[sooraj546]

I have studied the effect of air preheating on non-premixed combustion, keeping the mass flow rate and inlet tube diameter constant at inlet air-fuel stoichiometry. Only the fuel flow rate is always constant, the air volume flow rate changes (mainly velocity) with temperature ?
3 conditions are formulated:
air preheated to 566 K,
with no preheating, i.e. the air is at 295 K
and air preheated to 1000 K.
All the Inlet condition are shown in the attached figure.
On comparing the overall stoichiometric conditions - So when there is a decrease in the temperature of air supplied, there is a simultaneous decrease in the amount/volume flow rate of air required as well. This happens due to ideal gas law;
My results are quite strange :
1. air preheated to 295 K i am getting the furnace temperature around 1500 K
2. air preheated to 566 K i am getting the furnace temperature around 1450 K
3. air preheated to 1000 K i am getting the furnace temperature around 1600 K
can anyone explain to me y is it happening? is this due to dilution of as less the preheating temperature - less will be the amount/volume of air going in so fuel dilution effect will be less no higher temperature development etc.
Few papers have also reported such phenomena as well. but with no explanation:

1. Effect of air preheating and fuel moisture on combustion characteristics of corn straw in a fixed bed Wei Zhao, Zhengqi Li *, Guangbo Zhao, Fangshi Zhang, Qunyi Zhu Energy Conversion and Management

2. The Effects of Air Preheating and Fuel/Air Inlet Diameter on the Characteristics of Vortex Flame Mostafa Khaleghi,1 S. E. Hosseini,1 M. A. Wahid,1 and H. A. Mohammed2 Journal of Energy
Any help will be highly regarded.........

[vinerm]

This would not be straightforward to answer since the temperature is a function of multiple parameters. Since you have a fixed mass flow rate, if the inlet temperature of the gas is increased, the density reduces, hence, more volume goes in. However, the properties are mass based and not volume based. Even if more volume goes in, it will not be able to consume more energy or vice-versa.

Similarly, reaction rates are a function of temperature but the heat generated due to reaction may not be. So, hotter gas will react faster but will still generate same energy. However, if it is hotter, it will consume heat at a smaller rate. But this consumption and further temperature increase depends upon the specific heat of the gas. If it is temperature dependent, which it most likely is, situation is highly non-linear, and, hence, is not straightforward to answer that higher temperature of inlet gas will lead to lower or higher flame temperature.

You may plot the specific heat of the gas over the temperature and observe if it is higher in the regime where you are getting lower final temperature. If it is so, you have at least one causation for the predicted behavior.

[sooraj546]

Thanks a lot for replying..........i will surely let you know after checking into its details.........Sorry last time i forgot to attach my inlet conditions. Kindly have a look at it as well.
My concept is little different......................................... ... based on the attached table .................................................. .the residence time for each case was calculated:
Residence time can be simply defined as the time available for the air and fuel to mix properly and combust.

The residence time decreases with an increase in temperature. Thus it’s quite evident that at 295 K, the fuel is getting more time to mix properly with air and hence better combustion,

this is correct way to explain it?

[vinerm]

As far as non-premixed combustion model is concerned, it is only about mixing. As soon as mixing is done, combustion takes place. However, the rates at which this happens is dependent upon whether you are assuming chemical equilibrium or non-equilibrium, whether the system is adiabatic or non-adiabatic. Mixing is purely due to the turbulence (and small role is played by the molecular diffusion coefficients). However, do note that this model is based on the assumption of Lewis number being 1, i.e., all species mix at same rate. At higher temperature, diffusion coefficients for gas usually increase, so, mixing is faster as you mentioned. But molecular mixing is not much significant.

[sooraj546]

i have simulated this using EDDY DISSIPATION MODEL of ANSYS-FLUENT still whose rates are mixing dependent rather than on chemical kinetics. Is lewis number =1 for this model also....i doubt it Kindly help me out.
Plz find the attachment below:
https://www.researchgate.net/profile...hane-Flame.pdf

I have repeated it a using different grades of meshes- fine very fine etc........a number of times, still i am receiving the same result every time. That's what confuses me......and one more thing its not just at one point..... temperature is increasing...........all over the combustion chamber the obtained temperature is higher when air is supplied at 295 compared to air preheated to 566 K.

so finally is there any other valid explanation that could justify it...........

[vinerm]

Mesh will not have any effects as long as it is fine enough to not bring in any mesh artifacts. You will find explanations in two places, one in specific heat for the gas. It always increases with temperature, however, whether it is included or not included in the simulation makes a difference. Second is PDF tables. You should look at the tables. It is possible that there is something wrong with the tables. It is difficult to identify the issues, except abnormal ones, but you can compare with the literature. Furthermore, if the system is non-adiabatic, then I would suggest you run the simulation with adiabatic to get the adiabatic flame temperature and then compare those.

[sooraj546]

I am using EDM from species transport not from non premixed combustion side where adiabatic, non adiabatic, chemical equilibrium and PDF tables etc excites.....

Sorry

with regards
sooraj

[vinerm]

Since you mentioned non-premixed in your first post, I assumed that you are using non-premixed model. If you are using EDM, then there can only be one explanation for that, varying with temperature. If you observe that is it not just the maximum temperature but also the temperature field is different, then it is not just but other properties as well are responsible.

[sooraj546]

Another explanation is,

area of pipe constant in all cases :-

air at 295 K - the volume of air going in is velocity*area of pipe = x.
so air at 566 K - the volume of air going in = 2x, as velocity is almost double.
and air at 1000 K - the volume of air going in = 4x, as velocity is too high. as shown in table below.

lets assume by combustion 2000 K is attained inside the combustion chamber in all the cases. But the amount of gas (nitrogen) is double that in 566 K air preheat case so in order to raise its temperature before combustion to the furnace temperature consumes some heat as well. May be that y the sensible heat part is getting consumed here

Is this concept correct?

[vinerm]

Heat consumption is not a function of volume. All properties, such as , k, etc. are mass related and not volume related.