Experimental Investigation of an Unconfined Swirl-Stabilized Turbulent Premixed Flame

Abstract

A turbulent, swirl-stabilized jet emanating from an unconfined, premixed burner is investigated experimentally by means of optical (OH* chemiluminescence), acoustic (microphone), and laser-optical measurement techniques (Particle Image Velocimetry) for various swirl intensities. It is shown that even in case of an unconfined swirl flame, combustion-induced vortex breakdown (CIVB) occurs which, on one hand, contributes to the stabilization of the flame and stable combustion due to the increased recirculation zone and, on the other hand, can promote flashback if the recirculation zone travels upstream or is extended upstream, especially for high swirl intensity flows. Another effect associated to CIVB is the generation of vortex breakdown for low Reynolds number, reacting flows which corresponding non-reacting, isothermal flows at the some operating conditions do not create a recirculation zone at all. After crossing a threshold of injected momentum, i.e. Reynolds number, normalized flow fields become Reynolds number independent. It is found that noise emissions from the burner grow with increase in different parameters: equivalence ratio of the injected and burnt mixture, Reynolds number, and number of vanes on the swirler disc. All three parameters cause an extended area of heat release which is supposed to generate larger pressure oscillations and, hence, more noise. Nomenclature d = nozzle diameter f = frequency G' x = axial flux of linear momentum G'  = axial flux of axial momentum R = radius of the nozzle outlet R H = radius of the swirler hub Re = Reynolds number r = radius r/d = relative radial location with respect to nozzle diameter S = swirl number T = preheat temperature Tu = degree of turbulence/ turbulence level u = axial velocity u bulk = bulk velocity of the jet u/u bulk = normalized axial velocity u RMS = fluctuation of axial velocity 1 Ph.D. student, Chair of Fluid Dynamics, Hermann-Föttinger-Institut, 2 Figure 1. Schematic of a swirler insert (top left), test stand (top right), and sectional view of the outlet section of the test stand (bottom) u  = tangential velocity u  /u bulk = normalized tangential velocity u ,RMS = fluctuation of tangential velocity v = radial velocity v/u bulk = normalized radial velocity v RMS = fluctuation of radial velocity x/d = relative axial location with respect to nozzle diameter  = vane angle  = equivalence ratio