Dear Kevin and JFloyd,
first, thank you so much for your kind answers.
Kevin, I do not understand why I should add the produced CO amount in the lines of file
TestBurnSugarOK.fds you pointed out when adopting the simple chemistry approach in the REAC line. If I assume the complete conversion of sucrose, products are only H2O and CO2, following the reaction:
C12H22O11 + 12 O2 -> 12 CO2 + 11 H2O .
Probably it is not fully true that the only products are CO2 and H2O, but for the moment this is my assumption. So the schematization of the process is that solid sugar transforms (pyrolysis) into "gas sugar" (SugarFUEL in my example file TestBurnSugarOK.fds) at a rate defined by the properties of the two materials Sugar1 and Sugar2 in MATL lines (whose parameters were valued from a TGA analysis found in the literature and following the procedure explained in the FDS User Guide). Sorry for not having well explained my assumptions in my original post.
JFloyd, you are absolutely right, the process of sugar burning is much more complex than the one I am trying to implement: I neglected the release of water vapour before the flame ignition. The problem is that I do not know, and I did not find, enough data about the amount of water that would be released at a given heating rate nor about the energy required (I am continuing to search the literature). The steps of sucrose transformation before burning lead to the caramelization with the release of H2O as you say. Unfortunately, the products left in the caramel are at least three as explained at this link:
https://www.scienceofcooking.com/caramelization.htm.
Which composition exactly I would obtain before the flame, as a function of the heating rate, is not clear to me at the moment.
Nevertheless, I am interested in a fire scenario, so in cases where the heating rate is high enough to assume that even caramel, whatever its composition, will immediately transform into the final products, CO2 and H2O, that is what I am assuming in the file TestBurnSugarOK.fds. I thank you so much for suggesting further investigation of sucrose's physical and chemical reactions. I will continue to investigate this complex process.
However, let me go back to my original questions. Whatever intermediate products would be created by the heating and pyrolysis of sucrose, why, if I assume that CO is the gaseous fuel produced in the pyrolysis steps (described in the MATL lines of file TestBurnMaterial.fds), this setup does not ignite this gaseous fuel? The question is relevant to me because even if I change the composition of the gaseous fuel, I expect that, being my setup wrong in some way, I would obtain the same behaviour. What is it the relation between the HEAT_OF_COMBUSTION specified in the MATL lines and the HEAT_OF_COMBUSTION of the gaseous phase? Should not they be independent of each other?
I am sorry for the numerous questions I am contemporary asking you!
Warmest regards,
Franco