by Dr Latif Bouhadji
ASL-AQFlow Inc.
ASL-AQFlow Inc. has developed the Acoustic Scintillation Flow Meter (ASFM), a non-intrusive instrument for monitoring velocities and total flow rates in the intakes of low-head hydroelectric plants. These measurements allow hydro engineers to evaluate turbine operation efficiency and measure changes due to turbine performance degradation and/or the use of fish protection devices within the intake. To address the operational requirements, accuracies approaching 1% in the total flow measurements are sought.
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Deployment of the ASFM transducers in the intake through available gate slots: The ASFM uses a selected number of acoustic paths placed across the intake to collect the average value of the entire mean velocity profile at each elevation. |
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Factors such as the presence of upstream obstructions in the form of trash racks, fish diversion screens or difficult hydraulic conditions, such as flow separation, may affect the flow accuracy of the ASFM. These factors cause large variations in the distribution of velocity and turbulence in the measurement zone. The CFD studies carried out with ANSYS CFX (CFX-5.6) software, using ANSYS ICEM CFD for grid generation, provided a better understanding of the effect produced by these factors on the mean flow velocity and turbulence characteristics sensed by ASL's acoustic system and helped to identify optimum performance conditions. ANSYS ICEM CFD was able to mesh the large regions involved and the turbulence models and parallel ability of ANSYS CFX provided fast and accurate results.
Turbulent flow in hydroelectric intakes was investigated in large complex 3-D geometries including trash racks, fish-screens and circular pipes supporting the ASFM frame. The parallel simulations were carried out in a cluster of three PC’s using a Linux platform. K-ε model and k-ω based Shear Stress Transport (SST) turbulence models in ANSYS CFX were compared to predict the mean and turbulent flow characteristics.
The ASFM and CFD studies were undertaken at several North American hydroelectric power plant intakes.The first 3-D study was carried out at the Lower Monumental (WA, US) power plant, where the ASFM measurements were particularly successful, in order to gain confidence in the numerical model. An oblique or turning flow at the intake entrance can lead to a separated flow which produces irregular distributions of velocity and turbulence in the intake. This scenario may produce significant biases in the velocity measured by an ASFM. An opportunity came along to conduct a CFD study and analyze these issues when measurements were scheduled at the Hydro-Kennebec (ME, US) power plant intakes. At this particular plant a turning flow is present at the entrance of the intakes. The simulations showed the presence of large wakes produced by the trash rack as the flow descends the intake. As expected, large flow recirculation starts at the intake entrance of Unit 2. The velocity profile predicted with ANSYS CFX shows a very good agreement with both the ASFM data and Nortek Doppler Velocimeter data collected in Unit 1, where no separation was present. However, Unit 2 presented a discrepancy between the ASFM and the CFD data. This observation has led to a thorough analysis of the ASFM technique and subsequently improvement was made to the ASFM accuracy when dealing with difficult flow conditions such as those encountered in Unit 2.
ANSYS CFX was able to model the turbulent flow in large hydroelectric power plant intakes. The model results were then used to optimize the placement of the ASFM transducers and predict possible ASFM biases in difficult hydraulic conditions.
Lower Monumental Power Plant
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The trash rack is composed of six panels placed one on another up to the top of the intake entrance. Each panel consists of six horizontal I-beams and three vertical bars. |
Distribution of the mean horizontal velocity at a selected section at a flow rate ~ 113 m3/s in the Lower Monumental Intake: The trash rack clearly disturbs the flow far downstream in the intake. The circular pipes supporting the ASFM units are about 20 meters distance from the trash rack. |
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Distribution of the mean horizontal velocity field in the Lower Monumental Intake |
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Comparison of the mean horizontal velocity and the turbulent kinetic energy with collected ASFM data in the Lower Monumental Intake. The velocity profile shows different test cases carried out: 2-D and 3-D intake configurations, with and without trash rack and different turbulence models. |
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Hydro-Kennebec power plant
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Showing wall |
Without Wall |
The Hydro-Kennebec power plant consists of two units: Unit 1, where a large inlet curvature dives into the straight intake and Unit 2 where a straight wall is present prior to the entrance. Both units, separated by a 1.8 m wall, are about 15 m high at the intake entrance and ~ 7 m wide. The trash rack consists of five thick 25cmx69cm I-Beams and other thin vertical and horizontal bars which were not taken into account in the computational domain. The semi-circular front section of the turbines has a ~ 3m diameter. The cylindrical cross members supporting the frames on which the acoustic transducers are mounted are located at about 9 m distance from the trash rack. |
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Distribution of the horizontal mean velocity at selected flow rates in Hydro-Kennebec intakes: Large wakes are produced by the top I-beams as the flow descends the intake. These wakes diffuse, interact and merge by the slot location. A large flow recirculation is present at the intake entrance of Unit 2. The right figure shows the distribution in the ASFM plane of measurements: the signature of the trash rack and the turbine section on the flow field are clearly noticeable. |
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Comparison of the ASFM and CFD normalized velocity magnitude profiles in Hydro-Kennebec intakes. The discrepancy in Unit 2 allowed for improvement to be made to the ASFM accuracy. |
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