I simulate a reforming reaction (endothermic) in a tubular micro reactor (L=30 cm D=1.3 mm). when I contour H2 mol or mass fraciton along the tube length, H2 frac is increasing through the first 2 cm then it starts to decrease abnormally...why this happens?

this is a wall reaction (zero thickness wall)and this is a irreversible rxn. material of construction is Al. there is no problem with volume rxn or non-zero wall thickness when they are tried.

C8H18+ 8H2O- 8CO+17H2

thanks in advance

problem may be with incorrect representation of reaction. how the Reaction Rate contour look like. Try plotting Reaction rate along the wall. It can give some clue. how H2 Concentration contour looks.