AbstractAn upgrade to the ATLAS silicon tracker cooling control system may require a change from C3F8 (octafluoro-propane) to a blend containing 10-30% of C2F6 (hexafluoro-ethane) to reduce the evaporation temperature and better protect the silicon from cumulative radiation damage with increasing LHC luminosity. Central to this upgrade is a new ultrasonic instrument in which sound transit times are continuously measured in opposite directions in flowing gas for continuous real-time measurement of the C3F8/C2F6 mixture ratio and the flow rate.

Problem and objectiveIt is intended to evaluate, through the CFD (Computational Fluid Dynamics) techniques, the best geometry of the ultrasonic Flow Meter among:

•the Axial geometry•the Angled geometry

The Axial geometry (Fig. 1) consists of a straight pipe in which two ultrasonic transducers are set.

The Angled geometry (Fig. 2) consists of a transversal secondary pipe, intersecting at a certain angle with the main tube. The secondary pipe is closed at both ends by a pair of transducers.

The aim of the present set of simulations is to support the design of the flow meter by searching for the best geometrical configuration.The optimal solution in terms of both measurement reliability and pressure drop in the device is the one which less influences the velocity distribution.

CFD modelThe CFD software OpenFOAM was used.Geometry simplification and preparationMesh generation (from 4M to 8M polyhedral

cells, depending on the geometry, Fig. 3)

Applying physics model:•3D•Steady-State•Incompressible (since the speed is <0.3Mach)•SST k-ω turbulence model

Assuming boundary conditions•Velocity at the inlet•Pressure at the outlet

Simulations6 different combinations for the Axial Flow Meter and 18 different combination for the Angled Flow Meter have been simulated. The following geometrical parameters have been optimized:Diameter of the pipeDistance of the transducers from the elbow for

the Axial Flow MeterIntersecting angle for the Angled Flow MeterDistance among the transducers for the Angled flow Meter

Results The Axial Flow Meter:

The axial velocity contours ( Fig. 4) show a clear separation of the boundary layer downstream of the first transducer, with the consequent formation of a number of eddies, right by the sound wave path. Also, the axial velocity distribution in the cross section is greatly influenced by the transducers (Fig. 5). As a result, the transducers would measure a velocity noticeably different from the average one, and a calibration wouldn’t be possible since both the velocity field and the shape of the whirling area vary with the flow rate.

The Angled Flow Meter:The axial velocity contours (Fig 6) show a negligible influence of the secondary tubes over the main stream for the maximum transducers distance case, since the stream lines remain parallel to each other even next to the secondary tubes intersection. Due to the drag force of the main stream towards the still fluid in the secondary pipes, a major counterclockwise-rotating eddy occurs in the upper secondary pipe, whereas a clockwise-rotating eddy occurs in the lower secondary pipe. However, in spite of the presence of these eddies, the average speed in the secondary pipes must be zero, for the conservation of mass. Also, a wake occurs downstream of the upper secondary pipe, but it doesn’t influence the measurement as it is outside the sound wave path. The result is a uniform velocity distribution in the cross section of the main pipe (Fig. 7)

It is also necessary to discard the configuration with the transducers partially immersed in the main stream, due to the whirling area occurring within the sound path (Fig 8).

ConclusionCFD studies have considerably sped up the design of the Flow Meter by simulating, with high accuracy and reliability, its operation with different geometries, fluids, and thermo-fluid dynamic parameters. Simulations have shown that the Angled Flow Meter can provide a reliable measurement of the average velocity of the flow.

EN/CV/DC CFD TeamCERN, CH

www.cern.ch/cfdcfd-team@cern.ch

Section Leader: Michele BattistinTel. +41.22.767.80.87 cfd-team@cern.ch

Technical Coordinator: Enrico Da RivaTel. +41.22.767.67.98

Contacts:Authors: Gennaro Bozza, Enrico Da RivaTel. +41.22.767.06.44 +41.22.767.67.98

Optimization of a Combined Ultrasonic Flow MeterCFD-2011-04_Sonar

Figure 1: CAD modeling of the Axial Flow Meter

Figure 2: CAD modeling of the Angled Flow Meter

Figure 3: Mesh of the Axial Flow Meter: 3.8M cells

Figure 4: Axial velocity contours and stream lines of the axial flow meter

Figure 5: Axial velocity at the axis among the transducers versus dimensionless distance, for the axial flow meter.

Figure 6: Axial velocity contours and stream lines of the angled flow meter

Figure 7: Axial velocity at the axis among the transducers versus dimensionless distance, for the axial flow meter.

Figure 8: Axial velocity contours and stream lines of the angled flow meter, minimum transducers distance configuration