Reply To: Kylchap Exhaust Systems

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Martin Johnson

    Thanks for that link, Chris. You have partly saved me the trouble of replying. The Thermopedia data follows very closely the approach in the Engineering Science Data Unit Report 85032 – Ejectors and Jet Pumps Design and Performance for Incompressible Flow. There is another report covering compressible flow (sonic choking of blast nozzle) where the maths gets even heavier – Oh joy of joys!

    It is report 85032 on which I based my comments. Incidentally, as far as I can untangle the maths, the basic approach is the same as Porta uses.

    ESDU 85032 contains the following graph:
    ESDU Report Fig 3a

    which assumes certain typical loss coefficients for the nozzle, secondary inlet and diffuser, and also assumes that density of the motive fluid (exhaust steam) is equal to the density of the entrained fluid (combustion gas) which is approximately true. It shows the envelope of best basic design against volume ratio and pressure ratio.

    The graph is expressed in terms of volume ratio (gas / steam) and pressure ratio (draught / exhaust back pressure) and area ratio (blast pipe / choke). For a typical locomotive M is around 2 (varies with degree of superheat, grate area, working point etc. etc.)

    You will see that for M = 2, travel across to the dropping “M” line and you are well to the left of peak effy. You will also see that dropping down to the x axis suggests an area ratio of 0.12. With that set up you can expect an N value around 0.15. You would then need to size a blast pipe to give a back pressure 1/0.15 times the total draught. The choke would be 1/0.12 times the blast pipe area.

    The calculation would then be repeated using refined values for individual losses and density ratio etc. to optimise the design.

    I have recently waded through the Everett G Young paper which seems to confirm much of what a retired fluid dynamicist knows about diffusers, settling lengths and maximum diffusion angles. There are some very subtle design compromises to be made in terms of choke geometry and diffusion geometry to get the best out of a chimney, much of which will be down to experimental results – not calculation.