Locomotive Exhaust Visualisation

[John Hind]

Today there are tools and techniques that developers of locomotive exhaust systems never imagined. The tools and techniques are used for better understanding of fluid dynamics.

Paste the link into your browser and see the basic mechanism that creates flow in a smokebox. The vortices created by the jet coming out of the blastcap entrain the flue gas from the fire and creates the pressure difference between the smokebox and the firebox that creates the air flow needed to burn the coal.

The video also shows how ‘Jimmies’ work or any

[Martin Johnson]

Well yes but………….
I am not sure if there are any other explanations, but I could not get any sound on the link, but it looks to me as though the Reynolds number is very low in this visualisation – I think we are looking at the Laminar / Turbulent transition as the flow regime changes shortly after the nozzle to the vortex producing turbulent. At low Reynolds numbers, viscous forces dominate and it is viscosity that generates vortices – vortices are impossible in a completely inviscid fluid.

Real blastpipes will be working at high Reynolds numbers, not the lazy flow you see in the video.

Also note that the accepted theory of blast pipe design relies on Bernoulli, Momentum exchange and Conservation of Energy – notice that list does not mention vortices.

The case of the “Jimmy” is also misleading, because the Jimmy is probably reducing efficiency of the system, but such devices also increase back pressure and hence the energy input to the system and hence the overall performance increases. That is not to be confused with well designed multiple nozzles, which will have no or very marginal efficiency drop.

So while the flow visualisation is interesting, I would be wary of reading too much into it.

Martin

[John Hind]

Chapter 12.1 of Applied Gas Dynamics by Ethirajan Rathakrishnan and published in 2010. Gives an explanation.

https://books.google.co.uk/books?id=2KG03ir8Z9UC&lpg=PA485&ots=13BQUPCqZn&dq=%22differential%20shear%20at%20the%20jet%20boundary%22&pg=PA485#v=onepage&q=%22differential%20shear%20at%20the%20jet%20boundary%22&f=false

There has been lots of work on understanding the physics of jets because of their applicability in a number of fields.

The chapter goes onto explain why ‘Goodfellow Tips’ work.

Through practical experience steam locomotive engineers developed solutions that worked and it is only now through research in other fields that we are starting to understand how they work.

• This reply was modified 5 years, 11 months ago by John Hind.
• This reply was modified 5 years, 11 months ago by Chris Newman.
• This reply was modified 5 years, 11 months ago by John Hind.
• This reply was modified 5 years, 11 months ago by John Hind.

[Chris Corney]

Thanks for posting that John. It gives useful information for the sizing and proportions of a blastpipe/exhaust system, but I still agree with Martin that the fundamentals are Bernoulli and the conservation of momentum.

[Chris Corney]

The above link (to the book) describes various vortex generators, but it doesn’t say much about how much back pressure these generate. I w3as also thinking about the visualisation video. There is some evidence that a convergent mixing chamber is better that a parallel one, and in quantitative terms its interesting to mull this over in conjunction with turbulent entrainment theory.

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