Head losses and influence length of sharp-edged and rounded orifices under steady flow conditions

Orifices have been largely studied either for flow metering or for producing head losses. An orifice which is used as a flow meter is well defined in ISO 5167. All the geometrical parameters are defined to obtain discharges as accurate as possible. However, providing an orifice geometry which produces a certain amount of head losses could be useful as well. For example, during a refurbishment, throttling a surge tank of a high head power plant with an orifice allows the management of maximum and minimum water level followed by the management of downstream discharge control and transient events. When the discharge increase is limited, it is often efficient to place an orifice at the entrance of the existing surge tank allowing the same safety level to be kept. Actually, surge tank modifications have to be designed case-by-case. Normally, the placed orifice should produce asymmetric losses, which are defined by performing transient simulations for relevant flow directions in the whole water way system and hydropower plant.This research focuses on the evaluation of head losses and the length of the orifice influenced zone of 5 different contraction ratios (from 0.4 to 0.6) with two different orifice geometries. The influence length allows the determination as to which part of the surrounding pipe should be reinforced to avoid damages caused by cavitation.The main goal of this research is to provide the guidelines for the design of orifices in terms of head losses to the practical engineers and to avoid damages.