Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Exhaust System - theoretical

I honestly don���?�t wanna re-open a can of worms guys. To anyone that was offended by the last iteration of this post, I sincerely apologize. Further, I am not meaning to circumvent the lock on the other post. I just want your comments on the below email I got from my brother who is doing his PhD in engineering physics at Stanford (his PhD has nothing to do with fluid dynamics), I am merely giving you his background to gauge the relevance of his remarks.

He wrote:

���?The answer to your question isn't all that easy to come up with -- as the gas is compressible in general the flow behavior can be pretty complicated. However if we assume that the flow rate is always well below the sound velocity (though near a nozzle this isn't a great assumption), but the flow is still fast enough prevent significant heat transfer, then you can treat the gas as incompressible. In that case it is only the more restrictive of the two choke points that matters.

The turbulence induced by the converter mesh isn't all that relevant -- gas flow in channels is almost always turbulent.â�?�

I���?�m guessing his assumption that the gas flow is fast enough is incorrect (I would think that it probably is not fast enough at all.) I don���?�t wanna get into a super technical discussion that is probably over my head but if anyone is interested, could they run an analysis to ascertain if the speed of the gas flow would result in practical heat transfer in a fully warmed up exhaust or if such transfer is negligible?

Thanks

Exhaust gas is indeed VERY compressible. Therefore his assumption that it can be considered as incompressible is very incorect.

Proof?
For example - look at some exhaust backpressure comparisons (you can find them on the net for some tests done on the SRT-4, for example) in turbocharged cars when they change the size of downpipe or turbine size.

More Proof?
Nonetheless, if it weren't true then turbochargers would not be real - as turbochargers convert heat energy into kinetic and in the process they drop exhaust gas temperature as the gases pass through them and the pressure on the inlet side or a trubo is typically 20-30psi higher than on the outlet side (taking one example in mind) - although this varies proportionally to the amount of boost being used.