Measurement of velocity in nozzle jet

[es335]

How do I measure the velocity after a nozzle? The flow is not stationary.

I was hoping to use some variation of a optical measurement like LDA, but as I recall LDA is using tracer particles (right ?), and that is not an option because I can not be sure that the feed particles is uniformly distributed in the nozzle-air.

Any suggestions to what method I can use.


Cheers

 

[hacksaw]

the best measurement method is dictated by the application specifics and the quantiy that you are trying to characterize (mass flow, local velocity variations, volumetric flow, etc.) and the process fluid and whether or not you are after an industrial measurment or a "wind tunnel" measurment...

 

[es335]

I am not sure that I understand your answer.


>the best measurement method is dictated by the application >specifics
The "thing" is a standard nozzle. M=1 is obtained.

>and the quantiy that you are trying to characterize (mass >flow, local velocity variations, volumetric flow, etc.)
I want to quantify the velocity

>and the process fluid
The fluid is air. Temp. approx. 40C

>and whether or not you are after an industrial measurment >or a "wind tunnel" measurment...
I dont know the difference.

I would like to chart the velocity profile downstream of the nozzle. The nozzle is active in an on-off process where it is supposed to blow dirt off a plate. If I can chart the velocity profile in free air in different distances from the nozzle it would be of use to me.

Cheers

 

[SooCS]

I used a pitot tube to measure the static pressure in a pipe and convert it into velocity. Dont know if it work for you. But you can refer to Loss Prevention in Process Industries by Frank Lee Section 15.29 which give a good description of a momumtum jet with details equations of velocity profile and momemtum. There are some reference listed too.

I used to collect some of the references on momemtum jets but have to serch for them.

 

[es335]

Thanks guys for your help.

But I don’t think that a pitot tube can measure the velocity. Pitot is only applicable in stationary flows (or slowly changing flows), and mine isn’t.

And hot wire is also best for stationary flows. Well, some may react quicker than a pitot, but the nozzle opening time is approx. 0.5sec and a hot wire cannot react so quickly.

Laser doppler velocimetry (LDV) isn’t that the same as laser doppler anemometry (LDA) which requires some sort of reflecting media in the fluid. And I don’t think that I can dose a media uniformly in my nozzle air.

I need a measurement on pure air in a short transient flow. Not an easy task And I don’t know what I shall do.

Cheers

 

[hacksaw]

LDA is the best bet for a detailed characterization in an R&D environment. The pitot measurment can given you a combination of static and stagnation pressures along with the localized flow. The speed of response is a function of your design, but generally that give a local average reading (10-100 milliseconds at best).

The Hot wire measurements are also very good and fast. They have been used to characterize turbulence down stream of bluff bodies in wind tunnels. The response time again is limited by your design, but is on the order of a few milliseconds. You'll need high speed data capture and recording.

The LDA is a costly option, but the most comprehensive and least invasive) in any event you will also probably want to have some CFD software to cross-check some of the measurements.

Good luck, on tough problems, I've found a pint or two essential to developing solutions..

 

[SooCS]

Found the article by Birch, Hughes and Swaffield, Combust. Sci & Tech 1987, V52 pp 161-171. Velocity Decay of High Pressure Jets.

The apparatus used was "a hot film anemometer probe" calibrated for the velocities over the range of 10 m/s to 310 m/s. Have no idea how it looks like.

 

[es335]

Hi guys

I always thought that hotwire was too slow, but I guess I have to revise that viewpoint.
I’ll try looking for that article and for companies that produce that kind of instrument. If you have any in mind, please share.

Otherwise, thanks. Your help is greatly appreciated.