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From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Metal Autoignition Temperatures
Date: Mon, 16 Aug 1999 13:30:16 GMT

In article <19990810202737.24259.00005284@ngol02.aol.com>,
JHare10079 <jhare10079@aol.com> wrote:
>       I am looking for any information on autoignition
>in oxygen rich environments.

In general it's not so much a matter of something spontaneously bursting
into flames, but of impact, friction, etc. causing ignition.  As Jeff has
already noted, testing for this is problematic and the results tend to be
somewhat inconsistent.  (The orthodox tests also are probably rather too
conservative; they tend to show aluminum alloys as being unsafe, but
that's what's normally used for flight-weight oxygen tanks.)

>I'm playing with an engine
>concept that involves film cooling by oxygen that vaporizes
>next to the chamber wall. I am reliably informed that stainless
>steel or any other metal will burn at the right pressure and
>temperature.

True, for a suitable definition of "the right".  But do remember that the
whole point of any sort of cooling system is that it doesn't let the walls
get all that hot.  There have been tests of similar ideas; done well, they
work.

>       An equally good thing for me would be if someone knows
>of an inexpensive material that will not burn with oxygen at any
>temperature and pressure that could be bonded to the structural
>engine wall.

The only things which absolutely will not burn with oxygen under any
conditions are oxides.  (Indeed, many ordinary metals, such as aluminum,
react quite enthusiastically with oxygen, and are stable in air only
because they spontaneously form oxide films on the surface.)  But that
just changes the details of the problem:  now you have to worry about the
reliability of the bond and whether the metal will be attacked by the
oxide when things get really hot.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)


From: "Andrew Higgins" <higgins@mecheng.mcgill.ca>
Newsgroups: sci.space.tech
Subject: Re: Metal Autoignition Temperatures
Date: Mon, 16 Aug 1999 03:35:18 GMT

In article <19990810202737.24259.00005284@ngol02.aol.com> ,
jhare10079@aol.com (JHare10079) wrote:
>
>       I am looking for any information on autoignition
>in oxygen rich environments. I'm playing with an engine
>concept that involves film cooling by oxygen that vaporizes
>next to the chamber wall. I am reliably informed that stainless
>steel or any other metal will burn at the right pressure and
>temperature. I need to find out if the concept will work or
>need to be scrapped. Best for me would be a reference with
>pressure and temperature curves for various candidate metals.
>

There is no such thing as "autoignition temperature."

The temperature and pressure at which a metal (or any combustible
material) will ignite are complex functions of the material's shape and
size.  An obvious example is magnesium or zirconium:  In sub-micron
powdered form, these materials are pyrophoric (will ignite spontaneously
in contact with air), but in bulk form, they are extremely difficult to
ignite.  You can take a torch to an aluminum or zirconium rod and they
will not ignite, because heat is lost to the bulk metal faster than it
can be supplied by the torch.

Powdered metal autoignition has been studied in great detail, mostly for
application in solid rockets; there is an extensive literature on this
topic.  But the conditions under which *bulk* metal shapes (engines, for
example) will ignite is a poorly understood and little studied
phenomenon.  There was a project I worked on earlier this year for which
I needed to know if supersonic projectiles of a simple shape (spheres)
made from various metals (Al, Mg, Zr, etc.) would ignite due to
aerodynamic heating at supersonic speeds in air.  I ended up doing the
experiments myself with projectiles launched from a high velocity gas
gun, as there was really nothing in the literature to provide any advice
on this.

>
>       An equally good thing for me would be if someone knows
>of an inexpensive material that will not burn with oxygen at any
>temperature and pressure that could be bonded to the structural
>engine wall.
>

Copper isn't too bad and is easy to coat onto steel.  It can oxidize,
but it doesn't have an energetic enough reaction with oxygen to readily
"burn" under most conditions.  Nickel alloys are good too.

Ceramics, being oxides, do not in general burn, but are more difficult
to coat onto metal.  Aluminum oxide and various carbides have been tried
in the past with limited success.  There is a current project at Boeing
Rocketdyne to make a rocket engine combustion chamber out of silicon
nitride.  Probably outside your budget, though...
--
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University
     higgins@mecheng.mcgill.ca    Montreal, Quebec


From: Jeff Greason <jgreason@hughes.net>
Newsgroups: sci.space.tech
Subject: Re: Metal Autoignition Temperatures
Date: Fri, 13 Aug 1999 15:50:48 -0700

John,

  Almost anything will burn at *some* temperature and pressure.  I
have a lot of recent experience in this environment; unfortunately
for you, I'm going to keep some of that to myself :-)

  You are correct that stainless steel is out of the question if
you're going to be at any reasonably high pressure.

  I have a list like the one you're talking about around
somewhere, I'll see if I can dig it up.  However, there are *many*
different tests for oxygen compatibility of materials, and they
do *not* agree on how they should be ranked.  However:

  Copper, Nickel, and Monel (a copper-nickel alloy) are generally
accepted by all these tests to be good up to pretty extreme
ignition environments.  There are certain copper-based and nickel
based alloys which are also good, but then which ones are preferred
becomes very sensitive to your test procedure.

                                   -- Jeff

P.S.  Your concept has almost certainly been done; try looking
a bit deeper.  Remember, "Ten minutes in the library can save
you a year in the laboratory" -- though in this case it might
take a bit more.  If you want to e-mail me off-line, I might be
able to point you to a reference or two of interest.

JHare10079 wrote:
>
>        I am looking for any information on autoignition
> in oxygen rich environments. I'm playing with an engine
> concept that involves film cooling by oxygen that vaporizes
> next to the chamber wall. I am reliably informed that stainless
> steel or any other metal will burn at the right pressure and
> temperature. I need to find out if the concept will work or
> need to be scrapped. Best for me would be a reference with
> pressure and temperature curves for various candidate metals.
>
>        An equally good thing for me would be if someone knows
> of an inexpensive material that will not burn with oxygen at any
> temperature and pressure that could be bonded to the structural
> engine wall.

----------------------------------------------------------------
"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      ex-Rotary Rocket
thinking in quite the same way." --L. Yau  Propulsion Manager
   (Hughes is my ISP, not my employer)     <jgreason@hughes.net>


From: Doug Jones <random@qnet.com>
Newsgroups: sci.space.tech
Subject: Re: Metal Autoignition Temperatures
Date: Sat, 14 Aug 1999 20:44:32 -0700

Thermal conductivity is the key.  Steek cannot conduct excess heat
away from local hot spots fast enough, and ignites.  Copper alloys
usualy hold up quite nicely at huge heat fluxes, given good cooling.

A high strength copper alloy is the only useful choice.

JHare10079 wrote:
>
>        I am looking for any information on autoignition
> in oxygen rich environments. I'm playing with an engine
> concept that involves film cooling by oxygen that vaporizes
> next to the chamber wall. I am reliably informed that stainless
> steel or any other metal will burn at the right pressure and
> temperature. I need to find out if the concept will work or
> need to be scrapped. Best for me would be a reference with
> pressure and temperature curves for various candidate metals.
>
>        An equally good thing for me would be if someone knows
> of an inexpensive material that will not burn with oxygen at any
> temperature and pressure that could be bonded to the structural
> engine wall.
>
>
> John Hare

--
Doug Jones, Freelance Rocket Plumber


From: "Andrew Higgins" <higgins@mecheng.mcgill.ca>
Newsgroups: sci.space.tech
Subject: Re: Metal Autoignition Temperatures
Date: Tue, 17 Aug 1999 22:49:08 GMT

In article <37B9F157.397@netvision.net.il> , Boaz Ben-David
<bbd355@netvision.net.il>  wrote:
>
>Andrew Higgins wrote:
>>
>>  There was a project I worked on earlier this year for which
>> I needed to know if supersonic projectiles of a simple shape (spheres)
>> made from various metals (Al, Mg, Zr, etc.) would ignite due to
>> aerodynamic heating at supersonic speeds in air.  I ended up doing the
>> experiments myself with projectiles launched from a high velocity gas
>> gun, as there was really nothing in the literature to provide any advice
>> on this.
>
>Ah, and the results were?
>You ain't going to leave us as it is, are you?
>

I found out that aluminum, magnesium, and zirconium spheres (1.27 cm diameter)
fired at 2 km/s into pure oxygen at 5 bar pressure will burn quite nicely.  Mg
would even ignite at velocities as low as 1.4 km/s (about Mach 4) in 5 bar of
pure ox.

But none of these materials ignited in atmospheric pressure air at that
velocity, and the gas gun I was using couldn't push to higher velocities.  But
if a Zr sphere was firing first through a tube of pure oxygen at 1 bar, then
transitioned into ambient air, it could continue to burn.

I did not have time to do a thorough investigation of the ignition limits, and
won't be publishing anything soon, as the application of this work is kind of
sensitive.
--
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University
     higgins@mecheng.mcgill.ca    Montreal, Quebec


From: "Andrew Higgins" <higgins@mecheng.mcgill.ca>
Newsgroups: sci.space.tech
Subject: Re: Metal Autoignition Temperatures
Date: Tue, 17 Aug 1999 23:12:12 GMT

In article <37B9F157.397@netvision.net.il> , Boaz Ben-David
<bbd355@netvision.net.il>  wrote:
>
>Andrew Higgins wrote:
>>
>> The temperature and pressure at which a metal (or any combustible
>> material) will ignite are complex functions of the material's shape and
>> size.
>
>I would think that the protective oxide bond strength, will play a role too.
>The ratio of oxide density to its metal density is suspicious for the determination
>of what will happened.
>

Yes, that is another good point I should have mentioned.  There is an entire
classification scheme of metal combustion based on the metal and metal oxide
melting and boiling points, their coefficients of thermal expansion, and their
mutual solubility.  Messy stuff, but the tiny details can contain some
unpleasant surprises.

A classic example is boron metal, which is a truly wonderful fuel on paper
(volumetric energy density nearly four times as great as kerosene, and a mass
energy density comparable to the best cryogenic liquid fuels, e.g., liquid
hydrogen).  But a layer of molten, sticky boron oxide (B2O3 b.p. = 2580 K) on
the burning boron particle forms a significant barrier to ignition and rapid
combustion.

This subtle point of boron combustion (along with a few other subtle points in
the chemical kinetics) prevented it from ever being realized as a practical
fuel, despite hundreds of millions of dollars being spent on it in the 1950's
and 60's.

A good starting point in metal combustion is the review paper:

    Price, E.W., "Combustion of Metalized Propellants,"
    in "Fundamentals of Solid-Propellant Combustion,"
    ed. by K.K. Kuo and M. Summerfield, AIAA Progress
    in Astronautics and Aeronautics, Vol. 90, 1983,
    pp. 479-513.

available in your engineering library.
--
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University
     higgins@mecheng.mcgill.ca    Montreal, Quebec


From: "Andrew Higgins" <higgins@mecheng.mcgill.ca>
Newsgroups: sci.space.tech
Subject: Re: Metal Autoignition Temperatures
Date: Sun, 22 Aug 1999 06:46:31 GMT

In article <37BC85CE.1C39@netvision.net.il> , Boaz Ben-David
<bbd355@netvision.net.il>  wrote:
>
>Andrew Higgins wrote:
>>
>>I found out that aluminum, magnesium, and zirconium spheres (1.27 cm
>diameter) fired at 2 km/s into pure oxygen at 5 bar pressure will
>>burn quite nicely.  Mg would even ignite at velocities as low as
>>1.4 km/s (about Mach 4) in 5 bar of pure ox.
>>
>What was the calculated [or measured] T at those velocities?
>I vaguely recal that 0.9 Mach delivers about 170C.
>Don't remember the function of T vs V for supersonic-hypersonic velocities.
>Any help?
>

For supersonic flow, the stagnation temperature (the temperature where
the flow is brought to zero velocity with respect to the vehicle) is:

    Tstag = Tatmo * [1 + [(gamma-1) * M^2]/2]

Where "gamma" is the ratio of specific heats (gamma = 1.4 for air) and
"M" is Mach number.  This relation is good up until hypersonic
velocities (Mach 5 or so), when significant dissociation begins.

So, for air at Tatmo = 300 K, the stagnation temperatures are:

    Mach    Velocity    Tstag
    (V/a)   (m/s)       (K)
    =====   ========    =====
     1       340         360
     2       680         540
     3      1020         840
     4      1360        1260
     5      1700        1800
     6      2040        2460
     7      2380        3240

I found that Al projectiles have an ignition limit around Mach 5, while
Mg projectiles have a Mach 4 limit.  The stagnation temperatures at
these Mach numbers agree pretty well with these materials' boiling
points, suggesting that you have to reach the boiling point before
ignition.  The fact that zirconium burned, however, shows that this is
not a hard rule:  zirconium can burn without first boiling.

>
>If not too sensitive, what happened to the target? which was made from what?
>What was the penetration depth?
>

We were not interested in the impact dynamics.  The projectiles were
just shot into a big pipe packed with plates of steel, aluminum, and
carpeting.  We did not even clean out the pipe until all the experiments
were done.

I did these experiments using the University of Washington Ram
Accelerator Facility:

    http://students.washington.edu/~buckwadl/RAM/ram.html

where they fire hypervelocity projectiles to 2+ km/s every day of the
week, so they have a lot of experience stopping them.  But the
projectiles are always stopped destructively, and there is not much to
see after an experiment (fragments only).
--
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University
     higgins@mecheng.mcgill.ca    Montreal, Quebec

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