Christopher M. Heath, Robert C. Anderson, and Yolanda R. Hicks

Glenn Research Center, Cleveland, Ohio

Randy J. Locke

ASRC Aerospace, Inc., Cleveland, Ohio 

Excitation/Detection Strategies for OH Planar LaserInduced Fluorescence Measurements in the Presence 

of Interfering Fuel Signal and Absorption Effects

NASA/TM—2011-216947

January 2011

AIAA–2010–6667
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Christopher M. Heath, Robert C. Anderson, and Yolanda R. Hicks

Glenn Research Center, Cleveland, Ohio

Randy J. Locke

ASRC Aerospace, Inc., Cleveland, Ohio 

Excitation/Detection Strategies for OH Planar LaserInduced Fluorescence Measurements in the Presence 

of Interfering Fuel Signal and Absorption Effects

NASA/TM—2011-216947

January 2011

AIAA–2010–6667

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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NASA/TM—2011-216947 1

Excitation/Detection Strategies for OH Planar Laser-Induced 

Fluorescence Measurements in the Presence of Interfering 

Fuel Signal and Absorption Effects

Christopher M. Heath, Robert C. Anderson, and Yolanda R. Hicks

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Randy J. Locke

ASRC Aerospace, Inc.

Cleveland, Ohio 44135

Abstract

Planar laser-induced fluorescence (PLIF) excitation/detection methods have been applied to obtain 

spatial distributions of the hydroxyl [OH] reacting intermediary and hydrocarbon [HC] primary species in

laminar and turbulent combustion reactions. In this report, broadband and narrowband excitation/filtering 

techniques are explored to identify an optimal experimental configuration yielding significant fluorescent

signal with low absorption losses. The combustion environments analyzed include 1) a laminar nonpremixed methane/air flame and 2) a turbulent, non-premixed Jet-A/air fueled flame within a lean flame 

tube combustor. Hydrocarbon-based fuel and OH were excited via the R1(1), R1(10) and R2(7) transitions

of the A2

Σ+ ← X 2Π (1,0) band using a broadband Nd:YAG pumped optical parametric oscillator (OPO) 

and narrowband Nd:YAG/dye laser with ultraviolet frequency extension (UVX) package. Variables tested

for influence on fluorescent signal and absorption characteristics were excitation line, laser energy, 

exciting linewidth, combustion reactants, and test flow conditions. Results are intended to guide the 

transition from a dye/UVX laser to an OPO system for performing advanced diagnostics of low-emission 

combustion concepts.

Nomenclature

AR anti-reflective

FWHM full-width at half maximum

HTHP high temperature, high pressure

ICCD intensified, charge-coupled device

LDI lean direct injection

LSF laser saturated fluorescence

NCC national combustion code

Nd:YAG neodymium: yttrium aluminum garnet

NOx oxides of nitrogen

OH hydroxyl radical

OPO optical parametric oscillator

PAH polycyclic aromatic hydrocarbons

PLIF planar laser-induced fluorescence

UHC unburned hydrocarbon

UV ultraviolet

UVX ultraviolet frequency extension
NASA/TM—2011-216947 2

1.0 Introduction

Emission regulations for the next-generation of air-based propulsion systems call for the development 

and evaluation of advanced initiatives in low-NOx combustion research. Candidate emission reduction 

technologies, including advanced fuel injection hardware and alternative fuels, are often validated using

non-intrusive and in-situ measurement techniques (namely, remote laser diagnostics conducted in a

combustion chamber with optical access). Unfortunately, these types of flow interrogation methods often

suffer from optical complications that must be characterized and, if possible, corrected. Countering such

complexities requires a thorough investigation of interfering factors, including the development of error 

mitigation techniques and corrective procedures to improve the quality of experimentally acquired data.

Planar laser-induced fluorescence (PLIF) is an optical imaging process for obtaining spatially and 

temporally resolved two-dimensional chemical species, temperature, pressure and velocity measurements

(Ref. 1). In standard PLIF, a targeted molecular species is excited from a ground-level electronic state of

specific rotational and vibrational energy to a higher electronic state via photon absorption of incident 

laser radiation. Various decay processes, including fluorescence, provide a means for the laser-excited

molecules to revert to a stable energy level. Emitted fluorescence from this process is captured and related 

to the number density of the excited molecular species. PLIF has been reported in many studies as an 

effective means to describe flame location, flow structure, regions of recirculation and chemical 

composition in reacting flows (Refs. 1 to 5). The technique is most conventionally used to image 

concentration of minor and intermediate combustion species including CH, OH, NO and certain unburned

hydrocarbons (UHCs). Of particular interest to aircraft emission research is imaging of the hydroxyl [OH]

and UHC groups, which together may be used to describe fuel patternation, mixing performance and 

efficiency for a given fuel injection system or combustor architecture. Successful imaging of the OH

radical by means of laser-induced fluorescence under realistic engine operating conditions requires 

overcoming several adverse effects. Problems include interference from overlapping fuel signal, pressureinduced line broadening and laser attenuation. Techniques to mitigate these complications and produce 

useful, more quantitative OH imaging data are the primary focus of this study.

Several characteristics make the OH radical a valuable species for indirectly gauging combustor 

system level performance. There exists a large spectroscopic database for OH; there is good separation 

between its rovibronic lines with limited spectral interference (Ref. 4), and it is more easily and safely 

resolved than species like NO. OH is capable of providing insight into the physical flow structure, 

chemical composition and temperature distribution of reacting flows. In the exothermic hydrocarbon-air

reaction, OH most readily forms at the flame front and its spatial distribution marks locations of heat 

release. Regions of high temperature increase thermal NOx formation and may reduce the lifetime of

critical engine components (Ref. 2). The ability to track thermal intensity and sites of non-uniform mixing

through the presence of OH is useful for testing operating methods, evaluating hardware designs and

influencing future modifications to reduce emissions and further enhance combustor performance. 

Species and temperature are also important parameters for validating and improving the fidelity of 

computational models that simulate the complex, multiphase interactions and turbulent flow structures 

typically found in the combustion pathways of gas turbine engines.

Historically, OH PLIF has been employed effectively in low and atmospheric pressure flame 

experiments using gaseous fuel and oxidizer (Ref. 4). Such conditions are atypical of the multiphase and 

high-pressure environments of aircraft combustion systems. Known complications encountered in

performing quantitative PLIF measurements in real combustor settings include laser attenuation attributed 

to absorption, pressure-induced line broadening, spectral interferences, collisional fluorescent quenching 

and spectral overlap between species containing similar emission signatures (Refs. 2 and 6). For example, 

polycyclic aromatic hydrocarbons (PAHs)—common constituents in aviation fuels that include

naphthalene and its derivatives—absorb and emit energy over a broad spectrum often competing with the 

fluorescent emission bands of the OH molecule (Ref. 2). Spectral interferences may also arise due to

elastically scattered light from liquid fuel and, if the flame is luminous, blackbody radiation (Ref. 4). To 

reduce the influence of such phenomena, a variety of OH excitation lines have been tested with laminar 
NASA/TM—2011-216947 3

and turbulent reacting flows to identify a least absorbing transition that produces optimal OH fluorescent

signal. Additionally, narrowband and wideband spectral filters have been applied in each transition case 

to identify an excitation/detection combination that most effectively isolates spectral emissions of the OH 

radical from other constituents.

A variety of investigations (Refs. 2 to 4, and 6 to 8) report the use of PLIF and spectroscopic

techniques to study flame structure, composition and absorption effects in atmospheric as well as high 

temperature and high pressure combustion environments. Much of the laboratory work has been aimed at 

refining methods to produce quantitative species and local temperature measurements for use in practical 

combustion applications. Common areas of research to enhance measurement integrity include

excitation/detection strategies, laser energy/attenuation corrections and background subtraction methods 

for increasing signal-to-noise ratios. 

Seitzman et al. (Ref. 3) assessed a variety of techniques for exciting OH with the objective of

identifying a method for minimizing combined systematic and random experimental errors. Solution 

of a time-dependent rate model for population densities demonstrated that dye laser excitation of the 

A2

Σ+ ← X 2Π (1,0) band produced high signals with low overall error, ideal for quantitative single-shot 

imaging of hydrocarbon-air flames.

Hicks et al. (Refs. 2 and 4) used PLIF for flame structure visualization in an optically accessible Jet-A 

fueled lean flame tube combustor to simulate a working gas turbine engine environment. Excitation of 

OH, NO and fuel was performed while also exploring the effects of narrow and broadband filtering 

techniques to remove unwanted interfering spectra. Operating conditions were varied to simulate realistic 

flight conditions with overall equivalence ratios near 0.5. Frame averaging was applied to provide 

qualitative flow field characteristics, flame structure and fuel patternation information. Complexities 

noted in obtaining quantitative results included spectral interferences, turbulence interactions and highpressure effects such as spectral line broadening. Using a narrow passband filter centered on a region 

more isolated from the fuel fluorescent emission signature was suggested to reduce elastic scattering and 

interference. Real time laser energy absorption measurements were also recommended for future 

quantitative measurements. The need for an independent parametric study was mentioned to gain a better 

understanding of fluorescent temperature and pressure dependence.

Cooper et al. (Ref. 6) performed quantitative laser-saturated fluorescence (LSF) measurements of 

nitric oxide (NO) in a turbulent spray flame produced by a heptane fueled lean direct injector (LDI) at 

atmospheric pressure. The flame was operated at an equivalence ratio of unity in order to influence the 

NO formation mechanism. NO was excited via the Q2(26.5) transition of the γ(0,0) band at 225.58-nm 

and observed at the γ(0,1) band using a 2-nm wide detection window centered at 235.58-nm. Preliminary 

excitation and detection scans were conducted using a C2H6/O2/N2 flame doped with 80 ppm NO to reveal 

a lack of fluorescent interference from UHCs and PAHs within the narrow detection range. An off-line 

background subtraction method was applied to remove elastic scattering effects and isolate NO 

concentration. 

Cavity-ring down spectroscopy has been reported by Lozovsky et al. (Ref. 7) to obtain absolute 

temperature and concentration profiles of OH in methane/air flames. The work focused on the analysis of 

methane/nitrogen/oxygen flames (with nitrogen shroud gas) at low pressures (~30 torr). Flames with 

equivalence ratios of 0.8, 1.0 and 1.2 were studied and compared with combustion models demonstrating 

discrepancy between experimental and predicted results. This disagreement suggested the need for future 

analysis and model refinement.

In 2005, Baranger et al. (Ref. 8) reported experiments using spectroscopy to measure the influence of 

temperature, pressure and mixture composition on the fluorescence of kerosene vapor contained in a high 

temperature and pressure cell. The optically accessible vessel was operated between 400 and 700 K, at 

pressures between 0.1- and 0.7-MPa. Fluorescent spectra were acquired for excitation wavelengths of 

266-, 282- and 308-nm demonstrating a double fluorescent band structure that was attributed to the 

presence of several pure single and two-ring aromatics. A major loss in fluorescent signal was noted for 

the 282- and 308-nm excitation wavelengths and attributed to the broadband absorption properties of fuel 

constituents. Increasing temperature exhibited a decrease in kerosene fluorescence with spectral distortion 
NASA/TM—2011-216947 4

noted at the upper wavelengths of the spectra. Fluorescent signal was found to decrease linearly across the 

entire spectrum with a respective increase in pressure. Collision quenching by O2 measurements indicated 

a near-linear relationship between fluorescent intensity and the experimental oxygen molar fraction. The 

findings were used to perform quantitative fluorescent imaging of local equivalence ratio and temperature 

on a preheated kerosene/air jet exiting in ambient air.

The following text describes and compares results from PLIF measurements made during combustion 

testing at both atmospheric and high-pressure conditions. Test venues included a Hencken burner and a 

flame tube combustor. Laser configurations included a Nd:YAG/dye pumped UVX system and a 

Nd:YAG pumped OPO. Optical configuration variations included use of a narrowband and wideband 

filter on the detector. Other parameters varied during these tests included fuel flow rate and, in the case of 

the flame tube facility, inlet pressure and temperature. Data from all testing has been processed similarly. 

Specifically, processing of all images has included off-line background subtraction, frame averaging and 

noise removal. PLIF measurements of OH are the focus of this study with the objective of optimizing 

laser and optical configurations for future experiments.

2.0 Experimental Hardware

2.1 Laser Systems

PLIF imaging was performed using 1) a Continuum Nd:YAG (NY-81C) laser set to pump a tunable 

dye system (ND6000) employing Rhodamine 590 (R-590) dye with output frequency doubled by means 

of an ultraviolet frequency extension (UVX) package and 2) a Continuum Nd:YAG (9010) pumped 

Optical Parametric Oscillator (OPO).

For the dye laser configuration, the Nd:YAG was frequency doubled (532-nm) and the second

harmonics produced ~750-mJ/pulse at a repetition rate of 10-Hz. The dye laser operated with an 

energy/pulse of ~180-mJ at 564-nm. Using a frequency doubler/mixer (UVX) in a UVT-1 configuration, 

output from the dye laser was frequency doubled to produce the desired wavelength near 282-nm with

output linewidth near 0.05-cm–1

. Pulsed dye laser drift was monitored and controlled using a dye tracking 

system. UV light was separated from the fundamental output wavelength using a Pellin-Broca prism

mounted at the exit of the UVX. The 282-nm laser output energy was maintained near ~16-mJ/pulse. 

For the OPO system, the fundamental wavelength of the pumping Nd:YAG was frequency tripled 

(355-nm), producing a third harmonic at 300-mJ/pulse with a repetition rate of 10-Hz. The OPO linewidth 

specification is less than 6-cm–1 and the output energy was maintained near 6-mJ/pulse.

The two laser systems were tuned to excite one of three specific rovibronic transitions of the OH 

A←X (1,0) band: R1(1), R1(10) and R2(7). The accepted wavelengths corresponding to each transition line 

can be found in Ref. 9. Figure 1 shows fluorescence excitation scans about these lines. The OPO was 

scanned at a rate twice that of the dye/UVX and covers a larger range of wavelengths to access the low 

absorption (off-line) region in front of the OH A←X (1,0) band head. The off-line wavelength for the 

dye/UVX laser was between the R1(1) and R1(10) excitation lines. Previous research has indicated that 

this excitation and detection scheme has helped to reduce fluorescent trapping, absorption of the emitted 

OH fluorescence by other OH molecules and elastic laser scattering effects (Refs. 3, 5, and 10). Resonant 

and off-resonant excitations were confirmed by observing the level of OH fluorescence emitted from a 

laminar methane/air Bunsen burner flame using a photomultipler tube/boxcar average system. 

One of the goals of this research is to measure and compare the capabilities of the dye/UVX and OPO 

lasers, with the intention of using the OPO system exclusively in future laser diagnostic experiments. 

Several factors make the OPO laser more attractive than the dye/UVX system including: all solid-state; 

environmentally benign materials (no toxic chemicals are required for operation); reduced size and 

footprint compared to dye/UVX configuration; broad tuning range (from 205- to 2250-nm) without 

requiring a change of dye; and higher power at ~225-nm for NO fluorescence. The dye-UVX combination 

does, on the other hand, produce greater power output at a narrower linewidth near the OH excitation 

wavelengths, which is evidenced by the high quality UVX PLIF images contained later in this report.
NASA/TM—2011-216947 5

Figure 1.—OH excitation scans for UVX (left) and OPO (right) identifying the R1 (1), R1 (10) and R2 (7) rovibronic 

transitions.

2.2 Detector Systems

The detector used in these experiments was a 16-bit Princeton Instruments gated and intensified 

charge coupled device (ICCD) camera with detector array of 1024×1024 pixels. A Princeton Instruments 

programmable timing generator was used to synchronize the camera intensifier with the fluorescence 

induced by the laser pulse. The detectors were gated for 50-ns exposures using Princeton Instruments ST133 control units.

OH fluorescence from the (1,1) band near 315-nm was collected using an f/4.5, f = 105-mm UV 

grade lens. To achieve spectral isolation, either a 315-nm center wavelength, ~3.4-nm full-width at half 

maximum (FWHM) (narrow passband) filter with peak transmittance of ~16.1 percent, or a 314.4-nm

centered, ~11.8-nm FWHM (wide passband) filter with peak transmittance near 16.8 percent were 

applied. Previous experiments have indicated that unwanted or excessive fluorescence from fuel may be

permitted to pass to the detector with exclusive use of a wide (>10-nm FWHM) passband filter. Although 

the effects of signal from fuel can be reduced using background subtraction, further restriction by 

applying a filter with narrower passband aids in minimizing noise from fuel fluorescence. 

3.0 Image Processing and Analysis

For each planar OH measurement, both on-line and off-line images were taken. Image processing 

software was used to subtract the off-line or background image. Image pixel values were scaled from 

0 to 255 for displays using a 256-color palette. Anomalous pixels were removed and reset to the average 

intensity value of the directly surrounding region using a software routine developed to detect and reduce

image noise spikes. The source of unusually high intensity pixels is unknown, but particle emissions, 

signal noise, detector chip defects and scattered laser radiation onto the detector are all potential causes

(Ref. 2). All images were rebinned to 512×512 pixels. Intensity profiles were extracted and used for 

calculating average photon absorption through the flame. Because the output power of the UVX and OPO 

laser systems varied significantly (by a factor near three), images were scaled separately for each laser 

and only general trends between the two sets of images are comparable. The full process is represented 

diagrammatically for a single OH PLIF image in Figure 2. Similar methods were used to process all PLIF 

images contained herein.
NASA/TM—2011-216947 6

Figure 2.—Diagrammatic image processing description for methane/air fueled Hencken burner experiments.

Figure 3.—Optical setup for OH PLIF using a methane/air fueled Hencken burner at atmospheric pressure.

4.0 Methane/Air Flame Experiments

OH PLIF measurements were acquired using a non-premixed methane/air flame produced by a 

Hencken burner (30-mm square multi-element flow path) at atmospheric pressure to obtain qualitative 

distributions of the reacting OH intermediary. A Hencken burner consists of a square matrix of capillary 

tubes that alternately carry fuel or oxidizer. The device produces a two-dimensional flat diffusion flame. 

All images were acquired by averaging 10 frames, each consisting of on-chip accumulations of 100 gates 

(laser pulses). The reactant flow rates were regulated using a pair of digital mass flow control units.

Fluorescent intensity and absorbance were derived from the PLIF images at the equivalence ratios listed 

in Table 1. The total volumetric flow rate was kept constant for each flame at 6-SLM.

TABLE 1.—FLAME PARAMETERS & VOLUMETRIC FLOW RATES (SLM)

Reactant gas Φ = 0.75 Φ = 1.00 Φ = 1.25 Φ = 1.50 Φ = 1.75 Φ = 2.00

CH4 0.44 0.57 0.69 0.81 0.94 1.04

Air 5.56 5.43 5.31 5.19 5.06 4.96

The setup for the Hencken burner experiment is displayed in Figure 3. OPO and dye/UVX laser 

optics were matched and collinear overlap was ensured. Each beam was transformed into an optical sheet 

by transmission through a 200-mm cylindrical lens. The UVX planar beam measured ~30-mm in height 

by ~0.3-mm in thickness while the OPO beam measured ~50-mm in height by ~0.3-mm in thickness. All 

lenses were AR coated for 283-nm. The lasers were not operated simultaneously and the in-use input 

beam was blocked on the far side of the flame to ensure it did not enter the optics of the opposing laser.
NASA/TM—2011-216947 7

Figures 4 and 5 display the matrix of OH PLIF images acquired over the full range of equivalence 

ratios. High OH signal has been false colored as red. Each set of sixty-four images is scaled collectively 

for each laser to aid comparison by excitation line and filter. In each case, the flame centerline has been 

marked by a vertical white line. The laser sheet height for the dye/UVX is slightly greater than for the 

OPO. The two sets were acquired on two consecutive days. Some variation in flame structure exists 

between the two sets of images at each fuel/air ratio and may be attributed to imperfect flow rates and 

outside (ambient) air disturbances. Despite these variations, the images from the two laser systems reveal 

the same flame structure for a given equivalence ratio.

Figure 4.—OH PLIF images from the Hencken burner experiments using the OPO configuration. Images are scaled 

collectively. A white line denotes approximate flame centerline. Laser beam transmitted right to left. 
NASA/TM—2011-216947 8

Figure 5.—OH PLIF images from the Hencken burner experiments using the UVX configuration. Images are scaled 

collectively. A white line denotes approximate flame centerline. Laser beam transmitted left to right.

Qualitative analysis of the methane/air flame OH PLIF images also shows, as expected, lower signal 

for the narrow passband filter than for the filter with wider passband. Thus, use of a filter with wider 

detection range will result in greater fluorescent signal but may permit unwanted noise from fuel 

fluorescence. Another feature that is observable is the non-uniformity of the laser energy distribution, 

which manifests as horizontal striping in the images. While it occurs for both laser systems, it is 

particularly evident for the dye/UVX system and somewhat problematic. Improvements in the laser 

energy profile should result from measuring and correcting for the beam energy profile distribution or 

from using a more uniform, expanded portion of the beam.

Comparing data from one laser configuration with the other reveals apparent differences in coupling 

between each laser and the OH fluorescing species. For the OPO laser, regardless of filter, the R1(1) line 

appeared to induce the strongest fluorescent signal, followed by the R2(7) and R1(10) lines, in that order.

The dye/UVX laser produced differing results as the R2(7) line exhibited the strongest OH fluorescence 
NASA/TM—2011-216947 9

followed by the R1(1) and R1(10) lines. These results indicate that the R1(10) OH excitation line provided 

the weakest fluorescent yield for both laser systems and either filter. Continued efforts used only the R1(1)

and R2(7) transition lines.

For the dye/UVX laser, comparison of the resonant and off-resonant images suggests that average offresonant signal comprises less than 5 percent of the on-resonant signal, indicating the relative signal to 

non-signal ratio is on the order of 19. Average background ranges for the OPO are much higher, 

comprising 25 to 75 percent of the on-line signal intensity and indicating a significantly lower signal to 

background ratio than that reported for the dye/UVX system. Although the signal/noise ratio for the OPO 

laser is comparatively low resulting from low fluorescence levels, the system still provides sufficient 

detail of OH location and flame structure for the atmospheric methane-air flame.

A region 225 pixels in height (shown in Fig. 6) was used to obtain the intensity profiles representing 

the general fluorescent characteristics of each flame. The region was averaged by column to obtain the 

profiles, which plot the average signal against the distance in pixels from the burner centerline 

(represented by the 0 location on the horizontal axis). The results from this analysis are displayed in 

Figures 7 and 8. The intensity profiles were used to aid in determining average flame structure as they 

reveal a level of asymmetry that is less apparent by visual examination of the OH PLIF images alone.

To obtain a sense of which excitation lines are least absorbing in a flame environment where no 

absorption by fuel or PAH molecules is expected, the profiles were used to derive an absorption estimate. 

(A thorough absorption analysis will be conducted to consider absorption by the PAH in jet fuel as a 

function of temperature and pressure at a later date.) The areas on either side of the centerline were 

computed by numerical integration to generate average input-side and output-side fluorescence values. 

Approximate absorbance for each laser, flame, filter and flow rate combination have been calculated 

using Equation (1) where I corresponds to the average output-side fluorescence and I0 represents the 

average fluorescence on the beam input-side. 















 = − 0

Absorbance ln

I

I (1)

The approach for calculating absorbance is used only for the purpose of general analysis and should 

be considered approximate. The OH distribution is assumed to be symmetric about the flame centerline so 

that differences in output signal may be solely attributed to absorption. Although the Hencken burner is 

considered to produce a well-characterized, stable, and symmetric flame, there may be images in which 

the flame appears non-symmetric. This asymmetry may be due to flow irregularities within the burner 

itself, or to outside (ambient) air flows interfering with the unshrouded flame. The estimated absorbance 

values are displayed above each profile. Occasionally, the calculated absorbances reported are negative, 

indicating higher OH signal on the laser output side of the flame. In these cases, flame asymmetry is also 

apparent with higher OH formation on the output side. (See for instance the OPO wide passband image 

for line R2(7) at φ = 1.)

Figure 6.—Highlighted region selected and averaged to generate intensity 

profiles.
NASA/TM—2011-216947 10

The dye/UVX results were generally consistent across all equivalence ratios and between filters. 

R1(10) is the least absorbing excitation line on average, (but also produced lowest fluorescence yield) 

followed by R2(7) and R1(1). In most cases, R2(7) is less absorbing than R1(1) by roughly 40 percent while 

also providing the highest fluorescent signal, thus making R2(7) the best excitation line in this portion of 

the study. 

The OPO results are much less conclusive, most likely due to the low signal/background levels. Only 

the narrow passband filter produced fairly consistent results that distinguished the R1(10) line as least 

absorptive, though with the same caveats as for the dye laser system. The R2(7) and R1(1) lines appear 

comparable.

Figure 7.—Average OH intensity profiles scaled collectively for OPO laser system. (X-axis: Distance in pixels to flame 

centerline, Y-axis: Average intensity). Laser beam transmitted right to left.
NASA/TM—2011-216947 11

Figure 8.—Average OH intensity profiles scaled collectively for UVX laser system. (X-axis: Distance in pixels to flame 

centerline, Y-axis: Average intensity). Laser beam transmitted left to right. 

5.0 Power Survey

In addition to the matrix of excitation/detection schemes, the effect of laser energy on flame imaging 

was studied for the dye/UVX system using the R2(7) excitation line and the filter with narrow passband.

Results for the 1.25 equivalence ratio flames are displayed in Figure 9. While all power settings resulted 

in OH PLIF images, the lowest power levels do not provide detail of the local OH distribution within the 

flame. Rather, as with the low output power OPO laser (6-mW average power), the images demonstrate 

only the existence of OH and a flame zone. As might be expected, the average induced OH fluorescence 

scales linearly with laser power. (See Fig. 10). Saturation is not evident for the laser powers used in these

experiments. 
NASA/TM—2011-216947 12

Figure 9.—Power survey for Hencken burner flame using dye/UVX laser at the R2 (7) excitation line. Laser 

beam transmitted from left to right.

Figure 10.—Image averaged OH PLIF signal at various laser power settings 

demonstrating that the fluorescent signal response is within the linear regime for the full 

range of powers. The solid line is the linear fit to the dye/UVX laser system power 

survey.

6.0 Flame Tube Combustor Experiments

Two sets of experiments were conducted using an optically accessible flame tube combustion rig. In 

the first, an atmospheric pressure propane/air Bunsen burner flame was positioned in the test section of 

the combustor to examine the quality of the beams after expansion and transmission into the rig. The 

flame was operated at atmospheric pressure. The second set of data presented was obtained during testing 

at elevated pressure and temperature using Jet-A fuel at conditions representative of an aircraft gasturbine combustor.
NASA/TM—2011-216947 13

Figure 11.—Schematic drawing illustrating the optically-accessible 

flame tube and window assembly.

6.1 Facility

The flame tube used for this set of experiments is a high temperature and pressure combustion rig 

with partial optical access directly downstream of the fuel injection site. The optical access allows nonintrusive optical and laser-based techniques to measure chemical and physical processes during 

multiphase combustion. The test cell is designed to supply non-vitiated inlet air to the combustor at 

temperatures between 450 and 1100 °F (505 to 866 K) and pressures between 150 and 250 psia (1034 to

1724 kPa) with flow rates up to 10-lbm/s (4.54 kg/s). Total equivalence ratios for stable flame tube 

operation are generally between 0.33 and 0.55, depending on simulated engine conditions. Because the 

data are acquired from a region directly downstream of the fuel injection site(s), local equivalence ratios 

can often be much higher, even surpassing unity. The rig is operated remotely from an adjacent control 

room and laser facility. (See Fig. 13). To ensure personnel safety, physical access to the facility is 

restricted during operation.

An isolated full-section view of the window assembly and aft flame tube are displayed in Figure 11. 

The combustor housing measures 0.74-m in length and is lined with a castable ceramic to withstand 

average temperatures reaching 3200 °F (2033 K). UV-grade fused silica windows (38.1- by 50.8-mm) 

are mounted just downstream of the fuel injection site and provide optical access to the primary reaction 

zone from three sides of the square combustion chamber. The chamber cross section measures 76.2- by 

76.2-mm. The bottom of the chamber is typically reserved for a spark igniter and provides no optical 

access. The grade of fused silica used in the windows offers a minimum light transmission of 90 percent

within a spectral range from 250- to 3300-nm, making it ideal for supporting optical measurements. 

Nitrogen film cooling is used internally to regulate window temperature and accounts for no more than 

10 percent by mass of the total inlet air flow. The external housing and downstream flange section are 

water cooled to reduce thermal and material stresses at high temperature and pressure conditions during 

operation.

6.2 Optical Setup

Figures 12 to 14 illustrate the complete beam transport system and optical paths of the OPO and 

dye/UVX lasers prior to transmission into the flame tube. Both beams were initially sent through irises to 

isolate the primary UV light and then reflected off three mirrors before being directed vertically upward 
NASA/TM—2011-216947 14

into a beam tube assembly, transported across the ceiling and sent back down into the flame tube 

combustor. The second mirror in the UVX path could be moved to project the UVX laser on the same 

collinear path as the OPO. The beam transport system consists of remotely-operated mirrors connected to 

traversing stages that permit three-dimensional PLIF data to be acquired by scanning across the flow 

field. The beams were allowed to expand ~15-m (by divergence) before being transformed into laser 

sheets. The long axis of the laser sheet was aligned with the test rig flow direction. The beams were

transformed using a crossed cylindrical lens pair, one with f = 3-m to focus the sheet in the test rig (to a 

width of ~0.3-mm) and the other with f = 400-mm to extend the length of the laser sheet so that it 

measured ~ 3 times the length of the window. The lens pair was used to improve the laser energy 

distribution across the window from the dye/UVX laser (which as shown previously, has a fairly nonuniform energy distribution). Extending the length of the laser sheet ensured use of the most uniform 

portion of the dye/UVX laser energy profile. Despite significant energy loss from this process, past work 

has indicated that using the full beam energy sometimes results in partial saturation, producing a nonlinear signal response. Although the OPO laser energy distribution was fairly uniform, its beam also 

passed through the cylindrical lens pair, thus losing considerable energy. Because the dye/UVX is 

currently the primary laser used for aircraft combustor tests, effort was made to optimize the dye laser 

energy distribution at the expense of the OPO.

Figure 14 is a diagram demonstrating the optical setup used for PLIF measurements in the optically 

accessible flame tube combustor facility. The ICCD camera used for this effort was the same one used for 

the methane/air burner study. 

Figure 12.—Optical Path from UVX and OPO lasers to beam tubes of transport system.
NASA/TM—2011-216947 15

Figure 13.—Beam transport system from laser facility to flame tube combustor test cell.

Figure 14.—Experimental PLIF setup relative to flame tube combustor rig.
NASA/TM—2011-216947 16

R1 (1) R2 (7)

Figure 15.—OH PLIF images from propane-air

flame positioned inside a flame tube combustor 

rig. Top row: Dye/UVX OH PLIF. Bottom row: 

OPO OH PLIF. Each row scaled separately.

Laser beam transmitted from top to bottom.

6.3 Bunsen Flame OH PLIF Images

As determined from the methane-air flame experiments, the R1(10) excitation line was eliminated 

from the flame tube part of this study because of low fluorescent signal. The wide passband filter 

(314.4-nm, 11.8-nm FWHM) was used in all remaining tests and images were obtained by accumulating 

on-chip averages of 200 single shots with 50-ns gates.

Figure 15 displays a comparison between the two laser systems using an atmospheric pressure 

Bunsen flame positioned inside the flame tube combustor. This preliminary test was used to compare the 

OH PLIF signal after the laser beams had been fully expanded and transported into the test section of the 

combustor rig. The propane-air burner offered a second gaseous flame with fewer complexities than the 

multi-component, liquid-fueled aircraft combustor flame. 

The propane-air flame images duplicate the results of the methane-air flame for fluorescence 

intensity. Trends between the image sets are comparable to the Hencken burner data, with greater image 

clarity observed for the dye/UVX configuration. The OPO was able to provide only enough detail to 

describe the general features of this atmospheric propane/air Bunsen flame.

6.4 Liquid-Fueled OH PLIF Flame Results

A nine-element fuel injector was studied for the liquid-fueled PLIF imaging portion of this work. The 

injector is described in Reference 11 and consists of a 3×3 array measuring 76.2- by 76.2-mm with 

injectors spaced 25.4-mm (center-to-center) on a planar rectilinear grid. Each injector element is 

comprised of a set of six helical, axial vanes angled at 60° surrounding a simplex nozzle that atomizes and 

injects liquid fuel (Jet-A) at the throat of a converging-diverging venturi. The calculated swirl number is 

1.0. A single injector element is displayed in the full-section view of Figure 16. The complete array is 

shown in Figure 17.

PLIF was used to image OH and fuel at two different operating conditions inside the combustion 

chamber of the flame tube. Test conditions are described in Table 2 with subscript 3 denoting the inlet 

conditions for air upstream of the injector array. During this set of tests, only the center injector was 

fueled. The surrounding eight injectors were supplied with air only. All data were acquired along the 

vertical centerline (Y = 0-mm plane) for the UVX and OPO laser configurations. R1(1) and R2(7) lines 

were tested at each operating condition.
NASA/TM—2011-216947 17

Figure 16.—Spatial positioning of the angled 

vanes, fuel nozzle and venturi for each 

injector.

Figure 17.—End view of nine-point LDI hardware (Left). Isometric view of nine-point with 

imaging plane (Right).

TABLE 2.—FLAME TUBE COMBUSTOR OPERATING CONDITIONS.

Test point P3

(psia)

T3 

(°F)

ΔP/P3

(%)

WA3

(lbm/s)

Φcenter Φtotal

A 150 1000 4 1.34 1.32 0.15

B 150 650 4 1.53 1.47 0.16

Figure 18 shows the processed PLIF images from both laser systems, with the dye/UVX results in the 

top row and the higher inlet temperature results (Point A) on the left. Because the fuel now contains 

naphthalene and its derivatives, significant fluorescent signal from fuel is present in the on-line and offline images. Subtracting the (OH) off-line excitation images from the on-line images results in areas with 

comparatively low OH signal, shown in black. These locations are most clear in the top row images for 

Point B, which show a conical structure from the incoming fuel spray. There may also be regions within 

the fuel spray zone that have strong signal due to either fuel, OH, or both.

Results indicate notable differences between the two operating test points and each laser. The 

dye/UVX system produced a much higher OH fluorescence signal than the OPO laser for each condition, 

compared to the atmospheric pressure flames, which may indicate that OH pressure-broadening effects 

are worse for the wider linewidth OPO than for the dye/UVX system. On the other hand, the lack of 

conical structures in the OPO images might indicate relatively low fluorescence from fuel. The OH signal
NASA/TM—2011-216947 18

Figure 18.—PLIF images for Jet-A fueled nine-point lean direct injector array. Center injector 

only fueled. Each row and test point is scaled individually. Flow is from left to right. 

seems to begin farther upstream in the OPO images compared to the dye/UVX images. Another feature 

that differs for the two laser systems is that the excitation line that provided the highest fluorescence 

intensity at atmospheric pressure (R1(1)) for the OPO changed at the higher pressure condition, but did 

not change for the dye/UVX system. For example, at Point A, the OPO R2(7) line gives higher 

fluorescence signal than the R1(1) line and for Point B, the fluorescence intensities are comparable; 

whereas for the narrow linewidth dye/UVX, the OH excitation results continue to show that the R2(7) line 

gives greater fluorescence signal. This result may be an indicator that R2(7) couples to OH better at higher 

pressure for the OPO laser. These results reinforce the need to perform more controlled experiments in a 

high temperature, high pressure vessel for absorption and fluorescence yields to compare the laser power, 

linewidth and coupling (overlap) to both OH and fuel molecules.

7.0 Summary

OH PLIF excitation/detection strategies have been tested and compared in laminar and turbulent reacting 

flows using a Nd:YAG pumped dye laser with UVX and a Nd:YAG pumped OPO. OH generators have 

included an atmospheric methane/air flame from a Hencken burner, an atmospheric propane-fueled Bunsen 

burner and a high temperature and high pressure Jet-A fueled lean flame tube combustor. Results indicate that 

the Nd:YAG pumped OPO with significantly lower power and broader linewidth is inadequate for resolving

the flame structure via qualitative OH concentration measurements in the high pressure combustor. A low 

signal-to-noise ratio for the OPO along with apparent high-pressure induced effects cause a reduction in the 

total OH induced fluorescence signal. The higher energy and narrow linewidth dye/UVX combination is

capable of resolving OH at atmospheric conditions and performs reasonably well at high pressure and 

temperature. In the majority of experiments from this work, the R2(7) line appears to induce the strongest OH 

fluorescence, followed by the R1(1) and R1(10) transition lines. The wide (11.8-nm FWHM) passband filter 

permitted the majority of OH fluorescence to pass, while still restricting much of the strong interference from 
NASA/TM—2011-216947 19

fuel fluorescence. Absorbance calculations indicate that R1(10) is the least absorbing line on average in the 

methane/air flame experiments, followed by the R2(7) and R1(1) lines.

8.0 Future Work

In the next phase of this research, absorption measurements will again be performed to analyze the 

properties of samples of Jet-A fuel in an optically accessible high temperature, high pressure (HTHP) vessel. 

The vessel is designed to support sample temperatures from ambient to 800 °C at a maximum pressure of 

1000-psi.

Refined OH PLIF will be performed using the same flame tube combustor rig. The OPO laser will be 

retrofitted with an injection seeder system that will produce a ~25 percent increase in laser energy with a 

~50 percent reduction in linewidth. Results will be compared with those contained herein and improvements in 

performance will be noted. A beam profiler will be used so that imperfection in the laser energy distribution 

over the length of the laser sheet can be corrected.

The methane/air flame images, HTHP vessel and preliminary flame tube combustor results will be used to 

select a final excitation, extraction and correction method for acquiring advanced OH PLIF measurements. 

With the establishment of this optimal configuration, OH PLIF will continue to be used to characterize the 

operation of advanced low-emission combustion strategies in a variety of high temperature and pressure 

reacting environments. 

References

1. Hanson, R.K., Seitzman, J.M., “Planar Laser Fluorescence Imaging of Combustion Gases,” Journal of 

Applied Physics B, Vol. 50, No. 6, 1990, pp. 441–454. 

2. Hicks, Y., Locke, R., Anderson, R., Ockunzzi, K., “Planar Imaging of Hydroxyl in a High 

Temperature, High Pressure Combustion Facility,” NASA TM–107074, Oct. 1995.

3. Seitzman, J., Hanson, R., “Comparison of Excitation Techniques for Quantitative Fluorescence 

Imaging of Reacting Flows,” AIAA Journal, Vol. 31, No. 3, Mar. 1993, pp. 513–519.

4. Hicks, Y., Locke, R., Anderson, R., Zaller, M., Schock, H., “Imaging Fluorescent Combustion 

Species in Gas Turbine Flame Tubes: On Complexities in Real Systems,” NASA TM–107491, 

Jun. 1997; also AIAA Paper 97–2837, July 1997. 

5. Dyer, M., Crosley D., “Two-Dimensional Imaging of OH Laser-Induced Fluorescence in a Flame,” 

Optics Letters, Vol. 7, No. 8, 1982, pp. 382–384.

6. Cooper, C., Laurendeau, N., “Quantitative Laser-Saturated Fluorescence Measurements of Nitric 

Oxide in a Heptane Spray Flame,” Journal of Combustion Science and Technology, Vol. 127, 

Jun. 1997, pp. 363–382.

7. Lozovsky, V., Cheskis, S., Derzy, I., Kachanov, A., Romanini, D., “Cavity Ring-Down Spectroscopy 

of OH Radicals in Low Pressure Flame,” Journal of Applied Physics B: Lasers and Optics, Vol. 66, 

No. 3, 1998, pp. 377–381.

8. Baranger, P., Orain, M., Grisch, F., “Fluorescence Spectroscopy of Kerosene Vapor: Application to 

Gas Turbines,” Proceedings of the 43rd AIAA Aerospace Sciences Meeting and Exhibit, AIAA–

2005–828, Jan. 2005.

9. G.H. Dieke and H. M. Crosswhite, “The Ultraviolet Bands of OH Fundamental Data,” J. Quant. 

Spectrosc. Radiat. Transfer, Vol. 2, pp. 97–199, 1962. 

10. Allen, M., Hanson, R., “Digital Imaging of Species Concentration Fields in Spray Flames,” 21st

Symposium (International) on Combustion, Combustion Inst., Pittsburgh, PA, 1987, pp. 1755–1761.

11. Tacina, R., Lee, P., Wey, C., 2005, “A Lean-Direct-Injection Combustor Using a 9 Point SwirlVenturi Fuel Injector,” ISABE 2005–1106, 2005.
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Excitation/Detection Strategies for OH Planar Laser-Induced Fluorescence Measurements in 

the Presence of Interfering Fuel Signal and Absorption Effects 

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Heath, Christopher, M.; Anderson, Robert, C.; Hicks, Yolanda, R.; Locke, Randy, J. 

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WBS 984754.02.07.03.19.03 

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John H. Glenn Research Center at Lewis Field 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Planar laser-induced fluorescence (PLIF) excitation/detection methods have been applied to obtain spatial distributions of the hydroxyl [OH] 

reacting intermediary and hydrocarbon [HC] primary species in laminar and turbulent combustion reactions. In this report, broadband and 

narrowband excitation/filtering techniques are explored to identify an optimal experimental configuration yielding significant fluorescent 

signal with low absorption losses. The combustion environments analyzed include 1) a laminar non-premixed methane/air flame and 2) a 

turbulent, non-premixed Jet-A/air fueled flame within a lean flame tube combustor. Hydrocarbon-based fuel and OH were excited via the R1

(1), R1(10) and R2(7) transitions of the A2Σ+ X 2Π(1,0) band using a broadband Nd:YAG pumped optical parametric oscillator (OPO) and 

narrowband Nd:YAG/dye laser with ultraviolet frequency extension (UVX) package. Variables tested for influence on fluorescent signal 

and absorption characteristics were excitation line, laser energy, exciting linewidth, combustion reactants, and test flow conditions. Results 

are intended to guide the transition from a dye/UVX laser to an OPO system for performing advanced diagnostics of low-emission 

combustion concepts. 

15. SUBJECT TERMS

Laser diagnostics; Aircraft gas turbine combustion; Planar imaging 

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