The transverse jet is a canonical flow field used in several ground-based and aeroderivative gas turbine systems. The setup of the flow field is simple, with a jet of fluid injected at an angle (usually perpendicularly) to another stream of fluid. The interaction between these two streams generates a complex array of vortical structures, including shear layer vortices formed in the near field and counter-rotating vortex pairs observed in the far field of the transverse jet. Although implementing this flow configuration is straightforward, incorporating the flow field in high-performance systems necessitates a thorough understanding of how the flow topology influences key phenomena such as penetration, dispersal, and mixing of the jet fluid. Previous studies have shown that the mixing characteristics of the transverse jet can be linked to the formation and unsteady dynamics of the vortical structures present in both the near and far fields. These studies have also established empirical laws that describe the mixing behavior of the jet.
Although the single transverse jet is a well-studied flow field, actual engineering systems usually employ a large array of closely spaced jets. For example, film cooling technologies utilize homogeneous distribution of large number of transverse jets on gas turbine combustor surfaces and turbine blades to create a protective barrier between the metal components and the hot combustion products. However, in literature, arrays of transverse jets are often overlooked, and only a handful of studies exist that characterize the interactions of a small number of transverse jets. As a result, engineers frequently rely on existing scaling laws obtained from the single transverse jet flow field for the design of gas turbine systems. Additionally, studies on multi-elements flow fields, such as multi-jets and multi-wakes, have shown that the stability boundaries of the multi-element flow fields are often different from their single element counterparts (single wake and single jet) because of competing pressure gradients and the interacting regions of concentrated vorticity.
To that end, this dissertation presents an experimental study of the interactions between an array of transverse jets, utilizing spatially and temporally resolved velocity measurements. Distinct planar cuts of the multiple transverse jet flow field through particle image velocimetry (PIV) were used to quantify both its time-averaged and unsteady behaviors. The velocity data was obtained for systems containing a linear array of jets with varying number of jets (𝑛𝑛= 2 or 3), different jet-jet spacing (𝐿𝐿𝐷𝐷𝑗𝑗⁄∈[2,4]; where 𝐷𝐷𝑗𝑗 is the diameter of the jet) and system configurations (tandem vs inline). In the tandem configuration, the jets are offset relative to one another in the direction of the mean crossflow, while in an inline configuration, the jets are displaced perpendicular to this direction. Additionally, the fluid dynamic properties of the jets were also varied by varying the transverse jet momentum flux ratio ( 𝐽𝐽 ∈[4,25] ), while keeping the jet-to-crossflow density ratio (𝑆𝑆=0.35) and the crossflow properties fixed. These experiments were performed in a low-speed wind tunnel under non-reacting conditions at atmospheric pressure and room temperature. Baseline data of the single jet operated at the same momentum flux ratio was also obtained to compare their flow fields.
The low-speed PIV data, collected at 5 kHz, provided a larger field of view (typically of size 10𝐷𝐷𝑗𝑗×12𝐷𝐷𝑗𝑗), and was used to capture the global characteristics of the multiple transverse jet flow field. Measurements
taken in the center plane of the tandem jet system reveal evidence of jet fluid merging, that originated from the different nozzles within the system. The spatial location of fluid merging is sensitive to both system properties (𝐿𝐿/𝐷𝐷𝑗𝑗) and the fluid dynamic properties of the jets (𝐽𝐽). Additionally, increased jet fluid penetration was also observed in the tandem jet systems due to the increased lift generated from the downstream jets. An empirical jet trajectory law that accounted for the flow physics associated with increased lift generation observed in the tandem jet system was developed to better collapse the jet trajectory data of the different jets present in the tandem jet system.
On the other hand, the center plane measurements of the inline jet system showed that the near-field jet fluid penetration is comparable to that of the single jet. As a result, single jet length scales such as 𝐷𝐷𝑗𝑗,𝐽𝐽𝐷𝐷𝑗𝑗 and √𝐽𝐽𝐷𝐷𝑗𝑗, were adequate in collapsing the near-field jet trajectories. However, in the far field, the jet fluid penetration was consistently lower compared to the corresponding single jet. Spanwise planar velocity measurements of the flow field indicated that the counter-rotating pairs of jets in the inline system often interacted with each other, creating a complex topology of interacting vortex arms in the far-field of the flow field. The competition between inviscid mechanisms, such as mutual induction, and viscous mechanisms, like vorticity annihilation, led to these topological changes and reduced the overall lift generated by the counter-rotating vortex pairs, resulting in the observed decrease in jet penetration.
The differences observed in the near-field trajectories of the tandem jet system can be attributed to the varying stability of the individual jet’s shear layers. High-speed measurements of the center plane velocity, obtained at rates of 25 kHz or 40 kHz within a smaller field of view of size 3𝐷𝐷𝑗𝑗×4𝐷𝐷𝑗𝑗, were used to identify the shear layer vortices from the instantaneous velocity fields. Vortex metrics such as swirling strength, circulation and area were estimated for the individual vortices and the streamwise variation of the ensemble-averaged strength and size metrics of the shear layer vortices were obtained as 𝐽𝐽, 𝐿𝐿/𝐷𝐷𝑗𝑗, 𝑛𝑛 and system orientation were varied. These measurements enabled the extraction of the spatial amplification (growth rate) and decay rate of the shear layer vortices and helped in the identification of their breakdown region. These extracted spatial growth rates were found to closely depend on the stability properties of the individual jet’s shear layer.
The high-speed PIV measurements were further used to extract the characteristic frequencies and enable the classification of the jets’ shear layer instability behavior (convective vs absolutely unstable) based on the spectral characteristics of the transverse velocity fluctuations associated with the shear layer instability. Attempts to collapse the dependencies of the characteristic frequencies with a Strouhal number (𝑆𝑆𝑆𝑆) led to the understanding that the interaction between the different jets in the multiple transverse jet system changes the local time-averaged flow field at the vicinity of the individual jet’s shear layer. These changes resulted in jets having different stability properties that cannot be simply explained using single jet properties such as 𝑆𝑆 and 𝐽𝐽. Thus, the counter-current shear layer (CCSL) model was used to map out the changes to the stability boundaries of the different jets in the system by mapping the stability properties using the counter-current shear ratio, 𝑅𝑅1. The stability properties of the different jets in both the inline and tandem jet systems collapse well when mapped onto 𝑅𝑅1 and show good agreement with theoretically predicted stability transition values. The 𝑅𝑅1 mapping demonstrated that the transition to absolutely instability occurred through the presence of reverse flow and large adverse pressure gradients located right upstream of the jet. To that effect, the addition of jets to the multiple transverse jet system changed the stability boundaries of the jets by altering the local velocity profiles close to the jet nozzle.
To summarize, this dissertation reports on the changes to the transverse jet flow field with the addition of more jets to the system by evaluating the effects of the jet momentum flux ratio, the jet-jet spacing, number of jets and the system orientation. As part of this work, a large dataset of temporally and spatially resolved velocity fields of the multiple transverse jet flow field were obtained that can be used for future validation studies. Improved empirical laws were developed to better describe the jet trajectories and jet fluid mixing rates. Topological changes to the near-field and far-field vortical structures are characterized using flow visualization and vortex detection techniques. Finally, it also examines the control implications of the multiple transverse jet flow field by identifying key parameters and physical processes that govern the stability of the flow field.