The effect of flow orientation on nitriding process

• Nitriding reactions on a single body positioned parallel to the flow were studied.
• Rate model exploits mechanism appropriate for high concentration ammonia.
• Coverage of adsorbed N was half the value of a perpendicularly oriented body.
• The decrease in reaction speed resulted in a more uniform nitriding potential.
• Creation of grid turbulence seemed to compensate the decrease in reaction speed.

The effect of flow direction on the ammonia decomposition rates in a nitriding process has been studied with CFD simulations. A complex body is used as a steel catalyst and exposed parallel to a gas flow. Gas temperature and concentration and nascent nitrogen coverage over the surfaces are presented. Results are compared with an earlier work where the parts are positioned perpendicular to the flow. The computations show that as long as the part is streamlined, a rather uniform reactivity over the whole body is observed with very similar flow features. However, the uniformity is achieved at the expense of reduced reaction speed compared to the case where the part is positioned perpendicular. The low surface concentration and high gas phase gradient in the boundary layer indicate a transport controlled catalytic process. Thus, the decrease in reaction speed seems to be recoverable by the creation of grid turbulence and this requires special basket design with bars larger than the part diameter.