Characterization of the dissipation of elbow effects in bubbly two-phase flows

doi:10.1016/j.ijmultiphaseflow.2014.07.012

International Journal of Multiphase Flow

Volume 66, November 2014, Pages 101-109

https://doi.org/10.1016/j.ijmultiphaseflow.2014.07.012 Get rights and content

Highlights

•
Characterizes the dissipation of elbow-effects due to 90° vertical-upward elbow in two-phase flow conditions.
•
Developed correlation to predict the dissipation length of a 90° vertical-upward elbow in air–water bubbly flow.
•
Experimental database of local two-phase flow parameters measured with four-sensor conductivity probe.
•
Effect of flow parameters on dissipation characteristics of vertical-upward elbow.

Abstract

This study develops a method to characterize the elbow-region for vertical-upward-to-horizontal air–water bubbly two-phase flows through a 90° elbow with a non-dimensional centerline radius of curvature, R_C/D, of three. The dissipation of the elbow-effects in the horizontal region downstream of the elbow is characterized by the change in the local void fraction distribution. To quantify the dissipation characteristics, the elbow-strength, S, is defined as the inverse of the second moment of the local void fraction distribution. It is found that the elbow-strength decreases exponentially with increasing development length in the elbow-region. The slope of decrease of the elbow-strength in the elbow-region is determined by the dissipation parameter, β. The dissipation parameter determines the development length required for the elbow-effects on the two-phase flow parameters to become negligible. It is found that the dissipation parameter is a linear function of the mixture Reynolds number for the flow conditions investigated in the present study.

Introduction

The study of two-phase flow in straight pipes with different orientations and through various flow restrictions is very important for both practical applications and scientific advancement of the subject. Most energy systems, for example nuclear reactors, contain coolant channels that are arranged in varying orientations and that are interconnected by various flow restrictions such as elbows, valves, and tees. Flow restrictions can induce significant changes in the two-phase flow interfacial structures and their transport phenomena (Salcudean et al., 1983, Wang et al., 2004, Kim et al., 2007, Talley et al., 2009). Since the mass, momentum, and energy transfer in two-phase flow systems is governed by the interfacial structures, it is important to perform experiments and to develop models that capture the effects of flow restrictions on the interfacial structures. Most of the detailed studies on the interfacial structures have focused on two-phase flows in separate vertical-upward (Ishii and Kim, 2004), vertical-downward (Ishii et al., 2004), or horizontal (Talley et al., 2012) straight pipe geometries. However, flow restrictions such as vertical-upward elbows are essential to connect the vertical and horizontal pipes in practical two-phase flow systems.

Therefore, it is important to characterize the elbow-effects on two-phase flow parameters and develop a method to determine the length of the elbow-region. The elbow-region describes the section of the two-phase flow system where the elbow-effects are dominant. In general, the elbow-effects dissipate as the flow develops downstream of the elbow. A few studies in the literature have considered the dissipation of elbow-effects in single-phase flow conditions (Sudo et al., 1998, Mattingly and Yeh, 1991). Moreover, these studies characterize the elbow-effect in terms of the strength or intensity of the secondary flow generated by the elbow (Sudo et al., 1998), which can be utilized to predict the length of the elbow-region in single-phase flows.

However, there is a complete lack of studies that address the dissipation characteristics of the elbow-effects in two-phase flow conditions and model development for the elbow-region. In view of this, the objectives of the current study are: (1) to characterize the effects of a 90° vertical-upward elbow on two-phase flow parameters, (2) to define and identify the elbow-region in two-phase flow conditions, and (3) to develop a correlation to predict the length of the elbow-region in two-phase flow conditions.

The study is divided into three major sections. The first section describes the experimental studies on the geometric effects of a 90° vertical-upward elbow on two-phase flow parameters. The second section presents a modeling strategy for two-phase flow systems with elbows. Both the experimental study and modeling strategy form the basis for the development of a correlation for the dissipation length of the elbow-effects to define the elbow-region in vertical-upward-to-horizontal bubbly two-phase flows; this is presented in the third section.

Section snippets

Experimental studies

This section describes the experimental study that was performed to investigate the effects of a 90° vertical-upward elbow on local two-phase flow parameters in air–water bubbly two-phase flow. The experimental results are discussed to show the geometric effects of the elbow on the two-phase flow parameters, which are crucial toward the development of models for the elbow-region. A detailed discussion of the experimental study is given by Yadav et al. (2014).

Model development strategy

This section presents a modeling strategy for two-phase flow configurations with elbow restrictions. It also highlights the importance of characterizing the elbow-effects and the need for an accurate prediction of the elbow-region. The current discussion is presented in the context of the vertical-upward-to-horizontal two-phase flow configuration connected by a 90° vertical-upward elbow as described in the section on the experimental studies. However, one can easily extend the proposed modeling

Characterization of the elbow-region

As discussed, an accurate prediction of two-phase flows in systems with elbow restrictions requires an accurate prediction of the elbow-region. Since the elbow-effects on two-phase flow parameters are negligible in the upstream section, the elbow-region includes the length between the inlet of the elbow and a downstream location in the horizontal section where the elbow-effects are significantly dissipated. The objective of the present work is to establish a quantitative definition of the

Summary and conclusions

This study presents a method to characterize the elbow-region for vertical-upward-to-horizontal bubbly two-phase flows through a 90° vertical-upward elbow. Furthermore, a correlation is developed to predict the dissipation length to define the elbow-region. The correlation is developed based on experiments performed in an air–water two-phase flow facility. The facility has a loop configuration and consists of three vertical and two horizontal sections that are interconnected by four 90°

Acknowledgement

This work is supported by the United States Nuclear Regulatory Commission Office of Nuclear Regulatory Research.

This report was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party’s use, or the results of such use, of any information, apparatus, product, or process disclosed in

References (16)

M. Ishii et al.
Interfacial structures and interfacial area transport in downward two-phase bubbly flow
Int. J. Multiphase Flow
(2004)
S. Kim et al.
Development of the miniaturized four-sensor conductivity probe and the signal processing scheme
Int. J. Heat Mass Transfer
(2000)
S. Kim et al.
Geometric effects of 90° elbow in the development of interfacial structures in horizontal bubbly flow
Nucl. Eng. Des.
(2007)
G.E. Mattingly et al.
Effects of pipe elbows and tube bundles on selected types of flowmeters
J. Flow Meas. Instrum.
(1991)
M. Salcudean et al.
Effect of flow obstruction geometry on pressure drop in horizontal air-water flow
Int. J. Multiphase Flow
(1983)
C.-C. Wang et al.
Influence of horizontal return bend on two-phase flow pattern in small diameter tubes
Exp. Therm. Fluid Sci.
(2004)
Q. Wu et al.
Sensitivity study on double-sensor conductivity probe for the measurement of interfacial area concentration in bubbly flow
Int. J. Multiphase Flow
(1999)
M.S. Yadav et al.
Experiments on geometric effects of 90° vertical-upward elbow in air water two-phase flow
Int. J. Multiphase Flow
(2014)

There are more references available in the full text version of this article.

Cited by (24)

Numerical simulation of two-phase hydrodynamics and mass transfer inside a serpentine tubular gas–liquid reactor
2021, Chemical Engineering Science
The multiphase behavior in gas–liquid reactors is a focused topic in chemical engineering. In this paper, the TFM-PBM method is implemented to study hydrodynamics and interfacial mass transfer characteristics of bubbly flow in a novel serpentine tubular reactor. The calculated evolution of bubble diameter and the gas phase distribution which converts between the ‘wall-peak’ and ‘core-peak’ pattern while flowing across the serpentine tube are in reasonable agreement with experiment results. Both the slip penetration model and the eddy cell model are employed to investigate the local mass transfer process, and the overall $k_{l} a$ predicted by the two distinct models coincides with the measured data under various operating conditions. It was found that the deviation between the two distinct models becomes more pronounced as the turbulent dissipation rate increases, which is opposite to the results in the numerical study of bubble columns.
Progress in two-phase flow modeling: Interfacial area transport
2021, Nuclear Engineering and Design
The interfacial area transport equation (IATE) employs a dynamic approach to predict interfacial area concentration (IAC) in two-phase flows. It eliminates the need for the static flow regime transition criteria and flow regime dependent IAC correlations used in closing the two-fluid model and therefore any potential artificial bifurcation or numerical oscillations stemming from these static correlations. The efforts to develop the IATE are reviewed in this work, focusing on studies within the past decade. The current state-of-the-art in IATE is a two-group interfacial area transport equation that is applicable to predict interfacial area concentration from bubbly to churn-turbulent flows. An extensive experimental database has been established in literature for various two-phase flow conditions. The database is used to develop constitutive models to close the IATE and to validate its performance. These include adiabatic and heated conditions, vertical and horizontal flow orientations, channels with flow restrictions, round, rectangular, annulus, and rod-bundle geometries, and normal-gravity and reduced-gravity conditions. IATE has been implemented into practical system analysis codes such as TRACE and computational fluid dynamics (CFD) codes such as CFX and Fluent to evaluate its performance. In addition, the recent efforts of extending the IATE beyond churn-turbulent flow to churn-annular transition and annular flows are also reviewed. These include the new instrumentation, experimental database, and additional constitutive models to predict annular flows.
Numerical simulation of single and two-phase flow across 90° vertical elbows
2021, Chemical Engineering Science
Citation Excerpt :
They confirmed that the modified Froude number proposed by Gardner and Neller (1969) to determine the stratification after the bend can be applied to the air-silicone-oil three-phase flow. Yadav et al. (2014a, 2014b) investigated the effects of a 90° vertical-upward elbow on void fraction distribution and interfacial area concentration transport in the air–water two-phase flow using four-sensor conductivity. They developed a preliminary dissipation length model to quantify the length of the region affected by the elbow.
This study investigates elbow effects on single-phase and two-phase flows. ANSYS CFX is used to simulate the evolution of the single-phase velocity distribution and two-phase interfacial structure downstream of vertical elbows. Simulations are benchmarked with existing LDA data and detailed four-sensor conductivity probe data. It is found that elbows create secondary flow, containing two counter-rotating vortices in both single-phase and two-phase flows. The vortices shift to pipe center and develop into parabolic velocity profile downstream. The elbow effect dissipates exponentially and dissipates 90% of its initial maximum value at approximately 11D downstream, no matter the elbow direction. In two-phase flow, bubbles are entrained by the secondary flow to form double-peaked void fraction and interfacial area concentration distributions. The double-peaked distribution moves upward and becomes single-peaked distribution downstream. Due to the additional buoyant force, the elbow effect takes much longer length to dissipate in two-phase flow than in single-phase flow.
Separate-effect experiments and modeling for two-phase flow under geometric restrictions
2020, Nuclear Engineering and Design
While significant efforts have been made to investigate two-phase flows in vertical upward channels, limited work has been performed to study the effects stemming from geometric configurations on two-phase flow transport, such as flow restrictions and change in flow orientation. This presents significant shortcomings in the application of predictive models to practical systems, including nuclear power reactors. It has been well observed that geometric restrictions (e.g., elbows, spacer-grid, and inlet configurations) and change in flow orientation (e.g., upward-to-horizontal, horizontal-to-downward) can cause significant effects on the hydrodynamics of two-phase flow. In view of these, high-resolution separate-effect experiments have been performed to establish a two-phase flow database that is accurate, extensive and good enough for multi-phase CFD codes development, to develop predictive models accounting for geometric effects, and to improve the performance of nuclear reactor system analysis codes. Recent studies on the effects of the vertical-upward and vertical-downward 90-degree elbows, spacer grid, inlet configurations, and change in flow orientation are summarized. Some of the major two-phase flow phenomena investigated in the present study include flow regime transition, pressure loss, two-phase flow parameters, flow development, and interfacial drag. To demonstrate the functionality of the newly developed models, the one-dimensional steady-state one-group interfacial area transport equation is employed to predict two-phase flow transport under geometric effects.
Experimental study on void fraction, pressure drop and flow regime analysis in a large ID piping system
2019, International Journal of Multiphase Flow
Citation Excerpt :
In terms of the two-phase flow void fraction and pressure, many studies focus on the pipe flow without a flow direction change. For those studies on the characteristics of the two-phase flow through the elbow, Kim et al. (2010),Yadav et al. (2014) and Kim et al. (2014) studied the effect of the elbow on the two-phase flow in regard to the pressure loss, two phase dissipation and the secondary flow induced by the elbow. Spedding and Benard (2007) also studied the two-phase flow pressure drop through the 90-degree elbow.
An experimental study of the characteristics of air-water two-phase flow in a piping system with an inner diameter of 152.4 mm is presented. Both horizontal and vertical pipes (with the lengths of 15.09 m and 6.42 m) are included in the system and two 90-degree elbows are used for their connections. 71 flow conditions are performed in this experiment, covering plug, slug, pseudo slug, and stratified flow for the horizontal flow, and bubbly, cap bubbly, churn turbulent, and falling film/annular flow for the vertical flow. Detailed flow regime analysis for the flow regimes of the horizontal and vertical pipe flow and the effects of the elbow on the flow regime transition are discussed. The flow regime maps proposed generally agree with the existing maps. A significant flow regime shift in the vertical section is also observed and discussed. The area averaged void fraction development is presented and a sharp drop of void fraction in vertical downward test section is observed which results from the kinematic shock phenomenon. The result of the frictional pressure drop is discussed, and it agrees with the Kim et al.’s correlation. With the database, the drift flux model for Goda et al.’s vertical downward flow is evaluated. Large difference has shown at low void fraction which may indicate further modeling investigation.
Air-water two-phase bubbly flow across 90° vertical elbows. Part I: Experiment
2018, International Journal of Heat and Mass Transfer
This study performs experimental investigation on two-phase bubbly flow through 90° vertical elbows going from vertical-upward to horizontal and horizontal to vertical-downward orientations. The elbows have an inner diameter of 50.8 mm and a radius of curvature of 152.4 mm. A detailed database for local and global two-phase flow parameters in different flow orientations and across vertical elbows is established with a state-of-the-art four-sensor conductivity probe, a pressure transducer, and a high-speed video camera. The evolution of void fraction, interfacial area concentration, bubble velocity, and two-phase pressure drop along the axial development length in different orientations and across elbows are presented. It is observed that elbows create secondary flow and cause redistribution of the bubbles, which significantly affect the development of two-phase flow parameters. The effects occur across the elbows and remain for a significant length downstream. The effects on the void fraction distribution downstream of the elbow are discussed in detail for both elbow orientations. Characteristic differences in the elbow effects due to the elbow orientation are observed. For the vertical-upward elbow, a similar trend in the evolution of the void fraction distribution is observed for all flow conditions investigated while two different trends are observed downstream of the vertical-downward elbow. It is found that the effects of the vertical-downward elbow dissipate faster than that of the vertical-upward elbow. The elbow effects on interfacial area concentration, bubble velocity, and two-phase pressure drop are also discussed. The evolution of the interfacial area concentration is similar to that of the void fraction evolution in the straight pipe sections. However, it increases significantly across both elbows due to the promotion of the bubble breakup induced by the secondary flow. The void-weighted area-averaged bubble velocity is subject to significant changes across and immediately downstream of the elbows due to the redistribution of the bubbles. Additionally, the two-phase pressure gradients across the vertical-upward and vertical-downward elbows are approximately 1.5 and 2 times of that in the straight pipe sections.

View all citing articles on Scopus

View full text

Characterization of the dissipation of elbow effects in bubbly two-phase flows

Highlights

Abstract

Introduction

Section snippets

Experimental studies

Model development strategy

Characterization of the elbow-region

Summary and conclusions

Acknowledgement

Int. J. Multiphase Flow

Int. J. Heat Mass Transfer

Nucl. Eng. Des.

J. Flow Meas. Instrum.

Int. J. Multiphase Flow

Exp. Therm. Fluid Sci.

Int. J. Multiphase Flow

Int. J. Multiphase Flow