My question is. If i treat only gas phases, what kind of difference it is between multicomponent or multiphase? Why a multicomponent analisys need a background fluid? I've two stream one of h2 and the the other of O2 each containing H20 (vapour)I want know the concentration of each specie... How can setup this kind of model... multiphase approach or multicomponent ? Thank's

Multiphase flow (in your case) means the fuel is in the liquid phase, if your H2 is injected liquid, you have to use this kind of model, otherwise, in case of an all-gaseous analysis you don't need that. Multicomponent flow means you are using gases with different properties. The background fluid is convenient since air is the background fluid almost everywhere in the physical world. For your problem - define Air for example as the background fluid (even if it's not really present in your case), define the properties of all species present, set the proper concentrations of all species at the inlets and solve.