| dc.description.abstract | Combustion in gas turbine combustors features complex coupling of fluid dynamics, heat release, and chamber acoustics. Most combustion systems have a tendency to self-excite large amplitudes of pressure and heat release oscillations, which are also known as thermo-acoustic instabilities. Such oscillations can lead to vibration in mechanical systems and oscillatory heat load on combustor walls that can have detrimental effects. Thermo-acoustic instabilities can affect combustor performance, emission characteristics and a higher amplitude can cause flame-blow-out. The physical mechanism that couple heat release and pressure oscillations to excite these instabilities is quite complex. The acoustic waves in an annular gas turbine can have multiple modes - longitudinal, transverse (or azimuthal), and radial mode. The transverse acoustic waves travel transverse to the direction of flow through each burner and can affect the flame in ways that are different than longitudinal or radial waves, which necessitates their investigation over swirling flow and flame. In this thesis, we experimentally investigate the acoustically forced response of non-reacting swirling jet and partially-premixed swirl-stabilized methane−air flame to external transverse acoustic excitation. We also investigate the effect of varying the inter-nozzle spacing on the flow structures in a three-injector inline configuration under non-reacting conditions.
In the first part of the thesis, we investigate the effect of in-phase (IP) transverse acoustic excitation over a non-reacting swirling jet (denoted as columnar-jet: CJ). It is observed that, at a fixed excitation frequency, with an increase in the amplitude of excitation the annular jet widens, and the central recirculation bubble (CRB) widens and strengthens in magnitude. Further increasing the amplitude of excitation above a critical value, the columnar-jet (CJ) transitions to a wall-jet flow state (WJ) (or, conical vortex breakdown). In the WJ state, a large recirculation bubble (acquiring an almost complete observation window) is observed around the geometric centerline, and the flow enters the combustion chamber in the form of radial jet grazing along the combustor dump plane. An increase in the swirl number of the flow is observed during the transition from CJ to WJ state. Further, an increase in fluctuations in streamwise velocity and radial velocity is observed at the centerline and at one radius away from the centerline, respectively. The proper orthogonal decomposition (POD) analysis shows a global streamwise oscillation of the entire field-of-view as the most energetic mode in the WJ state. We hypothesize that the combined effect of the change in the distribution of mean azimuthal velocity and a strong radial velocity fluctuations at one radius away from the centerline as a precursor to the transition from CJ to WJ state. The strong could be responsible for deflecting the shear layers of the swirl jet axisymmetrically outward causing CJ to WJ transition. Further, we observe that as the flow the transitions from CJ to the WJ state, the contribution of the centrifugal force term to the radial pressure gradient drops, and the contribution of the convective term dominates. The response of the flow to turning OFF the acoustic excitation is observed to be a function of flow Reynolds number (Re). Below a critical Re, upon turning OFF the acoustic excitation the flow returns to the initial flow state – CJ state. However, above the critical Re, even after turning OFF the acoustic excitation the flow stays in the newly acquired wall-jet state (WJ). Thus, an observation of hysteresis in non-reacting swirling flows to external transverse acoustic excitation is observed.
We believe that these observations are the first instance of visualization and measurement of hysteresis phenomenon (leading to a bistable state) in non-reacting swirling flows due to transverse acoustic excitation (while keeping the Reynolds number and geometric swirl number of inlet flow fixed). This observation of hysteresis phenomenon is found consistent with two different types of injectors investigated in this thesis.
In the second part of the thesis, the effect of in-phase (IP) and out-of-phase (OP) transverse acoustic excitation on a partially premixed swirl-stabilized methane−𝑎𝑖𝑟 flame is investigated. The work primarily focuses on the flame response to high amplitude forcing. It is observed that for forcing amplitudes in the range of 0-2.9% of chamber pressure, during OP excitation an initial V-shaped flame transitions to an M-shaped as the acoustic excitation amplitude is increased at a fixed excitation frequency. The distribution of peak OH* intensity shifts from along the inner shear layer in the V-flame to a circular lobed structure on either side of the geometric centerline in the M-flame. Through forced acoustic response analysis, we hypothesize that the transition to M-shape is due to acoustically induced velocity and vortical disturbances near the nozzle exit during transverse OP forcing. These acoustically induced velocity fluctuations induce strain on the flame and the induced vortical disturbances cause the flame to roll. If the acoustically induced velocity gradients at the nozzle exit are higher than the extinction strain-rate of the flame, the flame simply rolls outward and extinguishes in the inner shear layer (ISL) at nozzle exit and tries to find a suitable location to stabilize in the outer recirculation zone (ORZ). These observations are supported by the instantaneous time snapshots of OH* chemiluminescence and velocity field. During IP excitation, at most excitation frequencies, the flame continues to remain in the V-shape with the increase in forcing amplitude (ranging between 0-2.9% of chamber pressure). The fluctuations in OH* field at one radius away from the centerline are found to be stronger during OP excitation than IP excitation. Proper orthogonal decomposition (POD) shows streamwise oscillations and radial oscillations as the most dominant mode during in-phase and out-of-phase transverse acoustic excitation, respectively. At higher excitation amplitude (>4% of chamber pressure) during IP excitation, a highly transient phenomena is observed where a repetitive transition between a V-shaped flame to a wall-jet flame is observed and correspondingly the pressure field shows a burst of repetitive epochs of high amplitude pressure oscillations. The velocity measurements of the flame indicate the flow continuously oscillating between columnar jet and wall jet state (similar to the flow transition from CJ to WJ state observed in non-reacting swirling flows). Therefore, the mechanism behind the continuous change in flame topology is attributed to the change in the vortex breakdown states at these high amplitude oscillations. Other transient events observed during high-amplitude IP transverse acoustic excitation include repetitive transition between V-flame and lifted-V flame, and observation of steady wall-jet flame. Further increase in forcing amplitude (~5% of chamber pressure) during IP excitation causes the flame to completely blow-out. Such a high-amplitude acoustic-induced blow-out is attributed to the transition of the swirling jet to a steady wall-jet state. We hypothesize that proximity of the flame to the dump wall during the wall-jet state leads to significant heat loss from the flame causing the blow-out.
The third part of the thesis investigates the effect of inter-nozzle spacing (S = 2D, 2.5D and 3D; D being the diameter of the injector at the combustor dump plane) between three adjacent non-reacting swirling jets over the flow structure of individual swirling jets. Simultaneous 2D-2C PIV and unsteady pressure measurement are performed to understand the structure and dynamics of the flow states during the interaction with the adjacent injectors. It is observed that due to interaction of flow-field of adjacent injectors, the flow of central (or side) injector transitions from columnar-jet (CJ) to wall-jet (WJ) state at higher flow Reynolds number (Re) and large inter-nozzle spacing (S). An intermittent behaviour at one of test conditions (Re = 18000, S = 3D) is observed where the flow through central injector transitions from CJ to WJ and back to CJ. Thus, the study shows that the interaction of flow of adjacent injectors can lead to a transition from columnar-jet state (CJ) to wall-jet state (WJ) even without any external source of perturbation (such as external transverse acoustic excitation as discussed in previous chapters of the thesis).
In summary, we investigated the effect of transverse acoustic excitation on the flow-field of a non-reacting swirling jet and on a partially premixed swirl-stabilized methane−𝑎𝑖𝑟 flame. The response of an injector to a pressure node/antinode excitation is found to be different that can cause non-uniform flame response during azimuthal instability in an annular gas turbine combustor. This can affect the azimuthal uniformity of the flow and temperature field in an annular combustor, which can have severe repercussions on the performance of a GT engine. | en_US |
