Calculate mass flow rate from inlet area

[vbottle28]

Hi all. I expect this problem is simpler than I think but i'm going around in circles and would appreciate a handy push in the right direction.

I have a combustion situation as shown in the image, note that the blue line represents a grate on which the wood sits, the red lines are an inlet pipe for air

Tw (Temp of wood) is known
Tair (Temp of air) is known
h (vertical displacement between air inlet and fire) is also known
A is the area of opening in the pipe under the fire. (known)
a is the area of opening between the pipe and the outside world and is the variable in my calculation.

From this, does anybody know the link between 'a' and the mass flow rate of air through the pipe and into the fire?

Thanks for any help in advance.

 

[vbottle28]

Sorry, i left out an important piece of information. the fire is inside a firebox, air can only enter the system through the red pipe and flue gas leaves through an outlet further above the diagram.

 

[renderu]

I don't think it's possible to determine the mass flow rate of the required air coming from the underside for the inputs given. Why wouldn't the wood burn without any air from underneath since it does have some air from above? I also don't see the importance of the openings A and a, since the air required by the wood shouldn't be dependent on these openings (unless your problem is so complex that air velocity enters into play). The amount/mass of wood is also not given and is critical to determine the combustion air requirements.

 

[chicopee]

I would check out older versions of Kent ME Hbdk "power". Their current edition would not bother with wood burning boilers but older editions would.I got one 1940 edition given to me by my uncle however I am out of the office for a while so I can't help you.

 

[MintJulep]

Impossible with only the information provided.

 

[IRstuff]

see:

http://en.wikipedia.org/wiki/Stack_effect

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[vbottle28]

Hi,

Renderu, aside from the i let and outlet, the wood is inside a sealed container, hence no air will enter from above. I should also mention that its not the air required by the wood that is key, it is an experiment to see the effect on combustion when adjusting how much air is allowed to the fire.

Mintjulep, I could also mention that the firebox has a constant cross section that is known, what other information would you require?

 

[EnergyMix]

vbottle, if it's an experiment, sometimes the best way to find out is to actually measure it!

Want to know the do's and don'ts of Eng-Tips? Read FAQ731-376.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529.

 

[MintJulep]

I'm reasonably sure that there is no information that makes this a solvable problem.

If you are willing to make enough assumptions and guesses about a dozen or so parameters then I think someone smarter than me could develop some equations that would allow the calculation of an estimate.

 

[vbottle28]

Alright, thanks for looking for me. I feel better knowing that it is a difficult problem. EnergyMix, any ideas how I could measure this? im using a gas analyser but it wont give mass flow rate, the just percentage of the constituents in the flue gas.

 

[MintJulep]

The direct measurement of air flow in a pipe or duct is not particularly difficult.