[Stoves] Heat transfer and in-line water heater

[Dean Still]

Dear AJH,

And it's convenient that along with the first list of variables adjusting
the air/fuel ratio can do a lot of good things simultaneously:

Lower CO and PM
Raise temperature of hot gases to the pot (convection)
Raise temperature in fuel bed (radiation)
With increased temperatures time to boil decreases
Then less fuel is used so even if emission factors are constant less
pollution is made per task, etc.

We adjust the air/fuel ratio in a Rocket type stove by increasing or
decreasing four things:

Size of the opening for air above the fire
Size of the opening for air below the fire
Size of the gaps below the pot
Size of the gaps at sides of the pot

All Best,

Dean


-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of andrew
Sent: Sunday, December 02, 2007 9:17 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Heat transfer and in-line water heater

On Saturday 01 December 2007 18:32, Dean Still wrote:

> The velocity of flue gases has a large impact on heat transfer to
> the pot (surface to be heated). Higher velocity flue gases get
> closer to the pot, narrowing the boundary layer of still air close
> to the pot.

I'm told that pulse combustors are actually able to disrupt this 
boundary layer but in general the boundary layer is the limiting 
factor in convection.

Of course despite Crispin not considering radiant heating being 
significant it is not limited by this boundary layer.

> Higher velocity flue gases get more heat into the pot. 

All other things being equal I'm sure you are right but it could lead 
us down the logical path that Frank was pursuing, the idea that 
higher massflow would give a higher flue velocity and thus increase 
heat transfer. The thing is the temperature of the flue gas is 
inversely proportional to the massflow and heat transfer at the pot 
is directly related to the temperature difference.

Intuitively I'd say going for a high combustion temperature (which 
enhances radiative transfer greatly) minimises the massflow and 
velocity and increases heat transfer. So to get the higher 
velocities means narrower passages, as you suggest, longer path in 
contact with pot and probably forced draught to overcome the 
increased drag.

<snip>

>
> Heating bath water is easy because 30C is a lower target.

It's a cool bath too! 41C suits me better.

> Covering 
> the square top tin is not necessary because steam is not made at
> 30C which is nice because we don't have to worry about pressure.

I'd still use a lid because a small convection above the pot will 
evaporate from the surface and I would expect the normal practice to 
be to heat a small amount of water to >80C and then add this to a 
larger cool volume of water in the bath. We could of course argue 
the merits of a bath versus a shower.
>
> Yes, heat transfer efficiency is the ticket,

It strikes me that after minimising indoor pollution by achieving 
good combustion this heat transfer is the next weak link.


> See you at ETHOS?

I'm sure my globetrotting friend will get there!

AJH

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