Rosebud grates

[malcolm hearneParticipant @malcolmhearne84128]

Is there an optimum angle for the venturi hole for a rosebud grate?

I notice that some people have used a centre drill on the underside of the grate.

[malcolm hearneParticipant @malcolmhearne84128]

Venturi angle for rosebud grates

[Andrew TinsleyParticipant @andrewtinsley63637]

The ones I have seen have the countersunk holes at right angles to the grate. I would guess that angling the holes will make little or no difference. But there again I have been known to get things wrong!!!

Andrew.

[SillyOldDufferModerator @sillyoldduffer]

Posted by malcolm hearne on 06/04/2023 09:53:33:

Is there an optimum angle for the venturi hole for a rosebud grate?

I notice that some people have used a centre drill on the underside of the grate.

My guess is yes, but it depends on so many variables it would be hard to predict. The size and temperature of the fire alters the pressure on which the venturi effect depends, and both vary during a run.

I suspect an optimum grate would be bespoke to a particular engine running in a particular way. To find the optimum, unless someone knows better, I think it would be necessary to experiment for best results, including the possibility of finding that a Rosebud isn't the answer!

As a starter for ten, I believe the lower angle should be slightly wider than the upper, and sharper than a typical 90° countersink bit. Say 30° underneath and 20° on top. There should be a straight section between the cones too but I've no idea how long it ought to be as a proportion of cone length, or even if it matters.

I'm not aware that much research has been done into model grates and fireboxes. Instead we have a scaling down of full-size practice with sensible adjustments for reasonable running rather than fine-turning for best possible performance. Be interesting to see the result of a team of professional engineers giving one of "our" engines the Tornado treatment, systematically reviewing the entire design. I suspect the result wouldn't be much better than a conventionally well-made model but no-one's tried it yet!

Dave

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