Rohan Gejji

A Parametric Study of Combustion Dynamics in a Single-Element Lean Direct Injection Gas Turbine Combustor: Part II: Experimental Investigation

The current paper is an account of the experimental investigation of a model high pressure lean direct injection (LDI) combustor that exhibits self-excited combustion dynamics. The combustor uses a single nozzle that injects liquid fuel into a subsonic venturi section to rapidly atomize and distribute fuel, and a modular design that allows discrete changes in geometry between tests so that a range of unstable frequencies and amplitudes can be measured. A target frequency of a few hundred Hz and target amplitudes of about 2% of mean pressure was obtained with a configuration comprising a 3/8-wave air plenum and 1/2-wave combustor geometry. In this paper, results from a broader parametric investigation of the combustor with varying geometry, inlet air temperature and equivalence ratio is presented, along with a more focused study of the configuration presenting the target characteristics. In the latter study, higher equivalence ratios (> 0.45) show higher pressure fluctuation amplitudes with a dominant 4L mode in the combustor whereas, at lower equivalence ratios, the instability amplitudes are lower with energy spread across the first few modes in the combustor. In a concurrent study, the experimental data are compared with high fidelity simulations, which showed excellent agreement with mode shapes and reasonable agreement with mode amplitude. The simulation indicated presence of complex interactions between the combustor acoustic modes, heat release modes, and a hydrodynamic precessing vortex core (PVC) mode similar to what has been observed in lowpressure, premixed gas systems, indicating the LDI concept is successful in its goal of rapidly distributing the liquid fuel, and providing direction for future study.

Computational Study of Combustion Dynamics in a Single-Element Lean Direct Injection Gas Turbine Combustor

Abstract : Simulations of self-excited combustion instabilities in a model configuration of a lean direct injection (LDI) gas turbine combustor were performed and investigated with different operating conditions (air temperature and equivalence ratio). Concurrently, experimental data were obtained at the same conditions in a well-instrumented test combustor with the same configuration to validate the simulation results. The simulations are used to investigate the coupling between the acoustic and heat release modes and the important flow dynamics to understand the physics that lead to combustion instabilities in the LDI combustor. A Precessing Vortex Core (PVC) hydrodynamic instability was found to be significant in driving spray and flame responses. Detailed and systematic studies of the PVC instability are also performed using non-reacting simulations of an acoustically-open combustor to minimize the acoustic and combustion effects on the flow field.

Computational Investigation of Combustion Dynamics in a Lean-Direct Injection Gas Turbine Combustor

Abstract : Combustion dynamics is investigated using an integrated computational/experimental approach for a laboratory-scale, single-element lean direct injection model combustor in which self-excited pressure oscillations are produced. The present study focuses on physics based computational simulations that fully describe the turbulence, spray, combustion and acoustics phenomena in the combustion chamber. Baseline three-dimensional results at an equivalence ratio = 0.47 confirm the self-excitation of acoustic modes in the chamber and also indicate the presence of precessing vortex core instabilities. Preliminary comparisons of the pressure oscillations with experimental measurements are also presented. Further, the effects of multi-dimensionality, equivalence ratio and secondary atomization are computationally investigated. In contrast to the 3D simulations, two-dimensional models capture the pressure oscillations with reasonably similar amplitudes, but show inherent limitations in describing the vortex breakdown process. Pressure oscillations are also shown to be intensified when the equivalence ratio is increased and damped when the secondary atomization effects are included.

Computational investigation of combustion instabilities in a laboratory-scale LDI gas turbine engine

Abstract : Self-excited combustion instabilities in a lean direct injection (LDI) gas turbine combustor are computationally investigated. The model LDI combustor under study was developed to produce combustion dynamics on demand using a single LDI element in an axisymmetric combustor. Three simulation cases for this combustor are considered: non-reacting and reacting flow in an acoustically open combustor and reacting flow in an acoustically closed combustor. We studied the dynamic flow features in the LDI combustor for both cases and investigated the important modes using a dynamic mode decomposition method. Precessing vortex core (PVC) instabilities are indicated as the critical hydrodynamic mode and lead to strong spray and flame response. In the acoustically close chamber simulation, we were able to capture self-excited combustion instabilities and the dominant modes from simulations, which qualitatively agree with the experimental results. The appearance of pressure peaks in both simulation and experiment at about 1400 Hz corresponding to the 4L acoustic mode and at 6000 Hz are explained by the nonlinear coupling of the PVC and acoustics modes and the associated feedback loop.

Effect of Aviation Fuel Type and Fuel Injection Conditions on Non-reacting Spray Characteristics of a Hybrid Airblast Fuel Injector

I njector spray characteristics have a significant influence on the combustion performance in a gas turbine engine, including an impact on dynamics, emissions and component life. Furthermore, commercial aviation faces fuel cost, environmental, and energy security challenges that arise from the use of petroleum based jet fuels. Sustainable alternative jet fuels can help address these challenges and need to be characterized in their spray performance. The present work describes the detailed characterization of several alternative fuels using a hybrid airblast atomizer on the basis of spray shape, droplet size and velocity distribution at a range of operating conditions including fuel temperature, injector pressure drop and spray chamber pressure and temperature. The characterization is done using optical patternation, phase Doppler anemometry (dual-PDPA) and high speed back-lit imaging. The measurements obtained as part of this work provide the validation data-set for computational modeling of the spray behavior which forms a critical part of the broader project. The results show a strong influence of the fuel temperature on the spray, with lower temperature (290 K to 240 K) decreasing the atomization quality by 14%, while the effect of fuel injection pressure on the spray is minimal. A large effect of pressure drop across the injector is seen on the spray, with a change from 2% to 6% leading to a decrease in drop size of up to 36%, which can result of a shift in the secondary breakup regime of the spray.

Combustion Dynamics Behavior in a Single-Element Lean Direct Injection (LDI) Gas Turbine Combustor

Abstract : A concurrent computational and experimental study of self-excited combustion dynamics in a model configuration of a lean direct injection (LDI) gas turbine combustor is described. Incoming air temperature and equivalence ratio were varied. Simulation at low equivalence ratio compared better with measurement and thus this condition was selected for a more detailed study of the underlying combustion dynamics mechanisms. First, hydrodynamic modes are investigated by conducting the simulation with an acoustically-open combustor so that acoustic effects on the flow field are minimized. The Vortex Breakdown Bubble (VBB) proves to be an important flow structure that can easily interact with the acoustic field to sustain instability. Second, detailed cycle studies of the acoustically closed combustor simulation reveals enhanced mixing and vaporization of the JP-8 fuel spray due to acoustic compression wave. Dynamic Mode Decomposition (DMD) analysis is used to identify the coupling between axial acoustics and the vortex breakdown bubble in the lower frequency region. Presence of another important hydrodynamic mode, the Precessing Vortex Core (PVC) is also identified from the DMD analysis. The possibility of nonlinear coupling between the acoustics and PVC modes is indicated.

Design and Testing of a High Pressure and High Temperature, Optically Accessible, Entrained Flow Coal Gasifier

Improved understanding of coal gasification chemical kinetics is needed to increase thermodynamic efficiency and to reduce undesirable CO2 emissions. This work describes an optically-accessible entrained-flow coal gasifier designed and built to allow measurements of the major species at various stages of the chemical reactions. The 2-meter tall gasifier consists of five subsystems: the optical diagnostics, steam generator, coal feeder, external heaters, and gas sampling and analysis. A stoichiometric H2 -O2 flame generates superheated steam, the gasifying agent, which reacts with pulverized coal fed from a variable feed-rate pressurized powder feeder. To sustain the endothermic coal gasification reaction, radiant heaters provide 15 kW of external heating. Diagnostics to determine the major species concentrations consist of tunable diode laser absorption spectroscopy (TDLAS) measurements within the reactor vessel assembly and analysis of dry product gases using a gas chromatograph.Copyright

An Experimental Study of a H 2 -O 2 Flame based Superheated Steam Generator for Coal Gasification

Chemical steam generation or direct steam generation has been extensively used over several decades for various applications including chemical processing and rocket propulsion. It carries the advantage of minimal heat loss during energy transfer between the heat source and water or steam. The hydrogen-oxygen (H2-O2) combustion reaction is one way steam has been generated chemically. Subject to availability of hydrogen and oxygen, steam generation using direct H2-O2 combustion offers advantages such as zero pollutant emissions, high thermodynamic efficiency, and high specific thermal power output. This paper is an account of the design and an experimental study of a 40 kW steam generator for application to coal and biomass gasification. Near stoichiometric combustion of H2-O2 is carried out in combustion chamber at a design pressure of 1.5 MPa and a peak vapor temperature of 3500 K. The heat of the reaction is used to produce additional steam by direct but staged injection of high pressure liquid water into the post-combustion zone to achieve a range of gasification process temperatures (1100 - 1400 K). The design of the present steam generator includes appropriate ignitor location and timing and staging and mixing of the H2, O2, and H2O streams. The experiments results of pressure and temperature obtained from the operation of the steam generator show close agreement with equilibrium calculations Steady 2-D axisymmetric calculation was performed to understand the profiles of scalars inside the steam generator. Detailed simulations show close agreement with the equilibrium calculation and experimental results.

Parametric investigation of combustion instabilities in a single-element lean direct injection combustor

Self-excited combustion dynamics in a liquid-fueled lean direct injection combustor at high pressure (1 MPa) are described. Studied variables include combustor and air plenum length, inlet air temperature, equivalence ratio, fuel nozzle location, and fuel composition. Measured pressure oscillations were dependent on combustor geometry and ranged from about 1% of mean chamber pressure at low equivalence ratio, up to 20% at high equivalence ratio. In the most unstable cases, strong pressure modes were measured throughout the frequency spectrum including a band around 1.2–1.5 kHz representing the 4th longitudinal mode, and another band around 7 kHz. The oscillation amplitudes have a non-monotonic dependency on air temperature, and are affected by the placement of the fuel nozzle relative to the throat of the subsonic swirling air flow. The parametric survey provides a rich dataset suitable for validating high-fidelity simulations and their subsequent use in analyzing and interpreting the complex combustion dynamics.

A model combustor for studying a reacting jet in an oscillating crossflow

This paper discusses a novel model combustion experiment that was built for studying the structure and dynamics of a reacting jet in an unsteady crossflow. A natural-gas-fired dump combustor is used to generate and sustain an acoustically oscillating vitiated flow that serves as the crossflow for transverse jet injection. Unlike most other techniques that are limited in operating pressure or acoustic amplitude, this method of generating an unsteady flow field is demonstrated at a pressure of 10 atm with peak-to-peak oscillation amplitudes approaching 20% of the mean pressure. An optically accessible test section designed for these conditions provides access for advanced laser and optical diagnostic measurements. Detailed measurements provide insight into the complex acoustic-hydrodynamic-combustion coupling processes and offer high-quality, high-resolution validation data for numerical simulations. Careful instrumentation port design considerations for the higher amplitude acoustics are detailed. As a whole, this paper focuses on select representative segments of the experiment operational space that highlight our strategy of providing an oscillatory flowfield. This includes presenting the acoustic operational space such as acoustic amplitudes, frequencies, and mode shapes. Select imaging results are then reported to support our strategies capability to produce high-fidelity measurements.