CFD Modeling of Cavitation in Hollow Jet Dispersive Valves

Cavitation phenomena currently occur in hydraulic equipment like pumps, turbines blades, propellers and valves, causing damages when certain operating conditions of the equipment are presented. Effects of pressure fluctuation, noise, vibration and erosion, can occur in these cases. Dispersing valves, which are widely used, for example, in stilling basins of hydroelectric power plants, due to the function exerted by them and to the operating conditions of the system, as well as to the high flow rate, can be submitted to the conditions of cavitation occurrence with damages to the valve structure. Physical modeling for the study of cavitation in dispersive valves may not be economically feasible, taking a lot of time to perform tests and to provide reliable data from measurements to be applied in a prototype. As a consequence, the use of CFD technique is an alternative methodology for these studies, with the advantages of being more economically feasible and able to reduce the time required for analysis, although, normally, it is necessary the numerical modeling calibration . Cavitation process in Hollow-Jet type dispersing valves, which are commonly used in hydroelectric power plants around the world, was taken as the object of the study herein.

Morassi and Ortiz (2016) studied the influence of the dissipating chamber dimensions of hydroelectric power plant in dispersing hollow jet valve flow and took as reference the data resulted from Falcon Dam physical model studies.

Following the work of Morassi and Ortiz (2016), the behavior of the cavitation occurrence for different flowrates was assessed by using 3D numerical simulation - CFD. The single phase simulation with no cavitation was first performed. The results were used in a transient multiphase case in order to improve the convergence. The density contours lines were obtained and the distribution of density and pressure for different sample locations were analyzed during time evolution. It was verified that cavities with mixture of vapor and liquid were formed at needle tip. These regions are characterized by having the smallest pressure around the valve.