Calculate velocity of product of combustion

[vibh]

Hi,

gas flow rate :- 40 cubic meter per hour at 1.35 kpa

Gas:- Natural gas(Methane:- 100 percent)

Air flow:- 400 cubic meter per hour at 17 kpa

Burner type:- High velocity gas burner

After combustion gas flows in a combustion chamber transferring heat to steel wall of length:- 20 metres of the crossection(350mm wide x 2500mm high)

i would like to calculate the speed of the gas of product of combustion

Please let me know the best way to calculate this

 

[danw2]

Does this thread shed any light?

Link

 

[georgeverghese]

Start from first principles -
a)write out the chemical reaction for CH4 combustion, then extrapolate for the no of mols per sec for this case
b)compute the adiabatic flame temp
c)compute the flame temperature for the given rate of air
d)calculate the density of flue gas and the actual volume of flue gas at the calculated flame temp
e)compute the space velocity for this case.

Ask a chemical engineer or industrial chemist to help if unsure.

Can you tell us why you need to know flue gas speed in the combustion chamber?

 

[vibh]

@danw,

the link you suggested is great gave me some insights

georgeverghese (Chemical)

I can write balanced chemical equation

from that i can derieved Adiabatic flame temperature

Computing flame temperature at given rate of air? Not sure how to do that

Please check if my procedure below is right to calculate the flame temperature for given rate of air

After combustion , based on stochiometric calculation i can calculate no. of moles of co2 and no. of moles of H20

every gas at NTP occupies 22.4 litres per mole

based on that i will calculate the volume of the gas at adibatic flame temerature, using the formulae V1/T1 = V2/T2

V1 is volume of product of combustion at NTP = (NO. of moles of co2 x 22.4 litres + No. of moles of H2O X 22.4 litres)

T1 is 25 deg c = 298 k

T2 is adibatic flame temperature

V2 = (V1/T1)XT2 (Assuming constant pressure process)

Based on V2, i can calculate volumetric flow rate per sec

Volumetric flow rate divided by cross-sectional area of the flow will give me Average velocity of the gas

Why do i need to know the speed of the gas

product of combustion transfer heat to the steel plate, which in turn melts the zinc on other side of the plate

the faster heat flows, the more heat transfer will happen

flue velocity will help me to calculate the convection heat transfer coeficient, based on dimesionless nos.

best regrads,

Vib

 

[georgeverghese]

Pls note with flames, the major component of heat transfer is by radiation, not by forced convection. See your Uni heat transfer text for calculating radiant heat transfer based on emissivity of flue gas products.
Your calcs for calculating actual flame temp is not correct.
First work out heat of reaction - combustion for this case
You've got to write out the chemical reaction for the actual moles of air, then work out the flame temp based on

(Σ n.MW.Cp for products).T2 - (Σ n.MW.Cp for reactants).T1 = Heat of reaction

It does look like you need help with basics - ask a chemist or chemical engineer to help with details.

 

[vibh]

@georgeverghese

hi george,

i think i didnt explain my point properly, As i said i can calculate the AFT,

What i am facing issues in understanding dynamics of the flow when the product of combustion is coming out the burner

My AFT is 2065 deg C, the furnace temperature is never that much

i am adding combusted product to the combutsion chamber at very high temp, enters a big room where the air is at room temperature

there will be heat transfer and the pressure drop, thats what i am interested in knowing the dynamics of the flow

best regards,

vib

 

[georgeverghese]

AFT calc seems okay. And it looks like the air quantity of 400sm3/hr is exactly the stoichiometric quantity required for combustion of 40sm3/hr of CH4.

If what you want to know is about heat transfer to some zinc plates on the other side of the combustion chamber, radiant heat transfer is the main mode of heat transfer, not convection. So space velocity of the flue gas through combustion chamber has little to do with this heat transfer.