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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.

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.

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

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

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.