Hydraulic Characteristics of Multistage Orifice Tunnels

In China many large-scale hydraulic projects have been designed with multistage orifice tunnels to dissipate a large amount of energy at the design discharge and water level. Based on two hydraulic-model tests and theoretical analysis about multistage orifice tunnels, this study further examined orifice geometry to meet energy-dissipation requirements and to minimize cavitation risk, as well as to systematically investigate factors influencing the energy dissipation and cavitation coefficients of the multistage orifices. The results indicate that measures, such as enlarging the tunnel diameter, decreasing the tunnel outlet area, and lowering the design water level of the reservoir, could be taken into consideration to increase the orifice cavitation coefficient sigma for a given orifice geometry when the incipient cavitation coefficient sigma i of the orifice is large. For the same orifice contraction ratio beta, the values of orifice dissipation coefficient k and sigma i are closely related to orifice geometry, especially to the radius r at the top edge of the orifice and to the ring fixed in front of the orifice plate.