Search

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 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.

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.

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

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

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.

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.

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

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.

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

An experimental study has been conducted to study the behavior of liquid jets injected into a crossflow containing a shear layer. The shea r layer was generated by merging two codirectional air streams of differing velocities. Th e injection liquid was water, and an injection diameter of 0.5 mm was used. An initial p art of this study used PIV and MieScattering to investigate the jet penetration and v elocity distribution. The current work furthers the investigation using PDPA to study the droplet size and volume flux distributions in the jet flow field. The parameters of interest w ere the ratio of the velocity of the two crossflowing streams creating the shear layer (R), and the liquid-air momentum flux ratio (q). The jet penetration increased for R > 1 an dde creased for R 1, higher SMD values were observed, but were uniformly distributed throughout the jet plume. For R < 1, SMD values were smaller, indicating better atomization. An increase in q increases the jet penetration and creates high SMD regions near the top of the jet periphery.

*† An experimental investigation was conducted to study the aerodynamic characteristics of the non-reacting, swirling flow field associated with a Multipoint Swirl-Venturi Lean Direct Injection (LDI) combustor. Nine fuel injectors were designed to fit within a 76.2 mm square area, which is approximately the same area of a conventional fuel injector. Each individual fuel injector has a helical axial-vaned swirler with a short internal convergent-divergent venturi to provide rapid mixing of the fuel and air and to provide a small recirculation zone to improve combustion characters. All the swirlers have same tip vane angles of 60o with the calculated swirl number of 1.0. A new design, in which the center swirler and the center fuel injector are recessed, is conducted to improve the range of the flame stability of multipoint LDI combustors. Two different swirler arrangements were investigated experimentally, which include a co-swirling array, where all swirlers acted in the same swirling direction, and a counter-swirling array, where the swirl direction alternated for adjacent swirlers. A 3-D Particle Image Velocimetry (PIV) system was employed to investigate the instantaneous flow structure of the swirling flow and to measure the mean velocity components and Reynolds stresses. Detailed experimental data are provided to improve understanding of the multipoint swirler generated flow field. The experimental results indicate that arrays with the recessed center swirler will alter flow structure significantly. There is a short strong central recirculation zone in both co-swirler and counter-swirler recessed arrays. The baseline arrays have the higher turbulent activity near the swirler exit as compared with the recessed arrays.

An experimental investigation was conducted to study the aerodynamic characteristics of the non-reacting, swirling flow field associated with a Multipoint Swirl-Venturi Lean Direct Injection (LDI) combustor. There are 9 fuel injectors designed to fit within a 76.2 mm square area, which is approximately the same area of a conventional fuel injector. Each individual fuel injector has a helical axial-vaned swirler with a short internal convergentdivergent venturi to provide rapid mixing of the fuel and air and to provide a small recirculation zone for burning. The fuel injector is inserted through the center of swirler and the fuel tip is at the throat of the venturi. All the swirlers have same tip vane angles of 60o with the calculated swirl number of 1.0. Two different swirler arrangements were investigated experimentally, which include a co-swirling array, where all swirlers acted in the same direction, and a counter-swirling array, where the swirl direction alternated for adjacent swirlers. A 3-D Particle Image Velocimetry (PIV) system was employed to investigate the instantaneous vortex structure of the swirling flow and to measure the mean velocity components and Reynolds stresses. A two-component Laser Doppler Velocimetry (LDV) system was employed to get the benchmark quality data which are planned for validating the state-of-the-art numerical models in addition more advanced LES approach. Detailed experimental data are provided to improve mechanistic understanding of the multipoint swirler generated flow field. The experimental results indicate that the flow structures change significantly as compared with the single swirler test. There are very strong interactions among the adjacent swirlers near the swirler exit.

** † An experimental study has been conducted to study the behavior of liquid jets injected into a crossflow containing a shear layer, generated by merging two co-directional air streams of differing velocities. The injection liquid was water, and a nozzle diameter of 0.5 mm was used. Two streamwise locations were available for jet injection, 6.35 cm and 15.24 cm from the location of the merging of the two airstreams. Mie-scattering and Particle Image Velocimetry (PIV) were used to investigate the jet behavior. The structure of the flow containing the shear layer, and the jet penetration and the droplet velocity distribution were studied. The parameters of interest were the ratio of velocity of the two air streams (R), the aerodynamic Weber number (We), the liquid-air momentum ratio (q) and the streamwise location of jet injection (Xi). For R > 1, the jets penetrate further into the crossflow, but spray coverage gets confined to the upper portion of the jet trajectory. For R < 1, the penetration is lesser, but uniform spray coverage was obtained. As the injection location moves further downstream, more mixing occurs between the two airstreams, weakening the shear layer. As a result, the effects of the shear layer on the jet also get dampened.

A research initiative encompassing the fields of computational simulation and physical experimentation incorporates this continued study of liquid sprays combined with complex swirling airflow. Two cases are examined in which a spray field generated by a simplex nozzle is influenced by the coaxial counter-rotating flow structure created by a production CFM56 swirl cup. The two cases involve atomization of water, as previously reported, and Jet-A aviation kerosene at an effective equivalence ratio of 0.7 (air-to-liquid loading ratio of 16.8 for the water case) and an air temperature of 100°F. For comparison, the volumetric flow rates of the dispersed phase were matched in each case. Measurements are conducted with a two-dimensional Phase Doppler Particle Analyzer (PDPA) system within close proximity to the swirl cup exit plane. The gathered data provides the three component velocities and a diameter distribution throughout the spray field. These velocities and corresponding RMS velocities of the dispersed phase averaged across all diameters are plotted for overall spray comparison. Furthermore, the droplet distributions at each spatially resolved measurement location are compared between the two cases utilizing the arithmetic mean, the Sauter mean, and the Rosin-Rammler distribution parameters. Finally, volumetric flux of the spray is displayed throughout the measured flow field.

An experimental study has been conducted to research the behavior of liquid jets injected transversely into a subsonic crossflow of air. Three injection liquids, water, Jet-A and N- Heptane, and two injection diameters of 0.381 and 0.762 mm, have been used. Pulsed shadowgraphy and Phase Doppler and Particle Analyzer (PDPA) were used to take measurements. The behaviors of the jet column, the post-breakup spray, as well as the penetration and breakup have been studied. Two breakup modes have been observed, column and surface breakup. The streamwise location of breakup is constant while the transverse location increases with momentum ratios. Jet penetrations have been correlated with momentum ratio, injector diameter and the streamwise distance. Volume flux exhibits a maximum in the spray core. Droplet sizes decrease with an increase in the crossflow velocity while penetration increases with an increase in injector diameter or momentum ratio. Droplet size exhibits a maximum in the spray core for low momentum ratios and near the periphery for high momentum ratios.

An experimental study was conducted to characterize the two-phase nonreacting and reacting flow characteristics of a mixer comprised of concentric counter-swirlers installed in a 3-inch square test section. A two-component Phase Doppler Particle Analyzer (PDPA) system was used to measure mean and fluctuating velocity components in the three directions, as well as the droplet size distribution and volume flux. Detailed measurements have been used to improve mechanistic understanding of reacting counter-swirling complex flow-field with attendant changes expected in future modeling direction. As known empirically for the mixer studied, the present indepth experimental results show that the reacting flowfield characteristics are significantly different from that of the isothermal flow in regard to, for example, the recirculation zone, and decay of the counter-swirl velocity component.Copyright © 2005 by ASME

An experimental investigation was conducted to study the aerodynamic characteristics of the confined, non-reacting, swirling flow field. The flow was generated by a helicoidal axial-vaned swirler with a short internal convergent-divergent venturi, which was confined within 2-inch square test section. A series of helicoidal axial-vaned swirlers have been designed with tip vane angles of 40 deg., 45 deg., 50 deg., 55 deg., 60 deg. and 65 deg.. The swirler with the tip vane angle of 60 deg. was combined with several simulated fuel nozzle insertions of varying lengths. A two-component Laser Doppler Velocimetry (LDV) system was employed to measure the three-component mean velocities and Reynolds stresses. Detailed data are provided to enhance understanding swirling flow with different swirl degrees and geometries and to support the development of more accurate physicaVnumerica1 models. The data indicated that the degree of swirl had a clear impact on the mean and turbulent flow fields. The swirling flow fields changed significantly with the addition of a variety of simulated fuel nozzle insertion lengths

This paper presents a computational analysis of flow in simplex fuel atomizers using Arbitrary-LagrangianEulerian method. It is well established that the geometry of an atomizer plays an important role in governing its performance. In this paper, we have investigated the effect on atomizer performance of four geometric parameters, viz., inlet slot angle, spin chamber convergence angle, trumpet angle and trumpet length. For constant mass flow rate through the atomizer, the effects on atomizer performance are shown in terms of variations 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. The geometric parameters considered in this study have not been addressed in available semi-empirical correlations and as such the present correlations do not provide guidance about these parameters to atomizer designers. It is seen that our computational code is able to predict influence of these geometric parameters on the atomizer performance well and can act as a very useful design tool for simplex atomizers.

The non-reacting fluid flow fields generated by multi-swirler arrays with two different port configurations were investigated experimentally. These two cases include a co-swirling array, where all swirlers act in the same direction, and a counter-swirling array, where all swirlers alternate between clockwise and counter-clockwise rotations. Each configuration consists of a three by three arrangement of swirlers, which use analogous geometries to generate the swirling flows, i.e. eight discrete jets. For each array, the mean and turbulence velocities were measured at approximately 25,000 discrete spatial locations by a two-component LDV system. Furthermore, the third velocity component was derived based on the symmetric geometry of the arrays. In both cases, the coalescence distance, distance for eight discrete jet streams to form uniform swirling flows, is very short; and each swirler within the arrangement possesses a central recirculation zone. These nine zones have varying strengths depending on the individual swirler location, and some rotate with the swirler direction. In comparison, the co-swirler configuration provides a large unified rotational flow in the same direction as the individual swirlers, while a counter-rotating flow pattern is formed by the other case. Lastly, interactions between the unified flow with that of each swirler in the array are discussed and further comparisons are made between the two cases.Copyright © 2002 by ASME

A 3-D analytical model has been developed to predict the instability of a swirling annular liquid sheet sandwiched by swirling air streams. The model incorporates essential features of the prefilming and subsequent atomization process inside pressure-swirl airblast atomizer. Effects of relative axial velocity, liquid swirl, inner and outer air swirl on the growth of various disturbance modes were examined. Effect of relative swirl orientation of the air swirl with respect to liquid swirl has also been investigated. It was found that increasing the relative axial velocity between the liquid and the gas phases significantly improves the fuel atomization. A combination of the inner and outer air leads to higher growth rates and larger most unstable wave numbers than a single air stream. Swirl has a significant effect on the instability of the liquid sheet. It not only increases the maximum growth rate and the most unstable wave number, but also switches the dominant mode from the axisymmetric mode to a helical mode. A combination of inner and outer air swirl leads to better atomization than a single air swirl while the inner air swirl is more effective than the outer air swirl in enhancing the instability of the liquid sheet. Liquid swirl is far more effective than air swirl in promoting the breakup of the liquid sheet. In regard to the effect of swirl orientation on atomization, co-swirl leads to the highest growth rate, but outer coand inner counterswirl leads to the largest most unstable wave number thus finest spray. Inner and outer counter-swirl results in worst atomization quality. NOMENCLATURES A Vortex strength m/s h Ratio of inner and outer radii /„ n* order modified Bessel function of first kind Kn n^order modified Bessel function of second kind k Axial wave number i/m n Azimuthal wave number * Member, AIAA Copyright ©2001. American Institute of Aeronautics and Astronautics, Inc. All rights reserved. P Mean pressure N/m p Disturbance pressure N/m Ra Inner diameter of liquid sheet m Rh Outer diameter of liquid sheet m r Radial coordinate m t Time s U Mean axial velocity m/s V Mean radial velocity m/s W Mean tangential velocity m/s u Disturbance axial velocity m/s v Disturbance radial velocity m/s w Disturbance tangential velocity m/s We Weber number (We = pUR/a) X Axial coordinate m Greek letters T| Displacement disturbance m 9 Azimuthal angle radian p Fluid density kg/m n Angular velocity 1/s co Temporal frequency 1/s Subscripts / Inner gas / Liquid phase o Outer gas s Based on swirling component INTRODUCTION In the past years, extensive research efforts have been made to optimize fuel atomization process and combustor aerodynamics aiming at improving combustion efficiency and reduce pollutant emissions. It was realized that it is crucial to achieve rapid and uniform fuel-air mixing. As pressure-swirl airblast atomizer combines desirable attributes of simplex and airblast atomizers, it leads to remarkable improvements in combustor aerodynamic and fuel/air mixing. In addition, it has such advantages as larger flow turndown ratio, lower pollutant emissions, and better patternation. As such, it is widely used in aircraft engine combustors and power generation furnaces. Inside a pressure-swirl airblast atomizer as shown in Fig. 1, liquid fuel emanates from a pressureswirl or simplex atomizer in the form of a conical 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. liquid sheet and impinges with the swirier cup. Due to strong shear action of the fast moving inner air, a thin annular liquid film is formed on the cup surface. The kinetic energy of the high-speed swirling air streams and the pressure energy of the liquid fuel act together to breakup the liquid sheet. As a result, pressure-swirl airblast atomizer has improved spray quality compared to simplex atomizer. However, our knowledge on its internal prefilming process and the subsequent breakup of the liquid sheet is very limited. Understanding factors that influence the instability and breakup of the liquid sheet will not only provide guidance for atomizer design but also improve spray model for engine combustion simulations. Pressure swirl atomizer Outer venturi Inner Swirler Outer Swirler Figure 1 Schematic of a pressure-swirl airblast atomizer It is well known that disintegration of liquid sheet is due to growth of unstable waves at the interface between the gas and the liquid sheet. Among all unstable waves, there is a so-called most unstable frequency (or wavelength) as illustrated in Fig. 2. This wave has the maximum growth rate and dominates the breakup of the liquid sheet. Its wavelength and growth rate are related to the mean size of droplets formed and the breakup length. Figure 2 A dominant frequency exists among unstable waves at liquid/air interface There are numerous studies" on the instability of two-dimensional liquid sheets. Flow conditions and liquid film geometry considered are nevertheless over simplified and can not be applied to atomization process in practical devices. These two-dimensional models were later extended to annular liquid sheets" 16 to simulate liquid breakup. However, flow conditions in these studies ~ were in the low speed regime and disturbances are assumed to be twodimensional. More importantly, swirl effect on the instability of the annular liquid sheet had not been considered. Theoretical, experimental, and computational efforts have been made to study internal flow field, liquid filming process, instability and breakup of liquid sheet, and resultant spray characteristics such as droplet size". Recently, we developed a theoretical model considering both inner and outer air swirl and found that air swirl has a destabilizing effect on the annular liquid sheet. Based on our previous model, in a more recent study, we developed a 3-D viscous instability model for airlast atomization. It was found that liquid viscosity not only reduces the growth rates of unstable waves, but also shifts the dispersion diagram towards long waves. However, the model does not consider the effect of liquid swirl. As such, the effectiveness of liquid and air swirl in enhancing the instability of the liquid sheet remains unclear. Furthermore, effects of the relative air swirl orientation with respect to liquid swirl are not understood. The objective of present paper is to develop a 3-D instability model for pressure-swirl airblast atomization by incorporating features such as three-dimensional disturbance, liquid swirl, inner and outer air swirl, and an annular geometry with a finite film thickness. The flow conditions and liquid film geometry considered here are very similar to their practical counterparts. Therefore, it is expected that this model is capable of providing insights into the underlying physics of airblast atomization and can be applied to predict drop sizes at the nozzle orifice. MATHEMATICAL MODEL We consider an inviscid swirling annular liquid sheet surrounded by coaxial swirling air streams as shown in Fig. 3. In the instability model, the gas phase is assumed to be inviscid and incompressible. Mean flows of the liquid, the inner and the outer air are assumed to be (£/,, 0, A/r), (Uit 0, Or} and (U0, 0, A/r), respectively. This combination of solid body rotation and free vortex profile is quite similar to the practical tangential velocity profile inside gas turbine combustors. Due to the swirl effect, to maintain an annular shape of the liquid surface, a constraint on the American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. mean pressure of the inner and outer air streams must be imposed by Eq. (1).

The National Combustion Code (NCC) was used to calculate the steady state, nonreacting flow field of a prototype Lean Direct Injection (LDI) swirler. This configuration used nine groups of eight holes drilled at a thirty-five degree angle to induce swirl. These nine groups created swirl in the same direction, or a corotating pattern. The static pressure drop across the holes was fixed at approximately four percent. Computations were performed on one quarter of the geometry, because the geometry is considered rotationally periodic every ninety degrees. The final computational grid used was approximately 2.26 million tetrahedral cells, and a cubic nonlinear k - epsilon model was used to model turbulence. The NCC results were then compared to time averaged Laser Doppler Velocimetry (LDV) data. The LDV measurements were performed on the full geometry, but four ninths of the geometry was measured. One-, two-, and three-dimensional representations of both flow fields are presented. The NCC computations compare both qualitatively and quantitatively well to the LDV data, but differences exist downstream. The comparison is encouraging, and shows that NCC can be used for future injector design studies. To improve the flow prediction accuracy of turbulent, three-dimensional, recirculating flow fields with the NCC, recommendations are given.

The upstream and downstream flows of a radial inlet swirler were investigated by using a 2-D Laser Doppler Velocimetry (LDV) probe. The radial profiles of axial and tangential velocities at various axial locations were obtained. The effects of Reynolds Number, swirler exit configuration and combustor outlet condition were examined. The results reveal that the combustor outlet condition has strong influence on the downstream flow field of the swirler. The effect of Reynolds Number depends on the swirler exit configurations. The influence of the convergent configuration from swirl chamber to orifice on the flow fields is very small whereas the influence of the divergence at the swirler exit is significant. The upstream and downstream flows of the radial inlet swirler are of sub-critical nature, thus the swirler exit configuration and combustor outlet condition significantly affect the flow fields.Copyright © 2000 by ASME

The flow fields of large-scale simplex nozzles were investigated by 2-D back-scattered Laser Doppler Velocimetry (LDV). The internal flow structures of a simplex nozzle at two different density ratios of the working fluid and the ambient medium were obtained. The effects of the density ratio, Reynolds Number and orifice geometry on the flow structure were examined. The results revealed that the density ratio only affects the internal flow field in the region where the radius is smaller than the orifice radius. The density ratio and Reynolds Number have stronger influence on the internal flow structure of a sudden contraction and 45° expansion orifice configuration than on that of a 45° contraction and sudden expansion orifice configuration. When the density ratio is one, the effect of the contraction geometry from swirl chamber to orifice on the internal flow is very small compared to the effect of the expansion geometry.Copyright © 2000 by ASME

The performance of liquid fuel atomizer has direct effects on combustion efficiency, pollutant emission and stability. Pressure swirl atomizer, or simplex atomizer, is widely used in liquid fuel combustion devices in aircraft and power generation industry. A computational, experimental, and theoretical study is conducted to predict its performance. The Arbitrary-Lagrangian-Eulerian method with finite volume scheme is employed in the CFD model. Internal flow characteristics of the simplex atomizer as well as its performance parameters such as discharge coefficient, spray angle and film thickness are predicted. A temporal linear stability analysis is performed for cylindrical liquid sheets under 3-D disturbance. The model incorporates swirling velocity component, finite film thickness and radius which are essential features of conical liquid sheets emanating from simplex atomizers. It is observed that the relative velocity between liquid and gas phase, density ratio and surface curvature enhance the interfacial aerodynamic instability. As Weber number and density ratio increase, both the wave growth rate and the unstable wave number range increase. Combination of axial and swirling velocity components is more effective than single axial component for disintegration of liquid sheet. A breakup model for conical liquid sheet is proposed. Combining the breakup model with linear stability analysis, mean drop sizes are predicted. The theoretical results are compared with measurement data and agreement is very good.Copyright © 1998 by ASME

A parametric experimental study has been conducted to measure the discharge coefficient, the flow number, the film thickness, the spray angle, the velocity coefficient and droplet size of a large-scale simplex nozzle using ultrasonic and optical techniques. Seventeen nozzle geometries have been studied for three mass flow rates. The large-scale nozzle provides adequate resolution for measurements of film thickness, spray angle, and droplet size. The experimental data collected have been used to derive new and improved correlations for nozzle flow and breakup parameters. It is found that the atomizer constant (ratio of total inlet area to product of the swirl chamber and orifice diameter) is the primary parameter affecting the atomizer performance. As the atomizer constant increases, the discharge and velocity coefficients increase and the spray angle decreases.

Evaluation of an efficient statistical transport model for turbulent droplet dispersion is made for a dilute spray of methanol droplets injected into a turbulent, axisymmnetric methane-fuelled diffusion flame burning in stagnant air. In the dispersion model, a computational parcel representing a group of physical particles (droplets) is considered to have a normal probability density function (pdf) in space. The mean is determined by Lagrangian tracking through a sequence of stochastically generated turbulent eddies and the variance is evaluated from a statistical formulation based on the linearized particle equations of motion. The basic validity of this model is established through comparison with available experimental data and with theoretical predictions using a conventional stochastic direct modeling approach. The conclusion of the evaluation is that the proposed dispersion model compares favorably with experimental data and provides a valid technique for simulating turbulent combusting sprays with significant computational savings over conventional methods.

A computational technique for the nonlinear analysis of longitudinal-mode liquid propellant rocket combustion instability is examined based on the unsteady, quasi-one-dimensional Euler equations with appropriate source terms introduced to account for interphase transport coupling with the spray. The method is first assessed for unsteady, nonreacting, isentropic duct flow with specified admittances at the outflow and inflow boundaries. For small amplitude disturbances, numerical results based on a sufficient number of grid points compare favorably with predictions from a small-disturbance linearized analysis.

An extensive numerical experiment has been conducted to evaluate rocket thruster performance using a laser-sustained hydrogen plasma as the propellant. The plasma was sustained using a 30 kW CO2 laser beam operated at 10.6 microns focused inside the thruster. The steady-state Navier-Stokes equations coupled with the laser power absorption process have been solved numerically. A pressure based Navier-Stokes solver using body-fitted coordinate was used to calculate the laser-supported rocket flow which included both recirculating and transonic flow regions. The local thermodynamic equilibrium (LTE) assumption was used for the plasma thermophysical and optical properties. Geometric ray tracing was adopted to describe the laser beam. Several different throat size thrusters operated at 150 and 300 kPa chamber stagnation pressure were studied. It was found that the thruster performance (vacuum specific impulse) was highly dependent on the operating conditions, and a properly designed laser supported thruster can attain a specific impulse around 1500 secs. The heat loading on the thruster wall was also estimated and was in the range of that for a conventional chemical rocket.