The influence of evaporation on the autoignition-delay of n-heptane air mixtures under gas turbine conditions

In this work, autoignition-delay times of liquid fuel sprays for flow situations similar to those in premixing ducts are calculated. An intensive parameter study was conducted to identify the influence of the evaporating spray on autoignition delay. The parameter variation covers duct conditions relevant to gas turbines. Three monodisperse sprays with droplet sizes of 10,50, and 100 μm and two sprays with Rossin-Rammler droplet size distribution are investigated. A full 3-D Navier-Stokes code is used for the prediction of the turbulent flow. It is coupled to a code based on a Lagrangian formulation for the prediction of the motion and evaporation of the droplets. The evolution of the chemical kinetics is predicted with the CHEMKIN package for n-heptane, which is selected as fuel. A detailed n-heptane low-temperature mechanism including 168 species and 904 reactions describes the chemical kinetics.

For initial temperatures inside the negative temperature coefficient region (NTC), the only spray parameter influencing autoignition delay is the spray evaporation time. If the initial temperature is on the lower boundary of the NTC region, the strong temperature dependence of autoignition in this region leads to a substantially longer autoignition delay due to the cooling of the gas phase caused by evaporation. A delaying effect of evaporation time is only present if the evaporation time is higher than the first induction time. Generally, the safety margin between autoignition and the end of evaporation is enhanced by utilization of a spray with small droplets and a narrow droplet size distribution. Also, a minimum autoignition delay for lean conditions at Φ=0.5 is identified.