Raman I Sujith

The present study investigates the flow field characteristics of a gas turbine swirler in a model combustion chamber, using particle image velocimetry. Detailed mean and RMS velocities, vorticity, Reynolds shear stress, and pseudoturbulent kinetic energy were obtained at various cross sections downstream of the swirler and in a plane along the inlet flow direction. The experiments were performed in a sudden expansion square geometry. A central toroidal recirculation zone and corner recirculation zone was observed and characterized. Another instability caused by swirl, called precessing vortex core, has been observed far downstream of the swirler, in the plane located at Z/D = 2.5 and 1.25 (D, diameter of the swirler) depending on the pressure drop across the swirler. High RMS velocity magnitudes are observed in several cross-sectional planes indicating high levels of turbulence generated by the swirling effect which promotes rapid mixing. The structure of the complex swirling flow field has been investigated both qualitatively and qualitatively. Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Most gas turbine engines in service use a prefilming airblast atomizer in combination with strong swirling flow. An experimental investigation of the characteristics of a hollow cone prefilming airblast atomizer in a strong swirling air flow Is presented in this paper. The measured quantities include the spray cone angle by planar imaging, patternation using planar laser-Induced fluorescence, planar droplet velocity field using two-component particle image velocimetry, and droplet size and velocity distributions using a phase Doppler particle-size analyzer and laser Doppler velocimetry. Patternation of the spray field showed regions of high droplet concentration and volume flux fluctuations in various cross-sectional planes. The investigation revealed the presence of droplets of narrow size range up to 25 μ around the axis of the atomizer in the central toroidal recirculation zone of the swirler. The presence of vortical structures entraining the droplets is observed from the instantaneous velocity vector fields. The velocity data shows entralnment of the droplets in the central toroidal recirculation zone. Examining the size and the velocity data reveals that the centrifugal action of the swirling air flowfleld from the atomizer carries the bigger droplets outward, toward the periphery of the spray.

This paper explores some fundamental issues involved in flame-acoustic interaction in the context of non-premixed flames. The combustion model considered is a two-dimensional co-flowing non-premixed flame. Both finite rate and infinite rate chemistry effects are examined. First, the velocity-coupled response of the flame to an externally imposed velocity fluctuation is studied at various frequencies of interest. The Damköhler number plays an important role in determining the amplitude and phase of the heat release fluctuations with respect to the velocity fluctuations. Second, the combustion model is coupled with the duct acoustics. The one-dimensional acoustic field is simulated in the time domain using the Galer kin method, taking the fluctuating heat release from the combustion zone as a compact acoustic source. The combustion oscillations are shown to cause exchange of acoustic energy between the different natural modes of the duct over several cycles of the acoustic oscillations.

This paper explores some fundamental issues involved in flame-acoustic interaction in the context of non-premixed flames. The combustion model considered is a two-dimensional co-flowing non-premixed flame in a uniform flow field, as in the Burke-Schumann geometry. Both finite-rate and infinite-rate chemistry effects are examined. First, the velocity-coupled response of the flame to an externally imposed velocity fluctuation is studied at various frequencies of interest. The Damköhler number plays an important role in determining the amplitude and phase of the heat release fluctuations with respect to the velocity fluctuations. Second, the combustion model is coupled with the duct acoustics. The one-dimensional acoustic field is simulated in the time domain using the Galerkin method, taking the fluctuating heat release from the combustion zone as a compact acoustic source. The combustion oscillations are shown to cause exchange of acoustic energy between the different natural modes of the duct over several cycles of the acoustic oscillations.

This paper reports an experimental investigation of the acoustic field in a duct due to vortex shedding from a single orifice located in the middle of the duct. These experiments were intended to simulate the acoustic field created by the flow past the protruding inhibitors in large segmented solid rocket motors. The effect of orifice plate thickness on the strength of the acoustic field in the duct is studied. It is shown that the phenomena of lock-on occurs in a certain velocity range for a limited range of thickness. Further more, it has been found that an optimum thickness exists for a given velocity and orifice diameter where the acoustic amplitude is maximum. The thickness range shifts to a higher value for larger orifices. Larger orifices excite lower amplitudes than smaller orifices. Lock-on occurs for a larger range of thickness for ducts with larger diameter. The paper closes with an examination of the physical mechanisms involved, and a suggestion for a strategy for passive control of such oscillations. © 2000 by B. Karthik, S. R. Chakravarthy and R. I. Sujith. Published by the American Institute of Aeronautics and Astronautics, Inc.

This paper reports about a preliminary investigation on the characteristics of acoustic radiation by a two-dimensional confined supersonic flow past a wall-mounted cavity. Rectangular cavities of nine different L/D ratios were considered: three “deep”, three “shallow'’ and three “square” shapes. Inlet Mach numbers of 1.5 and 2.0 were tested. Results obtained are in the form of (a) acoustic measurements at different locations on the wall of the test section opposite to the cavity wall and (b) visualisation of the flow in and around the cavity. A wide range of acoustic amplitudes was recorded at different locations with different cavities. The results help in the identification of cavity shapes that are most suited for further consideration in future studies on mixing, secondary fuel injection, and combustion. The visualisation experiments suggest more locations corresponding to oscillating shocks where acoustic amplitudes could be larger than recorded so far.

The objective of this work is to develop an acoustically excited solid waste incinerator that is of the continuous feed type. The combustor is essentially a tunable dump combustor, similar to the combustor of a liquid fuel ramjet. In this study, sawdust was used as a surrogate waste. The saw dust is fluidized and is fed to the combustor along with the air supply. The amount of unburned sawdust is determined by weighing the sawdust that is left behind after the carbon particles are separated out. © 2000 by B. Ramanarayanan, S. V. Ross, U. Darvemula, R. I. Sujith, and S. R. Chakravarthy.

This paper describes some experiments performed with cold flow of air at atmospheric pressure in a duct with a single orifice intended to simulate the protruding inhibitors in the ieter-segmental regions in large solid rocket motors. The length of the duct was closely varied in most experiments, and tests were performed with the orifice at different locations along the axis of the duct at each length ievei. Other parameters that were varied include the orifice size and flow velocity. Measurements were mainly in the nature of acoustic diagnostics, with a microphone or a transducer located at different axial locations along the duct in different runs. The combined results indicate that, although no audible sound of distinct frequencies were excited by vortex shedding at the orifice, the mechanism of pressure-coupled feedback-based vortex shedding is prevalent in the geometry adopted. The mechanism manifests in abrupt shifts in frequencies related to the different possible duct acoustic mode, and is accompanied by oscillatory fluctuations in the amplitude of the duct acoustic modes, as the duct length is varied. The results are thought to have implications in the context of abrupt frequency shifts experienced in large rocket motors. © 2000 by the authors.

A combustor with a divergent inlet passage and an axially movable bluff-body flame-holder has been developed in order to enable self-excitation of highamplitude acoustic oscillations, for the purpose of enhancing the rate of evaporation of water sprays. Commercially available liquid petroleum gas is used as fuel. The acoustic pressure levels and modal contents are controlled by varying the location of the bluff-body along the axis of the combustor relative to its inlet. Locating the bluff body closer to the inlet yields higher acoustic pressure amplitudes and over a wider range of air-fuel mass ratios. The sound pressure level is found to increase with increase in mass flow rate of air and airfuel mass ratio. The excitation modes are found to shift from the fundamental to the first harmonic for certain air-fuel mass ratios for a given mass flow rate of air. Measurement of pollutant emission levels indicate low CO concentrations in the presence of high-amplitude acoustic oscillations, but slightly higher NOx concentration than at low acoustic amplitude conditions. When high sound pressure levels are excited in the combustor, the heat transfer due to water cooling of the combustor walls and across the walls of the tailpipe increased considerably - up to 40% for an increase in SPL from 145 dB to 160 dB. Water spray evaporation experiments are performed with low and high sound pressure levels of acoustic excitation, for different water injection rates over a range of mass flow rates of fuel and air. For a given mass flow rate of air and fuel, it is found that the water evaporation rate increased significantly above a certain threshold acoustic pressure - up to 107% for a sound pressure level of 156 dB, despite the considerable increase in heat losses due to the acoustic oscillations. © 2003 by Dr. R. I. Sujith & Dr. S. R. Chakravarthy.

The present study investigates the effect of acoustic oscillations on swirl flow characteristics, using 2C and stereo PIV. The experiments were performed in a sudden expansion square geometry. A high amplitude acoustic field was established in the duct using external acoustic drivers (woofers) that were mounted on the test section downstream of the swirler. A constant acoustic pressure of 600 Pa was maintained at the exit of the swirler. The experiments were performed for different Strouhal numbers. It was observed that the fluctuations in axial velocity in the recirculation zone are much higher than that of the acoustic velocity. However, no significant change in the tangential velocity was observed. It was seen that the maximum fluctuation amplitude increases with Strouhal number in both low and high acoustic velocity fluctuations regimes. It was found that the use of vane passage size as the characteristic dimension leads to Strouhal numbers in the range of 0.2 to 0.5, where the axial velocity fluctuations in the recirculation zone are high. Copyright © 2009 by E. Arunraj, R. I. Sujith and Chakravarthy.