Turbomachinery Developments and Cavitation

After a brief review of flow-induced instabilities in turbopumps for liquid propellant feed systems of modern rocket engines, the lecture illustrates some recent results of the work carried out at Alta on the hydrodynamics and unsteady cavitation phenomena of these machines. A reduced order model for preliminary design and noncavitating performance prediction of tapered axial inducers is illustrated. In the incompressible, inviscid, irrotational flow approximation the model expresses the 3D flow field in the blade channels by superposing a 2D cross-sectional vorticity correction to a fully-guided axisymmetric flow with radially uniform axial velocity. Suitable redefinition of the diffusion factor for bladings with non-negligible radial flow allows for the control of the blade loading and the estimate of the boundary layer blockage at the specified design flow coefficient, providing a simple criterion for matching the hub profile to the axial variation of the blade pitch angle. Carter’s rule is employed to account for flow deviation at the inducer trailing edge. Mass continuity, angular momentum conservation and Euler’s equation are used to derive a simple 2nd order boundary value problem, whose numerical solution describes the far field axisymmetric flow at the inducer discharge. A closed form approximate solution is also provided, which proved to yield equivalently accurate results in the prediction of the inducer performance. Finally, the noncavitating pumping characteristic is obtained by introducing suitably adapted correlations of pressure losses and flow deviation effects. The model has been verified to closely approximate the geometry and noncavitating performance of a number of tapered-hub high-head inducers for space application. The results of a series of tests conducted in water under similarity conditions on the four-bladed DAPAMITO4 inducer, designed and manufactured by means of the above reduced-order model, are illustrated. Several non-synchronous instabilities have been observed on the inducer, including an axial surge, a backflow oscillation and, at higher temperatures, incipient rotating cavitation and backflow vortex instability. In addition, synchronous rotating cavitation (leading to the characteristic “one step” shape of the cavitating performance curve near head breakdown conditions) has been detected at all the flow conditions investigated. It has been found that the amplitude of the flow oscillations associated to this instability generally tends to decrease at higher water temperatures. The characterization of the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch high-head inducer, under different load and cavitation conditions is presented. The results have obtained in the Cavitating Pump Rotordynamic Test Facility at Alta by means of a novel experimental technique, allowing for the continuous measurement of the rotordynamic force spectra as functions of the whirl ratio. Comparison with simultaneous high-speed movies of the inducer inlet flow highlighted the relationship between the cavitation dynamics in the inducer backflow and the spectral behavior of the rotordynamic force as functions of the whirl ratio. Finally, the future perspectives of the work carried out at Alta on the hydrodynamics and unsteady cavitation phenomena of high performance turbopumps for liquid propellant feed systems of modern rocket engines are briefly illustrated.