[Gasification] Burning velocity

[Arnt Karlsen]

On Wed, 10 May 2006 16:25:58 -0300, Daniel wrote in message 
<1147289159.18091.232.camel at strangejava.wort.ca>:

> On Wed, 2006-10-05 at 15:14 -0300, Kevin Chisholm wrote:
> > 
> > We all know that IC engines with 4" strokes can work at 3600 rpm,
> > but the  above "flame speed analysis" says they can't.
> > 
> > Could someone please tell me what I am missing here?
> 
> I suspect that the quoted flame speeds are at atmospheric pressure and
> ordinary temperature.  In the higher pressure (density) and higher
> temperature gas in a cylinder at the moment of the ignition spark, the
> flame speed is likely higher.  And as combustion progresses, not only
> is the continuing compression of the rising cylinder increasing the
> pressure density and temperature, but the volume is being reduced too.
> And the heat addition of the combustion is in turn increasing pressure
> and temperature (though not density).
> 
> If I had to guess at the mechanics of gas flame speed, I would SWAG
> that flame speed is proportional to density (closer particles bump
> into each other more frequently), and also to the square root of the
> absolute temperature (i.e. mean particle speed - faster particles bump
> into each other more frequently).
> 
> So if you ignite a charge at ~35BTDC, let's just assume that the
> temperature is 400C (from compression), and the volume is about 1/4 of
> max.
> 
> the flame speed increase due to faster molecules would be
> sqrt(700K/300K) = 1.5X
> 
> and the increase due to increased density would be 4X (gas is 4X
> denser).

..I thought they'd slow down on compression?  Compression also means
"shorter lenghts to burn", so the end result remains workable.
(read up on combustion>>todo-list)

> Combining these two, this would increase flame speed by 1.5*4 = 6X. 
> So hydrogen (300cm/sec at atmos. temp and dens.) would have a flame
> speed of about 1800cm/sec, and pure CO (really slow at 30 cm/sec)
> would be 180cm/sec.
> 
> To burn through that inch of headspace (call it 3cm), assuming the
> worst case of ignition at the end rather than the centre of the
> charge, would take (for slow-burning CO) 3cm/180cm/sec = 1/60th of a
> second.
> 
> But it would actually take less than 1/60th of a second, that 180 cm/s
> is the _initial_ burn speed at ~35BTDC condition - remember, the burn
> speed is going to be increasing (hotter, denser gas), and the volume
> decreasing (piston still compressing), as the piston approaches TDC.
> 
> So you see, I've just shown that an engine can run at 3600 rpm.... ;-)
> 
> 
> 
> (for an extreme example of this, compare the burn speed of say a
> teaspoon of smokeless rifle powder in the open, to the burn speed of
> the same amount inside a loaded rifle cartridge.  The former is about
> one second of a soft, quiet yellow flame.  The latter is about
> 0.2-0.5ms, and anything but a soft quiet flame.  Same thing going on -
> the burn rate increases as the charge burns, causing the pressure and
> temp to rise, and feed back into still further increasing the burn
> rate... there is an initial exponential growth rate)



-- 
..med vennlig hilsen = with Kind Regards from Arnt... ;o)
...with a number of polar bear hunters in his ancestry...
  Scenarios always come in sets of three: 
  best case, worst case, and just in case.