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combustion/ionisation physics

combustion/ionisation physics - Physics Forum

combustion/ionisation physics - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 06-18-2005, 01:55 AM
Peter Lowrie
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Default combustion/ionisation physics



Hi

Can anyone help me determine formulae of combustion. I wonder if the
"ionisation" values of various fuels and oxidiser relate back to combustion
of the fuel. One other person described (the flame of) combustion as visual
evidence of a chain reaction.

Here's a model: Lets suppose we want to calculate the combustion output in
Joules of rocket fuel; Hydrogen and Oxygen. The Ionisation energy of
Hydrogen is given as 1312kj/(mole^-1) and it has just one energy level, the
oxidiser has three ionisation energy levels namely 1313, 3388 and
5300kj/(mole^-1). There are two Hydrogen atoms that have to be conjoined to
the single Oxygen atom. The second two oxidiser energy levels are unlikely
to factor into this event due to the Hydrogen bond taking up only outer
orbitals upon the creation of H2O. Therefore would it be reasonable to
assume that two times Hydrogens' ionisation energy times the first layer of
Oxygens ionisation energy equals the totalenergy output...or...(2 x 1312) x
1313 = E.

What do you think?

--
Regards,
Peter.
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  #2  
Old 06-18-2005, 03:35 PM
tadchem
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Default combustion/ionisation physics


"Peter Lowrie" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...
combustion
visual
the
to
of
x

Sounds to me like you really need a chemist, and I just happen to be one.

"Combustion" is a chemical reaction. Ordinarily, it can mean any reaction
that releases heat. More commonly, it refers to a reaction of a fuel
(usually a solid or liquid) with a gas (usually oxygen or air) to release
*significant* amounts of heat. The ionization energies of the reactants
(materials that are consumed in the reaction) are only part of the story -
the energy released by forming the products is also important. Sometimes in
a chemical reaction it takes more energy to produce the products than it
does to ionize the reactants. That results in an endothermic
(heat-consuming) reaction).

In combustion science there are four main types of reaction.

The 'flash' reaction produces an effect that extinguishes the combustion, to
the reaction cannot be sustained.

The 'sustained' reaction occurs when the physical and chemical conditions
allow the reaction to continue at a fairly constant rate - such as in a
common fire or flame.

The 'explosion' occurs when the pressure wave produced by the reaction (via
heating and greater volume of the reaction products) is sufficient to
initiate combustion in the unburned portion of the mixture, producing a wave
front (in which combustion occurs) that propagates at the speed of sound, or
maybe a little faster.

A 'detonation' occurs when the unburned portion of the mixture is ignited by
the heat radiation from the reaction, so the 'combustion front' moves much
faster than the speed of sound.

Both the explosion and the detonation are 'chain reactions.'

Calculating the heat output of a reaction requires knowledge of several
things, including the exact proportions of the reactants and products - for
example, burning carbon in oxygen can produce either carbon monoxide or
carbon dioxide, depending on the proportions.

Energy is either an ingredient or a product, as well, so one must calculate
the energy content of the various materials before and after the reaction.
The difference between the two is the energy absorbed or released by the
reaction.

One must also know the starting temperature and the final temperature to be
truly precise in the calculation, as it takes different amounts of energy to
heat different materials from the starting temperature to the final
temperature.

In general, the main quantity to account for is the "heat of formation" -
the heat absorbed or released when the material of interest is formed from
the pure elements at standard conditions. Since hydrogen and oxygen are
pure elements, their heats of formation are *defined* to be zero.

If necessary, you can also account for changes in the entropy and
temperature of the materials as these also can affect the change in the
'Gibbs' Free Energy' (a true thermodynamic quantity, not a Utopian dream or
a capitalist scam). It is the change in the free energy that decides
whether the reaction adsorbs or yields energy, and exactly how much.

HTH


Tom Davidson
Richmond, VA


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  #3  
Old 06-19-2005, 11:44 PM
Peter Lowrie
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Default combustion/ionisation physics

Dear Tom

tadchem wrote:


I see a situation here where the term air is exchanged for oxygen in the
air. The combustion of some fuels will never be complete because there
simply isn't enough oxygen in air and so some of the fuel ends up unburnt.

in

It is known that Hydrogen combustion is concussive, or a detonation...

or maybe a little faster. A 'detonation' occurs when the unburned portion
of the mixture is ignited by the heat radiation from the reaction, so the
'combustion front' moves much faster than the speed of sound.

H in O has a flame front approaching 3900 metres a second, curiously nothing
about the reaction leads to the fuel being consumed but rather it is
transmuted to water wherein the combustion constituents retain their atomic
identities

calculate

And so is this energy content equivalent to the atoms' ionisation energies?

be truly precise in the calculation, as it takes different amounts of
energy to heat different materials from the starting temperature to the
final temperature.

I've worked out that a tiny spark of some 200kj is enough to ignite any
amount of water gas (no air) and the final temperature is given in the
texts as 6000 degrees, albeit only for a fraction of a second

or a capitalist scam). It is the change in the free energy that decides

That's something I'll have to look up. As I understand it Gibbs Free Energy
is a quotient related to coefficiency of energy.

--
Regards,
Peter.
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  #4  
Old 06-20-2005, 01:56 PM
tadchem
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Default combustion/ionisation physics



Peter Lowrie wrote:


The term "air" is not always interchangeable with "oxygen", especially
in combustion science. The Space Shuttle booster rockets are fueled by
liquid hydrogen and use liquid oxygen as an oxidizer. The chemical
reaction is still called 'combustion.'


Beware. It is known that the phrase "it is known" often introduces an
idea based on unverified hearsay, is used by some people who can't be
bothered to do the research to discover what really happens, and
precedes misinformation frequently enough to be a 'red flag.'

Hydrogen combustion is not always 'concussive' or a 'detonation.' It
depends entirely on how the hydrogen and the oxidizer are pre-mixed (or
not). Hydrogen combustion is not concussive or a detonation in the use
of hydrogen-oxygen torches, for example:
[Only registered users see links. ]
[Only registered users see links. ]


That is a most curious statement, sir. You are telling me that the
reaction does not consume the fuel!


If you are talking about transmutation of one element to another, that
is a process that transcends any energies available from chemical
reactions - to the frustration of millenia of alchemists! Changing one
atom into another had to await the development of nuclear physics in
the early 20th century. That is a whole different problem from
combustion chemistry.


No. All the reactants and the products have an intrinsic energy
(actually enthalpy, but for discussions with those not trained in
thermodynamics, it is sufficient to call this property an 'energy')
called "heat of formation." The "heat of reaction" is the difference
between the heats of formation of the products and the heats of
formation of the reactants.


Given a mixture of the gases that lies within between the upper
explosive limit (75% H2/air) and the lower explosive limit (4% H2/air),
the hydrogen-oxygen reaction can be initiated by the *smallest*
electrical spark (number of kJ notwithstanding), by a heat source of
400C, by UV light sufficient to ionize either H2 *or* O2, or the
presence of either metallic platinum or palladium.

Look up the hydrogen MSDS (a PDF file) on this site:
[Only registered users see links. ]


Please do look it up. Gibbs' Free Energy is a true form of energy, and
not a ratio of anything to anything else. You have been led to a
misunderstanding.

[I have heard of 'coefficients' and I have heard of 'efficiency' in the
context of thermodynamics, but I have never heard of 'coefficiency' in
any context.]

Tom Davidson
Richmond, VA

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