Developing an acoustic discharge measurement technique for hydroelectric performance testing

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Gawne, Kevin D.
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The efficient operation of hydroelectric generating plants requires an accurate definition of the performance relationships of each turbine/generator unit. Of the information obtained by performance testing, discharge is the most difficult to measure accurately. It is envisioned that acoustic transit-time velocity measurement technology can be applied at low-head plants to obtain more accurate discharge measurements over current practices. The technique proposed herein involves continuously traversing a number of acoustic paths, each providing a chordal average velocity measurement, across the turbine intake to obtain a complete integration of the complex velocity profile typical of low-head hydroelectric plant conditions. Hydraulic laboratory testing of a single acoustic cell is the focus of this study. The acoustic cell was traversed across a laboratory intake structure to measure discharge; this measurement was compared to known laboratory flowrate to assess the accuracy and repeatability of the proposeddischarge measurement technique. Various continuous sampling strategies (i.e., traverse rates) and discrete sampling strategies (Gauss-Legendre positioning) are evaluated. A number of flow perturbances were also added to the intake structure to evaluate the technique in complex flow conditions. Based on the results described herein, it is concluded that the proposed continuous traversing technique can provide efficient, accurate and repeatable discharge measurements under complex flow conditions relative to discrete sampling techniques. In disturbed flow conditions, testing revealed that discrete sampling strategies were subject to considerable systematic integration error. Further laboratory testing of a multiple cell system is recommended because of the promising results obtained from this study. Also, two key recommendations pertaining to field testing of low-head plants are made: (1) low order discrete sampling techniques should not be practiced; and (2) careful consideration should be devoted to the geometry of the acoustic paths as conflicting effects on accuracy are dependent on the acoustic path lengths and path angles employed.