Numerical Modeling of Compressor and Combustor Flows

1
Numerical Modeling of Compressor and Combustor
Flows
Suresh Menon, Lakshmi N. Sankar Won Wook Kim S.
Pannala, S. Niazi, C. Rivera, A. Stein School of
Aerospace EngineeringGeorgia Tech, Atlanta, GA
30332-0150
2
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.

3
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.

4
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.

5
REDUCED ORDER MODEL
Flow Field is divided into Macro-zones. In each
zone, there are 4 states - r, u, v and T
6
Reduced Order Model II
In each zone, the governing equations are applied
A coupled system of ODEs result.
7
Reduced Order Model III

• This system of simultaneous nonlinear
  ordinary differential equations couples
  states from all the zones

• Steady state solution yields performance map.
• The unsteady solution may be used to analyze the
  nonlinear dynamics of the system.

8
Compressor Performance Map
9
REDUCED ORDER MODEL
Throttle effects may be inexpensively modeled,
and system transients studied.
Incoming Disturbances may be inexpensively
modeled.
10
NASA Low Speed Centrifugal Compressor
SIMULATION SETUP

• 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

11
Single Passage Grid Modeling
3-D SIMULATION SETUP
Grid Size 129x61x41 322,629 points
12
3-D SIMULATION SETUP
Boundary Conditions
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.
13
Surface Pressure Distribution Computations Vs.
Measurements
14
Surface Pressure Distribution Computations Vs.
Measurements
15
Compressor Performance Characteristics
16
Grid Sensitivity Impeller Performance Map for
LSCC
17
Velocity Field (Colored by Pressure)
RESULTS (Design Conditions)
Diffuser Region is Well BehavedNo Separation
18
RESULTS (Off-Design Conditions)
Velocity Field (Colored by Pressure)
Diffuser Region Shows Small SeparationOnset of
Instabilities
19
Effects of Bleeding on Diffuser Performance
With bleed
Without bleed
20
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.

21
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.

22
Droplets below a cut-offradius are modeled in
thesubgrid till vaporizationis complete.
Energy, Mass Transferred to subgrid.
Momentum transferredto the supergrid.
Droplets see local flow properties(Temperature
and Velocity).
Droplet Trajectory
23
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.

24
Mixing Layer Simulations with Droplets
3-D Shear layer, on which diturbances
corresponding to first unstable mode are
imposed.
25
Present Model Correctly ModelsLarge and Small
Particles
StStokes No.
26
Simulation of a Mixing Layer, where the upper
stream is laden with medium size particles
(Stokes No. 1). Experiment by Lazaros and
Lasheras (1992)
27
Conventional LES Scheme Vs. Present 5 Micron
Cut-Off
Product mass Fraction
28
Conventional LES Scheme Vs. Present5 Micron
Cut-Off
Temperature
29
Conventional LES Results are sensitive to
Droplet Cut-Off Size
4 to 5 times expensive than present approach
30
Present Approach is less sensitive toDroplet
Cut-Off Size
31
Experimental Set Up for LES/LEM Validation
main Air
Fuel
Coflow Air
Main Air
32
Comparisons with GA Tech Experiments
Measured inflow velocities, droplet distribution
and turbulence levels are input into the code
33
Comparisons with Ga Tech Experiments
34
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.

35
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.