Structure of attached and lifted gas jet flames in hysteresis region

Abstract

The flame structure and hysteresis chacracteristics of lifting and reattaching propane jet flames over contoured nozzles were studied. High-speed schlieren photography was employed to investigate the flow field in the near-nozzle region of cold jets, attached flames, lifted flames, and their transitions. Fine-wire thermocouples were used in conjunction with a computer data acquisition system to record temperature field in the stabilization region. Gas sampling and analysis were employed to determine the mean concentrations of oxygen and carbon monoxide The mean velocities were determined with a total pressure tube. The flow in the flame stabilization region is observed to be laminar in attached flames and dominated by organized structures in lifted flames. The characteristics of those structures have been measured during liftoff and reattachment transitions and compared with cold jet data. The average dimensionless celerities and Strouhal numbers of the organized structures in the burning parts of the flames were measured as 0.94 and 2.8 during liftoff and 0.62 and 1.30 during reattachment, respectively. The schlieren photographs and radial profiles of temperature and concentrations establish that the flame zone is located away from the shear layer. Further, they show that the molecular diffusion controls the lifting process. The dynamics of the organized structures that result in a wider shear layer govern the reattachment process. This difference between the structures of the flame base in the two configurations accounts for the hysteresis phenomenon. The variations of liftoff and reattachment velocities with nozzle diameter, fuel molecular weight and addition of inert diluents to the jet fluid were measured to substantiate these observations.