Design of a turbine flowmeter to measure beer

[jolyont]

I've been given a project to improve the design of a turbine used to measure the flow of beer! I've trawled the web and libraries to find previous work in the hope of finding some handy pointers, rules of thumb and simple equations to guide me. However, I've had little joy.

Basic parameters are:
- flow rate range: 5 to 60 seconds per pint (0.56812 litres)
- inner diameter of bore = 8.9mm

The design variables that I've got to play with include:
- blade angle (should I go for high or low angle of attack to improve response at low speeds?)
- hub diamater
- tip clearance
- shape of leading and trailing profile of the hub (hemisphere on the front? flat on the back? what effect does the wake profile have?)

If you can point in the right directions then that would be great! e.g. journal articles, textbooks, web resources etc.

 

[minger]

Do you have to use a turbine-style measuring device? Turbines work by removing energy from the working fluid, either by a change in pressure (Reaction), or by fluid momentum (Impulse). In either way, I'd be concerned about "flattening" the beer (OH GOD NOOO!), either through cavitation or simply by "splashing".

A quick google gave me this link:
http://www.flowmeters.co.uk/turbine_meters.htm

They use a Pelton-style (Impulse) turbine to measure. For maximum power extraction, Pelton wheels are designed such that the cups move at 1/2 the jet velocity. For an application like this, I would think that the wheels would move closer to the jet velocity in order to minimize power extraction and pressure loss.

Calibration will be key.

 

[jolyont]

I've seen these pelton wheels before and they're great for lots of torque at low flow rates. However, it has to be a turbine. I'm told that these pelton wheels make bad yeast traps which are hard to clean and it makes the beer taste bad.

 

[FredGarvin]

Turbine flow meters are the way to go if you need fast response. They are a bit cumbersome and temperamental in a few ways.

First off, what do you think needs improving over the current design? We use these flow meters a lot and they are pretty tried and true. You need to better define what your end need really is other than "make it better." Make it better HOW?

The thing is that you can make a pretty good flow meter easily. The tough part comes in minimizing the drop across it and making the turbine stable. Usually they a bearing mounted and have a very tight tip clearance (obviously leakage past the tips is a no no unless accounted for in calibration).

What kind of pick up are you planning on?

The only other thing I am thinking is that technically, beer is two phase flow. You may want to do some research on measuring two phase flow with turbine meters. I have never had to do that.

 

[minger]

For the leakage part, perhaps you could shroud the turbine and use some sort of axial magnetic seal.

 

[jolyont]

Hi All,
To help things, I've attached a photograph of an example rotor, and the sort of response that can be seen. My mission (which I have decided to accept) is to flatten out the response characteristic i.e. reduce the low speed roll-off, and beware of the high speed roll-off.

What I really want from you good people, are
> pointers to good reference material.
> recommendations onto the ideal profiles for the front and rear of the hub.
> mistakes to avoid

Thanks for the tip on two-phase flow. Bubble break-out is something we have to keep an eye on!

 

[jolyont]

The pick-up is optical. So, I'm not too worried about the pick-up slowing down the rotor! (unless we use a 1TW laser, of course!).

 

[FredGarvin]

Start here:

http://www.sensors-research.com/articles/turbines.htm

http://www.flowmeterdirectory.com/flowmeter_turbine.html

As far as the hub design, that is going to be for a CFD guy to tackle. I would think anything streamlined would be fine. Just no blunt bodies.

Turbine flowmeters rely on one very important thing for their accuracy: proper installation design. You need to have at least 10D upstream and downstream of the meter to make sure you have fully developed flow through the meter.

The only other thing I can think of off the top of my head is not not turn them using compressed air. They use the fluid flowing through them to cool and lubricate the bearings.

 

[minger]

A google book search turned up:
http://books.google.com/books?id=pw...resnum=2&ved=0CAsQ6AEwAQ#v=onepage&q=&f=false

It (page 262 if link didn't work properly) listed a few things that while directly applicable to centrifugal pumps, may give you some places to start:

- Increasing impeller width flattens the curve
- Decreasing the number of vanes steepens the curve and moves peak efficiency down and to the right
- Milling the blade tips can also flatten the curve