate passage of elongated bubbles and water slugs led to expansion and shrinkage of the dry patches. At the DNB, the dry patch size continued to increase, but the developing pattern was dependent on the flow rate. The peripheral temperature of the dry patches at the DNB was found to be ∼150°C.
