You're not really missing anything. It's a mental picture issue. We view
these systems as having smooth flows when in reality everything that's going
on in an engine is violent and fast. Our mental pictures (and for that
matter, anyone's flow equations) don't describe this kind of flow, it's way
too complex.
For that matter, even in a very simple system there's no way to predict
turbulent flow effects except in very approximate simulations. I know people
think physics is very predictive, and something as simple as the onset of
turbulence in a smooth pipe of known diameter with a homogeneous fluid at
known temperature, pressure, viscosity and any other parameter you choose to
add should be predictable. NOT.
All anyone is trying to do with a flow bench is create a relative
improvement with something that we can measure--in this case a differential
pressure across the part we're testing. Turbulent flow, laminar flow, and
all that other stuff isn't relevant in any way, because these aren't simple
pipes with simple flows. At 7000 RPM in a four stroke engine flow is
starting and stopping dead in a single intake tract 8 times a second. The
instant pressure in the short distance of a typical intake varies hugely,
from substantial vacuum to several times atmospheric even in an
unsupercharged engine because of inertia at the intake valve and sonic
effects. The exhaust pulse is an explosion (literally) of extremely hot gas
that cools incredibly quickly as it travels down the pipe, also dropping in
pressure from about 100 PSI when the exhaust valve opens to atmospheric
pressure a few feet down the pipe. All that happens eight times a second
also. Laminar or turbulent? Huh?
It's really worthwhile to read Ricardo, even if you don't get the math. He
was directly measuring all this stuff 70 years ago. The tools have changed
but the complexity of the problems haven't. These systems are highly
chaotic, and simple descriptions are about as accurate and useful as a
second grade teacher telling kids that Columbus discovered America because
he was the first to believe that the earth was round. One sentence and three
major factual errors. Par for the course.
Bill Babcock
Babcock & Jenkins
_____
From: Group44TR7@aol.com [mailto:Group44TR7@aol.com]
Sent: Monday, December 06, 2004 1:35 PM
To: Bill Babcock; ryoung@navcomtech.com
Cc: fot@Autox.Team.Net
Subject: Re: large runner 1500 (single ZS) manifold needed (longish)
In a message dated 12/6/2004 7:51:22 AM Pacific Standard Time, BillB@bnj.com
writes:
But isn't the whole idea to develop laminar flow, not turbulent? At the
point where it changes, there should be a fairly big discontinuity in flow
rate, no?
Avoiding turbulence is the whole reason we try to keep passages straight,
smooth changes in cross-section, etc.
Its been along time since I open a chemical engineering book. However, I
seem to recall that the fundamental reason for keeping piping straight and
rounded was to minimize the resistance to any type of flow. I can understand
that would be very important to the passages leading to the combustion
chamber be straight and smooth. But I would think that we would want some
substantial mixing (turbulence) created as the fluid/gases entry in into the
combustion chamber itself. Thereafter we would want to remove the gases as
easily as possible, meaning as straight and smooth as possible. Seems I am
missing something.
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