Application of the Scale-Adaptive Simulation to a Hot Jet in Cross Flow

The simulation of a hot jet in cross flow is of crucial interest for the aircraft industry as it directly impacts aircraft safety and global performance. Due to the highly transient and turbulent character of this flow, simulation strategies are necessary that resolve at least a part of the turbulence spectrum. The high Reynolds numbers for realistic aircraft applications do not permit the use of pure Large Eddy Simulations as the spatial and temporal resolution requirements for wall bounded flows are prohibitive in an industrial design process. For this reason, the hybrid approach of the Scale-Adaptive Simulation is employed, which retains attached boundary layers in well-established RANS regime and allows the resolution of turbulent fluctuations in areas with sufficient flow instabilities and grid refinement. To evaluate the influence of the underlying numerical grid, three meshing strategies are investigated and the results are validated against experimental data.

The reduction of full and part load emissions and the increase of the turndown ratio are important goals for gas turbine combustor development. Combustion techniques, which generate lower NOx emissions than unstaged premixed combustion in the full load range, and which have the potential of reducing minimum load while complying with emission legislation, are of high technical interest. Therefore, axial-staged combustion systems have been designed, either with or without expansion in a turbine stage between both stages. In its simpler form without intermediate expansion stage, a flow of hot combustion products is generated in the first stage of the premixed combustor, which interacts with the jets of premixed gas injected into the second stage. The level of NOx formation during combustion of the premixed jets in the hot cross flow determines the advantage of axially staged combustion regarding full load NOx emission reduction. Employing large-eddy simulation in openfoam, a tool has been developed, which allows to investigate staged combustion systems including not only temperature distribution but also NOx emissions under engine conditions. To be able to compute NOx formation correctly, the combustion process has to be captured with sufficient level of accuracy. This is achieved by the partially stirred reactor model. It is combined with a newly developed NOx model, which is a combination of a tabulation technique for the NOx source term based on mixture fraction and progress variable and a partial equilibrium approach. The NOx model is successfully validated with generic burner stabilized flame data and with measurements from a large-scale reacting jet in cross flow experiment. The new NOx model is finally used to compute a reacting jet in cross flow under engine conditions to investigate the NOx formation of staged combustion in detail. The comparison between the atmospheric and the pressurized configuration gives valuable insight in the NOx formation process. It can be shown that the NOx formation within a reacting jet in cross flow configuration is reduced and not only diluted.

Correlation analysis of oxy-fuel jet in cross-flow on thermoacoustic instability in a model gas turbine combustor

Injection of the liquid fuel across the incoming air flow is widely used in gas turbine engine combustors. Thus it is important to understand the mechanisms that control the breakup of the liquid jet and the resulting penetration and distribution of fuel droplets. This understanding is needed for validation of Computational Fluid dynamics (CFD) codes that will be subsequently incorporated into engine design tools. Additionally, knowledge of these mechanisms is needed for interpretation of observed engine performance characteristics at different velocity/altitude combinations of the flight envelope and development of qualitative approaches for solving problems such as combustion instabilities (Bonnel et al., 1971). This chapter provides an introduction and literature review into the subject of cross-flow fuel injection and describes the fundamental physics involved. Additionally highlighted are experimental technique and recent experimental data describing the variables involved in fuel spray penetration and fuel column disintegration. In recent years, there has been a great drive to reduce harmful emissions of oxides of Nitrogen oxides (NOx) from aircraft engines. One of the several approaches to achieve low emissions is to avoid hot spots in combustors by creating a lean homogeneous fuel-air mixture just upstream of the combustor inlet. This concept is termed as Lean Premixed Prevaporized (LPP) combustion. Creating such a mixture requires fine atomization and careful placement of fuel to achieve a high degree of mixing. Liquid jet in cross flow, being able to achieve both of these requirements, has gained interest as a likely candidate for spray creation in LPP ducts (Becker & Hassa, 2002). Since the quality of spray formation directly influences the combustion efficiency of engines, it is important to understand the fundamental physics involved in the formation of spray. As seen in Fig. 1, the field of a spray created by a jet in cross flow can be divided into three modes: 1) Intact liquid column, 2) Ligaments, and 3) Droplets. The liquid column develops hydrodynamic instabilities and breaks up into ligaments and droplets (Marmottant & Villermaux, 2004; Madabushi, 2003; Wu et al., 1997). This process is referred to as primary breakup. The location where the liquid column ceases to exist is known as the column breakup point (CBP) or the fracture point. The ligaments breakup further into smaller droplets and this process is called secondary breakup. The most relevant parameter for drop breakup criterion is the Weber number,

Axial staging in premixed gas turbine combustors is a promising option for the increase of firing temperature without NOx-penalty and for the improvement of turndown ratio, which is limited by the onset of CO-emissions. The configuration of greatest interest is the combination of state of the art premixed combustion in the primary stage with secondary injectors near the turbine inlet, which feed additional jets of premixed combustible mixture into the hot cross flow. Regarding NOx this configuration is particularly beneficial (1) if the overall mixing quality in the first stage is limited, (2) if the difference between primary zone flame temperature and turbine inlet temperature due to air addition along the combustor is large and (3) if a high degree of mixing in the second stage is achieved. The potential of this promising combustion concept was investigated in a large scale atmospheric test rig. For the study presented below scaling of the second stage according to Karlovitz number similarity was chosen. This leads to smaller jet diameters and higher injection velocities compared to scaling based on Damköhler number applied in an earlier study. The impact of the higher velocities at the injector outlet on the flow field, on the lift-off height of the flame and on NOx-formation is analyzed. A chemical network model is presented, which illustrates the effects of pre-flame and post-flame mixing on NOx-formation under atmospheric and high pressure conditions. In addition this model is used to study the interactions of chemistry with mixing between the reacting jet and cross flow. On the basis of atmospheric testing and reactor modeling, predictions for engine pressure are made assuming similar lift-off as well as pre- and post-flame mixing. These results are further analyzed regarding the NOx-reduction potential at different equivalence ratios and residence times. Finally, it is discussed under which conditions the investigated configuration can be beneficially applied to reduce NOx-emissions of real engines.

The present paper describes a synthetic jet interaction with the cross flow in low Reynolds number condition by flow visualization and the wall static pressure measurements. The primary focus of the current study is to examine the possibility on the interaction of the synthetic jet with the cross flow in low Reynolds number viscous dominant flow. The low bulk velocity of the cross flow is set in a small scale of the wind tunnel with a high aspect ratio. A wide range of Reynolds number based on the tunnel height and the bulk velocity is covered. The flow visualization at Reynolds number of 1,000 is conducted in X-Y and Y-Z planes to clarify the development of the interaction process in the downstream. Both the time averaged and phase averaged wall static pressure were obtained downstream of the jet injection. The synthetic jet has a diameter of 0.5 mm and a frequency of 100 to 400 Hz. The penetration of the jet in the cross flow depends on the jet velocity ratio, and the deepest penetration occurs at the phase of π/2 at the highest jet velocity ratio. The counter rotating longitudinal vortex pair is generated even in low Reynolds number and can be observed at 100d downstream from the injection. The vortex pair shows the up-wash motion at the center of the jet core and the down-wash motion at the outsides of the jet. For the synthetic jet in cross flow, the fluctuated wall static pressure is increased, and the wall static pressure has similar frequency to the synthetic jet.

The digital particle image velocimetry (DPIV) technique has been used to investigate the flow fields of an elliptic jet in cross flow (EJICF). Two different jet orientations are considered; one with the major axis of the ellipse aligned with the cross flow (henceforth referred to as a low aspect ratio (AR) jet), and the other with the major axis normal to the cross flow (henceforth referred to as a high aspect ratio jet). Results show that the vortex-pairing phenomenon is prevalent in the low aspect ratio jet when the velocity ratio (VR)≥3, and is absent in the high aspect ratio jet regardless of the velocity ratio. The presence of vortex pairing leads to a substantial increase in the leading-edge peak vorticity compared to the lee-side vorticity, which suggests that vortex pairing may play an important role in the entrainment of ambient fluid into the jet body, at least in the near-field region. In the absence of vortex pairing, both the leading-edge and the lee-side peak vorticity increase monotonically with velocity ratio regardless of the aspect ratio. Moreover, time-averaged velocity fields for both AR=0.5 and AR=2 jets reveal the existence of an “unstable focus” (UF) downstream of the jet, at least for VR≥2. The strength and the location of this focus is a function of both the velocity ratio and aspect ratio. In addition, time-averaged vorticity fields show a consistently higher peak-averaged vorticity in the low aspect ratio jet than in the high aspect ratio jet. This behavior could be due to a higher curvature of the vortex filament facing the cross flow in the low aspect ratio jet, which through mutual interaction may lead to higher vortex stretching, and therefore higher peak-averaged vorticity.

This article experimentally studies the synchronous control of combustion instability and NOx emission in a model gas turbine combustor. The application of tabular jet in cross flow was adopted to test the effectiveness of synchronous control. The flow rate, height, direction, and relative molecular masses were studied in this research. Results suggest that the ideal damping ratio of the sound pressure amplitude can be as high as 73.7% under the carbon dioxide jet in cross flow case. The carbon dioxide jet in cross flow case can lead to a better thermoacoustic instability suppression result than the instances of argon or helium. Besides, amplitude and frequency switching of the unsteady flame emerged during the experiment. Adding minimal inert gaseous can change the local oscillation of equivalence ratio. The suppression ratio of NOx emission can reach as high as 44% under the carbon dioxide cases, which performs better than the argon or helium cases. Moreover, the flame length declines as the tabular jet in cross flow rate increases. When the flow rate increases (or the height decreases), the size of the flame front or root will decrease. The average flame length dropped from 122 to 41 mm. The macrostructure of the flame changes significantly under different injecting heights or directions. This article can serve as a specific guideline for the passive control of thermoacoustic instability in gas turbine, rocket, and industrial burners.

One of the most used spray configurations for gas turbines and power combustors is liquid jet in crossflow. The process of breakup of liquid jet is very complex and understanding this mechanism is of paramount importance in engine design. This has led to the commencement of several studies from leading research groups [1–6]. Several new modeling methods such as the Madabhushi breakup model or more detailed VOF and Level set methods have been used successfully to understand and describe these complex breakup processes. However, most of these studies have been restricted to liquid jet in uniform single stream crossflow. In reality, these jets could be subjected to several gaseous streams and the breakup mechanism may vary significantly. Recently there have been some studies to understand the effect of non-uniformities on the crossflow velocity distribution and the droplet diameter. In the current work, we attempt to extend the scope of the Madabhushi breakup model to jets subjected to non-uniform crossflow. Non-uniform crossflow is created by co-directional and parallel gas flow using several hollow tubes. Locally, the momentum flux ratio changes by a factor of 4 and uniformity ratio (the ratio of the velocities of the two gas streams) of 2. A modified version of the Madabhushi model as proposed by Lambert et. al is used here to simulate the jet breakup. Model tuning has been conducted using University of Cincinnati Research data specifically designed for this configuration in partnership with General Electric Company. For turbulence, realizable k-ε with scalable wall function is used. The droplets are tracked using Ansys Fluent Discrete Particle Model (DPM). A second modeling approach VOF-to-DPM is also used which uses VOF equation along with LES with Dynamic Kinetic Energy Subgrid-Scale Model. This model requires no fine tuning of parameters and is more accurate but comes with more computational expense. Various simulations are performed with pure water, pure diesel and emulsified diesel and water with uniform and non-uniform cross flows inside a chamber at a pressure of 50psi. Overall, the trends due to difference in material properties of the two liquids especially on penetration and Sauter mean diameter are well captured. The droplet characteristics such as axial velocity, Sauter mean diameter and volumetric flux are compared with experimental measurements and shows reasonable agreement. Overall, the liquid penetration is within reasonable accuracy. Discrepancies were seen in the spatial variation of the spray quantities such as Sauter mean diameter, droplet axial velocity etc. The simulation revealed a more averaged field whereas in experiments some layering was observed with bigger droplets at the edge of the spray, away from the wall.

This paper presents the results of a series of laboratory experiments, by placing a turbulent wall jet in the upstream and downstream of a 90° channel bend, to investigate the effects of secondary flow on formation of local sediment erosion and deposition. The effects of jet and current flow intensities on the erosion pattern were characterized by the jet's Reynolds number, Re, channel Froude number, Fr, and the ratio of water depth to nozzle diameter, h/do. The geometrical characteristics of local erosion such as jet trajectory, maximum scour depth, ds, and maximum mound height, hm, were measured. The area and volume of scours and mounds were calculated from measurements, and the results were correlated with the controlling parameters. Experimental observations indicated the existence of distinct scour patterns due to variations of channel Froude number. Two additional scour holes were identified due to the effect of stream interactions and formation of vortex regions immediately downstream of sediment mound. It was found that both channel and jet flows contributed to trajectory of the jet and the footprint of erosion. A threshold Froude number of Fr=0.091 was identified at which the current and jet flow were in equilibrium. The results showed that for a constant value of Fr, all scour parameters increased with increasing Reynolds number. However, Re did not have any effects on the jet's deflection. Comparing the erosion profiles in the upstream and downstream bend indicated that the secondary current and turbulent redistribution altered the erosion and mound formation in the downstream of the bend in relatively high Reynolds (i.e., Re≥23,000), high channel Froude numbers (i.e., Fr>0.091) and high jet submergence (i.e., h/do>10.53).

In this work, an experimental study of a jet issuing in cross flow is reported. Support is offered to a theory on the origin of Karman-like vortices, in the wake of jets issuing in boundary layers. Further, a coherent structure with strong contribution to the energetics of the flow field is identified here. This structure has not been reported earlier, according to the present authors' knowledge, and it is shown to be related to engulfment of external fluid at the bottom of the jet, and to interesting stochastic and spectral characteristics of the flow field. Three-dimensional plots of the coherent quantities, based on experimental data, reveal a double-helical morphology of the coherent structure. The same morphology has recently been proposed for the far field of jets issuing in stagnant fluid (i.e., without cross flow). The results of this study are expected to support theoretical and numerical work on jets issuing in cross flow.

The gas turbine combustion chamber is a vital part of a gas turbine engine. Proper mixing of air in the combustor plays an important role in combustion. Increasing mixing rate is an important factor for better combustion efficiency. The injection of air in crossflow is widely studied over the years. The air injected at an angle in upstream direction gives better mixing by colliding with the crossflow. The computational analysis of the injected jet in cross flow is performed with different angles in the upstream direction. The k-omega SST turbulence model was used to investigate the mixing behavior. The air is injected at different angles and observed that with an increase in angle from 60° to 135°, the rate of mixing and turbulent intensity increased. The jet inclination in the upstream direction greatly influenced the mixing behavior. The jet penetration in perpendicular direction was almost the same for 120° and 135°. But there is added penalty in the form of the pressure loss at the angle 135°. So considering the pressure loss and ease of manufacturing the 120° jet inclination is preferable for better mixing among the four cases studied here. The idea of inclining jet in upstream direction can be implemented on the combustor for increased performance and shorter size.

Effects of coolant supply arrangement on double wall cooling: Hot-side effusion performance and cold-side Nusselt numbers at different initial blowing ratios

In connection with the problems of the ingestion of hot exhaust gases in engines of V/STOL and STOVL aircraft in ground effect, a series of studies have been undertaken. Ground impinging, two- and three-poster jets operating in the presence of cross flow were studied. The current paper is divided into two parts. The first part is a comparison of the low speed, two-, three-, and four-poster jet cases, with respect to the flowfield in the region of interaction between the forward and the jet flows. These include cases with mass balanced inlet suction. An analysis of the inlet entry plane of the low speed two- and three-poster jet cases is also given. In the second part, high speed results for a two jet configuration without inlet suction are given. The results are based on quantitative, marker concentration distributions obtained by digitizing video images.

Direct numerical simulation (DNS) of jets in cross-flow (JICF) has been carried out in this study, aiming for the investigation of vortex structure formation and evolution process associated with JICF. A recently developed DNS code is used, which solves three-dimensional (3D) compressible unsteady Navier–Stokes (NS) equations using high-order finite differences and multi-block structure grid treatment for complex geometry. Jet flow from a square duct, perpendicular to the mainstream flow, is introduced and the flow Reynolds number is 100 based on the jet duct diameter (D) and free-stream quantities. Two-dimensional (2D) calculations using various jet to free-stream velocity ratio (R = V jet/V free) reveals different vortex patterns and a further 3D study continues focusing on a velocity ratio of R = 2, for which complex vortex structure is produced. It is found from the 3D simulation that a counter-rotating vortex pair (CRVP) forms immediately after the jet exits, as observed from the experimental test and reproduced by other numerical simulations. The CRVP originates from the near wall viscous layer and its core position moves away from the wall as it evolves downstream. For the condition simulated, the CRVP is finally weakened (due to viscous diffusion) at about 1.6D downstream from the centre of the jet exit. No asymmetric CRVP has been observed, which was reported by other researchers for high-Reynolds number simulations.