Haut PDF A critical heat flux model for flow boiling in the ivr conditions

A  critical heat flux model for flow boiling in the ivr conditions

A critical heat flux model for flow boiling in the ivr conditions

Email Address: hm-park@kaist.ac.kr (Hae Min Park), sofia.carnevali@cea.fr (Sofia Carnevali), fabrice.gaudier@cea.fr (Fabrice Gaudier), jeongyh@kaist.ac.kr (Yong Hoon Jeong) A semi-empirical model, which is based on the CHF model developed in KAIST (Park, 2014), is developed to predict the critical heat flux (CHF) in the In Vessel Retention (IVR) configuration. The model consists of five theoretical equations describing the principal CHF variables: relative velocity, liquid velocity, micro layer thickness and the slug length. The CHF mechanism of liquid film dryout underneath slug is basically considered. Velocities of vapor and liquid are given by the Karman velocity distribution and the force balance between buoyancy and drag force. The micro layer thickness is defined by Cheung and Haddad (1997) model, based on the Helmholtz instability for the vapor stem located in the micro layer. Solution is obtained starting from seven scattered input parameters: mass flux, local quality, pressure, inclination angle, gap size, working fluid and heater material. Some assumptions are made concerning the premature CHF, the minimum length of slug and different types of the heater material and the working fluid. URANIE code, developed by Commissariat à l'Energie Atomique (CEA) is used to optimize the solution of the system. The optimization process is based on the integrated IVR-CHF database, including experimental data from KAIST, CEA (SULTAN experiments), UCSB (ULPU experiments) and MIT. For 278 experimental data, the developed CHF model has a root-mean-square (RMS) error of 14 %. The CHF predicted by the model is in good agreement with the experimental IVR-CHF database, except for the condition of high mass flux conditions (>500kg/m²s) and low inclination angle (<10°). A further improvement of the code may be suggested to cover this range and try to reduce the RMS error basing on the future worldwide experimental campaigns.
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Flow boiling in tube under normal gravity and microgravity conditions

Flow boiling in tube under normal gravity and microgravity conditions

microgravity conditions can be simulated by means of a drop- tower, parabolic flights on board an aircraft or a sounding-rocket. Although flow boiling is of great interest for space applications under microgravity conditions, few experiments have been con- ducted in low gravity. These experiments provided a partial under- standing of boiling phenomena and have been mostly performed for engineering purposes such as the evaluation of ISS (‘‘Interna- tional Space Station’’) hardware or two-phase loop stability. More- over, flow boiling heat transfer experiments in microgravity (referred to as l ÿ g) require large heat loads and available space. They are subject to severe restrictions in the test apparatus, do not last long and offer few opportunities to repeat measurements, which could explain the lack of data and of coherence between existing measurements. Nevertheless, several two-phase flow (gas–liquid flow and boiling flow) experiments have been con- ducted in the past forty years and enabled to gather data about flow patterns, pressure drops, heat transfers including critical heat flux and void fraction in thermohydraulic systems. Previous state of the art and data can be found in the papers of Colin et al.
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Flow boiling in tube in microgravity

Flow boiling in tube in microgravity

G=50 kg/m 2 /s, HTC is always lower in 0-g than in 1-g, even for quality larger than 0.15 corresponding to annular flow regimes. The experimental results are compared to two correlations. The correlation of Kim and Mudawar [19] takes into account the contribution of nucleate boiling and convective boiling in the evaluation of the heat transfer coefficient. This correlation is in good agreement with experimental data at low mass flux in normal gravity where both nucleate and convective boiling play a significant role. It seems to overestimate the HTC at low quality probably because the nucleate boiling contribution is limited in our experiment due to the very smooth surface of the sapphire tube. For high quality and high mass flux, dominated by Two-phase Forced Convection, the experimental results are in better agreement with the model of Cioncolini and Thome [20] predicting the heat transfer coefficient for an evaporating turbulent liquid film. These trends are in agreement with previous results of Baltis et al. [17] and Ohta and Baba [4]. At low quality in the nucleate boiling regime, the HTC is lower in
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Gas microflows in the slip flow regime: A critical review on convective heat transfer

Gas microflows in the slip flow regime: A critical review on convective heat transfer

Unsteady convection with CHF due to a sudden heat flux change or due to a sudden inlet pressure change were numerically investigated by Sun and Jaluria [47], taking into account viscous dissipation and thermal creep effects. Roughness effects Roughness effects in the CWT case have been taken into account by Khadem et al. [8], with slip boundary conditions that should be adapted to curved surfaces, but unfortunately as mentioned in section 3, with an inappropriate velocity derivative term leading to Eq. (6) instead of the correct Eq. (5) . The authors also considered thermal creep in their numerical simulation. The wall roughness was modeled with periodically distributed triangular elements and random shaped micro peaks. It was found that roughness resulted in a decrease of Nusselt number and had more significant effect on higher Knudsen number flows with higher relative roughness. In another paper [9], Hossainpour and Khadem investigated the role played by the roughness shape (rectangular, triangular, trapezoidal or make of random peaks) and shown that the Nusselt number is more sensitive to the roughness density than to the roughness shape. As well as in [8], however, Eq. (6) was used instead of Eq. (5) and the consequence on the results should be further investigated. Ji et al. [48] considered the same problem with second-order boundary conditions and drew qualitative similar conclusions. Croce and d’Agaro [49] analyzed the competition between rarefaction and compressibility effects for high pressure drop flows in rough microchannels. They took into account viscous dissipation and covered a wide range of the Mach number. The slip boundary condition was similar to Eq. (5) but without the thermal creep term:
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Distributions of critical flux: modelling, experimental analysis and consequences for cross-flow membrane filtration

Distributions of critical flux: modelling, experimental analysis and consequences for cross-flow membrane filtration

However, such a sharp transition is not often observed in practice and discrepancies between the concept and experiments are observed. Even experimental works carried out with well characterized suspensions and membranes often exemplify this discrepancy [5,6] and this is further presented in the experimental section of this paper. To take this into account, the concept of critical flux has sometimes evolved by distinguishing a weak form of critical flux from the original strong form of critical flux [7]. The weak form of critical flux is based on the subtle difference between slow fouling conditions (inducing permeability smaller than that obtained with a clean membrane filtering pure water) and faster fouling (inducing a deviation from the initial linearity of the J vs TMP curve). The weak form of critical flux thus shows its ability to describe experiments with numerous fluids from model fluids to complex ones [7]. However, this weak form of critical flux loses the original significance of the previous concept of critical flux and has no direct theoretical grounding.
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Distributions of critical flux: modelling, experimental analysis and consequences for cross-flow membrane filtration

Distributions of critical flux: modelling, experimental analysis and consequences for cross-flow membrane filtration

However, such a sharp transition is not often observed in practice and discrepancies between the concept and experiments are observed. Even experimental works carried out with well characterized suspensions and membranes often exemplify this discrepancy [5,6] and this is further presented in the experimental section of this paper. To take this into account, the concept of critical flux has sometimes evolved by distinguishing a weak form of critical flux from the original strong form of critical flux [7]. The weak form of critical flux is based on the subtle difference between slow fouling conditions (inducing permeability smaller than that obtained with a clean membrane filtering pure water) and faster fouling (inducing a deviation from the initial linearity of the J vs TMP curve). The weak form of critical flux thus shows its ability to describe experiments with numerous fluids from model fluids to complex ones [7]. However, this weak form of critical flux loses the original significance of the previous concept of critical flux and has no direct theoretical grounding.
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Assessment of Critical Heat Flux correlations in narrow rectangular channels

Assessment of Critical Heat Flux correlations in narrow rectangular channels

channel is discretized with 150 computational volumes of 4 mm each. The simulations were proven to be mesh-independent. The outlet pressure, mass flow-rate and imposed power together with the inlet liquid temperature at CHF were used as boundary conditions. The heat transfer and friction modeling in CATHARE were optimized for narrow rectangular channels, according to the suggestions in [14]. CATHARE computes the critical heat flux at each axial location; however the minimum CHF is always found at the end of the heated test section, due to the uniform heat flux distribution. Thus, only the outlet CHF values were retained for the following analysis.
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Fast Transients and Critical heat flux for experimental reactors applications

Fast Transients and Critical heat flux for experimental reactors applications

 Effects of the parameters of interest (Subsaturation, flow, pressure, exponential period of the transients) ?  How to enhance the transient (for reasonable reactivity insertions) CHF in experimental reactors (surface state / operation conditions, …) ?

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Flow Boiling in straight heated tube under microgravity conditions

Flow Boiling in straight heated tube under microgravity conditions

Toulouse, 31400, FRANCE E-mail: colin@imft.fr Boiling two-phase flow can transfer large heat fluxes with small driving temperature differences, which is of great interest for the design of high-performance thermal management systems applied to space platforms and on-board electronics cooling in particular. However, such systems are designed using ground-based empirical correlations, which may not be reliable under microgravity conditions. Therefore, several two-phase flow (gas-liquid flow and boiling flow) experiments have been conducted in the past forty years and enabled to gather data about flow patterns, pressure drops, and heat transfers including critical heat flux and void fraction in thermohydraulic systems. Previous state of the art and data can be found in the papers of Colin et al. [1], Ohta [2], and Celata and Zummo [3]. However, there is still a lack of reliable data on heat transfer in flow boiling in microgravity. Therefore, the purpose of our study is to clarify gravity effects on heat transfer characteristics and provide a fundamental description of boiling heat transfer for space applications.
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Transient flow boiling in a semi-annular duct: From the Onset of Nucleate Boiling to the Fully Developed Nucleate Boiling

Transient flow boiling in a semi-annular duct: From the Onset of Nucleate Boiling to the Fully Developed Nucleate Boiling

3.10. Impact of test parameters on the behaviour For a same sub-cooling and for 3 different flow rate values, the level of power has been varied as illustrated by the reduced wall to fluid heat flux space and time variation on Fig. 16 . It shows that the behaviour is very similar for the different test conditions. The degradation of the wall to fluid heat flux (blue part) before the transition toward nucleate boiling is not seen in some high flow rate and relatively low power cases (d,g,h). In those latter cases, it can be seen on the high speed camera images, that the vapor pocket incoming from the nucleation site is not as large as in the other cases. This could be interpreted by the lack of any dry-out and therefore no heat transfer degradation. In some cases, it is seen that the wall upstream a nucleation site can be heated up during a relatively long time until another nucleation event leads to its cool- ing (like in the bottom part (z P 4 cm) of cases d or f). In those cases, several successive events of heat transfer degradation and front propagation can be identified. From a more general point of view, the onset of nucleate boiling is all the more delayed than
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Transient flow boiling in a semi-annular duct: From the Onset of Nucleate Boiling to the Fully Developed Nucleate Boiling

Transient flow boiling in a semi-annular duct: From the Onset of Nucleate Boiling to the Fully Developed Nucleate Boiling

3.10. Impact of test parameters on the behaviour For a same sub-cooling and for 3 different flow rate values, the level of power has been varied as illustrated by the reduced wall to fluid heat flux space and time variation on Fig. 16 . It shows that the behaviour is very similar for the different test conditions. The degradation of the wall to fluid heat flux (blue part) before the transition toward nucleate boiling is not seen in some high flow rate and relatively low power cases (d,g,h). In those latter cases, it can be seen on the high speed camera images, that the vapor pocket incoming from the nucleation site is not as large as in the other cases. This could be interpreted by the lack of any dry-out and therefore no heat transfer degradation. In some cases, it is seen that the wall upstream a nucleation site can be heated up during a relatively long time until another nucleation event leads to its cool- ing (like in the bottom part (z P 4 cm) of cases d or f). In those cases, several successive events of heat transfer degradation and front propagation can be identified. From a more general point of view, the onset of nucleate boiling is all the more delayed than
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Extension to various thermal boundary conditions of the elliptic blending model for the turbulent heat flux and the temperature variance

Extension to various thermal boundary conditions of the elliptic blending model for the turbulent heat flux and the temperature variance

key variable in the temperature variance equation; the modeled equation must also be valid for all thermal boundary conditions at the wall; (iii) validating a posteriori, i.e., by full computations, the new model in the forced convection regime. In section 2, the thermal models that are used as a starting point are presented for the turbulent heat flux and the temperature variance, with a particular focus on the modeling of the near-wall region using the elliptic blending approach. In section 3, the asymptotic behavior of the terms involved in the transport equation for the turbulent heat flux is analyzed using Taylor series expansions, depending on the thermal boundary condition at the wall. The last part of this section is devoted to the development and a priori tests of a new turbulent heat flux model that satisfies the asymptotic behavior in the near wall region whatever the boundary conditions. Section 4 is dedicated to the development of a new transport equation for the dissipation rate of the temperature variance which is essential to obtain both an accurate thermal-to-mechanical time-scale ratio and an accurate temperature variance. Finally, in Section 5, the new model is numerically validated against the recent DNS database of Flageul et al. (2015) for a channel flow with three types of wall boundary conditions: an imposed temperature, an imposed heat flux and CHT.
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Flow boiling study in mini-channels

Flow boiling study in mini-channels

Figure 13: Pressure drop versus outlet quality (D h = 2 mm). 9. Conclusions Forced flow boiling heat transfer in minichannels in similar conditions as encountered in automobile air condi- tioners has been studied. Higher heat transfer coefficients than in conventional tubes are achieved but dry-out occurs at low vapour qualities thus decreasing performances. However the average heat transfer coefficient re- mains higher than in conventional tubes. These observations support literature studies which predict that bubble confinement leads to higher heat transfer coefficients and dry-out at low vapour quality in minichannels. The new Kandlikar (2004) general correlation for flow boiling in tubes was found to predict the present results be- fore dry-out occurs. The Thome et al. (2004) and Dupont et al. (2004) three zone model for microscale boiling predicted most of the observed trends, including dry-out and the lack of mass velocity influence. Using it for predictions still requires testing over a consequent database.
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Critical heat flux around strongly-heated nanoparticles

Critical heat flux around strongly-heated nanoparticles

PACS numbers: (68.08.De Liquid-solid interface structure: measurements and simulations; 44.35.+c Heat flow in multiphase system;65.80.+n Thermal properties of small particles, nanocrystals, and nanotubes ) Sub-micron scale heat transfer is attracting a growing interest [1], motivated by both fundamental and techno- logical issues. The fast emergence of this field is, to a large extent, associated with the development of micro and nano technologies. In some cases, thermal transfer is part of the system function (e.g. the use of nanofluids for heat transport or of multilayered materials for ther- mal insulation). In other cases, the enhancement of heat transfer is a key to a proper operation of the microsys- tem (e.g. microprocessors) and involves the integration on ever smaller scales of devices such as micro heat pipes. Although these systems are of micrometer size, the re- gions that limit heat transfer -interfaces, constrictions - are often characterized by even smaller lengths, bring- ing heat transfer issues into the domain of nanosciences. Recent interest in heat transport around nanoparticles has arisen in part from the particular properties of the so called ”nanofluids” [2, 3], i.e. colloidal suspensions of solid nanoparticles, which exhibit improved thermal transport properties. On the fundamental side, a number of laser heating studies were performed demonstrating even melting of metal nanoparticles without macroscopic boiling of the embedding liquid. [4, 5]. The physics of this phenomenon involves a complex interplay between boil- ing, heat transfer, and particle-fluid interactions (wet- ting), and is still poorly understood.
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Experimental study of boiling in porous media: effect of the void fraction on the critical heat flux

Experimental study of boiling in porous media: effect of the void fraction on the critical heat flux

given element due to the increase of the surrounding porous matrix heating has been experimentally demonstrated. The ongoing objective is to link this critical heat flux variation to the void fraction, using two-phase flow visualizations. So far, image processing is limited to the estimation of indicators linked to the local time-averaged void fraction, and to the calculation of the surface void fraction based upon the projection of the bubble areas into a vertical plane. Work is in progress to refine the analysis in order to estimate the volume void fraction. To this purpose, a calibration of the various parameters used in the image processing algorithm is being performed thanks to isothermal bubbling experiment visualizations for which the injected gas flow rate is known. When such a calibration will be performed, the influence of the void fraction on the critical heat flux will be quantified and investigated. These results will have to be taken into account in the reflooding numerical simulation tools used at the IRSN, where critical heat fluxes are so far estimated by using correlations established for convective boiling around one single heating tube [20, 21] and not in a bundle of heating tubes.
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Critical heat flux maxima during boiling crisis on textured surfaces

Critical heat flux maxima during boiling crisis on textured surfaces

Despite being an active area of research for over half a century, the boiling crisis is one of the least understood among known thermal and hydraulic instabilities 8 . Prior studies on increasing CHF include modification of fluid and surface properties 1,5,9 , and use of active techniques such as applied electric fields 10,11 . Surface modifications are particularly attractive because often there are constraints on selection of other fluids and operational conditions. Although most previous studies have identified surface roughness, wettability and porosity as important properties affecting CHF, the physical mechanisms governing the phenomenon remain largely unclear, with widely varying hypothetical explanations such as hydrodynamic instabilities 12,13 , liquid macrolayers 14,15 , vapour recoil 6,16,17 and surface wetting 18–20 . These studies, although informative, have mostly been limited to randomly textured surfaces, such as particle coatings 12,14 , nanowires 21 , graphene 22 and anodized porous materials 18,19 , and therefore do not provide a clear under- standing of the effect of texture on CHF. The conventional hydrodynamic instability formulation of CHF 23 does not fundamentally account for surface effects, whereas the mostly empirical macrolayer dryout theory 24 is in question due to recent experimental studies 25 challenging the existence of the macrolayer. A static porous media flow approach similar to conventional heat pipe analysis 26 has been attempted but found unable to explain CHF enhancement on porous surfaces 27 . Other recent studies employing parametric textures 16,17 have combined a static force balance approach at the liquid–vapour contact line of a bubble 28 with roughness parameters to predict a monotonic increase in CHF with surface texture density. Although this approach to explaining texture-induced CHF enhancement is promising, it does not take into account the dynamics of the contact line, which is essential to capture the physics of the phenomena. After experiments 18–20 indicated that it is surface wetting and not roughness per se that enhances CHF, more recent studies 29,30 (especially Rahman et al. 29 ) have come up with strong correlations between experimental data on CHF and liquid imbibition (or wicking) into the microstructures. However, they propose no physical mechanism detailing the role of liquid imbibition in the boiling crisis, adding essentially a data-correlated term to existing CHF models 23,28 .
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Experimental study of boiling in porous media: effect of the void fraction on the critical heat flux.

Experimental study of boiling in porous media: effect of the void fraction on the critical heat flux.

given element due to the increase of the surrounding porous matrix heating has been experimentally demonstrated. The ongoing objective is to link this critical heat flux variation to the void fraction, using two-phase flow visualizations. So far, image processing is limited to the estimation of indicators linked to the local time-averaged void fraction, and to the calculation of the surface void fraction based upon the projection of the bubble areas into a vertical plane. Work is in progress to refine the analysis in order to estimate the volume void fraction. To this purpose, a calibration of the various parameters used in the image processing algorithm is being performed thanks to isothermal bubbling experiment visualizations for which the injected gas flow rate is known. When such a calibration will be performed, the influence of the void fraction on the critical heat flux will be quantified and investigated. These results will have to be taken into account in the reflooding numerical simulation tools used at the IRSN, where critical heat fluxes are so far estimated by using correlations established for convective boiling around one single heating tube [20, 21] and not in a bundle of heating tubes.
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Alumina Nanoparticle Pre-coated Tubing Ehancing Subcooled Flow Boiling Cricital Heat Flux

Alumina Nanoparticle Pre-coated Tubing Ehancing Subcooled Flow Boiling Cricital Heat Flux

Keywords: Critical Heat Flux, Nanofluid, Boiling-induced Deposition 1. INTRODUCTION With the constant increase in energy demand, high capacity and efficient power plants will be needed. The power density of them will be higher; therefore, higher heat transfer rate is required to remove heat from the power source efficiently and safely. Nucleate boiling, with its high heat of vaporization, is usually the main heat transfer mechanism to remove heat from such systems. However, nucleate boiling is limited by critical heat flux (CHF), the level at which the heat transfer coefficient drops tremendously due to transition to film boiling. This usually causes damage to the heater. In nuclear reactors with very high power density fuel, reaching CHF is a key safety concern because of possible fuel damage. Therefore, many methods, such using wire wrap, swirl flow, twisted tapes…, have been applied to increase CHF. In light water reactors, a common way to enhance CHF is to use mixing vanes. Recently, nanofluids, colloidal dispersions of nanoparticles in a base fluid [1], have been shown to increase CHF. Even at less than 1.0 vol% particle concentration, up to 200% CHF compared to that of the base fluids were measured for pool boiling (You et al. [2] and Kim et al. [3]). Other researchers, such as Vassallo et al [4], Tu et al. [5], Kim and Kim [6], Moreno et al. [7], Bang and
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Calculation of thermal conductance based on measurements of heat flow rates in a flat roof using heat flux transducers

Calculation of thermal conductance based on measurements of heat flow rates in a flat roof using heat flux transducers

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Boiling heat transfer in mini-channels: influence of the hydraulic diameter

Boiling heat transfer in mini-channels: influence of the hydraulic diameter

In spite of numerous publ ica tio n s on t he experimental study of heat transf e r in b oi l ing flows, still few concern the specific study of the inftuence of the hydra u l i c diameter on the bo iling heat transfer coefficient. Consequently classical c orrel at i o n s of boili n g heat transfer coefficient in macro-tubes take in t o account the effects of the h ydrau li c di a m e ter rathe r poor l y. The Steiner a nd Taborek (1992) c o rre l atio n includes a Dt;0.4 term based on some experimental re su l t s for h yd ra u l ic diameters as low as l mm. Ishibashi and Nishikawa (1969) ex perim en ta l results within a c onfi n ed annular space showed that when the bubbles were confined, the heat transfer c o efficien t w as p ro p ort i o n al to Dt;0 67. Aritomi et al. (1993) obtained Dt;0·75 for boiling flows in annular space s with Dh < 4 m m . Tran et al. (1997) proposed a correlation for flow b oi li ng heat transfer in mi ni-ch a m1 e ls with a Dt;1 fo r the confinement e ffect . F ina ll y, Kew and C o rnwell (1994) a r gu ed that the confinement should have a sizable influence on the heat transfer coefficient when a new dimensionless number, t h e confinement number defined as Co = ( cr / (D; · g · (p1 - Pv)) )0·5, is greater than 0.5.
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