Numerical Modeling of Liquid Jet in Non-Uniform Crossflow Using Enhanced Madabhushi Model

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,

Multi-Scale methodology of breakup and atomization for liquid jets in crossflow

Effect of injector geometry in breakup of liquid jet in crossflow – insights from POD

For gas turbine combustors using liquid fuel injectors, introducing the spray into crossflowing air to achieve superior fuel-air mixing characteristics can improve performance and reduce emissions. Robust numerical design tools are needed to accelerate the development of low-emissions technologies. CFD models are used to assess the design, performance, and emissions of novel combustion systems. Atomization modelling is often facilitated by using the Lagrangian approach with breakup models rather than the more complex and computationally expensive higher-fidelity Volume-of-Fluid (VOF) approach. However, breakup models used in state-of-the-art CFD rely on empirical constants that have not yet been fully calibrated for jets in crossflow or for representative gas turbine conditions. Coupling the Lagrangian and VOF approaches could deliver a better compromise between accuracy and computational cost. Among the few studies conducted on the VOF-Lagrangian approach for jets in crossflow, validation was often limited to the near-field spray trajectory under low Weber numbers using either non-turbulent jets or water at ambient pressure. The predictive capabilities in the far-field region were only validated using deterministic breakup models and for non-turbulent jets under low pressure conditions. This study proposes and validates a novel numerical methodology for coupling the Lagrangian and VOF approaches using a Stochastic Secondary Droplet (SSD) breakup model with Adaptive Mesh Refinements (AMR) for turbulent liquid fuel jets in high-speed crossflow at more representative conditions. The predictions within both near and far-field breakup regions were validated using experimental datasets at high pressures (1–8 bar), Weber numbers (720–1172) and momentum flux ratios (6–33). The predictive capabilities and computational cost were also compared to the Lagrangian approach used with LES and used with an unsteady RANS numerical methodology previously developed and validated by the authors. The effects of different VOF-Lagrangian transition criteria on the computational cost were also assessed and recommendations have been provided for further improvements. The overall predictive capabilities of LES were significantly improved by the novel hybrid methodology proposed by the authors. Although it tends to underpredict the spray trajectory and Sauter Mean Diameter (SMD), it better captures the SMD in the wake region of the jet and the overall droplet velocities compared to the URANS methodology.

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.

High fidelity simulation was carried out by ANYS software to investigate the primary breakup characteristics of liquid jet in crossflow. The influence of nozzles shapes on the breakup features and deformation characteristics of liquid column was analyzed. Air velocity ranged from 40 m/s to 80 m/s and the shape of nozzles included circular, Rectangle-A, and Rectangle-B. The results indicated that, at the higher velocity of crossflow, the surface breakup of liquid column was mainly affected by the air velocity passing through the sides, rather the sharp corner of the rectangular nozzles. As compared with the circular nozzle, the boundary of breakup regime was expanded for Rectangle-A, while it was shrunk for Rectangle-B. In contrast to circular nozzles, the deflection of edges of liquid column was avoided for rectangular nozzles, thus preventing the shrinkage of liquid column during injection. The results of the analysis of the flow structures of crossflow showed that the air velocity in the flow direction of liquid column might be the reason for the difference of breakup regime of nozzles. In addition, the difference of windward trajectory between rectangular nozzles with the different widths was decreased as the air velocity increased.

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

This paper describes an experimental investigation of the spray created by Jet A fuel injection from a plate containing sharp edged orifice 0.018 inches (457 μm) in diameter and L/D ratio of 10 into the crossflow of preheated air (555 K) at elevated pressure in the test section (4 ata) and liquid to air momentum-flux ratio of 40. A 2 component Phase Doppler Particle Analyzer used for measuring the characteristics of the spray. The Weber number of the spray in crossflow was varied between 33 and 2020 and the effect of Weber number on spray properties was investigated. It was seen that shear breakup mechanism dominates at Weber number greater than about 100. Droplets’ diameters were found to be in the range of 15-30 microns for higher values of Weber numbers, while larger droplets (100-200 microns) were observed at Weber number of 33. Larger droplets were observed at the periphery of the spray. The droplet velocities and diameters were measured in a plane 30mm downstream of the orifice along the centerline of the spray at an incoming air flow Mach number of 0.2 and liquid to air momentum-flux ratio of 40. The droplets reach a maximum of 90% of the flow velocity at this location. The velocity of droplets in the directions perpendicular to the air flow direction is higher at the periphery of the spray possibly due to the presence of larger droplets. The RMS values of the droplet velocities are highest slightly off center of the centerline of the spray showing the presence of strong vortices formed due to the liquid jet in crossflow. The data presented here could serve as benchmark data for CFD code validation.

For lean combustion technology, sufficient mixing between fuel and air helps to move away from stoichiometric combustion, the flame temperature of which enormously promotes the production of harmful pollutants. Good fuel-air mixing is also essential to avoid fuel-rich pockets, where unburnt hydrocarbon and soot are likely to be produced. For low emissions gas turbines using liquid fuels, such as diesel, jet in crossflow is one the approaches to achieve efficient liquid jet breakup and evaporation as well as superior mixing characteristics. In order to develop low emissions fuel spray technologies that use transverse jets, it is important that the current numerical tools have the ability to accurately predict the jet breakup, both primary and secondary, evaporation mechanisms, to properly assess the performance of an injector design. However, because the numerical tools often use model constants that are calibrated with experimental data mostly only applicable to a specific range of operating conditions, a particular test fluid or injection method, it is very challenging to assess the accuracy of the predicted results without further experimental validation. To the author’s best knowledge, the breakup mechanism and the predictive capabilities of the breakup models used to guide the design of novel injectors for fuel transverse jets at elevated pressure and high-speed turbulent crossflow have not been reported in full detail. Thus, the main focus of this study is to compare and assess the capabilities of the breakup models used in state-of-the-art CFD, including a stochastic approach that relies less on experimental model constants, to predict the spray characteristics of turbulent jets in crossflow at different pressures, gas Weber numbers and momentum flux ratios that are more representative of gas turbine applications. This study will be conducted by using Simcenter STAR-CCM+. The simulated results will be compared to experimental data to validate and calibrate the breakup models through a parametric and sensitivity study.

PIV and LDV Measurements for Liquid Jets in Crossflow

Trajectory Modeling of Liquid Jets in Crossflow with Solid Obstructions

Liquid jets in crossflow are common ways of delivering large amounts of fuel in a rapidly atomized form to modern aircraft combustor systems. Unfortunately, they are not able to deliver the necessary droplet sizes for the demands of future advanced propulsion systems without modification. In this study, an ultrasonically excited solid obstruction, or pintile, is added to the liquid jet in crossflow system. This addition is intended to introduce additional instabilities in the liquid jet, improving on jet breakup characteristics across a wide range of Weber numbers. This setup is then subjected to a range of crossflow conditions, while maintaining the momentum flux ratio of the jet, and imaged using Mie scatter techniques in both the streamwise and transverse planes. Lagrangian droplet tracking is applied to the streamwise plane to obtain distributions of droplet sizes, velocities, and droplet generation rates to compare the excited and unexcited liquid jets. The sheet breakup distance is tracked in the transverse plane and compared. Across each condition studied, the introduction of the ultrasonically excited pintile led to improvements in the breakup characteristics of the liquid jet. For low Weber conditions, excitation led to a 45% decrease in average droplet size, and a corresponding 380% increase in droplet production rate. The sheet breakup distance was additionally shortened by more than 25% for each case studied, demonstrating substantial improvement to the inactive pintile, proving to be a strong candidate for next generation fuel injection technologies.

Advanced additive manufacturing opens ways to improve gas turbine fuel nozzles to enhance engine durability and performance while reducing gas emissions including NOx, CO, and soot. Additively building complex fuel passages enables engineers to better control the fuel-air mixing and its burning, not only in gas-only but also in dual fuel engine configurations. Dual fuel engines have additional requirements for proper liquid jet trajectory and spray quality and droplet evaporation that needs to be met through the careful injector configuration within the liquid fuel cartridges. Additionally, recent liquid fuel cartridges in the combustor inject a set of discrete liquid jets into a non-uniform cross flow, which can lead to complex interactions between liquid jets and incoming air during the jet breakup and evaporation phases. This can be an engineering challenge during engine development because the classical evaporation models need to be calibrated to get the accurate evaporation profile. This paper investigates the evaporation profile of liquid jets in non-uniform cross flow along the combustor axis numerically and experimentally. Three different crossflow temperatures are simulated and the effect of crossflow temperature on the spray evaporation and residual droplet characteristics is documented. Two different evaporation models, diffusion controlled, and convection/diffusion controlled based evaporation models have been utilized to predict the spray trajectory and spray density after the evaporation. Results were compared with experiments conducted at University of Cincinnati Research Institute (UCRI) with reasonable agreement. The fundamental physics and understanding of spray evaporation can help the combustor engineers to configure liquid fuel injectors that provide a suitable fuel/air mixture distribution for flame stabilization as well as complete combustion with minimal soot and CO emission in gas turbine engines.

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.

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.