Turbulent Flow

[hydroman247]

After reading this thread I have been wondering about turbulent and laminar flow in hydraulics and whether a similar principle applies.

The basic idea is that if after a certain component in a system, there is turbulent flow, whether or not a 90 degree elbow will help cure this. I am no expert in fluid dynamics but I can't see it making much of a difference apart from the obvious pressure drop across the elbow.

In the thread above, he says that the exhaust gases in a 4m system could travel up to 15m because of the spiraling. I suppose it is possible the individual particles could be traveling 15m but the volume of gas would be the same and surely the elbow would just increase the pressure drop making it less efficient. I am aware this would be more of an issue in hydraulics.

Your opinions?

 

[btrueblood]

Hydro,

That thread was discussing having gross axial swirl flow in a pipe, i.e. downstream from a rotating component like a turbine wheel or pump, and whether the effect of a 90 deg. elbow would break up the swirl into fully turbulent flow.

Generally, the swirl will break down due to friction losses in the pipe in any event. Short of turning vanes or de-swirlers, I can't imagine that any great benefit is obtained by simple expansions or elbows, vs. letting pipe wall friction do its thing. But I have no direct data to support either way.

 

[zdas04]

I went over there and read the OP and then a couple of the responses. It simply amazes me how much people yearn for fluids to act in a logical manner and how bizarre so many people's logic seems to be.

Bottom line--the mass flow rate at any plane in the flow stream normal to the pipe centerline must be the same as the mass flow rate at any other normal plane unless fluids are added or removed. That is known as the "continuity equation" and it is one of the basic foundations of fluid mechanics.

How does that apply to this case? Well let's say that a simple 90[°] elbow will remove swirl (it won't but let's pretend), then before the elbow we have the tortuous helical path for the fluids and after the elbow we have straight flow with the maximum velocity in the center of the pipe. Great, but how do you satisfy the continuity equation when before the elbow we have to do all this swirlly stuff and go farther? Well, what happens is the continuity equation is satisfied by a combination of changing pressure and changing velocity. There is a tiny pressure drop and half a square root of tiny increase in bulk velocity. Net effect on friction is zero.

There are devices like the Gallagher Flow Conditioner (or the Laws Plate for the Brits) that actually will dampen swirl. If you put one of these in a swirling stream then you'll get a fully developed 9th power flow stream (95% of flow in the middle 80% of the pipe) and no swirl. This has been tested thousands of times (flow conditioning is a really big deal in gas measurement) and every time it is found that the pressure drop per unit length is the same before as after the plate to about 3 decimal places. This result shows that for a given mass flow rate in a given pipe size, roughness, and length there is no pressure drop improvement to eliminating swirl.

The engineer who claimed there you could lower the backpressure on an engine by putting an elbow in the exhaust pipe was making stuff up because he so wanted it to be true.

David Simpson, PE
MuleShoe Engineering

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