Fluids

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14 pages, 2995 KiB

Open AccessArticle

Prediction of Critical Heat Flux for Subcooled Flow Boiling in Annulus and Transient Surface Temperature Change at CHF

by Wei Liu

Fluids 2022, 7(7), 230; https://doi.org/10.3390/fluids7070230 - 07 Jul 2022

Cited by 1 | Viewed by 1467

The ability to predict critical heat flux (CHF) is of considerable interest for high-heat equipment, including nuclear reactors. CHF prediction from a mechanistic model for subcooled flow boiling in rod bundles still remains unsolved. In this paper, we try to predict the CHF [...] Read more.

The ability to predict critical heat flux (CHF) is of considerable interest for high-heat equipment, including nuclear reactors. CHF prediction from a mechanistic model for subcooled flow boiling in rod bundles still remains unsolved. In this paper, we try to predict the CHF in an annulus, which is the most basic flow geometry simplified from a fuel bundle, using a liquid sublayer dryout model. The prediction is validated with both water and R113 data, showing an accuracy within ±30%. After the CHF in an annulus is calculated successfully, a near-wall vapor–liquid structure is proposed on the basis of the liquid sublayer dryout model. Modeling of heat transfer modes over the heating surface at CHF is performed, and predictions of the changes in liquid sublayer thickness and heater surface temperature at the CHF occurrence point are carried out by solving the heat conduction equation in cylindrical coordinates with a convective boundary condition, which changes with the change in flow pattern over the heating surface. Transient changes in the liquid sublayer thickness and surface temperature at the CHF occurrence point are reported. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology)

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20 pages, 2349 KiB

Open AccessArticle

Numerical Simulation of Departure from Nucleate Boiling in Rod Bundles under High-Pressure Conditions

by Sai Raja Gopal Vadlamudi and Arun K. Nayak

Fluids 2022, 7(2), 83; https://doi.org/10.3390/fluids7020083 - 18 Feb 2022

Cited by 2 | Viewed by 2617

Abstract

In subcooled boiling flows beyond a certain heat flux, heat transfer is hampered due to a phenomenon known as Departure from Nucleate Boiling (DNB). Conducting DNB experiments at one-to-one nuclear reactor operating conditions is highly challenging and expensive. Another alternative approach is to [...] Read more.

In subcooled boiling flows beyond a certain heat flux, heat transfer is hampered due to a phenomenon known as Departure from Nucleate Boiling (DNB). Conducting DNB experiments at one-to-one nuclear reactor operating conditions is highly challenging and expensive. Another alternative approach is to use Look-up table data. However, its applicability is limited due to its dependence on rod bundle correction factors. In the present investigation, a state-of-the-art Eulerian-Eulerian two-fluid model coupled with an extended heat flux partitioning model is used to predict DNB in tubes and rod bundles with square and hexagonal lattices (relevant to Pressurized Water Reactors). In this approach, bubble departure characteristics are modeled using semi-mechanistic models based on force balance analysis. The predicted DNB values are compared with experimental and Look-up table data and found out to be within 1.8% to 20%. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology)

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12 pages, 15461 KiB

Open AccessArticle

Rim Breakups of Impacting Drops on a Superhydrophobic Surface and a Superheated Surface

by Minori Shirota, Masaki Kato and Ai Ishio

Fluids 2022, 7(2), 79; https://doi.org/10.3390/fluids7020079 - 15 Feb 2022

Cited by 2 | Viewed by 2041

Abstract

The rim breakup of an impacting drop is experimentally investigated by comparing the impacts on superheated and superhydrophobic surfaces. The objective of the present study is to experimentally examine whether the Bo = 1 criteria holds for the rim breakups of drops impacting [...] Read more.

The rim breakup of an impacting drop is experimentally investigated by comparing the impacts on superheated and superhydrophobic surfaces. The objective of the present study is to experimentally examine whether the Bo = 1 criteria holds for the rim breakups of drops impacting on the surfaces. A transparent sapphire plate was heated to achieve the Leidenfrost impact, which enables us to observe with a high-speed camera from below. The characteristics of the rim breakup were evaluated quantitatively using a particle tracking velocimetry method for both the rim and the drops generated. As a result, we clarified that Bo of the rim increases in the spreading phase and marks the highest value of 0.5 on a superheated surface, which is smaller than that on a pillar, where Bo ≈ 1. On a superhydrophobic surface, the highest Bo was 1.2, which is smaller than that on a wettable solid surface, 2.5, but close to the value on a pillar. We also revealed that diameters of generated drops collapse on a master curve when plotted as a function of pinch-off time for both the impacts on superheated and superhydrophobic surfaces. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology)

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23 pages, 7233 KiB

Open AccessArticle

Single Contaminated Drops Falling through Stagnant Liquid at Low Reynolds Numbers

by Kosuke Hayashi, Yuya Motoki, Matheus J. A. van der Linden, Niels G. Deen, Shigeo Hosokawa and Akio Tomiyama

Fluids 2022, 7(2), 55; https://doi.org/10.3390/fluids7020055 - 25 Jan 2022

Cited by 1 | Viewed by 2392

Abstract

Numerical simulations of contaminated spherical drops falling through a stagnant liquid at low Reynolds numbers are carried out using the finite difference method. The numerical results are used to describe the behavior of the surfactant concentrations and to understand the surfactant effects on [...] Read more.

Numerical simulations of contaminated spherical drops falling through a stagnant liquid at low Reynolds numbers are carried out using the finite difference method. The numerical results are used to describe the behavior of the surfactant concentrations and to understand the surfactant effects on the fluid motions in detail. The predicted interfacial surfactant concentration,

Γ

, is almost zero for angles,

θ

, below a certain value (the stagnant-cap angle,

θ_{c a p}

), whereas it steeply increases and reaches a large value for

θ > θ_{c a p}

(the stagnant-cap region). The increase in the initial surfactant concentration,

C_{0}

, in the drop enhances the adsorption from the drop to the interface, which results in the increase in

Γ

and the decrease in

θ_{c a p}

. Peaks appear in the predicted Marangoni stresses around

θ_{c a p}

, which causes similar peaks in the pressure distribution. The high-pressure spots prevent the fluid motion along the interface, which results in the formation of the stagnant-cap region and the attenuation of the tangential velocity in the continuous phase. The surfactant flux from the bulk to the interface decreases C in the vicinity of the interface for

θ < θ_{c a p}

and the weak diffusion cannot compensate for the reduction in C by adsorption, which results in C at the interface smaller than

C_{0}

. The pattern of the low C region is determined by the advection and does not smear out because of a small diffusive flux. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology)

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21 pages, 3528 KiB

Open AccessArticle

Experimental Correlation for Splashing Condition of Droplets on Solid Substrates

by Yukihiro Yonemoto, Kanta Tashiro, Minori Yamashita and Tomoaki Kunugi

Fluids 2022, 7(1), 38; https://doi.org/10.3390/fluids7010038 - 16 Jan 2022

Cited by 3 | Viewed by 2451

Abstract

Droplet splashing behaviors of water-ethanol binary mixture liquids on roughened solid surfaces were experimentally observed in order to investigate the effects of surface tension, viscosity, and wettability/surface roughness on the splashing occurrence. The range of the droplet volumes was from 1.7 μL to [...] Read more.

Droplet splashing behaviors of water-ethanol binary mixture liquids on roughened solid surfaces were experimentally observed in order to investigate the effects of surface tension, viscosity, and wettability/surface roughness on the splashing occurrence. The range of the droplet volumes was from 1.7 μL to 32.6 μL. The ranges of the surface tension and the viscosity were from 21.1 mN/m to 71.9 mN/m, and from 1 mPas to 2.91 mPas, respectively. The surface roughness range was from 0.03 μm to 1.25 μm for R_a. The present experimental data were evaluated on the basis of the existing models. Resulting from these experiments, a simple model using the Ohnesorge number evaluated by the capillary length was proposed and the accuracy of the predicted critical values such as the critical Weber and Reynolds numbers were discussed. The result indicated that the liquid properties and the quantification of the surface condition such as surface roughness are important factors for the prediction of the splashing behavior. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology)

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10 pages, 4590 KiB

Open AccessArticle

Visualization and Sound Measurements of Vibration Plate in a Boiling Bubble Resonator

by Junichiro Ono, Noriyuki Unno, Kazuhisa Yuki, Jun Taniguchi and Shin-ichi Satake

Fluids 2021, 6(12), 443; https://doi.org/10.3390/fluids6120443 - 09 Dec 2021

Cited by 3 | Viewed by 2076

Abstract

We developed a boiling bubble resonator (BBR) as a new heat transfer enhancement method aided by boiling bubbles. The BBR is a passive device that operates under its own bubble pressure and therefore does not require an electrical source. In the present study, [...] Read more.

We developed a boiling bubble resonator (BBR) as a new heat transfer enhancement method aided by boiling bubbles. The BBR is a passive device that operates under its own bubble pressure and therefore does not require an electrical source. In the present study, high-speed visualization of the flow motion of the microbubbles spouted from a vibration plate and the plate motion in the BBR was carried out using high-speed LED lighting and high-speed cameras; the sounds in the boiling chamber were simultaneously captured using a hydrophone. The peak point in the spectrum of the motion of the vibration plate and the peak point in the spectrum of the boiling sound were found to be matched near a critical heat-flux state. Therefore, we found that it is important to match the BBR vibration frequency to the condensation cycle of the boiling bubble as its own design specification for the BBR. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology)

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Graphical abstract

16 pages, 5848 KiB

Open AccessArticle

Pressure Change for Single- and Two-Phase Non-Newtonian Flows through Sudden Contraction in Rectangular Microchannel

by Masaki Toshimitsu, Yukihiro Yonemoto and Akimaro Kawahara

Fluids 2021, 6(12), 440; https://doi.org/10.3390/fluids6120440 - 07 Dec 2021

Viewed by 2050

Abstract

The flow characteristics of the single-phase liquid and the gas–liquid two-phase flows including the Newtonian and non-Newtonian liquids were experimentally investigated in a horizontal rectangular micro-channel with a sudden contraction—specifically the pressure change across the contraction. The rectangular cross-sectional dimension has W_u [...] Read more.

The flow characteristics of the single-phase liquid and the gas–liquid two-phase flows including the Newtonian and non-Newtonian liquids were experimentally investigated in a horizontal rectangular micro-channel with a sudden contraction—specifically the pressure change across the contraction. The rectangular cross-sectional dimension has W_u × H_u (width × height) = 0.99 × 0.50 mm² on the upstream side of the contraction and W_d × H_d = 0.49 × 0.50 mm² on the downstream side. The resulting contraction ratio, σ_A (=W_d/W_u), was 0.5. Air was used as the test gas (in the case of the gas–liquid two-phase flow experiment), distilled water and three kinds of aqueous solution, i.e., glycerin 25 wt%, xanthangum 0.1 wt% and polyacrylamide 0.11 wt% were used as the test liquid. The pressure distribution in the flow direction upstream and downstream of the channel was measured. The pressure change and loss at the sudden contraction were determined from the pressure distribution. In addition, the pressure change data were compared with the calculation by several correlations proposed by various researchers as well as a newly developed correlation in this study. From the comparisons, it was found that calculations by the newly developed correlations agreed well with the measured values within the error of 30%. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology)

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13 pages, 4908 KiB

Open AccessArticle

Flow Structure and Deformation of Two Bubbles Rising Side by Side in a Quiescent Liquid

by Hiroaki Kusuno and Toshiyuki Sanada

Fluids 2021, 6(11), 390; https://doi.org/10.3390/fluids6110390 - 01 Nov 2021

Cited by 1 | Viewed by 1790

Abstract

In the motion of two spherical bubbles rising side by side, the bubbles are known to attract each other at a high Reynolds number (Re = ρUd/μ). Furthermore, spherical bubbles kiss and bounce under certain conditions; however, deformable bubbles [...] Read more.

In the motion of two spherical bubbles rising side by side, the bubbles are known to attract each other at a high Reynolds number (Re = ρUd/μ). Furthermore, spherical bubbles kiss and bounce under certain conditions; however, deformable bubbles repel each other without kissing. This paper experimentally and numerically presents the flow structures and shape of the nonkissing repulsion of deformable bubbles. For the experimental analysis, we organized bubble behaviors by Galilei number (Ga = ρg^1/2d^3/2/μ) and Bond number (Bo = ρgd²/σ). The bubbles repelled each other without kissing near the unstable critical curve of a single bubble. The curvature inside the gap, which is similar to the shape of a zigzag behavior bubble, was large. For the numerical analysis, the velocity of the equatorial plane inside the gap was larger due to the potential interaction, although the velocity behind was the opposite due to the strengthened vorticity generated at the surface. Furthermore, the double-threaded wake emerged behind the interacting bubbles, and it showed that the rotation direction was repulsion regardless of whether the bubbles attracted or repelled each other. The streamline behind the bubbles in the 2D plane was from the outside to the inside. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology)

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