Just a tidbit that may be of interest or food for thought in this Norwegian thesis on pressure piling.
"The presence of “inert species” for example nitrogen or solid surfaces pose further complications. They are generally not recognized to
participate in the reaction, but may have a catalyzing role in some reactions and may affect the process as heat sinks or alter diffusivity. The
replacement of nitrogen with helium in a methane-air mixture will for instance triples the burning velocity (Glassman 1987)."
Although interesting, seems like an awfull waste of precious helium...
[Edited on 14-2-2018 by nitro-genes]Morgan - 14-2-2018 at 10:02
Maybe a balloon filled with methane and air and then another filled with an equivalent of methane, oxygen, and helium substituted for N2 would be an
entertaining auditorily detectable comparison/demonstration on the rate of combustion?
Or would hydrogen/air balloons have a dramatic increase in flame speed as well if helium were substituted for nitrogen (and hydrogen for methane) -
that is if some increase in the flame speed would be apparent to our senses with a hydrogen/air bang that already goes pretty fast?
Some of the highlights or aspects from the thesis ...
In experiments, radiation will be much larger for rich mixtures due to the formation of soot, which have high emissivity.
Since the laminar burning velocity of hydrogen is roughly 6 times that of methane, the time needed for the flame to arrive at the orifice is much
shorter. In the current geometry, the short distance, and thereby the small time span between primary and secondary ignition did not allow for a
significant amount of gas to flow into the secondary chamber. Consequently the level of pressure piling was severely reduced.
In nearly all conducted experiments, methane gave higher peak pressure in secondary chamber than hydrogen. The main reason for this is that methane
has a slow laminar burning velocity that causes late ignition in the secondary chamber and high peak pressure. Due to of hydrogen’s high (laminar)
burning velocity, less time is available for precompression of the secondary chamber and consequently peak pressures are lower for this gas. This
trend is expected to be valid for all geometries resembling the ones used in the experiment, but might not be invalid for large-scale situations.
In these situations peak pressure and rate of pressure rise can be several factors higher than in comparable single vessel explosions. The term
pressure piling or pre-compression is used to describes explosions that show such characteristics pressure development.
The computed values of the turbulence intensity in both chambers demonstrated that turbulence induced in the secondary vessel is a major factor
affecting explosion violence.
4.7 Sources of error
Resonance effects
In general the fast burning mixtures showed considerable local variation often of periodic character. Such effects could have been caused by resonance
effect in the geometry ...
Condensed water
After a few tests water will typically condense on the inside of the vessel walls and may represents a significant source of error. Water may
evaporate from the warm vessel walls during gas filling and the subsequent period of turbulence settling, altering the gas composition. Water in the
gas mixture may affect reaction mechanisms and heat capacity, whereas a small portion
of the water at the vessel walls may evaporate during the explosion. It is generally assumed that the explosions will be to rapid for significant
amounts of water to evaporate.
Some important geometrical factors are:
• Volume ratio between vessels
• Cross-sectional area of connection
• Scale
• Shape of vessel
• Point of ignition
Same physics as that driving choice of LOX/LH for rocket fuel. Why would physicists be reporting this as anything novel?
More interesting to me...
If a weapons program will be what it takes for the US government to reinstate the long term policy of incentivizing the stockpiling all the Helium we
can reasonably separate from natural gas and oil production (as they damn well should have been doing, no matter what it does to the Bureau of Land
Management's budget requirements or energy company inconvenience).
Fuck your quarterly statement. We're going to need that gas in the long term and we are not getting access to more than what's built up under those
domes any time soon.
(Rant mode disengaged)
Attachment: Light Gas Gun.pdf (6.1MB) This file has been downloaded 811 times
[Edited on 14-2-2018 by Bert]unionised - 14-2-2018 at 10:31
Same physics as that driving choice of LOX/LH for rocket fuel.
[Edited on 14-2-2018 by Bert]
No it isn't.
Rocket motors don't mess about with diluents.Bert - 14-2-2018 at 10:35
Not what I meant at all, but as long as you bring it up?
Lighter exhaust gas = higher velocity, given similar heat. Yes, skewing the mix does change heat as well.
Check and see if NASA uses a stoichiometric mix, or if they run RICH. Which equates to a light molecule diluent.
(Edit)
Yes. They run their LOX/LH engines fuel rich. Stoichiometric O/H = 8. Space shuttle is variable, runs between 4.1 and 4.8.
There are several other reasons running the engine with a light gas dilluent is done as well, a complex calculation of interaction between fuel
weight, hardware requirements and attendant weight needing to be brought all the way to orbit vs. combustion energy, physical characteristics of the
working fluid and more.
