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1Christopher R. Johnson, Turbomachinery Aerodynamics, GE Aviation, additional contributing author 2Brian F. Hilbert, Institute for Flow Physics and Control, University of Notre Dame, additional contributing author

                    ISABE-2015-20236

ANALYSIS AND METHODOLOGIES FOR REDUCING VARIATION IN TURBINE AERODYNAMIC TESTING1,2

Tamuto Takakura, Scott C. Morris, & Joshua D. Cameron

Institute for Flow Physics and Control University of Notre Dame

Notre Dame, Indiana 46556 USA

Ruby L. Precourt Turbomachinery Aerodynamics

GE Aviation Cincinnati, Ohio 45215 USA

Abstract The Transonic Research Turbine (TRT) located at the University of Notre Dame provides aerodynamic conditions similar to those of real-world aircraft and power generation turbines for experimental research. The TRT design uses a number of unique features that allows for warm-air inlet conditions and variable operating points. This paper will describe the critical issues that are necessary to reach the required test-to-test, day-to-day, and build-to-build repeatability. These have been identified through several years of research activities, including a recent program where repeated builds of the same experiment were conducted. Some characteristics that were considered to be important included steady state achievability, tip clearance steadiness and control, and mass flow measurement accuracy. The lessons learned will be discussed, followed by several recommended best practices for aerodynamic turbine measurements. Nomenclature   A Inlet Area cp Specific Heat M Axial Mach Number m Core Mass Flow PT Total Pressure R Ideal Gas Constant T Torque

TT Total Temperature Greek Letters γ Specific Heat Ratio δ Pressure Ratio η Efficiency θ Temperature Ratio π Total Pressure Ratio τ Total Temperature Ratio ω Rotor Angular Speed Abbreviations MFP Mass Flow Parameter TCL Tip Clearance TRT Transonic Research Turbine Subscripts adia Adiabatic inlet Inlet Averaged mech Mechanical ref Reference Introduction High-speed rotating test turbine facilities are used to transition new technologies to air-breathing and power generation turbines. These high-speed facilities provide realistic evaluation at conditions closer to operating point. Short duration rotating test facilities (see, e.g. Dunn 2013, Anthony 2013, Epstein 1984) can achieve full flow similarity but operation time is on order of fraction of seconds. This limits the ability to obtain

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detailed flow information under various operating conditions and control parameters. Continuous duration facilities (see, e.g. Palafox 2013, Erhard 2000, Schmitz 2010) provide an environment for obtaining detailed flow information but typically at a higher operating cost compared to short duration facilities. These facilities help develop and understand novel axial turbine enhancing concepts. The Transonic Research Turbine (TRT) recently completed a multi-year, multi-phase program. The program examined the facility’s ability to obtain test-to-test, day-to-day, and build-to-build repeatability. The testing schedule showed that mechanical efficiency measurements are resolvable to ±0.05% or better in the facility. Precise measurements can allow for delta comparisons to baseline configurations that have helped to decipher design change benefits. This paper will document the various TRT procedures and techniques facility operators have recognized are needed to operate this test rig. Various metrics have been identified that has benefited in acquiring quality data measurements. These realized processes could be useful in operation of other high-speed continuous duration facilities and hopefully can be adapted elsewhere to achieve similar or better data repeatability measurements. Experimental Apparatus

The facility schematic is shown in Figure 1. Station 1 indicates where air is drawn from the atmosphere and continues through stations 2 through 9. The circuit splits into two paths at Station 10 that allows for a path to station 2 and mixes with fluid from station 1. Two throttling valves control the recycling process. The remaining portion is

ejected to the atmosphere. The inlet of the turbine is able to reach a moderately warm temperature (∼375 K) through this recycling process. A major benefit of a warm turbine inlet is that a higher pressure and temperature ratio can be achieved as the expanded fluid temperature aft of the experimental turbine can be kept above the dew and freezing point temperatures. Aerodynamically relevant test conditions can be reached in the test facility with higher pressure and temperature ratios. Injecting fluid at station 11 can simulate forward hub, forward tip, and aft hub cooling flows. The entering quantities are measured through a calibrated orifice tube. The sum of the stage mass flow and cooling flow was measured by a Venturi flow meter identified at station 6 located aft of the turbine test section and before the surge prevention inlet/compressor. The Venturi mass flow meter was manufactured by Lambda Square Inc. and calibrated by Colorado Engineering Experiment Station Inc. A Setra Model 270 pressure transducer measured the Venturi inlet pressure and a Setra Model 290 pressure transducer measured the inlet-throat differential. A custom variable speed AC electric motor is capable of producing 500 HP at 5000 RPM. It is mechanically linked on both side to the turbine and compressor. This configuration allows the turbine torque to supplement the motor torque to drive the compressor that requires up to 890 HP at full operation. The motor is capable of powering the compressor at 74% speed continuously at design point. It has an overload torque condition that can deliver 150% its designed torque rating for 40 seconds. This will allow the facility to reach up to 97% speed.

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The performance characteristic map for the compressor at design speed is shown in Figure 2. The square mark indicates the design point of the compressor. The compressor is equipped with variable inlet guide vanes (IGV) and diffuser guide vanes (DGV). This feature allows the compressor pressure ratio to be altered while maintaining a fixed turbine speed. The permissible compressor operating points are constrained by the curves labeled surge limit, 10%, and 100% IGV/DGV settings. A surge prevention inlet is denoted by station 12. The inlet helps to keep a sufficient surge margin from the prescribed safety limits of the compressor as indicated in Figure 2. TRT is capable of rotational speeds of up to 15000 RPM and producing pressure ratios up to 2.4. Previous experiments have demonstrated continuous operation with tip clearances as small as 0.13 mm. Repeatability in Venturi based adiabatic and mechanical efficiency measurements has been demonstrated to be better than ±0.25% and ±0.05% respectively as presented later in this paper. This includes full hardware re-builds.

The facility utilizes active magnetic bearings manufactured by SKF. The advantage is that magnetic bearings have minimal power dissipation since the bearings are not in direct contact with the rotating shaft. This allows for precise measurements of shaft torque from the torquemeter. The magnetic bearing control system can also easily be utilized for simulating eccentric or off-centered axes of rotation that could offer additional opportunities for experimental research.

Figure 1: ND-TRT Facility Schematic

 

 Figure 2: Compressor Performance

Curves

 

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A further detailed discussion of the facility specifics has been previously presented by Ma et. al. (2006) and Schmitz (2010). Facility Instrumentation

The facility is integrated with a data acquisition system capable of acquiring various pressure, temperature, and analog voltage measurements. The following sections will explain in detail information about the MasterDAQ suite as well as certain measurement devices. Total pressure rakes were installed forward and aft of the turbine stage. Each rake made by Aerodyn Engineering Inc. consisted of a five or six, depending on axial location, Kiel-type total pressure probes at different spanwise locations. Three inlet rakes were located 1.75 rotor chords upstream of the nozzle and thirteen exit rakes located 2.5 rotor chords downstream for a total of 15 inlet and 78 exit total pressure measurements. Each pressure measurement was acquired with an Esterline NetScanner 98RK-1 scanner consisting of 8 modules capable of 16 pressure measurements each for a total of 128 measurements per scanner. Each pressure port is capable of measuring differential pressures of ±34.5 kPa (±5 PSID), referenced to an upstream or downstream static port as appropriate. The reference ports were measured using a Setra Model 270 pressure transducer with an operating range of 0-137.9 kPa (0-20 PSIA). Each of the previously mentioned pressure rakes was paired with a K-type thermocouple to obtain total temperature measurements. The temperatures were acquired using a National Instruments cFP-1804 chassis equipped with cFP-TC-120 modules. The current DAQ configuration is capable of accepting 128 temperature

measurements. Analog voltages such as Setra pressure transducers and torquemeter outputs were measured using two different National Instruments modules. A total of 37 analog voltages were measured using NI9205 modules and 16 analog voltages using NI9215 modules. The NI9215 modules are true differential DAQ systems and were used for measurements requiring higher precision. The turbine torque output was measured using a Torquetronics ET1303 phase-shift torquemeter. The ET1303 is capable of measuring torque up to 300 Nm at a full-scale accuracy of 0.1% to a speed of 36000 RPM. The torquemeter was assembled in series with the mechanical drive system and was linked to the turbine shaft and high-speed gearbox pinion shaft as shown in the facility schematic. The torquemeter shaft was attached to the aforementioned shaft by two identical Coupling Corporation of America flex couplings. The torquemeter measures torque via the phase displacement of the torsion shaft. Each pick-up located on either side of the torsion shaft produces a train of waves as the shaft rotates and the phase relationship between these waves is translated to measured torque. A static calibration shows a nearly linear relationship in the upper half of voltage ranges. The torquemeter outputs greater accuracy in the upper range of the torsion shaft by design. This error is reflected in Figure 3 from previously obtained calibration data. A 0.6096 m (2 ft) moment arm was used to apply known torque at 0.9072 Nm (2 lbf-ft) intervals and the torquemeter output voltage was recorded. Each applied calibration weight is accurate to 0.907 g (0.002 lbm) or better, with a total uncertainty of 1.905 g (0.0042 lbm).

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Four tip clearance probes are circumferentially spaced 90 degrees flush to the shroud and placed axially at approximately 40% axial chord. The tip clearance probes were manufactured by Capacitec, Inc. and were connected to a signal conditioner created by Aerogage Corporation. Testing Methods TRT has several operational protocols that have been instituted to insure quality data acquisition.

 Figure 3: Torquemeter Calibration

Error

 

 Figure 4: Start Up Data for Establishing Heat Soak Criteria

 

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Weather and time of day could result in data drifts over time for a semi-open loop system. Low- and high-pressure weather fronts can change inlet pressures within small time periods. This can result in keeping a constant inlet condition difficult. Early mornings and evenings can similarly cause inlet condition variations as ambient temperature rapidly fluctuates as the sun rises or sets. Thunderstorms can cause temporary voltage surges in instrumentation that can alter correct measurement readings. The facility when first started follows a heat soak procedure. The heat soak process took approximately 30 minutes for facility components to reach thermal equilibrium. A sample start up data series is shown in Figure 4 with various metrics used to establish operating point as a

function of time. The compressor exit temperature required the greatest time in the facility to reach thermal equilibrium. Other facility mean values such as stage efficiency, pressure ratio, and temperature ratio seem to steady as the compressor exit fluid temperature begins to steady. A normalized circumferentially averaged spanwise radial profile using the inlet and exit rakes were then compared for each test point. This is shown in Figure 5. This comparison succinctly shows that the spanwise profiles in blue are nominally equivalent suggesting comparable data acquisition. The red profile in Figure 5(a) shows a slight increase in normalized inlet total pressure at immersion 4 indicating that the pressure is higher than normal and could be a result of a blocked pressure port. Noticeable deviation such as the

 Figure 5: (a) Normalized inlet total pressure (b) Normalized inlet total

temperature (c) Normalized outlet total pressure (d) Normalized outlet total temperature

(a) (b)

(c) (d)

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profile in red could be an indication of damaged instrumentation or systematic differences. TCL values were monitored during the length of the testing period. Previous experimental programs have shown that TRT is capable of achieving TCL-to-span ratios as low as 0.3% on continuous run-time basis with variations of less than ±0.01%. Figure 6 shows six data series each representing the same nominal test condition. The target TCL-to-span ratio for this program was 0.72%. The blue lines show example sets from data series within the acceptable target variation of ±0.013% for this particular test. The green shows an upward trend as a function of time. This usually indicates that the facility has not reached steady state. This can typically result when the outer rotor casing continues to warm and expands. The black data set shows a slight upward trend similar to the green but at an overall lower TCL-to-span ratio. The blade stretch on warmer days seems to result in a reduction of the TCL-to-span ratio. The red series show an interesting phenomenon at approximately 2250 seconds. There is an immediate jump in TCL. The ambient room temperature near the test article was warmer than normal with high humidity during this testing

period. The control room access door located near the test article was opened to enter the facility. The control room is climate controlled at or near room temperature and with relatively low humidity. This could have contributed to a disruption in the facility room equilibrium and resulted in an abrupt TCL change. Data Analysis

A metric used in turbine testing is adiabatic and mechanical efficiency to quantify different turbine designs. The adiabatic efficiency is defined as (Mattingly 1996)

𝜂!"#!$!%#& =1 − 𝜏

1 − 𝜋!!!

! (1)

and represents a ratio of non-isentropic expansion to ideal expansion. τ is the exit-to-inlet total temperature ratio and π is the exit-to-inlet total pressure ratio. The mechanical efficiency is a ratio of power output of the turbine to the enthalpy change across the stage and is defined as (Mattingly 1996)

𝜂!"#! =𝑇 ∙ 𝜔

𝑚𝑐!𝑇! 1 − 𝜋!!!

! (2)

where T is the torque produced by the rotor, ω is the angular speed of the rotor, 𝑚 is the core mass flow, cp is the heat capacity, and TT is the inlet total temperature. Adiabatic and mechanical efficiencies are shown in Figure 7 for a baseline configuration. The baseline configuration target is shown as an x. Each repeated test point is signified by a repeated symbol within a given build. The two different builds of the baseline configuration is represented as * and o. Figure 7(a) shows the adiabatic efficiency of multiple test points and builds as

 Figure 6: Sample Tip Clearance Data

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a function of Venturi (blue) and inlet mass flow parameter (MFP) (red) based corrected mass flow. A section on Venturi and MFP mass flow measurements is presented later in this section. The temperature and pressure ratio terms used in calculating adiabatic efficiency was obtained from rakes located upstream and downstream of the stage. The adiabatic efficiency is a discretized area-averaged metric and appears to result in greater scatter in the data than compared to the mechanical efficiency measurement shown in Figure 7(b). The torque was measured from the torquemeter and results in a mass-averaged measurement. This seems to contribute to less scatter. The Venturi based adiabatic and mechanical test-to-test and build-to-build repeatability is within approximately ±0.25% and ±0.05%. A useful metric was the ratio of the adiabatic efficiency to mechanical efficiency called the temp-to-torque ratio. A value close to unity was ideal since both efficiency values should independently measure the same. A

significant deviation could suggest that mass flow, torque, or temperature, for example, might be taken incorrectly. A sample time series of temp-to-torque for a single test point is shown in Figure 8. This figure indicates that on average the adiabatic efficiency measured only about 0.7% higher compared to the mechanical efficiency measurement.

 Figure 7: (a) Adiabatic and (b) mechanical efficiency based on Inlet MFP (red) and Venturi Mass Flow (blue) Measurements. The baseline configuration target is shown as a large black x. The * and o is represents two different builds.  

 Figure 8: Sample Temp-to-Torque

Ratio

 

(a) (b)

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Quantifying and correcting mass flow measurements is necessary when comparing test-to-test, day-to-day, and build-to-build measurements. A semi-open loop system can vary the inlet pressure depending on the atmospheric condition. The physical mass flow through the system changes accordingly for the same operating condition. A corrected mass flow rate is expressed as (Farokhi 2009)

𝑚!"##$!%$& =𝑚 𝜃𝛿

(3)

where

𝜃 ≡𝑇!,!"#$%𝑇!"#

(4)

𝛿 ≡𝑃!,!"#$%𝑃!"#

(5)

to account for this physical mass flow variation. The reference pressure and temperature are defined for a standard day at sea level or Pref = 101.325 kPa and Tref = 288.2 K. Both the Venturi and the inlet MFP based corrected mass flow

measurements can be compared to determine the possibility of test stage leaks. MFP is defined as (Farokhi 2009)

𝑀𝐹𝑃   =𝛾𝑅

𝑀

1 + 𝛾 − 12 𝑀!!!!

!(!!!) (6)

where M is the axial Mach number at the inlet. MFP is then related to corrected mass flow rate by

𝑚 =𝑃!"#𝐴𝑇!"#

𝑀𝐹𝑃 (7)

where A is the inlet cross sectional area. The effects of mass flow increases to adiabatic and mechanical efficiency is shown in Figure 9. The baseline configuration target is shown as an x. The mass flow increases are shown as Δ, ◊, and ∇ in increasing mass flow rates. These leaks were located between the turbine stage inlet and Venturi mass flow meter. A leak in TRT will result in a mass flow increase as air is drawn into the flow circuit because TRT operates sub-atmospherically.

 Figure 9: (a) Adiabatic and (b) mechanical efficiency based on Inlet MFP (red) and Venturi Mass Flow (blue) Measurements. The baseline configuration target is shown as a large black x. Leaks in the test region is shown as Δ, ◊, and ∇

in increasing leak sizes.

 

(a) (b)

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The MFP based mass flow measurements were insensitive to the downstream leaks and efficiencies measured within ±0.125% for all configurations. The Venturi located downstream of the leaks was able to capture the corresponding increases. This suggests that both measurement devices were accurately measuring the correct mass flow. A significant difference between the inlet MFP and Venturi mass flow would indicate that a possible leak could exist within the test section region of interest. The adiabatic efficiency was less sensitive to leaks compared to the mechanical efficiency measurements likely due to area- and mass-averaged differences. A constant torque line is shown in black in Figure 9(b). The constant torque line is obtained from Equation 2 by determining a constant torque and varying mass flow rates. The constant torque line shows that the efficiency changes likely result only from the mass flow increases and the resulting constant torque line can be used as a correction factor. Conclusions

A recent experimental aerodynamic turbine program in the Transonic Research Turbine at the University of Notre Dame resulted in critical systematic protocols for data acquisition. The critical precautions for pre, during, and post testing phases were presented. Environmental factors such as weather and ambient facility temperatures were discussed for the pre-data acquisition phase. Obtaining stable operating conditions were then discussed to prevent any equilibrium shifts during the testing period. Real-time data processing has helped in determining facility steadiness. The data was processed when testing concluded to determine if the

obtained series was acceptable within a prescribed bound. Certain troubleshooting methods such as analyzing adiabatic and mechanical efficiency or inlet MFP and Venturi based mass flow have been presented for testing phases when data seem to deviate from anticipated results. An execution of these procedures resulted in test-to-test, day-to-day, and build-to-build repeatability in adiabatic and mechanical efficiencies of ±0.25% and ±0.05%. A 0.72% TCL-to-span ratio with continuous run-time variations less than ±0.01% was attained. These practices instituted in TRT has resulted in successful completion of a multi-phase, multi-year program and could be helpful in obtaining similar repeatable data in other comparable high-speed test facilities. Acknowledgements

The authors would like to acknowledge GE Aviation in funding and approving the publication of this work. A special recognition is due to TRT technicians James J. Hock, Jr. and David E. Hipskind for their technical assistance in daily facility operations. References R.J.Anthony and J.P.Clark. A Review of AFRL Turbine Research Facility. ASME Paper No. GT2013-94741, 2013. M.Dunn and R.Mathison. History of Short-Duration Measurement Programs Related to Gas Turbine Heat Transfer, Aerodynamics, and Aeroperformance at Calspan and OSU. ASME Paper No. GT2013-94926, 2013. A.Epstein, G.Guenette, and R.J.G.Norton. The MIT Blowdown Turbine Facility. ASME Paper No. 84-GT-116, 1984. J.Erhard. Design, Construction, and Commissioning of a Transonic Test-

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Turbine Facility. Ph.D. Thesis, Institute of Thermal Turbomachinery and Machines Dynamics, Technical University of Graz, Austria, 2000. S.Farokhi. Aircraft Propulsion. Wiley, 2009 R.Ma, S.C.Morris, and T.C. Corke. Design of a Transonic Research Turbine Facility. AIAA Paper 2006-1331, 44th AIAA Aerospace Sciences Meeting and Exhibit, 9-12 January 2006, Reno, Nevada. J.D. Mattingly. Elements of Gas Turbine Propulsion. McGraw-Hill, Inc. 1996. P.Palafox, Z.Ding, J.Bailey, T.Vanduser, K.Kirtley, K.Moore, R.Chupp. A New 1.5-Stage Turbine Wheelspace Hot Gas Ingestion Rig (HGIR) – Part I: Experimental Test Vehicle, Measurement Capability and Baseline Results. ASME Paper No. GT2013-96020, 2013. J.T.Schmitz, Experimental Measurements in a Highly Loaded Low Pressure Turbine Stage. Ph.D. thesis, University of Notre Dame, 2010.