Assessment of Fan Response to Distortion

David Hall

Massachusetts Institute of TechnologyCambridge, MA 02139

March 10, 2011

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Research objectives

I Determine aerodynamic, structural behavior of D8 fanI define distortion transfer characteristicsI define environment that creates aeromechanical response

I Develop conceptual guidelines for distortion tolerant propulsorsI fan responseI use of other system components (e.g. non-axisymmetric stators)

Outputs

I characterization of propulsion system in terms of distortion transferI benefits of BLI accounting for fan off-design characteristicsI effect of distortion transfer on propulsion system performanceI design, performance assessment of distortion tolerant D8

configuration

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Preliminary research plan

1. Analysis of internal propulsion system flow with specified inletdistortion

2. Assessment of the effects of aerodynamic coupling ofpropulsion system components

3. Assessment of aeromechanical response of propulsorturbomachinery to inlet distortion

4. Assessment of the aerodynamic characteristics of integratedBLI propulsion system

5. Preliminary assessment of off-design performance of aboundary layer ingesting propulsor

6. Development of design guidelines for distortion tolerantpropulsors

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Turbomachine model to capture throughflow properties

Quantities of interestI Propulsive efficiency

I Power savings: compare to isolated design for same mission

I Evolution of boundary layer distortion through propulsorI Change in shape of fan map or operating line

I function of shape of inlet/upstream distortionI loss in fan adiabatic efficiencyI possible decrease in stable operating range

I Unsteady forcing on propulsor structures (due to BLI)

I Sensitivity of the above to design of propulsion system

Model turbomachinery with body force field

I Circumferential average of forces in real flow → source term

I Function of local flow properties (dependent on θ) at each(x, r) location in blade row

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Body force approach

Figure 1. Shematic illustration of the quasi-axisymmetric concept. Image courtesey of Jon Kerner, used withpermission.

Fx

Fy

Fz

=

1 0 0

0 cos θ − sin θ

0 sin θ cos θ

Fx

Fr

Fθ

(2)

Regardless of which set of governing equations are solved, the body forces must be computed in the cylindrical

system and then if necessary transformed to Cartesian coordinates.

Gong’s model1 is also used for the formulation of the forces Fx, Fr and Fθ. To simplify the energy

addition term, if there is no heat source in the fluid, the energy source is simply the work done by the blade

row, and thus simplifies to:

F · V + q = FθΩr (3)

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American Institute of Aeronautics and Astronautics

I Replace blade row with force fieldI Capture viscous forces → Euler calculationI Low reduced frequency distortion → locally axisymmetricI Local force function of local flow properties

I based on off-design response to axisymmetric flow(“quasi-axisymmetric”)

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Uses of body force model at MIT GTL

I Short wavelength stall inception in multistage compressorsI Distortion transfer, MPT noise in embedded BLI propulsion systemsI Low FPR fan noise and performance in ultra-short nacelles (P&W)

A. Peters, Z.S. Spakovszky

Low FPR Propulsor Noise and Performance in Ultra-Short Nacelles

Background

Body Force Representation of Turbomachinery Blade Rows Body Force Method – Enabling Approach

Body Force Method – Validation

Next Step: Parametric Study – Short-Inlet NacellesFull Domain Coupled Fan-Nacelle Analysis

Ground tests 2009 (AviationWeek.com).

• Engine designs trends tend towards higher bypass ratio, lower pressure ratio fan designs for improved fuel burn, reduced emissions and noise

• Innovative nacelle design concepts required to limit weight/drag increase for larger diameter fans short inlet and exhaust ducts

Challenges: Limited noise shielding and increased inlet distortion susceptibility

Low fan pressure ratio, high-bypassratio turbofan (MTU.de).

Problem Statement

Baseline configuration, takeoff operating condition(M = 0.25, angle-of-attack = 17 )

Absolute Mach Number

• Blade force field dependent on local flow conditions – use quasi-axisymmetric body force field to represent blade row influence on flow

• Determine functional dependence of force field on local flow from steady flow field, force definition independent of angular position

• Body force field responds to local flow properties instantaneously (non-axisymmetric flow field leads to non-axisymmetric force field)

Kerner 2009Gong 1999

Compute speedline

Single set of body force distribution captures fan performance over entire speedline

Fn = Kn(x,r) f(local flow, blade geometry)

Determine analytical description of blade force dependence on local flow

Remove blade row and place body force field in swept volume of blade row

Body force coefficient Kn

Single passage RANS

Stagnation pressureBlade mesh

Full domain body force (baseline configuration)

Stagnation pressure

body force domains

• Single set of BF description

• Good agreement over large range of operating conditions on T/O speedline

• BF model captures effects of bulk inlet swirl

Body Force

RANS

Body Force

RANS

Single Passage RANS Body Force

Relative Flow AngleFan efficiency

Fan pressure ratio

Control parameters tobody force model

• Blade metal angle

• Blade lean angle

• Solidity

• Rotational speed

Unsteady RANS Body Force

body force domainsfan

bladesM = 0.25 T/OAoA = 17

Internal RANS

grid

Internal body force

grid

Far field grid - internal flow domains exchangeable

Absolute stagnation pressureAbsolute stagnation pressure

Determined performance of baseline fan/nacelle configuration (inletL/D = 0.45), validated body force method in full domain simulations

• Use BF method to answer following research questions:

– How short an inlet design can be achieved while meeting performance, noise, and operability requirements? What is the key limiting factor in the design of short inlets?

– How does the fan blade design change if a coupled inlet-fan design approach is adopted for short nacelles?

– What is the potential of asymmetric geometries?

Cruise (M = 0.8)

InletL/D = 0.45

Takeoff (M = 0.25)

InletL/D = 0.1

MTFLOW axisymmetric analysis – preliminary scoping study

• Project goal: Define advanced fan/nacelle system to meet performance, noise, and operability requirements of future turbofan configurations

• Determine underlying mechanisms and necessary technologies (e.g. endwall treatment or asymmetric geometries) to reduce inlet distortion susceptibility/stability issues and fan source/radiated noise

• Need computationally cost-effective, coupled fan-nacelle analysis tool enabling parametric variations in fan/nacelle geometry

Body force method

Determine blade forces and flow field data at multiple

operating points on speedline

Tangential component of normal blade force, Fn,

Above: body force validation withaxisymmetric RANS calculations for lowFPR fan in ultra-short nacelle (A. Petersw/Pratt & Whitney)

Right: conceptual model of embedded BLIpropulsion system and calculated stagnationpressure distortion transfer across fan usingaxisymmetric Euler calculations with bodyforce model (Plas et al, SAI)

A 3-D inviscid calculation is done to estimate the flow in the computational domain

using Gong's body force code. The net thrust is then calculated using equation (5.1).

The fan power corresponding to this thrust is:

P = cp (Ttd -Tt) drh. (5.2)

This model is of higher fidelity than the parallel compressor and integral models,

not only because it is based on three-dimensional calculations, but also because it

now captures inlet losses and duct curvature.

Semi-circular

3-D viscous calculation 3-D fan distortion transfer(Madani's results) (Gong's code)

Expansion throughideal nozzle

Figure 5-2: Sketch of the calculation procedure for the propulsion system

5.3 Discussion of the results

5.3.1 Fidelity of the model

The fidelity of the body force calculation was assessed by comparison with the 3-D

results of Crichton [3]. Crichton designed a fan assuming a uniform incoming flow

of stagnation pressure 0.96 times the freestream stagnation pressure (to account for

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Power balance metrics

ProblemUse of BLI propulsor → what is thrust? drag?

SolutionDon’t worry about it

I Sources and sinks of powerI Power added to flow by propulsorI Viscous dissipation

I in shear layers (boundary layers, wakes, jets), shocks

I wake transverse excess energy

I Performance metricsI propulsive efficiency: (propulsive power)/(power added to flow)I propulsion system efficiency: (propulsive power)/(fuel power)

Consistent bookkeeping, reduces to (momentum)·V, conventionalefficiencies for isolated propulsor

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Aerodynamic coupling effects

I upstream influence of fan on boundary layer, inlet distortion

I interaction between components: rotor, stator, struts, etc.

I distortion mitigation through non-axisymmetric design(stators, nozzle, wall treatments)

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Integrated BLI propulsion system

D8.x Engine/Tail Configuration

• Rear fuselage and tails function as flow-aligning nacelles– greatly reduced nacelle size, weight, drag

– shield fan faces from ground observers

• Enables low-distortion BLI– local potential flow M 0.6 matches fan requirement

– no additional BL diffusion – no streamwise vorticity into fan

• Fin strakes synergystically exploited:– function as pylons carrying engine loads and tail surface loads

– shield fan faces from ground observers

N+3 Phase II

• Detailed design of rear fuselage/engine installation

• Test 1:4 powered model of fuselage + stub wings

• Investigate performance of small-core engines (Pratt)

22x14 ft

D8.1 Low Speed1/4 scalePowered model

“Aerodynamic gotcha”s of proposed D8 configuration. . .

I Interactions between engines?

I Interactions between engine, fuselage, and tail surfaces?

. . . and their effect on turbomachinery distortion transfer, efficiency

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Moving forward

I Strategy for determining body forces for 3D fan geometryI Assessment of body force model vs. other models

I extracted BF, parametric BF, AD, unsteady N-S with geometry

I Internal flow calculations, propulsor (fan) only (i.e. no core)I choose fan geometryI parametric axisymmetric geometry (inlet, duct, nozzle)I specified inlet distortion

I Gradually increase detail, extent of fluid domainI upstream influence of engine on incoming boundary layerI non-axisymmetric internal geometry (possibly)

I Structural response of components to distortion

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