History and Future of TDLAS for Combustion and Propulsion

Ronald K. Hanson Department of Mechanical Engineering, Stanford University

AFOSR-ARO Contractors’ Meeting Arlington, VA, June 2-5, 2014

Outline

Vision and historical perspective Fundamentals Historical highlights Recent developments The future

TDL Sensor Vision for Aeropropulsion

– Diode laser absorption sensors offer prospects for time-resolved, multi-parameter, multi-location sensing for performance testing, model validation, feedback control

Exhaust (T, species, UHC, velocity, thrust)

Inlet and Isolator (velocity, mass flux, species,

shocktrain location)

Combustor (T, species, stability)

l1 l2 l3 l4 l5

Diode Lasers

Fiber Optics

Acquisition and Feedback to Actuators

l6

Sensors developed for T, V, H2O, CO, CO2, O2, & other species Prototypes tested and validated at Stanford Several successful demonstrations in ground test facilities Opportunities emerging for use in flight: sensing and control

2

The History of TDL Absorption for Aeropropulsion: 35 Years: From the Laboratory to Flight

1977 – TDL absorption of NO and CO: fundamental spectroscopy, shock tubes, flames 1990 – Mass flux sensor using O2 absorption 1993 – Multiplexed measurements: H2O, T and momentum flux (thrust) 1998 – Laser-based combustion control 1998 – T, H2O and V in shock tunnels (@ CUBRC and HEG) 2001 – Multiplexing for multiple species in flames: CO, CO2, NH3, H2O 2002 – Wavelength-agile scanning for T, species, V, non-uniform flows, and PDEs 2004 – T, O*, N* in high-enthalpy arc-jets (@ NASA Ames) 2006 – Precision T and species: shock kinetics and aerosols 2007 – WMS-based Mass Flux, T at large scale (P&W engine, NASA Langley) 2009 – T, H2O & CO2 w/ sensing of incipient unstart in scramjet combustor (@ AFRL) 2010 – In-cylinder gasoline and T in production IC-engines (@Nissan North America) 2011 – First application in flight tests (@ AFRL) 2013 – New paradigm for scanned, normalized WMS (2f/1f) in dense flows 2013 – Extension to fiber-coupled mid-IR for hydrocarbon-fueled scramjet (@UVa) 2013 – Enthalpy flux in PDE (@NPS) 2014 – T in high-T, high-P air (to 150 atm, 2800 K) 2014 – Low-finesse cavity-enhanced absorption

Absorption Fundamentals: The Basics

• Scanned-wavelength line-of-sight direct absorption • Beer-Lambert relation

• Spectral absorption coefficient

High-quality spectroscopic data for S & Φ required!

( )LnLkII

io

t ⋅⋅−=⋅−=≡ ννν στ exp)exp(

PPTTSk ii ⋅⋅Φ⋅= χχν ),,()(absorbance

4

Absorption of monochromatic light

Absorption Fundamentals: The Basics

• Shifts & shape of Φ contain information

5

• V from Doppler shift • P from Line width • χi (species) from integrated area • T from line ratio

• T from ratio of absorption at two wavelengths of same species

• Potential to infer Trot ≠ Tvib

Absorption Fundamentals: The Basics

6

• Shifts & shape of Φ contain information

• To monitor multiple parameters or species • To assess non-uniformity along line-of-sight

Absorption Fundamentals: Summary

7

Combustion gases an opportunity for TDLAS

• Wavelength multiplexing is also effective

TDL Sensors Provide Access to a Wide Range of Combustion Species/Applications

Small species such as NO, CO, CO2, and H2O have discrete rotational transitions in the vibrational bands

Larger molecules, e.g., hydrocarbon fuels, have blended features

Different strategies needed to monitor discrete lines or blended absorption features

8 8

Two primary TDLAS sensor strategies

Two Absorption Sensor Strategies: Direct Absorption (DA)

WMS

Direct Absorption

Gas sample

Io It

Direct absorption: Simple, if absorption is strong enough WMS: More sensitive especially for small signals (near zero baseline)

WMS with TDLs improves noise rejection Normalized WMS, e.g. 2f/1f cancels scattering losses! What wavelengths can we access?

Injection current tuning

+ Injection current modulation @f

i’

i 0.4 0.5 0.6 0.7

0.00

0.25

0.50

0.75

Abso

rban

ce

Wavelength (relative cm-1)

Direct absorption lineshape

0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3 Direct Absorption Scan

Lase

r Inte

nsity

Sign

al

Time(ms)

Baseline fit

for Io

Lockin @1f, 2f

-0.02

0.00

0.02

0.04

WMS-2f lineshapeNorm

alize

d 2f

sig

nal

Wavelength (relative cm-1)0.4 0.5 0.6 0.7

0

2

4

6

8

WMS Scan

WM

S Si

gnal

Time (ms)0.4 0.5 0.6 0.7 0.8 0.9 1.0

& Wavelength Modulation Spectroscopy (WMS)

9

Allows access to many atoms and molecules Growing use of fiber coupling for remote sensing

Next: Historical Highlights

TDLs Provide Visible to Mid-IR Access

10

O,

10 11

Alkenes

11

Historical Highlights

TDLAS sensing for: 1. Flames and shock-heated flow 2. Combustion control 3. Hypersonic shock tunnels 4. Pulse-detonation engines 5. Precision thermometry 6. Large aeroengines 7. Scramjets Next: First hi-res spectroscopy in high T gases

12

TDL Measurements of CO and NO circa 1977: Using mid-IR lead-salt lasers Flames Cell Shock tube

CO 3340 K 0.20 atm

First use of rapid-scanning TDLs Typical result for shock-heated CO Powerful new strategy for

quantitative spectroscopy of high-T gases

13

Rapid-Scanning TDLAS enabled first determination of lineshape parameters at high T (1981)

Collision Width vs Rotational Quantum Number

Data quality sufficient to measure Dicke narrowing and develop accurate line shape models (Voigt and Galatry)

Quantified collision width as a f(quantum numbers) Next: Combustion controlled by TDL sensor

Methane/Air Flame 1850 K CO Absorption Lineshape P(23) v= 10 (2048.26 cm-1)

Philip Varghese (1980)

First Combustion Control using Laser Sensing

• TDL 2-line temperature used as a control variable

50-kW Forced Combustor US Navy, China Lake, CA

1997

5-kW Forced Combustor HTGL Stanford

1996

Ted Furlong (1996)

Compact Incinerator Using Forced Combustion

• ONR forced-combustor required real-time control

Klaus Schadow, Effie Gutmark, Ken Yu @ China Lake

Fuel and products of waste pyrolysis forced at sin(2πf0t+θ)

Secondary Air

Primary air forced at sin(2πf0t)

In situ T from ratio of two H2O transitions CO and UHC measured at exhaust

TDL Sensor for Incinerator Control Multiplexed lasers for multiple parameters

Results: Efficiency Optimized with TDL Control

TDL control reduced CO and UHC more than 10X Note ppm detection demonstrated Control established in 0.1 seconds!

Ted Furlong , 27th Combustion Symposium, 1998

Next: T, H2O and V in shock tunnels

18

Use of TDL Probe to Characterize Hypersonic Shock Tunnel @ CUBRC (1997)

First demonstration of remote, fiber-coupled TDL probe Time-resolved sensing of T, PH2O, V and composition demonstrated Species probed: H2O & K

Facility Test Area

Laser Lab

TDL System Optical Table

8' Test Section

Fiber Optic and Signal Line Conduit

4' Nozzle M = 8-16

TDL Probe

8" Reflected Shock Tube

Data Acquisition

Sean Wehe (1997)

4’ Nozzle M= 8-16

19

Two-Color H2O Absorption Probe for T, V and PH2O

Species: H2O Scan Rate: 10 kHz Scan Range: 0.3 cm-1 Laser Power: 10 mW

Ttran, Trot, PH2O, V determined from absorption lines at λ1 and λ2

Nozzle Exit pitot probe

Ttot. Probe H2O Probe

beam paths

Flow Direction

Detectors 18 cm

λ1

λ2

20

Measurements of Trot, Ttran, V, and PH2O @ CUBRC

• Pitot-probe pressure trace

• Single-sweep lineshapes (@ t = 3.6 msec)

ho 10 MJ/kg V∞ 4500 m/sec T∞ 650 K P∞ 8 Torr Test Gas air w/ H2O tracer

Facility conditions:

Area ratio → Trot = 560 K Note: Trot = Ttran

PH2O=0.43 Torr

21

Measurements of Trot, Ttran, V, and PH2O @ CUBRC

ho 10 MJ/kg V∞ 4500 m/sec T∞ 650 K P∞ 8 Torr Test Gas air w/ H2O tracer

Facility conditions:

Next: Multiplexed Measurements for Multiple Species in PDE

TDL sensor provided first time-resolved data for tunnel performance

Pitot P

T

V

PH2O

TDLAS Sensing in Pulse Detonation Engines (PDE)

First ONR measurement campaign at China Lake

Sensor operator in bunker 100m away

22 John Liu (1998)

Measurements allowed test of detonation models @ SU, NPS and China Lake

Wavelength-Multiplexed Tgas and XH2O Sensor in PDE

Scott Sanders et al. Silver Medal, 28th Combustion Symposium, 2000

5-Laser System

Wavelength-Multiplexed Sensor for T and H2O: Detonation of Stoichiometric C2H4 / air

• Excellent agreement validates NRL model Next: First application of fast-scanning VCSELs

Temperature XH2O

25

First Application of Fast-Scanning with VCSELs

PDE Schematic with Cs Addition

• Temperature measurement by lineshape: Tkinetic & Telectronic

26

First Application of Fast-Scanning with VCSELs

Example Fast-Scanning Data

• Cs lineshape measured at 2 µsec intervals!

• Linewidth provides P, Tkinetic

• Telect from ratio of emission and absorption

Next: Comparison of Telect & Tkinetic

27

First Application of Fast-Scanning with VCSELs

• Good agreement between Telect & Tkinetic

• Scanning approach works up to 4000 K!

• Good agreement with NRL model

Next: VCSELs for non-uniform T

28

Application of Fast-Scan Sensor for Non-Uniform T Distribution

O2 A-Band Absorption

Concept: Multi-line absorption data allows quantitative measurement of temperature distribution

Scott Sanders. Xiang Liu

Temperature binning strategy: no. of temperature bins ~ no. of lines scanned

29

Next: Precision T in shock-heated gases

Application of Fast-Scan Sensor for Non-Uniform T Distribution

Temperature binning strategy: no. of temperature bins ~ no. of lines scanned

Successful demonstration in multi-zone furnace and flames

Scott Sanders. Xiang Liu

Parabolic T Distribution

Concept: Multi-line absorption data allows quantitative measurement of temperature distribution!

30

Precision Time-Resolved T from WMS-2f/1f of CO2

• Validate fast, sensitive strategy for CO2 and T using a shock tube

Shock wave Test mixture

InSb Detector

DFB laser

2752nm

2743nmAperture

Detector

100 kHz100 kHz

T & CO2@ 40 kHz

Data Analysis

Data Acquisition

Detector

Shock Tube15 cm diameter

2752nm

2743nmAperture

Detector

100 kHz100 kHz

T & CO2@ 40 kHz

Data Analysis

Data Acquisition

Detector

Shock Tube15 cm diameter

Virtue: CO2 linestrengths near 2.7 µm much stronger than NIR lines

Aamir Farooq (2009)

31

Temperature vs Time in Shock-Heated Ar/CO2 Mixtures

Temperature data agree with T5 determined from ideal shock relations Temperature precision of ±3 K demonstrated! Unique capability for real-time monitoring of T in reactive flows WMS 2f/1f T sensor also insensitive to particle scattering

0 3 6 9 12-30

-20

-10

0

10

20

30

5 K

Diffe

renc

e of

Mea

sure

d T

& T 5 [

K]

Time [ms]

0 K

Reflected shock arrival

0 3 6 9 120

300

600

900

1200

0.0

0.6

1.2

1.8

2.4

Time [ms]Pr

essu

re [a

tm]

Tem

pera

ture

[K]

Reflected shock arrival

Incident shock arrival

1.2 atm, 2%CO2 in Ar

Tideal

Aamir Farooq (2009)

Next: Gas temperature in aerosol-laden gas

32

1f-Normalized WMS-2f for T with Particulate Scattering Validate in Aerosol-Laden Gases Aerosol shock tube experiment:

2% CO2 /Ar in n-dodecane aerosol, L=10 cm, P2=0.5 atm; P5=1.5 atm

2f/1f WMS sensor measures T in presence of aerosol!

Wei Ren (2010)

Next: Recent Extension to Cross-Band T Sensor

New Temperature Sensor: Cross-Band CO2

Lines from different bands

R(28) E” = 316 cm-1

R(96) E” = 3622 cm-1

CO2

Example: First Shock Tube Demonstration

33

Strategy: Use of lines in different CO2 bands allows large ∆E” with similar absorbances

First Shock Tube Demonstration of Cross-Band CO2 T Sensing

34

Example: Non-Reactive Shock (CO2/Ar)

Observed measurement precision: ± 5 K ≈ 0.5% at 1100 K!

2.0 2.5 3.0 3.50.0

0.5

1.0

Abso

rban

ce

Time [ms]

2752 nmE" = 316 cm-1

2% O2/Ar1124K, 2 atm

4175 nmE" = 3622 cm-1

2.0 2.5 3.0 3.5200

400

600

800

1000

1200

Measured T

Pressure [atm]

Tem

pera

ture

[K]

Time [ms]

FROSH T

0

1

2

3

4

5

Temperature Absorbance

Next: WMS-based Velocity & Mass Flux

Mitch Spearrin (2014)

35

Mass Flux Sensing in a Full-Scale Aero-Engine

Kent Lyle (2007)

Air Density ρ from measured absorbance of O2

A α ρO2 α ρair

Velocity u from Doppler shift

O2 A-Band Spectra

Mass Flux = (Air Density) x (Inlet Velocity)

Tests in Pratt and Whitney PW6000 Turbofan Engine Inlet

36

Bellmouth installed on inlet of commercial engine (Airbus 318) Sensor hardware remotely operated in control room TDL beams mounted in engine bellmouth

Mass Flux Sensing in a Full-Scale Aero-Engine

Kent Lyle (2007)

P & W Mass Flux Results

37

TDL data agrees well w/ test stand instruments (1.2% in V and 1.5% in ρ) Flow model employed to account for non-uniformities

Mass Flux Sensing in a Full-Scale Aero-Engine

Next: Application to Supersonic Flows

Air Flow

38 38 38

Supersonic Velocity@NASA Langley via TDLAS: Direct-Connect Supersonic Combustion Test Facility

DCSCTF: Simulates atmospheric supersonic and hypersonic flight conditions

M=2.65 nozzle: Tstatic~ 990K & Pstatic~ 0.7 atm; simulates M=5 flight Optical access added to isolator Measure V, T, mass flux

Diverging nozzle Combustor

Fuel injection

Isolator

Nozzle

Vitiated inlet air

38 Leyen Chang (2007)

Supersonic Velocity @NASA via TDLAS

Sensor translates to probe vertical and horizontal planes

Supersonic test facility at NASA Langley (2009)

Downstream window

Upstream window

Translating sensor

7cm

13cm

Tunnel cross section

39 Chang et al., AIAA, 2011

Supersonic Velocity @NASA via TDLAS

Sensor translates to probe vertical and horizontal planes

Downstream window

Upstream window

Translating sensor

40 Chang et al., AIAA, 2011

Vertical Scan

Supersonic test facility at NASA Langley (2009)

Supersonic Velocity @NASA via TDLAS

Fast sensor captures start-up transients in V and T

Downstream window

Upstream window

Translating sensor

41

Next: Fluctuations in T signal used to signal unstart in Scramjet

Supersonic test facility at NASA Langley (2009)

TDL Sensor for Scramjet Combustor Performance Example: Fluctuations in T Uniformity Signals Unstart

Simultaneous measurements on 4 H2O lines Two lines to monitor Tlow & two lines to monitor Thigh

AFRL, WPAFB, 2007

42 Greg Rieker (2007)

Sensor Monitors Time-Resolved Tlow vs Thigh

Tlow ≠ Thigh indicates temperature is not uniform

Low-frequency fluctuations anticipate inlet unstart Fluctuation sensing : A new paradigm for control!

Running FFT of Tlow & Thigh

Ratio low- frequencies

43 Greg Rieker (2007)

TDL Applications: 2011 to Present

New sensing strategies for measurements of 1. H2O, CO & CO2 in HC-fueled scramjet

2. Enthalpy flux in PDEs

3. Temperature in hi-P, hi-T air

4. Ultrasensitive absorption

44

Mid-IR Absorption Sensing in Propulsion Flows

Goal: Spatially-resolved CO, CO2 and H2O in supersonic combustion

45

UVa Supersonic Combustion Facility Charlottesville, VA

Mach 5 flight condition Mach 2 in combustor

C2H4/Air, φ ≈ 0.15

Spearrin, Shultz, Goldenstein (2013)

First use of Fiber-Coupled Mid-IR for Aero-applications Optical Setup: Translating Line of Sight (LOS )

46

Translating LOS

Fuel Injection

Recirculation Cavity

Reaction Zone

Two Measurement

Planes

Air Flow

Y

X

Four lasers Two for H2O and T

One for CO and one for CO2

SIDE VIEW Supersonic flow path

Time-resolved CO and T Measurements

TDL sensor applied in cavity

47

UVa Combustor

C2H4 + Air: ϕ ≈ 0.15

Two-line CO T measurement @ 6 kHz

• Sensor used to measure cavity dynamics

Comparison of TDL Data with CFD (NCSU)

48

P1

UVa Combustor

C2H4 + Air: ϕ ≈ 0.15

P2

P1 P2

CO2 CO2

• CFD and measured CO2 column density (CD) in good agreement

Comparison of TDL Data with CFD (NCSU)

49

P1

P2

UVa Combustor

C2H4 + Air: ϕ ≈ 0.15 CO CO P1 P2

Next: Enthalpy flux in PDE

• CFD overpredicts CO in cavity

Spearrin et al., Applied Phys B (2014)

TDL Sensing of Enthalpy Flux in PDE Goal: Time-resolved species and T in nozzle for flux

50

Pulse Detonation Combustor

CO SM Fiber Pitch

Detector Catch Systems

H2O T-sensing in chamber

CO2 SM Fiber Pitch

At Naval Post-graduate School in Monterey, CA

Measurement locations in throat and chamber Inner diameter = 3.5 cm

Gas Exhaust

2482 nm 2474 nm

2678 nm

Flow from Engine

Nozzle Exit

Fiber-Coupled Light to Engine

Detectors

Pitch Optics

H2O & T

CO2

Nozzle Entrance

4854 nm CO OR

Pressure Port

PDC Nozzle Schematic

Measured parameters: T, XH2O, XCO, XCO2

TDL Measured Time-resolved T, Xi

51

Pulse Detonation Combustor

0204060

0.00

0.05

0.0

0.1

0 20 60 800.0

0.1

P

P [a

tm]

T [K

/50]

X CO

T

X H2O

X CO2

Time [ms]

20 Hz Ethylene/Air Detonation Cycles

• Measurement locations in throat and chamber • Inner diameter = 3.5 cm • Assume flow choked at throat

CO SM Fiber Pitch

Detector Catch Systems

H2O T-sensing in chamber

CO2 SM Fiber Pitch

At Naval Post-graduate School in Monterey, CA

Gas Exhaust

Time-resolved measurements of T, Xi Mass Flux and Enthalpy Flux

Enthalpy Flux to Characterize PDC Performance

52

• TDL Sensor successfully monitors key PDE performance parameters

3 Consecutive Cycles

Chris Goldenstein, Comb. Symp (2015)

Next: High-T/High-P Air for AF

T Sensor for High-T/High-P Air Exploits Strong T Variation of NO in Equilibrium Air at High T

Concentration (mole fraction) of NO highly sensitive to T, insensitive to P At 2500K, 5% measurement of NO can yield 1% uncertainty in T!

53

Air Equilibrium XNO vs. Temperature (Cantera – GRI-Mech)

Next: Measure NO by TDLAS at 5 µm

NO-Seeded T Measurements

54

2% NO/N2

Best Fit: Tmeas=1.001Tknown±1.5%

• Accurate thermometry achieved across large T,P range!

T from NO absorption in non-reacting NO/N2 mixture with known χNO : demonstration behind reflected shock waves

Next: Application of TDLAS to ultra-sensitive species detection

55

Ultra-sensitive Species Detection: Low Finesse (LF) Cavity-Enhanced Absorption Spectroscopy (CEAS)

Cavity increases effective absorption path length, Leff

Leff = D/(1-R) Off-axis insertion enables easy, robust alignment

• Off-axis CEAS superior to traditional multipass

Example: Detection limits for CO using Mid-IR source

Goal: Ultra-sensitive species detection in ST experiments

5 mm

Sun, et al., Optics Express 22 (2014)

56

CEAS Detection Limits: Mid-IR detection of CO

• R = mirror reflectivity

• Gain = 𝑳𝒆𝒆𝒆𝑫

= 𝟏𝟏−𝑹

• Sub-ppm detectivity at 1000-2000 K with R=0.985

10-3 10-2 10-10.01

0.1

1

100.90.99

λ = 4.566 µm P = 1 atm; D = 15cmAssume minimum detectable absorbance = 0.5% @ 1MHz

CO d

etec

tion

limit

[ppm

]

1-R

1000K 1500K 2000K

0.999R

Sun, et al., Optics Express 22 (2014)

Next: first application to shock tube kinetics

57

Off-Axis CEAS of CO formation using Mid-IR

• Demonstrates good CO sensitivity and good temporal resolution

• Further improvements: 10 – 100x in progress

First highly-diluted CO formation experiment

Detection limit ~ 0.3ppm at 100kHz time resolution

-500 0 500 1000 15000

5

10

10ppm acetone in ArT = 1380K, P = 1.5atm

CO c

once

ntra

tion

[ppm

]

Time [µs]

TDL CO concentration Exponential fit

CO formation by acetone (10ppm) pyrolysis

Sun et al., May 23, 2014

Many Opportunities for Propulsion and Combustion Science • Emergence of new lasers and optical fibers in the mid- & far-IR • Many new applications!

• Propulsion and combustion control • Sensing in dense media (sprays, aerosols and high pressures) • Sensing in shock tube for combustion kinetics • Sensing in ground and flight tests

• New sensor strategies • Multi-quantum-state probing (for nonequilbrium T) • Mega-multiplexing and tomography • Multi-harmonic & multi-amplitude WMS methods • TDLIF and combined TLAS/LIF methods • New cavity-enhanced concepts

(e.g. with WMS, multi-color, broad sources) 58

A Promising Future for TDL Absorption Sensors

Acknowledgements

Research staff: Dr. David Davidson and Dr. Jay Jeffries Many former TDLAS students (~ 35) Recent students: Aamir Farooq, Wei Ren, Hejie Li, Greg Rieker Current students: Mitch Spearrin, Chris Goldenstein, Ian Schulz Sponsors: AFOSR, ARO, ONR, NASA and NSF

59