Experimental and Numerical Investigation of Unsteady Structures Within the Rim Seal Cavity in the Presence of Purge Mass Flow

Abstract

Flow within the space between the rotor and stator of a turbine disk, an area referred to as the rim seal cavity, develops azimuthal velocity component from the rotor disk. The fluid within develops unsteady structures that move at a fraction of the rotor speed. A measurement strategy is developed to measure the number of unsteady structures and the rotational speed at which they are moving in the rim seal cavity of an experimental turbine research rig. Data manipulation was developed to extract the speed and the numbers of structures present using two fast response aerodynamic probes measuring static pressure on the surface of the stator side rim seal cavity. A computational study is completed to compare measured results to a transient Unsteady Reynolds Averaged Navier-Stokes (URANS). The computational simulation domain consists of 8 vanes and 10 blades (the full test facility consisted of 29 vanes and 36 blades), carefully picked to reduce error caused by blade vane pitch mismatch and to allow for the structures to develop correctly, and the rim seal cavity to measure the speed and number of the structures. The experimental results found 15 structures moving at 77.5% of the rotor speed, computational results are based on the size of the domain and guidance from the experimental results found 14.5 structures are moving at 81.7% rotor speed. The agreement represents the first known test of its kind and the first known agreement between computational and experimental work performed in the large scale and rotating turbine research facility AFTRF at the Pennsylvania State University.

Flow mechanisms in axial turbine rim sealing

“…It may be significant that simple higher clearance axial seals show generally low frequencies, and the higher frequency results are for more complex tighter clearance seals. Note though that Town et al's results [36] do not fit this trend. Dependency on cavity volume might be expected, but it is again difficult to see any clear trend.…”

“…[33], Savov et al [34], Schädler et al [35], Jakoby et al [24] and Town et.al. [36] have all indicated unsteady rim seal cavity flow features which are unrelated to blade passing. Figure 5 includes these results with blue squares or bars indicating the approximate observed frequency range.…”

“…Radial rim seals were studied experimentally by Roy et al [32], Savov et al [34], and Town et al [36], as shown in figure 5 and table 2. The seal and cavity geometries were quite different in these cases, but all tests were conducted with a highly swirling annulus flow.…”

“…This was compared to low frequency unsteadiness observed by Cao et al and its effect on ingestion was said to be not understood. Town et al [36] reported measurements in the buffer cavity formed between two radial clearance seals in a turbine test rig. Two fast response pressure transducers were spaced 5 degrees apart.…”

“…Figure 13 shows pressure contours for a low flow case. The influence of the 22 vanes is clear in the outer annulus, the outer seal rotating mode is also visible, and a less regular structure is seen in the outer disc cavity Town et al [36] compared URANS solution for a 99 o sector with blades and vanes with their experimental measurements at Pennsylvania State University. The computations and unsteady pressure measurements were consistent with about 15 flow cells rotating at about 80% disc speed in their shallow disc cavity.…”