MORPHOLOGY OF UNCONFINED AND CONFINED SWIRLING FLOWS UNDER NON-REACTING AND COMBUSTION CONDITIONS

Abstract

Swirl is used in practically all types of combustion systems, including gas turbine combustion, furnaces and boilers. In combustion systems, the strong favorable effect of swirl to combustion air and/or fuel has been extensively used for flame stabilization, high heat release per unit volume, and clean efficient combustion. Flow and combustion characteristics of non-reacting and reacting swirl flowfields are characterized using a simulated Lean Direct Injection (LDI) method in a double concentric swirl burner. The LDI scheme had a large number of small size holes for fuel injection to provide rapid fuel mixing into the surrounding combustion air. The double concentric burner allowed examination of radial distribution of swirl in the burner (co- and counter-flow) under unconfined and confined conditions, both without and with combustion. The input thermal loading to the burner was held constant at 33 kW for all flames.

Particle image velocimetry (PIV), optical emission spectroscopy (OES), infrared (IR) thermometry, gas analysis and computer compensated micro-thermocouple measurements, were used for diagnostics. These diagnostics provided information on spatial and temporal distribution of flowfield, flame generated radicals, mean gas species concentration, and mean and rms temperatures compensated to high frequencies as well as the associated integral- and micro-thermal time scales, respectively.

Unconfined co-swirl flows had generally wider (except non-reacting) and longer internal recirculation zones, slower velocity decay, smaller reverse flow velocity, lower intensity of flame generated radicals, higher temperatures, and longer integral- and micro-thermal time scales as compared to its counter part. Confinement altered the global flame structure dramatically by rapid radial expansion of the flame to the combustor walls. It increased length and decreased width of the internal recirculation zone, delayed the velocity decay, increased temperatures, amplified intensity of flame generated radicals over a greater region, and enlarged turbulence levels. The vortical structures associated with the instantaneous flow for all cases revealed significant dynamical behavior in the flow as compared to the mean flow case. Infrared thermometry results supported the micro-thermocouple data on mean temperatures. The trend for NOx emissions was higher for the confined case in both co- and counter-swirl cases.