1. 1
Experimental Investigation of Hub Leakage Flow
through Stator Knife Seals
Jordan K. Lewis
Purdue University, West Lafayette, IN, 47907, USA
When looking at the efficiency of a high speed compressor, secondary leakage flow through the
stator hubs can influence the compressor’s performance. Compressors work by passing a fluid between
alternating rotational and stationary reference frames further diffusing the flow in each stage. The static
pressure in the system increases with each stage as the flow diffuses. This state of high pressure is not
easy to achieve or maintain because compressors create an adverse pressure gradient which makes the
fluid want to resist the energized state. The fluid will always want to flow from high pressure to low
pressure, resulting in flow leaking backwards through the compressor.
Figure 1: Schematic showing leakage flow through stator knife seals
These leakage flows are considered secondary flows because they are not the primary flow
through the outlet of the compressor. However, these secondary flows still impose a significant effect on
the overall efficiency of the compressor. Computational fluid dynamics (CFD) programs are useful tools
for predicting how a compressor design under various operating conditions will affect the performance.
CFD works with set boundary conditions for the flow through the system; however, boundary conditions
modeling secondary leakage flows are difficult to model and implement. For this reason, CFD is less
accurate when modeling flow through a compressor because the CFD mesh will be unable to account for
the secondary flows leaking through the stator hub [1].
The compressor used in this study is the Purdue 3 Stage Axial Research Compressor. It consists
of an Inlet Guide Vane followed by 3 stages of rotors and stators with a 24 in. tip diameter. The design of
the Purdue 3 Stage consists of shrouded stators in which the inner stator shroud sits below the inner
diameter of the primary flow path. The leakage flow through the shrouded stator cavity is minimized with
the use of knife seals to reduce the area between the spinning hub and stator vane.

2. 2
Figure 2: Diagram of Purdue 3 Stage shrouded stator cavity and knife seals.
The purpose of this experiment was to quantify how much flow is leaking through the shrouded
stator cavities and to provide data that can be used as boundary conditions to calibrate and validate a CFD
mesh of flow through the compressor.
The primary measurement device in this experiment was a piezoresistive Kulite pressure
transducer. The transducer is activated when mechanical strain is implemented on piezoresistors placed in
a Wheatstone bridge configuration because the mechanical strain caused by pressure alters the resistivity
of the transducer. The change in resistance modifies the voltage output from the transducer which can be
calibrated to determine the pressure imposed on the transducer [3]. An LQ-062 Kulite transducer was
placed on each side of the knife seal for stators 1 and 2. Each pressure transducer measured the resulting
voltage at each operating condition and through a voltage pressure calibration, the difference between the
upstream and downstream pressure measurements was calculated. A former study by Wellborn in a low-
speed axial compressor suggested that a difference in static pressure could be used to determine the
leakage mass flow rate through the knife seal clearance. Wellborn suggested using an equation derived
from the conservation of linear momentum and scaled with the primary mass flow rate through the
compressor [4].
The experimental setup consisted of machining a rectangular cavity into each stator vane. A
rectangular block with two holes for each transducer was also machined to fit precisely in each stator
cavity. It was important to use well-machined parts within a tolerance of 0.001 in. so that the effect of the
transducers in the cavity would be minimal on the overall performance of the compressor. The two
transducers were positioned inside of the rectangular block and a black epoxy resin was poured over the
top to provide a secure backing. The epoxy filled rectangular blocks were then baked at 160 degrees
Fahrenheit to further set the epoxy.
Once the epoxy hardened, the rectangular plates were placed in a sealed calibration chamber to
determine the conversion between the voltage output of the transducers and the actual pressure acting on
them. The chamber was slowly filled to 5 psi with data points being taken intermittently in a LabView
code relating the sensor output voltage to the pressure inside the chamber. The pressure in the chamber
was then decreased from 5 psi back to zero to account for any hysteresis in the system. With a complete
calibration data set, the points were plotted and a line of best fit was used to determine the calibration
factor for all four sensors. The coefficient of determination for the best fit line was 1.0000 verifying a
linear relationship for the calibration.

3. 3
Figure 3: Voltage to pressure calibration plot for LQ-062 Kulite transducers.
With the sensors fully calibrated, the blocks were installed in the stator assembly and the 36
gauge wire was carefully glued along the stator vane and passed through an exit hole in the casing. The
operating conditions for the compressor were controlled through a LabView code and the data collected
from the transducers was relayed through the LabView code to be saved for processing. The data were
sampled at a rate of 300 kHz with an anti-aliasing analog low-pass filter at a cut-off frequency of 100
kHz. The testing consisted of running the compressor at speeds of 68%, 80%, 90%, and 100%. Data
points from the transducers were taken intermittently as the throttle was adjusted to provide a range of
corrected mass flow rates. The experiment was repeated multiple times on different days and on opposite
sides of the compressor to account for leakage deviations from eccentricity of the rotor, manufacturing
tolerances, and other geometric defects. The data were processed and the change in pressure across the
knife seals in stators 1 and 2 were plotted. The solid red and dashed black lines represend the mean values
for experiments conducted on opposite sides of the compressor. Range bars were included on the plots to
represent the variation between the maximum and minimum changes in pressure with day to day
repeatability.
Figure 4: Plots of the change in pressure across the knife seals for stators 1 and 2 with respect to the normalized corrected flow
rate.

4. 4
Figure 5: Total pressure ratio map of the Purdue 3 Stage [2].
The resulting lines denoting the knife seal pressure difference at constant speeds can be combined
with a total pressure ratio map to quantify the flow leaking through the knife seals at each operating point
of the compressor [2]. The measured secondary leakage flow can then be used to calibrate and validate a
CFD model of secondary flows because the CFD has comparable experimental data allowing known
boundary conditions to be created for the mesh. These data are significant for the future development of
CFD due to the ability to better predict losses in a compressor system. In addition to the opportunities for
future CFD technology, some trends in the leakage flow were also discovered through this experiment.
Particularly, in stator 2, there is a slight decrease in the knife seal pressure difference as the stability limit,
represented by the dashed line in Figure 5, is approached. Further experimentation and analysis are
recommended to determine the reasoning for why this phenomenon occurs.

5. 5
Sources
1. Ball, P.R., 2013, "An Experimental and Computational Investigation on the Effects of Stator Leakage
Flow on Compressor Performance," MS Thesis, School of Aeronautics and Astronautics, Purdue
University.
2. Berdanier, R.A., 2015, "An Experimental Characterization of Tip Leakage Flows and Corresponding
Effects on Multistage Compressor Performance," PhD Dissertation, School of Mechanical
Engineering, Purdue University
3. Carter, S., Ned, A., Chivers, J., & Bemis, A. (n.d.). Selecting Piezoresistive vs. Piezoelectric Pressure
Transducers. Retrieved April 23, 2016, from
http://www.kulite.com/docs/technical_papers/Piezoresistive_vs_Piezoelectric.pdf
4. Wellborn, Steven Robert, "Effects of shrouded stator cavity flows on multistage axial compressor
aerodynamic performance" (1996). Retrospective Theses and Dissertations. Paper 11344