Simulation of pre-rotation in the flow at the inlet of a Reversible Pump Turbine
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In this master thesis, CFD simulations of an imposed pre-rotation in the flow at the inlet of a reversible pump turbine (RPT) were carried out in ANSYS CFX. The simulations were conducted on the RPT model designed by Olimstad and experimental data from Stranna's measurements were used as boundary conditions. The objective of this thesis is to conduct an investigation of the effect of pre-rotation with respect to cavitation. Adding a rotation to the flow before it enters the inlet of the impeller might be an effective countermeasure for cavitation as it might decrease the presence of low-pressure zones on the blades. Simulations for three different volume flows, a low volume flow, the volume flow for BEP and a large volume flow, were conducted. The magnitude of the low and large volume flows were 0.4775 and 1.3823 times the flow at BEP, respectively. Pre-rotation was imposed by adding a rotational velocity component to the flow at the inlet, and simulations with a rotational velocity component of 16 different magnitudes were carried out for every volume flow. In total 48 simulations were conducted and compared with each other. First, the 16 simulations with the same volume flow and different rotational components were compared with each other, and then the trends found for the three different volume flows were compared. The area of the low-pressure zones decreased for all volume flows when more pre-rotation was added. As the flow develops from the inlet of the domain to the inlet of the blade the magnitude of the rotational component of the velocity as it approaches the blade is unknown. The way the pre-rotation in the flow at the inlet was imposed was not optimal and this contributes to a development of the rotational velocity. Because of this, the direct relation between the actual magnitude of the imposed pre-rotation and the decrease in the area of the low-pressure zones can not be evaluated in this master thesis.