Particulate Formation, Evolution, and Fate -Michelson Day 3 Part 1

Summary

The speaker discusses various diagnostics techniques for analyzing particles in flames, emphasizing the challenges of sampling without perturbing the reactive environment. They delve into methods like using a quartz probe, stagnation plate extraction, and jet entrainment sampling, each with its advantages and limitations. The talk also explores the application of X-ray techniques, such as SAXS and WAXS, for measuring incipient particles and the use of Laser Induced Incandescence (LII) for sensitive soot detection. The summary highlights the importance of accurate data analysis and the need for further research in this complex field.

Takeaways

• 🔬 The speaker discusses different diagnostic methods for analyzing particles in flames.
• 🔥 Ex-situ diagnostics involve extracting samples from the flame using a quartz probe, which can perturb the flame.
• 🔍 Perturbations caused by the probe can affect temperature distribution, flow field, and radical distribution in the flame.
• 🧪 Extracting particles without perturbing the flame is challenging, and various methods have been tried to mitigate this.
• 💡 A stagnation plate technique is used to stabilize the flame and model the temperature distribution during sampling.
• 📊 The speaker mentions the importance of accounting for probe effects when analyzing experimental data.
• 🌡️ Sampling techniques like the entrainment method use inert gas jets to collect particles with minimal flame perturbation.
• 🛠️ Different sampling methods, including thermophoretic sampling and jet entrainment, have their own advantages and challenges.
• 🌟 The goal is to obtain accurate particle characteristics without significantly altering the flame environment.
• 🧬 In-situ diagnostics like laser-induced incandescence (LII) are used for real-time measurement of soot in various conditions.

Q & A

What are the challenges associated with sampling particles from a flame?

Sampling particles from a flame is challenging because it often perturbs the flame, affecting the temperature distribution, flow field, and radical distribution. Extracting particles without disturbing the reactive environment is difficult.

How do quartz probes impact the sampling process?

Quartz probes, when inserted into the flame, can cause perturbations that affect the temperature, flow field, and radical distribution. These perturbations can lead to less accurate measurements of the particles being sampled.

What is a stagnation plate and how is it used in sampling?

A stagnation plate is placed above the flame with a hole through which gas is passed, sucking up particles into the stabilization region. It helps model the probe's effect and account for temperature changes during sampling.

What are the benefits and drawbacks of using a stagnation plate for sampling?

The benefits include better modeling of the probe effects and temperature accounting. However, it can quench the chemistry at the sampling region, leading to less accurate measurements of the radical distribution.

What is the jet entrainment sampling technique?

The jet entrainment sampling technique involves using a jet of cold inert gas, such as nitrogen or argon, to entrain particles from the flame into a collector tube. This method minimizes perturbations to the flame.

How does the jet entrainment technique minimize flame perturbations?

The technique avoids placing a probe directly in the flame, reducing heat conduction and catalytic reactions. The cold gas jet quickly cools and dilutes the particles, minimizing perturbations.

What issues still need to be addressed with the jet entrainment technique?

The main issue is coagulation of particles at lower heights in the flame. Improved dilution methods are needed to prevent particles from sticking together and to achieve better spatial resolution.

What is Laser-Induced Incandescence (LII) and how is it used?

LII is a technique used to measure soot by heating particles with a laser until they emit incandescence. It is sensitive to soot particles and can be used for volume fraction measurements, particle sizing, and assessing particle maturity.

How does LII help in measuring primary particle size?

LII measures the cooling rate of particles after they are heated by the laser. The cooling rate, which depends on the surface-to-volume ratio, can be modeled to determine primary particle size.

What are the advantages of using scattering techniques for particle measurement?

Scattering techniques, such as elastic scattering, can provide information on primary particle size, volume fraction, aggregate size, and the number of primary particles in aggregates. They offer a non-invasive way to measure these properties in situ.

Outlines

00:00

🔬 Flame Diagnostics and Sampling Challenges

The speaker discusses the complexities of flame diagnostics, particularly focusing on the issue of perturbations caused by sampling methods. They mention the use of a quartz probe to extract particles from a flame, a method they have employed for many years. The perturbations can affect temperature distribution, flow field, and radical distribution, which has been demonstrated over several decades. Despite the challenges, there is often no alternative to sampling without some form of perturbation. The speaker also addresses the problem of condensation within the probe and the potential for chemistry to occur within it, which can affect the accuracy of the data. They highlight the importance of considering what happens inside the probe when analyzing experimental data.

05:02

🌡️ Effects of Stagnation Plate on Flame and Particle Sampling

The speaker explores the use of a stagnation plate in flame sampling, discussing its benefits and drawbacks. They explain how the stagnation plate can affect the temperature distribution and radical distribution, leading to a depletion of radicals near the plate. This can quench the chemistry at the sampling region, making it difficult to accurately capture the true nature of the particles. The speaker also mentions the use of a metal tube with a hole for sampling in both pre-mixed and diffusion flames, noting the perturbations and potential clogging issues associated with this method. They discuss their own experiments and the decision to stop using this technique due to concerns over the impact on temperature distribution and the risk of oxidation.

10:04

🌪️ Jet Entrainment Sampling Technique for Flame Particles

The speaker introduces a new sampling technique involving a jet of inert gas to entrain particles from the flame. This method aims to minimize perturbations to the flame by avoiding direct insertion of a probe. They describe the setup, which includes a small tube generating a nitrogen jet that captures particles without significantly disturbing the flame. The speaker discusses the collaboration with Matthias Simon's group to characterize this technique, showing that it causes less perturbation than other methods. They present data on temperature distribution and particle sampling, highlighting the advantages of not having a probe in the flame, which reduces the risk of catalyzing reactions and quenching radical species.

15:05

📊 Comparison of Sampling Techniques and Particle Analysis

The speaker compares different sampling techniques, including rapid insertion and jet entrainment, using TEM (Transmission Electron Microscopy) for particle analysis. They discuss the challenges of achieving the right spatial resolution and the issues of coagulation in the sampling probe. The speaker notes the need for better dilution to prevent particles from sticking together. They also mention the limitations of the jet entrainment technique in terms of sampling specific regions of the flame, such as the edges or center, and the difficulty in isolating edge particles for analysis.

20:06

💡 Innovative Ideas for Flame Sampling and Diagnostics

The speaker suggests innovative ideas for improving flame sampling, such as using a slot burner to reduce end flame effects by injecting nitrogen at the ends. They also propose the idea of shooting down the center of the flame to avoid perturbation. The speaker acknowledges the many ways to improve current techniques and the need for further research in understanding soot formation and evolution. They also address a question about accounting for ambient oxygen in the co-flow, explaining the challenges and some strategies they use to minimize oxidation issues.

25:07

🚀 Synchrotron-Based X-Ray Techniques for Soot Diagnostics

The speaker discusses the use of synchrotron-based X-ray techniques for soot diagnostics, highlighting the potential of these methods despite their complexity. They explain the process of obtaining beam time at a synchrotron facility and the challenges of setting up and conducting experiments. The speaker also explores the possibility of using X-ray techniques to measure incipient particles, which are difficult to detect with laser-based techniques due to their small size. They share their experiences and the complications encountered in trying to subtract the gas phase background signal from the X-ray scattering data.

30:10

🌌 Synchrotron Facilities and X-Ray Scattering Experiments

The speaker provides an overview of synchrotron facilities and the process of conducting X-ray scattering experiments. They describe the challenges of accounting for temperature and composition effects on the background signal and the time-consuming process of subtracting this background to isolate the signal from incipient particles. The speaker also discusses the experimental setup, including the scale of the equipment and the remote operation from a control room. They mention the concerns of beam-line scientists when introducing a flame into their facility and the importance of communication in alleviating these concerns.

35:12

🔎 Small Angle Scattering Techniques for Particle Analysis

The speaker compares small angle X-ray scattering (SAXS) with small angle neutron scattering (SANS) for analyzing particles in a flame. They discuss the advantages of each technique, such as better resolution and sensitivity with SAXS, and the ease of background determination with SANS. The speaker also mentions the potential of combining these techniques to gain a better understanding of particle behavior in flames. They present results from experiments using both SAXS and SANS, highlighting the differences in the measurements obtained from each method.

40:13

🌟 Synchrotron Light Sources and Laser-Induced Incandescence

The speaker explains the workings of a synchrotron, detailing how electrons are accelerated in a circular path to produce light through a relativistic effect called bremsstrahlung. They also discuss free electron lasers as a coherent source of light. The speaker then transitions to laser-induced incandescence (LII), a technique used to measure soot volume fraction, particle sizing, and maturity. They describe the process of LII, where a laser heats up the soot particles, causing them to emit incandescence that is measured. The speaker also mentions the use of LII in engines to study soot formation and the need for a good energy balance model to interpret the data.

45:16

🌈 Advanced Optical Techniques for In-Situ Particle Analysis

The speaker discusses advanced optical techniques for in-situ particle analysis, such as laser-induced incandescence (LII) for measuring primary particle size and multi-angle wide-angle scattering for obtaining aggregate size and volume fraction. They describe how these techniques can be used to understand particle maturity and how to extract meaningful data from the signals obtained. The speaker also touches on the challenges of implementing these techniques in varying conditions, such as turbulent systems, and the importance of having accurate models for energy balance and particle behavior.

Keywords

💡Diagnostics

Diagnostics in the context of this video refers to the methods and tools used to evaluate and study the properties of flames and particles within them. It is central to the video's theme as it discusses various techniques for analyzing combustion processes and particle synthesis. For example, the script mentions 'exitu Diagnostics' and 'sampling' as part of the diagnostic process.

💡Perturbations

Perturbations are disturbances or changes caused in a system, which in this video, refer to the alterations in the flame's properties due to the act of sampling. The term is integral to understanding the challenges of obtaining accurate samples from a flame without affecting its natural state. The script discusses how inserting a probe into a flame can cause 'perturbations to the flame from temperature' and other factors.

💡Quartz Probe

A Quartz Probe is a specific tool used for sampling particles within a flame. It is mentioned in the script as a method for extracting particles, which can introduce perturbations. The use of a quartz probe exemplifies the practical aspects of flame diagnostics discussed in the video.

💡Reactive Environment

A Reactive Environment in this video pertains to the conditions within a flame where chemical reactions are taking place. The term is key to understanding the complex nature of火焰 diagnostics, as it highlights the dynamic and interactive processes that occur within flames. The script discusses the difficulty of extracting particles from a 'reactive environment' without causing changes to the chemistry.

💡Mass Spectrum

A Mass Spectrum is a type of data representation used to measure and plot the mass-to-charge ratio of ions. It is a concept discussed in the script in relation to analyzing the particles extracted from a flame. The script mentions a 'Mass Spectrum of that experiment' which shows different particle masses and their distribution, indicating the complexity of particle composition in flames.

💡Stagnation Plate

The Stagnation Plate is a component used in some sampling techniques to stabilize the flame and allow for the extraction of particles. It is a critical concept in the video as it represents an alternative approach to sampling that aims to minimize perturbations. The script describes how the stagnation plate is used to 'account for that so then you know everything' in the sampling process.

💡Soot

Soot is a black carbonaceous material produced by the incomplete combustion of organic matter. In the video, soot is a primary subject of study, with various diagnostic techniques aimed at understanding its formation and characteristics within flames. The script discusses 'soot formation' and 'soot distribution', emphasizing its importance in combustion research.

💡Aerosol Mass Spec

Aerosol Mass Spectrometry (AMS) is a technique used to measure the mass and chemical composition of aerosols, including particles found in flames. The script mentions sending extracted particles 'into the aerosol Mass Spec', indicating its use for detailed particle analysis in the context of flame diagnostics.

💡Jet Entrainment

Jet Entrainment is a sampling technique where a jet of inert gas is used to entrain particles from the flame for analysis. This technique is highlighted in the script as a method that potentially reduces perturbations to the flame. The script describes how particles are 'entrained into that jet', offering a glimpse into the practical application of this method.

💡Laser Induced Incandescence (LII)

Laser Induced Incandescence is a diagnostic technique used to detect and measure the properties of soot particles. It is a significant concept in the video as it represents a laser-based method for studying soot in flames. The script mentions using LII to show 'where the particles are in our flame', demonstrating its application in visualizing soot distribution.

Highlights

Discussion on the challenges of flame diagnostics and perturbations caused by sampling methods.

Use of quartz probe for extracting particles from a flame and its associated issues.

Importance of understanding perturbations in temperature, flow field, and radical distribution in flame sampling.

The complexity of extracting particles from a reactive environment without altering their chemistry.

Analysis of mass spectra of extracted particles and the potential for probe-induced condensation.

Concerns about chemistry occurring within the sampling probe and the need for rapid dilution and cooling.

Introduction of the stagnation plate technique for particle extraction and its modeling.

The impact of the stagnation plate on radical distribution and chemistry near the plate.

Differences in sampling between pre-mixed and diffusion flames and the use of metal tubes.

Challenges with clogging and perturbations when using metal tubes for flame sampling.

Transition to using a jet of inert gas for particle entrainment and collection.

Collaboration with Matthias Simon's group for characterizing the jet entrainment sampling technique.

Comparison of different sampling techniques including TEM and their spatial resolution.

The need for improved dilution to prevent particle coagulation in sampling probes.

Innovative ideas for modifying flame geometry to better accommodate sampling techniques.

Strategies for minimizing oxygen contamination from the co-flow in sampling processes.

Exploration of X-ray techniques for soot diagnostics and the challenges of gas phase background subtraction.

Advantages of using SAXS (Small Angle X-ray Scattering) for particle measurements.

Comparison between SAXS and other scattering techniques like SANS (Small Angle Neutron Scattering).

The potential of using a free-electron laser for Wax measurements in flames.

Introduction to Laser Induced Incandescence (LII) for soot diagnostics and its sensitivity to mature particles.

Techniques for using LII to measure primary particle size and aggregate size in flames.

Utilization of elastic scatter for obtaining information on particle size and volume fraction.

Transcripts

yesterday we were talking about uh the

different Diagnostics remember we talked

about

um exitu Diagnostics you know extracting

from a flame now we're kind of I I think

uh we stopped yesterday kind of smacked

in the middle of sampling how people do

sampling so we were kind of talking

about perturbations like so a lot of

times we like will put a quartz probe

right into the middle of the flame and

um and extract using the quartz probe so

um I've been doing this for many years

and I know this is an issue that we have

perturbations to the flame from from

temperature in the temperature

distribution the flow field

um a radical distribution and people

have demonstrated this multiple times

over the last many decades but we don't

have a lot of choice in terms of

sampling it's it's really hard to to

extract particles from a flame or

reactive environment

um without perturbing it so so uh there

are ways that we can try to think about

how to do this and this is a challenge

you know this is a challenge for you a

challenge for all of us if you're going

to try to look at particles say you're

looking at particle synthesis under

reactive conditions not even in a flame

you it a lot of times you really want to

know those particles are what they look

like what their characteristics are

um

but if you're going to perturb the

chemistry around your sampling then you

you're maybe not going to get the most

realistic example of what your particles

really are at the time and you want to

see them in your flow so uh

and this is this is another example of

I'm not sure maybe this is so this you

may have seen this I think I showed this

on the first day this uh extraction and

they're looking at the different

particles I think there are different

explanations for this for these data

this is a Mass Spectrum of that

experiment I showed also yesterday

like I showed this experiment two times

um where they extract the full particles

and then saw these like really high

masses in this kind of bumpy like you

know distribution and maybe those are

aggregated particles because they're

ionizing the full particle and then

getting their Mass Spectrum but it's

also possible that when they stick when

they use their probe in inside their

probe they're actually getting some kind

of condensation this some of this may be

happening in their probe so that's a

question and that's a question that I

often have for our experiments is what's

happening inside that probe you know I I

said yesterday we have we sometimes

worry about even chemistry that's going

on in the probe if we're not diluting

and and dropping the temperature fast

enough

so

um so here's an example of where there

is a probe Pro you know once you suck

the particles in the probe so this is a

the smps results that we've looked at a

couple of times actually

um and you see a particle size

distribution but that Peak to the left

they think is is uh something that

happens inside the probe you're

generating particles inside your probe

so I think that you know this is

something we always have to think about

and and be careful of when we're

analyzing the data

um so here's um

this is the extraction technique

um where you have the stagnation plate

above the plane we talked about this

yesterday a little bit but this is what

it looks like so here's your your

um

pre-mixed flame and then there's the

stable the stagnation plate or

stabilization plate above that and it

has a hole in it and then there's a an

open like

um tube shape in the plate itself and

they send gas through that hole

um and suck up the particles into the

stabilization player stagnation plate

and that theory is you can if you know

the temperature of the stagnation plate

then when you do the modeling you can

account for that so then you know

everything in and so this is a nice

approach is to actually just model your

whole probe in with your experiment so

this is another way of dealing with

probe effects is to actually take that

take the probe into account but if

you're really trying to sample what's

what's happening at the higher

temperatures then

um it's really hard to do you can't just

model it if you don't know what you're

you know that you're actually trying to

get data to validate your model

um so uh so this is a but I do think

this is a really nice approach

this is what the temperature

distribution looks like

um so you can

um so you see the experimental data or

are those symbols and then they go and

model the temperature distribution

um with that stagnation played in there

and then they change the height of that

relative to the burner and and do the

modeling

so this is this is a the bonus of this

type of sampling

um this is with the stagnation plate

um

and looking at the effects of having the

stagnation plate in there so you

actually see there's soot so there's o h

distribution this is really important so

what you see is the radical distribution

is depleted near the stagnation plate so

the chemistry you're trying to

um freeze and Sample right at the

stagnation plate or sampling region is

just kind of being quenched right there

so you have to like

um if you don't know what that chemistry

is and you're trying to figure out what

it is it's hard to do that when you have

something that's perturbing it so much

so

um let's see oh

is the right maybe I didn't label this

didn't keep the labels oh yeah oh is the

right hand column and you can see the um

the bar chart and you can see that near

the stagnation plate which is on the uh

right hand side you that the oh is

dropping

okay yeah

and this is um some more like actually

drawn out results with the stagnation

plate sitting at 15 millimeters which is

what it is in the the figure to the left

as well

um so you see that the oh when you do oh

the stagnation plate near the stagnation

plate and that's a few millimeters like

that's a big spatial difference where

you see the the oh dropping right next

to the stagnation plate that's actually

really significant

um so there's uh so that's in a

pre-mixed flame and a diffusion flame

oftentimes people will take a a metal uh

tube and drill a hole in it and then use

that metal tube with the hole and then

you can move

um because the uh

with a pre-mixed frame often you have

kind of you know homogeneous uh flame uh

across most of your flame again the very

edges you'll have a a different

distribution but through most of your

flame is going to be the same so it's

quasi one-dimensional

um with height in the flame for you know

diffusion Flame the soot is going to be

is going to have a radial distribution

between the center and the edge of the

flame it's going to vary dramatically so

um it so you can actually use this

technique to not only move the burner up

and down relative to that hole but you

can move it back and forth so you can

try to sample just individual locations

in the flame but this this tube can have

very large perturbations on the flame

itself so um in fact and and clogging is

a real problem in this technique so when

we do this experiment we have an

automatic like if you try this we have

we actually have a little scrubber that

every few seconds goes across like the

tube it's just you know automated to get

to sweep across the tube and clean the

soot that accumulates on the outside of

the tube so you can tell the tube is

actually probably really perturbing

um or the distribution inside the flame

itself

um

and we think that this tube heats up and

catalyzes reactions especially if we

have the tube um anywhere close enough

to the edge or near the tip of the flame

where we can get some oxygen up into

that tube we get oxidation of our

species that are collected in the tube

even though we're running a very high

flow of nitrogen of cold nitrogen

through that tube we still have enough

oxidation because a tuba cell warms up

because it's sitting in the flame

warming up and it gets really pretty

darn hot like you can't easily touch it

it's really hot too

um so and then people have demonstrated

that this tube also has a big impact on

on temperature distribution so again I

really worry we do these experiments but

I really worry about the results so

we've actually stopped using this

technique we recently stopped using it

because we use it I'm going to show you

a different Technique we use but it it

has its own complications

so this is the technique we started to

use

um and we we

um basically and train particles in a

jet of inert gas cold inert gas so we

often will use nitrogen sometimes argon

it doesn't really matter too much as

long as you can entrain some of the gas

and particles from the flame so we have

we basically have a very very tiny tube

and and generate a jet of of nitrogen

for instance and then we collect it so

um the gas comes from the left hand side

on here there's a burner in between that

you don't can't the flame isn't lit

right in this picture and then on the

other side is our collector tube so the

jet of gas goes into our collector tube

along with the flame sample

um and then we can do whatever we you

know want with that often we'll send it

into the aerosol Mass Spec or we'll

collect on a grid um to do Tem

um and we think that this technique

seems to be better so this is what it

looks like with the flame there

we get it as close to the flame as we

can without

um

with the with the gas off without it

pertur visibly perturbing the flame so

um what we don't want if we can help it

is to we try to avoid getting a lot of

oxygen from the co-flow into our probe

because we don't want to have the

problems associated with with um

oxidation but we it doesn't we haven't

seen that so much with these with this

probe maybe

um the material has an effect because it

doesn't heat up as much

um

so we collaborated with Matthias Simon's

group to do

um a net you know characterization of

this technique so this is what our flame

looks like that's the same same linear

Hank and burner we are looking at

earlier uh yesterday

um this is what the flame looks like

with the tube in place with our sampling

or tube in place without the jet on this

is what it looks like with the jet on

and what you can see is it kind of you

know wherever it is in the flame it cuts

out a little section of the flame but

the rest of the flame it just is

unperturbed just by eye so we thought

well that doesn't mean that it's not

perturbed

um and and what this helps is that we

don't have a probe sitting in our flame

so if it's not sitting in the flame it's

not conducting

um uh heat isn't conducted to it so it's

not cooling and you don't have the

surface in the flame to catalyze

reactions and to um quench your your

radical species

so this is uh what it looks like with

the um jet on so this is what the when

we do the calculations or when

matthias's group does the calculations

this is a temperature distribution

looking down the end of the flame on the

left hand side and then looking at the

side of the flame close to where we are

we do our sampling

um and and so this is without the jet on

and you can see in left hand side the

little

um

our little probe things

um sitting there we actually usually

have the probes closer together than

this

um

and then this is what the jet turned on

so the flow is coming from the left to

the right and you can see our jet of

nitrogen is cold is shooting through the

flame instead vecting the um the

particles and gas phase species just

right at the bottom of that jet tube

into our probe

um and so that's it from the the end on

and then if you're looking straight down

the jet you see there's you know uh at

zero that's where our tube is

um so that's cold but right to the right

of it

um the set the temperature is

unperturbed you know there's a little

bit of perturbation right to the right

of it

um but you know it's mostly it's less

perturbations than if you have a probe

sitting in the flame

uh

so this is what it looks like if you

um follow the flow

um like a lagrangian type calculation

you have uh temperature on the left hand

side and then residence time so if

you're following you know particle Mass

um you're you if you start out you know

at the bottom so basically this follows

the um a sample coming up from the

burner and when it hits so that if we

have the the probe sitting at six

millimeters in the flame

um if you once it gets to six

millimeters the temperature will drop

because it's cold okay so it's it starts

to hit that nitrogen and it cools off

dramatically right at six millimeters

um

so your your particle has come up and

now it's being drawn into that probe um

tube and it's cooler uh so

and then if you're looking at the

dilution effect so your particles are

coming up and then once you hit the jet

in this configure you know for this

calculation they're immediately diluted

and here they're diluted by 50 so you

can change the flow and and change the

dilution effect if if you want but

there's a lot of characterization that

we didn't do um this is just kind of

like a basic characterization kind of it

was a proceedings paper so it was one of

those hey do you want to collaborate on

this like quick get a paper in

okay so uh so it has promised but it

still needs some characterization but

here are some

um here's a comparison we did with tem

where we did rapid insertion so we had

our TM grid you know

um injected into the flame and pulled

out quickly

um uh so on the so these are you saw

these data yesterday right

um they we have lii and sax showing

where the particles are in our flame

right

um and then on the right hand side we

have the particle diameter of the

primary particles in the aggregate

um assessed with sacs measurements with

our analysis using sacs and on in the

green it's tem using the jet entrainment

sampling

um technique where we take that those

particles that are entrained into that

jet and hit a um a tem grid and then do

TM on it

so just like you know showing you

yesterday in the middle of the flame we

have mature soot like our Aggregates

that look pretty typical have the right

um fractal Dimension that or the the

typical fractal dimension for Aggregates

um

higher up where we see them going away

in the Lei and sacs that's where we have

oxidation and the particles look like

they're oxidized they've gotten smaller

um and then lower down I think I

probably didn't show all of you know all

of these images yesterday but lower down

we think we have coagulation in the

sampling probe so this is one of the

areas that we need to figure out how to

address and that's probably what we need

to do is better dilution

um somehow to to incorporate kind of

maybe

um instead of a straight tube maybe a

bigger a tooth that can goes bigger and

we inject more nitrogen or something

some some way to keep the particles from

crashing into each other and sticking

together

um uh and then we can compare that with

tem from the rapid insertion technique

thermophoretic sampling

so that's on the left so the Permian

particle size from that is on the left

and you see that you know there's

actually pretty good agreement for some

of the heights but in the middle we have

some issues and this is what we're

seeing is

um even at five millimeters we don't see

mature soot

um

uh at six millimeter it's seven

millimeters we do

um and at eight millimeters we do even

um where we think there should be

oxidation

um there's no coagulation at the bottom

so that's a good thing and we don't see

any particles below four millimeters and

I think actually what's really happening

here with this technique is the grid is

actually pretty large our flame is

pretty small and we basically kind of

have like a it's hard to get the as I

was saying before it's hard to get the

right resolution for particles when you

have a big grid and you're you need a

better spatial resolution so it's not

that this technique is is is completely

bad or anything it's just that it's

really hard to get the right spatial so

this is relative to the center of the

grid right and I just think that it's

hard to do that with this technique in

it and it does perturb the plant

especially this flame is not a very big

flame if you have a bigger flame it's

probably easier to do the more reliable

measurements using this technique

this is definitely a standard technique

that we've used a lot over time

okay so the conclusions for this is um

there are advantages we're not sticking

a probe in the flame and perturbing the

um radical distributions temperatures so

much we don't have services for for

quenching

um

um and catalytic you know reactions

um but there's definitely room for

improvement like especially we need to

figure out how to deal with the

coagulation at the lower uh Heights in

the flame and the other the other issue

with this technique is you notice we if

so when you have a hole in the tube you

can actually place it like if you want

to just look at the particles that are

in the center of the flame they're going

to have different you know they're going

to be very different from the particles

at the edge but when we send the jet

through the flame we're actually

sampling the edges and the center so we

we don't have that distribution if we

want to do the edge just look at Edge

particles we could move it over to the

edge but if we want to look at the

center particles it's really hard to do

it with this technique we have done we

have used it in a pre-mix flame where we

actually put the probes in the flame and

shot through the flame so it's a little

bit easier because you don't have these

Edge effects you don't have that

distribution radial distribution but um

so that's a real drawback with this

technique yes

oh that's a really good idea yeah that's

a really good idea the another thing I

thought about is not having a coanular

flame

and having so we we started to try to do

this

um but so if you have a slot a slot

burner

and on the ends so you get end flames

um but if you put nitrogen on the ends

instead of the oxidizer you can reduce

that effect on the ends and then you

could shoot down the center of the flame

so you basically don't have a you know a

cylindrical setup and that might be

another way to do that but we haven't

you know had time or chance to try it

there there's so many things so go for

it do it do it tell me what you see

um so yeah yeah so there I think there's

so many ways to improve this and this is

really an area that we need

um you know we need people working in

helping to understand soot formation and

evolution

okay so

um are there any other like thoughts or

questions on sampling

like the night begins

uh so how do you account for that so the

question is how do you account for

sucking an ambient oxygen from the

co-flow right into the flame itself

um so that's a really good question and

um we often will actually put the probe

so close to the flame that we can see it

you know perturb the plane like we'll

put it all the way up that's I mean this

is the hardest one of the hardest

problems is getting rid of the oxygen

um and you're right this is uh

um something that we really need to work

on and

um so we will put them it right up at

the edges of the flame but then that

also perturbs the co-flow and we don't

want to perturb the clothes it perturbs

the flow field for the co-flow and we

don't want to do that either

um but uh

I think with we have so much dilution so

quickly and it is cold so it's not in a

hot tube as it's flowing down the

nitrogen itself isn't getting hot you

know as we're I think it really helps

that we're actually quenching it so

quickly that we don't have oxidation so

so far we haven't seen oxidation with

this jet entrainment technique yeah

yeah you're welcome thank you

okay so yes

yeah

so you're are you talking about the

nozzles that are the sampling

of the fuel injector oh the the should

um accumulation on the fuel on on the

fuel injector yeah I don't know of a

good technique to look at that that's a

that's a really interesting problem

um and you mentioned Lai uh you

um the thing is metal can give you lii

as well so you know your whole system

can you know is is if you're trying to

look just at the so it is really kind of

complicated to do

um you might be able to uh look at the

spectral emission if you use use laser

Heating and look at spectral emission

because the metal and the soot will have

different emissivities and different

wavelength

um dependencies that would be an

interesting thing to try the other issue

is when you start to heat up that hot

with like when you're trying to do Lai

you actually blow things apart right if

you're hitting a surface it's gonna

really blow this off of your probe right

yeah so that's off of your fuel tube

that you're look trying to look at so

that's a really difficult problem

um

I don't have a good answer for you yeah

yeah now I'm going to start thinking

about it that's really interesting yeah

um

uh did someone else have another

question I saw another hand up I thought

yeah no

okay okay so let's talk about

um uh Institute Diagnostics so

um an area I spent quite a bit of time

like trying to uh figure out different

ways to measure set you know it's

actually a hard thing to do because

spectroscopically there are no like

signatures right it's just broad

um so it's it's hard to

um and except for the fact that it

changes the evolution of the spectral

signatures change over the lifetime the

evolution of the particles and the flame

so you can use that we talked about

um but there are Diagnostics we use and

I'll I'll talk about those and then talk

about ways that we can add new

techniques so one of the things that

I've been working on is as I was saying

before is x-ray techniques because we've

done a lot with um

with laser-based techniques and we're um

you know there I figure there have to be

some other uh opportunities to do other

techniques the problem with x-ray

techniques is they're hard to implement

you have to go to a synchrotron usually

or have some kind of X-ray Source in

your lab

um if you want everyone to do an

experiment of synchrotron it's actually

really pretty straightforward as long as

you know the the whole process of

getting beam time

um it's uh so that processes you

basically wherever you you want to go

there are synchrotrons in China and

Europe and U.S and you just figure out

like what kind of technique you want to

do look in the literature see what other

people are doing and then

um

there's a whole process for getting beam

time you just write a very short

proposal three-page proposal and you

submit it there's like two you know for

the synchrotrons I work work at it's

it's like two times a year you can

submit a proposal and then the Cycles go

like six months like sometime within the

next six if it if you get scored well

enough sometimes within the next six

months you can usually do your

experiment

um and then that proposal is valid

usually for two years or something like

that and so you can do experiments every

six months for the next few years if

your proposal scored well enough and

it's free like you know this beam time

is paid for it's a community

um uh facility

um and it's it's open to anyone like so

you can it doesn't matter if you live in

the US you can go to the one ones one of

the US synchrotron so any of these

techniques I'm talking about at a

synchrotron or is something you could do

um the what's uh the and and I'd say

there it's kind of like a big adventure

like and you're given beam time and it

starts at a certain time on a certain

day and you better make sure you have

everything working and you're all set to

go at that time that's the most

stressful part is like I we usually take

a ton of equipment with us and it takes

a couple days to set up and we're like

madly trying to get everything to work

before being the beam turns on

um and then you're doing these often

you're doing these shifts at

um or like night shifts or you know

sometimes 24 hour shifts sometimes 48

hour shifts you know it can be grueling

um so when I it was talking about

um working with people it's um this is a

time where you want to choose your

co-workers well because when things

aren't working at three in the morning

you don't want a grumpy co-worker you

want to have someone who's like oh yeah

we can solve this let's try this you

know um so uh it's it's a really

interesting experience if if you think

of something you want to try it's a

really interesting thing to do and again

touchingly I can tell you how to like

try to go through the process of getting

beam time

so so here's a technique that I've been

looking at trying to figure out if we

can use it to measure incipient

particles because you can't use lasers

to measure incipient particles because

the wavelength is too long compared to

the size of the particle

um with the you might you in theory

should be able to use this sax

measurements to measure these nanometer

size particles so far I haven't been

successful that doesn't mean it can't be

done but I'll tell you show you kind of

the complications of the technique but

basically you take your your X-ray beam

you send it into your flame it scatters

off of the particles

it also scatters off of the gas phase so

that's like one of the big complications

onto a 2d detector

and you know the 2D detector is

something that's sitting at the

synchrotron you know it's not it's that

it's part of the facility so you don't

have to worry about detection part you

don't have to worry about the com the

data collection part you know it's all

set up for people to use

um and and you look at the angle of the

X-ray scatter

um so this is what you know you might

get for Signal

um

and on your 2D detector

um and then you integrate as a mutually

so you integrate over the angle that

goes around you know the circle and you

end up getting

um a a curve that looks like this which

is signal as a function of this

parameter Q

which is the momentum transfer parameter

and that is basically this equation

Lambda is the synchrotron the wavelength

you're using in the experiment so from

the synchrotron the X-ray wavelength

um and uh Theta is the angle relative to

the

um the beam the scattered angle relative

to the beam

so so you end up with this

um

uh curve and you can see we did this as

a function of height in the flame and

you get different signals as a function

of a height of a burner

and then when we go and subtract the um

the gas phase background which is the

monster and the problem here

um we get something that looks like this

is what your signal looks like it

doesn't look all that pretty like it's a

kind of like noisy

um over in the right hand side and this

is exactly where you have the incipient

particles and so the problem is when you

have incipient particles they're not

that much different in size from your

gas phase species your gas phase species

are getting pretty big once you have

particle Inception right

um there it doesn't seem like they're

that big but they're they're big enough

to cause a problem when you go to

subtract the signal from the gas phase

species when you go try to subtract that

signal you also run into the problem

that um different temperatures which are

different heights in the flame different

heights and Flame will have different

temperatures so you can't you can't just

take a background with no soot and with

soot and subtract that background so you

have to figure out what is the

temperature effect you know the density

of gas will change as a function of

temperature and you're scattering off

different you know gas phase part and

and also what is the composition effect

because you have composition is is also

changing as a function of height in the

flame so it's not like you can just turn

off the burner make a background

measurement you turn off the flame make

a background measurement and turn the

flame back on again you just can't do it

you have to account for the um the

composition and temperature of the

background so that's where it gets

really hard

um and

um and and there are ways that we we try

to do that um and and did that for this

you know it took it took me about three

years to figure out an approach to

subtract the background for this for

this Flame

um so this is what it looks like when

you're running an experiment you have

like this huge tube that is your scatter

where your your light scatters down that

tube the detector on on this picture is

on on the left hand side the flame is

that little tiny orange dot where the

arrow is it's tiny compared to the rest

of the experiment

um and then you're not in there with the

experiment you're actually in a control

room it's like if being at Nasa and

you're you're running this whole thing

from a control room in this experiment

you actually have a little window so you

can even see if your flame is lit in a

lot of places like you don't actually

have any way you know maybe there's a

camera that shows your your flame to

make sure it's it's lit and you're not

flooding the the hutch with um your fuel

and oxidizer because that could be a

disaster

um often you'll run into the the beam

line scientists will

um be a little bit nervous if you have a

flame flame at their beam line for the

very first time because people have had

had issues with like lighting things on

fire in the different synchrotrons and

and people get really really nervous

about this

you have to just keep talking talk them

down from the cliff and and say that you

you can handle it

um

okay so so you so then you could take

that signal you have some kind of model

and there are different types of models

for looking at particles

um so uh then you go ahead and and uh

analyze the data using the model

um for scattering

um

so

um this is what the background signal

looks like as a function of temperature

like this is just a whole bunch of

different temperatures

um for the and you can see like that's

no sit that's like not the

uh part doesn't even include soot and

then you can see now

um I've broken it down so you can see

the relative signals depending on where

you are in the flame but five

millimeters that's where we saw a lot of

signal right up um with the Lai

um that's the bottom panel on the right

hand side you can see the signal is

really really low in that compared to

the gas the set signal is in blue the

gas phase signal is in green so this gas

based signal is higher than the set

signal in this region right where you

want to see incipient particles so

that's really the problem and then

there's like the instrument

um background just get random scattering

instrument and that's also pretty high

up there um in the pink and dotted line

um yeah so it it gets really kind of

kind of complicated so then there are

other techniques so here's this is what

you do for a sax measurement

um there's also

um uh small angle scattering Neutron

scattering so you can scatter electrons

or you can get our neutrons

um in electrons you're

um uh sensitive to electron density with

neutrons you're sensitive to the um the

nucleus so you might get different

information what depending on whether

you did

um sorry sorry x-ray scattering versus

Neutron scattering right so so comparing

actually doing these experiments and

comparing them is really interesting so

these people did this group did both

sacs and sands on the same configure

flame configuration so these are these

are actually two different papers

um and you can see uh the uh there are

advantages to sacs that you have better

resolution

um and and sensitivity

um

the measurements for both of them are

really difficult and the advantages of

of Sands Neutron scattering is you can

it's easier to actually figure out what

the background is relative to your

signal

so um so I feel like I want to try Sans

just to figure out if if we can under

like use that measurement to understand

the sax measurements better

okay

so that's um x-ray scattering oh yeah

and here are the results a comparison of

the results for sacks on the left and

Sans on the right

um so that so I've pointed out like two

um curves that are so and they're both

as a function of radius right on the

left hand side is

um the I tried to point out where this

where the measurements were made at the

same height of a burner and that same

flame and you can see that there's a big

there's kind of a difference between the

two measurements as a function of radial

position

um where

um sacks actually sees particles a lot

like the kind of the highest density of

particles is at the center where Sands

there's no part of you know goes the

signal goes to zero at the center of the

flame so it's kind of kind of

interesting so I kind of I really want

to understand more there's not a lot

that's been done using Sans but I think

it might be a really interesting

experiment uh to try yes what's your

question

okay so the question is how does

um is it are you talking about sacks or

um Sands how does that so let me tell

you how sex differs from xrd

um uh first so remember x-ray

diffraction is is uh we talked about

x-ray diffraction yesterday and that's a

way if you remember

um that can give you constructive and

destructive interference when you

scatter off of a material and you can

map out the different um the spacing

between the uh

um the the different layers for instance

in graphite or you can even see the

distance between the like the repeated

separation of the of one side of the

six-membered Rings in a sheet of six

membered rings so it gives you different

um kind of resonant uh like

constructively interfering

um building up of signal at a particular

locations

um the the difference between x-ray

diffraction and sax is that your um your

you have your detector pulled way out on

on sacs relative to x-ray diffraction so

you're looking at the small angles for

x-ray diffraction you're looking at

wider angles and x-rayed a fraction is

actually a lot like almost it is

basically uh wet what we call wax wide

angle x-ray scattering so that's what

xrd is x-ray to fraction is that what

your question was

yeah yeah okay so so that was that's a

really good question yes you're right

it's they're both x-ray scattering

um and I think I have something on wax I

actually went through it this morning I

don't remember seeing it

um but that's what wax is so wax is

x-rayed a fraction in situ so you can do

wax in a flame

yeah exactly

it's hard it's hard to do you have a lot

of the same problems that you have with

um sex and um and only only a maybe one

one very brave group has tried it

um uh and it's been hard for them to

extract uh information from the data but

but I think you could probably do a

better job if you used a free electron

laser so

um we're instead of having I should have

put in a a figure or of the Sinker house

so how does a synchrotron work right

this is kind of interesting I don't

think I I remember to put this in there

so so when you took physics a physics

class do you remember that if you

accelerate a charge in a field

um you'll get light admit emitted and

it's a relativistic effect it's called

bremstrom right so it's so you're

accelerating a charge in a field so if

you have

um

electrons

so a synchrotron is circular it's like a

big uh ring

um and you inject electrons into this

ring

and you you basically you often like

accelerate them in another ring and you

inject them to this ring so these

electrons are going around this ring

um but the way a real synchrotron is set

up is you basically have it doesn't the

electrons don't want to go in a circle

you have to make them go in a circle

right so you have these

um regions these um that bend the

trajectory so they're basically a

magnetic field the bends the trajectory

um so when you change the direction of a

of a moving object that's an

acceleration right it's changing the

direction it is an acceleration so every

time you change the direction of an

electron light will come off so what

happens is you have all these what they

call bending magnets or or you know

there there are other ways to bend

electrons but say they're you have every

time you put one of these magnet

magnetic fields here here you will get a

beam of photons okay so and then the

photons will have a distribution of

wavelengths and you and usually

different these in in a real synchrotron

each of these would be called a beam

line and each beam line has a different

wavelength they they kind of select a

different wavelength region for each

each beam line and then usually they put

you know the experimental stations over

here those are called end stations and

so when you go to try to get beam time

you you decide okay I'm going to work I

want to work with

um hard x-rays so that's you know like

say 20 kilo electron volts

okay and and you'll go contact the

person who's on that be mine or you know

there are a lot of other wave Photon

energies that you might want to work

with so that's how

um a synchrotron Works

um with a synchrotron then you have all

these beam lines and when it's running

there are maybe a huge number of groups

that are using the synchrotron for a

free electron laser is kind of

um an interesting thing where you

basically have electrons going linearly

usually down this like

um long maybe two two kilometer long

path and you wiggle you have these like

fields that will um make the electrons

wiggle and every time they wiggle they

emit some photons

um so it's like a it's like a uh a laser

with distributive feedback is what you

would call it in in laser science

that makes one long beam of coherent

photons where these are not coherent uh

lay a free electron laser is coherent so

so that's the difference so if you have

a a coherent Source you might be able to

do some really interesting wax type

measurements um with a free electron

laser

okay I mean that was an aside

um but uh I should have added some of

that in into into this lecture of

material

um anyway so that's the difference

between

so let's move on to some something that

you can probably do in your lab

um so laser induced incandescence is a

really

um popular really useful technique

um and people I think really started

using it um initially when when they

figured out how to do this in engines to

look at soot is sit

formation in engines and it's really

sensitive to soot and not to so if you

so what they did is they made

um some windows on say made a special

experimental cylinder with a quartz um

cylinder wall

so they could inject a laser or they had

an a laser beam coming from the bottom

where they reflected it along up into

the cylinder but either way you have

light coming into your cylinder you make

a light sheet if you're to look at just

the signal of the scattering you'd be

scattering off the field droplets plus

the particles that are generated

when you use

um laser induced incandescence you only

see the this formed because what has to

happen is you send your laser in it

absorbs the light it heats up and emits

incandescence and that's what you

measure

so you you don't get that from field

droplets one they don't absorb very

strongly and two if they absorb and heat

up they're just going to vaporize so

there are a couple of characteristics of

of the particle that you have to have it

has to absorb strongly and has to be

refractory can go to high temperatures

before it kind of breaks apart okay so

um so people use it for volume fraction

measurements

for particle sizing I'll talk about this

and then for understanding particle

maturity

okay

um yeah so those are the characteristics

um

and an incipient particles don't absorb

uh strongly enough

um and get to high enough temperatures

for you actually to be able to see them

so you only see mature particles

um okay

so this is kind of you know like what a

basic Lai type of signal would look like

where you have the laser profile in the

red dotted you know you have we usually

use something like a nanosecond type

time scale shorter time scales like um

femtoseconds too short to have the

process happen where you actually heat

the particle up

so it has to absorb and and

um that energy that's absorbed has to be

turned into thermal energy okay so

um at low fluences which is very often

where we use this technique

um you or it depends on what you're

trying to do

you have during the laser so the signal

Li signal is in blue the solid Blue Line

without symbols so um during the laser

pulse the um particle the signal

increases it's not instantaneous with

the signal because it takes a while to

heat up and then emit light the

temperature is the purple with dots

um so what happens is you can see the

temperature of the particle heating up

during the laser pulse as it is

absorbing and heating and then it stops

heating after the laser pulses over

and the bottom you actually have the

particle heats up really fast and then

it starts to blow apart like it gets to

the sublimation point and boom you start

to lose your signal because you loot the

particle vaporizes it sublimes so the

signal is dependent if you look at the

top the volume of the particle times

temperature to the fifth power

so your highly sensitive temperature of

the particle so you really want to get

that particle to high temperatures in

fact if you want to do volume fraction

measurements you want to get it to the

sublimation point then you'll then if

you know what you're you there all your

particles are coming to the sublimation

point the same temperature they're going

to stop there you'll be sensitive to the

volume fraction so right when the

particle hits its maximum that's going

to be your volume fraction so if you

take the peak I'm going to show you a

whole bunch of figures where you take

the peak of that Lei signal as a

function of time and plot that as a

function of laser fluids

um

and this is what that looks like

um so that's a laser fluids on the

bottom for both of these figures the top

is the peak particle temperature and the

bottom is the Pico laa signal so you can

tell a lot just from this um this

fluence curve

so this is so this is you know you know

you we'd have to modify how you

implement the technique if you're going

to do this in a sample that's varying

like a turbulent system and you want to

have instantaneous particle measurements

this is with a steady Flame

okay

um so this is you know an example of

what some of the people have started

with with Lai and notice that you know

you can you send your laser and and you

do it at like say 1064 nanometers where

you're not gonna it's not if you have a

shorter laser pulse a shorter laser

wavelength what's going to happen is

you'll excite

laser-induced fluorescence of the gas

phase species but you can take advantage

of that fact do a lower fluence

um simultaneously send in a lower

fluence shorter wavelength so 532 so a

yag laser has 1064s is fundamental

wavelength 532 is a doubled you double

it with a doubling Crystal so lii is at

the top so that's 1064.

um and this is a sheet of light so

you're looking at the Lai from this is

actually in um an engine cylinder

um in a direct injection diesel engine

um looking at where the soot is being

formed this the second image is the

laser-induced fluorescence

um from the 532 nanometer light and the

or that's actually 355.

um so that's again you do another stage

of of mixing

um and to get 355 from the yag and then

the bottom is particle image

philosometry with a double pouts

um to map where particles are moving to

get looking at scatter from particles as

they move okay so so that's actually

really cool to be able to send laser

pulses in and and get all this

information in an engine cylinder this

is under high pressure yeah

you know I don't know the answer to that

question that's a really so the question

is were they using the Sip particles as

a seed or are they using

um other the usually people will put

um particles into the flow to image it I

actually think they weren't I think they

were putting particles into the flow

yeah

um okay so so that's uh

um Lai

so it's it's a nice because it can be

used under a lot of different conditions

um this is how you would use

um Lai if you want to get primary

particle size

um so uh notice on the top it's the like

signal is a function of time so those

are you know you

like relative to the laser pulse it

would be nanoseconds on the very left

hand side and they've expanded the time

frame

all the way out I didn't show you that

in the figure I showed you as a function

of time but if you wait long enough the

particles are actually going to be

conductively cool

so um so that's why the particle the

signal is going away they're actually

cooling

um and in in this

experiment what they did is look at

different heights above the burner and

then looked at the time trace for the

Lai and then they used a model that did

a good job of conductive cooling because

conductive cooling is going to be

dependent on the surface to volume ratio

and you have a model that basically

solves the energy balance and mass

balance equations

um for so once you you heat it up with

the laser and you can back out primary

particle size

so that's what they did here so as you

increase in particle size the conductive

cooling will be slower right and the

smaller particles will cool faster

okay so that's that's a um I mean

there's a drawback in that you have to

have a really good model energy balance

model to do this there is another

drawback I'll introduce in a after

probably after the Break

um but uh people use this technique a

lot to measure particle size in situ

without having to extract from the flame

um this is another technique

um it's just a elastic scatter but you

can get a lot of information uh from

scattering like uh multi-angle wide

angle scattering this is these are 2D

images though just from looking scatter

you can get um primary particle size

here they've gotten volume fraction from

the scatter they have an estimate for

the radius of generation so how big that

that aggregate is

um

uh and then uh using all that

information then you can back out what

the number of primary particles is in

the um in your sample

um so what's nice I'd say the biggest

advantage of using these Gathering

techniques is you get aggregate size

it's it's a nice technique for getting

aggregate size

um in your lab without having to go to a

synchrotron

um yeah so we should take a break now

um and uh we'll reconvene in 15 minutes