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>>Okay so I'm back with my area of expertise actually,
[laughter] which is microgravity combustion science.
And so I'm David Urban again and the, like Brian,
my branch manages the Microgravity Combustion Research
program that's conducted both, currently on the Space Station
and previously on the Shuttle.
And we also do work pertinent to that exploration,
particularly looking at spacecraft fire safety
as our main area of the applicability
of reacting systems to exploration but we also do work,
for example, In-Situ resource utilization
where you're reacting, in most cases,
soil to produce useful materials.
And these are a bunch of collage of pictures from various things.
Some of these expose the fire safety issues.
This is smoke particular from spacecraft materials.
This is a burning droplet so this is one
of the big reasons people like to do combustion research
and low gravity is it gives you access
to one dimensional systems.
So it's perfectly spherical, flames are possible
and it makes a lot of the study a lot easier.
Things change a lot.
This is a 1G flame spreading across liquid
and this shows you the zero G1, very different.
You don't have the buoyant flow lifting the flame up.
I'll discuss this in more detail but next slide please.
So first off the question is why does NASA care about combustion?
And despite all the progress we've made in energy, etcetera,
our primary energy source is still from combustion.
Roughly 85 percent
of the delivered energy they use every day comes
from a combustion system of one kind or another.
It's a primary cause of global warming and air pollution.
It affects people directly every day.
It's also an inherent part of many industrial processes,
really glass making, steel manufacturer, etcetera,
count on there being a combustion process.
You couldn't do it the same way
with an electrical system, for example.
And it's also a major source of the loss of property and life,
both in you know building fires and in wildfires.
It's the power source for portable applications,
you know cars, trucks, trains and it's a catastrophic hazard
for the man space flight program.
Also many new materials we're looking at, nanotubes, diamonds,
ceramics, etcetera, are made in combustion reacting systems
and so it really gives you a chance
to make a carefully controlled environment,
you know high temperature sensors conditions.
And people often argue that it's man's first technology
but it's also one of our most complex
and still incompletely understood.
Next slide please.
So on that note I think the biggest challenge
of the discipline is everybody goes oh,
fire, we understand that.
You know, I've made fires as a kid.
It's been so pervasive in the human life for so long
that people think it's well understood.
But the reality is that substantial improvements
in the quality of life in space, or here on earth,
will require substantial improvements in our ability
to predict and control combustion.
And that's a wildfire picture.
This just shows, I don't have a companion photo
but this is soot coming out of the earlier generation aircraft
and that's, soot control is one
of the major problems we have with air pollution.
The small particles you get
out of combustors are the very hazardous range,
are the PM2.5 particles are a big concern.
And this is a recreation of the MIR fire.
We did a ground base.
That's an estimate of what it was like.
Unfortunately they were a little too busy
at the time to take pictures.
And this is actually the hardware looking back from MIR.
It's showing you the dramatic destruction
of the oxygen generator so it's a real concern
for manned space flight.
Next slide please.
So things I'm gonna try to cover,
what areas can microgravity research contribute
to the fields of combustion and what progress have we made
and is there room for more progress?
And also, what can microgravity combustion tell us about
and contribute to exploration goals?
Next slide please.
So there's a lot of detail
within the general concept of fire.
And so there's, well spending too much trouble,
people that talk about combustion worry
about is it premixed or not premixed.
For example, a gasoline spark injection engine is premixed.
Your diesel engine is not premixed.
Is it a spray like in a jet engine or in diesel
and are we talking about particle cloud fires
like in coal mines and the grain elevators or are you talking
about flames spread across the surface in a building fire
or a smoldering, which is the initiation source of many fires.
And also finally are you trying to synthesize something
with the reaction system?
And the reason why we'd like to study the zone of gravity is
that within a flame the temperatures rise
from you know room temperature to as much
as 2,000 degrees centigrade and that causes a huge difference
in the density and so you have huge buoyancy driven flows
which dramatically overwhelm many
of the other things occurring within the flame
and you have settling stratification
of the components.
And the other reason is you can get a one dimensional spherical
flame, such as I mentioned,
which makes numerical modeling much easier.
And then you can also stretch the flame space in ways
that make it much easier to model
and so it gives you better opportunities
to make detail measurements of what's going on.
And the applications of all this are things
like explosion control.
A big one economically is increase deficiency
and reduce pollution.
For spacecraft we've got the big reduced fire [inaudible]
spacecraft and finally new materials in nano products.
We ensure one of the new areas
of industrial production these days.
Next slide please.
So what do we see out of it so far and there's some examples
that work to date has already appeared in a number
of textbooks, these are all sort
of fundamental combustion textbooks.
There's also a recent book on safety designed
for space systems, has a big section
on spacecraft fire safety incorporated
in the latest results.
And I recommend any of these if you want to look at,
this is a nice overview of all the aspect of combustion,
microgravity combustion, fire free fall
but there's also this book if you want to focus
on spacecraft fire safety.
This is the example that is most prevalent in the media,
is a good example for the lay person what they were
talking about.
This is a 1G flame and this is a candle flame,
picture taken of MIR.
And as you see with no up, there's no preferred direction.
The flame assumes the shape more or less like hemisphere.
And the other thing that's significant is
that there's dramatic reduction in the brightness of the flame
which means there's very little soot,
which is what makes flames bright
in this low gravity flame.
And so this has been very neat
because it's a very good cogent example,
it's a lot of educational material
and school publications.
It's really taking a life of its own, two simple photographs.
Next slide please.
So what do we care about flames spread?
The interesting thing is you know fire makes its own flow
in 1G so if you have a flame, the hot gas, as I mentioned,
rises rapidly and gives you a minimum velocity below
which you can't go.
Whereas in spacecraft you don't have buoyancy
and the only airflow you have is whatever the ventilation
system provides.
And that's lower than the airflow you get
from the flame itself.
So you have conditions of very low airflow exists
where you have the flame and low gravity.
And it turns out those are conditions
of increased flammability compared to 1G.
Airflow caused by the buoyancy supports the flame
but it also blows it out.
So the answer is that we have a condition,
this is oxygen concentration, this is air velocity,
this brown region is where it's flammable
and this is the minimum air velocity you'll get in 1G.
You'll see this lower velocity region
where you can have a fire, sustain a fire in low G.
And so we also see a similar non monotonic dependence
on gravity level.
So it turns out the most flammable conditions are
somewhere between the lunar and margin in gravity level
and so more flammable here on Earth
because you don't have the airflow blowing it out.
Another issue of concern is the actual ignitability
of material can increase with reduced pressure.
The burning rate doesn't necessarily increase
but things are more easy to ignite.
So the prevalent assumption that 1G is always the worst case
for fire safety is not really the, may be incorrect.
And so the area, there's a whole lot of work development
of predicted understanding of ignition of flames spread,
particularly for thick fuels is quiet incomplete.
Next slide.
So an example that's most you know for flame spread is
if you ignite a piece of material in the middle in 1G
and blow across it, the flame always goes downwind
and so forest fires, everything always spreads downwind.
In low gravity again, if you have, you can have conditions
of lower airflow and the fact
that it spreads quite the opposite
and it will spread upwind because it's seeking oxygen
and the reduced airflows you have in low gravity.
The wind, in this case, is coming this way.
This is the leading edge of the flame
and there's an oxygen shadow behind it
and it's actually pursuing upwind.
And so it's quite contrary, again,
to normal gravity experience.
And likewise at very limited airflows we have this bizarre
spider shaped flame spread
where again it's seeking oxygen boundaries.
Next slide please.
So in the very fundamental area, looking at premix systems
where you mix the oxidizer in the fuel
and then you try to ignite it.
Again the example in your life is your car engine but if you go
to very low energy conditions, a Russian mathematician,
Zoldovich whose name is in combustion literature
but he also was the one of the fathers of their Atom bomb,
over half century ago predicted you could get these steady flame
ball structures where the right conditions of the connectivity
and diffusivity of the gas, the flame would not grow.
It would just be supported by diffusion of the products back
in but no one could ever test it
because of the buoyant gravitational effects.
But Shuttle experiments demonstrated indeed this kind
of flame structure was possible and could persist
for you know tens of minutes without moving.
But likewise we've also looked
at other structures including cool flames.
And you can also establish a gradiences of reactivity
where you have different concentrations
across the mixture,
which wouldn't be possible, one, due to buoyancy.
So this gives you big opportunities
for real fundamental studies.
Look at again flame propagation to rate of activity,
cool flames which are particular case
of flame before the normal admission you think of.
They're still quite hot but they're cooler,
only a few hundred degrees C which is sort of initial phase
of ignition and it's of interest in, for example,
aircraft engine tank fires cause the ignition flame is going
to be a cool flame and then other limiting behavior.
Next slide please.
So looking at non premix systems like a Bic lighter type flame,
the buoyancy in the flame structure even
at high foode numbers and so it would be high velocity.
So simple flame shape models have been validated
by ground base microgravity testing providing classical data
for textbooks.
And a good example is even if Reynolds number, even if you go
into the turbulent regime where you think your velocity is
so high how can gravity matter?
It turns out that the low gravity flame boundary is way
up here compared to the 1G flame height
so there's a big difference, even in systems
where people would predict that the velocities or high end
of gravity shouldn't matter.
So it gives you a real opportunity
for ideal flames geometry for flame structure measurements.
Next slide.
Another area of big importance is metal combustion
and it was a very surprising result,
there has been much research of this type up in space
but it's an important area cause high oxygen systems you know
they have a lot of fresh oxygen up on the spacecrafts
to support the crew and one
of the standard materials used is copper bearing alloys
because they're deemed, they're found to be at low flammability
in 1G but it turns out whereas copper is considered the gold
standard of non flammability in oxygen systems,
it's quite flammable in low gravity in high oxygen systems.
And this is because the melted fuel behaves differently.
It tends to drip away, take energy out of the system in 1G
where it doesn't happen in low G.
So reduced gravity enables steady melt layer
and gas flow conditions you don't see in 1G.
And it's an area that's really poorly understood
and worthy of more work.
Next slide.
So again classic system, there's a candle flame
and this has been an excellent tutorial for the public
and classic diffusion system.
In 1G the airflow comes up.
The hot air rises up here and it draws air
in the bottom supporting the flame,
which gives that upward shape.
In low G the oxygen diffuses in
and the chemical products diffuse out
and so that's why you've got the nearly spherical shape.
And the net result of that slower process means there's
less soot which is why the flame is less bright.
But it demonstrated, also contrary, there's another belief
that in long term flames couldn't exist
in low gravity cause they'd run out of oxygen but it turns
out actually it's not quite the case.
If you have a reasonably flammable material they can
support themselves by diffusion alone in air environment.
In fact this particular test the candle finally went
out because it ran out of wax,
not because there was any shortage of oxygen supply.
[inaudible audience comment] Well we didn't have a good
measurement system in there but that's gonna be on the order
of a thousand C to get the blue colors.
But given the simplicity of the system
of these experiments we didn't have a temperature sensor
in there.
That's something that we would like to address in the near term
but we haven't had a chance to do it for that system.
Next slide please.
I've been mentioning soot and those that don't know
about combustion don't necessarily care
or don't understand why they should care
but soot is the dominant, is physical particles
of carbonaceous material that form in a flame
and it dominates the radiant heat
from the flame and the light.
And so if you're trying
to control the flames you've really got to control the soot
in the flame, both from the pollution standpoint
and also for the heat load.
It's why you feel warmth from your campfire
and why you can read by a candle,
whereas your gas stove has no soot in it
which is why you couldn't read by that.
And so going to diffusion flames gives you longer
residence times.
It gives you an opportunity
to study the soot processes more completely
and get a better understanding of the chemical kinetics
and also gives you large scales
which allow better measurements of the structure.
Next slide please, so I mentioned droplet combustion
and this shows, it's neither one of our more complex experiments
but it's really amazingly cool.
You have two needles, you generate a droplet of fuel
and then the needles pull away symmetrically
and you get a free floating droplet of fuel.
And you can only do this in a spacecraft type environment.
You've got a long term [inaudible] gravity
and then you can come in and with igniters ignite the flame
and then you have a burning droplet of fuel floating
in space with no disturbances.
It doesn't always work.
It sticks to the needle sometimes
but we've got a very good success rate releasing these
droplets and then you can have a beautiful one
dimensional system.
That's the droplet in there and that's the flame boundary
around it, versus the 1G
where you'd have the teardrop flame such as you'd expect.
And this gives you a beautiful model for environment
for testing your chemical kinetic models which you use
to predict the combustion.
And the applications are very substantial
because again 85 percent of our energy comes from fossil fuels
and about 90 percent
of the transportation sector is liquid fuels
of which a huge fraction of that is burned
in liquid spray droplet into configurations.
So gives you an idealized geometry
to develop fundamental data to predict this
and provide the building blocks
for detailed combustion engine modeling for systems
like jet engines, which again are sprayed.
Next slide.
So question was are there future,
is there significant potential for future progress
in combustion science?
And our experience was the research community was not
ideal limited.
A broad range of topics were pursued ranging
from fundamental combustion theory,
applied combustion topics
and exploration related investigations.
And it's really due to budget constraints
that reduced the amount of research in this area.
There's still many unexplored opportunities in these areas.
Next slide.
This just breaks it down into one way to split it up
and again I talked about this, the gaseous flames are the wide.
A lot of interesting things going on there
like the premixed and the non premixed.
You've got these triple flame structures, flame vortices,
depression, etcetera, droplet sprays, particles and dust.
It's very interesting both for single droplets and but also
in low gravity you can create arrays of droplets
or even stabilize in look at it whereas
in 1G you couldn't create these systems.
Likewise you create particle clouds
to study particle cloud combustion.
There was quite a bit of interest
in combustion synthesis, giving you an opportunity
to make unique materials that are important
for new industrial processes.
And then as I've mentioned fire safety is a big area.
I'll talk a little more about specific cases in there
but that's directly relevant
on future exploration is understanding the crew
fire safety.
And there are other miscellaneous areas looking
at propellants and flames with cold boundaries and development
of diagnostics for future experiments.
Next slide.
So terrestrial issues
where microgravity combustion can have an impact,
again I've mentioned energy.
If you truly understand these flames it can help us build more
high efficiency, low efficiency flames which can operate closer
to the flammability limits,
which means we can control the emission better.
And again, 85 percent of our energy economy
and small changes are very significant.
One area we're looking at is changing the structure
of the flame to make it easier
to sequester the CO2 coming out of the flames.
Typically you're gonna go to high oxygen flames.
And also reducing CO2 use of flames
that are high in hydrogen.
Soot controls are incredibly important.
And in other areas if we're talking
about hydrogen fueled cars,
hydrogen safety is a huge problem.
It gives, operating low gravity it gives you a chance
to create some of the stratified ignition scenarios you're
worried about for hydrogen refueling stations.
Next slide please.
So now the other question is can microgravity research make a
significant contribution in NASA's exploration goals?
Next slide.
So there's several areas affected by reactive systems,
one is fire prevention, detection, and suppression.
This ties into extravehicular activity.
It turns out the big tradeoff trade space
for spacecraft design really pitted fire safety
against the EVA because, as we'll describe in a minute,
this tradeoff in the atmosphere preferences.
We're also supporting insidual research utilization reactor
systems to produce useful materials on other planets
and again the environmental monitoring of controls,
specifically sensor design
and post fire cleanup for the spacecraft.
Next slide.
So the and particularly in fire safety are atmosphere selection
and material flammability.
How do you select an atmosphere so you have enough materials
to build your spacecraft so that it will be safe?
Detecting a fire in low gravity
where you don't have the buoyant flow to move the smoke all
to the ceiling where you can detect it easily
and how do you suppress a fire in low gravity?
And this is really based on all this long history
of ground based and space based fundamental research.
We've also accumulated enough understanding
to make substantial improvements in the very practical issues
of spacecraft fire safety.
Next slide please.
I mentioned the tradeoff with EVA.
This gives you an example of what is the atmosphere
like on the spacecraft?
Now this is the cabin total pressure ranging
from you know vacuum to one atmosphere
and this is volume percent oxygen from zero to 100
and so the Shuttle and the MIR
and the Space Station all operate
at basically sea level air so one atmosphere 21 percent oxygen
or as the early Mercury Gemini Apollo was pure oxygen
at about five PSIA.
And then working back Skylab was only
about 70 percent oxygen and coming back.
Now it turns out the human physiology
to the first order cares most
about the partial pressure of oxygen.
So they have a curve, it's called the normoxic curve
which is the constant partial pressure curve
and so the interesting problem is that fires care most
about concentration and the actual absolute pressure is less
important to a fire than the amount of nitrogen oxygen ratio.
So there really is a different response between the two.
So what happens is if you want
to go EVA you can get decompression sickness
from the suit so they pump up the oxygen concentration
and reduce the amount of nitrogen
and so normally they operate at 30 percent oxygen to prepare
for the Shuttle and Space Station.
But the problem is if you're going to the Moon
or someplace you want to be doing EVA's rapidly,
you don't want the crew to be getting decompression sickness
so the goal is can we find a hemisphere we can tolerate
from fire safety perspective that will provide,
reduce the risk of decompression sickness?
If you go too high on oxygen you can actually get
in trouble on oxygen toxicity.
If you go too far this way, away from that green line you go
into conditions where it's hypoxic
and the crew can't function as well.
And so there's a general region
where it's deemed unimpaired performance.
Blue line is roughly one mile elevations you know Denver type
oxygen partial pressure.
And that was found to be sort of the limit they could participate
in for long term low gravity human performance.
Next slide.
So after much discussion trading, the trade space
for the Constellation vehicles was gonna put us at average
of 30 percent oxygen but really the upper limit was 34 percent
oxygen at a reduced pressure.
And this would provide for the spacesuit conditions
which is pure oxygen three to four pounds.
You have no decompression risk.
But the concern was really actually we had relatively
little knowledge about low gravity
and material flammability in these conditions
of combined reduced pressure and increased oxygen.
So looking at it fairly quickly we did find indeed
as I mentioned that contrary to some intuitive understanding
as you come down in lower total pressure,
things become more ignitable,
although the burning rate is somewhat reduced.
But the increased oxygen concentration was
very significant.
And so it end up that the data we had,
data made it clear it was a fairly safe condition
but we really needed to get more, a complete understanding
and be more careful about our material selection
if we're going to be continuing to operate
in enhanced oxygen conditions.
Next slide.
So it's a new condition.
We have very limited material data in those conditions
because none of the U.S. spacecraft will operate
in those conditions and pressure effects had gone limited study
so the big question is do the flammability limits change
and do the ones we, system that NASA uses to measure this,
really represent the legitimate behavior?
And that's the big problem because all the materials
that NASA picks are tested in 1G
and so there's really not been extensive tests to verify
that NASA's test methodology is really the best choice
for approving materials for low gravity.
And some things were completely not examined,
actually for example, flammability of human hair,
which turned out to be sort of surprising
when we took a quick look at it.
Next slide.
So given the further areas
or given the increased flammability challenges imposed
by the new space of atmospheres, early detection is
of increased importance.
And until very recently virtually no work has been
conducted looking at detail of the detection
of fires in low gravity.
Whereas right now we're doing a test
on the Space Station this moment looking at the changes
in the particle size distribution
where if you're trying to detect a fire from the smoke particles
but there's really not enough understanding
of how you'd integrate that into a vehicle system.
Next slide.
So looking at hair, as I mentioned that, you know,
these are always dramatic.
Due to style reasons, women are always the offenders
with the longest hair and normally they keep it tied up
but a lot of them have actually ponytails where it fans
out quite a bit and if you undo it all it's quite a lot.
And the question is how big an issue is that?
There's a lot of experience in the shuttle and Space Station
that 21 percent is not a big deal.
But we're concerned going to higher level concentrations
about what the issues were with that.
Next slide please.
So we did a relatively limited study
but in using drop tower work and we really need
to extend this further.
But actually the results were dramatic.
This is oxygen concentration so this is 21 percent,
this is 30 percent, and this is flame spread rate.
And the important thing about this graph is its log rhythmic
and so you know it goes up by virtually in order of magnitude,
if you go from 21 percent to 30 percent.
So this huge increase in the flammability, both concurrent
and opposed, which means whether you're blowing
into the flame or with the flame.
So even in low gravity in those conditions it's faster than 1G
for one condition at 30 percent oxygen.
So it really goes from a situation where usually
when people's hair ignites you've got enough time
to respond and do something about it,
whereas in low gravity really you would not have enough time
to respond to put out a fire.
So it's really an actually frightening concern.
So that was one thing that was actually completely unstudied
and we're willing to pay more attention to it
as we talk seriously about going
to long term space travel in these conditions.
So in conclusion there's a whole lot of areas
of interesting research, there's been a lot of research
to date that's really benefitting pollution control
and increased combustion efficiency
in systems such as jet engines.
And we're now producing substantial results
that are applicable
to spacecraft fire safety conditions.
Any questions?
[silence]
>>Seems like some of the earlier slides you showed seems
to indicate that our Test 1 in NASA's 6001 seems
to be not conservative.
Is that your opinion?
Cause that's you know Shuttle Station
and Orion Constellation design is based on Test 1.
>>Well we're not sure.
The test one, conservatism is unknown,
I guess is the big concern.
I think it represented a good selection system for ruling
out more flammable materials
but it doesn't give you much flexibility as far
as cause it's currently still go no go test
so you don't know how close you are to the boundaries
so it doesn't give you as much flexibility
in applying materials as you want.
I think that's why we'd like to extend
>>But we have a way
of establishing what is called a flammable threshold
for every material.
That is a more extensive testing.
You can find the flammability threshold for every material.
>>For any given geometry you can do that and we're working
on systems that are better founded
in the combustion physics in that test which we'd
like to develop more completely.
But right now it's a reasonable choice but we'd
like to get a better system
where we do a better applicability
but right now we're okay.
>>The other thing that scared me a little bit when you said
about the copper alloy's because all our high pressure oxygen
systems are, as you know, made of monel and so
if these are true safety risks,
I mean I think you know your team needs to work
with the design standard team and I'm part of that so
>>Those results were developed by the White Sands group as far
as coppers and they're very aware of it.
The reality is the solution is make it thicker and so that's,
it turns out you don't have the wonderful security you have
with copper bearing alloys in 1G but any other questions?
Okay thank you.
[applause]