[Stoves] FW: Heat transfer and in-line water heater

[Crispin Pemberton-Pigott]

Dear John TF

>...consider the 
>industrial application of turbulators. In a boiler heat exchanger, 
>turbulators are used in the flue gases,  to passively break the laminar 
>flow, encouraging the scrubbing of the hot gases against the tubing 
>walls, and increase residence time.

You are quite correct, and the effect on vertical surfaces is much more
pronounced than on horizontal surfaces.  So you will sometimes find
'turbulators' or turbulence inducers mounted in a way that uses the chimney
draft to obviate the need for fans.

>...Resulting in an increase in heat transfer.

It is important to look _very_ carefully at why they work.

You mention residence time (most important) and breaking laminar flow (not
nearly as important) and 'scrubbing of hot gases against the tubing walls'.
So the debate is over the addition a phrase that includes 'stationary air'
which needs to be moved or removed.

It is the mixing (stirring, really) and delay in getting through the system
that gives nearly all the improvement.  Making the gap smaller just makes
the stirring easier.

If you looking the meanings of laminar and boundary layer (etc) you will
find that there really isn't any laminar flow in a heat exchanger and the
entire gas path is boundary layer. But it is certainly possible to stir up
the air to make sure heat in the centre of the gas path is made available to
the absorbing surfaces, esp if they are vertical.


>This may not be directly applicable to small stoves, but I wonder if  a 
>stainless scrubbing pad inserted into the annular gap, between the pot 
>and stove would improve the heat transfer?

I think it is worth trying, especially on a gasifier (because it will clog
with soot) and perhaps it should be copper/brass wool.  I think Dale A is
doing some tests on getting more heat into the pot by attaching things
underneath.  I don't know if he is going that fine on the mesh though.

It is a very good idea.  I have suggested (and tested) spiral channels that
are also tapered in depth to compensate for volume reduction.  Christa Roth
tried this in Malawi with a clay top deck.  In the end they have gone to a
conical plate made in a press that does account for the temperature drop.

Regards
Crispin