Gas Turbine Relevant Conditions Research Articles

Structure and dynamics of the wake of a reacting jet injected into a swirling, vitiated crossflow in a staged combustion system

Secondary injection of the fuel, also referred to as staged combustion, is being studied by gas turbine manufacturers as a means of increasing the power output of the gas turbine systems with minimal contribution to NO x emission. A reacting jet issuing into a swirling, vitiated crossflow operating at gas turbine relevant conditions was investigated as a means of secondary injection. In this study, the flow field of the reacting jet was investigated using high repetition rate (HRR) (5 kHz), two-component particle imaging velocimetry and OH-PLIF. In applications similar to the one currently studied in this work, viz. secondary injection of fuel in a gas turbine combustor, rapid mixing and chemical reaction in the near field of jet injection are desirable. Based on our analysis, it is hypothesized that the shear layer and wake field vortices play a significant role in stabilizing a steady reaction front within the near wake region of the jet. Premixed jets composed of natural gas and air were injected through an extended nozzle into the vitiated flow downstream of a low-swirl burner that produced the vitiated, swirled flow. The jet-to-crossflow momentum flux ratio was varied to study the corresponding effect on the flow field. The time-averaged flow field shows a steady wake vortex very similar to that seen in the wake of a cylindrical bluff body which helps to stabilize the reaction zone within the wake of the jet. The HRR data acquisition also provided temporally resolved information on the transient structure of the wake flow associated with the reacting jet in crossflow.

Influence of steam dilution on the ignition of hydrogen, syngas and natural gas blends at elevated pressures

This paper presents the influence of steam dilution on the autoignition behaviour of hydrogen, carbon monoxide, syngas, methane, and natural gas mixtures under gas turbine-relevant conditions. Rapid compression machine experiments were performed for fuel/air mixtures at equivalence ratios of 0.5, 1.0, and 2.0, in the temperature range 895–1140K for the H2 and CO mixtures and 730–1060K for the natural gas mixtures and at pressures of 10 and 30bar. Shock-tube experiments were performed for CH4–O2–Argon mixtures with and without H2O addition, highly diluted in argon (98% by vol.). The parameters were varied using an L9 Taguchi matrix for equivalence ratios of 0.5, 1.0, and 2.0; pressures of 1.6, 11, and 30atm; and water contents of 0%, 10%, and 30% of the fuel by volume. It was found that significant changes in the thermal properties of the mixtures affect the reactivity, whereas no chemical effect of the steam addition was observed for the majority of the mixtures investigated. Only mixtures of pure carbon monoxide were strongly influenced by water addition. In this case, the presence of water in the mixture allows the formation of relatively reactive ȮH radicals which enhances the possible oxidation chemistry of carbon monoxide leading to a greater observed reactivity of CO in the presence of water.

Turbulent flame speed for hydrogen-rich fuel gases at gas turbine relevant conditions

In this paper, correlations of the global consumption-based turbulent flame speed (ST) for hydrogen-rich fuel gases are presented. Interpretations on the derived ST are supplemented by a fractal analysis of the flame front. The findings are based on an experimental investigation of lean-premixed, dump-stabilized axisymmetric flames at gas turbine conditions (preheated up to 623 K and pressurized up to 2.0 MPa). Depending on the turbulent Damköhler number (Da), distinct characteristics of a normalized turbulent flame speed, ST/SL0, are observed (SL0is the unstretched laminar flame speed). While the dependence of ST/SL0 on the turbulent Reynolds number (ReT) is revealed for the flames with fast chemistry (Da > 1), flame stretch becomes dominant in determining ST/SL0 for those with slow chemistry (Da < 1). The transition from flame front wrinkling to flame stretch as the dominant factor is also evidenced by the fractal characteristics of the flame front.

Turbulent Flame Speed as an Indicator for Flashback Propensity of Hydrogen-Rich Fuel Gases

The turbulent flame speed (ST) is proposed to be an indicator of the flashback propensity for hydrogen-rich fuel gases at gas turbine relevant conditions. Flashback is an inevitable issue to be concerned about when introducing fuel gases containing high hydrogen content to gas turbine engines, which are conventionally fueled with natural gas. These hydrogen-containing fuel gases are present in the process of the integrated gasification combined cycle (IGCC), with and without precombustion carbon capture, and both syngas (H2 + CO) and hydrogen with various degrees of inert dilution fall in this category. Thus, a greater understanding of the flashback phenomenon for these mixtures is necessary in order to evolve the IGCC concept (either with or without carbon capture) into a promising candidate for clean power generation. Compared to syngas, the hydrogen-rich fuel mixtures exhibit an even narrower operational envelope between the occurrence of lean blow out and flashback. When flashback occurs, the flame propagation is found to occur exclusively in the boundary layer of the pipe supplying the premixed fuel/air mixture to the combustor. This finding is based on the experimental investigation of turbulent lean-premixed nonswirled confined jet flames for three fuel mixtures with H2 &gt; 70 vol. %. Measurements were performed up to 10 bar at a fixed bulk velocity at the combustor inlet (u0 = 40 m/s) and preheat temperature (T0 = 623 K). Flame front characteristics were retrieved via planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) diagnostics and the turbulent flame speed (ST) was derived, accordingly, from the perspective of a global consumption rate. Concerning the flashback limit, the operational range of the hydrogen-rich mixtures is found to be well represented by the velocity gradients prescribed by the flame (gc) and the flow (gf), respectively. The former (gc) is determined as ST/(Le × δL0), where Le is the Lewis number and δL0 is the calculated thermal thickness of the one-dimensional laminar flame. The latter (gf) is predicted by the Blasius correlation for fully developed turbulent pipe flow and it indicates the capability with which the flow can counteract the opposed flame propagation. Our results show that the equivalence ratios at which the two velocity gradients reach similar levels correspond well to the flashback limits observed at various pressures. The methodology is also found to be capable of predicting the aforementioned difference in the operational range between syngas and hydrogen-rich mixtures.

Experimental Analysis of Soot Formation and Oxidation in a Gas Turbine Model Combustor Using Laser Diagnostics

Sooting ethylene/air flames were investigated experimentally in a dual swirl gas turbine model combustor with good optical access at atmospheric pressure. The goals of the investigations were a detailed characterization of the soot formation and oxidation processes under gas turbine relevant conditions and the establishment of a data base for the validation of numerical combustion simulations. The flow field was measured by stereoscopic particle image velocimetry, the soot volume fractions by laser-induced incandescence, the heat release by OH chemiluminescence imaging and the temperatures by coherent anti-Stokes Raman scattering. Two flames are compared: a fuel-rich partially premixed flame with moderate soot concentrations and a second one with the same parameters but additional injection of secondary air. Instantaneous as well as average distributions of the measured quantities are presented and discussed. The measured soot distributions exhibit a high temporal and spatial dynamic. This behavior correlates with broad temperature probability density functions. With injection of secondary air downstream of the flame zone the distributions change drastically. The data set, including PDFs of soot concentration, temperature and flow velocity, is unique in combining different laser diagnostics with a combustor exhibiting a more challenging geometry than existing validation experiments.

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

Investigation of the Syngas Flame Characteristics at Elevated Pressures Using Optical and Laser Diagnostic Methods

The effect of pressure on the characteristics of syngas flames is investigated under gas turbine relevant conditions using planar laser induced fluorescence of OH radicals and OH* chemiluminescence imaging. An optically accessible combustor fitted with a swirl burner was operated with two different syngas mixtures, preheated air at 700 K, and pressures ranging from 5 to 20 bars. The thermal load varied from 15 to 25 kW/bar at an equivalence ratios 0.5. The OH-PLIF measurements show that the flames under all conditions exhibited two reaction fronts, one at the shear layer between the inner recirculation zone and the fuel inlet, and one between the fuel inlet and the air nozzle. The more or less continuous reaction front at low pressure turned into a highly corrugated flame front at higher pressures, with isolated regions of ignition and extinction. The probability density distribution of the flame curvature for the mixtures studied showed that the inner and outer flame responded differently to the pressure increase, with the mean curvature magnitude also depending on the mixture composition and thermal load. The measurements clearly shows the limitations associated with the use of OH* chemiluminescence images as a marker for the heat release rate especially in case of syngas mixtures.

FLOX® Combustion at High Power Density and High Flame Temperatures

In this contribution, an overview of the progress in the design of an enhanced FLOX® burner is given. A fuel flexible burner concept was developed to fulfill the requirements of modern gas turbines: high specific power density, high turbine inlet temperature, and low NOx emissions. The basis for the research work is numerical simulation. With the focus on pollutant emissions, a detailed chemical kinetic mechanism is used in the calculations. A novel mixing control concept, called HiPerMix®, and its application in the FLOX® burner are presented. In view of the desired operational conditions in a gas turbine combustor, this enhanced FLOX® burner was manufactured and experimentally investigated at the DLR test facility. In the present work, experimental and computational results are presented for natural gas and natural gas+hydrogen combustion at gas turbine relevant conditions and high adiabatic flame temperatures (up to Tad=2000 K). The respective power densities are PA=13.3 MW/m2 bar (natural gas (NG)) and PA=14.8 MW/m2 bar(NG+H2), satisfying the demands of a gas turbine combustor. It is demonstrated that the combustion is complete and stable and that the pollutant emissions are very low.

Turbulent flame speed for syngas at gas turbine relevant conditions

Modifications of conventional natural-gas-fired burners for operation with syngas fuels using lean premixed combustion is challenging due to the different physicochemical properties of the two fuels. A key differentiating parameter is the turbulent flame velocity, S T , commonly expressed as its ratio to the laminar flame speed, S L . This paper reports an experimental investigation of premixed syngas combustion at gas turbine like conditions, with emphasis on the determination of S T / S L derived as global fuel consumption per unit time. Experiments at pressures up to 2.0 MPa, inlet temperatures and velocities up to 773 K and 150 m/s, respectively, and turbulence intensity to laminar flame speed ratios, u′/ S L , exceeding 100 are presented for the first time. Comparisons between different syngas mixtures and methane clearly show much higher S T / S L for the former fuel. It is shown that S T / S L is strongly dependent on preferential diffusive-thermal (PDT) effects, co-acting with hydrodynamic effects, even for very high u′/ S L . S T / S L increases with rising hydrogen content in the fuel mixture and with increasing pressure. A correlation for S T / S L valid for all investigated fuel mixtures, including methane, is proposed in terms of turbulence properties (turbulence intensity and integral length scale), combustion properties (laminar flame speed and laminar flame thickness) and operating conditions (pressure and inlet temperature). The correlation captures effects of preferential diffusive-thermal and hydrodynamic instabilities.

High-pressure experiments and modeling of methane/air catalytic combustion for power-generation applications

The catalytic combustion of methane/air mixtures is investigated experimentally and numerically at gas turbine relevant conditions (inlet temperatures up to 873 K, pressures up to 15 bar and spatial velocities up to 3×10 6 h −1). Experiments are performed in a sub-scale test rig, consisting of a metallic honeycomb structure with alternately coated (Pd-based catalyst) channels. Simulations are carried out with a two-dimensional elliptic fluid mechanical code that incorporates detailed transport and heat loss mechanisms, and realistic heterogeneous and homogeneous chemistry description. The methodology for extracting heterogeneous kinetic data from the experiments is presented, and the effects of catalytic activity and channel geometry (length and hydraulic diameter) on reactor performance are elucidated. A global catalytic kinetic step provides excellent agreement (at temperatures below 950 K) between the measured and predicted fuel conversion, over a wide range of parameter variation (channel hydraulic diameter and length, pressure, and inlet temperature). It is shown that, under a certain combination of catalytic activity and channel length, the absolute temperature rise across the catalyst becomes essentially independent of pressure, a feature highly desirable for many practical systems. Even though the computed catalyst surface temperatures remain well below the decomposition temperature of PdO, a significant section of the catalyst—amounting up to 30% of the total reactor length—contributes minimally to the total fuel conversion, suggesting catalytic activity design improvements in the reactor entry.