school of aerospace engineering mite numerical modeling of compressor and combustor flows suresh...

School of Aerospace Engineering

MITE

Numerical Modeling of Compressor and Combustor

FlowsSuresh Menon, Lakshmi N. Sankar

Won Wook Kim S. Pannala, S. Niazi, C. Rivera, A. Stein

School of Aerospace EngineeringGeorgia Tech, Atlanta, GA 30332-0150


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RESEARCH OBJECTIVES

• Develop first-principles based tools for modeling flow through axial and centrifugalcompressors.

• Develop first-principles based tools formodeling two-phase reacting flowwithin combustors.

• Use these tools to explore control strategiesfor stable operation of compressors and combustors.


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Compressor Modeling: Progress To Date

• A two-dimensional rotor-stator Navier-Stokes code has been developed, and used to model rotating stall.

• A reduced order model based on 2-D simulations has been developed, and validated.

• 3-D Navier-Stokes simulations have beencompleted for a NASA centrifugal compressor configuration.

• Stable operation of the 3-D configuration has beenachieved at low mass flow rates using passive control devices.


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Two-Dimensional Flow Solver

• Solves compressible Navier-Stokes equations for Rotor-Stator Configurations.

• Can model oscillating blades, inflowand downstream disturbances.

• Has been extensively validated. (Rivera, Ph. D. Dissertation, May 1998.)

• Some validation studies were presented last year.

• Forms the basis for the new Reduced Order Model.


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REDUCED ORDER MODEL

Flow Field is divided into Macro-zones.In each zone, there are 4 states - , u, v and T


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Reduced Order Model II

Current Zone

Neighbor Zone

Neighbor ZoneNeighbor Zone

Neighbor Zone

In each zone, the governing equations are applied:

t

qdV Fi Gj ndS

Viscous Losses from

CFDsimulations

A coupled system of ODEs result.


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Reduced Order Model III• This system of simultaneous nonlinear

ordinary differential equations couples states from all the zones

dq

dtA q Viscous Losses

• Steady state solution yields performance map.

•The unsteady solution may be used to analyze the nonlinear dynamics of the system.


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Compressor Performance Map

0

0.1

0.2

0.3

0 0.2 0.4 0.6

Non-Dimensional Mass Flow Rate

No

n-D

imen

sio

nal

Pre

ssu

re

Rat

io

CFDCalculations

Measured Data

Reduced OrderModel


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REDUCED ORDER MODEL

Incoming Disturbances may be inexpensively modeled.

Throttle effects may be inexpensively modeled, and system transients studied.


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NASA Low Speed Centrifugal CompressorNASA Low Speed Centrifugal Compressor

Perspective View of the NASA Low Speed Centrifugal Compressor

• 20 Full Blades with 55° Backsweep

• Inlet Diameter 0.87 m

• Exit Diameter 1.52 m

• Design Conditions:– Mass Flow Rate 30 kg/sec

– 1862 RPM

– Total Pressure Ratio 1.14

SIMULATION SETUPSIMULATION SETUP


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Single Passage Grid Modeling3-D SIMULATION SETUP3-D SIMULATION SETUP

Grid Size:

129x61x41

= 322,629 points


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Inlet:p0,T0,v,w specified; Characteristic equation solved to model acoustic waves leaving the domain.

Diffuser Exit:pback specified;entropy and vorticity are extrapolated from Interior.

Periodic Boundaries:Flow properties are periodic from blade to blade.

Blade Surface:no-slip velocity conditions.

3-D SIMULATION SETUPBoundary Conditions

p

nT

n

0

0


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Surface Pressure Distribution

Computations Vs. Measurements5% Blade Span From Hub

0.9

0.92

0.94

0.96

0.98

1

0 0.2 0.4 0.6 0.8 1

Meridional Distance

p/ps

td

suction side-cfdpressure side-cfdsuction side-exppressure side-exp

49% Blade Span From Hub

0.9

0.92

0.94

0.96

0.98

1

0 0.2 0.4 0.6 0.8 1

Meridional Distance

suction surface-cfd

pressure surface-cfd

suction surface-exp

pressure surface-exp


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0.9

0.92

0.94

0.96

0.98

1

0 0.2 0.4 0.6 0.8 1Meridional Distance


0.9

0.92

0.94

0.96

0.98

1

0 0.2 0.4 0.6 0.8 1

Meridional Distance

p/ps

td

Surface Pressure Distribution

Computations Vs. Measurements


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1.09

1.11

1.13

1.15

1.17

1.19

10 15 20 25 30 35 40 45

Mass flow (kg/s)

Tot

al P

ress

ure

Rat

io

CFD

CFD with bleeding

Experiment

Controlled,Stable Operation

Compressor Performance Characteristics

CFD without bleeding


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1.05

1.07

1.09

1.11

1.13

1.15

1.17

1.19

1.21

20 25 30 35 40 45

Mass flow (kg/s)

Tot

al P

ress

ure

Rat

io

CFD - Coarse Grid

CFD - Fine Grid

Grid Sensitivity Impeller Performance Map for LSCC


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Velocity Field (Colored by Pressure)RESULTS (Design Conditions)

Diffuser Region is Well

Behaved

No Separation


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Velocity Field (Colored by Pressure)RESULTS (Off-Design Conditions)

Diffuser Region Shows

Small Separation

Onset of Instabilities


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Effects of Bleeding on Diffuser Performance

Without bleed With bleed


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Compressor Simulations: Conclusions

• A new CFD based reduced order model has been developed and validated.

• A 3-D unsteady compressible flow solver for modeling centrifugal compressors has been developed and validated.

• Good agreement with experiments have been obtained for a Low Speed Centrifugal Compressor (LSCC) tested at NASA Lewis Research Center.

• For the LSCC, flow instabilities were found to originate in the diffuser region.

• Stall control by the use of bleed valves on the diffuser walls has been computationally demonstrated.


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Combustor Modeling- Progress To Date

• A stand-alone methodology for droplet convection,vaporization, turbulent mixing and chemical reaction has been developed, and was reported last year.

• During the current period, this methodology wassuccessfully coupled to gas-phase unsteady flow solvers.

• Incompressible and compressible versions of thetwo phase flow solvers have been developed.

• Ability of the methodology to track particles injected into a vortex has been verified.

• Validation against Ga Tech experiments are in progress.


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Droplet TrajectoryDroplets see local flow properties(Temperature and Velocity).

Energy, Mass Transferred to subgrid.

Momentum transferredto the supergrid.

Droplets below a cut-offradius are modeled in thesubgrid till vaporizationis complete.


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Features of the Present Approach

• Present subgrid approach is more efficient than other LES schemes where a very fine multi-dimensional subgrid is needed to model the droplets.

• In conventional Lagrangian schemes, all the coupling between the droplet and the gas phase is via the supergrid. In the present approach, only the momentum of gas and liquid phase is coupled via the supergrid.

• Conventional Lagrangian schemes assume droplets vaporize instantaneously, below a cut-off radius.This can give erroneous results.


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Mixing Layer Simulations with Droplets

3-D Shear layer, on which diturbances corresponding to first unstable mode are imposed.

Seed Particles


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Present Model Correctly ModelsLarge and Small Particles

St=Stokes No.

Particle Response Time

Flow Response Time


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Simulation of a Mixing Layer, where the upper stream

is laden with medium size particles (Stokes No. = 1).

Experiment by Lazaros and Lasheras (1992)


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Conventional LES Scheme Vs. Present

5 Micron Cut-OffProduct mass Fraction

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0.25

0.0 0.2 0.4 0.6 0.8 1.0Y/Ylen

Pro

duct

Mas

s F

ract

ion

LEM/LES

Conventional LES


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Conventional LES Scheme Vs. Present5 Micron Cut-Off

Temperature

280.0

300.0

320.0

340.0

360.0

380.0

0.0 0.2 0.4 0.6 0.8 1.0Y/Ylen

Tem

per

atu

reLEM/LES Conventional LES


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Conventional LES Results are sensitive to Droplet Cut-Off Size

0.00.20.40.60.81.0

0.0 0.2 0.4 0.6 0.8 1.0

Y/Ylen

Prod

uct D

ensi

ty (

p/0)

Conventional LES (Cut-off 5 microns)



4 to 5 timesexpensive thanpresent approach


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Present Approach is less sensitive toDroplet Cut-Off Size

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1.0

0.0 0.2 0.4 0.6 0.8 1.0Y/Ylen

Prod

uct D

ensi

ty ( p

/0)

LEM/LES (Cut-off 5 microns)LEM/LES (Cut-off 10 microns)LEM/LES (Cut-off 20 microns)


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main Air

Main Air

Honeycomb

FuelCoflow Air

TurbulenceGenerator

MeasurementPlanes

Optical Access

Optical Access

Experimental Set Up for LES/LEM Validation


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Comparisons with GA Tech Experiments

Measured inflow velocities, droplet distribution and turbulence levels are input into the code

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30.0

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Radial Distance (m)

Vel

ociti

es (

m/s

)

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Inflow Umean (Expt.) Inflow Umean (LES)

Inflow Urms (Expt.) Inflow Urms (LES)


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Comparisons with Ga Tech Experiments

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Radial Distance (m)

Axi

al v

eloc

ity (m

/s)

Umean - Expt. (X/D = 13)

Instantaneous Axial Velocity (Compt.)


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Combustor Modeling- Conclusions

• Incompressible and Compressible Two-Phase Reacting Flow Solvers have been developed.

• Droplet convection, evaporation, turbulent mixing and reaction are all modeled from first principles.

• Present approach is less expensive than conventional LES, but more accurate.

• Flow solver has been validated with experiments.


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Research Plans for Next Year

• Extend the new CFD based reduced order model to 3-D centrifugal configurations. Validate.

• Study stall and surge control of the Ga Tech centrifugal compressor configuration using CFD, and using the 3-D reduced order model.

• Perform further validations of the LES/LEM two-phase flow method with Georgia Tech data.

• Perform two-phase reacting flow simulations for a dump combustor configuration.

school of aerospace engineering mite numerical modeling of compressor and combustor flows suresh...

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