pressure measurements with piezoelectric transducer

[Darrell Norquay]

At 12:15 PM 8/15/96 +1000, Craig wrote:

>Wow, I bet that made a bang just right for 4am ;-)

Ya well, we ha a BIG muffler on the end (an old 300 barrel oil tank filled
with firefighting foam)  Still, you felt it more than heard it...


>What's the flame arrestor? I've heard that a fine mesh will stop a flame
>front (specifically, a friend had a 13B turbo rotary on propane, but the
>gas mixer was before the turbo. It wrecked the engine when 15PSI of
>compressed mixture ignited (& it bent the pipe from the turbo to the
>manifold.. I'm wondering if some mesh will stop this).

It was basically a tube of tightly rolled stainless steel mesh.  The
principle was that the stainlees steel absorbed the heat of combustion as
the flame front propogated through it.  In fact, it absorbed enough heat to
put out the flame.
This is the principle behind most flame arrestors, they consist of a mesh,
screen, or other series of small holes which break up the burning gas into
small "packets", which are easily reduced below the temperature necessary to
sustain combustion.  There are other problems, however.  If the flame front
is allowed to propogate down a tube, it will eventually reach a velocity
which causes the mixture to detonate, rather than burn.  If this happens,
pressures skyrocket.  The main reason for extinguishing the flame is to
prevent the whole pipeline from being destroyed if an accidental fire
occurs.  The entire system must be sized to contain the resulting pressures,
however.

As far as using this in an automotive application, even if you put out the
flame, you still have a residual pressure pulse (which does all the damage).
Also, the mesh needs to be several layers thick to be effective, which would
give far too much restriction to flow for any practical sized unit.  Some
propane carburetion systems have some sort of a rubber flapper valve which
is held closed by normal manifold vacuum, but can blow open if you have a
backfire, thus preventing damage like you describe.

IMHO, having a suck-through turbo setup with gaseous fuel is just asking for
trouble.  Temperatures inside the turbo housing can reach the autoignition
temp of propane, turning the whole thing into a rather large pipe bomb...

 


regards
dn
dnorquay at awinc.com