Detailed measurements of the structure of a jet diffusion flame in a cross flow

Abstract

Measurements were made of the structure of a turbulent jet diffusion flame for propane discharged normal to a free stream of air with uniform velocity profile. Attention was mainly paid to the influence of the velocity ratio, which is defined as the ratio of cross-flow velocity to fuel jet velocity, on the flame structure. Laser Doppler velocimetry and fine wire thermocouples were employed to measure the velocity and temperature profiles in the flame, respectively. The concentration profiles of the major chemical species in the flame were measured by using gas chromatograph. In the discussion, we describe the time-averaged flame structure for different velocity ratios in order to explore the relationship between the visible flame length and the flame structure. Photography of the visible flame shows that the flame length reaches a minimum as the velocity ratio increases from 0. The measured results suggest that, for the higher velocity ratio, the entrainment of air proceeds actively in the upstream, which is due to the vortex motion formed in the lower region of the jet. The entrainment of air suppresses in the downstream because the flame is fully bent and it looks like the coflowing jet. For the lower velocity ratio, the flame is not fully bent even in the downstream, which leads to the increased entrainment of air in this region. These behaviors of air entrainment result in the occurrence of a minimum in the flame length.