Tractorsport Flowbench Forum Archive

Posted: **Mon Feb 19, 2007 11:46 am**

Flow depression and testing???

I have been looking at both single and two stage motor configurations from both a theoretical and an application standpoint. In testing and trying to learn to listen from others on the board I have learned that with motors flowing in series below a given retraction point (my guess is the diameter of the impeller opening) this configuration can generate larger depressions than in parallel.

So am I correct to assume that for a given restriction as depression increases across the restriction that velocity is also increasing? At some rate?

If so has anyone exceeded the supersonic boundary during flow testing?

Just working on a test theory here,

Rick

Posted: **Mon Feb 19, 2007 6:52 pm**

Posted: **Mon Feb 19, 2007 8:25 pm**

Posted: **Mon Feb 19, 2007 10:15 pm**

[color=#000000]Ok; here is the avenue I am heading down, correct me if I

Posted: **Mon Feb 19, 2007 11:40 pm**

[quote="1960flh"][/quote]
[color=#000000]Rick,

[quote]Ok; here is the avenue I am heading down, correct me if I

Posted: **Tue Feb 20, 2007 3:44 am**

You can have sonic blowdown,given that you make a restriction (read: rocket nozzle) just below the valve seat,and keep the CSA of the exhaust port slightly expanding all the way to the port exit. If the sonic wave (shock wave,not the pressure) have area to expand so it doesn't "bounce off the walls" and converge on itself,it won't choke (noticeably,that is). That's my take on T.O.O's "critical area rule". On the flowbench,such a port screams...it seems very turbulent for the first couple mm's of lift,then it howls. When that happens the test pressure "locks". You can open the valve 0.1" or 0.2" more,if it was at 28" before,it stays at 28". But the flow increases... The pulse generated in the running engine is then strong enough to make positive pumping work. It sucks the piston up. That's why LW found power at 22:1 CR even though it "should" not make more power with that high CR (I read somewhere that 17:1 is "The limit") due to pumping losses,it has less PW than you'd imagine...

Posted: **Tue Feb 20, 2007 3:27 pm**

North Ameican Aviation (Rockwell, now part of Boeing) has a windtunnel that is "blowdown" from very large tanks into an 8 foot square throat. Max speed is 3 times the speed of sound. It takes some very careful design and construction, but possible.

Doug

Posted: **Tue Feb 20, 2007 3:55 pm**

It has been my understanding (or misunderstanding) for a long time now that port velocities in excess of mach 0.55 tend to hurt flow and performance.
On the other hand, sonic wave tuning has to do with tuning the runner or pipe length so the sonic wave is arriving at the intake valve just as it opens to influence cylinder fill, and to time the exhaust length to catch the pulse going out to help pull the blowdown. Oversimplification, but I think it's correct.

Posted: **Wed Feb 21, 2007 8:23 am**

Posted: **Wed Feb 21, 2007 11:19 am**

My head was stuck in the intake port when I made that post! Sonic velocities in the intake is a bad thing, the exhaust is a different story.

Posted: **Wed Feb 21, 2007 6:01 pm**

I used to work in a nationally accredited standards laboratory here in Australia. The physics lab had an airflow bench that would make you guys drool. Anyhow, they used "sonic nozzles" as airflow standards.

The way this worked, you fitted the sonic nozzle onto the top of the flow bench, just as you guys would fit one of Bruce's standard calibration plates. A very strong depression is then created beneath the sonic nozzle by cranking up the airflow through the bench. The nozzle roars, as the air pours through the smooth venturi, and this air roar gets louder and louder as the depression and airflow is increased. Then suddenly it goes dead silent !!!

When the nozzle goes sonic no sound can escape back out through the entrance. Reduce the airflow just sightly, and the sound is deafening. It is an absolutely fascinating demonstration to watch. Once the nozzle goes sonic the airflow cannot increase any further no matter how high the depression behind the nozzle is raised. That critical sonic flow is the rated flow for that particular flow standard.

And it works the other way too. Pressure discharging through a nozzle can go sonic, and limit the flow to some fixed known flow rate. A very common application of this principle, is to set the flow through very large steam turbines. The steam turbine discharge nozzles go sonic, and steam flow then becomes completely independent of boiler pressure.

Another familiar application is the convergent/divergent cone at the tail end of a rocket. The throat of a rocket motor always goes sonic, it would not work otherwise. The flow is limited, and the internal pressure inside a rocket engine can reach thousands of psi. Without a sonic nozzle to limit flow, the exhaust would just empty out at the back, without generating any significant internal pressure inside the engine. It is the very high unbalanced internal pressure that provides the thrust. And the only way to hold all that pressure in, is with a sonic nozzle.

Exhaust valves, and notably turbo exhaust housings can go sonic too. If your turbo goes sonic in the turbine housing, exhaust manifold back pressure rises incredibly high, but boost pressure can rise no further, because turbine flow reaches a plateau and can increase no further.

Exhaust valves always go sonic during the first few thou of lift. It is often called "critical flow".

Ever run an engine with no exhaust manifold, just open ports ? Even just at idle, that really harsh exhaust crackle, is a sonic shock wave. At the very first instant of valve opening. Fairly similar to the very sharp "crack" you get from the muzzle of a high velocity rifle.

So exhaust valves don't just go sonic at full power, they always do.

Posted: **Wed Feb 21, 2007 6:58 pm**

Posted: **Wed Feb 21, 2007 8:09 pm**

Tony,

We, too, use sonic nozzles to calibrate our Ford Research bench.

Tom V.

Posted: **Wed Feb 21, 2007 9:01 pm**

[color=#000000]I know I started this thread but have been enjoying the ___ out of reading the responses, Thanks. It was my gut (and some college physics) that told me the Crack of the exhaust was a sonic wave and has lead me to really think that the critical point of valve opening could play a big role in improved performance. Don

Posted: **Wed Feb 21, 2007 9:16 pm**

Tractorsport Flowbench Forum Archive

Flow depresion - Effect on velocity