Introduction
One of the most important issues discussed in the field of reservoir management is the release of accumulated sediments. According to documented reports, 1% of the available volume of reservoirs is lost annually to sedimentation, which can increase to 3% in semi-arid regions. Sedimentation causes undesirable consequences such as increased maintenance costs, inability to control floods, reduced power generation capacity, etc. Various methods have been proposed to manage sediment inflow into the reservoir and removal of accumulated sediment.
Among these methods, an innovative VMHS hydrosuction method has significant advantages which consists of the vertical multi-hole pipe connected to a pipeline continued to downstream of the reservoir. In this method, the energy is obtained by the difference in water level between the reservoir and the outlet of the system is used to discharge the water/sediment mixture flow through the holes and drive it to the downstream. The other considerable advantages of this system are the minor loss of the reservoir water, durability and the ease of use as well as environmentally friendliness.
In this research, it is attempted to provide quantitative understanding of hydraulic characteristics of flow through inlet, holes and connections of the pipeline system for designers of VMHS.
Methods
This study was conducted at the Hydraulics Laboratory of the College of Agriculture and Natural Resources of the University of Tehran. A physical model consisting of a tank and a hydro-suction pipe system. The system included a perforated vertical suction pipe with various hole configurations and dimensions, designed to evaluate hydraulic behavior for different flow and pressure conditions. Experiments were categorized into three stages to provide sufficient data for estimating longitudinal and local loss coefficients with a no-hole pipe and multi-hole vertical pipe based on flow discharge and pressure measurements. A total of 28 suction pipe types were tested, varying in hole number (1, 2 and 3 holes), position, spacing, and diameter (14.25, 28.5 and 42.75 mm). Dimensional analysis using the Buckingham π method was employed to define key dimensionless parameters influencing the system's flow characteristics.
Results and Discussions
This study focuses on determining head loss coefficients in a VMHS system across various configurations and flow rates. The first stage involved measuring frictional and minor losses using Bernoulli’s and Darcy-Weisbach equations for different energy head conditions (ΔH1, ΔH2, ΔH3 respectively 1.5, 2 and 2.5m). The experiments encompassed a wide range of flow discharges, and associated Reynolds numbers revealed that all flows were turbulent. Frictional loss coefficients varied based on pipe roughness and Reynolds number, while minor loss coefficients were evaluated at bends, flowmeter, inlet, and globe valves. These coefficients were then averaged and summarized for further use in subsequent stages of the study.
In the third stage, the flow was restricted to only pass through pipe holes by sealing the pipe inlet, showing reduced discharge rates that were unaffected by the ΔH. The suction performance improved with increased hole diameter and number, and the discharge coefficient (CD) for each configuration was calculated. Lower CD values indicated smoother flow entry and reduced wall resistance. Notably, “Type 3” pipes with the largest holes showed negative minor loss coefficients (Kh), indicating that perforations served as primary inlets. Hydraulic pressure analyses showed that in high flow discharges, negative pressures developed along the pipe walls due to inlet flow curvature, and averaged pressures were used for analysis when local measurements were unreliable. The influence of hole sizes and layout on inlet pressure was also assessed, indicating minimal effects for smaller diameters.
Conclusion
In the first phase of the experiments, energy losses in the hydrosuction system were analyzed, including both friction loss along the pipeline and minor losses from components such as bends, flowmeter, control valve, and the pipe inlet. In the second and third stages, the flow discharge coefficients and minor loss coefficients at the suction pipe holes were calculated using experimental data and hydraulic energy equations. Results showed that negative pressure at the pipe inlet wall is due to flow curvature and compression, and at higher discharges, the averaged inlet pressure becomes negative, with larger hole diameters leading to increased inlet pressure and greater flow contribution from side holes.