APPLICABILITY OF FLOW SIMULATION MODELS TO THE CASE OF RADIAL INFLOW ONTO A ROTATING CORE

Abstract

The paper presents the results of fluid flow simulations carried out by means of the FIDAP7.6 program (a fluid dynamics FEM package) for the case of radial inflow onto a rotating shaft. The particular geometric configuration has been chosen to resemble a generalized inlet chamber of twin low-pressure steam turbines, but with the axial outlet section extended to allow better observation of flow instabilities in that region.

The calculations were carried out for the same channel geometry for both compressible and incompressible flow, using the same or slightly varying boundary conditions. Extensive variation in nonphysical parameters of the model was explored, such as applying different meshes over the region, as well as utilizing different turbulence and upwinding models.

The intent of this research was to evaluate the relative applicability of the various available flow models to the simulation of axisymmetric flows with steep velocity gradients, and to discover the limitations of these models. The calculations have in fact established significant differences in the behavior of the simulated flow for the different meshes and models. Some results were characterized by extensive areas of recirculating flow whereas others, for the same boundary conditions, showed no recirculation. Correct near-wall meshing as well as the choice of the upwinding scheme were established as the critical factors in this regard. There was also noticeable variation in outlet velocity profiles.

An extensive zone of separation within the investigated channel as well as a standing annular vortex near the point of stagnation are flow features of some interest. These patterns of flow change in response to the changing non-physical parameters; the separation zone in particular is absent or slow to develop under some setups.

The influence of inflow parameters, the initial velocity distribution and turbulent intensity in particular, on flow behavior in contact with the rotating shaft have also been an area of investigation, as these are often defined with considerable uncertainty in practical applications. It was observed that some latitude in assuming these parameters did not significantly alter the relevant flow parameters at outlet (the velocity and pressure distributions), although it did induce variation in other aspects of the flow (such as the extent of the standing vortex).