Air Turbine Design -- advice regarding a Hall sensor type air flow meter

[jT990]

Hello all,

I am after some advice regarding a hall sensor type air flow meter.

I know from experience that most hall sensor flow meters allow a bypass of fluid around the turbine/fan. However I would like to know what would happen if this was the case of the design below.

Specifications;
Inlet air velocity 0-11.5ms^-1
Inlet ID 1.5mm
Turbine diameter: 5mm
Air gap between turbine and housing: 0.1mm

I assume two things will happen,
1.) the turbine itself will impede intake air velocity and eventually reach zero.
and/or 2.) the turbine will rotate too fast and the turbulence created inside the housing will in turn impede intake air.

I will attach calculations shortly.Thanks in advance.

 

[Baluncore]

First a belated welcome to PF, and apologies for the delay.

I am after some advice regarding a hall sensor type air flow meter.

I assume two things will happen,
1.) the turbine itself will impede intake air velocity and eventually reach zero.
and/or 2.) the turbine will rotate too fast and the turbulence created inside the housing will in turn impede intake air.

You are correct.
In order for the turbine to spin, there must be a significant pressure drop across the turbine, and because the system is regenerative, with flow back upstream, the rotation velocity will be high with little net flow. This will not make a good airflow meter.

Since the volume passing through the paddle wheel is the same as the volume being returned on the other side of the wheel, there will be no net mass flow, apart from leakage, unless the absolute output pressure, p2, is less than the absolute input pressure, p1.

With a slight pressure difference, the wheel will need to turn many times to transfer one wheel volume of air, hence the higher speed.
The RPM will be proportional to; 1 / ( 1 - p2/p1 ).