Forwarded: Re: Airflow measurement - again...

[Evert Rosseel]

> 1)  How would the system determine the direction of travel of the 
> particles?  More importantly, how does it detect direction changes in 
> dynamic flow, or what kind of errors are caused by turbulance and eddies 
> in the air flow, etc.?

The classical systems measure the velocity along one axis (1D). For 
2D or 3D more lasers are needed, and different colors are used to 
measure in the different directions (expensive !).

LDA systems measure the real particle velocity, which - normally - is 
equal to the flow velocity. This means turbulence is measured if 
present. In fact if turbulence has to be measured in flow in engines, 
very often LDA is used.

direction changes : see end of message

> 2)  Do you think this is applicable to inside the manifold, or on the air 
> inlet ducting somewhere.  It seems the laser lenses might be hard to keep 
> clean.

Yes. It has several disadvantages though :
- it measures at one location
- only when one and only one particle passes through the interference 
bands

This means that you need to do measurements over the whole cross 
section of the manifold, and calculate an average value to measure 
the flow rate.
If the flow is cyclic (as in engines) you also need to average over 
time (1 cycle). 

For these reasons LDA is only appropriate for research, if one is 
interested in the how the flow pattern changes over time for 
instance. This involves a lot of measurements, in several points in 
the flow. Only after computer processing the flow can be visualised.

Keeping the lenses clean is not a real problem for research, as it needs to be 
done only every few hours.

> > LDA requires particles in the air-stream to work.  Fortunately the
> > dust present is enough.  The system works by intersecting two laser
> > beams at the point you want to measure the velocity.  At the
> > intersection, the two beams form interference bands.  As particles
> > travel through the intersection, they reflect pulses of light at each
> > constructive interference fringe.  A photo-detector senses this
> > reflected light.  The frequency of the detected pulses is directly
> > related to the speed of the particle, which is related to the flow
> > velocity.  Note though that this is not a MASS flow detector

I also want to add that in the simple system described above, the 
sign of the velocity is not known. If the flow changes direction you 
need to give the  interference pattern a (sufficient) velocity of its own, 
which is done by shifting the frequency of one of the beams.

In some engines this flow reversion occurs even in the location where 
the mass flow meter (MFM) is installed. OPEL has a 4 cylinder 2 litre 
engine where this happens in a certain rpm and load range. The 
measurements of the MFM need to be compensated 
for this effect (some air passes the MFM 3 times !).

Also, at low load SI engines will have flow reversal in the inlet 
ports (valve overlap combined with a low inlet manifold pressure: the 
exhaust pressure is higher !).

Evert
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*    Dr. ir. Evert Rosseel                                             *
*    Laboratory for Machines                                   *
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