Effects of scaling laws on the combustion and NOx characteristics of hydrogen burners

Abstract

The effect of constant velocity and constant residence time scaling on the local nitric oxide (NOx) emissions and flame characteristics of complex partial premixed hydrogen burners were investigated numerically and theoretically. A previously developed and validated computational fluid dynamic (CFD) model was employed to conduct in total 11 simulations at various burner scales ranging from a base case of 10 kW to an up-scaled burner design at 500 kW. The flame characteristics were investigated by means of a novel CFD based regime diagram and compared to Damköhler and Karlovitz numbers obtained from scaling theory. The flame is at laboratory scale mainly characterized by the thin reaction zone regime. Employing constant velocity scaling was predicted to overall decrease the Karlovitz number, which causes the combustion to appear partially in the corrugated flamelet regime and at scales exceeding 250 kW also in the wrinkled flamelet regime. Constant residence time scaling on the other hand leads overall to a combustion with constant Damköhler numbers. However, a constant Karlovitz number close to unity was observed for a significant part of the flame-sheet, which leads in this flame regions to a variable Damköhler number. Both investigated scaling principles lead to an increase of the overall NOx emissions, with constant velocity scaling resulting in the highest emissions. This is mainly attributed to the larger volumes and longer residence times of the flame and immediate post flame region compared to constant residence time scaling. The total NOx formation in the inner recirculation zone, on the other hand, is lower for constant velocity scaling and is found to be dominated by the local oxygen atom (O) and hydroxyl (OH) concentration. Constant velocity scaling causes a breakup of the inner recirculation zone at the 500 kW scale, which leads to a fundamentally different flow field and causes the flame to impinge onto the combustion chamber wall, whereas constant residence time scaling maintains the inner recirculation zone at all investigated scales. The breakup of the recirculation zone is attributed to the different effect of the scaling principles on the velocity to length scale ratio and momentum of the annular jet flow. © 2020