Quote:
Originally Posted by Lasse
So I don't see much difference in
blowing or sucking air, but I really like to know more.
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If you look at the flow equations used to calculate flow across a test restriction such as an orifice plate you need to know, amongst other things, the upstream fluid density and the pressure drop across the orifice. The fluid density is calculated from the fluid pressure and temperature. When testing inlet flow you simply measure ambient barometric pressure and temperature and you know those aren't going to change during the course of a single flow test.
The upstream pressure at the orifice i.e. between it and the test piece will then be ambient pressure minus the pressure drop across the head being tested. Temperature can be assumed not to change during the air's brief passage through the flowbench. The upstream fluid density is then a simple calculation.
When testing exhaust flow all that goes out of the window. The blower motors are not only pressurising the air but also heating it, often quite considerably as you can tell by putting your hand over the exhaust of a domestic vacuum cleaner, and the amount of heating is going to change during the course of the test as the motors get hotter or work harder.
So at minimum you now also need a temperature probe between the orifice and the motors and you're going to have to read that at every valve lift. The pressure upstream of the orifice (what would be downstream of it in an inlet flow test) can still be calculated from ambient pressure plus the pressure drop across the head plus the pressure drop across the orifice.
Assuming I've got the numbers right a 40 degree centigrade rise in the air temperature through the blower motors would change the air density by 12% and alter the final corrected flow figures by 7%. That's a pretty big error if it isn't measured and adjusted for.
Finally, although in fairness it's a very small factor, as the temperature inside the flow bench changes the orifice itself will be expanding or contracting and so its flow capability will change.
I have no idea how different commerical flow benches take temperature into account or even if they correctly allow for the fact that upstream air density is not going to be the same as in an inlet flow test. I solved the problem with my own flowbench very easily. I don't have enough interest in exhaust flow to even want to test it so my bench works only in the inlet direction. My opinion about exhaust flow is it's pretty easy to get any exhaust port modified to a high efficiency from basic principles and any extra flow you're missing by not doing flow development is going to make very little difference to power anyway.
For all the above reasons if you are going to test exhaust flow it makes life much easier and removes several possible sources of error if you do it in the inlet flow direction by sucking through the exhaust port.
Dave