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Summary The NexGen (Sonic) burner is the new burner developed by the Federal Aviation Administration, FAA, to replace old oil burners used for the required fire certification tests on power plant-related materials, as it provides the capability to control both air and fuel flow rates. During a fire test, the burner is supposed to simulate a certain fire condition, so the flame properties should be robust and repeatable. The NexGen burner can achieve this due to the precise fuel and air controls. However, the current calibration criterion (ISO2685:1998 and AC20-135) may not be good enough to ensure consistent flame properties. In the presented work, the sensitivity of the burner performance to air and fuel flow rate, as measured by the temperature and heat flux for calibration purposes, was studied. Additionally, the influence of the turbulator and the thermocouple size used for flame calibration was also studied. The impact of varying fuel/air ratio and thermocouple sizes was studied by conducting fire tests on aluminum samples, to show the inadequacies in the current calibration standards.

The altitude relight of a gas turbine combustor is an FAA and EASA regulation which dictates the successful re-ignition of an engine and its proper spool-up after an in-flight shutdown. Combustor pressure loss, ambient pressure, ambient temperature, and equivalence ratio were all studied on a full-scale, 3-cup, single-annular aviation combustor sector to create an ignition map. The flame development process was studied through the implementation of high-speed video. Testing was conducted by placing the sector horizontally upstream of an air jet ejector in a high altitude relight testing facility. Air was maintained at room temperature for varying pressure, and then the cryogenic heat exchanger was fed with liquid nitrogen to chill the air down to a limit of −50 deg F, corresponding with an altitude of 30,000 feet. Fuel was injected at constant equivalence ratios across multiple operating conditions, giving insight into the ignition map of the combustor sector. Results of testing indicated difficulty in achieving ignition at high altitudes for pressure drops greater than 2%, while low pressure drops show adequate performance. Introducing low temperatures to simulate the ambient conditions yielded a worse outcome, with all conditions having poor results except for 1%. High-speed video of the flame development process during the relight conditions across all altitudes yielded a substantial effect of the pressure drop on ignitability of the combustor. An increase in pressure drop was associated with a decrease in the likelihood of ignition success, especially at increasing altitudes. The introduction of the reduced temperature effect exacerbated this effect, further hurting ignition. High velocity regions in the combustor were detrimental to the ignition, and high area, low velocity regions aided greatly. The flame tended to settle into the corner recirculation zone and recirculate back into the center-toroidal recirculation zone (CTRZ), spreading downstream and likewise into adjacent swirl cups. These tests demonstrate the need for new combustor designs to consider adding large recirculation zones for combustor flame stability that will aid in relight requirements.

An experimental study was conducted to perform an analysis of the effect of the geometric modifications of the venturi on the non-reactive and reactive flow behavior using a counter-rotating radial-radial swirler. In the non-reactive flow tests, measurements were taken in a central vertical plane and horizontal (cross-sectional) plane at the exit of the swirler, using a High-Speed, Two Dimensional, Particle Image Velocimetry (2D PIV) system. The size of the swirler used in the non-reactive flow tests is a 4.76X scaled size of the swirler used in combustion. The 4.76X swirler models were tested in air flow seeded with olive oil at Re = 51,500, corresponding to the pressure drop across the 1X swirler models of 4% of atmospheric pressure at ambient conditions. Compared with the 1X swirler models, the 4.76X swirler models provide high spatial and temporal resolutions from the enhanced visibility of the flow characteristics and lower velocities at the same Re. Four swirler configurations of high swirl number (SN ≈ 1.0) were used, with no modification for the baseline configuration (configuration 1), and with the chevrons on the venturi for the straight chevrons configuration (configuration 2). The design of the inclined venturi was used for the converging venturi configuration (configuration 3), and chevrons were added on the converging venturi for the converging chevrons configuration (configuration 4). In the combustion tests, the 1X swirler models were tested using 478K preheated air at 4% pressure drop across the swirler, and different chamber lengths. Measurements were conducted using a regular camera to capture the flame image, and dynamic pressure transducers to obtain the acoustic pressure oscillations. Four configurations were studied and compared in the non-reactive and reactive flows with the objective of understanding the mechanisms responsible in reducing the extent of the combustion instabilities. Results of this study show that the converging venturi in configuration 3 appears to be the best design in eliminating the combustion instabilities in the fuel-lean region as compared to the other configurations. This indicates that the prevention of the frequencies coupling between the heat release rate and acoustic oscillations has been achieved by using the design of the converging venturi.

A generic novel injector was designed for multi-Lean Direct Injection (M-LDI) combustors. One of the drawbacks of the conventional pressure swirl and prefilming type airblast atomizers is the difficulty of obtaining a uniform symmetric spray under all operating conditions. Micro-channels are needed inside the injector for uniformly distributing the fuel. The problem of non-uniformity is magnified in smaller sized injectors. The non-uniform liquid sheet causes local fuel rich/lean zones leading to higher NOx emissions. To overcome these problems, a novel fuel injector was designed to improve the fuel delivery to the injector by using a porous stainless steel material with 30 μm porosity. The porous tube also acts as a prefilming surface. Liquid and gaseous fuels can be injected through the injector. In the present study, gaseous fuel was injected to investigate injector fuel-air mixing performance. The gaseous fuel was injected through a porous tube between two radial-radial swirling air streams to facilitate fuel-air mixing. The advantage of this injector is that it increases the contact surface area between the fuel-air at the fuel injection point. The increased contact area enhances fuel-air mixing. Fuel-air mixing and combustion studies were carried out for both gaseous and liquid fuel. Flame visualization, and emissions measurements were carried out inside the exit of the combustor. The measurements were carried out at atmospheric conditions under fuel lean conditions. Natural gas was used as a fuel in these experiments. Fuel-air mixing studies were carried out at different equivalence ratios with and without confinement. The mass fraction distributions were measured at different downstream locations from the injector exit. Flame characterization was carried out by chemiluminescence at different equivalence ratios and inlet air temperatures. Symmetry of the flame, flame length and heat release distribution were analyzed from the flame images. The effects of inlet air temperature and combustion flame temperature on emissions was studied. Emissions were corrected to 15% O2 concentration. NOx emissions increase with inlet air temperature and flame temperature. Effect of flame temperature on NOx concentration is more significant than effect of inlet air temperature. Fuel-air mixing profile was used to obtain mass fraction Probability Density Function (pdf). The pdfs were used for simulations in Chemkin Pro. The measured emissions concentrations at the exit of the injector was compared with simulations. In Chemkin model, a network model with several PSRs (perfectly stirred reactor) were utilized, followed by a mixer and a PFR (plug flow reactor). The comparison between the simulations and the experimental results was investigated.

The Combustion and Fire Research Laboratory (CFRL) at the University of Cincinnati (UC) is working on the development of advanced next generation injectors for DLN combustors. Several inputs were received from the project partners during the development phase. In the present paper, developmental work on two novel injectors with Porous Injection Technology (PIT) is presented. The technology has the potential to reduce NOx emissions to single digit PPM level with a stable combustion across wide range of load conditions. One of the key factors that are essential for lowering NOx levels is the efficient mixing of fuel-air in both spatial and temporal domains. The porous injection technology has the potential to reduce the spatial and temporal gradients to a minimum. In the present paper, two measurement techniques were used to evaluate the fuel-air mixing under atmospheric conditions. The CO2 mixing technique was used to quantify the spatial variations in the fuel mass fraction. Planar Laser Induced Fluorescence (PLIF) was used to obtain both spatial and temporal fuel mass fractions. The CO2 mixing measurements were used to validate the PLIF data for quantification. The RMS fluctuations in spatial and temporal domains were quantified from PLIF data. The combustion experiments were carried out at atmospheric pressure with a preheated temperature of air of 500–650 K and equivalence ratio of 0.5–0.8. The pressure drop across the injector was 4%. Natural gas with 90% methane and 9% ethane was used as fuel. The results show a stable flame for both injectors without combustion instabilities. Both injectors show low NOx levels. For conventional swirl stabilized design with PIT, the NOx levels were of the order of 1.5 ppm at the firing temperature of 1866 K whereas for the novel micromixer design, the NOx levels were of the order of 4 ppm @ ∼1866 K.

An experimental investigation was conducted to study the effect of chevrons on the dynamic behavior of the swirling flow generated by a counter-rotating radial-radial swirler. 3X models of a low swirl number swirler (SN ≈ 0.6) were used to achieve lower velocities for the same Reynolds number (Re) and enhanced visibility of the flow characteristics by enabling high spatial and temporal resolutions. Three swirler configurations were used, including the baseline with no chevrons. Configuration 2 features chevrons on the trailing edge of the primary swirler, and configuration 3 has chevrons on the trailing edge of both primary and secondary swirlers. The swirlers were tested in water flow at Reynolds number (Re) = 51,500 which corresponds to the typical operational pressure drop of 4% of atmospheric pressure for the corresponding 1X model of the swirler at ambient conditions. Water testing was used since it allows additional slowing down of the flow dynamic features so that they can be captured and analyzed. Measurements were conducted in a vertical plane passing through the swirler centerline, and two horizontal (cross-sectional) planes using a High-Speed, Two Dimensional, Particle Image Velocimetry (2D PIV) system to obtain the mean, turbulent and dynamic behavior of the flow. Results of this study introduce the concept of chevrons on swirlers as a promising approach to change the flow dynamic behavior and thus, affect combustion dynamics. The results show that the presence of chevrons break down the region of high modal energy into several smaller regions. However, configuration 2 has few regions of the highest modal energy among the configurations, whereas the modal energy values for configurations 3 has the lowest magnitudes. Thus, the secondary chevrons in configuration 3 play an important role to eliminate these high-energy local spots as well as meet the requirement to break down the large scale structures.

An experimental investigation was undertaken to develop a lean direct injection (LDI) combustor concept. The LDI concept consisted of a 9-point fuel injection system setup in a 3×3 array, where each point is made of a fuel nozzle fitted into a counter-rotating radial-radial swirler. Each swirler consisted of an inner, primary swirler, and an outer, secondary swirler, with opposite flow rotation. The experimental investigation consisted of atmospheric combustion and lean blow-out (LBO) tests, and high pressure combustion. All tests utilized a pressure drop of 4% across the swirlers. Parameters tested included variable swirler strengths and fuel nozzle injection depth. Fuel staging was employed on all configurations. Two swirlers, with swirl numbers (SN) of 0.64 and 1.07, were used in the swirler array, with the baseline configuration consisting entirely of the low-SN swirlers, and configurations 2 and 3 utilizing 1 and 3, respectively, of the high-SN swirlers. Configuration 2 has the high-SN swirler at the center, whereas Configuration 3 uses three high-SN swirlers in the center row. From the atmospheric tests, flame stability and lean blowout occurred at lower equivalence ratios for configuration 2 and 3, indicating that the presence of the high-swirl swirler improved flame anchoring. Two different nozzle insertion depths were tested, with a deep insertion depth providing partial prefilming on the venturi, and a shallow insertion depth with no pre-filming. Partial pre-filming greatly aided in fuel/air mixing and produced a shorter, blue flame, which was stable, and had lower LBO limit as compared to full direct injection (shallow insertion), which resulted in more unstable, longer, yellow flames. Emission measurements were conducted at high pressures using water cooled sampling probe, and a gas analyzer system to measure the emission indices of NOx and CO. The results showed that this LDI design produced emissions values comparable to those produced by current lean premixed prevaporized (LPP) combustor designs, well below current ICAO standards.

In the present investigation, a novel fuel injection concept is developed for Dry Low NOx combustors. A multi-injector block is used for fuel-air mixing and flame stabilization. The injectors in the multi-injector block are equally spaced in a rectangular grid of 3×3. Each injector of the multi-injector block has a porous concentric tube through which fuel is injected into the annular space around the porous tube. The porous tube is made up of stainless steel with 30 μm porosity. Air enters the annular space around the porous tube through eight tubes that surround the porous tube. The fuel and air mix in the annular space between the injector wall and the porous tube. A CO2 mixing technique is adopted to investigate the fuel-air mixing quality under non-reacting atmospheric conditions. CO2 is used to simulate the fuel. The CO2 concentrations are converted to fuel mass fractions for comparison. The experiments are carried out at pressure drop of 4% across the injector. The fuel mass fraction contours show an annular ring of low fuel concentration downstream of the individual injectors with a high concentration in the Central Toroidal Recirculation zone (CTRZ). Furthermore, PIV measurements are conducted to study the injector aerodynamics under the same operating conditions. The PIV measurements also show CTRZ downstream of the individual injector. The jet emanating from the injectors expands gradually downstream. An outer weak recirculation zone (ORZ) is also observed between the jets. Preliminary combustion experiments are carried out to observe the flame shape and heat release distribution. The flame shape follows the velocity contours. The combustion studies show that the flame length is longer. The long flame length is due to the high velocities at the exit of the injector. A probability density function (pdf) is generated for the fuel mixture fraction. The pdf is based on the mixture fraction and velocity data obtained at the exit of the injector block. Chemkin analysis is carried out to estimate the emissions based on the experimentally obtained pdfs.

Novel advances in gas turbine combustor technology, led by endeavors into fuel efficiency and demanding environmental regulations, have been fraught with performance and safety concerns. While the majority of low emissions gas turbine engine combustor technology has been necessary for power-generation applications, the push for ultra-low NOx combustion in aircraft jet engines has been ever present. Recent state-of-the-art combustor designs notably tackle historic emissions challenges by operating at fuel-lean conditions, which are characterized by an increase in the amount of air flow sent to the primary combustion zone. While beneficial in reducing NOx emissions, the fuel-lean mechanisms that characterize these combustor designs rely heavily upon high-energy and high-velocity air flows to sufficiently mix and atomize fuel droplets, ultimately leading to flame stability concerns during low-power operation. When operating at high-altitude conditions, these issues are further exacerbated by the presence of low ambient air pressures and temperatures, which can lead to engine flame-out situations and hamper engine relight attempts. To aid academic and commercial research ventures into improving the high-altitude lean blow-out (LBO) and relight performance of modern aero turbine combustor technologies, the High-Altitude Relight Test Facility (HARTF) was designed and constructed at the University of Cincinnati Combustion & Fire Research Laboratory (CFRL). This paper presents an overview of its design and an experimental evaluation of its abilities to facilitate optically-accessible combustion and spray testing for aero engine combustor hardware at simulated high-altitude conditions. Extensive testing of its vacuum and cryogenic air-chilling capabilities was performed with regard to end-user control — the creation and the maintenance of a realistic high-altitude simulation — providing a performance limit reference when utilizing the modularity of the facility to implement different aero turbine combustor hardware. Ignition testing was conducted at challenging high-altitude windmilling conditions with a linearly-arranged five fuel-air swirler array to replicate the implementation of a multi-cup gas turbine combustor sector and to evaluate suitable diagnostic tools for the facility. High-speed imaging, for example, was executed during the ignition process to observe flame kernel generation and propagation throughout the primary, or near-field, combustion zones. In the evaluation performed, the HARTF was found to successfully simulate the atmospheric environments of altitudes ranging from sea level to beyond 10,700 m for the employed combustor sector. Diagnostic methods found compatible with the facility include high-speed flame imaging, combustion emission analysis, laser light sheet spray visualization, phase Doppler particle analysis (PDPA), and high-speed particle image velocimetry (HSPIV). Herein discussed are correlations drawn — linking altitude simulation capability to the size of the implemented combustor hardware — and challenges found — vacuum sealing, low pressure fuel injection, fuel vapor autoignition, and frost formation.

A novel airblast injector is designed for gas turbine combustors. Unlike standard pressure swirl and prefilming/non-prefilming air blast atomizers, the novel injector is designed to improve the fuel injection delivery to the injector and improve atomization of the fuel by using a porous stainless steel tube. There are three swirling air streams in the injector. The liquid fuel is injected through the porous tube, with 7 micron porosity, between the swirling air streams, viz. an inner swirling air through the tube and the other two swirling air streams merging downstream of the tube. The swirl vane angles and the air split ratio are selected to increase the amount of air through the injector and facilitate the atomization process. The liquid fuel is injected through the outer surface of the porous tube, due to the permeability of the tube, produces a thin liquid sheet on the inner surface of the tube. The atomization occurs by surface stripping of the liquid sheet. The advantage of such an injector is that it produces a liquid sheet with uniform thickness around the circumference of the tube under all liquid loading. The porous tube also increases the surface area of contact between the fuel and air and produces a fine spray at engine idle conditions. An experimental approach is adopted in the present study to characterize the spray and aerodynamics of the injector for Jet-A and Gas-To-Liquid (GTL) fuels at atmospheric conditions. The effect of flare height on the Sauter Mean Diameter (SMD) is also studied. Spray characterization, droplet size and volume flux are investigated with PDI measurements. The effect of pressure drop and fuel properties on SMD distribution is analyzed. Velocity profiles at downstream of the injector are obtained from LDV measurements, and the velocity profile at the exit of the injector is also analyzed. A central toroidal recirculation zone (CTRZ) is observed at the exit of the injector. The effect of different configurations of the injector on spray characteristics is studied. A correlation for SMD is obtained.

to that of the confinement (85%). Instantaneous particle image velocimetry results reveal that the primary jets control the structure of the primary and the secondary combustor regions. The wake regions and central recirculation zone all maintained their absolute size at different pressure drops.

In the present study, two measurement techniques are adopted to evaluate the fuel–air mixing under atmospheric conditions using an industrial fuel–air premixer. These techniques are CO2 mixing and planar laser induced fluorescence (PLIF) in water. In these techniques, CO2 and fluorescent dye are injected as fuel simulants. CO2 measurements are used to validate PLIF in water. In the CO2 technique, CO2 concentrations are converted to fuel mass fractions, whereas in the PLIF technique, a modified post processing method is used to convert the LIF signal into fuel mass fraction. The experiments are conducted at the same Reynolds number and momentum flux ratio for two injection strategies. To study the effect of the flow aerodynamics on the mixing results, high-speed particle image velocimetry (PIV) measurements are conducted in water at the same Reynolds number. A comparison of fuel concentrations measured with the CO2 and PLIF techniques shows good quantitative agreement at all momentum flux ratios. However, deviations between the two techniques are observed at locations of high fuel concentration gradients. The unsteady mixing is evaluated using the PLIF technique with high temporal resolution. Analysis of PIV and PLIF data shows that unsteady mixing is lower at regions of high fluctuations in velocity. Moreover, it is found that there is high unsteady mixing at locations of high concentration gradient.

In the present study, two measurement techniques are adopted to evaluate the fuel-air mixing under atmospheric conditions using an industrial fuel-air pre-mixer. These techniques are CO2 mixing and Planar Laser Induced Fluorescence (PLIF) in water. In these techniques, CO2 and fluorescent dye are injected as fuel simulants. CO2 measurements are used to validate PLIF in water. In the CO2 technique, CO2 concentrations are converted to fuel mass fractions whereas, in the PLIF technique, a modified post processing method is used to convert the LIF signal into fuel mass fraction. The experiments are conducted at the same Reynolds number and momentum flux ratio for two injection strategies. To study the effect of the flow aerodynamics on the mixing results, high speed PIV measurements are conducted in water at the same Reynolds number. A comparison of fuel concentrations measured with the CO2 and PLIF techniques shows good quantitative agreement at all momentum flux ratios. However, deviations between the two techniques are observed at high fuel concentration gradients. The unsteady mixing is evaluated using PLIF technique with high temporal resolution. Analysis of PIV and PLIF data shows that unsteady mixing is lower at regions of high fluctuations in velocity. Moreover, it is found that there is high unsteady mixing at locations where there is high concentration gradient.

An experimental investigation has been conducted to study the mean and unsteady behavior of the non-reacting swirling flow using a water test rig. Water was used as the flow medium as for a given Reynolds number (Re), the flow dynamics are slowed down by about 18 times compared to atmospheric air making it easier to investigate the flow dynamics. The flow was examined using a 3X model of a counter rotating radial-radial swirler. 2D high speed Particle Image Velocimetry (PIV) measurements were employed to study the instantaneous and the mean velocity fields. Tests were conducted at Re values corresponding to an air pressure drop of 4%, 2.8%, 1.8% and 1% of atmospheric pressure for the corresponding 1X model of the swirler under atmospheric test conditions. The use of water to test the unsteady behavior of the swirling flow was validated by conducting tests on the same 3X model in the same test rig using air at the same Re values. The mean and turbulent behavior of the swirling flow in water and air showed excellent agreement over the range of Re tested. For this swirler, the normalized mean and RMS velocities did not change significantly with Re for the range of Re tested. Strong flow instability was observed at the exit of the swirler. This instability was created by a precessing vortex core (PVC). For air and water tests, the dominant frequency of this instability increased linearly with the increase in Re. For all Re investigated, the dominant frequency of water flow was 18 times less than that of air at the same Re. The Strouhal number was found to be nearly identical for air and water testing for all Re values. Maximum Turbulent Kinetic Energy (TKE) was found to exist on the boundaries of strong shear layers. The TKE decayed quickly downstream due to the quick decay of the PVC. The phase angle difference between the high TKE regions was 3.14 radians indicating a circumferential mode of instability. The results obtained demonstrate that water testing is an accepted method for studying the unsteady flows.

An experimental study has been carried out to investigate the isothermal aerodynamic behavior and to discern the effects on the flow-field resulting from interactions between low and high-swirl counter-rotating radial-radial air swirlers in three Lean Direct Injection configurations utilizing a 3 × 3 array of radial-radial swirlers. Configurations consisted of varying combinations of two swirlers featuring high and low swirl intensity. Two-dimensional velocity data is presented from the measurement of 37 planes spanning the width of the LDI array. Particle Image Velocimetry (PIV) was used to take velocity field measurements and to study the inter-swirler interactions. Three test cases were studied which utilized a combination of a low and high Swirl Number swirlers: the baseline case utilized 9 low swirl (SN about 0.6) swirlers, the second case used one high swirl (SN about 1.0) swirler in the center of the array, and the third case used 3 high swirl swirlers in a row within the array. The flow field developed by the three experimental cases differed significantly and inter-swirler interaction proved significant and highly complex. The velocity fields developed from swirlers in an array varied from that of the individual swirler, and as such, it should not be expected that the array have the same characteristics of the individual swirler. Placing a high-swirl swirler in a low-swirl array increased swirler interaction and led to substantial favorable changes in velocity fields and the recirculation zones developed downstream of each swirler in comparison to the baseline configuration including the development of a large CTRZ with weakened intensity for increased flame anchoring potential.Copyright © 2015 by ASME

A series of experiments have been conducted to study the effect of the flare expansion angle on the mean and unsteady behavior of the non-reacting swirling flow using a water test rig. The flow was examined in water using a 3X model of a counter rotating radial-radial swirler. Three flares having expansion angles of 30.9°, 35.9° and 40.9° with respect to the swirler centerline were tested. 2D high speed Particle Image Velocimetry (PIV) measurements were employed to study the instantaneous and mean velocity fields. Tests were conducted at a Reynolds number equivalent to an air pressure drop of 4% for the corresponding 1X model of the swirler under atmospheric conditions. The flare expansion angle was found to have a clear impact on the mean, turbulent and dynamic behavior of the swirling flow. With the increase in the flare expansion angle, the width of the Center Toroidal Recirculation Zone (CTRZ) increased. The length and width of the Corner Recirculation Zone decreased with increasing the flare angle. For the 30.9° flare, the high turbulence regions extended further axially compared to the other two flares. Strong flow instability was observed on the boundaries of the reverse flow zones. A temporal FFT approach was used to obtain the dominant frequencies of flow instability based on the instantaneous velocity data. The dominant frequency of this instability was slightly lower for the 30.9° flare angle. High turbulent kinetic energy (TKE) was located in the vicinity of the unstable shear layers originating from the CTRZ and CRZ. The TKE reached its maximum near the center for the 30.9° flare and near the walls for the 35.9° and 40.9° flares. The phase angle difference between the high TKE regions was 3.14 radians, indicating a circumferential mode of instability. The obtained results give a clear explanation on the mechanisms driving the flow instability and how these mechanisms change with the change in the flare angle. These results also serve as a tool to validate CFD models.Copyright © 2015 by ASME

The presented work focuses on the experimental spray structure and the combustion of a linearly-arranged 5-swirler array. The aerodynamics and spray characteristics of a non-reacting single swirler are reported first as a baseline, followed by those of a 5-swirler array to investigate the effect of swirling flow interactions on aerodynamics and combustion. For the baseline single swirler, the smaller droplets follow the air flow more closely and further dispersed away at the exit of swirler. Thus, the mean diameter of droplet increases with the flow developing further downstream. However, in the central portion of a 5-swirler array, the droplet size remains similar. It is attributed to that swirling flow interactions might provide better air/fuel mixing and the additional shear stress to break up droplet continuously and is evident by the higher turbulent intensity in the aerodynamic measurement. Due to the influence of gas phase, the distribution of liquid phase in center toroidal recirculation zone (CTRZ) is non-uniform in a 5-swirler array. The center swirler of a 5-swirler array features a larger CTRZ which is accompanied by two smaller CTRZs from its neighbors. The flame anchored by the center swirler of a 5-swirler array is richer than the other two neighboring flames when the inter-swirler spacing is 2D, where D is the diameter of swirler exit diameter. However, when the inter-swirler spacing is increased to 2.5D, all swirlers feature a similar flame, which is different from what is expected from non-reacting flow studies reported previously. The unexpected result should be attributed to the difference in swirling strength between non-reacting and reacting flows. Moreover, the high speed imaging is employed to investigate the flame spreading during ignition process for a 5-swirler array. The high-speed movies show that the directional mechanism of flame spreading along lateral direction remains basically the same and is independent of the investigated test parameters including: two inter-swirler spacings, five fuel flow rates, five air pressure drops across swirlers, and five upstream air temperatures. An empirical correlation incorporating normalized inter-swirler spacing, air/fuel ratio, Reynolds number, and normalized air temperature is proposed and validated through a normalization procedure within around ± 10% error. The increase of Reynolds number and normalized air temperature has favorable impact on the flame spreading, which is stated by the empirical correlation.

A numerical study of turbulent flow through 3×3 multi swirler arrangement has been performed using the realizable k-e turbulence model on a grid with about 19 million points. All co and alternate co/counter swirler configurations comprised of radial-radial swirler with counter rotating vanes are analyzed. The offset distances of swirler exit from the base wall of confinement of 0.02D and 0.31D are considered where D is the diameter of swirler exit. For both arrangements, a strong jet is issued as the flow exits individual swirl cup. Recirculation is observed at the walls and between each swirl cup along with the formation of central toroidal recirculation zone (CTRZ) at each individual swirler. It is observed that all co swirling arrangement has a stronger more compact individual CTRZ. On the other hand alternate co and counter arrangement produces more swirler-to-swirler interactions. When the offset between swirler exit and base wall of confinement is increased to 0.31D, longer but more compact CTRZ are formed at each swirler cup. The velocity gradient for 0.31D offset case is also higher than that of 0.02D. These differences in the flow field indicate better combustion performance, fuel breakup and flame anchoring for the higher offset case.Copyright © 2015 by ASME

A computational analysis of flow in simplex fuel atomizers using the arbitrary-Lagrangian‐Eulerian method is presented. It is well established that the geometry of an atomizer plays an important role in governing its performance. We have investigated the effect on atomizer performance of four geometric parameters, namely, inlet slot angle, spin chamber convergence angle, trumpet angle, and trumpet length. For a constant mass flow rate through the atomizer, the atomizer performance is monitored in terms of dimensionless film thickness, spray cone half-angle, and discharge coefficient. Results indicate that increase in inlet slot angle results in lower film thickness and discharge coefficient and higher spray cone angle. The spin chamber converge angle has an opposite effect on performance parameters, with film thickness and discharge coefficient increasing and the spray cone angle decreasing with increasing convergence angle. For a fixed trumpet length, the trumpet angle has very little influence on discharge coefficient. However, the film thickness decreases, and spray cone angle increases with increasing trumpet angle. For a fixed trumpet angle, the discharge coefficient is insensitive to trumpet length. Both the spray cone angle and the film thickness are found to decrease with trumpet length. Analytical solutions are developed to determine the atomizer performance considering inviscid flow through the atomizer. The qualitative trends in the variation of film thickness at the atomizer exit, spray cone angle, and discharge coefficient predicted by inviscid flow analysis are seen to agree well with computational results.

Multiple swirlers arranged in an annular fashion are used in modern day gas turbine engines. A section of this annulus can be considered as a straight line or what is referred to in the paper as a linear arrangement of swirlers. Three such linear arrangements are computationally analyzed and results are presented through this study. Study of linear arrangements is crucial and novel to the swirler aerodynamics research as it lays a foundation in understanding the flow physics when swirlers are arranged at a fixed distance next to each other. Swirling flows are complicated and when slight modifications are introduced in physical arrangements the flow is impacted drastically. In the present study observations have been presented on effect of changing the offset of exit plane of swirler from the base wall of confinement when there is a single swirler or a linear arrangement of swirlers. Computational simulations of flow through single and multi-swirler array have been carried out to understand the effect of the distance of exit plane of swirler from the base wall of confinement on the swirler aerodynamics. The swirlers used in this study are radial-radial swirlers with counter rotating vanes. The computational domain extended from the inlet manifold to 12 D downstream from the swirler where D is the diameter of swirler exit. Realizable k-ε turbulence model is used and the computational grid is about 4 million points for a single swirler arrangement, about 12 million points for a three swirler array and up to 22 million for the five swirler arrangement. The computational model is validated by comparing the results with velocity measurements carried out at three different planes downstream of the swirler exit using LDV technique. First, single swirler with the exit plane of swirler with an offset of 0.04 D and 0.02D with the base wall of confinement and that with no offset (swirler exit in-line with base wall of confinement) are analyzed. It is observed that flow development in region close to the swirler exit is highly sensitive to the offset condition. In case of 0.04D and 0.02D offset a strong jet is formed as soon as the air exits the swirler. The flow tends to progress vertically forming recirculation zones in the vicinity of corners of the horizontal and vertical walls. When there is no offset, the flow exiting the swirler tends to align with the base wall and then progresses vertically. Thus for no offset case a jet formation is not observed. Next, multi-swirler arrangements with 0.04D, 0.02D offset as well as no offset configurations are simulated. All the swirlers tend to show similar pattern as single swirler arrangements with a slight difference in intensity of the flow field. For swirlers with offset of 0.04D and 0.02D there is formation of a strong jet exiting the swirler and recirculation zones are formed in corners of the base and vertical walls of the confinement as was observed for the single swirler arrangement. Recirculation zones are also formed in areas between each swirler assembly in the multi swirler arrangement. For the no offset condition it is again observed that flow aligns with the horizontal base wall for each of the swirler assembly. The axial velocity of the flow in this arrangement tends to be lower than the offset case in regions between each swirler. An interesting phenomenon of multi swirler arrangement is an asymmetrical flow pattern that is observed at each swirler. While each swirler geometry is identical, the flow pattern as well as the strength of recirculation zone developed from each individual swirler differs significantly. Results show that alternate swirlers tend to exhibit similar flow characteristics.

In this work, a series of tests have been conducted to study the aerodynamics of two typical gas turbine combustion swirlers, using Laser Doppler Velocimetry (LDV). The first swirler used is a coaxial counter-rotating, radial-radial swirler, designated as the R-R swirler. The primary swirler accounts for 40% of the total effective area of the R-R swirler. The second swirler employed consists of an inner dual-axial swirler assembly with only around 20% of total swirler effective area and an outer coaxial radial swirler, and is designated as the A-R swirler. The aim of this work is to compare the aerodynamics performance of these two different types of swirlers under similar boundary conditions, such as confinement effect, downstream exhaust nozzle, and chamber length. The R-R swirler has a very high radial dispersing rate, and for the unconfined case, the swirling jet remained attached to the dome plate resulting in a very weak, unbound CTRZ. The swirling jet for the A-R swirler exited at a relatively low expansion angle. For confined cases, the swirling jet attached to the walls of the confinement at a certain distance downstream of the exit plane for both swirlers. When the exhaust nozzle was applied as a downstream boundary condition, a spinning vortex core with high turbulent intensity was detected along the swirler axis for the A-R swirler, whereas the only difference for the R-R swirler was a slight reduction in the expansion angle of the swirling jet. The proximity of the exhaust nozzle to the swirlers was changed by changing the chamber length. For the R-R swirler, the effect of chamber length was only detected in the near-field region. For the A-R swirler, a reduction in chamber length caused the spinning vortex core to become more intense. Additionally, an annular CTRZ was observed for the shorter chamber lengths.

In this experimental work, a series of tests have been conducted to further study the aerodynamics of linearly-arranged 5-swirler arrays, using Laser Doppler Velocimetry (LDV). Two major parameters have been investigated for the 5-swirler arrays in this work, including the inter-swirler spacing, and the end wall distance. An additional effect of dome recession was studied for a single swirler in order to provide some insight into the results for the 5-swirler arrays. The 5-swirler arrays with an inter-swirler spacing of 1.75D, 2D, 2.5D, and 2.75D were studied, where D is the diameter of swirler exit. For the inter-swirler spacing of 1.75D or 2D, the center swirler is shown to have a weak, large center toroidal recirculation zone (CTRZ). The swirlers next to the center swirlers have strong, compact CTRZs whereas the outer swirlers have weak, large CTRZs. Thus, starting from the center swirler, the CTRZs exhibit a large – small – large pattern in either direction. For the swirler spacing of 2.5D or 2.75D, the trend is reversed, with a strong CTRZ for the center swirler and a small – large – small CTRZ pattern. The end wall distances of 0.75D, 1D, 1.25D, and 2D cases were studied. The end wall distances are seen to affect the strength of each CTRZ and the corner swirling flow pattern, though the bulk flow structure did not change significantly. The dome recession shows a clear impact on the expansion of swirling flow generated by a single swirler. As the dome recession distance increases, the expansion angle of the swirling jet reduces significantly for the unconfined cases. A phenomenological description is discussed to understand the reason for the periodically alternating CTRZ pattern observed in the experimental results of the 5-swirler arrays.

A three-dimensional viscous stability analysis has been carried out to model airblast atomization. The model considers an annular liquid sheet downstream of an airblast atomizer and incorporates essential features, such as three-dimensional disturbances, liquid viscosity, innerand outerair swirl, and e nite e lm thickness. Effects of axial velocity, air swirl, and liquid viscosity on the growth rates of various disturbance modes have been examined in detail.Ithasbeenfoundthatincreasingtherelativeaxialvelocitybetweentheliquid andthegasphasessignie cantly improves the fuel atomization. The inner and outer air moving together enhances the instability of the liquid sheet more signie cantly than only the inner or outer air. When air swirl is absent, theaxisymmetricmode dominates the breakup process of the liquid sheet. Liquid viscosity is found to have a twofold effect: reduce the growth rates of unstable waves and shift the dispersion diagram toward long waves. Air swirl not only promotes the instability of theliquidsheet,butalso switchesthedominantmodefrom theaxisymmetricmodetoahelicalmode.Acombination of the inner and outer air swirl improves airblast atomization more signie cantly than a single air swirl.

A temporal linear stability analysis has been carried out to predict the instability of a viscous liquid jet surrounded by a swirling air stream with three-dimensional disturbances. The effects of flow conditions and fluid properties on the instability of the liquid jet are investigated via a parametric study by varying axial Weber number axial velocity ratio of the gas to liquid phase, swirl Weber numbers, density ratio and the Ohnesorge number. It is observed that the relative axial velocity between the liquid and gas phases promotes the interfacial instability. As the axial Weber number increases, the growth rates of unstable waves, the most unstable wavenumber and the unstable range of wavenumbers increase. Meanwhile, the increasing importance of helical modes compared to the axisymmetric mode switches the breakup regime from the Rayleigh regime to the first wind-induced regime and on to the second wind-induced regime. The predicted range of wavenumbers in which the first helical mode has higher growth rates than the axisymmetric mode agrees very well with experimental data. Results show that the destabilizing effects of the density ratio and the axial Weber number are nearly the same. Liquid viscosity inhibits the disintegration process of the liquid jet by reducing the growth rate of disturbances and by shifting the most unstable wavenumber to a lower value. Moreover, it damps higher helical modes more significantly than the axisymmetric mode. Air swirl has a stabilizing effect on the liquid jet. As air swirl strength increases, the growth rates of helical modes are reduced more significantly than that of the axisymmetric mode. The air swirl profile is found to have a significant effect on the instability of the liquid jet. The global, as well as local, effects of the swirl profile are examined in detail.

A numerical study is presented of the effects of changes in simplex nozzle geometry on its performance. A computational model based on the arbitrary-Lagrangian-Eulerian method with an adaptive grid-generation scheme is used. Three nondimensional geometric parameters are studied: the length-to-diameter ratio of the swirl chamber L s /D s and orifice l o /d o and the swirl-chamber-diameter-to-exit-orifice-diameter ratio D s /d o , The variations in the atomizer performance, caused by the changes in the geometric parameters, are presented in terms of the film thickness at the exit of the orifice, the spray cone angle, and the discharge coefficient. Results indicate that these geometric parameters have a significant effect on the internal flow and performance of simplex nozzles. With a constant mass flow through the nozzle over the range of parameters considered, an increase in L s /D s produces an increase in the film thickness at the orifice exit, a decrease in the spray cone half-angle, and a slight decrease followed by an increase in the discharge coefficient. Conversely, increasing l o /d o decreases film thickness, spray cone angle, and discharge coefficient. An increase in D s /d o results in a decrease in film thickness and discharge coefficient and a decrease in spray cone angle.

A theoretical model to predict the instability of an annular liquid sheet subjected to coaxial swirling airstreams is developed. The model incorporates essential features of a liquid sheet downstream of a prefilming airblast atomizer such as three-dimensional disturbances, inner and outer air swirl, finite film thickness, and finite surface curvature. Effects of flow conditions, fluid properties, and film geometry on the instability of the liquid sheet are investigated. It is observed that the relative axial velocity between the liquid and the gas phases enhances the interfacial aerodynamic instability by increasing the growth rate and the most unstable wave number. At low velocities, a combination of inner and outer airstreams is more effective in disintegrating the liquid sheet than only the inner or only the outer airstream. Also, the inner air is more effective than the outer air in promoting disintegration. Swirl not only increases the growth rate and the range of unstable wave numbers but also shifts the dominant mode from the axisymmetric mode to a helical mode. With the presence of air swirl, the most unstable wave number and the maximum growth rate are higher than their no-swirl counterparts

This paper describes the development and results of a mail survey to determine the views of industrial, government, and academic aerospace professionals on the required content of future aerospace engineering curricula and on the skills that will be needed by aerospace graduates in the early twenty-first century. The survey is one step in an ongoing process by the Department of Aerospace Engineering and Engineering Mechanics at the University of Cincinnati (UC) to restructure its aerospace engineering curriculum. This paper discusses the development of the survey, its results, and the preliminary conclusions that have been drawn from the results. The survey included questions on such issues as introducing design experience to the students throughout the curriculum and presenting some topics using a more interdisciplinary approach than has traditionally been followed. While the survey was constructed primarily to gather information specific to the UC curriculum redesign effort, its results are relevant to other aerospace engineering programs considering or pursuing curricular redesign.

A computational model for flow in a simplex nozzle has been established to predict the characteristics of the liquid sheet emanating from it. An important aspect of the numerical method is the accurate tracking of the liquid/gas interface. Because the interface geometry is not known a priori, it must be determined as part of the solution. The arbitrary-Lagrangian-Eulerian numerical method with finite volume formulation was employed for this purpose. To validate the computational and numerical modeling, experiments have been conducted on a large-scale nozzle using flow visualization techniques. The gas/liquid interface locations inside the nozzle, as well as just downstream of the orifice, have been determined for a range of mass flow rates and injector geometries. Using these measurements, the liquid film thickness and angle of the liquid sheet has been determined. Comparisons of the computational predictions with the experimental measurements show excellent agreement. Results indicate that the current theoretical correlations based on inviscid flow assumptions underestimate the film thickness and overestimate the spray angle significantly in large scale nozzles. It was found that an increase in the atomizer constant K [xAp/(D m d o )] results in decreasing the spray angle and increasing the liquid film thickness, where Ap is the total swirl slot area, D m is the effective spin chamber diameter, and d o is the orifice diameter. The discharge coefficient also increases with the atomizer constant.

Streaked particle imaging velocimetry and sizing (SPIVS), a nonintrusive laser-based optical probe for planar measurement of individual droplet velocities and sizes within a spray, is demonstrated. SPIVS, an outgrowth of particle imaging velocimetry, was developed to facilitate droplet velocity and size measurements. Inherent features of the technique are the provision of droplet image pairing and time sequencing of droplet image pairs during the image acquisition stage and measurement of droplet size from the recorded total light-scattering energy for each droplet streak. The ability of the SPIVS technique to accurately and reliably determine droplet size and velocity is evaluated by a comparison test between this probe and a phase/Doppler particle analyzer instrument. Measurements of droplet size and velocity at various radial positions within a test spray were compared.

An experimental study has been conducted to examine the effect of chamber length on the aerodynamic characteristics of an enclosed, non-reacting, swirling, flow field. The swirling flow was generated by a counter-rotating radial-radial swirler consisting of an inner, primary swirler generating counter-clockwise rotation and an outer, secondary swirler generating clockwise rotation. The enclosures used were square cross-section chambers of differing lengths. The internal cross section of all chambers was 50.8 mm × 50.8 mm (2 inch × 2 inch). 3 different lengths of chamber used for the tests were 76.2 mm (3″), 101.6 mm (4″), and 152.4 mm (6″) respectively. A nozzle was used at the downstream end of the enclosure to ensure the absence of reverse flow back to test chamber and to simulate the area reduction in typical combustor. The nozzle reduced the cross-section area from 50.8 mm × 50.8 mm (2″ × 2″) to 22.2 mm × 22.2 mm (0.875″ × 0.875″) via 45° slope. A two-component laser doppler velocimetry (LDV) system was used to measure the velocities in the flow fields. The chamber length has been observed to have a clear influence on the mean and turbulent velocity profile near the exit of swirler. However, this effect is not as evident further downstream in the flow field. For the short chamber length, higher values of axial and tangential velocities were observed in the swirling jet due to the proximity of the downstream nozzle to the swirler. For this chamber length, higher turbulence intensities were observed in the swirling jet and inside center toroidal recirculation zone. The magnitudes of the swirling jet velocity and the turbulence intensities decreased with an increase in the chamber length. Two counter-rotating flows could merge more complete in the exit of swirler with the chamber length decreasing.Copyright © 2013 by ASME

A series of experiments have been conducted to study the aerodynamic characteristics of a confined swirling flow generated by multiple rad-rad swirlers arranged linearly. The rad-rad swirlers used in this study are identical, and consist of an inner, primary swirler generating counter-clockwise rotation and an outer, secondary swirler generating clockwise rotation. A two-component Laser Doppler Velocimetry (LDV) system was employed to measure the velocity in the flow field. Initial measurements were conducted on unconfined and confined flow generated by a single swirler to serve as the baseline reference for the multi-swirler arrangements. Tests were conducted for 3 and 5 swirlers arranged in a line, with a spacing of 2D between the swirler centers, where D is the swirler exit diameter. An additional 5 swirler configuration was tested, where the exit plane of the center swirler was shifted 3.2 mm (1/8 inch) in the streamwise direction. The flow field generated by the multi-swirler arrangement is very complex, due to the interaction between the swirling jets of adjacent swirlers. The number of swirlers is seen to have a clear impact on the entire flow structure, as well as each recirculation zone. For the 3 swirler arrangement, a weak CTRZ is observed for the center swirler, whereas strong CTRZs are observed for the two outer swirlers. For the 5 swirler arrangement, the CTRZ pattern for the 3 inner swirlers is the same strong-weak-strong as seen for the 3 swirler arrangements, with weak CTRZs observed for the two outer swirlers. Higher interaction between swirlers is observed for the 5 swirler arrangement, as compared to the case with 3 swirlers. Since the swirlers are identical, the region between swirlers features merging of two opposing swirling jets, producing high turbulence intensity in the near field region. For the case with the offset center swirler, the swirling jet from this swirler did not merge with its neighbors in the near field region. This resulted in strong CTRZ for the center swirler, accompanied by weaker CTRZs at its immediate neighbors, which is reverse of the CTRZ strength pattern observed for the initial 5 swirler arrangement.

An experimental study has been conducted to study the effect of the dome geometry on the aerodynamic characteristics of a non-reacting flow field. The flow was generated by a counter-rotating radial-radial swirler consisting of an inner, primary swirler generating counter-clockwise rotation and an outer, secondary swirler generating clockwise rotation. The dome geometry was modified by introducing dome expansion angles of 60° and 45° with respect to the swirler centerline, in addition to the baseline case of sudden expansion (90°). The flow downstream of the swirler is confined by a 50.8mm × 50.8mm × 304.8mm (2″ × 2″ × 12″) plexiglass chamber. A two-component laser doppler velocimetry (LDV) system was used to measure the velocities in the flow field. The dome geometry is seen to have a clear impact on mean swirling flow structure near the swirler exit rather than the downstream flow field. For the configurations with 60° and 45° expansion, no corner recirculation zone is observed and the swirling flow structure is asymmetric due to the non-axisymmetric dome geometry. The cross-section area of central recirculation zone is larger for dome geometry with 60° expansion angle, as compared to the 90° and 45° cases. The configurations with 60° and 45° expansion have higher magnitudes of negative velocity inside the core of central recirculation zone, as compared to the configuration with 90° expansion angle.

As combustors are put in service, problems such as erosion, hot spots, and liner oxidation occur, and a solution based on lessons learned is essential to avoid similar problems in future combustor generations. In the present paper, a combustor flow structure control via combustor geometry alteration is investigated using laser Doppler velocimetry. Mainly, three configurations are studied. The first configuration is that of a swirl cup feeding a dump (rectangular cross section) combustor. The rectangular chamber is configured with a width to breadth (w/b) ratio of 85%. The second configuration is similar to the first one, but a combustion dome is installed. The dome is configured with a 9 deg difference in the expansion angle on both sides (asymmetric dome). The third configuration is that of a swirl cup and a combustion dome installed in a prototype combustor (single annular combustor (SAC) sector), with both primary and secondary dilution jets blocked. The SAC is configured with a cross sectional area that decreases toward the exit through the tilting of the inner combustor liner. The results show that the combustion dome eliminates the corner recirculation zone and the low velocity region close to the combustor walls. The combustion dome asymmetry results in a significant asymmetry in the velocity magnitude, as well as the turbulence activities and the tilting of the central recirculation zone (CRZ) toward the surface with the higher expansion angle. The liner tilting results in a 40% reduction in the length of the CRZ. However, once the primary jets are open, they define the termination point of the CRZ. The chamber w/b ratio results in a CRZ with the same diameter ratio (85%) in all configurations. Interestingly, the maximum reverse flow velocity is roughly constant in all measurement plans and configurations up to a downstream distance of 1R (R is the flare radius). However, with open primary jets, the CRZ strength increases appreciably. It appears that the confinement dictates both the flow field outside the CRZ and the size of the CRZ, while the swirl cup configuration mainly influences the strength of the CRZ. Regarding turbulence activities, the presence of the dome damps the fluctuations in the expanding swirling jet region. On the other hand, the primary jets increase the turbulence activities appreciably in the jet impingement region, as well as the upper portion of the CRZ (60% increase).

A streaked-particle-imaging-velocimetry (SPIV) technique for the instantaneous planar measurement of three-component velocity has been developed and demonstrated. In this system a camera images the scattered light from two laser sheets onto the same recording medium. One of the laser sheets, double pulsed with short pulse duration, freezes the tracer particle motion and records a pair of images from each tracer. The other laser sheet, cw, provides tracer trajectories whose length is controlled by the sheet thickness. The recorded image from each tracer is then its streak trajectory superimposed on its frozen paired particle image. The planar two components of velocity are deduced from the distance between image pairs and the time separation of the double light pulses. This information, combined with the tracer trajectory streak length, determines the trace particle staying time within the laser sheet. The tracer velocity normal to the laser sheet is then calculated from this staying time and the laser sheet thickness that can be calibrated from the measurements. The resultant SPIV technique was demonstrated with a free jet seeded with small particles, and the derived velocity was reported.

Transverse dilution jets are widely used in combustion systems. The current research provides a detailed study of the primary jets of a realistic annular combustion chamber sector. The combustor sector comprises an aerodynamic diffuser, inlet cowl, combustion dome, primary dilution jets, secondary dilution jets and cooling strips to provide convective cooling to the liner. The chamber contracts toward the end to fit the turbine nozzle ring. 2D PIV is employed at an atmospheric pressure drop of 4% (isothermal) to delineate the flow field characteristics. The laser is introduced to the sector through the exit flange. The interaction between the primary jets and the swirling flow as well as the sensitivity of the primary jets to perturbations is discussed. The perturbation study includes: effect of partially blocking the jets, one at a time, the effect of blocking the convective cooling holes, placed underneath the primary jets and shooting perpendicular to it. In addition, the effect of reducing the size of the primary jets as well as off-centering the primary jets is explained. Moreover, PIV is employed to study the flow field with and without fuel injection at four different fuel flow rates. The results show that the flow field is very sensitive to perturbations. The cooling air interacts with the primary jet and influences the flow field although the momentum ratio has a 100:1 order of magnitude. The results also show that the big primary jets dictate the flow field in the primary zone as well as the secondary zone. However, relatively smaller jets mainly influence the primary combustion zone because most of the jet is recirculated back to the CRZ. Also, the jet penetration is reduced with 25% and 11.5% corresponding to a 77% and 62% reduction of the jet’s area respectively. The study indicates the presence of a critical jet diameter beyond which the dilution jets have minimum impact on the secondary region. The jet off-centering shows significant effect on the flow field though it is in the order of 0.4 mm. The fuel injection is also shown to influence the flow field as well as the primary jets angle. High fuel flow rate is shown to have very strong impact on the flow field and thus results in a strong distortion of both the primary and secondary zones. The results provide useful methods to be used in the flow field structure control. Most of the effects shown are attributed to the difference in jet opposition. Hence, the results are applicable to reacting flow.Copyright © 2010 by ASME

The occurrence of combustion instability dynamics known, as “screech, howl and growl,” in the combustors of gas turbine engines is a very difficult challenge for engineers. The very high amplitude pressure oscillations caused by combustion dynamics, are not only detrimental to the operation of the engine and combustor, but the difficulty in predicting and remedying these problems can lead to significant costs and delays in engine development. The coupling of the unsteady heat release in the flame with the natural acoustic resonance modes of the combustor duct causes the phenomena of combustion dynamics. To improve our understanding of stability characteristics in such complex systems, encountered in many industrial applications, the flame structure of an atmospheric swirl-stabilized burner, containing dilution and cooling air holes and fed with natural gas fuel, was systematically investigated for various inlet temperatures, pressure drops and air-fuel ratios. Experiments were also designed and conducted with the goal to understand better the phenomena of combustion dynamics that were experienced. More specifically, six acoustic pressure transducers were incorporated in the combustor and in the upstream duct to measure the acoustic field and the acoustic impedance characteristics at specified locations of interest. A one-dimensional wave propagation model is presented to predict the acoustic frequencies and damping of resonance modes, based on the geometry of the test rig, the flow conditions, and the acoustic impedance characteristics of the terminations of the combustor. This paper will present the acoustic analysis of the test data in the light of the above-mentioned theoretical modeling. The limitations of the current test rig are pointed out and changes in the rig design are discussed for future research.Copyright © 2010 by ASME

The method of admission of the swirling flow to the combustion chamber has a strong influence on the flow field structure in Gas Turbine Combustors (GTC). Two different exit configurations are studied. The first configuration is that of a swirl cup that ends only with a splash plate such that there is a sudden unguided expansion as the flow emanates from the swirl cup. The second is a swirl cup that ends with a splash plate and an asymmetric combustion dome. Laser Doppler Velocimetry (LDV) measurements are conducted in the horizontal plane (X-Y), for both configurations, 5mm from the flare exit. Also, LDV measurements are conducted in two vertical planes passing by the combustor centerline (X-Z and Y-Z). The results reveal a significant difference in the flow structure for both configurations. The combustion dome appears to reduce the turbulence activities close to the exit of the swirl cup. In addition, the presence of the combustion dome eliminates the corner recirculation zone and the low velocity region close to the combustor walls. It is interesting to see that the asymmetry of the combustion dome (9° difference in the expansion angle on both sides) results in a significant asymmetry in the velocity magnitude as well as the turbulence activities. Moreover, the asymmetry in the combustion dome results in a tilting of the CRZ toward the surface with the higher expansion angle. The results highlight the importance of the proper and careful design of the GTC front section. The experiments are conducted in a dump combustor (rectangular cross section). To study the effect of the chamber geometry on the flow field, the base configuration is installed in an annular combustor sector and LDV measurements are conducted in the axial radial plane (X-Z). The flow field as well as the shape of the CRZ are significantly different in both cases. The CRZ height reduced by 40% with the swirl cup installed to the SAC sector. The results emphasize the strong influence of the confinement on the flow structure.Copyright © 2010 by ASME

The phenomena of combustion dynamics strongly depends on the energy release (flame) dynamics of combustion systems. The temporal Fast Fourier Transform (FFT) and spatial Proper Orthogonal Decomposition (POD) methods were used to analyze high speed energy release (chemiluminescene) videos on a natural gas powered gas turbine burner. The considered case has a single dominant acoustical frequency. Both methods provide an insight into the temporal and spatial distribution of energy release rate dynamics associated with the acoustic pressure oscillation. In the FFT analysis, the spatially distributed amplitude and phase angle at the selected acoustic frequency can be secured, and the energy release dynamics (motion) associated with the considered acoustics can be visualized. The trigger mechanism for combustion dynamics was unveiled in the spatial phase angle distribution at the acoustic frequency. In the POD analysis, the first few modes have been found to contain a significant portion of energy associated with the considered dynamics system. The first POD mode contains 19% of energy and its associated temporal and spatial feather is very similar to those calculated by the FFT analysis. The POD Mode #2 and #3 contains less energy, 7% and 5% respectively and their temporal behavior is also associated with the acoustical frequency. However, their spatial structures contain higher spatial frequency components compared to that of Mode #1. The unveiled spatial structure by Mode #2 and #3 cannot be easily discovered from the FFT amplitude and phase information since it was washed out by the higher energy spatial structure from Mode #1. A very significant finding in this research is that the POD can be used for reduced order set of modeling which can capture the energy release dynamics by truncating out the “noise.” The POD also represents an effective means of “spatial frequency filtering,” based on the POD related energy distribution, which enables the sharpening of the energy release rate spatial structure information.

This paper investigates the combustion dynamics of a single cup gas turbine combustor sector, a swirl-stabilized burner with dilution and cooling air holes. The burner was fueled by natural gas and was operated at one atmospheric pressure with various air inlet temperatures, air pressure drops, and air–fuel ratios. Acoustic emissions as functions of operating conditions were measured. Chemiluminescence imaging with a high-speed CCD camera and simultaneous dynamic pressure measurements has been used to characterize the combustor’s behavior. Imaging of the combustion energy release rate has provided an insight on the flame structure and its interaction with the dilution air jets. The combustion dynamics were correlated well with the heat release rate in the primary zone or in the dilution air jet region. Two low-frequency combustion dynamics modes (fuel-rich combustor domes with frequency around 420Hz, and fuel-lean combustor domes with frequency around 280 Hz) were identified by a dynamic microphone, dynamic pressure transducers, and a high-speed CCD camera. A two-dimensional distribution of the heat release rate dynamics (amplitude and phase angle) reveals different combustion dynamics driving mechanisms for fuel-lean and fuel rich modes. The acoustic emissions for the fuel-lean combustion mode correlated well with the energy release rate dynamics of the swirling jet inside the combustor dome. The acoustic emissions for the fuel-rich mode were associated with the unsteady chemical energy release rate from interactions between the dilution air jets and the fuel-rich mixture convected from the combustor dome.

The performance of a recently introduced statistical transport model for turbulent particle dispersion is studied here for rigid particles injected into a round turbulent jet. Both uniform and isosceles triangle pdfs are used. The statistical sensitivity to parcel pdf shape is demonstrated.

simplification in calculating the physical particle distribution over a grid could mean substantial gains in code efficiency. Moreover, there will be virtually no accompanying increase in numerical noise generation as compared with predictions using normal pdfs. In some cases, it may also be acceptable to apply the theory with the even simpler uniform pdfs where rapid computation of an approximate solution is desired.

A computational model based on an arbitrary-Lagrangian-Eulerian numerical algorithm is developed for flows separated by a free surface where the surface tension force is important. This model is used to study deformation and oscillation of cylindrical/spherical droplets with and without external forced convection. The calculated frequency of droplet oscillation agrees well with the analytical value derived from perturbation analysis, and the amplitude of oscillation does not decay with time, which indicates that numerical diffusion and damping do not exist in the adopted algorithm

A statistical transport model for turbulent particle dispersion is formulated having significantly improved computational efficiency in comparison to the conventional stochastic discrete-particle methodology. In the proposed model, a computational parcel representing a group of physical particles is characterized by a normal (Gaussian) probability density function (pdf) in space. The mean of each pdf is determined by Lagrangian tracking of each computational parcel, either deterministically or stochastically