Author(s): Balkrishna Shankar Chavan

Linked Author(s): Balkrishna Shankar Chavan

Keywords: Flat spillway; Air Demand; Offset; Ramp; Depth of overflow;

Abstract: Past decade has witnessed spurt in hydroelectric projects involving high intensity discharge(q) in excess of 300m3/s/m and high heads(h) exceeding 200m. Such situation leads to pressures on the surface of spillway to drop below atmospheric leading to cavitations damage. To mitigate cavitations damage, measures such as control of geometry, smoothness of the spillway surface, use of cavitation resistant materials are adopted to a limited success. As such increasing head and discharge there are limitations for adopting preventive measures and provision of aerator becomes only alternative to mitigate cavitations damage. Present study employs use of along with PC based Data Acquisition System (DAS) to measure pressures on spillway profile. There are various methods to measure air concentration in a spillway overflow like optical probe and resistivity probes. Most reliable being intrusive phase detection probes. Present study used resistivity probes to measure air concentration in a spillway over flow downstream of aerators. Two locations were selected to measure the air concentration. Two alternatives with offset of 1.5m and 2.5m namely without ramp and with ramp upstream of aerator were studied in the model. Comparison of various alternatives are presented in a table using non dimensional parameters jet length λ, air entrainment coefficient ᵝ and cavity pressure Δp. Model study indicated that air concentration along the bed is more for small gate openings and decreases drastically in the vicinity of 2-3% for decreasing Froude number. Flow depth is fully aerated for small gate openings implying no cavitation damages. ᵝ decreases with increase in gate openings. Addition of ramp improved air entrainment in the jet and pressures in aerator. Jet length λ increases with addition of ramp. Study also showed that functioning of an aerator on the orifice spillway is different from the flat slope chute spillway. Results of model study are presented in a tabular as well as graphical form. Mathematical equations proposed for ,jet length λ , air entrainment coefficient ᵝ in terms of Froude Number Fr, Proposed equations would be of help for designers ofshort and low spillway aerarors. Recommendations from model studies are enumerated in the concluding part.

DOI: https://doi.org/10.3850/38WC092019-0217

Year: 2019