Arnab Banerjee Mechanical Engineering IUPUI MSME Thesis Presentation Advisor: Prof. Razi Nalim

Wave Rotor Research Group-IUPUI

Prediction And Design Of Fuel-Air Mixing in a Combustion Wave Rotor Using Two-Dimensional

Unsteady Moving Mesh Flow Computation

Arnab BanerjeeMechanical Engineering

IUPUI

MSME Thesis PresentationAdvisor: Prof. Razi Nalim

November 27, 2005


Objectives of the present work

• Develop a methodology to study multidimensional effects of wave rotors and apply to NASA four-port pressure exchanger using commercial CFD code

• Predict the fuel-air mixing in an internal combustion wave rotor (ICWR)

• Determine key parameters that affect the fuel-air distribution in a wave rotor and improve understanding to obtain desired fuel distribution


Introduction

• Wave Rotor: A device for energy exchange efficiently within fluids of differing densities by utilizing unsteady wave motion

• Two configurations studied here– NASA four-port pressure exchanger– Internal combustion wave rotor (ICWR)


NASA four-port pressure exchanger

Inlet from the Burner

Inlet from the compressor

Exits to Turbine and Burner

Schematic of a gas turbine topped by a four-port wave rotor

Partially cut away 3D view

• Turbine inlet pressure is 15% -20% more than compressor exit pressure ideally

• Increased overall engine thermal efficiency and specific work


Internal Combustion Wave Rotor (ICWR)

Wave Rotor

Compressor Turbine

Schematic of ICWR

• Constant Volume Combustion


2D & 3D view of wave rotor


Pre- and Post- Processing Package

• Developed in-house by Khalid (2004-05)• Hexagonal unstructured grid• Parametric geometry and grid

– Grid and geometry stored in small portable files– Variable port/rotor channel counts and shape– Tailored grid clustering

• Imports and exports STAR-CD files• 3D and “unwrapped” simultaneous view• Runs easily on laptops (windows)


Results of two grid packages

Star-Design IUPUI in-house code


Past 1-D simulations

Paxson and Nalim 1-D code (1997) Berrak and Nalim Detonation 1-D code (2004)


Past 2-D simulations

Welch (1997) NASA 4-port

Kerem & Nalim (2002) single channel

Piechna et.al (2004) wave rotor


Solution Methodology

• Arbitrary Sliding Interface

• MARS (Monotone Advection Reconstruction Scheme) – 2nd order accurate

• PISO predictor-corrector algorithm– Corrector stages below specified limit (20)

indicates convergence reached for specified residual tolerance


Arbitrary Sliding Interface


Estimating Artificial Diffusivity

• Use shock tube with different grid resolutions representing the range of CFD simulations carried out

• Calculated artificial diffusion from known equation

• Compared these values with physical diffusivity in simulations


0

100

200

300

400

500

600

0.00E+00 1.00E-01 2.00E-01 3.00E-01 4.00E-01 5.00E-01 6.00E-01 7.00E-01 8.00E-01 9.00E-01

X-coordinate (m)

Tem

per

atu

re (

K)

Physical Diffusivities:

Thermal diffusivity for air ~0.00002

Turbulent diffusivity for ICWR case ~0.5

Distance along tube

Ti

T

Cell size (cm)

Artificial diffusivity

(m2/s)

2.50 ~1.5

1.00 ~0.5

0.25 ~0.05

Shock tube


Hardware Resources

• AVIDD Linux Cluster– Huge Scratch space– Batch Scheduling– Accessible from outside of network (SSH)

• Dual CPU PC– Quick turnaround– Debugging– Manual decomposition

15


Methodology Development

• Welch (1997) simulated NASA 4-port configuration using code validated against experiment– 2D unsteady, laminar, compressible, ideal

gas, adiabatic walls, no leakage

• IUPUI simulation– Same as above and also included passage to

passage leakage


Grid Resolution


Welch IUPUI

Rotor Passage Grid Dimensions (nodes) 115 x 41 123 x 41

Rotor Wall Tangential Spacing (in cm) 8.90E-03 9.00E-03

Rotor Wall Tangential Spacing (in cm) 6.40E-02 6.20E-02

Inlet & Outlet Port Grid Dimensions (nodes) 85 x 151 85 x 151

Low Pressure Exhaust Port Dimensions (nodes)

85 x 165 85 x 151

Port Wall Tangential Spacing (in cm) 8.90E-03 9.00E-03

Rotor/Port Interface Axial Spacing (in cm) 6.40E-02 6.00E-02

Rotor Interior Axial Spacing (in cm) 0.25 0.25

Grid discretization comparable to Welch (1997)


Computed instantaneous total temperature

400 1200

IUPUI Welch-2D


Interface skewing between cold driven flow and hot driver flow not seen in one-dimensional computations

Hot driver gas coats the trailing end of the high pressure exit port thus discharging more hot gas to the burner


Computed instantaneous static temperature contours showing close up view of passage gradual opening

process and 2D flow features

IUPUI Welch-2D


Fuel-Air Mixing in an Internal Combustion Wave Rotor (ICWR)

• Include multidimensional effects

• Include turbulence modeling (k-epsilon with wall functions)

• Include species transport equations

• Include property dependence on mixture composition and temperature

• Examine the effect of fuel-air distribution on combustion


• Boundary Conditions - from Alparslan, Nalim and Synder (2004)– Inlet was specified as total conditions

• Total pressure at inlet segments 109 KPa• Total temperature at inlet segments 291 K

– Exit port was specified as static conditions• Static pressure at 72 KPa

– Hot gas injection port• Static temperature 600 K

• Combustion using one-step reaction combined time scale model

C3H8 + 5O2 3CO2 + 4H2O– the reaction time scale is the sum of the dissipation and

chemical kinetics time scales.

kinlr

lrct

ctR

R

Yk

,

,


Rotational speed of the rotor (rpm) 4100

Number of cycles per revolution 1

Rotor angular velocity (rad/s) 429.2

Number of passages 20

Passage length (meters) 0.7747

Mean passage width (meters) 0.062

Mean radius (meters) 0.199

Gap b/w rotor end wall & blade (meters) 0.005

ICWR geometry


Grid Resolution

Ignition port


Inlet species compositions

Air Inlet

Fuel-air Inlet

Fuel or Air Inlet

Direction of Flow

Species Mass Fractions


Non-Combustion Pressure waves for time converged solution

10.5 KPa 182.6 KPa


Fuel distribution for one-dimensional and two-dimensional

Red indicates stoichiometric fuel-air mixture, the desired fuel fraction for the ignition region


Shape of fuel-air interface

• Fuel-air interface at the middle of the inlet has expected skew (tangential non-uniformity) due to passage opening to fuel over time

• Fuel-air interface forming at the beginning of the inlet is less skew

• The skew of interface maybe something useful to control


Close-up view of first inlet segment opening to rotor passage

“tufts indicate flow vectors relative to rotor”

Vabs

rVrel

General velocity diagram

Modified relative velocity diagram for present case

Vabs

rVrel


Developing more uniform fuel-air interface

• All the inlet port segments have the same total pressures

• First inlet segment has higher static pressure than other segments due to higher pressure from rotor passage

• Thus absolute velocity in the first inlet segment is lower than other segments

• Non-axial relative velocity forces more fuel into the trailing side of the passage


Reduced total pressure at first inlet segment


Increased total pressure at first inlet segment


Results of varying total pressure at first inlet segment

• Decreasing total pressure at first inlet segment has backflow not helping in the fuel distribution shape in other passages

• The fuel-air interface is skewed similar to fuel air interaction in middle of inlet ports

• Increasing total pressure at first inlet segment causes no backflow

• The fuel-air interface is skewed too


Adding air-buffer as first inlet segment


Results from air buffer case

• The non-axial relative velocity in the first inlet segment which doesn’t have fuel doesn’t influence the filling of fuel in passage

• The fuel-air surface is skewed similar to the fuel-air surface in the middle of the inlet port


Close-up view of inlet port opening to rotor passage – with & without air buffer

Fuel sent in from first inlet segment

Air sent in from first inlet segment


Setup - combustion case

• Boundary conditions obtained from 1-D detonation model.

• The present case is studied for deflagration and 2-D incompatible with 1-D BCs

• Modified BCs to velocity high flow causing choke exhaust

• Used case to study general effect of fuel-air distribution on combustion


Ignition port

Combustion with fuel-air coming in from first three inlet segments


Combustion air coming in from first inlet segment acting as air-buffer

Ignition port


Results of combustion case

• Premature ignition when fuel-air mixture from first three inlet segments due to hot products from previous cycle

• Presence of air buffer as first inlet segment prevents premature combustion


Skewness (tangential non-uniformity)


Comparison of penetration of fuel for both configurations


Conclusions• Developed methodology for 2-D wave rotor simulation

– Compared with published 2-D simulation results by Welch (1997)

– Used commercial solver for CFD simulations

• Applied methodology to ICWR– Studied multidimensional factors affecting fuel-air distribution on

few configurations– With no air buffer – skew can be affected by timing, total inlet

conditions– Premature ignition can be prevented by air-buffer– To do a higher fidelity simulation, of a given wave rotor

configuration, include a finer grid based on NASA 4-port wave rotor and geometry and boundary conditions obtained from one-dimensional deflagration.

Arnab Banerjee Mechanical Engineering IUPUI MSME Thesis Presentation Advisor: Prof. Razi Nalim

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