Two-phase Flow In Narrow Channel Research Articles

Numerical analysis of sodium-argon two-phase flow in narrow channels under severe accident conditions in pool type SFR

A two-phase flow model for sodium-argon mixture flow through the reactor vessel head penetrations under the energetic core disruptive accident in a sodium fast reactor (SFR) is presented. The model evaluates the total sodium mass in sodium-argon mixture released to the reactor containment building (RCB) through penetrations. The driving pressure at the penetration entrance is given as input. The two-phase frictional and local pressure drops, inertial and piezo-metric heads are accounted for in the model. Depending on the argon volume fraction, either a homogenous or separated flow approach is adopted to evaluate the mixture flow. Reactor-scale analysis show that the presence of argon in vessel head penetrations can significantly mitigate the total sodium release mass during the slug impact phase. Also, the two-phase frictional pressure drop plays a mitigating role and needs to be included in the model formulation to avoid overestimation of the sodium release mass. Fluid-structure interaction effects such as vessel head bending and hold-down bolt elongation have negligible influence on sodium release mass as long as the deformations are within the elastic limit. The model is conceived to be part of an integrated code system to evaluate the sodium fire and the maximum RCB pressure under severe accident conditions in a pool-type SFR.

Investigating the effect of the fluid properties on bubble dynamics and heat transfer in a tapered microgap with multiphase flow modeling

Nucleation and bubble dynamics on a heater surface contribute to high heat transfer rate in pool boiling. Introducing two-phase flow in narrow channels further improves heat transfer. Use of expanding taper microgap geometry further enhances heat transfer, and proper balancing of taper angles and flow lengths leads to self-sustained flow boiling in tapered microgap geometries. This paper focuses on understanding the underlying enhancement mechanism by studying the bubble behavior as they expand and accelerate in the direction of increased taper. The present study conducts a 2D simulation analysis of bubble growth in tapered microgaps with numerical simulations to identify the effect of the fluid properties and tapered angle in the bubble and fluid dynamics behavior. Ansys-Fluent is customized with user-defined-functions (UDFs) accounting for the interfacial heat and mass transport, including a sharp interface and direct calculation of mass transfer with temperature gradients. The study was conducted using air injection and boiling simulation from the conception to the departure of a bubble. The tapered angles were 5°, 10°, and 15°, with flowrates between 3 ml/min to 30 ml/min, 1 mm air inlet, and at 1 mm distance from the convergent end. The departure time of 10 subsequent bubbles was recorded to check the configuration with the quickest bubble removal. A critical flowrate and surface tension region was established for the escape direction of the bubble. In addition, the numerical simulation considered the tapered microgap with a nucleating bubble at atmospheric conditions with a wall superheats of 5 K. The results show that the bubble growing over the heated surface creates fluid circulations and interfacial conditions that suppress the thermal boundary layer leading to an increased local heat transfer coefficient within a range of 1 mm from the interface.

Mechanism Analysis of Falling Film Evaporation Two-Phase Flow in Narrow Channels

Abstract As an efficient heat and mass transfer technology, falling film evaporation has attracted more and more scholars' attention. In this paper, a falling film evaporation platform was established to discuss the mechanism of two-phase flow in narrow channels. The width of the channel varied from 0.5 to 2.0 mm. We focused on the relationship between heat transfer characteristics with flow patterns, channel width, and pressure loss. Three kinds of flow patterns were observed: bubble flow, restricted bubble flow, and dry area. In channels with the same width, the initial heat flux of subcooled evaporation increased linearly with the growth of flow rate. But at the same flow rate, the initial heat flux of subcooled evaporation decreased when the channel width became larger. The heat transfer coefficient on the wall was influenced by the bubble generation frequency and bubble separation diameter. Higher bubble generation frequency led to smaller bubble separation diameter. Through mathematical analysis, it was found that the heat transfer coefficient changed with the channel size. And a boundary was observed between the strengthening and weakening status. The comparison of theoretical and experimental values showed good agreement.

Mechanism Analysis of Falling Film Evaporation Two-Phase Flow in Narrow Channels

Mechanism Analysis of Falling Film Evaporation Two-Phase Flow in Narrow Channels

Experimental studies on the separation characteristics of the gas-liquid two-phase flow in narrow channel with T-junction

Experimental studies on the separation characteristics of the gas-liquid two-phase flow in narrow channel with T-junction

Numerical study of adiabatic two-phase flow patterns in vertical rectangular narrow channels

The current paper investigates an upward two-phase flow of air-water mixture in vertical narrow rectangular channels with two different cross-sectional dimensions of 25×5mmand25×2mm respectively by using VOF (Volume of Fluid) model. To facilitate the simulations of different flow patterns, a simple and practical inlet condition, which is validated by experimental results, is employed. In the VOF model, the inlet condition will make the gas phase to be air layers, and due to the entrainment effect, the layers would coalescent and break up to small bubbles. The simulation results reveal that the main flow patterns of the two-phase flow in narrow channels are bubbly flow, slug flow, churn-turbulent flow and annular flow, and the void fraction distributions of those flow patterns are quite different. The void fraction of bubbly flow is below 0.3, which is consistent with published literatures. As the Taylor bubbles are located at the center of the flow channel and the slug flow is intermittent, the void fraction of slug flow is more evenly distributed in the center. The flow pattern map has been drafted under various air–water superficial velocities. The central region of the annular flow is gas core entrained liquid droplets, thus the void fraction is almost constant in the central region. The two-phase flow pattern map of a vertical rectangular channel shows that the range of annular flow in the channel is larger, while the scopes of slug flow and bubbly flow in the channel are smaller in comparison with the flow pattern map of tube. In addition, the transitional boundaries between flow patterns occur at smaller gas velocity than those in the tube. While compared the 2mmgap rectangular channel with the 5mm ones, it is found that with the gap size reducing, the flow pattern transition boundary shifts to the left.

ICONE19-43913 VISUALIZATION OF TWO-PHASE INTERFACIAL STRUCTURES IN NARROW CHANNEL

The gas-liquid interfacial structure of two-phase flow in narrow channel was measured by a novel measurement method using electrical conductance. This method consists of two different conductance measurements. One measure the conductance between neighbor electrodes on the sensor, and the liquid film thickness on the channel wall can be estimated from the measured conductance. Another measures the conductance between the opposing electrodes of two sensors which are installed on the walls of narrow rectangular channel. This conductance is related directly to void fraction in the narrow gap. These two measurements are performed simultaneously. Thus, two-dimensional distributions of both liquid film thickness and void fraction can be measured by using the liquid film sensors with numerous electrodes. The gas-liquid interfaces were reconstructed by estimating the thickness of the gaseous phase in the gap. From the reconstructed data, the void fraction and interfacial area concentration were obtained.

Numerical simulation of bubbly two-phase flow in a narrow channel

An advanced numerical simulation method on fluid dynamics – lattice-Boltzmann (LB) method is employed to simulate the movement of Taylor bubbles in a narrow channel, and to investigate the flow regimes of two-phase flow in narrow channels under adiabatic conditions. The calculated average thickness of the fluid film between the Taylor bubble and the channel wall agree well with the classical analytical correlation developed by Bretherton. The numerical simulation of the behavior of the flow regime transition in a narrow channel shows that the body force has significant effect on the movement of bubbles with different sizes. Smaller body force always leads to the later coalescence of the bubbles, and decreases the flow regime transition time. The calculations show that the surface tension of the fluid has little effect on the flow regime transition behavior within the assumed range of the surface tension. The bubbly flow with different bubble sizes will gradually change into the slug flow regime. However, the bubbly flow regime with the same bubble size may be maintained if no perturbations on the bubble movement occur. The slug flow regime will not change if no phase change occurs at the two-phase interface.

Adiabatic two-phase flow in narrow channels between two flat plates

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