Hi Ahmed,

first off I would like to point out that you cannot really classify laminar flame speed purely on the basis of the chemical formula of a fuel. You have to examine flame speed within the context of its chemical structure. In general, fuel structure is known to greatly influence the laminar flame speed, with the rule of thumb being: unsaturated molecules > ring molecules > straight chain molecules > branched molecules. So taking as an example the C6 group, where you can have ring molecules and several degrees of unsaturation or different branching sites, you would get the following ranking in flame speed:

1) 1-hexyne, C6H10 (2 degrees of unsaturation - triple bond)

2) cyclohexene, C6h10 (2 degrees of unsaturation - double bond + ring)

3) 1-hexene, C6H12 (1 degree of unsaturation - double bond)

4) cyclohexane, C6H12 (1 degree of unsaturation - ring)

5) n-hexane, C6H14 (saturated, straight) - diffrence with cyclohexane v small, almost non-existent

6) isohexane, C6H14 (saturated, branched)

7) 2,2 dimethyl butane, C6H14 (saturated, double branched). 

We have actually recently published a paper on this: Fuel 167:347 · March 2016. 

This ranking would pretty much stand for all Cx groups. 

So why is this the ranking and how does it relate to the chemical formula or better structure of the fuel?

Well, firstly the adiabatic temperature of highly unsaturated fuels is slightly higher than that of saturated ones. The same stands for fuel diffusivity, primarily due to the slightly lower molar mass of the highly unsaturated fuels. However the effects of these two parameters are secondary. For instance, the difference in flame speed between e.g. 1-hexyne and 2,2 dimethyl butane can be as high as 50-60% in stoichiometric conditions, yet the difference in Tad or D is only ca. 5%. 

Hence the answer primarily lies in the different chemical kinetic mechanisms of the fuels and primarily in the ease of abstraction of allylic as opposed to primary H atoms during the first steps of the fuel breakdown mechanism. The H radical pool generated for unsaturated molecules better supports the flame and thus its speed. A great reference that explains why this is the case is this: Farrell, J.T., Johnston, R.J., Androulakis, I.P., Molecular Structure Effects On Laminar Burning Velocities At Elevated Temperature And Pressure, SAE Tech Paper (2004) 01-2936.

For further reading have a look at (and relevant cited articles within):

Davis, S.G., Law, C.K., Determination of and Fuel Structure Effects on Laminar Flame Speeds of C1 to C8 Hydrocarbons, Combust Sci Technol 140 (1998) 427

Wu, F., Kelley, A.P., Law, C.K., Laminar flame speeds of cyclohexane and mono-alkylated cyclohexanes at elevated pressure, Combust Flame 159 (2012) 1417

A.A. Burluka, R.G. Gaughan, J.F. Griffiths, C. Mandilas, C.G.W. Sheppard, R. Woolley, Turbulent burning rates of gasoline components, Part 1 – Effect of fuel structure of C6 hydrocarbons, Fuel, Volume 167, 1 March 2016, Pages 347–356

A.A. Burluka, R.G. Gaughan, J.F. Griffiths, C. Mandilas, C.G.W. Sheppard, R. Woolley, Turbulent burning rates of gasoline components, Part 2 – Effect of carbon number, Fuel, Volume 167, 1 March 2016, Pages 357–365

Hope I was of help,

Harry

Dear Harry 

Thanks for your answer.

I have already read your and Dr Burluka works and agree what you mention but my question is about the blending of fuel that when 3 fuels (1,2 and 3) have the different molecular structure. when blending fuels (1 +2) , (2+3) and (1+3)  then  calculate the adiabatic flame Temperature equivalent and other parameter you will find out that your conclusion not as the result you get.