Investigations of curved draft tubes with long exit cones

Abstract

The aim of achieving a maximum increase in the installed capacity of a hydroelectric station located in a narrow canyon without substantial additional expenditures for construction has led to the suggestion of a two-level (two-row) layout of the units in the powerhouse [1]. With such a layout the curved draft tubes of the lower level units reach a considerable length as a comequence of having to elongate the exit cone.The mutual effect of operating units and the hydraulic conditions in the lower pool of model blocks of twolevel powerhouses with long draft tubes were investigated" at the B. E. Vedeneev All-Union Scientific-Research Institute of Hydraulic Engineering (VNIIG) [2], research department of Gidroproekt, and V NIIGidromash. The effect o2 elongating the exit cone on the efficiency and discharge capacity of the turbine, except for minor investigations at VNIIGidromash, hasnot been studied.In the study made at the department of hydropower utilization at the V. V. Kuibyshev Moscow Institute of Civil Engineering (MISI), main attention was paid to an investigation of the effect of elongating the exit cone on the operation of the turbine and on the flow characteristics in the draft tube. The maximum length of the tested model robes, 15D I (Fig. 1), exceeded the length of the tubes installed in accordance with the designs of the Chirkey and Toktogul hydroelectric stations, i.e., about 9.5I) 1.At present the prospect of using draft tubes with long exit cones is limited to designs of powerhouses with a two-level layout of the units. These designs call for the installation of Francis turbines having Q~ max = 800 liters/ see. However, it is of interest to investigate the effect of elongating the exit cone over a wider range of turbine discharges characteristic not only for Francis but also for Kaplan turbines. Therefore, power tests were carried out on a model hydraulic turbine stand with a PL 20/661-25 runner.The flow structure in the entry section of the exit cone depends on the flow characteristics at the entrance to the draft re'be and on the character of this flow transformation in the entry cone and especially in the tube elbow. In the majority of turbine operating regimes the region of flow with maximum axial velocities in the tube entrance is located on the periphery of the cross section. This distribution is retained also at the entrance to the elbow. The turn of the twisted flow in the elbow with the cro~s sections gradually expanding in plan is characterized by the fact that, owing to the effect of centrifugal forces, the zone of maximum axial velocities is located at a maximum distance from the vertical plane of symmetry, i.e., near one or the other side walls of the elbow. In the case of a positive twist of the fl0w at the entry to the tube (in the direction of turbine rotation) the maximum axial velocities occupy a region near the left (with the current) wall of the elbow. As a result the greater part of the turbine discharge enters the left part of the exit cone. In the ease of a negative twist the zo...

Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 1 - Velocity Field

“…Despite the measurements showing the asymmetric flow, the simulation provides an almost symmetric flow after the draft tube elbow at the downstream section. Gubin et al (1974) investigated the flow at the outlet of curved draft tubes and found that the asymmetry in the axial velocity profiles is directly influenced by the swirl leaving the runner, similarly to Amiri et al (2016).…”

Optimization of Elbow Draft Tubes for Variable Speed Propeller Turbine

“…The existing literature does not present any uniform view of the parameterisation of the EDT geometry. For example, Gubin [24] states that there are 20 parameters that can be used to describe EDT. Other authors like Mun et al [13] or Lyutov et al [2] describe EDT using 9 independent parameters.…”

Influence of the Reshaped Elbow on the Unsteady Pressure Field in a Simplified Geometry of the Draft Tube

“…In addition, it allows to place the turbine above the tail water without losing head [3][4][5] and it reduces the total civil costs of the powerhouse, primarily in excavation and concreting [6]. The shape and dimensions of the elbow draft tube depend on the turbine type and size [2,7]. The shape and geometry of the draft tubes have been changed over time from a cone to an elbow draft tube [1][2][3][4][5][6][7].…”

“…The shape and dimensions of the elbow draft tube depend on the turbine type and size [2,7]. The shape and geometry of the draft tubes have been changed over time from a cone to an elbow draft tube [1][2][3][4][5][6][7]. Despite all these changes applied over time to the draft tubes, the most significant hydraulic losses in the reaction hydraulic turbines are still associated with decelerated swirling flows in the elbow draft tube under off-design conditions [8,9].…”