The effect of jet preheating on turbulence in a premixed jet in hot coflow

Abstract

Moderate or Intense Low oxygen Dilution (MILD) combustion is an important combustion regime, which utilises the combustion of gas in hot and often vitiated environment. MILD combustors are characterised by the use of exhaust gases recirculated into the combustor via dilution or mixing with the incoming fuel or alternately by rapidly mixing an incoming fuel stream into a hot environment of combustion products. However, the influence of preheating the fuel jet is not well understood. Current approaches rely heavily on assumptions about the role of heat on laminarisation of turbulence, due to viscosity. However, the density ratio between jet and environment is known to have a dominant effect on mixing. Velocity measurements are presented for a pair of turbulent premixed flames issuing into a premixed coflow. Two cases are considered, both with and without preheating of the central fuel jet. The resulting near-field velocity is measured using digital Particle Image Velocimetry (PIV). The resulting velocity maps are processed to find the turbulence intensity, u’/U. A two-point correlation technique is used to determine the turbulence integral length scale, lf. The influence of preheating the central jet on the resulting near-field turbulence characteristics are presented and discussed.