4 General Description Of The Measured Flow Field

The primary parametric variables for this study are seal flow rate, turbine stator vane position, and turbine rotor blade position. The distinct changes in the steady and unsteady flow field attributed to the variation of these parameters are the subject of this research. To better portray these changes, it is first necessary to understand the similarity in flow patterns present in all test cases.

First note the orientation of a measurement plane relative to the test hardware, typical of what is presented in Figure 4.1. The single measurement plane is presented with the mainstream flow from left to right. Upstream of the measurement plane is the vane trailing edge, with an approximate distance of 0.264 in. (6.7 mm) between the vane trailing edge and the plane at the hub wall. At the downstream edge of the measurement plane is the rotor leading edge. The measurement plane is positioned such that the seal is located at an axial distance of 0.287 - 0.307 in. (7.3 -7.8 mm) from the upstream edge of the measurement plane.

The measurement plane is subject to flow structures convected downstream from the vane row, to a pressure field developed by the vane row, to a fluctuating pressure field introduced by the passing rotor blade, and to the emergent seal flow. All this is in addition to the shear forces at the hub wall.

There are then specific flow patterns along the upstream edge, the hub wall and the downstream edge of the measurement plane. The upstream edge is affected by the vane exit flow field, and the downstream edge by the rotor potential field. Additionally, these upstream and downstream effects can extend well into the center of the measurement region and can be significant in the low momentum flow along the hub wall.

A typical velocity field is presented in Figure 4.1. Along the upstream edge is radial upward flow which is evident to a certain extent in all of the data sets. Along the downstream edge is a low momentum region which may or may not contain a vortex core, depending on the rotor blade position. Along the hub surface is reduced axial velocity, and a slightly large radial component of velocity at the axial location of the seal gap.

The information presented in Figure 4.1 includes both velocity vectors and the resolved vorticity values at those points. The scale on the vorticity plots is in m/s/mm, this scale being used because of the small physical size of the interrogation regions. The spacing from one velocity vector to the next is 13.8 mil (0.35mm). Thus the scale for each structure is relatively small.

The vortex core which exists at the leading edge of the rotor blade is evidence of the start of the rotor horseshoe vortex. The pressure field around the blade provides the potential for vorticity generated at the wall to contribute to a vortical structure at the blade leading edge. Flow acceleration around the blade blockage then sweeps the vortex around the blade. Additionally, the negative vorticity at the upstream edge of the measurement plane matches with the wake vortex structure presented in Figure 4.2 and described by Zaccaria and Lakshminarayana (1995). High values of vorticity are also common at the seal gap downstream edge since this is where the seal flow penetrates into the wall boundary layer and turns to align with the mainstream flow.

The upper half of the measurement plane is, in general, far removed from the rotor blade effects at the downstream edge, the vane exit flow effects at the upstream edge, and the seal flow at the hub wall. It is the lower half of the measurement region where fluid momentum is low and subject to influence by outside forces.

Figure 4.1 Sample of the typical intra-stage space flow field.
Figure 4.2 The vane exit flow field for the Purdue Turbine first stage vane.
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