SOLUTION SHEET

Time-resolved OH LIF System
High-speed flame front imaging in turbulent combustion

The state-of-the-art solution for time-resolved flame front imaging
Flame front visualization by means of OH planar Laser-induced Fluorescence (LIF) can
now be performed at frame rates high enough to capture the rapid dynamics in avariety of combustion applications, such as ignition phenomena, flame propagation andlocal extinction events. Our time-resolved planar OH LIF system integrates the latest
developments in high-speed imaging and laser technology with user-friendly softwarefor handling high-speed data acquisition and analysis of even the most demanding
turbulent combustion processes.

Key benefits•

Advanced Nd:YAG and tunable dye lasers designed for repetition rates up to 10 kHz•

High quality UV light-sheet optics•

Wide range of high-speed CMOS cameras•

UV-sensitive high-speed image intensifiers•

Synchronizer with ultra-high temporal precision in the sub-nanosecond range•

Complete software platform for system set-up, data acquisition, data analysis andvisualization of resultsA powerful tool for studies of turbulent combustion phenomena
Our time-resolved OH LIF system utilizes a non-intrusive (optical), two-dimensional
measurement technique based on the Laser-induced Fluorescence emission of
thehydroxyl radical, OH, produced during combustion. The beam from a high-speed
tunable laser is formed into a light-sheet which illuminates a thin planar sectionof the combustion process and excites the OH radicals, which subsequently emit
fluorescent light. The fluorescence is recorded by an intensified camera, creating amap of the OH or flame front distribution in the plane.
Through user-friendly hardware and software, the user quickly gains full confidence
to operate the system. As the backbone of our systems, DynamicStudio is the mostcomprehensive imaging software for optical diagnostics. From within our software,
you can easily control the measurement system and perform advanced image
data analysis to extract valuable information and a better understanding of thecombustion process being studied.
When required, the system can also control a second laser and camera e.g. forsynchronized velocity measurements by PIV.

The TR OH LIF solution in brief
The system is comprised of a state-of-the-art Nd:YAG-pumped, tunable dye laser
specially designed for applications at high repetition rates, a high-speed CMOS
camera equipped with a UV sensitive image intensifier, advanced electronics for
hardware synchronization, and software for straightforward data acquisition andanalysis.

Overview of the different components.

Laser light source and light sheet optical components
As laser pulse energy is a critical property for time-resolved LIF systems, the choiceof laser source is of great importance. Time-resolved OH LIF measurements arecarried out using an advanced Nd:YAG-pumped, tunable dye laser to generate laser
pulses for OH excitation at around 283 nm.
The Nd:YAG pump source is a diode-pumped laser with short pulse duration in therange of 10 ns and is designed to work in conjunction with a tunable dye laser. To
ensure that the laser fulfils functional requirements, every single laser is optimized
for the repetition frequency and pulse energy according to the application at hand.
To gain the highest energy level outputs, the dye laser is equipped with a dedicatedSecond Harmonic Generator (SHG) unit optimized for conversion of visible lightinto UV. In order to select the desired absorption peak of OH, the laser output isfine-tuned by scanning the wavelength. The SHG allows tuning of the output beam
within the range of 220-320 nm.
The SHG mount is motorized so that crystal phase matching is done automaticallythroughout the wavelength scan. This means that the you will always have
optimum pulse energy out of the dye laser without the need of manualadjustments. This allows you to focus on the application rather than on themeasurement equipment.
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The dye laser features:• Linewidth 0.06 cm-1 at 580 nm, providing more efficient excitation of 		
narrow absorption lines, and thus increased signal-to-noise ratio•

Digital wavelength selection making it easy to control from a computer•

Automated crystal phase matching throughout wavelength scanning andthus no need for manual adjustments

Optical devices used to form parallel UV light sheets are included with thesystem. Light sheet dimensions are approx. 50 mm x 0.1 mm (when used witha beam waist adjuster) with a focal length of 600 mm.

UV / visible parallel light sheet optics

To ensure a secure working environment, UV and visible light - from theNd:YAG laser to the location at which the light sheet is formed - is containedin a closed environment. Laser alignment goggles and protective goggles arealso available.

Cameras, Image Intensifiers and Lenses
We offer several types of CMOS cameras with different frame rate, pixel
size, light sensitivity and inter-frame shutter speed characteristics. Amongthe highest performance cameras recommended for time-resolved planarLIF measurements, you will find the SpeedSense 1212. With a full sensor
resolution of 1,280x800 pixels, it can handle acquisition speeds of up to 12.6
kHz. In combination with the large pixel size (with high light sensitivity), theSpeedSense 1212 is an excellent choice for time-resolved LIF measurements.
The camera and image intensifier are separate units which allows existing
hardware systems to be upgraded for combustion applications. This solution
is ideal for laboratories dealing with multiple applications such as PIV aswell as LIF in liquid and gaseous phases including combustion processes,
microscopy and spectroscopy.

SpeedSense CMOS camera

The image intensifier units are designed for UV, visible and near-IR
applications using a broadband photocathode in order to cover a wide
range of applications. The units offer full control over gain and gate time
options ranging from continuous to ultra-fast. For high-speed applications
two options are available: The L Series with a minimum gate time of 40 ns
and a maximum repetition rate of 100 kHz, and the H Series with a minimum
gate time of 10 ns and a staggering 200 kHz repetition rate. The short gate
time of the H series makes it the best choice for ultra-fast and light intensive
applications such as sparks and ignition events.
To match the intensifiers, a 100 mm camera lens with optimized transmission
for UV light is available. For OH fluorescence detection, a dedicated, narrow,band-pass interference filter is included.

High Performance Synchronizer
Synchronization between laser pulses, cameras and image intensifiers ishandled by our High Performance Synchronizer fully controlled via theDynamicStudio software.
The synchronizer is a multi-channel delay and signal generator with < 1 nsresolution. The 32 output channels are programmed independently; thereby
offering comprehensive synchronization and triggering facilities for highly
demanding processes. It also features four input channels for synchronization
with external devices or events, two analog inputs with up to 500 kHz sample
rate, and four analog outputs. For further details, please consult separate
data sheet on “Imaging Synchronizers”.

User-friendly imaging software platform - Straightforward data analysis
The image acquisition system is based on DynamicStudio, a comprehensive anduser-friendly software platform for imaging applications. It features an advanced
plug-and-play-based distributed system, making it easy for you to have fullcontrol over the hardware for image acquisition. Data acquisition as well asPage 3

The High Performance Synchronizer

analysis is supported within the same software, so there is no need to spend
time exporting the data for further analysis.
With easy access to numerous numerical methods as well as quick and accurate
calibration and processing methods, users will experience the benefits ofworking with a truly user-friendly software platform.
Numerical methods available with the software:•Processing based on:◊ LIF signal filtering and amplification correction◊ Energy correction (including light sheet profile correction, fluence andenergy budget)◊ The physics of fluorescence based on combustion radical being studied- Options include state population calculations, emission frequency, andabsorption cross-section coefficient.

•Statistics on LIF images and scalar maps•

Flame Front / Phase Boundary Detection•

For combined PIV and OH LIF measurments: Flame front detection followedby high resolution velocity analysis across the flame front.

Left: A single frame from simultaneous TR OH LIF and TR PIV in a lifted flame. Right: Flame front
detection using OH LIF (black line), allowing phase separated PIV analysis in the burnt (red) andunburnt (blue) regions, for improved PIV result.

The software features many automated options such as laser pulse energy correction(on each individual LIF image), hardware calibration and processing routines etc.
DynamicStudio also features an Image Processing Library (IPL), which is a comprehensive “electronic catalog” of numerical methods; offering quick access to image
quality enhancement, image filtering and other more advanced numerical processing
to get the best possible results from LIF images. Processing is fully automated andincludes macro-analysis capability (i.e. sequence of processing methods).

Boosting analysis capabilities with analysis sequences and MatLab® Link
The data analysis is further strengthened by the possibility of defining analysis
sequences and performing batch processing which significantly speeds up theprocess of working with large data sets. And with the MatLab Link in DynamicStudio,
you can tailor your analysis by calling up your personally designed MatLab scripts
directly from DynamicStudio, allowing you to process the data directly in thedatabase without the need of first exporting the data.

Animation and AVI movie creation
The acquired and/or processed image data can be browsed immediately at reduced
frame rates for quick visualization of the process and evaluation of the data quality.
When needed, AVI movies can be created (with full control over frame rate).
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Simultaneous control of multiple systems
The modular design of the systems allows the user to control several different
systems at the same time from one master computer. In this way, a combustion LIF
system can be combined with a PIV system for simultaneous combustion species
measurements and flow velocity measurements. Alternatively, two LIF systems canbe combined to measure two species simultaneously.

Specifications
Pump laser
Wavelength (nm)532

Pulse energy (mJ)

9 (from 0-10kHz)

Pulse duration (ns)<10Repetition rate (kHz)0-15

Tunable dye laser
Fundamental tuning range, 2400 l/mm grating
Linewidth
UV extension unit

< 0.06 cm1 at 580 nm
External frequency doubling unit incl. 4 Pellin-Broca

Frequency doubling crystal

CameraSpeed (fps)330-740 nm

BBO 220-320 nm

SpeedSense Lab 310

SpeedSense 1212

3,260

12,6001280 x 800

1280 x 800

Pixel size (µm)

2028

Lens mount type

FFSensor resolution (pixels)

Image intensifiers

H Series High-speed 1

L Series High-speed

Maximum repetition rate

200 kHz100 kHz

Minimum gate time

10 ns 1

40 ns

Photocathode diameter

23.9 mm

25 mm

Photo cathode material

Multialkali 2

S20 2

Phosphor screen

P46

P46

SoftwareDynamicStudioBase package
Combustion LIF Add-on for DynamicStudioLiquid and Gas LIF Add-on for DynamicStudio

12

Best choice for ultra-fast and light intensive applications such as sparks and ignition events.
Both Multialkali and S20 has a wide spectral response from the ultraviolet to near infrared region.

Solution Sheet 0568_v2 . Subject to change without notice. Copyright © 2019.
Dantec Dynamics. All Rights Reserved. www.dantecdynamics.com