[Stoves] Radiant heat and heat transfer

Wed Nov 28 00:51:07 EST 2007

Yes - it is very close to ideal.  Yes, the density of water vapour is lower
than air - that's why water vapour rises and forms clouds!

Have fun

Philip 

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of frank
Sent: 28 November 2007 02:32
To: Robert Penn Taylor
Cc: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Radiant heat and heat transfer

Dear Robert, Andrew and Philip and stovers,

Robert Penn Taylor wrote:

>
> Steam is the vapor state of H2O. Vapor = gas. It's unclear to me if 
> that's what you meant above. It is *not* liquid droplets suspended in 
> a gas.

So it obeys the gas laws? (18g per 22.4L) Somehow it seems water vapor is
different.

>
>
>
>> But what if the steam is produced from wet wood in a fire. It comes 
>> off at 100 deg into a 550 deg. environment. Will the water molecules 
>> excite to a greater state than that at 100 deg and carry more energy 
>> farther? Farther in distance than, say, the excited CO2, N, O2,  CO,  
>> molecules before it comes to a more 'relaxed' colder state? hopefully 
>> on the surface of a pot?
>
>
> See above. By "greater state" do you mean higher temperature? Steam at
> 101 C will carry more energy than steam at 100 C. If steam contacts an 
> object and drops in temperature from 101 C to 100 C, it will give up 
> about about twice as much energy to the object as dry air would in the 
> same situation. Distance carried will depend on a few other things 
> such as convective coefficients and flow patterns, all of which will 
> be different for different concentrations of gases (due to density, 
> viscosity, etc.)

THANKS - this is what I was wondering. I realize it takes energy to
evaporate water into steam (still below the pot) but is this offset buy the
heated air loaded with more energy,  able to travel farther?, and transfer
heat more efficiently to the pot, cleaner emissions? and more complete
combustion with less char?. And as you point out there are so many variables
(your list above) it is hard to calculate what will happen. There must be
some reason they use steam to transfer heat around a building. We are
transferring heat from the hottest part of the fire to the bottom of the
pot.

>
>
>> Experiment: Get a fire going using 50 grams dry wood then dump on 200 
>> more grams of dry wood to burn under a stack and measure the 
>> temperature of the gas coming out the top. Then get the fire going 
>> with 50 grams dry wood and add 286 g wood having 30% moisture (=200g 
>> dry wood) and measure the temperature at the stack. If the 
>> temperature coming out is higher with the wet wood the 'heat' will 
>> travel farther.
>
>
> The temperature will decrease noticeably with the moist wood because:
>
> 1. Energy must be taken from the combustion zone to convert the 
> moisture in the wood to the vapor state. The energetic penalty for 
> this is high due to the heat of vaporization of water.

Perhaps offset with the advantages?

>
> 2. Moist wood doesn't pyrolyze very well. There's a drying front that 
> precedes pyrolysis of a solid fuel. The longer this drying front 
> takes, the less fuel is available for pyrolysis at any given moment, 
> resulting in a decrease in the effective fuel rate. Lower fuel rate 
> results in lower heat flux.

I am also thinking of the water misting. The fuel remains dry as the mist
goes around the fuel. The flame is cooler as it evaporates the water but the
result is better energy to get heat to the pot??? The mister only in use
once the combustion is high enough to handle it. 
something like that

> <snip>
>
>> And another one (or two) : If one gram of water is converted to steam 
>> what volume of gas does that produce?
>
>
> Depends on the pressure inside the vessel. Or the atmospheric pressure 
> if not in a closed vessel.
>
This follows the law of 18g water =  22.4 liters gas (STP) ??

> <snip>
>
>> If we burn wet wood the gas going out is increased in velocity based 
>> on it being at a higher temperature plus the water turning into a gas?
>
>
> There's almost certainly a decrease in velocity because the lower gas 
> temperature as mentioned above results in lower volume, and 
> conservation of mass then dictates a lower velocity. The water turning 
> into gas has little effect because the expansion occurs equally in all 
> directions -- so the expansion pushes back against the incoming air as 
> much as it pushes out on the outgoing air.
>
I am confused. A steam pot produces gas (water vapor) and shoots out the
spout. I would think that since when using water we 'produce' gas to add to
the air flow already taking place. I realize the cooler temperatures of the
air decrease the volume of that dry air fraction- but doesn't the water add
so much more volume that total is a large increase?

>> and if using a mist the water droplets burst into a bubble of gas 
>> that causes mixing?
>>
>> Experiment: A procedure I think can be used to determine mixing is
>> to: at time zero inject CO2 into the bottom of a cylinder (mixing 
>> chamber of a TLUD) and determine the time required to leave a 
>> cylinder (peak height) using an IR CO2 detector. Knowing our flow 
>> rate, the volume in the cylinder and time it took to travel through 
>> we can determine if the CO2 channeled through or mixed with the gases 
>> in the cyl.
>
>
> You'd need to look at concentration to determine extent of mixing.

Then we need to use a gas not involved in combustion. Not CO2, CO, O2, N 
(from wood), not H. What can we use? I was thinking we could inject CO2 
without changing the flame conditions.

> -Penn Taylor
>
>
Thanks for your time and help.

Frank

-- 
Frank Shields
Soil Control Lab
42 Hangar way
Watsonville, CA  95076
(831) 724-5422 tel
(831) 724-3188 fax
frank at compostlab.com
www.compostlab.com

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