Accepted Manuscripts

Davide Laera and Sergio M. Camporeale
J. Eng. Gas Turbines Power   doi: 10.1115/1.4036010
Modern combustion chambers of gas turbines for power generation and aero-engines suffer of thermo-acoustic combustion instabilities generated by the coupling of heat release rate fluctuations with pressure oscillations. The present article reports a numerical analysis of limit cycles arising in a longitudinal combustor. This corresponds to experiments carried out on the LRIA (Longitudinal Rig for Instability Analysis) test facility equipped with a full-scale lean-premixed burner. Heat release rate fluctuations are modeled considering a distributed flame describing function (DFDF) since the flame under analysis is not compact with respect to the wavelengths of the unstable modes recorded experimentally. For each point of the flame, a saturation model is assumed for the gain and the phase of the DFDF with increasing amplitude of velocity fluctuations. A weakly nonlinear stability analysis is performed by combining the DFDF with a Helmholtz solver to determine the limit cycle condition. The numerical approach is used to study two configurations of the rig characterized by different lengths of the combustion chamber. In each configuration, a good match has been found between numerical predictions and experiments in terms of frequency and wave shape of the unstable mode. Time resolved pressure fluctuations in the system plenum and chamber are reconstructed and compared with measurements. A suitable estimate of the limit cycle oscillation is found.
TOPICS: Dynamics (Mechanics), Flames, Combustion, Fluctuations (Physics), Combustion chambers, Limit cycles, Oscillations, Pressure, Heat, Wavelength, Stability, Energy generation, Gas turbines, Numerical analysis, Acoustics, Waves, Shapes, Test facilities, Aircraft engines
Ilias Bosdas, Michel Mansour, Anestis I. Kalfas, Reza S. Abhari and Shigeki Senoo
J. Eng. Gas Turbines Power   doi: 10.1115/1.4036011
The largest share of electricity production worldwide belongs to steam turbines. However, the increase of renewable energy production has led steam turbines to operate under part load conditions and increase in size. As a consequence long rotor blades will generate a relative supersonic flow field at the inlet of the last rotor. This paper presents a unique experiment work that focuses at the top 30% of stator exit in the last stage of an LP steam turbine test facility with coarse droplets and high wetness mass fraction under different operating conditions. The measurements were performed with two novel fast response probes. A fast response probe for three dimensional flow field wet steam measurements and an optical backscatter probe for coarse water droplet measurements ranging from 30 up to 110µm in diameter. This study has shown that the attached bow shock at the rotor leading edge is the main source of inter blade row interactions between the stator and rotor of the last stage. In addition, the measurements showed that coarse droplets are present in the entire stator pitch with larger droplets located at the vicinity of the stator's suction side. Unsteady droplet measurements showed that the coarse water droplets are modulated with the downstream rotor blade-passing period. This set of time-resolved data will be used for in-house CFD code development and validation.
TOPICS: Pressure, Drops, Blades, Steam turbines, Unsteady flow, Airfoils, Rotors, Stators, Probes, Water, Steam, Supersonic flow, Test facilities, Renewable energy, Shock (Mechanics), Computational fluid dynamics, Flow (Dynamics), Suction, Stress, Backscattering
Gregoire Witz, Markus Schaudinn, Joerg Sopka and Tobias Buecklers
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035903
Continuously increasing hot gas temperatures in heavy duty gas turbines lead to increased thermal loadings of the hot gas path materials. Thermal barrier coatings are used to reduce the superalloys temperature and cooling air needs. Until now 6-8 wt% yttria stabilized zirconia is the first choice material for such coatings, but it is slowly reaching its maximum temperature capability due to the phase transformation at high temperature and sintering. New thermal barrier coating material with increased temperature capability enable the next generation of gas turbine with >60% combined cycle efficiency. Such material solutions have been developed through a multi-stage selection process. In a first steps, critical material performance requirements for thermal barrier coating performance have been defined based on the understanding of standard TBC degradation mechanisms. Based on these requirements, more than 30 materials were a pre-selected and evaluated as potential coating materials. After carefully reviewing their properties both from literature data and laboratory test results on raw materials, five materials were selected for coating manufacturing and laboratory testing. Based on the coating manufacturing trials and laboratory test results, two materials have been selected for engine testing, in a first step in GT26 Birr Test Power Plant and afterwards in customer engines. For such tests the original coating thickness has been increased such to achieve coating surface temperature ~100K higher than with a standard thermal barrier coating. Both coatings performed as predicted in both GT26 Birr Test Power Plant and customer engines.
TOPICS: Temperature, Thermal barrier coatings, Coatings, Engines, Manufacturing, Gas turbines, Power stations, Testing, High temperature, Combined cycles, Sintering, Raw materials, Strategic materials, Superalloys, Phase transitions, Cooling
Larry Lebel, Sylvain Turenne and Rachid Boukhili
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035906
This paper presents an experimental procedure developed to simulate the behavior of ceramic matrix composites (CMCs) under the cyclic thermal stresses of a gas turbine combustion chamber. An experimental apparatus was assembled that produces a temperature gradient across the thickness of a CMC specimen while holding the specimen at its two extremities, which simulates the bending stress that would be observed at the center of a combustor panel. Preliminary validation tests were performed in which A-N720 oxide-oxide CMC specimens were heated to a surface temperature of up to 1,160 °C using an infrared heater, which allowed for the calibration of heat losses and material thermal conductivity. The specimen test conditions were compared with predicted conditions in generic annular combustor panels made of the same material. Provided that a more powerful heat source is made available to reach sufficiently high temperatures and through-thickness temperature gradients simultaneously, the proposed experiment promises to allow laboratory observation of representative deterioration modes of a CMC inside an actual combustion chamber.
TOPICS: Simulation, Ceramic matrix composites, Thermal stresses, Combustion chambers, Gas turbines, Temperature gradient, Heat, Temperature, Calibration, Heat losses, High temperature, Thermal conductivity, Bending (Stress)
Ziliang Zheng, Tamer Badawy, Naeim A. Henein, Peter Schihl and Eric Sattler
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035910
Sasol isomerized paraffinic kerosene (IPK) is a coal-derived synthetic fuel under consideration as a blending stock with JP-8 for use in military equipment. However, Sasol IPK is a low ignition quality fuel with derived cetane number (DCN) of 31. The proper use of such alternative fuels in internal combustion engines (ICEs) requires the modification in control strategies to operate engines efficiently. With computational cycle simulation coupled with surrogate fuel mechanism, the engine development process is proved to be very effective. Therefore, a methodology to formulate Sasol IPK surrogate fuels for diesel engine application using Ignition Quality Tester (IQT) is developed. An in-house developed MATLAB code is used to formulate the appropriate mixture blends, also known as surrogate fuel. And Aspen HYSYS is used to emulate the distillation curve of the surrogate fuels. The properties of the surrogate fuels are compared to those of the target Sasol IPK fuel. The DCNs of surrogate fuels are measured in the IQT and compared with the target Sasol IPK fuel at the standard condition. Furthermore, the ignition delay, combustion gas pressure, and rate of heat release of Sasol IPK and its formulated surrogate fuels are analyzed and compared at five different charge temperatures. In addition, the apparent activation energies derived from chemical ignition delay of the surrogate fuel and Sasol IPK are determined and compared.
TOPICS: Fuels, Diesel engines, Ignition, Ignition delay, Engines, Internal combustion engines, Cycles, Pressure, Heat, Temperature, Combustion gases, Synthetic fuels, Simulation, Coal, Matlab, Military systems
Hua Xiao and Agustin Valera-Medina
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035911
To explore the potential of ammonia-based fuel as an alternative fuel for future power generation, studies involving robust mathematical, chemical, thermofluidic analyses are required to progress towards industrial implementation. Thus, the aim of this study is to identify reaction mechanisms that accurately represents ammonia kinetics over a large range of conditions, particularly at industrial conditions. To comprehensively evaluate the performance of the chemical mechanisms, 12 mechanisms are tested in terms of flame speed, NOx emissions and ignition delay against experimental data. Freely propagating flame calculations indicate that Mathieu mechanism yields the best agreement within experimental data range of different ammonia concentrations, equivalence ratios and pressures. Ignition delay times calculations show that Mathieu mechanism and Tian mechanism yield the best agreement with data from shock tube experiments at pressures up to 30 atm. Sensitivity analyses were performed in to identify reactions and ranges of conditions that require optimization in future mechanism development. The present study suggests that the Mathieu mechanism and Tian mechanism are the best suited for the further study on ammonia/hydrogen combustion chemistry under practical industrial conditions. The results obtained in this study also allow gas turbine designers and modelers to choose the most suitable mechanism for combustion studies.
TOPICS: Combustion, Gas turbines, Hydrogen fuels, Ignition delay, Fuels, Flames, Hydrogen, Sensitivity analysis, Shock tubes, Nitrogen oxides, Emissions, Energy generation, Optimization, Chemistry
Valentina Futoryanova
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035914
One of the common failure modes of the diesel-engine turbochargers is high-cycle fatigue of the turbine-wheel blades. Mistuning of the blades due to the casting process is believed to contribute to this failure mode. The turbine-wheel designs under investigation is radial and is typically used in 6-12L diesel engine applications. A continuous twistedblade model is developed in Matlab using finite-element techniques. The model is tested and validated against different symmetrical cases as well as ABAQUS results. It is proposed to use this model for statistical mistuning studies of turbine wheels in the product design stages.
TOPICS: Turbines, Blades, Wheels, Diesel engines, Failure mechanisms, Finite element analysis, Casting, Symmetry (Physics), Turbochargers, Matlab, Product design, High cycle fatigue
Ding Xi Wang and Xiuquan Huang
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035912
The paper presents an efficient approach for stabilizing solution and accelerating convergence of a harmonic balance equation system for turbomachinery flutter and blade row interaction. The proposed approach combines the Runge-Kutta method with the Lower Upper Symmetric Gauss Seidel (LU-SGS) method and the block Jacobi method. The LU-SGS method, different from its original application as an implicit time marching scheme, is used as an implicit residual smoother with under relaxation, allowing big CFL numbers (in order of 100s), within the Runge-Kutta explicit time marching loops. The block Jacobi method is introduced to implicitly integrate the time spectral source terms of a harmonic balance equation system, in order to reduce the complexity of the direct implicit time integration by the LU-SGS method. The implicit treatment of the time spectral source term by the block Jacobi method greatly augments the stability region of a harmonic balance equation system with grid reduced frequency well above 10. Validation of the harmonic balance flow solver was carried out using linear cascade test data. Flutter analysis of a transonic rotor and blade row interaction analyses for a transonic compressor stage were presented to demonstrate the stabilization and acceleration effect by the combination of the LU-SGS and the block Jacobi methods. The influence of the number of Jacobi iterations on solution convergence is also investigated showing that the two Jacobi iterations are sufficient for stability purpose, which is much more efficient than existing methods of its kind in the open literature.
TOPICS: Stability, Flow (Dynamics), Compressors, Relaxation (Physics), Cascades (Fluid dynamics), Flutter (Aerodynamics), Rotors, Blades, Runge-Kutta methods, Turbomachinery
Daejong Kim
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035913
The equation (7) in the original paper had a typo. However, all the discretizations and simulations were done properly using the correct equations and results do not change
TOPICS: Simulation, Design, Engineering simulation, Thrust bearings
Mateusz Golebiowski, John P.C.W. Ling, Eric Knopf and Andreas Niedermeyer
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035893
This article presents the application of statistical methods to guide the rotordynamic design of a turbo-generator shaft-line. One of the basic requirements is all shaft components must survive the event of a short circuit at the terminals of the generator. This is typically assessed via a transient response simulation of the complete machine train (including generator’s electrical model) to check the calculated response torque against the allowable value. With an increasing demand of a shorter design cycle and competition in performance, cost, foot-print and safety, the probabilistic approach is starting to play an important role in the power train design process. The main challenge arises with the size of the design space and complexity of its mapping onto multiple objective functions and criteria which are defined for different machines. In the presented paper, the authors give an example demonstrating the use of statistical methods to explore (Design of Experiment) and understand (Surface Response Methods) the design space of the Combined Cycle Power Train with respect to the typically most severe constraint (fault torque torsional response), which leads to a quicker definition of a turbo-generator’s arrangement. Further statistical analyses are carried out to understand the robustness of the chosen design against future modifications as well as parameters’ uncertainties.
TOPICS: Design, Cycles, Robustness, Trains, Combined cycles, Torque, Machinery, Safety, Turbogenerators, Simulation, Turbochargers, Transients (Dynamics), Generators, Circuits, Uncertainty, Statistical analysis
Carlos F. Montes and Roger L. Davis
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035848
The thrust vectoring performance of a novel nozzle mechanism was numerically investigated. The nozzle was designed for supersonic, air-breathing engines using published engine data, isentropic relationships, and piecewise quartic splines. The mechanism utilizes two staggered, adjustable ramps. A baseline inviscid numerical simulation without ramps verified the nozzle design by comparing the results to analytical data. Nine ramp configurations were analyzed under steady-state turbulent viscous conditions, using two sets of inlet parameters corresponding to inlet conditions with and without an afterburner. The realizable k-e model was used to model the turbulence field. Area-weighted integrals of the exit flow showed superior flow deflection with the non-afterburning inlet flow parameters. Calculations of the mean flow deflection angles showed that the flow can be deflected as much as 30° in a given direction with the largest ramp length and angle values. The smallest ramp length (less than 5% of the nozzle length) demonstrated as much as 21° in flow deflection.
TOPICS: Thrust, Flow (Dynamics), Nozzles, Deflection, Turbulence, Computer simulation, Engines, Steady state, Air-breathing engines, Splines, Design
H. Lian, J. B. Martz, B. P. Maldonado, A. G. Stefanopoulou, K. Zaseck, J. Wilkie, O. Nitulescu and M. Ehara
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035849
Diluting Spark-Ignited (SI) stoichiometric combustion engines with excess residual gas improves thermal efficiency, and allows spark to be advanced towards Maximum Brake Torque (MBT) timing. However, flame propagation rates decrease and misfires can occur at high Exhaust Gas Recirculation (EGR) conditions and advanced spark, limiting the maximum level of charge dilution and its benefits. The misfire limits are often determined for a specific engine from extensive experiments covering a large range of speed, torque and actuator settings. To extend the benefits of dilute combustion while at the misfire limit, it is essential to define a parameterizable, physics-based model capable of predicting the misfire limits, with cycle to cycle varied flame burning velocity as operating conditions change based on driver demand. A cycle averaged model is the first step in this process. The current work describes a model of cycle averaged laminar flame burning velocity within the early flame development period of 0 to 3 percent mass fraction burned. A flame curvature correction method is used to account for both the effect of flame stretch and ignition characteristics, in a variable volume engine system. Comparison of the predicted and the measured flame velocity was performed using a spark plug with fiber optical access. The comparison at a small set of spark and EGR settings at fixed load and speed, shows an agreement within 30% of uncertainty, while 20% uncertainty equals ± one standard deviation over 2,000 cycles.
TOPICS: Combustion, Flames, Exhaust gas recirculation, Cycles, Engines, Torque, Uncertainty, Brakes, Thermal efficiency, Physics, Fibers, Stress, Actuators, Ignition
Sebastian Bahamonde, Matteo Pini, Carlo De Servi, Antonio Rubino and Piero Colonna
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035841
Widespread adoption of renewable energy technologies will arguably benefit from the availability of economically viable distributed thermal power conversion systems. For this reason, considerable efforts have been dedicated in recent years to R\&D over mini organic Rankine cycle (ORC) power plants, thus with a power capacity approximately in the 3–50 kW range. The application of these systems for waste heat recovery from diesel engines of long-haul trucks stands out because of the possibility of achieving economy of production. Many technical challenges need to be solved, as the system must be sufficiently efficient, light and compact. The design paradigm is therefore completely different from that of conventional stationary ORC power plants of much larger capacity. A high speed turbine is arguably the expander of choice, if high conversion efficiency is targeted, thus high maximum cycle temperature. Given the lack of knowledge on the design of these turbines, which depends on a large number of constraints, a novel optimal design method integrating the preliminary design of the thermodynamic cycle and that of the turbine has been developed. The method is applicable to radial inflow, axial and radial outflow turbines, and to superheated and supercritical cycle configurations...
TOPICS: Fluids, Design, Turbines, High temperature, Organic Rankine cycle, Cycles, Power stations, Temperature, Design methodology, Economics , Thermal energy, Heat recovery, Thermodynamic cycles, Diesel engines, Renewable energy, Superheating, Trucks, Inflow, Outflow
Stefan Bauer, Balbina Hampel and Thomas Sattelmayer
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035842
The configuration investgated in this study is a tubular air passage with fuel injection from one single orifice placed in the side wall. In the range of typical gas turbine combustor inlet temperatures, the performance vortex generator premixers have already been investigated for natural gas as well as for blends. For highly reactive fuels, the application of VGPs in recuperated gas turbines is particularly challenging because the high combustor inlet temperature leads to potential risk with regard to premature self-ignition and flame flashback. An atmospheric combustion VGP test rig has been designed, which facilitates investigations in a wide range of operating conditions in order to comply with the situation in recuperated micro gas turbines, namely global equivalence ratios between 0.2 and 0.7, air preheating temperatures between 288K and 1100K, and air bulk flow rates between 6-16 g/s. Both the entire mixing zone in the VGP and the primary combustion zone of the test rig are optically accessible. High speed OH* chemiluminescence imaging is used for the detection of the flashback and blow-off limits of the investigated VGPs. Flashback and blow-off limits of hydrogen in a wide temperature range covering the auto-ignition regime are presented, addressing the influences of equivalence ratio, air preheating temperature and momentum ratio between air and hydrogen on the operational limits in terms of bulk flow velocity. It is shown that flashback and blow-off limits are increasingly influenced by auto-ignition in the ultra-high temperature regime.
TOPICS: Ejectors, Gas turbines, Generators, Hydrogen, Vortices, Temperature, Ignition, Combustion chambers, Combustion, Fuels, Flow (Dynamics), Momentum, Natural gas, Chemiluminescence, Flames, Imaging, Micro gas turbines, Risk
Andrew Marshall, Julia Lundrigan, Prabhakar Venkateswaran, Jerry Seitzman and Tim Lieuwen
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035819
Fuel composition has a strong influence on the turbulent flame speed, even at very high turbulence intensities. An important implication of this result is that the turbulent flame speed cannot be extrapolated from one fuel to the next using only the laminar flame speed and turbulence intensity as scaling variables. This paper presents curvature and tangential strain rate statistics of premixed turbulent flames for high hydrogen content fuels. Global (unconditioned) stretch statistics are presented as well as measurements conditioned on the leading points of the flame front. These measurements are motivated by previous experimental and theoretical work that suggests the turbulent flame speed is controlled by the flame front characteristics at these points. The data were acquired with high speed particle image velocimetry (PIV) in a low swirl burner (LSB). We attained measurements for several H2:CO mixtures over a range of mean flow velocities and turbulence intensities. The results show that fuel composition has a systematic, yet weak effect on curvatures and tangential strain rates at the leading points. Instead, stretch statistics at the leading points are more strongly influenced by mean flow velocity and turbulence level. It has been argued that the increased turbulent flame speeds seen with increasing hydrogen content are the result of increasing flame stretch rates, and therefore SL,max values, at the flame leading points. However, the differences observed with changing fuel compositions are not significant enough to support this hypothesis. Additional analysis is needed to understand the physical mechanisms through which the turbulent flame speed is altered by fuel composition effects.
TOPICS: Fuels, Flames, Hydrogen, Statistics as topic, Turbulence, Flow (Dynamics), Particulate matter
Aron P. Dobos and Allan T. Kirkpatrick
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035820
This paper presents an efficient approach to diesel engine combustion simulation that integrates detailed chemical kinetics into a quasidimensional fuel spray model. The model combines a discrete spray parcel concept to calculate fuel-air mixing with a detailed primary reference fuel chemical kinetic mechanism to determine species concentrations and heat release in time. Comparison of predicted pressure, heat release, and emissions with data from diesel engine experiments reported in the literature shows good agreement overall, and suggests that spray combustion processes can be predictively modeled without calibration of empirical burn rate constants at a significantly lower computational cost than standard multidimensional (CFD) tools.
TOPICS: Chemical kinetics, Combustion, Modeling, Diesel, Sprays, Fuels, Heat, Diesel engines, Emissions, Pressure, Calibration, Simulation, Computational fluid dynamics
Feijia Yin and Arvind Gangoli Rao
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035821
This paper focuses on the off-design performance of a turbofan engine with an Interstage Turbine Burner (ITB). The ITB is an additional combustion chamber located between the High-Pressure Turbine (HPT) and the Low-Pressure Turbine (LPT). The incorporation of ITB in an engine can provide several advantages, especially due to the reduction in the HPT inlet temperature and the associated NOx emission reduction. The objective is to evaluate the effects of the ITB on the off-design performance of a turbofan engine. The baseline engine is a contemporary classical turbofan. The effects of the ITB are evaluated on two aspects: firstly, the influences of an ITB on the engine cycle performance; secondly, the influences of an ITB on the component characteristics. The dual combustors of an ITB engine provide an extra degree of freedom for the engine operation. The analysis shows that a conventional engine has to be oversized to satisfy the off-design performance, for instance, the flat rating temperature. However, the application of an ITB eases the restrictions imposed by the off-design performance requirements on the engine design, implying that the off-design performance of an ITB engine can be satisfied without sacrificing the fuel efficiency. Eventually, the performance of the ITB engine exhibits superior characteristics over the baseline engine at the studied operating points over a flight mission.
TOPICS: Engines, Design, Turbines, Turbofans, Combustion chambers, Temperature, Pressure, High pressure (Physics), Cycles, Engine design, Flight, Nitrogen oxides, Degrees of freedom, Fuel efficiency, Emissions
Klaus Brun, Sarah Simons and Rainer Kurz
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035822
Pressure pulsations into a centrifugal compressor can move its operating point into surge. This is concerning in pipeline stations where centrifugal compressors operate in series/parallel with reciprocating compressors. [1], [2], and [3] provided predictions on the impact of periodic pressure pulsation on the behavior of a centrifugal compressor. This interaction is known as the "Compressor Dynamic Response" (CDR) theory. Although the CDR describes the impact of the nearby piping system on the compressor surge and pulsation amplification, it has limited usefulness as a quantitative analysis tool, due to the lack of prediction tools and test data for comparison. Testing of compressor mixed operation was performed in an air loop to quantify the impact of periodic pressure pulsation from a reciprocating compressor on the surge margin of a centrifugal compressor. This data was utilized to validate predictions from Sparks' CDR theory and Brun's numerical approach. A 50 hp single-stage, double-acting reciprocating compressor provided inlet pulsations into a two-stage 700 hp centrifugal compressor. Tests were performed over a range of pulsation excitation amplitudes, frequencies, and pipe geometry variations to determine the impact of piping impedance and resonance responses. Results provided clear evidence that pulsations can reduce the surge margin of centrifugal compressors and that geometry of the piping system immediately upstream and downstream of a centrifugal compressor will have an impact on the surge margin reduction. Surge margin reductions of over 30% were observed for high centrifugal compressor inlet suction pulsation.
TOPICS: Compressors, Surges, Pressure, Pipes, Geometry, Piping systems, Testing, Dynamic response, Suction, Pipelines, Excitation, Resonance
Liwei Zhang and Vigor Yang
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035808
A Large-Eddy-Simulation-based numerical investigation of a turbulent gaseous jet in crossflow is presented. The present work focuses on cases with a steady crossflow and two different jet-to-crossflow velocity ratios, 2 and 4, at the same jet centerline velocity of 160 m/s. Emphasis is placed on the detailed flow evolution and scalar mixing in a compressible, turbulent environment. Various flow characteristics, including jet trajectories, jet-center streamlines, vortical structures, and intrinsic instabilities, as well as their relationships with the mixing process, are examined. Mixing efficiency is quantified by the decay rate of scalar concentration, the probability density function, and the spatial and temporal mixing deficiencies. Depending on the jet-to-crossflow velocity ratios, the wake vortices downstream of the injector orifice can either separate from or connect to the main jet plume, and this has a strong impact on mixing efficiency and vortex system development. Statistical analysis is applied to explore the underlying physics, with special attention at the jet-center and transverse planes.
TOPICS: Flow (Dynamics), Turbulence, Scalars, Physics, Density, Eddies (Fluid dynamics), Simulation, Plumes (Fluid dynamics), Wake turbulence, Ejectors, Vortices, Probability, Statistical analysis
Liwei Zhang and Vigor Yang
J. Eng. Gas Turbines Power   doi: 10.1115/1.4035809
The present work extends Part I of our study to investigate the flow dynamics and scalar mixing of a turbulent gaseous jet in an oscillating crossflow. Attention is first given to intrinsic flow instabilities under a steady condition. Both power spectral density and proper orthogonal decomposition analyses are applied. For the case with a jet-to-crossflow velocity ratio of 4, the two most dynamic modes, corresponding to jet Strouhal numbers of around 0.1 and 0.7, are identified as being closely linked to the shear-layer vortices near the injector orifice and the vertical movement in the jet wake region, respectively. The effect of oscillation imposed externally in the upstream region of the crossflow is also examined systemically at a jet-to-crossflow velocity ratio of 4. A broad range of forcing frequencies and amplitudes are considered. Results reveal that the dominant structures observed in the case with a steady crossflow are suppressed by the harmonic excitations. Flapping-detaching motions, bearing the forcing frequencies and their subharmonics, become dominant as the forcing amplitude increases. The ensuing flow motions lead to the formation of a long, narrow jet plume and a relatively low mixing zone, which substantially alters the mixing efficiencies as compared to the case with a steady crossflow.
TOPICS: Flow (Dynamics), Jets, Bearings, Ejectors, Flow instability, Vortices, Principal component analysis, Excitation, Turbulence, Plumes (Fluid dynamics), Shear (Mechanics), Spectral energy distribution, Wakes, Scalars, Oscillations

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