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Induced flow in a T-junction

Application Challenge 3-02 © copyright ERCOFTAC 2004

Introduction

A dedicated test rig comprising a T-junction has been tested in the Research and Development Division of Electricité de France in Chatou, France. A high Reynolds number flow is maintained in the main pipe while very small incoming mass flow rates are imposed in the auxiliary pipe (or ‘dead leg’). In such a configuration, a vortex is generated at the junction. Due to the shear, the flow is recirculating in the dead leg. The symmetry of this recirculation with respect to the plane including the axes of the two pipes may break down, then a swirling flow extends along the dead legs.

The objective here is to predict the apparition of the swirl, and to describe it (for example predict its height in the dead leg).

Two RANS turbulence models have been used for calculations : the k-epsilon model and the Reynolds Stress Model (or “Second Moment Closure”, SMC).

Relevance to Industrial Sector

The primary circuit of Pressurized Water Reactors is connected to a large number of auxiliary lines in which the fluid is usually colder than in the main pipe. Most of the time, the mass flow rate is small in the part of the auxiliary line located between the main circuit and the first valve. Hence, this zone might show temperature fluctuations if hot fluid coming from the main pipe is recirculating due to the shear at the junction. Previous analyses in Robert (1992) have shown that the swirl power in the dead leg is directly affected by the Reynolds number in the main pipe and by geometric details of the junction, whereas the influence of thermal effects is comparatively negligible. This is the reason why the application challenge proposed here focuses on the isothermal study of this flow : the motivation, for safety reasons, is to understand and be able to model the hydraulic behavior of auxiliary lines connected to the primary circuit of Pressurized Water Reactors.

Design or Assessment Parameters

The main parameter which will allow to assess the quality of the calculations is the height of the swirl. We put the stress on the necessity to define properly the extremity of the swirl, since it is highly subjective, in the experiment as well as in the calculations. All definitions are of course purely conventional.

Velocity profiles along a diameter of dead leg sections could also be a good parameter. Unfortunately, these data are not available.

Flow Domain Geometry

The junction is a sharp edged one. The dead leg diameter D is 100 mm. Its dimensionless length H/D is 20. The origin is located at the main pipe centre.

Figure 1 : Schematic of T-Junction Application Challenge.

Flow Physics and Fluid Dynamics Data

The flow in the dead leg can be decomposed in three distinct zones :

• In a near-junction zone (between two and three diameters from the axis of the main leg) can be observed a recirculation whose axis is normal the junction plane.

• Higher in the dead leg this eddy becomes asymmetrical, and thus one finds the actual corkscrew.

• At last, the third zone, which begins where ends the latter one, is the dead leg zone where there is no more influence of the main pipe flow.

The fluid (water) is incompressible and its viscosity is n = 1.03x10^-6 m²/s (numerical value used in the simulations). The corresponding Reynolds number based on the main pipe bulk velocity and the diameter of the auxiliary pipe is 895000.

Table 1 provides values for the velocities and flow rate.

Table 1 Flow Rates

Main Pipe Characteristics
Flow rate (m3/s)	0.120
Bulk velocity (m/s) VM	9.2
Auxiliary Pipe Characteristics
Bulk velocity (m/s) VA	0.092	0.046	0.023
VA/VM	1%	0.5%	0.25%

Figure 2 : Schematic view of the flow in the dead leg

© copyright ERCOFTAC 2004

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Contributors: Frederic Archambeau - EDF - R&D Division

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