Test Rigs for Exhaust Systems

[John Hind]

These two links are to reports on exhaust system testing carried in the USA in the 1930’s on 1/4 scale test rig

https://www.ideals.illinois.edu/handle/2142/4435

&

https://www.ideals.illinois.edu/handle/2142/4484

These show what has already been done.

However, the engineering world has moved on since the 1930’s in terms of measurement techniques and understanding of fluid mechanics.

Would we do the same today or rely on Computational Fluid Dynamics

[Chris Corney]

Am I correct in thinking that CFD can only model steady flows? There are a number of components in an exhaust system whereby pulsating flows can be exploited, for example Kordinas and de Laval nozzles. Also I think the inertia of the gas in the long chimney of a Lempor will have an effect. If the velocity of the gas is rising and falling all the time, what happens to the energy when it is reducing? Atmospheric pressure isn’t going to change, but the “peristaltic or piston” effect could cause some additional vacuum in the smokebox.

[John Hind]

Hi Chris – CFD has been used for pulsing flows – see this paper from 2009.

Numerical simulation of transient flows
in a vacuum ejector-diffuser system
V Lijo1,HDKim1∗,G Rajesh2, and T Setoguchi3
1School ofMechanical Engineering, Andong National University, Andong, Republic of Korea
2Indian Institute of Space Science and Technology, Kerala, India
3Saga University, Saga, Japan

Abstract: The objective of the present study is to analyse the transient flow through the vacuum
ejector system with the help of a computational fluid dynamics method. An attempt is made
to investigate the interesting and conflicting phenomenon of the continuous entrainment into
the primary stream with limited mass supply from the secondary chamber. The results obtained
show that the one and only condition in which a continuous mass entrainment can be possible
in such types of ejectors is the generation of a recirculation zone near the primary nozzle exit. The
flow in the secondary chamber attains a state of dynamic equilibrium of pressure at the onset of
the recirculation zone. A steady flow assumption in such ejector systems is valid only after the
dynamic equilibrium state.
Keywords: compressible flow, ejector, internal

Proc. IMechE Vol. 224 Part G: J. Aerospace Engineering

[Chris Corney]

Hi John,

It’s difficult to imagine what is being described, just from the abstract, but it might help locomotives with excessive smokebox pressure pulsations. It also reminds me of the drawing of Porta’s Lemprex, with the auxiliary chambers to smooth out the flow. My feeling is that it would be less effective in terms of basic draughting, and there are other options we could consider.

[Martin Johnson]

I notice the Indian abstract refers to

a recirculation zone near the primary nozzle exit

That seems to imply major flow separation, which can only happen with uncontrolled diffusion – an inherently unstable process. When I stopped real work, the state of the CFD art had not cracked un-controlled diffusion. Has it moved on in the last 10 years or so?

Pulsations – 3 ways round that:
a storage volume in the steam exhaust – not much space “up front”
larger blast pipe orifice – may not be feasible to achieve smokebox vaccum.
Graduated exhaust opening on the valve system – may impose too much exhaust back pressure.
Take your pick.

Martin

[Chris Corney]

Pulsations are not necessarily detrimental to a locomotive exhaust, unless they become excessive. It has been noted that a locomotive with an off-beat exhaust can steam better than one with the valves set correctly, although the effect of back pressure is likely to be greater.

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