S. M. Jeng

Computational and Experimental Study of Liquid Sheet Emanating from Simplex Fuel Nozzle

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.

Parametric study of simplex fuel nozzle internal flow and performance

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 Comprehensive Model to Predict Simplex Atomizer Performance

The pressure swirl atomizer, or simplex atomizer, is widely used in liquid fuel combustion devices in the aerospace and power generation industries. A computational, experimental, and theoretical study was conducted to predict its performance. The Arbitrary-Lagrangian-Eulerian method with a 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 pray angle and film thicknes are predicted. A temporal linear stability analysis is performed for cylindrical liquid sheets under three-dimensional disturbance The model incorporates the swirling velocity component, finite film thickness and radius that are essential features of conical liquid sheets emanating from simplex atomizers. It is observed that the relative velocity between the liquid and gas phases, densit ratio and surface curvature enhance the interfacial aerodynamic instability. The combination of axial and swirling velocity components is more effective than only the axial component for disintegration of liquid sheet. For both large and small-scale fuel nozzles, mean droplet sizes are predicted based on the linear stability analysis and the proposed breakup model. The predictions agree well with experimental data at both large and small scale.

The effect of air swirl profile on the instability of a viscous liquid jet

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.

Effect of Geometric Parameters on Simplex Atomizer Performance

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.

Instability of an Annular Liquid Sheet Surrounded by Swirling Airstreams

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

Computational Model to Predict Flow in Simplex Fuel Atomizer

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.

Effect of Swirler Offset on Aerodynamics of Multiswirler Arrays

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-e 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.Copyright

Numerical Study of Flow Through 3x3 Multiswirler Arrays With Co and Counter Swirler Arrangements

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