A simple spray–flamelet model: Influence of ambient temperature and fuel concentration, vaporisation source and fuel injection position

Abstract

This work presents an analysis of the external structure of diffusion flames established with liquid fuel vaporisation (ethanol) in a counterflow configuration. The model assumes that part of the fuel is in the liquid phase (droplets) dispersed in the gas phase, which presents an initial fuel mass fraction. The spray–flamelet model is distinguished from the flamelet model by the presence of droplets in the reaction zone. There is no relative velocity between the droplets and the gas phase and the droplets vaporise completely before the stagnation plane. The mathematical description of the dilute spray–flamelet is made by the conservation equations with source terms corresponding to the fuel droplets. In the fuel species conservation equation, the source term has a strength proportional to the droplets vaporisation rate. In the energy conservation equation, the source term (heat sink) has a strength proportional to the product of the droplet vaporisation rate and the latent heat. The model exhibits a parameter defined as the spray combustion parameter M that combines the chemical reaction, flow field and spray properties involved in the problem. An analysis is performed to determine how the dilute spray–flamelet is affected by the liquid fuel injection position, the spray combustion parameter M and the temperature and fuel mass fraction Φ in the incoming fuel stream. A uniform distribution is considered to describe the droplets. The results confirm the expected influence of the spray combustion parameter M and the ambient temperature on the spray–flamelet. Effects on the temperature profile can be observed on the spray–flamelet by varying the liquid fuel injection position and the fuel mass fraction Φ in the incoming fuel stream. The results also indicate that, depending on the value of M, there is a value of Φ above which a burning regime is imposed that is almost totally controlled by the incoming fuel stream conditions.

Keywords:

• spray–flamelet
• spray combustion
• counterflow liquid–vapour fuel diffusion flame
• staged combustion
• droplet vaporisation

Acknowledgements

This work was partially supported by CAPES, CNPq Grant No. 303046/2010-4 and FAPESP Grant No. 2011/06673-1.