Abstract :

The efficient and enhanced mixing of heat and incoming reactants is achieved in modern gas turbine systems by employing swirling flows. This is realized by a low velocity region (internal recirculation zone -IRZ) zone resulting from vortex breakdown phenomenon. Besides, IRZ acts as effective flame holder/stabilization mode. Double concentric swirling jet is employed in plethora of industrial applications such as heat exchange, spray drying and combustion. As such, understanding essential features of vortex breakdown induced IRZ and its acoustic response in swirling flow/flame is important in thermo-acoustic instability studies.The key results of the present experimental investigation are discussed in four parts. In the first part, primary transition (sub-critical states) from a pre-vortex breakdown (Pre-VB) flow reversal to a fully-developed central toroidal recirculation zone (CTRZ) in a non-reacting, double-concentric swirling jet configuration is discussed when the swirl number is varied in the range 0.592 S 0.801. This transition proceeds with the formation of two intermediate, critical flow regimes. First, a partially-penetrated vortex breakdown bubble (VBB) is formed that indicates the first occurrence of an enclosed structure resulting in an opposed flow stagnation region. Second, a metastable transition structure is formed that marks the collapse of inner mixing vortices. In this study, the time-averaged topological changes in the coherent recirculation structures are discussed based on the non-dimensional modified Rossby number (Rom) which appears to describe the spreading of the zone of swirl influence in different flow regimes. The second part describes a secondary transition from an open-bubble type axisymmetric vortex breakdown (sub-critical states) to partially-open bubble mode (super-critical states) through an intermediate, critical regime of conical sheet formation for flow modes Rom ≤ 1 is discussed when the swirl number (S) is increased beyond 0.801. In the third part, amplitude dependent acoustic response of above mentioned sub and supercritical flow states is discussed. It was observed that the global acoustic response of the sub-critical VB states was fundamentally different from their corresponding super-critical modes. In particular, with a stepwise increase in excitation amplitude till a critical value, the sub-critical VB topology moved downstream and radially outward. Beyond a critical magnitude, the VB bubble transited back upstream and finally underwent radial shrinkage at the threshold excitation amplitude. On the other hand, the topology of the super-critical VB state continuously moved downstream and radially outwards and finally widened/fanned-out at threshold amplitude. In the final part, transition in time-averaged flame global flame structure is reported as a function of geometric swirl number. In particular, with a stepwise increase in swirl intensity, primary transition is depicted as a transformation from zero-swirl straight jet flame to lifted flame with blue base and finally to swirling seated flame. Further, a secondary transition is reported which consists of transformation from swirling seated flame to swirling flame with a conical tailpiece and finally to highly-swirled near blowout ultra-lean flame. For this purpose, CH* chemiluminescence imaging and 2D PIV in meridional planes were employed. Three baseline fuel flow rates through the central fuel injection pipe are considered. For each of the fuel flow cases (Ref), six different co-airflow rate settings (Rea) are employed. The geometric swirl number (SG) was increased in steps from zero till blowout for a particular fuel and co-airflow setting. A regime map (SG vs Rea) depicting different regions of flame stabilization are then drawn for each fuel flow case. The secondary transformation is explained on the basis of physical significance of Rom. Brief Bio-data R.Santhosh obtained his Bachelors of Engineering from the Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumkur, Karnataka affiliated to Visvesvaraya Technological University, Belgaum, Karnataka in 2009. He was awarded 5 gold medals including University gold medal for having secured first rank in the Mechanical engineering batch of 2005-2009. Later he joined (in 2010) the group of Dr. Saptarshi Basu, Department of Mechanical Engineering, Indian Institute of Science, Bangalore under direct PhD program where he extensively worked on swirling flows and flames in an unconfined coaxial burner. He is currently working as junior research associate, continuing the work on swirl flows with Dr. Basu in National Center for Combustion Research and Development (NCCRD) at IISc, Bangalore. His main research interests are vortex breakdown in swirling flows, spray-swirl interaction study, and study of soot formation in a flow laden with convecting line vortex. His experimental expertise involve tomographic PIV, 2D PIV in reacting and non-reacting flows, Chemiluminescence imaging, LIF, Shadowgraphy, Hotwire anemometry. He is a member of American physical society (APS).