Modelling NO_x emissions of single droplet combustion

Abstract

An approach for modelling and simulation of the generation of nitrogen oxide (NO_x) in the gas phase surrounding single burning droplets is presented. Assuming spherical symmetry (no gravity, no forced convection), the governing equations are derived first. Then simplifications are introduced and it is proven that they are appropriate. The influences of the initial droplet diameter, the ambient conditions, and the droplet pre-vapourisation on NO_x are investigated. The fuel of choice is n-decane (C₁₀H₂₂) as it resembles kerosene and diesel fuel best, and the complexity of the reaction mechanism is manageable. Combinations of C₁₀H₂₂ mechanisms and well-established NO_x kinetics are evaluated in detail and validated for their applicability in the context of this work.

The conducted simulations of droplet combustion in an atmosphere of hot exhaust gas show that NO_x formation (by mass of fuel) increases linearly with the droplet diameter. There is a trade-off between available oxygen and ambient temperature. Increasing the equivalence ratio of the exhaust gas leads to higher NO_x emissions in the very lean regime, but to lower emissions if the equivalence ratio exceeds 0.85. Pre-vapourisation of fuel at ambient conditions becomes beneficial with respect to NO_x emissions only if the degree of vapourisation is above a minimum limit. If less fuel is vapourised before ignition, the NO_x emissions remain almost unaffected.

Keywords:

• single droplet combustion
• n-decane (C₁₀H₂₂)
• nitrogen oxide (NO_x); kinetic modelling
• partial pre-vapourisation

Acknowledgments

[Acknowledgments]

This research is jointly supported by the German Aerospace Center (DLR) and the European Space Agency (ESA) under contract numbers 50 WM 0735 and AO-99-094, respectively. The authors would like to acknowledge the very helpful support of Bernd Rogg regarding the user-specific developments of the Cosilab software package.