Spray ignition and local flow properties in a swirled confined spray-jet burner: experimental analysis

Abstract

Laser ignition was investigated in the swirled, confined CORIA Rouen Spray Burner under ultra-lean conditions (=0.61) with n-heptane as the liquid fuel. Ignition probability was calculated for different spark locations and compared to the non-ignited local flow properties. Mean velocity components of the carrier flow were measured by PDA under spray presence and without spray, and are compared to mean values from PIV. PIV measurements provide information on the instantaneous airflow and the total strain rate. Fuel droplet size-velocity data was measured by PDA. Toluene-PLIF images were acquired to provide information on the local equivalence ratio and the flammability factor. Results show that the outer recirculation zone (ORZ) has a flammability factor close to 1 and the highest ignition probability (~80%). These results have a high correlation with the air velocity field and turbulent kinetic energy. Instantaneous equivalence ratio images and shear rate-velocity fields give important information on local segregation of the flow properties that help to understand the ignition process. The present work provides a useful database for numerical simulations and industry, plus new insight on spray ignition. KeywordsSpray ignition, PDA, PIV, Toluene PLIF, Ignition probability, Local flow properties. IntroductionThe ignition process in gas turbines involves a wide range of parameters which makes it a multi-physical complex problem. New aeronautical burner designs demand a better knowledge of the mechanisms here involved to assure, for instance, re-ignition in high altitude of lean-combustion engines. Real combustors work with two-phase flows with strongly varying local properties. Indeed, turbulence, flow velocity, droplet size and velocity, and fuel vapour repartition are parameters governing the ignition mechanisms. Several studies on spray ignition [1-3] have been carried out. Studies on spray ignition in industrial configurations are still scarce although some pioneer experiments and simulations in linear [4,5] and annular combustors [6,7] have been done. Investigations on the influence of parameters such as turbulence [8, 9] also exist. The knowledge is still not sufficient to identify and understand the real mechanisms of ignition. More experiments are also needed to validate numerical simulations. Ignition in aeronautical engines can be divided into four steps: (i) energy deposition through a spark and its evolution into a flame kernel; (ii) kernel propagation; (iii) flame stabilisation on one injector; (iv) injector-to-injector propagation of the flame. Sometimes one of these phases is not successful and leads to a missed ignition. Even with a sufficient energy supply at phase (i), phase (ii) appears to be a critical step, namely in lean conditions. During its propagation in the chamber, the flame kernel will be exposed to varying local properties of the flow that may be adverse for its survival. This depends on both, the initial location of the kernel (spark location) and on its subsequent trajectory...