Investigation on Combined Air and Water Injection in Francis Turbine Draft Tube to Reduce Vortex Rope Effects

Abstract

In this paper, the effect of water, air, and their combined injection from two different injection points is studied in order to reduce vorticity effects in a draft tube of prototype turbine working at three operating points. The flow from spiral case to the end of draft tube is simulated using the shear stress transport k–ω turbulence and two-phase models. Using an appropriate validation method, acceptable results were obtained under the noninjection condition. To determine suitable number of points and inlet flow rate for air injection as well as the appropriate nozzle diameter for air and water injection, a new method which considers the ratio of total loss to the pressure recovery factor is used, in addition to using the traditional method which calculates the total loss in the draft tube. Comparing results of the three types of injections shows air injection in the operating range greater than 70% of turbine design flow rate, is much more effective than water injection or the combination of air and water injection. However, in the operating range below 70%, either water or air injections are not suitable, but combination of these two fluids can improve system performance.

An experimental investigation on the effects of cylindrical rods in a draft tube at part load operation in down-scale turbine

“…These techniques include flow injection inside the draft tub as well as alterations of the draft tube geometry. Water and air injection have been investigated at the tip of the runner crown and the walls of the draft tube [8][9][10][11][12][13]. Bosioc et al [6] studied water injection from the runner crown into the vortex region.…”

Francis Turbine Draft Tube Troubleshooting during Operational Conditions Using CFD Analysis

“…In Table 2, the percent of partial discharge operating point is explained as the discharge of the turbine over optimum efficiency discharge (Q/Q opt × 100). A total cell number of 2,631,341 was selected, considering grid quality and the grid independence test, using an analysis of the grid convergence index (GCI) as mentioned in detail in [29,30]. Moreover, structured and unstructured grid types were used in the draft tube and the rest of the geometry, respectively.…”

“…The tangential, radial, and axial velocities, TKE (Turbulence Kinetic Energy), and ω at the beginning of the stay vanes were extracted at three operating points in the inlet of steady state. A total cell number of 2,631,341 was selected, considering grid quality and the grid independence test, using an analysis of the grid convergence index (GCI) as mentioned in detail in [29,30]. Moreover, structured and unstructured grid types were used in the draft tube and the rest of the geometry, respectively.…”

Experimental Studies of Pressure Pulsations in Draft Tube Diffuser of Pump-Turbine Models for Heads up to 200 m

“…Over the years, a large number of methods and technologies have been developed to bring the pulsation level to safe values [22]. The most commonly used ones are the installation of fi ns in the diff user of the draft tube [23] and grilles at the outlet of the runner [24], injection of an air jet [25,26], and changing the geometric charac te ristics of the runner blade system [27,28]. Among the latter, the method of lengthening the cone of the runner is widely used [29,30].…”