چكيده لاتين :
Introduction
Formation of air-entraining vortices in an intake leads to unsteady flow and cause problems
such as vibration in hydro mechanical equipment, abnormal noises, severe fluctuations in
local pressures and exacerbated cavitation conditions (Chen & Chen, 2015). As stated by
(Sarkardeh, 2017) the stronger vortex the greater will be its negative effects on intake
performance. There have been many studies on the critical submerged depth and vortex
formation in intakes. (Kocabas & Yildirim, 2002) investigated the effect of rotational flow
on the critical submerged depth in intakes and found that the vortex formation with the aircore vortex and the critical submerged depth was significantly dependent on the approach
flow conditions and the inlet geometry. Therefore, a separate case study should be
undertaken to address any structure with a particular geometry. In this paper, the effect of
flow rate deviation due to the use of a deflector on a vertical intake was investigated. In this
paper, also the variation of submerged depth, Froude number, vortex type, and critical
submerged depth was discussed.
Methodology
In this study, different scenarios were created by varying the deflector angle from 3 to 11
degrees. In order to measure the discharge rate, the flow passes an electromagnetic flow
meter with a full-scale accuracy of ±0.2%. A depth gauge with an accuracy of ±0.05 mm
was used to measure the water head on the intake crest. Given that the critical submerged
depth is in agreement with the nature of the type 4 vortex (Naderi & Gaskin, 2018).
Therefore, in experiments with the observation of type 4 vortex, the critical submerged
depth was determined. Then, according to the dimensional analysis and the results of the last
experiments, the key factors affecting the critical submerged depth, including deflector
angle (α) and Froude number (Fr) were identified. The range of variation of the tested
parameters included in deflector angle (α) of 3, 7 and 11 degrees, the discharge of 2 to 26
liter/s and the Froude number of 0.3 to 3.9.
Results and Discussion
After processing experimental observations, the trend of the rating curve changes, the
relative submerged depth versus the Froude number, as well as the vortex type were
evaluated. The results showed that the flow with a higher deviation angle α has a steeper
rating curve. In other words, the angle α has a direct effect on the spherical sink surface
sector, so that by increasing the angle α, the effective cross-sectional area of the flow
decreases, resulting in a reduced spherical sink surface sector, and leading to a weak
performance of the intake. This caused water to be stored in the reservoir for low
discharges. According to the experimental observations at a fixed Froude number, the
greater the angle α, the lower the relative submerged depth. To be more precise, the
deflector inside the intake, in turn, compresses the air-core vortex and moves it upstream,
thereby creating weak vortices and reducing the amount of rotation of the flow on the crest,
thus it was observed that with increasing the angle α at a constant Froude number, less
relative submersion depth was required. On the other hand, decreasing the angle of α causes
the vortex core to be emitted outward, so more rotation was needed to maintain this vortex.
Therefore, in this situation, the intake at a constant Froude number requires a greater relative
submersed depth.
Conclusions
The results of this study showed that the use of a deflector in the vertical intake, in addition
to reducing the critical submerged depth, weakens the formation of a vortex, stabilizes
reservoir water level changes, and prevents unsteady flow conditions. The results also
revealed that an increase in deviation angle α and consequently the Froude number, allowed
the vortex compression effect to impose the greatest impact on the relative submerged depth
(S/Di).
Acknowledgement
We are grateful to Jundi-Shapur University of Technology and for funding of present
project allowing access to the hydraulic and river engineering laboratory of the Department
of Civil Engineering.
