Chad,
Yes velocity increases as the pipe diameter goes down...(sheesh what a
gaff...) I should have said "velocity increases through the orifice and
back pressure also increases due to the restriction and its associated flow
separation and turbulence, effectively reducing the velocity in the
downstream remainder of the pipe by scrubbing the gas's energy off as heat.
It also decreases the mass flow potential of your system (engine + exhaust)
by leaving a lot of old gas in the pipe as the new gas is entering (like
using too large a pipe diameter only worse). Thats the big to small issue.
Whew an engineering ego is easily bruised...
In a sharp edged transistion to a larger pipe diameter, (cat outlet going
from 2.5 to 3") there is a significant loss due to the edge effects of the
orifice due to flow separation at the edges, effectively necking the flow
path smaller than the inlet. If the restriction was a smooth transistion
like a venturi, then velocity would rise, but minimal heat would be
generated and hopefully a minimal pressure head increase would result, and
the velocity would approximate the upstream velocity after a few pipe
diameters (50 or so in turbulent flow...)only minorly reducing your system
efficiency...
Nvr spout thery wen you dnt hve tme to type...
rip it up!
BKB
-----Original Message-----
From: RCPL-VAN CNIWR [mailto:RCPL-VAN_CNIWR@van.reid-crowther.com]
Sent: Wednesday, February 03, 1999 12:35 PM
To: dakota-truck@buffnet.net
Subject: DML: Attn: Bruce Bridges "Engineering Theory"
Hi, Bruce. I was reading your reply about restrictions to the cat and
differences in velocities between a 2.5" and 3". You had stated that
velocity slows when there is a restriction (like a smaller pipe). From
my days in mechanical engineering in college, the continuity equation
says that, if flow is the same, velocity INCREASES when there is a
reduction in area. This is the equation where A1v1 = A2v2. By
reducing the area, velocity goes up. You have a good point about the
boundary layer on the surface of the pipe. I guess you're talking
about friction loss and the resulting energy that the high velocity
air in the pipe loses.
Watching your velocity in the pipe is important for tuning an engine,
as I'm sure you'd agree. Scavenging can help produce more hp.
I'm not trying to flame anybody, but just bringing up some engineering
rules that are ever-present when working with fluid dynamics. Thanks
for the time.
Chad Niwranski
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 12:12:31 EDT