CFD Analysis and Full Scale Testing of a Complex Auxiliary ......the entire APU and cooling intake systems. The hardware was manufactured out of fiberglass shells, stereolithogrophy

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CFD ANALYSIS AND FULL SCALE TESTING OF A COMPLEX AUXILIARY POWER UNIT INTAKE SYSTEM

Bruce Bouldin and Kiran Vunnam Honeywell Aerospace Phoenix, Arizona USA

Jose-Angel Hernanz-Manrique and Laura Ambit-Marin

Airbus Industries Getafe, Spain

ABSTRACT

Auxiliary Power Units (APU’s) are gas turbine engines which are located in the tail of most commercial and business aircraft. They are designed to provide electrical and pneumatic power to the aircraft on the ground while the main propulsion engines are turned off. They can also be operated in flight, when there is a desire to reduce the load on the propulsion engines, such as during an engine-out situation. Given an APU’s typical position in the back of an airplane, the intake systems for APU’s can be very complex. They are designed to provide sufficient airflow to both the APU and the cooling system while minimizing the pressure losses and the flow distortion. These systems must perform efficiently during static operation on the ground and during flight at very high altitudes and flight speeds. An APU intake system has been designed for a new commercial aircraft. This intake system was designed using the latest Computational Fluid Dynamics (CFD) techniques. Several iterations were performed between the APU supplier and the aircraft manufacturer since each of their components affects the performance of the other. For example, the aircraft boundary layer impacts APU intake performance and an open APU flap impacts aircraft drag. To validate the effectiveness of the CFD analysis, a full scale intake rig was designed and built to simulate the tailcone of the aircraft on the ground. This rig was very large and very detailed. It included a portion of the tailcone and rudder, plus the entire APU and cooling intake systems. The hardware was manufactured out of fiberglass shells, stereolithogrophy components and machined plastic parts. Three different

airflows for the load compressor, engine compressor and cooling system had to be measured and throttled. Fixed instrumentation rakes were located to measure intake induced pressure losses and distortion at the APU plenum and cooling ducts. Rotating pressure and swirl survey rakes were located at the load compressor and engine compressor eyes to measure plenum pressure losses and distortion. Static pressure taps measured the flow pattern along the intake and flap surfaces. The intake rig was designed to be flexible so that the impact of rudder position, intake flap position, APU plenum baffle position and compressor airflow levels could be evaluated. This paper describes in detail the different components of the intake rig and discusses the complexity of conducting a rig test on such a large scale. It also presents the impact of the different component positions on intake performance. These results were compared to CFD predicted values and were used to calibrate our CFD techniques. The effectiveness of using CFD for APU intake design and its limitations are also discussed.

INTRODUCTION

Description of APU Intake System The APU intake system subject to the CFD analysis and rig ground test consists of two ducts, the APU intake duct and the cooling duct, as shown in Figure 1. The larger duct directs airflow into the APU. This intake contains a splitter that bisects the duct into forward and aft sections. The splitter is placed roughly in the middle of the intake duct, although its final position and orientation was optimized to achieve maximum pressure recovery with minimal distortion. The walls of this APU intake duct are lined with acoustic treatment, although the

Proceedings of ASME Turbo Expo 2011 GT2011

June 6-10, 2011, Vancouver, British Columbia, Canada

GT2011-46748

Copyright © 2011 by ASME

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aerodynamic impact of this treatment was not considered for either the CFD analysis or intake rig test.

Figure 1 Two Views of APU Intake Duct The APU intake connects directly into an APU plenum which contains structural elements as well as a cylindrical foreign object damage or FOD screen. The cylindrical screen is composed of a wire mesh which is designed to keep out potentially damaging compressor FOD. The APU engine compressor (E/C) and the load compressor (L/C), which provides air to the aircraft environmental control system (ECS), sit on opposite sides of the plenum, connected by a shaft supported by a mid-bearing structure. Both compressors pull air from this one plenum, so the APU intake duct feeds two compression systems, one for the APU and one for the ECS system. Figure 2 shows the plenum geometry and the different components that it contains.

Figure 2 View Into APU Plenum The smaller intake duct feeds cooling air directly into the APU compartment, where it is used to cool the engine compartment as well as provide cooling air for the oil cooler. An eductor system, which is a low pressure ejector system, is incorporated into the APU exhaust and is used to pull air through the APU compartment and oil cooler. The oil cooler is mounted directly to the eductor plenum. The eductor pumps cooling air during static ground operation in addition to in-flight conditions where it is further assisted by ram air provided by the open intake flap.

The flap has two open angle positions when the APU is running. The more open position is optimized for ground operation so that inlet airflow pressure losses may be kept to a minimum. This second position is at a lower angle for all flight conditions. The lower flap angle minimizes the drag impact by reducing blockage and spillage. The APU intake system is located aft of the vertical tail, rotated from aircraft top dead center between the vertical and horizontal tails. It is positioned to prevent being hit by the moving rudder and to avoid ingestion of a wake from either the vertical or horizontal tails. Figure 3 shows the APU intake position on the aircraft.

Figure 3 Location of APU Intake on Aircraft Tailcone Description of the APU The APU intake feeds a Honeywell HGT1700 engine. This APU is an 1100 horsepower engine which provides approximately 470 lbs/min of air and 150 kVa of electrical power under hot day conditions to the aircraft for cabin environmental controls and electrical needs. Pneumatic air from the APU is also utilized for starting the main engines. Figure 4 shows a view of the HGT1700 APU.

Figure 4 HGT1700 Auxiliary Power Unit

APU Intake

Splitter

Cooling Duct

APU Intake

Splitter

Cooling Duct

E/C Bellmouth

L/C IGV’sFOD Screen

Plenum Inlet

APU Inlet

Vertical Tail

Horizontal Tail

Tailcone

APU Inlet

Vertical Tail

Horizontal Tail

Tailcone


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DISCUSSION CFD Analysis Description To get an initial evaluation of the performance of this inlet system, an extensive CFD study was conducted. The CFD analysis was performed for in-flight conditions as well as static ground conditions, however, only the ground condition will be considered in this paper as the CFD results will be compared to the intake ground rig results. A CFD mesh was created to include all of the critical aircraft, intake and plenum geometries. Since in-flight conditions were included in the CFD study, a significant portion of the forward tailcone and vertical and horizontal tails were included in the CFD model since these components could have a significant impact on intake performance in flight. For simplicity, the same mesh was used for the static ground condition as the in-flight condition even though such a large section of the tailcone and vertical and horizontal tails was not needed for the static ground case. Likewise, the boundary layer grid on the tailcone to fully develop the boundary layer in flight was maintained even though it was not needed for the ground case. The CFD model included all of the detail on the intake flap including the side plates and splitter plate. The flap was positioned at the ground optimized angle for the static ground CFD analysis. The APU intake geometry includes the splitter and it is attached to the APU plenum. The APU plenum model included all critical geometry such as the E/C bellmouth and the L/C bellmouth and IGV’s, with the IGV’s fixed in the open position corresponding the max ECS condition for the APU. The mid-bearing housing and structural elements are also included. The cylindrical FOD screen is modeled as a porous jump boundary condition. A known pressure loss is applied to the boundary condition to simulate the pressure drop through the actual FOD screen. Figure 5 shows the detailed CFD geometry inside the APU plenum and the modeled components. Engine and load compressor mass flows are set by varying the exit static pressure at the end of cylindrical extensions which extend downstream of the E/C and L/C eyes. These extensions allow for constant static pressures to be applied at the mass flow outlets, but are far enough away that the flow field is not adversely affected at the area of interest at the compressor eyes. The extensions are modeled as a symmetry boundary condition, which is equivalent to a frictionless wall. Figure 5 also shows the extensions and the pressure outlet boundary conditions.

Figure 5 Plenum CFD Geometry

The cooling flow through the cooling duct is also controlled by varying the exit static pressure. But because the airflow through this duct dumps directly into the APU compartment, a simulated compartment was included in the CFD model. This compartment is then throttled down to a much smaller pressure outlet boundary condition where the exit static pressure is varied until the desired cooling air mass flow is achieved. The smaller exit area allows for decreased sensitivity of mass flow to exit static pressure, allowing for quicker solutions to the desired cooling airflow. The simulated APU compartment with the necked down exit boundary condition is shown in Figure 6.

Figure 6 Simulated APU Compartment

Baffle

FOD ScreenE/C Outlet

L/C Outlet

L/C Outlet

Extension

L/C Eye

E/C Eye (Not Visible)

Simulated APU CompartmentCooling Flow Outlet

Cooling Duct


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A farfield is placed around the entire modeled tailcone and intake system. A constant static pressure and temperature are applied to this boundary to simulate sea level and standard day ground conditions. Only one half of the aircraft is modeled, with a symmetry plane bisecting the fuselage, as shown in Figure 7 along with the farfield.

Figure 7 CFD Farfield and Symmetry Geometry The final CFD grid had 5.3 million tetrahedral and prismatic cells. Prismatic boundary layer grids were placed on the aircraft fuselage surfaces since boundary layer flow behavior along the aircraft skin is critical to obtaining accurate inlet performance estimates, particularly in flight. Prismatic boundary layer grids are not used on the intake geometry due to its complicated shape. Additionally, airflow here tends to be very turbulent and already separated. Consequently, boundary layer grids would have minimal impact on the predicted airflow behavior within the intake. This grid was solved using the CFD solver FLUENT. The realizable k-epsilon turbulence model was used with non-equilibrium wall treatment. The max ECS condition was modeled, which corresponds to the highest pneumatic and electrical load required by the aircraft’s ECS while on the ground. The appropriate E/C, L/C and cooling airflows were obtained from the engine cycle for this condition and applied to the three static pressure outlet boundaries described above. Intake Rig Ground Test Description In order to verify the CFD results and to calibrate the code, a full scale intake ground rig was designed and run at the Honeywell Aerospace facility in Phoenix, Arizona. The rig was designed to be installed in the flow facility, which has an array of vacuum pumps and a large steam ejector facility to produce the required airflow for the E/C, L/C and cooling systems. Figure 8 shows an aft looking forward view (from the aircraft perspective) of the intake ground rig setup. Several of the key

components are labeled including APU intake duct, the APU plenum and the cooling duct. Flow measurement bellmouths are located within the large tank for the E/C flow and in the smaller tank placed on the ground for the cooling flow. The data acquisition panel, where the pressure and temperature readings were hooked to the Honeywell facility’s data acquisition system is also shown.

Figure 8 Aft Looking Forward View of Intake Ground Rig Figure 9 shows a forward looking aft view of the intake rig setup. In this figure, the third tank, which contains a flow measuring bellmouth, is clearly shown. The L/C airflow is measured within this large tank. The tailcone section and the rudder system are mounted on top of this tank and are supported by a welded frame made of square steel tubing. This figure also clearly shows one of the flow control valves that were used to meter the three different airflows. These valves were activated remotely from the facility’s control room where the desired airflow could be dialed in. This figure also shows the underside of the rudder, which was designed to be manually deflected up to 20° and held in place by locking pegs.

Figure 9 Forward Looking Aft View of the Intake Ground Rig

Farfield

Symmetry Plane

Cooling Flow Measurement

E/C Flow Measurement

Data Acquisition Panels

APU Plenum

APU Intake DuctCooling Duct

L/C Flow Measurement

Flow Control Valve

Rudder Axis

Support Frame


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Figure 10 shows a clear view of the rudder and the intake flap and support cables that were attached to maintain the open position of the flap as it is subjected to a large suction force when the APU simulated airflows are at their maximum. The stepping motor for the rotating survey rakes, to be discussed in more detail later, is also clearly visible in this figure.

Figure 10 View of the Top of the Intake Rig Figure 11 shows a view looking down into the APU and cooling intake ducts. The ice fence is shown attached next to the inboard edge of the cooling duct. Simulated ice buildup on the intake splitter leading edge, composed of foam and held in place by speed tape, is also shown. Testing the impact of ice buildup was one of the goals of this intake rig. Tufts are also attached to many locations on the intake system. The movement of these tufts was monitored by a closed circuit video system. One of the two cameras is shown in Figure 11. The video outputs of these two cameras were monitored in the facility control room so that flow patterns could be monitored. The cameras were also used to monitor the health of the rig while it was being operated.

Figure 11 View Down APU and Cooling Intakes

Since aerodynamic parameters were to be measured at the L/C and E/C compressor eyes, it was necessary to include detailed geometry inside the APU plenum. Figure 12 shows some of this detail, including the FOD screen, the APU mid-bearing and support structures and the L/C IGVs, which were fixed in the full open position to match the configuration analyzed with CFD. A variety of rig suitable materials were used to create the different hardware components of this intake rig. The hardware shown in Figure 12 is made of machined plastic. Other components such as the APU plenum and cooling duct were made of Accura 60 stereolithography material. The tailcone and rudder were made of fiberglass and the intake flap was manufactured from machined plastic with a honeycomb stiffener backing. The rapid advances in prototype hardware creation such as stereolithography have significantly simplified the creation of rig-ready hardware. It is estimated that using the rapid prototyping to create rig hardware reduced the rig preparation time by as much as two months compared to more traditional rig manufacturing techniques.

Figure 12 View of Inside APU FOD Screen A baffle, which essentially consists of a thin piece of material spanning from the forward wall of the plenum to the aft wall outside of the FOD screen is an import piece of hardware. The baffle stabilizes the plenum airflow and depending on where the baffle is located, impacts the amount of compressor ingested bulk swirl generated by the intake system. Three different baffle locations were tested during this test program, bottom dead center (BDC) directly opposite the plenum opening and 30° rotated from BDC in the clockwise and counter-clockwise directions. The baffle at BDC is considered the baseline as this is the position at which the CFD analysis was conducted. The rig was designed so that different machined plastic baffles could be inserted in slots on the plenum wall at the three tested positions. Inserts were also created out of machine plastic so

Rudder

Intake Flap

Rotating Survey Rake Step-Motor

Ice Fence

Simulated Ice Build Up on Intake Splitter

Video Camera

FOD ScreenL/C IGVs


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that no leakage occurred through the two slots which did not hold the baffle. Figure 13 shows the three tested baffle locations and the machined baffles and inserts.

Figure 13 The Three Tested Baffle Locations and the Machined Plastic Baffle Parts

Intake Ground Rig Instrumentation The intake ground rig was equipped with a full set of instrumentation so that the intake recoveries, distortion and flow fields could be determined. To determine the plenum losses and distortion at the compressor eye, a rotating survey rake system was incorporated into the rig. One survey rake motor controlled three sets of survey rakes. The engine compressor had two survey rakes; one of which held 4 kiel probes which measured total pressure. These probes were placed at the axial location corresponding to the centrifigul E/C leading edge at radial locations corresponding to centers of equal area. Diametrically opposite of this rake at the same axial station and radii, another rake holding swirl measuring cobra probes is

placed. Cobra probes are three-element pressure probes which are calibrated to measure swirl based on pressure differences between the three pressure elements. Only tangential swirl was measured with these cobra probes as the radial component of swirl has a minor impact on compressor performance. The third survey rake is located at the axial station corresponding to the L/C leading edge and contains four total pressure kiel probes radially positioned at centers of equal area. Cobra probes were not used for the L/C since the IGV’s will dictate the amount of swirl entering the L/C and since the position of the IGV’s are fixed, there is no variation in swirl. Figure 14 shows a sketch of the three rotating survey rakes. The survey rake was rotated in increments of 5° for a full 360° sweep of the compressor faces for a select number of rig test points.

Figure 14 Sketch of Rotating Survey Rakes at the E/C and L/C Compressor Eyes

For APU intake performance, the most important set of instrumentation is located at the plenum inlet, which corresponds to the Aerodynamic Interface Plane (AIP) of the intake system. At the plenum inlet, a full array of total pressures, static pressures and total temperature probes is located. A set of 9 rakes holding 4 probes each span the plenum inlet opening. This instrumentation is used to calculate the APU intake recovery and plenum distortion. Figure 15 shows a sketch of this instrumentation array and shows the location of the different types of instrumentation. Total and static pressure probes are needed since the local velocity at each probe location must be calculated to determine the plenum distortion.

BDC

Machined Plastic Baffles and Inserts

Three Tested Baffle Locations

30° CW

30° CCW E/C Eye InstrumentationTwo Rotating Rakes

Four Pt Kiel and Cobra Probes per Rake

L/C Eye InstrumentationOne Rotating Rake

Four Pt Kiel Probes per Rake

Total Pressure Kiel Probes

Swirl Cobra Probes

R1

R2

R3

R4

R2

R3

R4

Viewed Forward Looking Aft

R4

R1

R2

R3

R1

Total Pressure Kiel Probes


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Figure 15 Sketch of Plenum Inlet Instrumentation Array The probes are located at the centers of equal area and each rake holds two probes located on each side of the intake splitter. Since the splitter effectively splits the APU intake into two distinct ducts, it was necessary to have the same amount and type of instrumentation in each channel. Figure 16 is a picture of the plenum inlet instrumentation array installed in the stereolithography (SLA) plenum top hardware.

Figure 16 Plenum Inlet Instrumentation Array Installed in Plenum

The cooling duct exit also contains a smaller rake array which is used to calculate the cooling duct recovery. This rake is shown in Figure 17. Additionally, static pressure taps were incorporated into the duct and flap walls to help better visualize the airflow patterns.

Figure 17 Cooling Duct Exit Instrumentation Array

CFD and Rig Test Results Three aerodynamic parameters are computed to determine the acceptability of APU and cooling intake performance. These three parameters are:

PT0

PT2Recovery Intake APU

Where: PT2 = Area averaged total pressure at the exit of the intake duct and the inlet to the APU plenum, measured by the rake array in Figure 15. PT0 = Ambient total pressure

PT0

PT2coolRecoveryDuct Cooling

Where: PT2cool = Area averaged total pressure at cooling duct exit, measured by the rake array in Figure 17. PT0 = Ambient total pressure

θ

V

Vr

DIcDistortion Plenum APUi

2ii

ri = The distance from plenum inlet centerline to the ith probe Vi = Velocity at the ith probe = TT0/Tref The APU plenum distortion parameter is a measure of the angular momentum entering the plenum, has units of m2/s and can have a positive or negative value. A positive distortion value indicates that the flow velocity entering the plenum is biased towered the side that will produce swirl in the plenum that is co-rotational with the engine and load compressors. A negative distortion value indicates distortion that will produce

= Total Pressure and Static Pressure Probes (24)= Total Pressure and Total Temperature Combo Probes (4)= Total Pressure, Static Pressure and Total Temperature Probes (8)

Dire

ctio

n o

f Flig

ht

(Lo

oki

ng

Dow

n In

to P

len

um)

A

B

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18

19 20 21 22 23 24 25 26 27

28 29 30 31 32 33 34 35 36

A/18 A/9 A/9 A/9 A/9 A/9 A/9 A/9 A/9 A/18

B/8

B/4

B/4

B/4

B/8

L/C

E/C

Trailing Edge of Splitter

Viewed Looking Down Into Plenum+R-R

Cooling Inlet Exit

W1

W1/4W1/2W1/4

W2

W2/4

W2/4 W2/2

Viewed Standing Forward and Looking Down Cooling Duct

Ou

tbo

ard

Sid

e

Inb

oa

rd S

ide

= Total Pressure Probes (8)= Static Pressure Tap (6)


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counter-rotational swirl in the plenum. Figure 18 illustrates this concept. Typically, a slight amount of counter-rotational swirl is desirable as this can help to increase the pressure rise of the compressor.

Figure 18 The APU Plenum Distortion Concept The comparison of CFD predicted and rig tested intake parameters is shown in Figure 19. For the APU intake recovery, the CFD matched the rig test data almost exactly. For the cooling duct recovery, the CFD over-predicted the recovery slightly. This is most likely due to geometric differences between the CFD model and the test rig at the cooling duct exit. The CFD solution included a simulated APU compartment into which the cooling duct emptied. Due to size and accessibility issues, a large simulated compartment was not possible on the intake rig. Instead, the cooling duct was connected directly to a collector duct which then carried the cooling airflow to the flow measuring tank and bellmouth. However, the difference in recovery is very slight (0.10% difference) so the correlation between CFD predicted and rig tested cooling duct recovery is considered to be excellent. The recoveries of Figure 19 and all subsequent recovery plots are plotted as normalized recoveries. The normalized recovery is the true measured recovery divided by a constant value. This allows for the proprietary intake performance data to be presented so that comparisons between different tested configurations can be made. The normalized recovery will sometimes exceed a value of 1.0, which is not possible with true measured recovery. A significant difference is seen for the APU plenum distortion. The inlet rig measured significantly more negative plenum distortion values than the CFD. The rig measured distortion is approximately 88% higher than what is predicted by CFD. The measured plenum distortion is still comparatively low and well with acceptable limits.

Figure 19 Comparison of CFD and Rig Test Intake Parameters

To better understand the differences in CFD predicted and rig measured plenum distortion values, the velocity at each individual probe location was compared for the rig data and the corresponding location in the CFD model (Figure 20). This is applicable since the plenum distortion parameter is primarily a function of airflow velocity at discrete points. A comparison shows that both the CFD and test rig data approximately follow the same trends from probe to probe, but there are slight differences at each of the individual probes. This may be partially due to the density of the CFD grid at the plenum inlet plane. A relatively coarse grid will cause data sampling at discrete coordinate locations to be slightly in error. Historically, the plenum inlet distortion parameter has proven to be very

E/C

L/C

Viewed Aft Looking Forward Looking Into Plenum

Viewed Aft Looking Forward

Compressor Rotation

Compressor RotationDistortion

Induced SwirlDistortion Induced Swirl

Positive Swirl and Positive Distortion Negative Swirl and Negative Distortion

+R-R

+R-R +R-R

Ple

nu

m C

en

terl

ine

0.975

0.98

0.985

0.99

0.995

1

1.005

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1.015

0 0.5 1 1.5 2 2.5 3 3.5 4

No

rma

lize

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oo

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ct R

ec

ove

ry -

PT

2c

oo

l/PT

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Cooling Duct Corrected Flow, Wc

Cooling Duct Recovery - PT2cool/PT0Ground Optimized Flap Angle

CFD Results

Rig Test Results

0.980

0.985

0.990

0.995

1.000

1.005

1.010

1.015

0 5 10 15 20 25 30

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rma

lize

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PU

Inta

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ocv

ery

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/PT

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APU Corrected Flow, Wc

APU Intake Recovery - PT2/PT0Ground Optimized Flap Angle

CFD Results

Rig Test Results

-2

-1.8

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-1.4

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AP

U P

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, DIc

APU Corrected Flow, Wc

APU Plenum Distortion - DIcGround Optimized Flap Angle

CFD Results

Rig Test Results

IncreasingNegative Distortion


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sensitive to small variations in velocity. This could possibly explain the differences in calculated distortion.

Figure 20 Comparison of CFD Test Data Velocities at Plenum Inlet

Additional Intake Ground Rig Results The test matrix of the intake ground rig test included investigations into several different aircraft and intake configurations. The impact of intake flap angle on intake aerodynamic performance is critical to understand. A range of flap angles higher and lower than the ground optimized flap angle were tested. A test of a full range of flap angles was necessary since it is possible that the intake flap actuator may malfunction, leaving the intake flap stuck at an angle lower than the ground optimized flap angle. Flap angles larger than the ground optimized flap angle were also tested to cover the entire Honeywell experience range. Figure 21 shows the impact of intake flap angle on both the APU intake and cooling duct recoveries. Normalized recoveries are plotted for different APU ECS power levels and the main engine start (MES). True to its name, the ground optimized flap angle exhibited the highest APU intake recovery.

Figure 21 Impact of Intake Flap Angle on Recovery The reason that the APU intake recovery dropped with flap angles larger than the ground optimized flap angle is that at more open angles, the gap between the trailing edge of the flap and the fuselage skin decreased, as shown in the sketch in Figure 22. This reduction in the gap prevented airflow from entering the APU intake from the aft side of the intake, reducing the overall recovery. This lack of aft intake airflow at the higher flap angles was confirmed by the behavior of the tufts placed along the intake aft wall. They showed strong attached flow at the ground optimized flap angle but indicated stalled or unsteady airflow at higher flap angles. This gap influence on intake recovery is not as pronounced on the cooling duct as the gap between the intake flap and the fuselage skin is larger behind the cooling duct due to the curvature of the fuselage skin.

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

-15 -10 -5 0 5 10 15

Ve

loc

ity

Ra

tio

(V

/Va

vg

)

+R

Velocity Profile at Plenum Inlet

Row 1 - Test

Row 1 - CFD

Row 2 - Test

Row 2 - CFD

Row 1

Row 2

Row 3

Row 4

-RDistance from Plenum Centerline

0.2

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1

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-15 -10 -5 0 5 10 15

Ve

loc

ity

Ra

tio

(V

/Va

vg

)

+R

Velocity Profile at Plenum Inlet

Row 3 - Test

Row 3 - CFD

Row 4 - Test

Row 4 - CFD

Row 1

Row 2

Row 3

Row 4

-RDistance from Plenum Centerline

Normalized APU Recovery - PT2/PT0vs. Flap Angle

0.980

0.985

0.990

0.995

1.000

1.005

Increasing Flap Angle

No

rma

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PU

Inta

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R

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-P

T2

/PT

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Max ECS Condition

-10% of Max ECS

-20% of Max ECS

+10% of Max ECS

MES

- Door Deflected an Additional 5° Due to Aero Forces

Ground Optimized Flap Angle

Normalized Cooling Duct Recovery - PT2cool/PT0vs. Flap Angle

0.984

0.986

0.988

0.990

0.992

0.994

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1.000

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Increasing Flap Angle

No

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PT

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Max ECS Condition

-10% of Max ECS

-20% of Max ECS

+10% of Max ECS

MES

- Door Deflected an Additional 5° Due to Aero Forces



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Figure 22 More Open Flap Blocks Aft Entering Intake Airflow

Both the APU intake and cooling duct recovery data shows a large drop off at flap angles lower than the ground optimized flap angle. For the higher flow conditions, data at the lowest flap angle could not be obtained. Due to the huge pressure suction force exerted on the flap at this low angle, there was a fear that the flap or its support structure would fail. Considerable bending of the intake flap was observed at the max ECS condition. It is estimated by visual observation via the video cameras that the flap angle was decreased an additional 5° due to this bending at these high flow levels. Data acquisition at flows higher than those at the max ECS condition was not attempted for these low flap angles. As shown in Figure 10 and Figure 11, the APU intake and aircraft rudder are located in close proximity to each other. When the rudder is deflected in the starboard direction, the trailing edge of the rudder sweeps in front of both the cooling duct and the APU intake. The intake rig was designed so that the rudder could be deflected about its correct axis. Most data was acquired with the rudder with zero deflection. However, intake data was also taken with the rudder at -5°, +5°, +10°, +15° and +20°. Negative deflection is swept toward the port side of the aircraft (away from the intake) and positive deflection is swept toward the starboard side of the aircraft (in front of the intake). The results of this test are shown in Figure 23. Since the cooling duct is located inboard, its recovery is more significantly affected by the deflection of the rudder. The cooling duct shows a small reduction in recovery with increasing rudder angle. However, this decrement is small and is well within acceptable limits. The APU intake recovery actually shows a very small increase in recovery with increasing rudder deflection. A possible explanation for this might be that a deflected rudder is positioned such that it helps to direct airflow into the APU intake.

Figure 23 Impact of Rudder Angle on Recovery The plenum baffle is a thin piece of metal or composite that spans from aft wall to forward wall of the plenum at the end opposite the plenum opening. The primary purpose of the baffle is to stabilize the airflow within the plenum. However, its circumferential location within the plenum can influence the bulk swirl formed inside of the plenum. Offsetting the baffle in either the clockwise or counter-clockwise direction can change the amount of bulk swirl that the engine compressors are exposed to, thus impacting compressor and APU performance. For the rig test, three baffle positions were tested, at bottom dead center (BDC) which is exactly opposite the plenum opening and 30° offset from BDC in the clockwise and counter-clockwise directions. Figure 24 shows a sketch of these three baffle locations and their resulting influence on normalized APU intake performance parameters. Baffle position had no impact on APU intake recovery, however, it did have a big impact on plenum inlet distortion. Plenum inlet distortion and plenum and compressor bulk swirl are related parameters since the plenum distortion parameter describes the velocity profile entering the plenum. Positive plenum distortion produces positive (co-rotational) compressor bulk swirl and negative plenum distortion produces negative (counter-rotational) compressor bulk swirl. The negative distortion measured with the baffle at BDC implies that the APU intake duct biases the airflow in the direction of negative


More Open Flap Angle

More Open Flap Blocks Airflow Entering Intake From Aft Direction

APU Intake Entry Airflow Pattern During Static Operation

To APU


More Open Flap Angle

More Open Flap Blocks Airflow Entering Intake From Aft Direction

APU Intake Entry Airflow Pattern During Static Operation

To APU

Normalized APU Intake Recovery - PT2/PT0Max ECS Condition - Ground Optimized Flap Angle

0.9990

0.9992

0.9994

0.9996

0.9998

1.0000

1.0002

1.0004

-10 -5 0 5 10 15 20 25Rudder Angle (deg)

No

rma

lize

d A

PU

Inta

ke

R

eo

cv

ery

- P

T2

/PT

0

Rig Data

Normalized Cooling Duct Recovery - PT2cool/PT0Max ECS Condition - Ground Optimized Flap Angle

0.9990

0.9992

0.9994

0.9996

0.9998

1.0000

1.0002

1.0004

-10 -5 0 5 10 15 20 25Rudder Angle (deg)

No

rma

lize

d C

oo

ling

Du

ct

Re

co

ve

ry -

PT

2c

oo

l/PT

0

Rig Data


11

distortion and bulk swirl. This is advantageous as a small amount of negative bulk swirl can result in increased engine compressor performance. Moving the baffle by 30° in either direction shifts the plenum distortion significantly. A baffle location of 30° CW changes the velocity profile at the plenum inlet enough to shift the distortion from the negative direction to the positive direction. This shows that moving this small piece of hardware, sitting at the bottom of a plenum, can have a profound influence on the airflow entering and within the plenum.

Figure 24 Impact of Baffle on APU Intake Aerodynamic Parameters

Ice buildup is a concern for any intake system. Previously run ice trajectory analysis indicated that on this APU intake system, ice would primarily accumulate on the intake splitter. A simulated ice buildup on the splitter leading edge was constructed out of foam and metallic tape, as shown in Figure 25. This buildup had a significant impact on APU intake performance as shown in Figure 26. A very large reduction in APU intake performance is observed with the presence of the ice buildup. However, the reduction was not so severe as to significantly impact APU performance for the few times that ice buildup of this severity is experienced. The ice buildup actually reduced the plenum distortion and eliminated any

influence of airflow on distortion. This is most likely due to the ice buildup producing separated airflow off of the splitter leading edge which eliminates any velocity profile at the plenum inlet. This flow separation is consistent with different airflows so no variation in plenum distortion with varying airflow levels is seen. This test is only a preliminary evaluation to get an early view of the impact of ice buildup on intake performance. It is not a substitute for a true intake test in an icing test facility which is scheduled later in the development program for this APU installation.

Figure 25 Simulated Ice Build-Up on Intake Splitter

Figure 26 Impact of Simulated Ice Build-Up on APU Intake Aerodynamic Parameters

Normalized Plenum Inlet Distortion - DIcGround Optimized Flap Angle

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30Normalized APU Corrected Flow

No

rma

lize

d P

len

um

Inle

t D

isto

rtio

n -

DIc

Baffle 30deg CCW

Baffle BDC

Baffle 30deg CW

Baffle BDCBaffle 30° CCW

Baffle 30° CW

Aft Looking Forward

Baffle BDCBaffle 30° CCWBaffle 30° CCW

Baffle 30° CWBaffle 30° CW

Aft Looking Forward

Normalized APU Intake Recovery - PT2/PT0Ground Optimaized Flap Angle

0.980

0.985

0.990

0.995

1.000

1.005

1.010

1.015

1.020

0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30Normalized APU Corrected Flow

No

rma

lize

d A

PU

Inta

ke

R

ec

ov

ery

- P

T2

/PT

0

Baffle 30deg CCW

Baffle BDC

Baffle 30deg CW

Baffle BDCBaffle 30° CCW

Baffle 30° CW

Aft Looking Forward

Baffle BDCBaffle 30° CCWBaffle 30° CCW

Baffle 30° CWBaffle 30° CW

Aft Looking Forward

Simulated Ice

Normalized APU Intake Recovery - PT2/PT0Max ECS Condition - Ground Optimized Flap Angle

0.980

0.985

0.990

0.995

1.000

1.005

1.010

1.015

1.020

0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30

Normalized APU Corrected Flow

No

rma

lize

d A

PU

Inta

ke

D

isto

rtio

n -

PT

2/P

T0

No Ice

With Ice

Normalized Plenum Inlet Distortion - DIcMax ECS Condition - Ground Optimized Flap Angle

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30

Normalized APU Corrected Flow

No

rma

lize

d P

len

um

Inle

t D

isto

rtio

n -

DIc

No Ice

With Ice


12

CONCLUSIONS This paper discusses an extensive CFD and rig test program used to evaluate a new APU installation design. It discusses, in detail, the CFD modeling and rig hardware required to properly evaluate an APU intake system. It shows that when modeled with enough geometric fidelity, CFD analysis can be used with a high degree of confidence. This paper also highlights how a well thought out and designed rig can produce a wealth of information. Different intake and aircraft configurations can be tested quickly and accurately. Such testing provides a reliable check on CFD predictions, especially when new or altered meshing or solution techniques are used in the rapidly evolving world of CFD. Quite possibly in the future, CFD will be used as the sole design tool and rig testing will not be necessary. Certainly, CFD should be used during the detailed design phase to develop one or two final intake configurations. However, with the advancements in rapid prototyping hardware tools, the same CAD models used to create the CFD meshes can be sent to a rapid prototyping machine to produce accurate models of intake or aircraft hardware. This enables rig testing as a viable and cost effective tool to verify CFD analyses. A full evaluation of an organization’s CFD capability combined with a cost and schedule assessment for creating and conducting a rig test should be performed before deciding to abandon rig testing all together during an intake design effort.


Documents

CFD Analysis and Full Scale Testing of a Complex Auxiliary ......the entire APU and cooling intake systems. The hardware was manufactured out of fiberglass shells, stereolithogrophy