Rajesh Sadanandan

Flow Field and Combustion Characterization of Premixed Gas Turbine Flames by Planar Laser Techniques

Lean premixed natural gas/air flames produced by an industrial gas turbine burner were analyzed using laser diagnostic methods. For this purpose, the burner was equipped with an optical combustion chamber and operated with preheated air at various thermal powers P, equivalence ratios Φ, and pressures up to p=6 bars. For the visualization of the flame emissions OH∗ chemiluminescence imaging was applied. Absolute flow velocities were measured using particle image velocimetry (PIV), and the reaction zones as well as regions of burnt gas were characterized by planar laser-induced fluorescence (PLIF) of OH. Using these techniques, the combustion behavior was characterized in detail. The mean flow field could be divided into different regimes: the inflow, a central and an outer recirculation zone, and the outgoing exhaust flow. Single-shot PIV images demonstrated that the instantaneous flow field was composed of small and medium sized vortices, mainly located along the shear layers. The chemiluminescence images reflected the regions of heat release. From the PLIF images it was seen that the primary reactions are located in the shear layers between the inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters.

Flow field and combustion characterization of premixed gas turbine flames by planar laser techniques

Lean premixed natural gas/air flames produced by an industrial gas turbine burner were analyzed using laser diagnostic methods. For this purpose, the burner was equipped with an optical combustion chamber and operated with preheated air at various thermal powers P, equivalence ratios Φ , and pressures up to p = 6 bar. For the visualization of the flame emissions OH* chemiluminescence imaging was applied. Absolute flow velocities were measured using particle image velocimetry (PIV), and the reaction zones as well as regions of burnt gas were characterized by planar laser induced fluorescence (PLIF) of OH. Using these techniques, the combustion behavior was characterized in detail. The mean flow field could be divided into different regimes: the inflow, a central and an outer recirculation zone, and the outgoing exhaust flow. Single-shot PIV images demonstrated that the instantaneous flow field was composed of small and medium sized vortices, mainly located along the shear layers. The chemiluminescence images reflected the regions of heat release. From the PLIF images it was seen that the primary reactions are located in the shear layers between the inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters.Copyright

Flame Characteristics and Emissions in Flameless Combustion Under Gas Turbine Relevant Conditions

Nomenclature d = inner diameter of the FLOX® nozzle, mm Da = Damkohler number dl = position of fuel nozzle with respect to air nozzle exit plane, mm f = focal length, mm k = rate coefficient NOx = oxides of nitrogen (NO and NO2) OH = OH chemiluminescence (electronically excited) Tad = global adiabatic flame temperature, K Tair = air preheat temperature, K v = velocity in air nozzle, ms 1 = air equivalence ratio = equivalence ratio

Experimental Analysis of Altitude Relight Under Realistic Conditions Using Laser and High-Speed Video Techniques

The altitude relight performance of a lean fuel injector and combustor was investigated at the altitude relight test rig at the Rolls-Royce Strategic Research Centre (SRC) in Derby. The studies were performed for different mass flow rates of air and kerosene, a combustor temperature and pressure of 278 K and 0.5 bar, respectively. Good optical access to the combustion chamber enabled the application of optical and laser measuring techniques. High-speed video imaging in the UV and visible wavelength range at a frame rate of 3.5 kHz was used to visualize the temporal development of the flame kernel. The observed differences between the UV and visible flame emissions demonstrate the different origins of the luminosity, i.e. OH* chemiluminescence and soot radiation. Further, laser-induced fluorescence of kerosene and OH radicals was applied at a frame rate of 5 Hz to visualize the fuel distribution and regions of hot and reacting mixtures. For two exemplary flames with different mass flow rates and fuel-to-air ratios, the steady burning flames after successful ignition are characterized in this paper by the distributions of kerosene, OH*, OH and soot luminosity. An example of the flame kernel development for a successful ignition is given by an image sequence from a high-speed video recording of the chemiluminescence. The importance of the upstream movement of the flame kernel as a condition preceding successful flame stabilization is identified.Copyright

Numerical and Experimental Investigation of a Semi-Technical Scale Burner Employing Model Synthetic Fuels

In this paper the development and the application of a numerical code suited for the simulation of gas-turbine combustion chambers is presented. In order to obtain an accurate and flexible framework, a finite-rate chemistry model is implemented, and transport equations for all species and enthalpy are solved. An assumed PDF approach takes effects of temperature and species turbulent fluctuations on the chemistry source term into account. In order to increase code stability and to overcome numerical stiffness due to the large-varying chemical kinetics timescales, an implicit and fully-coupled treatment of the species transport equations is chosen. Low-Mach number flow equations and k-e turbulence model complete the framework, and make the code able to describe the most important physical phenomena which take place in gas-turbine combustion chambers. In order to validate the numerical simulations, experimental measurements are carried out on a generic non-premixed swirl-flame combustor, fuelled with syngas-air mixtures and studied using optical diagnostic techniques. The combustor is operated at atmospheric and high-pressure conditions with simulated syngas mixtures consisting of H2, N2, CH4, CO. The combustor is housed in an optically-accessible combustion chamber to facilitate the application of chemiluminescence imaging of OH* and planar laser-induced fluorescence (PLIF) of the OH-radical. To investigate the velocity field, particle image velocimetry (PIV) is used. The OH* chemiluminescence imaging is used to visualise the shape of the flame zone and the region of heat release. The OH-PLIF is used to identify reaction zones and regions of burnt gas. The fuel composition is modelled after a hydrogen-rich synthesis gas, which can result after gasification of lignite followed by a CO shift reaction and a sequestration of CO2. Actual gas compositions and boundary conditions are chosen so that it is possible to outline differences and similarities among fuels, and at the same time conclusions about flame stability and combustion efficiency can be drawn. A comparison between experimental and numerical data is presented, and main strengths and deficiencies of the numerical modelling are discussed.Copyright

Effect of Swirl on the Flame Dynamics and Pollutant Emissions in an Ultra-Lean Non-Premixed Model Gas Turbine Burner

ABSTRACT The effect of varying swirl strength on the isothermal flowfield, flame dynamics, and pollutant emissions in a novel, unconfined and non-premixed swirl burner at atmospheric conditions was investigated using optical and laser diagnostic methods. The burner (IIST-GS1), operated with methane as fuel, produced stable ultra-lean flames at the two geometric swirl numbers investigated, S = 1.59 and 2.81. In both cases, the burner exhibited two flame stabilization zones with varying θglob, the bluffbody stabilized and swirl stabilized, with a transition region separating the two zones. Comparison of two-dimensional (2D) flowfield plots with OH* images revealed that the size of the two stabilization zones and, hence, the heat release distributions depended on the swirling strengths. Higher swirl strengths in S = 2.81 enhanced the reverse flow of burned gases towards the burner exit thereby aiding in the flame stabilization. Also, the increased mixing between the fuel, air, and burned gases for S = 2.81 resulted in lower NOx emissions when compared to S = 1.59 at all operating conditions. With decreasing θglob, the flame stabilized at axial locations closer to the burner exit for both swirling strengths. At very low θglob the increase in swirling strength enabled the flame to stabilize at regions where the local in-plane extensive strain rates exceeded the critical strain rates for non-premixed CH4/air flames, suggesting significant premixing of fuel-air with burned gases. The ultra-low NOx capability of the novel IIST-GS1 burner was evident throughout the investigated operating conditions.

2D-PIV Measurements in a Novel Swirl Burner Under Isothermal Conditions

2D-PIV measurements were performed in a novel non-premixed model GT burner designed to promote intense mixing of the fresh fuel/air with the product gases. The axial and tangential flow field measurements reveal three distinct zones, viz. bluffbody-stabilized, swirl-stabilized and an intermediate zone. The high vorticity fields generated by the converging–diverging flow field will promote intense mixing of the reactants and thereby favour a reduction in the thermal NO\(_{\text {x}}\) emissions. The measurements show the presence of fine-scale turbulent eddies in the intermediate zone. This, along the relatively low extensive strain fields, will aid in the flame stabilization at these locations.

OH* Chemiluminescence and OH-PLIF Measurements in a Micro Gas Turbine Combustor

In this work the combustion behavior of the Turbec T100 natural gas/air combustor was analyzed experimentally. For the visualization of the flame structures at various stationary load points OH* chemiluminescence and OH-PLIF measurements were performed in a micro gas turbine test rig equipped with an optically accessible combustion chamber. The OH* chemiluminescence measurements are used to get an impression of the shape and the location of the heat release zones. In addition the OH-PLIF measurements enabled spatially and temporarily resolved information of the reaction zones. Depending on the load point the shape of the flame was seen to vary from cylindrical to conical. With increasing thermal power load the maximum heat release zones shift to a lifted flame. Moreover, the effect of the optically accessible combustion chamber on the performance of the micro gas turbine is evaluated.Copyright