Air velocity measurement behind a small nozzle

BY saying behind a nozzle, do you mean to say, inside the nozzle.
Well if this is the case then you may be able to calculate the velocity using Bernouli's Equation
You will need to measure the pressure at the point inside the nozzle, where the velocity is to be measured. You may do so by using a suitable pressure guage at that point.
Now selsct a point just outside the nozzle where the velocity and pressure can be measured. Once this is obtained, apply Bernoulli's equation at those two sections. It should be



equate them together and apply a suitable loss of head due to frictional losses. since the only unknownhere would be the velocity of flow inside the nozzle, it can be easily deducted (h=distance of section from datum surface, can be taken as zero if nozzle axis horizontal and point of measurement lise on the axis, else measurement has to be made)

The International Standard Atmosphere gives a figure of 340.3m/s as the speed of sound at sea level. This means the velocity at the back of the nozzle is Mach 2.4! Is this correct?

well patents have been filed for accelerating compressed air to supersonic speeds

Compressed air nozzle

The discharge of air through the nozzle would remain constant. So using a method suited for your setup, measure the discharge of air at a point just outside the nozzle (like, at the vena contracta).

Once this has been achieved, from the geometry of nozzle, calculate the c.s area of the nozzle at the point where the velocity of fluid has to be measured.
You may then use the formula

Q=A*V

Sinve V is the only unknown, it can be evaluated

Hi ace,

If I read your question correctly and you want to measure inside a very small nozzle (too small even for pitot tubes -see below), to use the equations posted above (Bernoulli's etc.) the flow in the nozzle must be laminar.

If the design of the nozzle is such that you are able to maintain laminar flow at such high speeds, then measure pressures (& calculate velocity) a sufficient distance upstream of the nozzle (i.e. in the larger diameter pipe feeding the nozzle) and apply Bernoulli's equation (as Slugger suggests) to calculate the increase in velocity caused by the nozzle.

If flow is turbulent I don't think it can be calculated (anyone?).

A good method of measuring velocities of that order (and in confined spaces?) is by using a static-pitot tube system. They are used by aircraft (even supersonic) to measure air-speed.

Basically a static-pitot tube combination enables a precise measurement of air pressure(s) from which the air velocity can be derived.

To calculate air velocity you need to measure the 'ram' pressure using a pitot-tube which is positioned with the inlet pointing directly into the air-flow at the same time as the 'static' or ambient pressure.

These two measurements can be made from either two seperate tubes or a combination tube.

Once these two pressures are known then a calculation can be made to determine the speed of air.

As the pitot & static tubes are nothing more elaborate than hollow tubes, (albeit shaped) they are cheap.

If you are happy to have manual readings then you can use a manometer type arrangement to measure the pressures (which can be constructed to be as accurate as you need it) and a calculator (or slide rule!) to do the necessary calculations.

If you require an electronic readout (for display/datalogging etc.) then you will need pressure transducers, -here you will pay more for higher accuracy sensors so it is necessary to determine how accurate your measurement needs to be to determine equipment cost.

Another advantage of this method is that only the tubes (which only require very small openings) need to be in the airflow, -all your pressure measuring 'stuff can be loacated a convenient distance away at the other end of these tubes. Thus the tubes can be inserted into tight ducts, wind-tunnels etc.

There are plenty of 'Google' links to Pitot-static systems with formulae etc.

I hope the above helps.

To find the appropriate measurement device, we'll need a little more information about the setup. As stated previously, if this is air @ 800 m/s we're dealing with supersonic compressible flow; M=2.34. The next issue is the size of the nozzle since that will limit what types of probes/techniques could be used.

Instead of probes such as pitot tubes and not wire anemometers, there are some other techniques like LDV(Laser Doppler Anemometry), and PIV (Particle Imaging Velocimetry) that may work. Again, the resolution of these devices may limit their effectiveness based on the dimensions of your nozzle.

Also, what exactly are you trying to measure? - The avergae velocity?, the velocity profile across the jet?, the turbulence content?, the length of the jet core?, The centerline velocity?, The spreading of the jet? - all of this will help determine the best measurement device.

Any more info you can give us?

Hi everyone

Actually I am trying to test some thing at high velocities (Wind Tunnel kind of stuff) . I need to accelerate the air to 800 m/sec and little higher . I have with me the pressure available (air thru compressor). So I decided to design a CD nozzle in order to get the required velocity and place the object in it. The nozzle has been designed theoretically knowing the Inlet/Chamber pressure to get the required velocity.

Now the problem is that the nozzle jet is of 3in dia approx (open air, no test section of a Wind Tunnel))and I am placing the test object in it. I need to be sure that the average air velocity (approx.) is the required one for which the nozzle has been designed.

Regards

ace66

Sorry guys this was no guest but me ace66

Thanks for the extra info. Is there a reason you're ejecting the air into open space and not using a diffuser after the test section? If you're interested in the aerodynamics of the object that will be in your test section, you may want to consider a difuser aft of the test section in order to help control the expansion of the flow after the nozzle.

For measuring the core velocity, a simple pitot tube will be your best bet. You can find some good information on the theory and equations for the subsonic and supersonic Pitot tubes here:

http://www.efunda.com/designstandards/sensors/pitot_tubes/pitot_tubes_theory.cfm

Pulling the Mach number from the pressure ratio will require an iterative process. Once you have this, you will also need the temperature to determine the actual velocity.

With supersonic flow you need to be careful in that a shock will form ahead of the pitot tube so you need to account for the changes across the shock wave in order to determine the speed of the incoming flow. The other issue with this is that the pitot tube itself will interfere with the flow and generate its own shock, which will disturb the uniform flow you're trying to create so you may want to consider the ability to remove it or move it out of the way when the test article is installed.

You will also need to be very careful about the size of the model you install, since this will effectively shrink your cross-sectional area, and alter the flow speed arouind the object. If you can get your hands on the book, "High-Speed Wind Tunnel Testing," by Pope, it will cover all these issues anmd give you some good ideas to go with.

"you may want to consider a difuser aft of the test section in order to help control the expansion of the flow after the nozzle"

"Pulling the Mach number from the pressure ratio will require an iterative process. Once you have this, you will also need the temperature to determine the actual velocity".

If i have a CD nozzle with exit Mach no. of 2,3 or 4 in open air and I place the object say 3,4 inches back. Don't you think it is the right approach?. Why is the diffuser necessary? Its a jet in open atmosphere and no test section. and why is temperature measurement necessary

Many thanks

Regards

Ace66

It is also possible to use a thermal anemometer which could be smaller and would disturb less your flow. In general those probes have a warm element and are trimmed to maintain own temperature constant by increasing the current. Since heat transfer is proportional to speed of the fluid the current and the temperature are related and one gets a signal which can be directly recorded (voltage).

Hi Ace66

Compressed air escaping through an orifice will almost always travel through the orifice at the local speed of sound when the pressure ratio of P2 psia/ P1 psia is less than 53%. For example if the atmospheric pressure today is 14.7 psia (29.92) inches of mercury or 1.0135 Bars any gauge pressure above 13 psig will produce sonic velocity. The velocity of the air is limited by the sonic barrier, speed of sound, if the compressed air is 15 psig or 1500 psig. Trust me.

At higher pressure the air is more dense so the total weight of the discharge will be higher at high pressure but the velocity is pegged by the sonic barrier.

Hi

Do you mean to say that the exit Mach no. of the flow can't be greater than 1? Well you can have the exit Mach nos. in Convergent Divergent nozzles like 2,3, 4 etc. It all depends on area and pressure ratios.

Hi Ace66

You can create some wonderful things within the nozzle and we do with diffuser and expansion ratio but a sonic shock wave will bring the air flow back to subsonic velocity either in the diffuser or in the near vicinity of the exit to atmosphere. Some conditions produce the shock wave immediately past the exit. "The sonic shock wave is blown out." That sonic shock spoils all the fun.

Hi Everyone

thanks for the discussion, but I still donot have the solution. Any more suggestions?

Regards

Ace66

Guess I do not understand what you are trying to accomplish. I will watch the comments and may find out.