Reply To: Porta's Derivation of Lempor Theory

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Martin Johnson

    Hi Chris,
    Thanks for the reply.
    I had read this “bounce” stuff in Nigel Day’s writings, which claims that only those with understanding of boundary layers would understand it. But that is as scientifically rigorous as Abracadabra or perpetual motion. There do not seem to be any comparitive tests (physical or CFD) of axial or splayed nozzles. If anybody does know of such tests – please declare it!

    I also have problems with supporters of the “Conservation of Momentum” supporters taking on the “Bernoulli” supporters in some kind of pitched battle. In my universe, universal laws are universal and do not give way to other laws on a whim or fancy – In short both C of M and Bernoulli must BOTH apply at all times in all situations.

    So, you asked for my views. I think the splaying of the nozzles might be a method of injecting higher energy fluid toward the walls of the mixing chamber and diffuser. Higher energy fluid at the wall would help to suppress flow separation at the transition from the mixing chamber into the diffuser. If you look here at the bottom left sidebar “Lempor CFD?” then go to the first CFD velocity plot, there is dark blue at the walls as the mixing chamber transitions to the diffuser – as you would expect. That indicates incipient flow separation in the area, which means the diffuser is not working as well as it could. (Actually when I stopped working, CFD had not reached a stage where it could RELIABLY predict such separations, because you need a very fine grid in the area of separation to accurately predict the separation; a point that I suspect is still conveniently forgotten).

    However, in the extreme case of an annular jet pump (driving fluid injected in an annular ring around outside of pumped fluid) the peak efficiency drops a little – source ESDU report 85032. On the subject of conventional multi nozzle designs, ESDU 85032 says “the nozzles should be spaced equally across the mixing chamber entrance and should not be placed so as to form a ring close to the wall as this arrangement reduces efficiency”

    I have found in the layout of nozzle for my own steam lorry design that splaying the nozzles and just drawing out 1 in 3 steam cones, suggests a “hole” in the steam between the four jets, which would persist well into the mixing stage.

    Hence my simple question – does anybody know why we are doing it? The ASTT obviously believe in the theory as it is incorporated in the S160 lemporta design.