i

UNIVERSITY OF WATERLOO

Faculty of Engineering

Department of Chemical Engineering

Diffuse Two-Dimensional Line-of-Sight Attenuation method for Soot

Concentration Measurements using a Pulsed LED Source and an

Intensified-CCD Sensor Camera

National Research Council Canada

Ottawa, ON, Canada

Prepared by

Syed Mohammad Mukarram

20495674

September 14, 2015

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564 High Point Avenue Waterloo, ON N2l 4N1 September 15, 2015 Dr. Eric Croiset, department chair

Chemical Engineering

University of Waterloo

Waterloo, ON

N2L 3G1

Dear Sir,

This report, titled “Diffuse Two-Dimensional Line-of-Sight Attenuation method for Soot

Concentration Measurements using a Pulsed LED Source and Intensified CCD Sensor Camera”, is

written to fulfill the requirements of the WKRPT 300 course following my 3A academic term.

This report is based on my experience as a Black Carbon Metrology student in the Black Carbon

Metrology Group (BCM) at the National Research Council (NRC). This group focuses on the

metrology of black carbon, commonly known as soot. The team leader for the group is Dr. Kevin

Thomson and my supervisor, who is also a member of this group, is Dr. Meghdad Saffaripour.

I was responsible for setting up and conducting diffuse two-dimensional line-of-sight

attenuation (2D-LOSA) measurements to determine soot volume fraction in a laminar diffusion

flame and to show that beam steering does not affect the results. In addition, I was required to

improve upon the work done by a pervious co-op student and reproduce the results from

papers published by the researchers in the BCM group. The objective was to obtain results with

lower shot noise uncertainty.

I would like to thank Dr. Kevin Thomson for providing valuable insights, suggestions and

potential solutions and Dr. Meghdad Saffaripour for providing guidance, step-by-step

procedure for completing the project and proofreading this report. I would also like to thank

Dan Clavel for his help in the setup of various instruments and Bob Sawchuk for his suggestions

on improvements. I also confirm that this report was written entirely by me and has not

received any previous academic credit at this or any other institution.

Sincerely,

Syed Mohammad Mukarram 2049564

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Summary The main purpose of this report is to document improvements made to the 2-D LOSA soot

concentration measurement method by lowering the uncertainty and the shot noise associated

with the measurements. The uncertainty in question is of the soot volume fraction (SVF) and

optical thickness (OT). Another main objective was to prove that beam-steering effect is

negligible. Two wavelengths were used to calculate these data, 455 nm and 690 nm and so a

comparison of the data obtained from these wavelengths has also been made.

The beam steering effect was proven to be eliminated from the current 2-D diffuse LOSA

system as the plots of the transmissivity were observed to be constant lines with a maximum

difference of almost 0.1% among the data points. This meant that the deflected beams

followed the same path to the iCCD as the non-deflected beams after passing through the

flame. Eliminating the beam steering phenomenon meant that the accuracy of the

measurements is improved.

When quantifying the source-to-flame intensity ratio, it is noted that at the 690 nm LED

experiments, this intensity ratio does not increase if the on-CCD accumulations are increased.

The intensity ratio remains constant for a particular height of the flame. Although the intensity

ratio varies throughout the flame, it is not effected by increasing on-CCD accumulations. For the

455 nm LED experiments, the source-to-flame intensity ratio increases when the on-CCD

accumulations are increased.

Through various experiments, it was observed that increasing the number of frames

taken for averaging the measurements decreases the uncertainty in both the SVF and OT and

also, it decreases the shot noise in those measurements. When the number of frames were

increased from 25 to 100, the uncertainty of the OT and SVF decreased from 1.226*10-3 to

5.324*10-4 and from 0.113 to 0.05 respectively for the 455 nm LED and from 1.561*10-3 to

8.457*10-4 and 0.228 to 0.124 respectively for the 690nm LED experiments. Increasing the

number of on-CCD accumulations had a similar impact as increasing the number of frames. For

the 455 nm LED experiment, when the on-CCD accumulations was increased from 2000 to

6000, the uncertainty of the OT decreased from 1.275*10-3 to 6.723*10-4 and the uncertainty of

the OT decreased from 1.561*10-3 to 1.044*10-3 when the on-CCD accumulations where

increased from 300 to 900 for the 690 nm LED experiment.

For future experiments, it is recommended that more than 100 frames for averaging

should be used in order to further reduce shot noise and more than 900 on-CCD accumulations

should be used for the 690nm wavelength. When increasing the on-CCD accumulations, care

should be taken so as to avoid over saturation of the on-CCD camera.

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Contents Summary .......................................................................................................................................... i

List of figures ................................................................................................................................... iii

List of tables .................................................................................................................................... iii

1.0 Introduction ......................................................................................................................... 1

1.1 Project Background .......................................................................................................... 1

1.2 Objective .......................................................................................................................... 2

1.3 Theory .............................................................................................................................. 2

1.0 Experimental Setup .............................................................................................................. 4

2.1 Experimental Procedure ................................................................................................... 8

2.0 Results and Discussions ....................................................................................................... 9

3.1 Source-Flame Intensity for Ethylene-Air experiment ...................................................... 9

3.2 Beam-Steering ................................................................................................................ 11

3.3 Soot Volume Fraction and Optical Thickness ................................................................. 13

3.4 Uncertainties .................................................................................................................. 18

3.0 Conclusions ........................................................................................................................ 21

4.0 Recommendations ............................................................................................................. 22

Glossary ......................................................................................................................................... 23

References .................................................................................................................................... 23

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List of figures Figure 1: Beam-Steering.................................................................................................................. 2

Figure 2: Experimental Setup .......................................................................................................... 4

Figure 3: Schematic of diffuse 2D-LOSA setup (This is the 2007 setup and the image is taken

from [3]) .......................................................................................................................................... 4

Figure 4: Major components of the iCCD (image taken from Princeton Instruments PI-MAX User

Manual) ........................................................................................................................................... 6

Figure 5: Data acquisition using 690 nm wavelength LED .............................................................. 7

Figure 6: Twenty-five shot average images collected with diffuse 2D-LOSA for a coannular

ethylene-air nonpremixed Gülder burner flame at the first step height ....................................... 7

Figure 7: Radial profiles of source-to-flame intensity ratio using 690 nm wavelength LED at HAB

33 mm ............................................................................................................................................. 9

Figure 8: Radial profiles of source-to-flame intensity ratio using 455 nm wavelength LED at HAB

33 mm ........................................................................................................................................... 10

Figure 9: Image of a mixture of methane and nitrogen flame nonpremixed with air. The blue

region at the bottom of the flame does not contain soot and is used for data acquisition ........ 12

Figure 10: Normalized transmissivity curves at 455 nm. Plot (a) and (b) show data at HAB 10 mm

and HAB 30 mm respectively ........................................................................................................ 12

Figure 11: Normalized transmissivity curves at 690 nm. Plot (a) and (b) show data at HAB 10 mm

and HAB 30 mm respectively ........................................................................................................ 13

Figure 12: Effect of beam steering on diffuse 2D-LOSA (image taken from [3]) .......................... 13

Figure 13: (a) Ethylene flame used for SVF experiments and (b) SVF distribution in the flame

(image (b) is taken from [3]) ......................................................................................................... 15

Figure 14: SVF at various heights above the burner ..................................................................... 16

Figure 15: Radial profiles of optical thickness at various heights above the burner. (a) shows

optical thickness at a HAB of 10 mm, (b) at a HAB of 20 mm, (c) at a HAB of 40 mm and (d) at a

HAB of 50 mm ............................................................................................................................... 17

Figure 16: Effect of increasing number of frames ........................................................................ 19

Figure 17: Comparison of σoptical thickness at various heights above the burner. (a) shows σOptical

Thickness at a HAB of 10 mm, (b) at a HAB of 30 mm and (c) at a HAB of 50 mm ........................... 20

List of tables Table 1: Intensity values at 455nm and 690nm and at a HAB 33 mm ......................................... 11

Table 2: Parameter values for ethylene-air SVF experiment ........................................................ 14

Table 3: 𝑲𝝀(𝒆) values at different wavelengths, heights and radius ......................................... 14

Table 4: Uncertainties at step height 2 for ethylene-air experiment ........................................... 18

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1.0 Introduction

1.1 Project Background

Black carbon, commonly known as soot, is a major component of particulate matter which is

produced by emissions from industrial combustors and engines. It has a significant impact on

air quality and human health, and is now being considered as an important factor in global

warming [1, 3]. In order to reduce and regulate the amount of emitted soot, the metrology of

soot should be well understood.

One of the current methods that researchers are using to determine the soot

concentration in combustors is the two-dimensional line-of-sight attenuation (2D-LOSA). This

method involves directing a light source through an attenuating medium which contains soot.

As the light passes through the attenuating medium, some of it gets absorbed by soot and

other particulate matter, some of the light gets scattered and the rest of the light passes

through the attenuating medium and reaches the capturing device which measures light

intensity. The device that measures light intensity is usually a charge-coupled device (CCD) or

complementary metal-oxide-semiconductor (CMOS) sensor. Through this method, the soot

volume fraction (SVF) in the attenuating medium can be obtained using a correlation between

the loss of light intensity passing through the attenuation medium and the soot concentration.

The diffuse 2D-LOSA system used for the present study was developed by Thomson et

al. [3] which offers a high sensitivity for soot volume fraction, resolving optical thickness down

to 0.001. However, this system does suffer from sensitivity loss in heavily-sooting regions of the

flame because of a strong flame (attenuating medium) emission intensity which is comparable

to the light source intensity [5]. An improvement to this system involves using a pulsed LED

(light emitting diode) as a light source which would produce a much higher light source

intensity compared to the flame intensity. Also, in this report, an intensified CCD (iCCD) sensor

was used instead of a CCD sensor.

A phenomenon which is observed in the 2D-LOSA system is beam steering, which causes

the beam of light produced from the light source to deviate from its original path when it

passes through the flame. This occurs due to the presence of high temperature gradients within

the flame which causes refractive index gradients that bend light passing through the flame.

Beam steering can cause a loss in sensitivity and also creates a fixed noise in the measurements

as the noise remains unchanged when the measurements are repeated given that beam

steering remains unchanged [3]. A simple sketch of the beam-steering effect is shown in Figure

1.

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1.2 Objective

The main objectives of this study are to prove that the effect of beam steering on SVF results

can be removed using diffuse 2D-LOSA setup, repeat the soot volume fraction experiments

from previous research paper [3, 5] with improvements, using light source with two

wavelengths, 455 nm and 690 nm respectively and lower the uncertainties of the soot volume

fraction and optical thickness and the contribution of shot noise to those uncertainties. Other

objectives included quantifying source to flame intensity.

1.3 Theory

The governing equation to find the soot volume fraction, SVF, within the flame is

𝑓𝑣 =

𝐾𝜆(𝑒)𝜆

6𝜋𝐸(𝑚)𝜆

This equation is derived from the Rayleigh-Debye-Gans polydisperse fractal aggregate (RDG-

PFA) theory. 𝐾𝜆(𝑒) is the local extinction coefficient and 𝐸(𝑚)𝜆 is the soot absorption function.

Although 𝐸(𝑚)𝜆 changes depending on the wavelength being used, it will be held constant for

purposes of comparison. 𝜆 is the wavelength of the light source used for data acquisition. In the

Mathcad script, the flame is divided into two halves and the soot volume fraction of both the

halves is calculated and reported as an average. 𝐾𝜆 can be determined from the following

equation:

−ln𝜏 = ∫ 𝐾𝜆

(𝑒)𝑑𝑠∞

−∞

In the equation 2, 𝜏 is transmissivity and −ln𝜏 is the optical thickness (OT). OT is a measure of

how opaque the attenuation medium is to the incident radiation. 𝜏 is calculated using the

following equation:

𝜏 =

𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛 − 𝑓𝑙𝑎𝑚𝑒

𝑙𝑎𝑚𝑝 − 𝑑𝑎𝑟𝑘

In order to calculate the transmissivity, four different images need to be captured: one without

the light source and the flame (dark), one with the light source and without the flame (lamp),

(1)

LED Flame iCCD

Figure 1: Beam-Steering

(2)

(3)

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one with the flame and without the light source (flame) and one with both the light source and

the flame (transmission). These four images are used in equation 3 to calculate the

transmissivity 𝜏. Each image can contain several frames which are used for averaging the data

and are also used to provide the standard deviation (uncertainty) so as to assess the quality and

reliability of the final output. The negative natural log of 𝜏 is calculated and the result is

smoothed using a locally weighted polynomial regression (LOESS) method. The smoothed data

is then deconvoluted using a one-dimensional tomography method three-point Abel inversion

[2] to find 𝐾𝜆(𝑒). The image processing and the data calculation are done using an existing

Mathcad script.

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2.0 Experimental Setup The setup for the diffuse 2D-LOSA requires fine alignment of the optics, the iCCD and the

integrating sphere in order to perform data acquisition. Figure 2 shows an image of the

experimental setup.

Figure 2: Experimental Setup

Figure 3 below shows a schematic of a previous setup which was used in [3]. The figure

explains how the light beams from the LED reach the CCD. In both the setups, the light follows

the same path to the CCD/iCCD. The differences between this setup and the current setup from

figure 2 are that an LED connected to a driver was used instead of an arc lamp, the band-pass

filter was attached to the flame imaging lens and an iCCD was used instead of a CCD. A sigma

lens was used for the flame imaging lenses instead of a pair of lenses.

Figure 3: Schematic of diffuse 2D-LOSA setup (This is the 2007 setup and the image is taken from [3])

iCCD Flame imaging lens

Band-pass filter

Burner nozzle

Integrating sphere

LED driver

Light source imaging lenses

Exhaust fume

fan

PTG&

controller

Data acquisition

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In Figure 3, light shading represents the total light that exits the integrating sphere and dark

shading represents the rays for one line-of-sight chord through the medium. The dotted line is

the smaller solid angle of the line-of –sight chord that is collected by the flame imaging lens [3].

The advantage of having a controlled LED over an arc lamp is that the intensity of the light

emitted from the LED can be increased by using short duration pulses. An iCCD is a modified

version of a CCD which allows intensification of photons. The sigma lens operates in a similar

manner as the pair of lenses shown in figure 3 but the advantage of using the sigma lens is that

the magnification of the image can be varied and aperture size (f-number) can be varied.

Increasing the aperture size can allow more light to enter the iCCD.

In the current setup, the LED driver (HPLS-36DD7500) which sits on top of the

integrating sphere has an LED connected to it at the bottom. The LEDs used for these

experiments are of either 455 nm wavelength, which emits blue light, or of 690 nm wavelength

which emits red light. The LED driver has a duty cycle of up to about 3.3% and is capable of

driving the LED at a high power using short pulses, between 50 ns and 40 µs. The pulse width

used in these experiments was 39 µs. A higher pulse width results in a higher intensity recorded

by the iCCD. The LED driver is connected to the controller (ST-133B). The type of controller used

is called PTG and it also allows triggering the LED driver and the iCCD. Through triggering, the

pulse width of the LED and the frequency of pulses can be controlled and it also allows

synchronizing the iCCD and the light source.

The light from the LED reflects several times from the diffuse reflective surface inside

the integrating sphere after which it leaves the 25 mm diameter exit of the integrating sphere.

The exiting light is a highly Lambertian diffuse light source and is then collimated by the first

source imaging lens. The distance between the exit port of the sphere and the collimating lens

is 98 mm. The focal length of the collimating lens is 101.6 mm and the focal length of the

focusing lens is 152.4 mm which results in a 1.5 nominal magnification. Both the lenses have a

diameter of 50 mm and are plano-convex lenses. The distance between these two lenses is 105

mm. Upon reaching the second source imaging lens, the light is focused by this lens onto the

flame produced by the burner nozzle. After passing through the flame, the light enters the

flame imaging lens where it is focused onto the iCCD camera sensor. The distance between the

burner and the focusing lens is 113mm and the distance between the flame imaging lens and

the burner is 152 mm.

The laminar diffusion flame is created using a Gülder burner used in the previous diffuse

2D-LOSA system [3]. Two different flames are used in these experiments: a non-premixed

ethylene-air flame for soot volume fraction measurement experiments and a diluted methane-

nitrogen with non-premixed air flame for beam steering experiments. The fuels exit a circular

mild-steel tube (burner nozzle) with a 12.7 mm outer diameter and inside diameter of 10.9 mm.

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The burner produced a laminar diffusion flame which is a stable source of soot. The flame has

an axisymmetric profile. The air exits through a co-annular tube of 88 mm inner diameter. The

burner is surrounded by a steel mesh with viewing ports parallel to the optical axis of the

system. The air flows upwards the steel mesh and reduces flame movements caused by air

currents of the room near the setup.

The flame imaging lens is a commercial camera lens produced by Sigma. The f-number

used is 16 and the magnification ratio used is 1:1. For the 455 nm wavelength LED, a bandpass

filter centered on 442 nm with bandwidth 46nm was used and for the 690nm wavelength LED,

a bandpass filter centered on 684 nm and bandwidth 24nm was used. The bandpass filters only

allow a small range of wavelength of light to pass through depending on the bandwidth and the

wavelength on which the filter is centered. The bandpass filters are mounted onto the flame

imaging lens and the lens is mounted onto the iCCD camera.

Figure 4: Major components of the iCCD (image taken from Princeton Instruments PI-MAX User Manual)

Figure 4 shows an image of the process by which the photons are converted to data that

can be displayed on a computer monitor. The iCCD (PI-MAX 1300) uses a sensor with a 1340 by

1300 array (resolution) of wells that each correspond to a pixel when the data is read out of the

array. For experiment purposes, 833 by 929 arrays (resolution of region of interest) were used.

When the light enters the iCCD, the photons collide with the photocathode to release electrons

with each collision. These electrons are attracted to the microchannel plate (MCP) which is

more positive than the photocathode. The MCP end furthest from the photocathode is more

positively charged and the voltage of this MCP end can be increased by increasing the gain

setting of the iCCD. When the electrons move through the MCP, they collide with the channel

walls generating additional electrons, resulting in electron gain. Increasing the difference in

voltage between the MCP ends will result in an increasing number of addition electrons. The

electrons exit the channels and strike the fluorescent screen releasing protons. These photons

are transferred to the surface of the iCCD where they produce charge at the pixels they strike.

Charge accumulates in the pixel wells until the intensifier is gated off. Finally, the charge is read

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out of the array and converted from an analog to a digital signal which is recorded as an

intensity (count) value.

Winview 32 is the software used to for data acquisition for these experiments. Through

the winview software, the pulse width (gate width) of the LED and the iCCD, the gate delay of

the iCCD (a gate width of 9 ns and a gate delay of 1ns means that the iCCD is exposed to the

light for 8ns), the number of frames required for averaging, the gain setting, the frequency of

analog to digital conversion fA/D, the frequency of trigger fgate and the on-CCD accumulations

can be altered (on-CCD accumulations refers to the number of times the iCCD is gated on and

off during an exposure so as to store charge in the array well and read it out altogether. This

increases the value of the intensity of the recorded light and can be used for high sensitivity

soot volume fraction measurements in heavily-sooting regions of the flame). Figure 5 shows an

image of the setup during data acquisition and Figure 6 shows images of data acquisition as

seen in the Winview software.

Figure 5: Data acquisition using 690 nm wavelength LED

Flame Lamp Transmission Dark Transmissivity

Figure 6: Twenty-five shot average images collected with diffuse 2D-LOSA for a coannular ethylene-air nonpremixed Gülder

burner flame at the first step height

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2.1 Experimental Procedure

For the SVF experiments, the flame was divided into 4 equal step heights in the vertical

direction. This was done because the whole flame could not be observed by the iCCD. The

height of the flame is approximately 61 mm and the step size used to divide the flame is 17

mm. The height of the flame observed by the iCCD is approximately 18.5mm but a step size

of 17 mm ensures an overlap of about 1.5 mm from the previous image. This is to make

sure that all the flame is covered for data processing using the Mathcad script. For these

experiments, ethylene flowrate is set at 195 cc/min and pressure at 20 psi, air flowrate at

284 L/min and air pressure at 50 psi.

For the beam steering experiments, the flame height was divided into 2 step heights

using the same step size. These experiments require data acquisition of a soot free flame

and it was observed that the first 2 step heights of the flame are soot free. The air flowrate

is set at 370 L/min and air pressure at 24.5psi, nitrogen flowrate is set at 0.4 L/min and

nitrogen pressure at 7psi and methane flowrate and pressure are set at 353.7 cc/min and 5

psi respectively.

Before taking measurements, wait for the detector temperature to be locked at -20oC as

this will ensure low dark charge. Dark charge is charge accumulated within a well, in the

absence of light. Dark charge can also be reduced by lowering the detector temperature. It

is noted that the repeatability of the lamp increases after taking a few acquisitions with the

lamp only. Take 1 acquisition using 25 frames and then start taking the actual acquisitions.

In the Winview software, set the desired on-CCD accumulation depending on the

wavelength, number of frames, gate width and gate delay. Also, set the frequency of analog

to digital conversion to 100 kHz.

The first step height should contain about 1 mm of the burner nozzle. In order to

maintain accuracy and consistency, for the first step height, take acquisitions of the lamp

and the dark and then take the acquisitions of the transmission and flame. On the second

step height, take the acquisitions of the transmission and the flame and then switch off the

flow controls and take acquisitions of the lamp and the dark. Do the same as step height 1

for step height 3 and the same as step height 2 for step height 4. Make sure to wait for the

flame, pressure readings and the flowrates to stabilize before taking data acquisitions of the

transmission and the flame. While reporting data, certain heights above the burner (HAB)

are selected for comparison purposes.

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3.0 Results and Discussions The image processing to obtain data is done using a Mathcad script prepared by Kevin

Thomson. A comparison of different experiments which were done in different years namely

2007, 2014 and 2015 was made. The 2007 experiments used an arc lamp as the light source

whereas the 2014 and 2015 experiments used a pulsed LED as the light source. A CCD was used

in 2007 whereas an iCCD was used in 2014 and 2015. The 2014 setup was similar to the current

setup but a high gain value was used which increased the uncertainty of the lamp [5]. The

measurements reported in this section include source-to-flame intensity ratios, beam steering

effect, soot volume fraction, optical thickness and the uncertainties associated with the optical

thickness and soot volume fraction.

3.1 Source-to-Flame Intensity for Ethylene-Air experiment

Providing a sufficiently high source (LED) to flame intensity ratio will produce a high signal to

noise ratio. The sensitivity of the soot volume fraction measurements can also be improved

with a higher source to flame intensity ratio [4]. When using the 690nm wavelength LED, it was

noticed that by increasing the on-CCD accumulations, the intensity of both the source and the

flame increased by a proportionate amount, resulting in a slight increase in the source to flame

intensity ratio.

Figure 7: Radial profiles of source-to-flame intensity ratio using 690 nm wavelength LED at HAB 33 mm

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Figure 7 shows source-to-flame intensity ratio at a height of 33 mm above the burner

nozzle. Radial position is the distance from the centerline of the flame. Regions outside of the 2

minima represent analysis of where there was no flame present whereas the region inside the

two minima contains the flame.

The reason for the insignificant variation in source-to-flame intensity ratio by increasing

the on-CCD accumulations is that the emission of the ethylene flame is higher at 690 nm

compared to 455 nm wavelength [4]. Therefore, increasing the on-CCD accumulations increases

the intensity recorded of both, the lamp and the flame. The maximum source to flame intensity

ratio using on-CCD accumulations of 300 is 6.03, using an on-CCD accumulations of 600 is 6.211

and using an on-CCD accumulations of 900 is 6.308.

When the 455nm wavelength LED was used, it was observed that increasing the on-CCD

accumulations did have a great impact on the source to flame intensity ratio. This is because

the flame emission is lower at lower wavelengths and increasing the on-CCD accumulations

increases the flame intensity by a very small factor. The maximum source to flame intensity

ratio using on-CCD accumulations of 2000 is 70.91, on-CCD accumulations of 4000 is 99.63 and

on-CCD accumulations of 6000 is 117.6.

Figure 8: Radial profiles of source-to-flame intensity ratio using 455 nm wavelength LED at HAB 33 mm

11

When comparing Figure 7 to Figure 8, it is evident that increasing the on-CCD

accumulations will produce a higher source to flame intensity ratio at 455nm.

Table 1: Intensity values at 455nm and 690nm and at a HAB 33 mm

455nm LED

On-CCD accumulations Max flame intensity

Lamp intensity % Increase in lamp intensity

% Increase in max flame intensity 2000 215 13781

4000 321 27419 99 49.3

6000 431 41376 51 34.3

690nm LED

On-CCD accumulations Max flame intensity

Lamp intensity % Increase in lamp intensity

% Increase in max flame intensity 300 1748 9391

600 3401 18746 99.6 94.5

900 4974 28355 51.3 46.2

From Table 1 it can be seen that the intensity of the flame at 455nm is considerably than

that at 690nm. At both the wavelengths, the lamp intensity increases by the same factor as the

on-CCD accumulations. The lamp intensity is obtained at the same radius as the maximum

flame intensity.

3.2 Beam-Steering

The purpose of the experiment was to confirm the assumption of negligible beam-steering

effect. A mixture of methane and nitrogen non-premixed with air were selected as fuel for the

flame for this experiment. Methane was diluted with nitrogen to produce a soot free flame so

that there is no attenuation of the LED beams by soot. Although a completely soot free flame

could not be produced, image processing was only done for the region where there was no soot

present. Step heights 1 and 2 of the flame had no soot present and this can be seen in Figure 9

which shows an image of the flame used for the beam-steering experiment.

From equation 3, without an attenuating medium, the transmissivity vs. radius plots

should be constant. Figures 10 and 11 show plots of transmissivity and these plots are

normalized based on the two highest values of transmissivity. By observing these 2 figures, it

can be said that the beam steering effect has been eliminated by the current diffuse 2D-LOSA

setup. From plots (a) and (b) in Figure 9 and Figure 10,the maximum variation in the normalized

transmissivity data at 455 nm is 0.3% and 0.31% respectively and the maximum variation in the

transmissivity data at 690 nm for these heights is 0.39% and 0.36% respectively.

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Figure 9: Image of a mixture of methane and nitrogen flame nonpremixed with air. The blue region at the bottom of the flame does not contain soot and is used for data acquisition

Figure 12 shows how the beam steering effect is eliminated by the current setup. In the

collimated 2-D LOSA setup mentioned in [3], all the light approaching the lenses and aperture

(light shade) would be imaged onto the CCD but in the current system, a smaller region of the

light shade (dark shade) is imaged onto the CCD. This ensures that deflected beams (dotted

lines) are not imaged onto the CCD therefore eliminating the effect of beam steering. Any other

deflected beams which reach the iCCD follow the same path as the undeflected beams

represented by the dark shade. This allows eliminating the effect of beam-steering interference

[3]. The difference in the data points in Figures 10 and 11 occur because of the uncertainties of

mainly the lamp. These uncertainties are discussed in Section 3.4.

Figure 10: Normalized transmissivity curves at 455 nm. Plot (a) and (b) show data at HAB 10 mm and HAB 30 mm respectively

Air flow Air flow

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Figure 11: Normalized transmissivity curves at 690 nm. Plot (a) and (b) show data at HAB 10 mm and HAB 30 mm respectively

Figure 12: Effect of beam steering on diffuse 2D-LOSA (image taken from [3])

3.3 Soot Volume Fraction and Optical Thickness

The SVF experiments are performed and the results were compared with the results from [3]

and [5]. The current SVF experiments were done using the following parameters: The gain

setting was set to 0 as increasing the gain increases the uncertainty of the lamp images [5]. The

fuel used in this experiment is ethylene. The 2007 system ([3]) used a wavelength of 440nm and

𝐸(𝑚)𝜆 value of 0.26 and the 2014 system ([5]) used a wavelength of 530nm and 𝐸(𝑚)𝜆 value

of 0.27 for their experiments.

14

Table 2: Parameter values for ethylene-air SVF experiment

Wavelength Gain # of on-CCD accumulations

Pulse Width (µs)

fA/D (KHz) fgate (Hz) # of frames

455 nm 0 2000 39 100 200 25

690 nm 0 300 39 100 200 25

The absorption of light by soot particles increases as the wavelength of the LED

decreases. For the current experiments, 𝐸(𝑚)𝜆 is kept constant at 0.27 in equation 1, so the

only two variables in this equation are the wavelength 𝜆 and the local extinction

coefficient 𝐾𝜆(𝑒). Table 3 lists the values of 𝐾𝜆

(𝑒) at HAB 10mm and 40mm for 455nm and

690nm wavelengths. The radius at which this value is obtained is the radius at which the

maximum SVF value is obtained.

Table 3: 𝑲𝝀(𝒆) values at different wavelengths, heights and radius

Wavelength Height above burner Radius at max 𝑓𝑣 𝐾𝜆(𝑒)

455 nm 10 mm 3.72 mm 0.023

40 mm 2.04 mm 0.062

690 nm 10 mm 3.68 mm 0.012

40 mm 1.96 mm 0.038

Table 3 shows that values of the local extinction coefficient 𝐾𝜆(𝑒) increases with

decreasing wavelength. Therefore, at lower wavelengths, the soot volume fraction attained has

a higher value. It is also known that at lower wavelengths, polycyclic aromatic hydrocarbon

(PAH) molecules formed due to incomplete combustion also absorb light which can cause an

increase in the 𝐾𝜆(𝑒) values.

Figure 14 shows a comparison of the SVF at different wavelengths at heights of 6 mm,

10 mm, 20 mm, 30 mm, 40 mm and 50 mm above the burner. It is observed that at lower

heights of the flame, the SVF at and closer to the center of the flame is almost negligible

whereas the SVF in the borders of the flame is clearly detectable. This occurs because

combustion takes place at the annular region of the burner nozzle due to the upward air flow

and there is no combustion in the center of the flame at lower heights as air is not able to

diffuse into the flame. As higher heights of the flame are analyzed, the SVF at the center and

the borders of the flame increases. Air is now able to diffuse in the flame as the width of the

flame decreases and combustion takes place in the center of the flame. So this means that

moving up in the flame, more air is able to diffuse into the flame as the width of the flame

decreases with increasing height. From the Mathcad script, it was noted that beyond the height

of 44 mm above the burner, the SVF is more at the centerline of the flame and starts to

15

decrease at the borders of the flame. Figure 13 (a) shows an image of the ethylene flame used

for the SVF experiments. The brighter regions of the flame contain more soot than other

regions of the flame as more combustion takes place in those regions. Plot (b) in Figure 13

shows the SVF distribution in the flame. It only shows the SVF for one half of the flame. The SVF

values reported in Figure 14 are averaged values based on two halves of the flame.

Figure 13: (a) Ethylene flame used for SVF experiments and (b) SVF distribution in the flame (image (b) is taken from [3])

The SVF plots at heights of 6 mm and 10 mm have very similar values for 440 nm and 455 nm

but as the height increase, the difference in these two SVF curves also increases. The SVF curve

at 440 nm displays the lowest values at HAB 40 mm and HAB 50 mm. A comparison of 2007 and

2014 experiments were made in [5] and it was shown that there was a difference in heights in

the comparison. After adjusting the heights, the SVF plots were very similar to each other.

Air flow Air flow

(a) (b)

16

Figure 14: SVF at various heights above the burner

17

Figure 15 shows the optical thickness for the current system and the systems setup in 2014 and

2007. The peaks in the plots show optical thickness at the borders of the flame and the region

between the peaks shows the optical thickness between the borders of the flame. It can be

seen from the plots that the optical thickness decreases with increasing wavelength. This shows

that less light beams at lower wavelengths pass through the attenuating medium as compared

to higher wavelengths. At lower heights of the flame, the SVF is lower at the center and higher

at the borders and therefore, as shown by plots (a) and (b), the optical thickness is lower at and

closer to the center and it is higher at the borders of the flame. As the SVF increases at the

center of the flame at higher heights, the optical thickness also increases as shown by (c) and

(d). The more soot and other PAH particles present in the flame, the harder it is for the light

beams to reach the iCCD as the beams are either scattered or absorbed by these particles.

Figure 15: Radial profiles of optical thickness at various heights above the burner. (a) shows optical thickness at a HAB of 10 mm, (b) at a HAB of 20 mm, (c) at a HAB of 40 mm and (d) at a HAB of 50 mm

18

3.4 Uncertainties

An uncertainty analysis was performed to understand and find the potential areas for

improvement of errors on the measurements. Also, improvements were made to reduce the

shot noise in the measurements. In the Mathcad script, the mean and standard deviation of

each binned pixel was calculated and then the mean of those 2 quantities was calculated for all

the pixels to provide a mean and standard deviation of the whole binned image.

The uncertainty of the optical thickness (OT) was calculated using the following

equation.

𝜎𝑙𝑛𝜏 = √(𝜎𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛

𝑡𝑟𝑎𝑛𝑠 − 𝑓𝑙𝑎𝑚𝑒)

2

+ (𝜎𝑓𝑙𝑎𝑚𝑒

𝑡𝑟𝑎𝑛𝑠 − 𝑓𝑙𝑎𝑚𝑒)

2

+ (𝜎𝑙𝑎𝑚𝑝

𝑙𝑎𝑚𝑝 − 𝑑𝑎𝑟𝑘)

2

+ (𝜎𝑑𝑎𝑟𝑘

𝑙𝑎𝑚𝑝 − 𝑑𝑎𝑟𝑘)

2

Table 4 lists the uncertainties of the whole images at two wavelengths and the effect of

changing the on-CCD accumulations and the number of frames on uncertainties. The

uncertainty of soot volume fraction is calculated based on averaging the two halves of the

flame. Some of the experiments were repeated twice and the values are recorded with a

comma.

Table 4: Uncertainties at step height 2 for ethylene-air experiment

Wavelength On-CCD accumulations

# of frames

σlamp σtransmission σflame σdark σOT σSVF

455 nm

2000 25 4.794, 4.719

11.554, 11.969

0.517, 0.435

0.053, 0.051

1.226*10-3, 1.275*10-3

0.113, 0.118

2000 50 3.467 6.896 0.308 0.038 7.316*10-4 0.068

2000 100 2.475 5.154 0.233 0.032 5.324*10-4 0.05

3000 25 5.994 19.449 0.654 0.058 1.349*10-3 0.124

4000 25 6.894 19.15 0.795 0.065 1.025*10-3 0.097

6000 25 9.159 17.45 0.616 0.11 6.723*10-4 0.068

690 nm

300 25 3.993, 4.075

5.683, 4.3

8.428, 1.783

0.051, 0.051

1.561*10-3, 7.832*10-4

0.228, 0.12

300 50 4.043 5.259 1.555 0.053 8.543*10-4 0.132

300 100 2.032 3.162 4.75 0.045 8.457*10-4 0.124

600 25 5.713 8.869 14.751 0.05 1.3*10-3 0.189

900 25 7.379 10.769 17.243 0.051 1.044*10-3 0.152

From Table 4, it is noted that increasing the number of frames used for averaging

measurements will result in a decrease in the uncertainty of all the desired quantities.

Increasing the on-CCD accumulations also decreases the uncertainty of the optical thickness

19

and soot volume fraction but it increases the uncertainty of the lamp, transmission, the flame

and the dark. As stated in Section 3.1, increasing the on-CCD accumulations increases the

intensity of the lamp by a much greater factor than the intensity of the flame at 455nm and by

a much smaller factor at 690nm. This causes the denominator in the optical thickness

uncertainty equation to increase therefore resulting in a smaller standard deviation. For

455nm, the % decrease in the uncertainty of the optical thickness by increasing the number of

frames from 25 to a 100 at on-CCD accumulations of 2000 is 56.6% and the % decrease is

45.16% when the on-CCD accumulations were increased from 2000 to 6000 while using 25

frames for averaging. For 690nm, a repeat of the experiment at 300 on-CCD accumulations and

25 frames provided a much lower uncertainty than the rest of the values listed in Table 4.

Increasing the number of frames provides more data for averaging calculations and therefore

reduces the uncertainty of the measurements.

Figure 16 shows another favorable effect of increasing the number of frames at 455nm.

The plot produced with a 100 frames has much lower noise as compared to 50 frames and 25

frames. This results in the plot being much smoother than the rest.

Figure 16: Effect of increasing number of frames at HAB 22 mm

Figure 17 shows a comparison of σoptical thickness for the wavelengths 440 nm, 530 nm, 445

nm and 690 nm at various heights above the burner. The plots were created using 25 frames. It

was observed that the peaks were higher for the 2007 experiment at HAB 30mm and HAB

50mm. Both wavelengths for the current experiment had lower uncertainties than the 2014

experiment at all radial positions in plot (a). This maybe because the 2014 experiment was

using a gain setting of 128 which increased the uncertainty of the lamp [5]. The 2007

experiment always had the lowest uncertainty in between the borders of the flame. This may

occur because of the low uncertainty of the arc lamp images which was reported to be an

average value of 1.046 at a step height of 2. When compared to the 2014 system, plots (b), (c)

and (d) show that the current system has achieved lower σoptical thickness closer to the center of

the flame.

20

(a)

(b)

(c)

Figure 17: Comparison of σoptical thickness at various heights above the burner. (a) shows σOptical Thickness at a HAB of 10 mm, (b) at a HAB of 30 mm and (c) at a HAB of 50 mm

21

4.0 Conclusions Upon using the improvements suggested by [3, 5] and developing other improvements, the

effect of beam steering was proven to be negligible on the diffuse 2D-LOSA setup as the plots of

transmissivity were almost constant. It was concluded that increasing the number of on-CCD

accumulations for 690nm LED has very less effect on the source to flame intensity ratio

whereas the effect is more pronounced for the 455nm LED. The source to flame intensity ratio

at 455nm using on-CCD accumulations of 2000 is 70.91 and using 6000 is 117.6 which results in

a % increase of 65.8%. For the 690nm LED, increasing on-CCD accumulations from 300 to 900

resulted in a mere % increase of 4.6%.

Soot volume fraction experiments indicated that at the 455nm wavelength LED, the

local extinction coefficient is higher as compared to 690nm LED. This results in higher soot

volume fraction values at 455nm. The optical thickness measurements showed that the optical

thickness is higher at lower wavelengths and lower at higher wavelengths.

In the optical thickness uncertainty analysis, it was shown that at 455nm, increasing the

number of frames to a 100 from 25 and increasing the number of on-CCD accumulations from

2000 to 6000 resulted in a % decrease 56.6% and 45.16 % respectively. At 690nm, increasing

the number of frames decreased the uncertainty of the lamp, transmission and the dark but the

uncertainty of the flame was increased. This might be due to a randomness of the flame

emission. This resulted in a higher uncertainty of the optical thickness. As increasing the on-CCD

accumulations at 690nm increases the lamp uncertainty and the flame uncertainty, the optical

thickness uncertainty also increased. Therefore, at 690nm, it is advisable to perform at higher

number of frames. At 455nm, both the on-CCD accumulations and the number of frames can be

increased which will have a favorable impact on the uncertainties. It is to be noted that at even

at 455nm, increasing on-CCD accumulations does increase the uncertainty of the lamp.

From the uncertainties provided in table 4, it is evident that the intensity of the lamp

varies a lot from one pulse to another and has the maximum effect on the uncertainty of the

transmission and the uncertainty of the optical thickness.

22

5.0 Recommendations - For 690nm LED, it is recommended to use 300 on-CCD accumulations as increasing the

on-CCD accumulations increases the uncertainty of the optical thickness and svf but

does not increase the source-flame intensity ratio. For 455nm LED, a higher number of

on-CCD accumulations than 6000 should be used but care should be taken so as to not

over saturate the iCCD

- A higher number of frames than 100 should be used for data acquisition. This may not

be possible because of time constraints

- Switch off all the lights in the room and cover any other light so as to eliminate the

random effect of other light sources

- Decrease the detector temperature to below -20oC as this will decrease the dark charge

count

- Use a computer with a faster processor in the lab so that analysis through Mathcad can

be done simultaneously with data acquisition using the WinView software

- An engineered diffuser could not be used for these experiments due to time restrictions.

Use the engineered diffuser and compare the source to flame intensity ratio obtained to

the ratio obtained with the integrated sphere

- Use a high power LED so that lower on-CCD accumulations can be used which will

reduce the uncertainty of the lamp

- Increase the frequency of triggering the LED and the iCCD and check the impact on the

intensity of the LED and the flame

23

6.0 References

[1] T. C. Bond et al., "Bounding the role of black carbon in the climate system: a scientific

assessment,"Journal of Geophysical Research: Atmospheres, vol. 118, no. 11, 2013, pp.

5380-5552

[2] C. J. Dasch, "One-dimensional tomography: a comparison of Abel, onion-peeling,and

filteredbackprojection methods," Applied Optics, vol. 31, no. 8, 1992, pp. 1146-1152

[3] K. A. Thomson et al., "Diffuse-light two-dimensional line-of-sight attenuation for soot

concentration measurements," Applied Optics, vol. 47, no. 5, 2008, pp. 694-703

[4] Stephen R. Turns., “AN INTRODUCTION TO COMBUSTION”. Propulsion Engineering Research

Center and Department of Mechanical Engineering. The Pennsylvania State University:

McGraw-Hill, Inc

[5] Alex Frey, Work term report: analysis of a pulsed LED source and intensified CCD sensor-

based diffuse 2-dimensional line of sight attenuation system for soot concentration

measurement, 2014

24

Glossary Two-Dimensional Line-of-Sight Attenuation (2D-LOSA): A technique used to determine soot

volume fraction in the flame

Beam Steering: The refraction of light due to temperature gradient-induced refractive index

gradients

BCM: Black Carbon Metrology

Charge-Coupled Device (CCD): A device that generates electrical charge by capturing photons

and converting the charge to a digital signal

CMOS: Complementary Metal-Oxide-Semiconductor

F-Number: The ratio of a lens' focal length to its effective diameter

Gain: A parameter on an iCCD that is proportional to the amplification of signal

Integrating Sphere: An instrument with a diffuse reflective inner coating that produces diffuse

light

Intensified CCD (iCCD): A CCD that uses an image intensifier to amplify the number of photons

collected by the CCD

Micro-Channel Plate (MCP): A component in an iCCD that can have an electric potential

induced along its length and is composed of many channels through which electrons can travel

and dislodge additional electrons, amplifying the initial signal

NRC: National Research Council

LED: Light Emitting Diode

Soot Volume Fraction (𝑓𝑣): The volume of soot per total volume

fgate: the frequency of triggering the iCCD and the LED driver

fA/D: the frequency at which an analog signal is converted into a digital signal