[Stoves] Radiant heat and heat transfer

[IPC]

A small consideration.  If you get a release of steam as Crispin suggests,
then you can enter gasification reactions, and turn some of the steam into
hydrogen and carbon monoxide, with a bit of local cooling - but it then
burns quite nicely once it gets enough air.  You can see this happening in
wood fires made from small chips with restricted air flow, if you sample the
gases deep in the bed.

The net effect if some increase in gas flow (because the CO and H2 are of
larger volume than the water vapour) local cooling and overall greater heat
output.

(Dr)Philip Lloyd
Energy Research Centre
University of Cape Town
Private Bag Rondebosch 7701
South Africa
Tel +27 (0)21 650 3896
Fax +27 (0)21 650 2830
 


 

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Crispin
Pemberton-Pigott
Sent: 29 November 2007 12:47
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Radiant heat and heat transfer

Dear Frank

I compliment Penn on the contributions too.

There is a consideration that you should keep in mind as well (sorry this is
so complicated) but it is excess air and how the steam or wood-water-steam
affects the excess air ratio.

In general you cannot tell how the performance of the system has improved
with a thermometer alone.  If you reduce the air flow through the system by
generating a (sort of) explosive or 'expansive' supply of steam in the
middle of the fire, it will reduce air intake as Penn has described.  This
may have the effect of driving up the flame temperature - or would if there
was air alone.  There is extra heat.  This extra heat will be absorbed by
the water vapour at a rate of twice that of air per unit mass (not volume).
The final temperature will be lower - if the air supply was constant, but it
will probably not be. It will be lowered.  Interesting, huh?

The overall gas velocity may be reduced (either naturally or by you moving a
choke if you like) and the dwell time increased to effect a great heat
transfer in the (fixed) distance the gases pass against the pot.  At some
combinations of moisture content, excess air, gas velocity, flame
temperature and burn rate (which is affected by the air coming in) there is
an increase of heat transferred (few) and some combinations where it is
reduced (most).

Although the temperature may show a reduction with the steam present, it may
not necessarily contain less heat, and the quantity of heat transferred may
not necessarily be lower either.

The gain from reduced excess air (presuming there was some to begin with!)
can in some cases be enough to off set the boiling loss Penn mentioned.

The worthiness of this scenario is that general case statements can't be
made that depend on temperature measurements alone. You have to know the
excess air present in the system to have any clear idea what the system
efficiency is.  The very interesting question you have asked is a great case
study.

Regards
Crispin in Melville


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