# Effect on Flow measuring device due to turbulent flow of air

1. Jan 27, 2009

### Prasannak

effect on Flow and pressure measuring due to turbulent flow of air

Hi,

I want to measure the air flow rate using a device which will basically measure the velocity of air and converts that in terms of flow rate reading. For creating flow of air i have used vacuum pumps which is connected by using the L connector and T connectors with the tubings. In this test, i am worried about the turbulent flow in the L connector and T connectors , which can effecting in flow measurement because measurement of flow rate in terms of velocity. What i exactly wanted to know is whether turbulent flow effects the mass flow measurement? if yes, how to reduce the turbulence effect?

Last edited: Jan 28, 2009
2. Jan 27, 2009

### minger

First, what size are your tubes and what speeds are fluids? In many cases people talk about reducing turbulence when Reynolds numbers just aren't very condusive for that.

Also, many flow meters measure mass or volume flow, how exactly are you getting velocity?

Offhand, I would think you would want the flow to be turbulent. If you are measuring velocity at a point in the stream, and it's not turbulent, then there is a large velocity profile of which you could possibly measure.

3. Jan 28, 2009

### Prasannak

Thanks for the respons.

Tube dia= 40mm, Maximum velocity= 30m/sec
Principle of working of mass flow sensor:

An electrically heated thermoresistor
is kept at a constant higher temperature relative to the
medium (CTD mode). The heat dissipation into the medium
increases with the increasing mass flow. The heater voltage is
thus a direct measure of the standard flow velocity.

My assumption is, In laminar flow all the particles move in same velocity and the accurate measurement can be achieved. Please explain me how the turbulent flow helps in this application?

Basiccally, I need to test the component with the constant pressure difference @ the inlet and outlet. My pressure measuring device will measure the pressure @ single point. Due to turbulent flow pressure @ every point is different. Please let me know how to overcome this effect..

Last edited: Jan 28, 2009
4. Jan 28, 2009

### FredGarvin

What you're describing is a hot wire anemomenter. Are you making your own? If not, manufactures provide calibration curves to help account for variations in the run range and what heat transfer phenomena are happening at what velocities. You may have to pick a meter that is suited for your particular velocity range.

5. Jan 28, 2009

### minger

Yes, I am very aware at how those sensors work, velocity is backed out though; the real measurement as I mentioned is mass flow. ANYWAYS, your low will more than likely be turbulent anyways, however this is good. Laminar flow gives a velocity profile where the velocity at the center is much higher than the rest of the flow. Maybe someone has the equation, but IIRC, the velocity profile is exponential, i.e.
$$u(y) \propto U_{max}\left(\frac{y-h}{h}}\right)^2$$
You will get a more uniform velocity profile as the flow is tubulent. You are correct that the velocity at a single point will be slightly unsteady in time, however, time averaged will yield a more uniform velocity profile and laminar.

See the image here:
http://www.engineersedge.com/fluid_flow/flow_velocity_profiles.htm

6. Jan 29, 2009

### Prasannak

Thank U very muchhhh...

Last edited: Jan 29, 2009