Analysis of a Centrifugal Pump Equipped With an Axial Rotor With Variable Speed

Abstract

In the last two decades the energy market policy was focused on development of renewable energy. The renewable energy (solar and wind power) induces a fluctuating component of the electrical grid. The solution to compensate fluctuating energy is provided by the hydraulic turbines in order to produce energy in a short time and the pumps units in order to use the energy excess. In order to ensure higher flow rate for the storage pumps, the units have constructive differences besides regular. Consequently, the complex shape of the suction-elbow with symmetrical geometry generates unsteady flows which are ingested by the impeller. These phenomena induce stronger unsteady flow conditions, such as stall, wakes, turbulence and pressure fluctuations, which affect the overall behavior of the pump providing vibration, noise and radial and axial forces on the rotor. Alternatively, an axial rotor can be installed in front of the impeller. In this case, the flow non-uniformity will be decreased and the static pressure will be increased at the pump impeller inlet. Consequently, the efficiency behavior practically remains unchanged while the cavitational behavior is improved. Recently, a new concept was explored in order to assess the cavitational behavior on a wide range operation of the pump. The new concept proposes variable speed for the axial rotor, while the speed of the pump impeller remains constant. Accordingly, this new concept is experimentally and numerically investigated in the paper. First, the paper presents the 3D numerical investigation of the pump impeller combined with an axial rotor. The axial rotor is investigated at variable speed (between 2000 and 3000 rpm) while the pump impeller has constant speed of 2500 rpm. The minimum static pressure and the static pressure coefficient are analyzed in order to assess the new method. Second the mechanical design of the solution, for testing on the laboratory is presented. The solution contains an innovative clutch for controlling the variable speed of the axial rotor using magneto-rheological (MR) fluids. Thirdly, the experimental results concentrates on MR clutch operating regimes and the pump efficiency analysis. The MR clutch was installed between the driven electrical motor and the pump, three regimes were investigated: runaway speed for the axial rotor, clutch with MR fluid inside with and without magnetic field applied on it. The conclusions are drawn in the last section.