Дисертації з теми "Gas Turbine Swirl Injectors"

Hydrogen, as an alternative to conventional aviation fuels, has the potential to increase the efficiency of a gas turbine as well as reduce emissions of greenhouse gases. In addition to significantly reducing the number of pollutants due to the absence of carbon, burning hydrogen at low equivalence ratios can significantly reduce emissions of oxides of nitrogen (NOx). Because hydrogen has a wide range of flammability limits, fuel lean combustion can take place at lower equivalence ratios than those with typical hydrocarbon fuels.

Numerous efforts have been made to develop gas turbine fuel injectors that premix methane/natural gas and air in fuel lean proportions prior to the reaction zone. Application of this technique to hydrogen combustion has been limited due to hydrogen's high flame rate and the concern of the reaction zone propagating into the premixing injector, commonly referred to as flashback. In this investigation, a lean-premixing hydrogen injector has been developed for application in small gas turbines. The performance of this injector was characterized and predictions about the injector's performance operating under combustor inlet conditions of a PT6-20 Turboprop have been made.
Master of Science

Current lean-premixed fuel injector designs have shown great potential in terms of reducing emissions of pollutants, but such designs are susceptible to combustion instabilities in which aerodynamic instability plays a major role and also has an effect on mixing of air and fuel. In comparison to prototype testing with combustors running in operating conditions, computational approaches such as Large Eddy Simulations (LES) offer a much more cost-effective alternative in the design stage. However, computational models employed by LES require validation by experimental data. This is one of the main motivations behind the present experimental study. Combined particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) instrumentation allowed simultaneous measurements of velocity vector and a conserved scalar introduced into the fuel stream. The results show that the inner swirl shear layer features two pairs of vortices, which draw high concentration fuel mixture from the central jet into the swirl stream and causes it to rotate in their wakes. Such periodic entrainment also occurs with the characteristic frequencies of the vortices. This has clear implications for temporal variations in fuel/air ratio in a combusting flow; these bursts of mixing, and hence heat release, could be a possible cause of mixing-induced pressure oscillation in combusting tests. For the first time in such a flow, all 3 components of the turbulent scalar flux were available for validation of LES-based predictions. A careful assessment of experimental errors, particularly the error associated with spatial filtering, was carried out. Comparison of LES predictions with experimental data showed very good agreement for both 1st and 2nd moment statistics, as well as spectra and scalar pdfs. It is particularly noteworthy that comparison between LES computed and measured scalar fluxes was very good; this represents successful validation of the simple (constant Schmidt number) SGS model used for this complex and practically important fuel injector flow. In addition to providing benchmark data for the validation of LES predictions, a new experimental technique has been developed that is capable of providing spatially resolved residence time data. Residence times of combustors have commonly been used to help understand NOx emissions and can also contribute to combustion instabilities. Both the time mean velocity and turbulence fields are important to the residence time, but determining the residence time via analysis of a measured velocity field is difficult due to the inherent unsteadiness and the three dimensional nature of a high-Re swirling flow. A more direct approach to measure residence time is reported here that examines the dynamic response of fuel concentration to a sudden cutoff in the fuel injection. Residence time measurement was mainly taken using a time-resolved PLIF technique, but a second camera for PIV was added to check that the step change does not alter the velocity field and the spectral content of the coherent structures. Characteristic timescales evaluated from the measurements are referred to as convection and half-life times: The former describes the time delay from a fuel injector exit reference point to a downstream point of interest, and the latter describes the rate of decay once the effect of the reduced scalar concentration at the injection source has been transported to the point of interest. Residence time is often defined as the time taken for a conserved scalar to reduce to half its initial value after injection is stopped: this is equivalent to the sum of the convection time and the half-life values. The technique was applied to a high-swirl fuel injector typical of that found in combustor applications. Two test cases have been studied: with central jet (with-jet) and without central jet (no-jet). It was found that the relatively unstable central recirculation zone of the no-jet case resulted in increased transport of fuel into the central region that is dominated by a precessing vortex core, where long half-life times are also found. Based on this, it was inferred that the no-jet case may be more prone to NOx production. The technique is described here for a single-phase isothermal flow field, but with consideration, it could be extended to studying reacting flows to provide more insight into important mixing phenomena and relevant timescales.

A novel, fast switching, reliable, and economical fluidic gaseous fuel injector system designed for natural gas engines has been developed in this research. The system consists mainly of no-moving-part fluidic devices and piezo-electric controlling interfaces. The geometric parameters of a fluidic device seriously affect its performance. Traditionally, these parameters can only be optimised through "trial and error" exercise. In this research, a computer simulation model for the jet steady state attachment and dynamic switching has been developed. The good agreements between predicted results and experimental ones show that the model can not only explain the jet attaching and switching mechanism, but also optimise the design of geometric parameters of a fluidic device. The steady state and dynamic characteristics of the system were tested on a laboratory experimental rig. The results show that the system can handle the large gas volume flow rate required by natural gas engines and is capable of operating via pulse width modulation. A few typical commercial solenoid type gas injectors were also tested and the results were compared with those from the fluidic system. It was found that the fluidic gaseous fuel injector system has faster switching responses and smaller injection cycle-to-cycle variations.

The aerodynamic integration of an aero-engine intake system with the airframe can pose some notable challenges. This is particularly so for many military air- craft and is likely to become a more pressing issue for both new military systems with highly embedded engines as well as for novel civil aircraft configurations. During the late 1960s with the advent of turbo-fan engines, industry became in- creasingly aware of issues which arise due to inlet total pressure distortion. Since then, inlet-engine compatibility assessments have become a key aspect of any new development. In addition to total temperature and total pressure distortions, flow angularity and the associated swirl distortion are also known to be of notable con- cern. The importance of developing a rigorous methodology to understand the effects of swirl distortion on turbo-machinery has also become one of the major concerns of current design programmes. The goal of this doctoral research was to further the current knowledge on swirl distortion, and its adverse effects on engine performance, focusing on the turbo-machinery components (i.e. fans or compressors). This was achieved by looking into appropriate swirl flow descriptors and by correlating them against the compressor performance parameters (e.g loss in stability pressure ratios). To that end, a number of high-fidelity three-dimensional Computational Fluid Dynamics (CFD) models have been developed using two sets of transonic rotors (i.e. NASA Rotor 67 and 37), and a stator (NASA Stator 67B). For the numerical purpose, a boundary condition methodology for the definition of swirl distortion patterns at the inlet has been developed. Various swirl distortion numerical parametric studies have been performed using the modelled rotor configurations. Two types of swirl distortion pattern were investigated in the research, i.e. the pure bulk swirl and the tightly-wound vortex. Numerical simulations suggested that the vortex core location, polarity, size and strength greatly affect the compressor performance. The bulk swirl simula- tions also showed the dependency on swirl strength and polarity. This empha- sized the importance of quantifying these swirl components in the flow distortion descriptors. For this, a methodology have been developed for the inlet-engine compatibility assessment using different types of flow descriptors. A number of correlations have been proposed for the two types of swirl distortion investigated in the study.

In many industrial gas turbine combustors the injection of liquid fuel resembles the simple configuration of a jet in a rectangular channel with cross-flowing air, albeit with complex geometry both upstream and downstream from the channel. Therefore the detailed study of a jet in cross flow is an appropriate platform for the development of models for atomization and vaporization, both of which are key processes influencing efficiency and the emissions of pollutants from practical combustion devices. In the current study the breakup of a liquid jet and vaporization of droplets are modelled using an entirely Eulerian approach, where the liquid phase is treated analogously to a gas species in a multi-component reacting mixture. A novel boundary condition is proposed for the liquid surface area per unit mass at the jet inlet, and results are found to be insensitive to adjustments of the size parameter for this boundary condition. Validation is carried out in two stages: firstly turbulence closure via the Reynolds Averaged Navier-Stokes (RANS) approach with the standard constants is assessed for a gas-phase jet in cross flow with two different software packages; then predictions of the Sauter mean diameter of droplets are compared to measurements of a liquid jet in cross flow at 6 bar pressure. The turbulence model yields a reasonably accurate prediction of the flow field provided that the distribution of velocity across the jet inlet is specified. Droplet sizes agree well with the experiment except for a small region near the floor of the channel, where discrepancies can be attributed to the RANS closure. Application of the model is demonstrated for an industrial gas turbine combustor at its full load operating condition.

Gas turbines are extensively used in combined cycle power systems. These form about 20% of global power generating capacity, normally being fired on natural gas, but this is expected in the future to move towards hydrogen enriched gaseous fuels to reduce CO2 emissions. Gas turbine combined cycles can give electrical power generation efficiencies of up to 60%, with the aim of increasing this to 70% in the next 10 to 15 years, whilst at the same time substantially reducing emissions of contaminants such as NOx. The gas turbine combustor is an essential and critical component here. These are universally stabilized with swirl flows, which give very wide blowoff limits, and with appropriate modification can be adjusted to give very low NOx and other emission. Lean premixed combustion is commonly used at pressures between 15 to 30 bar, these even out hot spots and minimise formation of thermal NOx. Problems arise because improving materials technology/improved cooling techniques allow higher turbine inlet temperatures, hence higher efficiencies, but with the drawback of potentially higher emissions and stability problems. This PhD study has widely investigated and analysed two different kinds of gas turbine swirl burners. The research has included experimental investigation and computational simulation. Mainly, the flashback and blowoff limits have been comprehensively analysed to investigate their effect upon swirl burner operation. The study was extended by using different gas mixtures, including either pure gas or a combination of more than one gas like natural gas, methane, hydrogen and carbon dioxide. The first combustor is a 100 kW tangential swirl combustor made of stainless steel that has been experimentally and theoretically analysed to study and mitigate the effect of flashback phenomena. The use of a central fuel injector, cylindrical confinement and exhaust sleeve are shown to give large benefits in terms of flashback resistance and acts to reduce and sometimes eliminate any coherent structures which may be located along the axis of symmetry. The Critical Boundary Velocity Gradient is used for characterisation of flashback, both via the original Lewis and von Elbe formula and via new analysis using CFD and investigation of boundary layer conditions just in front of the flame front. Conclusions are drawn as to mitigation technologies. It is recognized how isothermal conditions produce strong Precessing Vortex Cores that are fundamental in producing the ii final flow field, whilst the Central Recirculation Zones are dependent on pressure decay ratio inside the combustion chamber. Combustion conditions showed the high similarity between experiments and simulation. Flashback was demonstrated to be a factor highly related to the strength of the Central Recirculation Zone for those cases where a Combustion Induced Vortex Breakdown was allowed to enter the swirl chamber, whilst cases where a bluff body impeded its passage showed a considerable improvement to the resistance of the phenomenon. The use of nozzle constrictions also reduced flashback at high Reynolds number (Re). All these results were intended to contribute to better designs of future combustors. The second piece of work of this PhD research included comprehensive experimental work using a generic swirl burner (with three different blade inserts to give different swirl numbers) and has been used to examine the phenomena of flashback and blowoff in the swirl burner in the context of lean premixed combustion. Cylindrical and conical confinements have been set up and assembled with the original design of the generic swirl combustor. In addition to that, multi-fuel blends used during the experimental work include pure methane, pure hydrogen, hydrogen / methane mixture, carbon dioxide/ methane mixture and coke oven gas. The above investigational analysis has proved the flashback limits decrease when swirl numbers decrease for the fuel blends that contain 30% or less hydrogen. Confinements would improve the flashback limit as well. Blowoff limits improve with a lower swirl number and it is easier to recognise the gradual extinction of the flame under blowoff conditions. The use of exhaust confinement has created a considerable improvement in blowoff. Hydrogen enriched fuels can improve the blowoff limit in terms of increasing heat release, which is higher than heat release with natural gas. However, the confinements complicate the flashback, especially when the fuel contains a high percentage of hydrogen. The flashback propensity of the hydrogen/methane blends becomes quite strong. The most important features in gas turbines is the possibility of using different kinds of fuel. This matter has been discussed extensively in this project. By matching flashback/blowoff limits, it has been found that for fuels containing up to 30% of hydrogen, the designer would be able to switch the same gas turbine combustor to multifuels whilst producing the same power output.

Power generation gas turbine manufacturers and operators are tasked increasingly with expanding operational flexibility due to volatility in global gaseous fuel supplies and increased renewable power generation capacity. Natural gas containing high levels of higher hydrocarbons (e.g. ethane and propane) is typical of liquefied natural gas and shale gas, two natural gas sources impacting gas turbine operations, particularly looking forward in the United Kingdom. In addition, hydrogen-blending into existing natural gas infrastructure represents a potential energy storage opportunity from excess renewable power generation, with associated combustion impacts not fully appreciated. This thesis aims to address the specific operational problems associated with the use of variable gaseous fuel compositions in gas turbine combustion through a combination of experimental and numerical techniques, with a focus on natural gas blends containing increased levels of higher hydrocarbons and hydrogen. Parametric experimental combustion studies of the selected fuel blends are conducted in a new fully premixed generic swirl burner at elevated ambient conditions of temperature and pressure to provide representative geometry and flow characteristics typical of a can-type industrial gas turbine combustor. New non-intrusive diagnostic facilities have been designed and installed at Cardiff University’s Gas Turbine Research Centre specifically for the characterization of the influence of fuel composition, burner geometry, and operating parameters on flame stability, flame structure, thermoacoustic response, and environmental emissions. Experimental measurements are supported through the use of numerical chemical kinetics and acoustic modelling. Results from this thesis provide an experimental validation database for chemical kinetic reactor network and CFD modelling efforts. In addition, it informs gas turbine manufacturers on potential burner design modifications for future fuel flexibility and provide enhanced empirical tools to power generation gas turbine operators for increased operational stability, reduced environmental impact, and increased utilization.

Thesis (M.S.)--University of Cincinnati, 2005.
Title from electronic thesis title page (viewed Apr. 18, 2006). Includes abstract. Keywords: Large Eddy Simulation; LES; Lean Pre-mixed; LPM; Gas Turbine Combustor; Combustion; Reacting Flows. Includes bibliographical references.

This thesis explores the effects of operating parameters on the location and shape of lifted flames in a Low Swirl Burner (LSB). In addition, it details the development and analysis of a CH PLIF imaging system for visualizing flames in lean combustion systems. The LSB is studied at atmospheric pressure using LDV and CH PLIF. CH* chemiluminescence is used for high pressure flame imaging. A four-level model of the fluorescing CH system is developed to predict the signal intensity in hydrocarbon flames. Results from imaging an atmospheric pressure laminar flame are used to validate the behavior of the signal intensity as predicted by the model. The results show that the fluorescence signal is greatly reduced at high pressure due to the decreased number of CH molecules and the increased collisional quenching rate. This restricts the use of this technique to increasingly narrow equivalence ratio ranges at high pressures. The limitation is somewhat alleviated by increasing the preheat temperature of the reactant mixture. The signal levels from high hydrogen-content syngas mixtures doped with methane are found to be high enough to make CH PLIF a feasible diagnostic to study such flames. Finally, the model predicts that signal levels are unlikely to be significantly affected by the presence of strain in the flow field, as long as the flames are not close to extinction. The results from the LSB flame investigation reveal that combustor provides reasonably robust flame stabilization at low and moderate values of combustor pressure and reference velocities. However, at very high velocities and pressures, the balance between the reactant velocity and the turbulent flame speed shifts in favor of the former resulting in the flame moving downstream. The extent of this movement is small, but indicates a tendency towards blow off at higher pressures and velocities that may be encountered in real world gas turbine applications. There is an increased tendency of relatively fuel-rich flames to behave like attached flames at high pressure. These results raise interesting questions about turbulent combustion at high pressure as well as provide usable data to gas turbine combustor designers by highlighting potential problems.

Les simulations aux grandes échelles sont devenues un outil de base pour la recherche de turbulence. La plupart des investigations LES traitent des écoulements à bas nombre de Reynolds et ont une discrétisation spatiale élevée, ce qui a comme conséquence des coûts de calcul élevés. Pour rendre la LES applicable aux cas industriels, les possibilités de fournir des LES avec de bas coûts de calculs sur des écoulements à nombre de Reynolds élevés doivent être étudiées. Le régime d'écoulement d'une turbine à gaz industrielle est dominé par plusieurs phénomènes d'écoulements. L'injection de carburant sous la forme d'un jet dans l'écoulement transversal et l'écoulement vrillé entrant dans une chambre de combustion sont deux configurations génériques représentatives des écoulements rencontrés dans des brûleurs de turbines à gaz. Afin de prouver la capacité de la LES de traiter ces phénomènes d'écoulement, deux investigations numériques ont été faites pour reproduire les résultats des études expérimentales. Le premier traite le JICF. Dans le second cas (écoulement vrillé), l'approximation axisymétrique est examinée en détail en la comparant à un calcul LES 3D complet et aux données expérimentales. Dans la dernière étape, après avoir examiné quelques détails du brûleur, toute les parties du brûleur qui ont été étudiées comme objets isolés sont réunies et un calcul d'un segment de 20° du brûleur est fait. La séparation de l'écoulement sur les ailettes et le mélange entre air et jet de carburant sont quantifiées à l'aide des résultats LES. En conclusion, on propose des modifications de brûleur et leurs effets sont discutés. L'objectif est d'éviter la formation des structures à grande échelle dans la chambre de combustion, qui sont suspectées d'être une source des instabilités de combustion.

The study of flow fields and heat transfer characteristics inside a gas turbine combustor provides one of the most serious challenges for gas turbine researchers because of the harsh environment at high temperatures. Design improvements of gas turbine combustors for higher efficiency, reduced pollutant emissions, safety and durability require better understanding of combustion in swirl flows and thermal energy transfer from the turbulent reacting flows to solid surfaces. Therefore, accurate measurement and prediction of the flows and heat loads are indispensable. This dissertation presents flow details and wall heat flux measurements for reacting flow conditions in a model gas turbine combustor. The objective is to experimentally investigate the effects of combustor operating conditions on the reacting swirl flows and heat transfer on the liner wall. The results shows the behavior of swirling flows inside a combustor generated by an industrial lean pre-mixed, axial swirl fuel nozzle and associated heat loads. Planar particle image velocimetry (PIV) data were analyzed to understand the characteristics of the flow field. Experiments were conducted with various air flow rates, equivalence ratios, pilot fuel split ratios, and inlet air temperatures. Methane and propane were used as fuel. Characterizing the impingement location on the liner, and the turbulent kinetic energy (TKE) distribution were a main part of the investigation. Proper orthogonal decomposition (POD) further analyzed the data to compare coherent structures in the reacting and non-reacting flows. Comparison between reacting and non-reacting flows yielded very striking differences. Self-similarity of the flow were observed at different operating conditions. Flow temperature measurements with a thermocouple scanning probe setup revealed the temperature distribution and flow structure. Features of premixed swirl flame were observed in the measurement. Non-uniformity of flow temperature near liner wall was observed ranging from 1000 K to 1400 K. The results provide insights on the driving mechanism of convection heat transfer. As a novel non-intrusive measurement technique for reacting flows, flame infrared radiation was measured with a thermographic camera. Features of the flame and swirl flow were observed from reconstructed map of measured IR radiation projection using Abel transformation. Flow structures in the infrared measurement agreed with observations of flame luminosity images and the temperature map. The effect of equivalence ratio on the IR radiation was observed. Liner wall temperature and heat transfer were measured with infrared thermographic camera. The combustor was operated under reacting condition to test realistic heat load inside the industrial combustors. Using quartz glass liner and KG2 filter glass, the IR camera could measure inner wall surface temperature through the glass at high temperature. Time resolved axial distributions of inner/outer wall temperature were obtained, and hot side heat flux distribution was also calculated from time accurate solution of finite difference method. The information about flows and wall heat transfer found in this work are beneficial for numerical simulations for optimized combustor cooling design. Measurement data of flow temperature, velocity field, infrared radiation, and heat transfer can be used as validation purpose or for direct inputs as boundary conditions. Time-independent location of peak location of liner wall temperature was found from time resolved wall temperature measurements and PIV flow measurements. This indicates the location where the cooling design should be able to compensate for the temperature increase in lean premixed swirl combustors. The characteristics on the swirl flows found in this study points out that the reacting changes the flow structure significantly, while the operating conditions has minor effect on the structure. The limitation of non-reacting testing must be well considered for experimental combustor studies. However, reacting testing can be performed cost-effectively for reduced number of conditions, utilizing self-similar characteristics of the flows found in this study.
Ph. D.

Low-emissions combustor design is crucially important to gas turbine engine manufacturers. Unfortunately, many designs are susceptible to unsteady oscillations that can result in structural fatigue and increased noise. Computational approaches that resolve flow unsteadiness, for example Large Eddy Simulation (LES), are being explored as one avenue to help understand such phenomena. However, in order to quantifY the accuracy of LES predictions, benchmark validation data in suitably chosen test cases are required. Comprehensive experimental data covering both time-averaged and timeresolved features are currently scarce. It was the aim of this thesis, therefore, to provide such data .in a configuration representing the near-field of a typical gas turbine fuel injector. It was decided to focus on the fuel injector since many unsteady events are believed to originate because of the transient interactions between the fuel injector flow and the main combustor flow. A radial fed two-stream fuel injector, based on a preexisting industrial gas-turbine Turbomeca design was used, since this geometry was known to be susceptible to unsteadiness. The fuel injector was investigated under isothermal conditions to place emphasis on the fluid mechanical behaviour of the fuel injector, including detailed capture of any unsteady phenomena present. Light Sheet Imaging (LSI) systems were used as the primary experimental technique to provide high quality spatially and temporally resolved instantaneous velocity and scalar field information in 2D planes (using ParticieImage Velocimetry (PIV) and Planar LaserInduced Fluorescence (PUF) techniques). Several methods were employed to extract information quantifYing the flow unsteadiness and improve visualisation of timedependent large-scale turbulent structures. Proper Orthogonal Decomposition (POD) analysis enabled clear identification of the dominant modes of energy containing structures. The results indicated that periodic high-energy containing vortex structures occurred in the swirl stream shear layer, emerging from the fuel injector. These formed a two-strong two-weak rotating vortex pattern which propagated down the main duct flow path. The formation of these vortices was found to be a function of the swirl number and originated due to an interaction between the forward moving swirl flow and the furthest upstream penetration point ofthe recirculation zone present in the main duct flow. Dependent on the magnitude of the swirl number (influencing the swirl stream cone angle) and the geometry of the fuel injector, the vortex formation point was sometimes found inside the fuel injector itself. If the vortices originated inside the fuel injector they appeared much more coherent in space and time and of higher energy. A second unsteady high energy containing phenomenon was also identified, namely a Precessing Vortex Core (PVC), which was damped out if the fuel injector contained a central jet. The dynamics of the PVC interacted with the dynamics of the swirl stream shear layer vortices to reduce there strength. Transient scalar measurements indicated that there was a clear connection between the unsteady vortex pattern and the rate of mixing, resulting in bursts of high heat release and is therefore identified as one source of combustor oscillations. Future fuel injector designs need to pay close attention to these unsteady features in selecting swirl number and internal geometry parameters.

Although pre-swirlers play a determinant role in the transport of air from stationary parts to rotating holes, knowledge about their actual performance is limited. Therefore, this paper aims to relate how the pre-swirler pressure drop affects the performance of different pre-swirlers in terms of discharge coefficient, adiabatic pre-swirl effectiveness, and swirl ratio. The results are extracted from numerical simulations carried out on three different designs, two guide vanes, and a nozzle. When available, the results are compared to experimental data. The guide vanes have shown similar responses to the pressure drop variations. Their discharge coefficients remain relatively insensitive with an average value of 97%. The swirl ratio range from 0.704 to 1.013 and 0.703 to 1.023 respectively for a pressure drop varying from 3 to 7 bars. The adiabatic pre-swirl effectiveness is of 96% and 94%, respectively, under steady state operation.The nozzle design has shown inferior performance as compared to the guide vane designs. Its discharge coefficient remains around 91% and the swirl ratio varies between 0.678 and 1.121 for a pressure drop ranging from 3 to 10 bars. Under steady state operation, the adiabatic pre-swirl effectiveness is 1.22. The influence of through-flows on the aforementioned parameters was also analyzed. It was observed that the through-flow deteriorates the performance of the pre-swirlers, whether in terms of dimensionless pre-swirl effectiveness, or swirl ratio. The discharge coefficient was however not affected.

Air velocity and fuel concentration data have been collected throughout the flow fields of two gas turbine mixers in an effort to better understand the mixing of fuel and air in gas turbine mixers. The two gas turbine mixers consisted of an annular flow profile and incorporated swirl vanes to produce a swirling flow to promote fuel/air mixing. The fuel was injected into the bulk flow from the pressure side of the swirl vanes. The first mixer had a swirl angle of 45°, while the second had a swirl angle of 55°. In order to examine the effect of the swirl angle on the mixing of fuel and air as the flow progressed through gas turbine mixers, axial and tangential air velocity data was taken using a laser Doppler velocimeter (LDV). Also, fuel concentration data was taken separately using a hydrocarbon concentration probe with methane diluted with air as the fuel. The data were taken at varying axial and varying angular locations in an effort to capture the spatial development of the fuel and velocity profiles. The spectra of the data were analyzed as well in an effort to understand the turbulence of the flow. It was found that the 55° swirler exhibited smaller variations in both velocity and fuel concentration values and that the fuel reached a uniform concentration at axial locations further upstream in the 55° degree mixer than in the 45° mixer. The RMS values of the velocity, which were influenced by the swirl vanes, were higher in the 55° mixer and likely contributed to the better mixing performance of the 55° mixer. The fuel concentration spectrum data showed that the spectra of the two mixers were similar, and that the fluctuations in fuel concentration due to flow emanating from the swirl vanes were seen throughout the length of the two mixers.
Ph.D.
Department of Mechanical, Materials and Aerospace Engineering
Engineering and Computer Science
Mechanical, Materials and Aerospace Engineering

While the gas turbine research community is continuously pursuing development of higher cyclic efficiency designs by increasing the combustor firing temperatures and thermally resistant turbine vane / blade materials, a simultaneous effort to reduce the emission levels of high temperature driven thermal NOX also needs to be addressed. Lean premixed combustion has been found as one of the solutions to these objectives. However, since less amount of air is available for backside cooling of liner walls, it becomes very important to characterize the convective heat transfer that occurs on the inside wall of the combustor liners. These studies were explored using laboratory scale experiments as well as numerical approaches for several inlet flow conditions under both non-reacting and reacting flows. These studies may be expected to provide valuable insights for the industrial design communities towards identifying thermal hot spot locations as well as in quantifying the heat transfer magnitude, thus aiding in effective designs of the liner walls. Lean premixed gas turbine combustor flows involve strongly coupled interactions between several aspects of physics such as the degree of swirl imparted by the inlet fuel nozzle, premixing of the fuel and incoming air, lean premixed combustion within the combustor domain, the interaction of swirling flow with combustion driven heat release resulting in flow dilation, the resulting pressure fluctuations leading to thermo-acoustic instabilities there by creating a feedback loop with incoming reactants resulting in flow instabilities leading to flame lift off, flame extinction etc. Hence understanding combustion driven swirling flow in combustors continues to be a topic of intense research. In the present study, numerical predictions of swirl driven combustor flows were analyzed for a specific swirl number of an industrial fuel nozzle (swirler) using a commercial computational fluid dynamics tool and compared against in-house experimental data. The latter data was obtained from a newly developed test rig at Applied Propulsion and Power Laboratory (APPL) at Virginia Tech. The simulations were performed and investigated for several flow Reynolds numbers under non-reacting condition using various two equation turbulence models as well as a scale resolving model. The work was also extended to reacting flow modeling (using a partially premixed model) for a specific Reynolds number. These efforts were carried out in order investigate the flow behavior and also characterize convective heat transfer along the combustor wall (liner). Additionally, several parametric studies were performed towards investigating the effect of combustor geometry on swirling flow and liner hear transfer; and also to investigate the effect of inlet swirl on the jet impingement location along the liner wall under both non-reacting as well as reacting conditions. The numerical results show detailed comparison against experiments for swirling flow profiles within the combustor under reacting conditions indicating a good reliability of steady state modeling approaches for reacting conditions; however, the limitations of steady state RANS turbulence models were observed for non-reacting swirling flow conditions, where the flow profiles deviate from experimental observations in the central recirculation region. Also, the numerical comparison of liner wall heat transfer characteristics against experiments showed a sensitivity to Reynolds numbers. These studies offer to provide preliminary insights of RANS predictions based on commercial CFD tools in predicting swirling, non-reacting and reacting flow and heat transfer. They can be extended to reacting flow heat transfer studies in future and also may be upgraded to unsteady LES predictions to complement future experimental observations conducted at the in-house test facility.
Ph. D.

In this study, the influence of inlet flow incidence on the aerodynamic and thermal performance of a film cooled linear nozzle vane cascade is fully assessed. Tests have been carried out on a solid and a cooled cascade. In the cooled cascade, coolant is ejected at the end wall through a slot located upstream of the leading edge plane. Moreover, a vane showerhead cooling system is also realized through 4 rows of cylindrical holes. The cascade was tested at a high inlet turbulence intensity level (Tu1 = 9%) and at a constant inlet Mach number of 0.12 and nominal cooling condition, varying the inlet flow angle. In addition to the reference incidence angle (0°), four other cases were investigated: +20°, +10°, -10° and -20°. The aero-thermal characterization of vane platform was obtained through 5-hole probe, endwall and vane showerhead adiabatic film cooling effectiveness measurements. Vane load distributions and surface flow visualizations supported the discussion of the results. On the vane, a significant movement in stagnation point happened when incidence angle varied, resulted in changing of the coolant distribution pattern between SS and PS of the cooled vane; which adversely affects the efficiency for both negative and positive inlet flow incidence angles. On the platform, however, a relevant negative impact of positive inlet flow incidence on the cooled cascade aerodynamic and endwall thermal performance was detected. A negligible influence was instead observed at negative incidence, even at the lowest tested value of -20°.

Coupling of spray with the coherent structures of a highly turbulent flow has been a long-standing problem especially in the context of liquid fuel delivery systems in gas turbine combustors. The atomizer in a gas turbine combustor usually has one or more (radial/axial entry) air swirlers with a fuel nozzle being mounted centrally along the longitudinal axis of swirler. It is well known that swirling flows are highly three dimensional in nature and often induce multiple aerodynamically unstable modes whose frequencies are several orders of magnitude. The basic understanding of flow dynamics in gas turbine swirl cup is critical to achieving clean and efficient combustion in modern-day gas turbine combustors. In this work, we analyze the evolution of the hydrodynamic topology and consequent spray-flow interactions in a coaxial swirl injector assembly. The key results of the present work are discussed in four parts. In the first part, the global evolution and temporal dynamics of various vortex breakdown modes are discussed. Experiments are carried out for three sets of co annular flow Reynolds number 𝑅𝑒𝑎=4896,10545,17546. Furthermore, for each 𝑅𝑒𝑎 condition, swirl number ‘𝑆𝐺’ is varied independently from 0≤𝑆𝐺≤3. Three distinct forms of vortex breakdown namely, pre-vortex breakdown (PVB), central toroidal recirculation zone (CTRZ; axisymmetric toroidal bubble type breakdown) and sudden conical breakdown are explored in greater details. Energy ranked, and frequency resolved / ranked robust structure identification methods – POD, DMD respectively is implemented over instantaneous time resolved PIV data sets to extract the dynamics of coherent structures associated with each vortex breakdown modes. The dominant structures obtained from POD analysis suggest the dominance of KH instability (axial + azimuthal; accounts ~ 80 % of total TKE) for both PVB and CTRZ while the remaining energy is contributed by shedding modes. On the other hand, shedding modes contribute to the majority of the TKE in conical breakdown. The frequency signatures quantified from POD temporal modes and DMD analysis reveals the occurrence of multiple dominant frequencies in the range of ~ 10 – 400 Hz with conical breakdown. This phenomenon may be a manifestation of high energy contribution by shedding eddies in the shear layer. Contrarily, with PVB and CTRZ, the dominant frequencies are observed in the range of ~ 20 – 40 Hz only. In addition, the current work explores the hysteresis (path dependence) phenomena of conical breakdown as functions of Reynolds and Rossby numbers. It has been observed that the conical mode is not reversible and highly dependent on the initial conditions. In the second part, we have reported how the liquid sheet behaves in such swirling flows. The air flow rate across the swirler is progressively varied to probe the two-phase flow interaction dynamics across weak, transition and strong momentum coupling regimes. The liquid sheet breakup and gas – liquid phase interaction dynamics suggests strong one way coupling at higher MR values. The POD analysis implemented over the shadow images clearly delineates the superimposing of gas phase instabilities with liquid sheet. The breakup length scale and liquid sheet oscillations are meticulously analyzed in time domain to reveal the breakup dynamics of liquid sheet. Furthermore, the large-scale coherent structures of swirl flow exhibit different sheet breakup phenomena in spatial domain. For instance, flapping breakup is induced by counter rotating vortices in the flow field induced by vortex breakdown phenomenon. The breakup regime map is also constructed based to illustrate the various forms of breakup mechanism as a function of MR values. Finally, the ligament formation mechanism and its diameter, size of first-generation droplets are measured with phase Doppler interferometry (PDI). The measured sizes scale reasonably with KH waves. In the third part, the fundamental mechanisms of vortex-droplet interactions leading to flow distortion, droplet dispersion and breakup in a complex swirling gas flow field are discussed. In particular, how the location of droplet injection determines the degree of inhomogeneous dispersion and breakup modes have been elucidated in detail. The droplets are injected as monodispersed streams at various spatial locations like the vortex breakdown bubble and shear layers (inner and outer) exhibited by the swirling flow. Time-resolved particle image velocimetry (3500 frames/s) and high-speed shadowgraphy measurements are employed to delineate the two-phase interaction dynamics. These measurements have been used to evaluate the fluctuations in instantaneous circulation strength 𝛤′caused by the flow field eddies and resultant angular dispersion in the droplet trajectories 𝜃′. The droplet-flow interactions show two-way coupling at low momentum ratios (MR) and strong one way coupling at high momentum ratios. The gas phase flow field is globally altered at low air flow rates (low MR) due to the impact of droplets with the vortex core. The flow perturbation is found to be minimal and mainly local at high air flow rates (high MR). Spectral coherence analysis is carried out to understand the correlation between eddy circulation strength 𝛤′and droplet dispersion 𝜃′. Droplet dispersion shows strong coherence with the flow at certain frequency bands. Subsequently, proper orthogonal decomposition (POD) is implemented to elucidate the governing instability mechanism and frequency signatures associated with turbulent coherent structures. POD results suggest the dominance of KH instability mode (axial and azimuthal shear). The frequency range pertaining to high coherence between dispersion and circulation shows good agreement with KH instability quantified from POD analysis. The droplets injected at inner (ISL) and outer shear layer (OSL) show different interaction dynamics. For instance, droplet dispersion at OSL exhibits secondary frequency (shedding mode) coupling in addition to KH mode, whereas, ISL injection couples only at a single narrow frequency band (i.e. KH mode). Finally, we have analyzed the spray- flow field dynamics in the realistic injector configuration (i.e. high shear injector). High shear injector usually consists of a series of air swirlers (primary and secondary) with diverging flare at the exit and centrally mounted fuel nozzle. It should be noted that to precisely probe the characteristic features, experiments have been also conducted with independent primary and secondary swrilers. A parameter named dynamic pressure ratio (𝜉) is used to quantify the monemtum transfer pathways between primary and secondary swirler flow field across various test cases. The test cases which exhbit 𝜉<1 are identified as primary swirler dominant flow and for 𝜉≥1 are delineated as secondary swirler dominant flow. In other words, for 𝜉<1 momentum exchange will be take palce from primary to secondary swirler and vice versa for 𝜉≥1 condtion. . The results revealed that flow pertaining to the secondary swirler exhibits sharp narrowband frequency in the range of 0 – 60 Hz, whereas, the primary swirler flow exhibits wideband frequencies with distinct peaks at 200, 800 Hz. The POD analysis extended over combined primary and secondary swirler flows shows the persistence of wide band oscillations for the test cases pertaining to 𝜉<1 (i.e. Regime I). This is due to the dominance of primary swirler flow in the Regime I. On the contrary, the frequenncy signatures shift to sharp narrow band (0- 70 Hz) for the secondary swirler dominant cases (i.e. 𝜉≥1; Regime II). In addition, we have also reported the sensitivity of the high shear swirl cup with respect to the geometrical parameters. The geometric parameters like flow split ratio (γ) between primary and secondary swirler, geometric swirl number, area ratio (Δ), flow orientation (i.e. co and counter rotation), exit flare angle (ϴ) etc are considered. The length scale (𝑊𝐷𝑓⁄), which embodies the radial extend of the recirculation zone is used as criteria to distinguish the various test cases. It is found that, the magnitude of (𝑊𝐷𝑓⁄) is governed by near field swirl number (SN10) and Reynolds number for the cases where SNgeo, γ, Δ have been varied. Here, SN10 represents the experimentally measured swirl number at ~ 10mm from the exit of swirl cup. On the other hand, for the cases with variations being θ and flow orientation, (𝑊𝐷𝑓⁄) founds to be only a function of near field swirl number f (SN10). Next, the spatial distribution of the spray perceived from patternation studies shows a linear relationship with the magnitude of 𝑊𝐷𝑓⁄. It is interesting to note that, though the spatial spread of the spray scales with 𝑊𝐷𝑓⁄, however, the spatial uniformity and measured droplet remains insensitive to the test variables.

The stringent pollution and emission norms due to the present climate change and global warming have pushed industries to meet these norms and cut down their emission levels. In the same pipeline, the aviation industries do not remain untouched. The emissions from aircraft engines burning fossil fuel, NOx, CO, unburnt hydrocarbon, etc., affect the atmosphere's upper layer temperature and air quality. These increasing number of air-transport demands and strict emission norms have forced the aviation industry to develop a next-generation aero-engine that can burn fossil fuel more efficiently and meet the emission norms. Liquid fuel is a major power source for most power-generating units, such as land-based and air-based gas turbine combustors, rocket engines, industrial burners etc. The high energy density per unit volume for liquid fuel makes it a better candidate than gaseous fuel for an air-breathing engine/combustor. The extraction of power from the liquid fuel involves various stages such as fuel injection, its atomization in smaller droplets, oxidizer and fuel droplet mixing and then ignition of this fuel-oxidizer mixture. The fuel injection process and technique are key to enhancing the gas turbine combustion performance and reducing the emission levels to meet the pollution norms in upcoming eras of aviation transport. However, optimization of the fuel injection system remains a key challenge, especially in liquid-powered gas turbine engines. Modern-day aircraft combustors utilize high shear fuel injectors that consist of multiple arrangements of swirlers along with a concentric fuel nozzle that generates coflowing swirling air to enhance atomization quality and get a homogeneous level of fuel-air reactant mixture prior to combustion. The key observations of the current work are discussed in four parts. In the first part, we have designed, developed and characterized the performance of a new class of high shear injector (HSI). The benchmark to evaluate the performance of HSI are Spray flow field, Droplet size distribution, and droplet dispersion across a wide range of air-to-liquid mass ratios (ALR; 4-14). In the first part of the study, the influence of the injector’s geometrical features over the time-averaged and dynamical spray characteristics are examined using high fidelity laser-diagnostics technique (high-speed PIV). These features are swirl numbers (SN_Prim), airflow split-ratio(γ), area ratio(Δ), flare angle(θ) and relative flow orientation of primary and secondary swirlers (co and counter-rotation). A Simplex pressure-swirl fuel nozzle is mounted at the center of the injector to deliver the liquid. The non-dimensional length scales (radial; W/Df and axial; L/Df) are used to distinguish the test cases. W/Df and L/Df are governed by nearby swirl number SN5 for both counter and co-rotation swirl configuration. Here, SN5 is the experimental swirl number calculated 5mm from the exit. The length scales, W/Df and L/Df, are more sensitive to the split-ratio of primary and secondary swirlers, flare angle, and it’s mixing length. The spray droplet size and spatial uniformity are insensitive to the test variables. Further, dominant dynamic spatial modes changes with a change in W/Df of the recirculation zone and the oscillation frequency of the most dominating modes shifts to a lower value with increasing W/Df. Simplex pressure-swirl fuel nozzle and dual orifice fuel nozzle are commonly used for fuel delivery at the center of the fuel injector/atomizer in the present-day gas turbine combustor. However, these fuel-nozzle have limitations such as hollow-cone liquid sheets collapsing at higher pressure, prone to plugging of narrow passages with contaminations over time, and high delivery pressure requirement. The second part of the work addresses these issues by replacing the same with a discrete liquid-jet fuel nozzle with a simple orifice design and low injection pressure. The performance of a high shear injector with a discrete liquid – jet mounted at the center is evaluated and compared to the performance achieved with a high shear injector using a simplex pressure-swirl fuel nozzle. The comparison shows the potential of a discrete liquid -jet fuel nozzle to replace the simplex pressure-swirl fuel nozzle with the proper design of high shear injector. The injectors have excellent atomization capability along with superior azimuthal distribution of spray. The Sauter-Mean Diameters (SMD) across all the test cases are in the range of 9-30µm,15-37µm, 15-50µm and 23-75µm at ALR 14.1, 9.44, 7.08, and 4.72 respectively. Further, the Std. Deviation of azimuthal spatial uniformity in an azimuthal plane is below 6 percent of the mean. A high shear injector consists of multiple swirler that produce swirl flow, and swirl flow is generally characterized by swirl number (SN). Above particular SN, the swirl flow creates a negative axial pressure gradient at the central axis which manifests a vortex breakdown bubble (VBB), also called the recirculation zone or central toroidal recirculation zone (CTRZ). The CTRZ help to stabilize the flame inside the gas turbine combustor. However, the onset of the vortex breakdown bubble is associated with a self-excited instability known as precessing vortex core oscillation. The PVC oscillation in a swirl flow-based combustor aids the thermoacoustic instability, resulting in severe hardware damage and poor emission characteristic of the engine. The third part of the work addresses the suppression of PVC oscillation to avoid the thermoacoustic -instability by modifying the fuel nozzle mounted at the center of the injector. A dummy cylindrical post is attached to the fuel nozzle that acts as the centerbody. The work shows the intermittent or absolute suppression of PVC oscillation with proper design of centerbody and variation of flow Reynold number. The diameters of the centerbody considered are Dc = 7;9 and 11mm. The results further demonstrate the suppression of loud whistle-like acoustic sound with the suppression of PVC oscillations in the flow. Considering the current global warming scenario and emission norms, the fourth part of the work addresses the soot formation study using Laser-induced Incandesce Imaging (LII) in a turbulent non-premixed ethylene swirl flame at a constant global equivalence ratio, ∅global=0.55 in a high shear injector. First, the impact of the split-ratio of primary and secondary swirler on soot formation is estimated at the given Reynolds number and pressure conditions for constant ∅global, which shows that a 60/40 swirl cup produces lower soot than a 40/60 swirl cup. Further, at constant pressure and ∅global=0.55, soot volume fraction reduces from ~4 ppm to ~0.8 ppm by increasing the Reynolds number from Re~5000 to Re~ 15000, and at Re ~20000, no soot is observed. At constant exit bulk velocity and constant ∅global=0.55, the soot volume fraction scales-up with pressure as p2.1 on log-log plots. Further, pressure increment increases the soot formation at a constant Re number. Overall, it is observed that pressure endorses soot formation. In addition, a large formation of soot particles is majorly observed in the annual jet’s region of the swirl flow field.

The research presented herein describes the coupling of acoustic and heat release fluctuations in a perfectly-premixed swirl-stabilized combustor by analysis of simultaneous high-repetition-rate laser diagnostics data. Nine cases are studied, varying the thermal power and the equivalence ratio. Proper orthogonal decomposition (POD) of the velocity data shows that cases with higher amplitude thermoacoustic oscillations have flow fields containing helical vortex cores (HVC); these cases are further analysed to determine the driving mechanisms of the oscillations. Flow and flame statistics are compiled as a function of both the phase in the thermoacoustic cycle and a phase representing the azimuthal position of the HVC relative to the measurement plane. These data are used to spatially map the thermoacoustic energy transfer field, as described by the Rayleigh integral. It is found that periodic deformations of the HVC cause large-scale flame motions, resulting in regions of positive and negative energy transfer.

Lean premixed combustion is preferred in gas turbines because of the reduced (NOx) emissions. However, the combustors operating in lean premixed regime could suffer from problems like flame flashback, flame blowoff and thermoacoustic instabilities. In the first part of this work, we have designed and developed a novel technique to mitigate the self-excited thermoacoustic instabilities inside a lab-scale combustor. The mitigation strategy is realized by rotating the otherwise static swirler, which is primarily meant for stabilizing the lean premixed flame. The proposed strategy is tested over a range of bulk flow velocities, mixture equivalence ratios, and swirler rotation rates for validating the robustness of this concept. A prominent reduction in the fundamental acoustic mode amplitude by about 25 dB is observed with this control technique for the cases that are studied. The physical mechanism responsible for the instability mitigation due to the rotating swirler is investigated by observing the distinct changes associated with the reacting flow field using Particle Image Velocimetry. The rotating swirler induces vortex breakdown and increased turbulence intensity to decimate strongly positive Rayleigh indices regions (and eventually the acoustic energy source) to render quiet instability mitigated swirling flames. In the second part, we have extended the studies to a more realistic combustor design, comprising three interacting swirl premixed flames, arranged in-line in an optically accessible hollow cuboid test section, which closely resembles a three-cup sector of an annular gas turbine combustor with a very large radius. Multiple configurations with various combinations of swirl levels between the adjacent nozzles and the associated flame and flow topologies have been studied. We observe that, for the cases where adjacent flames interact, there exists a dominant mode of oscillation whose amplitude is 30 dB more for 30-45-30 case as compared to the 45-45-45 configuration. PLIF measurements on the horizontal (x-z) plane confirmed the existence of intensely burning fuel-air pockets in the interaction zone. These pockets burn intermittently resulting in continuous evolution and annihilation of the flame surfaces in the flame-flame interaction regions, which eventually leads to fluctuations in the heat release rate. The combined effect of these heat release fluctuations gets coupled with natural frequencies of the combustor, which drives the self-excited thermoacoustic instability modes. In the third part, we present experimental results that provide insights into the flow-flame dynamics leading to flame blowoff inside a model gas turbine combustor housing three interacting swirling premixed flames. In particular, we focus on the mechanism of intermittent extinction-reignition phenomena and the final blowoff event i.e., complete flame extinction. The dynamics of the system close to blowoff conditions is monitored using simultaneous pressure and multi-kHz OH* Chemiluminescence measurements. These measurements reveal that there exists low frequency, large amplitude oscillations due to the large-scale extinction and reignition events occurring inside the combustor. sPIV and OH-PLIF techniques are used to probe the flow-flame interactions, especially the ones which are responsible for the flame extinction along the shear layers and the mechanisms of flame reignition.

Intimate knowledge of ammonia fueling gas turbines is of crucial importance for power generation sectors, owing to its carbon-free nature and high hydrogen capacity. Anticipated challenges include, among others, the difficulty to stabilize ammonia flames and on top of that the propensity of ammonia flames to produce large quantities of nitrogen monoxide emissions. In gas turbine devices, combustion in practice occurs in a turbulent swirl flow and at elevated pressure conditions. The stability of ammonia flames and the production of NO emissions are sensitive to such parameters. This body of work focuses on the development of a swirl combustor, ~30kW thermal power, for investigating behaviors of flame stability limits and NO emissions from the combustion of ammonia fuel with mixtures of hydrogen or methane at pressure conditions up to 5 bar. Data show that increasing the ammonia addition increases the equivalence ratio at the lean blowout limit but also reduces the flames’ propensity to flashback. If the volume fraction of ammonia in the fuel blend exceeds a critical value, increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blow-out occurs instead. This significantly widens the range of equivalence ratios yielding stable ammonia flames. Regardless of the fuel blend, increasing the pressure increases the propensity to flashback if the bulk velocity remains constant. Pure ammonia-air flames are stable under elevated pressures, and the stable range of equivalence ratio becomes wider as the pressure increases. The NO emissions are measured for large ranges of equivalence ratios, ammonia fractions, and pressures. Regardless of the ammonia fraction, data show that competitively low NO emissions can be found for slightly rich equivalence ratios. Good NO performance is also found for very lean ammonia-hydrogen-air mixtures, regardless of the pressure. NO mole fractions for lean ammonia mixtures can be reduced as pressure increases, demonstrating the strong potential of fueling gas turbines with ammonia-hydrogen mixtures.

The complex multiphase flow physics of spray-swirl interaction in both reacting and non-reacting environment is of fundamental and applied significance for a wide variety of applications ranging from gas turbine combustors to pharmaceutical drug nebulizers. Understanding the intricate dynamics between this two phase flow field is pivotal for enhancing mixing characteristics, reducing pollutant emissions and increasing the combustion efficiency in next generation combustors. The present work experimentally investigates the near and far-field break-up, dispersion and coalescence characteristics of a hollow cone spray in an unconfined, co¬annular isothermal swirling air jet environment. The experiments were conducted using an axial-flow hollow cone spray nozzle having a 0.5 mm orifice. Nozzle injection pressure (PN = 1 bar) corresponding to a Reynolds number at nozzle exit ReN = 7900 used as the test setting. At this setting, the operating Reynolds number of the co-annular swirling air stream number (Res) was varied in four distinct steps, i.e. Res = 1600, 3200, 4800 and 5600. Swirl was imparted to the co¬axial flow using a guided vane swirler with blade angle of Ф=45° (corresponding geometric swirl number SG = 0.8). Two types of laser diagnostic techniques were utilized: Particle / Droplet imaging analysis (PDIA) and shadowgraph to study the underlying physical mechanisms involved in the primary breakup, dispersion and coalescence dynamics of the spray. Measurements were made in the spray in both axial and radial directions and they indicate that Sauter Mean Diameter (SMD) in radial direction is highly reliant on the intensity of swirl imparted to the spray. The spray is subdivided into two zones as function of swirl in axial and radial direction: (1) near field of the nozzle (ligament regime) where variation in SMD arises predominantly due to primary breakup of liquid films (2) far-field of the nozzle where dispersion and collision induced coalescence of droplets is dominant. Each regime has been analyzed meticulously, by computing probability of primary break-up, probability of coalescence and spatio-temporal distribution of droplets which gives probabilistic estimate of aforementioned governing phenomena. In addition to this, spray global length scale parameters such as spray cone angle, break-up length, wavelength of liquid film has been characterized by varying Res while maintaining constant ReN.

The complex multiphase flow physics of spray-swirl interaction in both reacting and non-reacting environment is of fundamental and applied significance for a wide variety of applications ranging from gas turbine combustors to pharmaceutical drug nebulizers. Understanding the intricate dynamics between this two phase flow field is pivotal for enhancing mixing characteristics, reducing pollutant emissions and increasing the combustion efficiency in next generation combustors. The present work experimentally investigates the near and far-field break-up, dispersion and coalescence characteristics of a hollow cone spray in an unconfined, co¬annular isothermal swirling air jet environment. The experiments were conducted using an axial-flow hollow cone spray nozzle having a 0.5 mm orifice. Nozzle injection pressure (PN = 1 bar) corresponding to a Reynolds number at nozzle exit ReN = 7900 used as the test setting. At this setting, the operating Reynolds number of the co-annular swirling air stream number (Res) was varied in four distinct steps, i.e. Res = 1600, 3200, 4800 and 5600. Swirl was imparted to the co¬axial flow using a guided vane swirler with blade angle of Ф=45° (corresponding geometric swirl number SG = 0.8). Two types of laser diagnostic techniques were utilized: Particle / Droplet imaging analysis (PDIA) and shadowgraph to study the underlying physical mechanisms involved in the primary breakup, dispersion and coalescence dynamics of the spray. Measurements were made in the spray in both axial and radial directions and they indicate that Sauter Mean Diameter (SMD) in radial direction is highly reliant on the intensity of swirl imparted to the spray. The spray is subdivided into two zones as function of swirl in axial and radial direction: (1) near field of the nozzle (ligament regime) where variation in SMD arises predominantly due to primary breakup of liquid films (2) far-field of the nozzle where dispersion and collision induced coalescence of droplets is dominant. Each regime has been analyzed meticulously, by computing probability of primary break-up, probability of coalescence and spatio-temporal distribution of droplets which gives probabilistic estimate of aforementioned governing phenomena. In addition to this, spray global length scale parameters such as spray cone angle, break-up length, wavelength of liquid film has been characterized by varying Res while maintaining constant ReN.