[Stoves] Radiant heat and heat transfer

Tue Nov 27 15:02:59 EST 2007

"Steam" is water vapour, and at 1 atmosphere it is at 100 deg C - but like
any gas, it can be at any temperature.  It can be heated after leaving the
liquid state.  And, of course, the 1 atmosphere is important, too.  If you
increase the pressure then you increase the temperature at which water boils
(and vice versa). Higher pressure is the basis of the "pressure cooker" - it
cooks faster, because it cooks at higher temperature.  

Save your time on the experiments. The answers to your questions are readily
computed, and the experiments themselves have so many nasty little tricks up
their sleeves that getting them right takes a lot of work - for very little
reward. A "nice" experiment is to measure the length of the jet of
"invisible" steam as it comes out of the spout of the kettle.  More fuel
means faster boiling and a longer "invisible" section before the steam
condenses as a cloud of water droplets and is visible.  

Hope that helps.

Philip Lloyd

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of frank
Sent: 27 November 2007 08:40
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Radiant heat and heat transfer

Thanks all,
A few more questions.
Steam is water molecules in an excited state and a gas? Steam is produced
when water achieves 100 deg. so the steam must be at 100 deg. 
coming off a pot of water? 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?

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. Another experiment might be to get
the fire going with 50g dry wood, add on 200 g dry wood and mist the air
going in to see if the temperature increases (or
decreases) at the stack. OR perhaps the temperature along the side of the
stack should be measured to determine how far up the stack the heat will go
before the condensing on the surface takes place. Suggestions ?

And another one (or two) : If one gram of water is converted to steam what
volume of gas does that produce? Is that the same volume at 100 deg as it is
if it has a temperature of 550 deg? 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? 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. as it went
through. If at the bottom of the cyl we placed blocks of steel heated red
hot with propane torch then send in air with steam,  inject the CO2 just
above the hot blocks and determine mixing. Then heat the steel with propane
torch to red hot and add air with equivalent mist that turns to steam
(rapidly expands), inject CO2 and determine mixing while gases expand.
Problem is the expanding gases increases air flow coming out so we need a
flow meter at both ends to calculate back to starting flow. Or determine the
amount of gas expansion by the concentration difference of
CO2 with the steam (already expanded) compared to the water turning to steam
diluting the CO2 with the increased volume of gas. So we use both time and
concentration of CO2 flowing through the cylinder. Suggestions ?

I have a long list of experiments I want to do and it may be awhile before I
can get to them. It would be nice if the procedures of tests we think
important to us was worked out and available to labs so when someone has the
time to do them 

Frank

andrew wrote:

>As has been pointed out the pot that sees the hot combustion benefits 
>from the radiative heat transfer as well as convection via the flue 
>gases, the metal of the pot then transfers this heat to the water by 
>conduction through the metal, pots further down the line don't benefit 
>from the radiative effect and they also only experience cooler 
>convection because the first pot has also already taken some heat from 
>the flue gases.
>  
>
>>What I am wondering is if one was to start a fire using dry wood then, 
>>once going well, added wet wood the heat produced would travel via 
>>steam (radiant heat?)
>>    
>>
>
>No, the steam carries heat via convection but it has also robbed heat 
>in turning from liquid to vapour, this heat is generally not available 
>to any pots further down unless their surfaces are less than 100C. In 
>fact it's even worse than this because the humidity may be low, 
>depending on the fuel being used
>
>  
>
>>to the second and third pots (out
>>of combustion sight) and condense on the cold metal surface 
>>transferring the heat to the metal-water So using wet wood (or adding 
>>water mist to the hot gases produced) aid in getting heat to more 
>>places on the pot (back side) and to pots located at greater 
>>distances? It may take heat away to produce the steam but that is not 
>>lost heat unless it makes it past the pots without being used?
>>    
>>
>
>Yes, you've got that except you haven't allowed for the fact that the 
>flue gases are mixed with the steam, to transfer the heat efficiently 
>with the steam you need to prevent it mixing with the flue gases, 
>otherwise proportion will always be carried away as a gas even if the 
>pot is below 100C.
>
>Have a look at a heat pipe, also I believe in the early days of high 
>rise buildings before electric pumps were cheaply available this system 
>was used to carry heat to the upper floors where a thermo syphon  ( 
>what we in UK call gravity circulation) would need too wide bore 
>piping. Essentially you have a tall tube with liquid at the bottom and 
>vapour above it with no air in the tube. Heat the liquid till it boils 
>and the pressure goes up but if the top of the tube is cool the 
>increased pressure causes some vapour to condense at the cool top 
>surface, giving up its latent heat, it then runs back down the inside 
>wall of the tube and replenishes the reservoir.
>
>
>AJH
>
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>  
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--
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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