<p>The primary objective of this
research was to shed light on the changes in performance observed in a
high-speed, centrifugal compressor that occur during the transition from
subsonic to transonic operating conditions, using experimental data collected
on a research compressor developed by Honeywell Aerospace, as well as results
from a numerical model of the compressor.</p>
<p> An
understanding of the flow behavior in transonic centrifugal compressors is
critical as the drive for higher stage pressure ratios while maintaining a compact
size results in higher rotational speeds and increased aspect ratios in the
inducer of the impeller. Both of these design trends result in higher relative
Mach numbers near the impeller leading edge, resulting in the formation of
shocks and an increasingly complex flow field. Since it is necessary to maintain
high efficiency and adequate surge margin at these conditions—to ensure the compressor
is stable across the full operating range—it is important to understand the
effects of the transition from subsonic to supersonic flow on performance and
stability. Due to the limited availability of research in the open literature
regarding transonic centrifugal impellers, especially experimental studies, these
behaviors are still not fully understood.</p>
<p>Experimental data collected during
steady state operation as well as during speed transients, showed a sudden
decrease in the variance of the unsteady pressure field throughout the
compressor, but most dramatically in the inducer shroud. Analysis of the
performance also showed a significant increase in impeller efficiency of
approximately 2 points as speed was increased from 80% to 90% of the design
speed. Temperature measurements upstream of the impeller leading edge indicated
a dramatic reduction in the degree of flow recirculation in the same speed range,
indicating the increase in performance is related to a decrease in the blockage
near the impeller leading edge. A low pressure region was also observed in the inducer
passage, which disappeared upon transition to the transonic operating regime,
this coupled with decreased inducer static pressure rise and relative diffusion
at lower speeds, strongly indicates that increased loss in the inducer at lower
speeds is responsible for the observed performance deficiency during subsonic
operation.</p>
<p>Analysis of the numerical results
revealed that the low pressure region in the inducer may be attributable to the
interaction between the inlet shroud boundary layer and the low momentum tip
leakage flow in the impeller passage, which at lower speeds, results in the tip
leakage flow forming a large recirculation region in the inducer passage. It
was also determined that the step change in instability coincides with the
inducer shock extending to the shroud and reducing the strength of the
interaction between the low momentum regions in the inlet and impeller passage,
thereby allowing the tip leakage flow to form into a vortex and preventing the
development of the recirculation region in the inducer. </p>
<p>This research provides a possible
explanation for the observed instability in the compressor, which may allow for
further testing of techniques to mitigate the instability caused by the
blockage in the inducer, such as casing treatment, bleed, or flow injection
into the inducer shroud.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/13360211 |
| Date | 14 December 2020 |
| Creators | William Brown (9754892) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/EXPERIMENTAL_AND_NUMERICAL_EVALUATION_OF_THE_PERFORMANCE_OF_A_HIGH-SPEED_CENTRIFUGAL_COMPRESSOR_AT_OFF-DESIGN_CONDITIONS/13360211 |
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