An experimental investigation of the effects of nozzle ellipticity on the flow structure of co-flow jet diffusion flames

Abstract

The flow structure of cold and ignited jets issuing into a co-flowing air stream was experimentally studied using a laser Doppler velocimeter. Methane was employed as the jet fluid discharging from circular and elliptic nozzles with aspect ratios varying from 1.29 to 1.60. The diameter of the circular nozzle was 4.6 mm and the elliptic nozzles had approximately the same exit area as that of the circular nozzle. These non-circular nozzles were employed in order to increase the stability of attached jet diffusion flames. The time-averaged velocity and r.m.s. value of the velocity fluctuation in the streamwise and transverse directions were measured over the range of co-flowing stream velocities corresponding to different modes of flame blowout that are identified as either lifted or attached flames. On the basis of these measurements, attempts were made to explain the existence of an apparent optimum aspect ratio for the blowout of attached flames observed at higher values of co-flowing stream velocities. The insensitivity of the blowout limits of lifted flames to nozzle geometry observed in our previous work at low co-flowing stream velocities was also explained. Measurements of the fuel concentration at the jet centerline indicated that the mixing process was enhanced with the 1.38 aspect ratio jet compared with the 1.60 aspect ratio jet. On the basis of the obtained experimental data, it was suggested that the higher blowout limits of attached flames for an elliptic jet of 1.38 aspect ratio was due to higher entrainment rates

Development of a cylindrical burner comprising multiple pairs of opposing partially premixed or inverse diffusion flames

“…The resultant flow stretching increases the rates of heat release since more fuel and air are entrained into the flame zone thus providing a sustainable ignition source. 33,34 The current velocity limits exceed the maximum values between 30 and 45 m/s that were reported by Papanikolaou and Wierzba 19 for co-axial jets' normal diffusion flames with elliptical ports and those of Song et al. 35 for an elliptical jet flame in a cross flow.…”

“…The elliptical jets reduce the soot and NOx emissions from partially premixed flames, 18 while increasing the aspect ratio (defined as the major to the minor diameter ratio) enhances the flow stretch and provides more increase in the combustion efficiency of nonpremixed flames. 19 These features are favorably coupled to the strain rate corresponding to the jets' mutual impingement in both flame modes. Since high thermal outputs from the multiple opposing flames' burners can thus be achieved with minimized levels of pollutants, such burners can be commissioned for satisfying the increasing magnitudes of miscellaneous industrial demands.…”