I meshed with ICEM CFD the whole domain "at once". The domain is not "decomposed" per say, the different grid sizes are the different refinements (the smallest being around the nozzle). Now, with ICEM, the use of O-grids creates this weird cross shape, but I did not think this would a problem, especially since I used a very similar mesh (though slightly less refined) with RANS and FGM for a flame without wind and it burned well (I have attached the pictures of the RANS similar mesh and the good results).