•MIDI: an integral-scale experiment of a light-water-reactor spent fuel pool.•A study of the influence of the spent fuels loading pattern, decay heat and initial pool level.•A phenomenology which is ...characterized by three competing liquid vaporization modes.•The gravity-driven flashing is accidentally likely in a spent fuel pool.
Motivated by the 2011 accident at the Fukushima Daiichi nuclear power station, the French Institute for Radiation Protection and Nuclear Safety (IRSN) launched in 2014 the DENOPI project. This research program, supported by the French Government and carried out in collaboration with national and international partners, dealt with the open issues of spent fuel pool accidents. As part of this project, a test facility named MIDI was designed and built at the IRSN’s research unit of Cadarache, France. The MIDI facility is an experimental tool which aims at reproducing the thermal–hydraulic phenomena that may occur at an integral scale in a spent fuel pool undergoing a loss-of-cooling accident, before the stored fuels get uncovered. This paper first describes the MIDI facility and its instrumentation and the test matrix achieved to date. An overview of its main results is then provided. In the framework of the DENOPI project, nine tests were achieved and allowed investigating the phenomenology of this type of accident. The highlighted phenomenology is characterized by three competing liquid vaporization modes: the evaporation at the pool free surface, the nucleate boiling within the fuel bundles and the gravity-driven flashing of superheated water on top of the storage racks. The existence of the latter vaporization mode, uncertain at the launch of the DENOPI project, is now confirmed in the configuration of a spent fuel pool.
•Biphilic pattern enables vapor bubbles to move on the surface during boiling.•On-surface bubble movement affects boiling heat transfer.•Biphilic surface shows boiling enhancement for low heat ...fluxes.•Biphilic surface increases critical heat flux.
In pool boiling on a horizontal surface, the flow of vapor is mainly the departure of vapor bubbles from the boiling surface. If the surface is composed of specially laid-out areas with varied wettabilities, vapor can move on the surface and out of the boiling region. In the present work, a silicon surface with such a function is fabricated using laser ablation and silanization grafting methods, and pool boiling on the surface is experimentally studied. On the biphilic surface, the boiling region is super-hydrophilic (SHI), and two opposite sides of the SHI region each is in connection to a wedge-shaped SHO (super-hydrophobic) track. The two sides of the SHO track are bounded by HO (hydrophobic) areas. Tests with air bubbles underwater show that the SHO tracks enable the directional spontaneous movement of the air bubbles on the surface, and the driving force for the natural motion is analyzed. Boiling heat transfer on the surface is compared with a SHI surface without the SHO tracks. The biphilic surface shows noticeable enhancement of heat transfer for low heat fluxes (< ∼150W/cm2) but reduces heat transfer for high heat fluxes in nucleate boiling. In addition, the biphilic surface extends the heat flux and surface temperature ranges of nucleate boiling, showing ∼14% increase of critical heat flux. The effects on boiling heat transfer are related to the observations of vapor transport by the SHO tracks.
•A commercial CNT buckypaper is utilized to enhance boiling of dielectric liquids.•CHF and maximum HTC were increased by 74.2 % and 102 % compared to the flat surface.•Surface characteristics, ...wickability and bubble dynamics were investigated.•Predicted CHF based on a modified model agrees with the experimental result.•Low-cost and mass-producible buckypapers is promising for immersion cooling.
To further develop the two-phase immersion cooling technology with dielectric liquids and accelerate their applications in industrial practices, a comparative experimental study on saturated pool boiling heat transfer of FC-72 on the carbon nanotube (CNT) buckypaper and the smooth indium tin oxide (ITO) film is performed at atmospheric pressure in the present work. The commercially available buckypaper is selected as the nanostructured reinforced surface because of its low-cost, highly-scalable and mass-producible advantages. It achieves enhancements of CHF and maximum HTC up to 74.2 % and 102 %, compared to the smooth ITO surface. The underlying mechanism is well elucidated from the perspectives of surface characteristics, capillary wicking capability and bubble dynamics. The interconnected nanoporous network structure of CNT buckypaper can significantly improve bubble dynamics behavior and induce a marked nanoscale capillary wicking effect. The outstanding wickability can provide additional liquid replenishment on the boiling surface, facilitating the rapid generation and departure of bubbles as well as delaying film boiling. Interestingly, in terms of dielectric fluid (FC-72), it is found that the markedly increased bubble departure frequency on the CNT buckypaper surface primarily arises from a substantial reduction in the bubble waiting period. The present work not only provides an in-depth insight into the enhanced pool boiling characteristics of dielectric liquids on nanostructured surfaces, but also opens opportunities for the industrial applications of state-of-the-art two-phase immersion cooling technology based on nanostructures.
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The present investigation is focussed on the surface associated qualitative analysis for nucleate pool boiling heat transfer characteristics of alumina-water based nanofluids. An ...optimal ANN design has been developed with various training algorithms and hidden layer based on experimental results. The nanofluids were prepared using distilled water and alumina nanoparticles of 40 nm size. The particle concentrations varied form 0.01 wt.% to 1 wt.%. The characterisation of nanoparticles and nanofluids was done using FESEM, EDAX, zeta potential stability test and thermophysical analysis. The pool boiling setup validation was done by comparing experimental results with Rohsenow’s correlation outputs. The boiling heat transfer was performed by varying input heat flux and calculating heat transfer coefficients with surface temperatures. The post boiling analysis was also performed on the nanoparticle deposited surfaces. The effect of nanoparticle concentrations and microlayer particle deposition is studied using surface roughness analysis, contact angle measurement and microscopic images. The pool boiling heat transfer with nanofluids showed an enhancement of up to 70 % as compared to distilled water. The improved thermophysical properties of nanofluids and the enhanced surface wettability provided an overall increase in heat transfer coefficients. The particle deposited surfaces showed heat transfer deterioration of up to 40 % with water. The ANN model analysis showed that the LM (Levenberg Marquardt) algorithm provided the most optimised structure. The combination of number of epochs and MSEs (Mean square errors) were taken for selecting the best model.
•Review heat transfer/flow parameters from studies on server cooling technologies.•Perform direct quantitative comparison between cooling technologies.•Identify heat transfer limitations, power ...requirements, etc.•Summarize industry implementations of each cooling solution from a design perspective.•Identify the capacity of cooling solution relative to heat loads expected by 2020.
This review quantitatively examines and compares the heat transfer characteristics of several cooling technologies with potential application in the server electronics industry. Strategies that have been examined include traditional air cooling, single and two-phase indirect liquid cooling, heat pipes, pool boiling, spray cooling, and jet impingement. The characteristics that have been examined include heat flux values, coolant temperatures, and coolant flowrates; which serve as indicators of the heat transfer limitations and power requirements of each cooling solution. A direct comparison against anticipated server heat loads has shown that some form of liquid cooling is necessary in high performance computing applications; where individual processor heat loads are expected to reach 300W by the year 2020. While in the case of general purpose computing, where individual processor heat loads are expected to reach 190W, air cooling remains a viable option; although other factors such as operating costs, chip reliability, and waste heat recovery may still encourage the use of liquid cooling.
•Different pool boiling CHF mechanisms and parametric trends are reviewed.•Also reviewed are modifications to mechanisms proposed to improve predictive accuracy.•Available models are shown to focus ...on specific parametric influences but not others.•The study points to the need for better understanding of near-wall interfacial behavior.
Critical heat flux (CHF) is arguably the most important design and safety parameter for any heat-flux controlled boiling application. The present two-part study is focused on CHF for pool boiling from flat surfaces. The first part will review different CHF models and associated mechanisms and parametric trends, while the second part will be dedicated to assessment of CHF models and correlations. Aside from Kutateladze’s 1948 pioneering CHF formulation, which is based on dimensional analysis, five different CHF mechanisms are prevalent in the literature: bubble interference, hydrodynamic instability, macrolayer dryout, hot/dry spot, and interfacial lift-off. Additionally, many modifications to these mechanisms have been proposed to improve predictive accuracy in tackling the parametric influences of pressure, surface size and roughness, surface orientation, and contact angle. Among the five mechanisms, Zuber’s hydrodynamic instability theory has received the most attention because of both its mechanistic formulation and theoretical appeal. More recently, the interfacial lift-off mechanism, which is also theoretically based, has received significant experimental validation, and offers the advantage of tackling different surface orientations. Overall, it is shown that, despite the large body of published pool boiling CHF literature, there are major data gaps in the coverage of relevant parameters. This points to a need for more strategically planned future experiments that would also include microphotographic analysis of near-wall interfacial features, in order to validate or dispute proposed CHF mechanisms.
We conducted a comprehensive investigation into the mechanisms the enhancement of pool boiling heat transfer. Created a porous medium composite free particle reinforced structure (FPPM). FPPM is ...synthesized with copper foam (Cu foam) with a porous structure as the base material; it is characterized by free particles inside the cavity. We used deionized water (DI water) as the working fluid to study the heat transfer performance of pool boiling on different surfaces under atmospheric pressure. The boiling mechanism was verified by bubble dynamics. The findings showed that the addition of free particles to the copper foam sample significantly reduced the superheating of its onset of nucleate boiling (ONB). Compared to the polished surface, a maximum of 2.57 times the heat transfer coefficient was achieved; in addition, a maximum of 1.59 times the heat transfer coefficient was achieved compared to the copper foam surface without free particles. The visualization results showed the formation of nucleation sites on the surface with free particles, which decreased the energy barrier to easy nucleation. During the boiling process of the pool, the high thermal conductivity of free particles and the disturbance to the gas–liquid layer significantly reduced the bubble detachment frequency, resulting in good heat transfer performance over the whole range of heat fluxes.
•Pool boiling heat transfer characteristics of a ferrofluid were studied.•Presence of positive magnetic field gradient decreases the boiling heat transfer.•At higher concentrations of nanofluid, the ...effect of the magnetic field is boosted.
In this research, an experimental study was conducted to investigate the pool boiling heat transfer of Fe3O4/water nanofluid (ferrofluid) in the atmospheric pressure. This study also investigated the influence of the magnetic field on the rate of boiling heat transfer of nanofluid. Deionized (DI) water was used to examine the repeatability, integrity and precision of the experimental apparatus where a well agreement with the existing correlations was observed. The investigation of various volume concentrations of nanofluid revealed that boiling heat transfer in high concentrations decreases with an increase of concentration while it rises with the increase of concentration in low concentrations. The boiling heat transfer coefficient at 0.1% volume concentration nanofluid was evaluated as optimal (increasing up to 43%). In addition, experimental studies showed that the presence of positive and negative magnetic field gradients decrease and increase the boiling heat transfer, respectively. The findings of this study showed that at higher concentrations of nanofluid, the effect of the magnetic field on nanoparticles is boosted. The results of the experiments indicated that adding nanoparticles would not necessarily increase the boiling heat transfer coefficient. In fact, the surface roughness and the magnetic field gradient on the boiling surface were the main factors that could affect the boiling heat transfer coefficient, significantly.
