Influence of the Turbulence Length Scale and Intensity on Spark Ignition of Kerosene Jet-A1–Air Mixtures at High Altitude Relight Conditions

Abstract

In recent years Avio Aero and the Karlsruhe Institute of Technology (KIT) have significantly enhanced their available experimental technology level in order to consolidate and deepen the investigation of some critical phenomena in aero-engine gas turbine combustion. A key parameter in the development of a lean-burn combustor is the design of the ignition system. The increase in the amount of air that flows through the primary zone to approximately 60% of the overall air proves to be beneficial for emissions reduction. In return it causes shorter residence times in the primary zone and makes the flame kernel generation and propagation a real challenge. With the scope to enhance the fundamental understanding of the ignition process at altitude conditions, the ISCRA rig (Ignition in Subatmospheric Conditions – Rig for Altitude Relight Investigation) has been designed and manufactured at the KIT. The design of the rig allows the generation of altitude conditions at variable flow velocities and turbulence characteristics. An optical access for ignition recording by a CCD camera is also provided. The paper presents the results of a fundamental investigation at the KIT using a generic setup. It consists of a pressure atomizer with known atomization properties mounted in a test rig which allows the variation of several parameters that influence ignition, namely: air pressure; air temperature; velocity; Fuel-Air-Ratio (FAR); Sauter Mean Diameter (SMD); spark energy; turbulence intensity and length scale. Two grey spots that exist in the knowledge map of the ignition process are addressed in this paper. These are data on kerosene spray ignition at temperatures below 293 K, where data is scarce and the relationship between flame kernel generation and propagation, which has not been broadly investigated nor is well understood. The ignition probabilities of kerosene sprays as a function of ignition energy in the temperature range from 253 K to 293 K, pressure of 0.7 bar, flow velocities up to 6 m/s at different turbulence intensities and length scales are measured. Furthermore, the influence of the above mentioned parameters on the kernel generation and propagation is presented and discussed. Results show that the energy required for flame kernel propagation is higher than for flame kernel generation. This difference increases at lower temperature and higher turbulence intensity. The paper gives an insight in magnitude of this difference in energy and compares the tendencies with an earlier model. The absolute values need to and will be corrected by future in situ measurements.