Ibrahim Yimer

The strong-jet/weak-jet problem: new experiments and CFD

Abstract The strong-jet/weak-jet coupling is basic to a new generation of ultra-low-NOx burners that are diffusion type and rely on lower flame temperatures through controlled mixing of the air, the fuel, and recirculated combustion products. Further, to developing a theory on the isolated jet pair in a previous publication, where the jet trajectories were predicted and compared with experimental data with good agreement, new experimental results of cold modeling by using flow visualization and laser Doppler velocimetry (LDV) are reported in this paper. Mathematical modeling by using a commercial computational fluid dynamics code (CFD) was employed to predict the flow field. CFD is also used to predict a mixing progress variable, R12 for the three stream mixing of air, fuel, and combustion products for the first time. Flow visualization reveals very interesting behavior of large scale structures and engulfment as the fuel and combustion air interact and mix. The measured mean and root-mean-squared velocities on the plane of bilateral symmetry are compared with the CFD predictions. The profiles across the jet cross-sections and the behavior along the air jet axis closely follow the experimental results, however, the fuel jet trajectory is poorly predicted. Constraints in the model due to the assumption of normal or well-developed turbulence spectrum in concentration fluctuations are highlighted.

Flow Downstream of a Cluster of Nine Jets

The authors gratefully acknowledge the support of the Natural Sciences and Engineering Research Council of Canada NSERC , the Ontario Graduate Scholarship Program OGS , and the National Research Council of Canada NRC . The experiments were done at the Aerodynamics Laboratory, Institute for Aerospace Research at the National Research Council, Ottawa. The nozzle used in the investigation was provided by Parker Hannifin-USA.

Experimental Characterization of the Damping of Fuel-Air Ratio Fluctuations Using Transfer Function Analysis

This paper presents the theoretical and experimental framework used to characterize the capability of premixers used in Dry Low Emission (DLE) gas turbines to dampen fuel-to-air ratio (FAR) oscillations and thus serve as a passive control device for combustion noise. Based on a convection-diffusion volume model, transfer function analysis in the frequency-domain was used to describe the interaction between convection and turbulent diffusion mechanisms. The study showed that the best achievable damping was obtained when the ratio of convection to turbulent diffusion effects (expressed in terms of Peclet number) was unity. For this particular condition, the spreading of Residence Time Distribution (RTD) is optimal hence decreasing the coherence between incoming and outgoing perturbations. For large Peclet numbers, mixing mechanisms are not sufficient to dampen incoming FAR fluctuations and for very small Peclet numbers FAR perturbations can be communicated almost instantaneously to the premixer outlet, without attenuation.Copyright

Combustion Emission Measurements With Preheated Jet Fuel

Jet fuel thermal stability at high temperature is receiving increased attention recently as advanced aero engines are being pushed to high power, high pressure and temperature regimes for improved engine cycle performance and low emissions. This paper describes the rig experimental tests to assess the high fuel temperature effect on combustor emissions. A special test rig facility has been designed and set up for emission measurements with preheated fuel. The purpose of the tests is to evaluate the combustor emission characteristics under nominal and elevated fuel temperatures. The scope of the project is two fold: (1) to design, procure and establish a dedicated hot fuel deoxygenation, fuel preheat facility that can reach temperature up to 600 °F (589 K); (2) to measure combustion emissions, mainly NOx, CO and UHC, at normal and elevated fuel temperature under representative engine operating conditions. The test rig has run for extended duration and proved reliable over the whole test campaign. Measured emission results show that fuel temperature effect on NOx, CO, UHC emissions are marginal, possibly due to the low emission capability of the sector combustor that is less sensitive to fuel inlet condition changes than other combustor designs. These results indicate a manageable risk for engine development with elevated fuel temperature from the emission viewpoint.Copyright

Optimizing Mixing for Maximum Damping of Fuel-Air Ratio Oscillations in Gas Turbine Premixers

An analytical model based on advection-diffusion volume is analyzed with the objective to investigate the limits of achievable damping in fuel-air ratio oscillations over the range of frequencies at which combustor thermoacoustic instabilities are normally encountered. Results show that there exists an optimum degree of diffusion that will allow maximum damping in fuel-air ratio fluctuations. The upper bound on damping is found to be dependent on a constant value of a new non-dimensional number, defined as a ratio between Peclet Number and Strouhal Number. The analysis presented here is considered useful to evaluate the extent of damping that is inherently obtainable in practical premixers. Also presented are the results of experimental investigations, which were conducted to verify the analytically predicted behavior of premixers. The measurements were performed over a selected range of frequencies using a premixer configuration with square cross section and with fuel jet issuing in a co-flowing bulk air stream. The effects of modulation amplitude and frequency, and pressure drop across the premixer on the damping effectiveness of the premixer were examined. Results show that for the test conditions considered, the modulation amplitude has a profound effect on the premixer performance, whereas pressure drop effects are insignificant.Copyright

Study of Mixing Enhancement From a 12-Lobe Convoluted Mixer

Lobed mixers have been used in a variety of engineering applications, such as jet noise reduction, infrared suppression and improvement of propulsive efficiency for turbine engines. More recently, they have emerged as an attractive method to enhance mixing between fuel and air in advanced low-emission gas turbine combustors. The objectives of the present work were to assess the effectiveness of these devices for use inside the combustor and provide experimental data to validate CFD predictions. The mixing enhancements due to streamwise vortices generated from a 12-lobe convoluted mixer were characterized using Planar Laser Induced Fluorescence (PLIF) measurements, while 2-D PIV measurements established the underlying velocity field. The geometrical set-up of the mixing system is pertinent to many combustion systems using advanced lean premixed concepts with gaseous fuels. In addition to the benchmark case with no mixer, two different lobe geometries were considered, a semi-circle (or round) lobe and a square lobe. In this paper the experimental results are presented and discussed. Numerical predictions were performed for the semi-circle lobe geometry using a Reynolds-averaged Navier-Stokes (RANS) code and the results are compared with experimental measurements.© 2003 ASME

Two Phase Flow Downstream of a Multipoint Lean Direct Injection Low NO x Nozzle

This work examines the flow downstream of a Parker Hannifin low NOx LDI nozzle. The nozzle is a square matrix of 3 × 3 airblast simplex fuel ports. The air pressure drop was set to 5%, for a Reynolds number of 40,000. Liquid injection pressure was 2.28kPa. The nozzle is tested at atmospheric conditions without combustion. The objective of this work is twofold: first characterize the spray and the turbulent flow fields; and second examine the effect of the interaction between jets on turbulence and spray profiles. Jet-jet lateral impingement starts within ∼ 1–2 nozzle diameters downstream. The comparison of a single jet and the 3 × 3 matrix spray profiles shows some degree of coalescence due to the interaction between jets. Despite this, the Sauter mean diameter of the resulting spray field is in the 25–35 μm range. In the first few air swirl cup diameters downstream of the nozzle exit plane (down to z/D = 3), the droplets are still accelerating to the air velocity and turbulence is anisotropic. No–slip and turbulence isotropy assumption are accurate only well downstream of the exit plane (z/D = 7.5).Copyright

An Experimental Investigation of the Flow Downstream of a Low NOx Gas Turbine Nozzle

Achieving rapid mixing is the main challenge to reducing NOx levels in non-premixed liquid fired gas turbine combustors. Multipoint lean direct injection (LDI) is among the techniques used to achieve low emissions and good mixing by injecting fuel in small amounts through different injection ports. This study looks at the flow characteristics of a nozzle using this concept. The Parker Hannifin injector used is a square matrix of nine simplex fuel nozzles. Phase Doppler particle analyzer was used to measure both the spray characteristics as well as the gaseous phase turbulence flow field. The average velocities, Reynolds stresses and the Sauter Mean Diameter were measured at a series of positions downstream of the nozzle exit, x3 , ranging from 0.6 to 7 times the swirl cup diameters.Copyright

Parametric Study to Optimize Air/Fuel Mixing for Lean, Premix Combustion Systems

New designs of gas turbine combustors for power generation applications have to meet ever-tightening emission standards (mainly NOx, CO and UHC) while operating at high combustor pressures. This requires a detailed understanding of the physical processes involved. The air-fuel mixture preparation is a critical step in most advanced gas turbine combustion strategies to achieve lower emissions. It has long been established that the level of unmixedness between the fuel and air is strongly tied with NOx levels. The present paper applies the statistical technique of Design Of Experiments (DOE) to a generic mixer set-up that includes an axial swirler, with fuel injected at discrete locations and transverse to the flow. The objective is to identify influential design and operating parameters that will provide rapid and enhanced mixing. The parameters tested include Swirl strength as measured by the Swirl number, Swirl type (Constant angle vs. Free vortex), number and momentum of fuel injection sites and gas temperature. Planar Laser Induced Fluorescence of acetone (PLIF) was used to quantify mixing at various planar locations in the mixing section. Commercial CFD software is used to model the flow field and predict the spatial mixing at selected conditions. Comparisons are made with experimental measurements with the aim to validate the CFD code and also on comparing the model results with the measurements.Copyright