Data on maximum pressures under constant volume ignition for C8H18 and C12H23

[randini]

Looking for a resource that gives maximum pressure data under constant volume conditions for the following common fuels:

C8H18
C12H23

Basically I would like to compare the results of our experiment to previously published studies. We start with a spherical constant volume chamber "bomb" and add a stoichiometric mixture of fuel @ different compression ratios (bar)and mass to find the maximum pressure of combustion in a constant volume container. Any studies out there that give basic maximum pressure values for these two fuels @ varying charge mass and compression ratios?

Should be simple stuff to find but I am not coming up with easy data for it. Thanks,

Randy

 

[25362]

Hard to find published studies on flames and pressure characteristics; one link that hints at combustion of liquid hdrocarbons is:

http://web.ics.purdue.edu/~lqiao/Pu... jet A and S-8 in laminar premixed flames.pdf

For pressure developed on the confined ignition on vapors, there is some info in Crowl and Louvar's Chemical Process Safety: Fundamentals with Applications. Prentice Hall.

 

[randini]

Thanks Chemical,

I searched through the link and will follow up on the other references you gave. The link seemed to focus on flame speeds more than the maximum pressures achieved by constant volume combustion so I will continue my search. Have yourself a great day!

 

[IRstuff]

Maximum pressure, for a constant mass of combustibles, would seem to be high dependent on the actual volume, and what the walls are made of. Since you refer to compression ratio, it makes me think you've already done the calculations to determine the theoretical final pressure. It seems to me that you know your inputs, and you know the amount of energy released, which must go into the heats of fusion and raising the temperature of the combustion products, which leads you to the peak pressure, assuming instantaneous pressure equalization.

TTFN
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[randini]

Hi IRstuff,

Maybe another way of putting it: Given stoichiometric constant volume combustion @ stated compression ratios, does volume need to be included to obtain max pressures? If volumes were to get real large I could see the potential of misleading data but will there be much difference in our experiments compared to others if we are using relatively close volume samples? We are using a non-stretchable insulated container (bomb). Also our spherical volumes are in the 3 cm radius' range.

Thanks,

Randy

 

[IRstuff]

I don't understand what compression ratio you are referring to.

I would think that with as small a volume as you state, the volume is even more critical. To the first order, doubling the diameter would reduce the peak pressure by a factor of 8, wouldn't it?

TTFN
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[randini]

Hi Ir,

I'll put together another example:

Bomb 1- Total inside volume 3 cm, 18-1 compression ratio (18 atm), stoichiometric mix of air and C8H18. combustion is assumed to be instantaneous, constant volume (no stretching), and no heat lost to walls or outside environment.

Bomb 2- Total inside volume 9 cm, 18-1 compression ratio (18 atm), stoichiometric mix of air and C8H18. combustion is assumed to be instantaneous, constant volume (no stretching), and no heat lost to walls or outside environment.

Since both samples are at stoichiometric conditions, same compression ratio, and same assumed constraints, does volume matter? In my mind, I see both of them achieving very similar final pressures since they have proportional masses of combustibles per volume. Thanks again,

Randy

 

[25362]

Assuming complete, perfect and instantaneous combustion (deflagration) with liquid hydrocarbons (using a spark?) and stoichiometric dry air as sole participants, the excess moles of the combustion gases would be about 7.5% over the original air moles (neglecting the liquid fuel moles).

The adiabatic flame temperatures would be around 2300 K. Knowing the volume of the bomb and taking the gaseous mixture as ideal it wouldn't be too difficult to estimate the overpressure. Am I right?

These links may be helpful:

http://www.docstoc.com/docs/27567371/Lecture-1-Introduction-to-Combustion-flame-temperature

http://www.eng.su.ac.th/me/elearning/ThermodynamicsII/7ChemicalReaction.pdf

 

[IRstuff]

OK, now I see. In the ideal case, yes, the two ought to be similar, assuming no loss of heat. In reality, I would guess the smaller bomb may have the same peak pressure, but the pressure/temperature will fall faster due to the fact that there's 1/27th the thermal mass thermally diffusing its energy into ~1/9 the solid thermal mass of the container.

TTFN
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[randini]

Chemical and Ir,

I appreciate your input and insight into this. The last couple of links are very good and we will keep those in mind as we continue along. It would be great to find data for this by those who have performed similar experiments with bombs. I guess that is the difficulty of finding the needle in a haystack.

To find other experiments under these conditions would be helpful in terms of materials used for the bomb structure etc. We assume a lot in the theoretical (which I think will be close to reality), however it will become important for our application to extrapolate the data of actual experiments to certain types of materials used for the inner lining of the bomb etc. We will be experimenting with insulating materials, such as ceramics for the lining of our constant volume combustion chamber, as well as others to accomplish a more reliable combustion process.

Given the uniqueness of our application, we anticipate some deviance from theoretical. As an example, our constant volume chamber will have some very interesting swirling affects for turbulence during the combustion process.

These journey's keep us fresh...?

Thanks all!

Randy