V. Erenburg

Shock-Tube Ignition Study of Methane in Air and Recirculating Gases Mixture

To investigate the effect of recirculated combustion products on the combustion process of a typical gas-turbine engine, shock-tube and modeling investigation of ignition delay time was performed. Different mixtures of CH 4 -O 2 -Ar-N 2 -H 2 O were tested. Results showed that replacing N 2 by combustion product components, CO 2 and H 2 O, at concentrations and temperature range, which are typical for the flameless oxidation regime, did not affect the ignition delay time. The temperature range of the shock-heated samples was 1350-1800 K, and the pressure range was 5-10 atm. Modeling calculations were carried out with two mechanisms, and they both showed similar tendency. The correlation between the experimental data and calculations is discussed.

Effect of the Recirculating Gases on the Mild Combustion Regime

The present work is concerned with the thermodynamic and chemical kinetics of gas turbine combustor operating in the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime. The objective of the present study is to evaluate analytically the effect of the recirculation rate of combustion products within the FLOXCOM gas turbine combustor on a number of combustion parameters, mainly on the ignition delay time, NOX and CO emission, minimum ignition temperature, rate of pollutant formation and the dilution rate. The study also refers to the mechanism of influence of the recirculation rate on these values. Combustion pressure and inlet air temperature are used as parameters. Considered fuel was methane. The analysis is mainly based on CHEMKIN simulations where the calculation scheme of the combustion process in the combustor is modeled by a combination of plug reactors and mixers. Due to the unique characteristics of gas turbines, inlet air temperature is directly linked to combustion pressure while assuming conventional adiabatic compression efficiencies. It is shown that free radicals, which are part of the reaction products and exists for only a short period of time within the recirculating gases, decrease ignition delay time. The importance of shortening the ignition delay is further highlighted because of the adverse effect oxygen dilution has on this parameter (dilution of the reactants by the passive reaction products). It was found that there is an optimal recirculation rate, which corresponds to maximum heat density. In addition, results indicate that CO emission values rise with the recirculation rate; however the NOX values are more complicated. NOX depends on recirculation rate when flame temperatures are kept held constant. The NOx emission increases and the CO emission decreases with compressor pressure ratio. The CO concentration that is evaluated in the combustion process is further reduced during last dilution stage. Finally, basic rules for design optimization of the combustor are drafted. These are based on conventional onedimensional fluid and thermodynamic relations and on the CHEMKIN simulations. Nomenclature Krecirculation rate K= mr/(ma, + Mmass flow rate (kg/s), Ο oxygen content (mass percentage), r compressor pressure ratio, QR specific calorific heat of fuel (J/kg), Ttemperature (K), Vvelocity (m/s), Vn volume of the recirculation zone (m/s), μ, relative mass flow rate (/»,/ ma), ρ density (m / kg), Φ equivalence ratio, τ, stirring time (sec), τ2 ignition delay time (sec), τ? combustion time (sec), τ4 post combustion time (sec), TJ dilution time (sec), Subscripts: α-air c -combustion, d -diluted e exit, /-fuel r H-ecirculated, rz -recirculation zone s -stirred,

The Role of the Recirculating Gases at the Mild Combustion Regime Formation

The present work is concerned with the thermodynamic and chemical kinetics of gas turbine combustor operating in the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime. The objective of the present study is to evaluate analytically the effect of the recirculation rate of combustion products within the FLOXCOM gas turbine combustor on a number of combustion parameters, mainly on the ignition delay time, NOx and CO emission, minimum ignition temperature, rate of pollutant formation and the dilution rate. The study also refers to the mechanism of influence of the recirculation rate on these values. Combustion pressure and inlet air temperature are used as parameters. The gas turbine is fueled with methane. The analysis is mainly based on CHEMKIN simulations where the calculation scheme of the combustion process in the combustor is modeled by a combination of plug reactors and mixers. Due to the unique characteristics of gas turbines, inlet air temperature is directly linked to combustion pressure while assuming conventional adiabatic compression efficiencies. It is shown that free radicals, which are part of the reaction products and exists for only a short period of time within the recirculated gases, decrease ignition delay time. The importance of shortening the ignition delay is further highlighted because of the adverse effect oxygen dilution has on this parameter (dilution of the reactants by the reaction products). It was found that there is an optimal recirculation rate, which corresponds to maximum heat density. In addition, results indicate that CO emission values rise with the recirculation rate, however the NOX values are more complicated. NOX depends on recirculation rate when flame temperatures are kept held constant. The NOX emission increases and the CO emission decreases with compressor pressure ratio. The CO concentration that is evaluated in the combustion process is further reduced during last dilution stage. Finally, basic rules for design optimization of the combustor are drafted. These are based on conventional one-dimensional fluid and thermodynamic relations and on the CHEMKIN simulations.Copyright

Fundamentals of Low-NOx Gas Turbine Adiabatic Combustor

The present work is concerned with the thermodynamic, chemical and geometric parameters of gas turbine combustor operating in the flameless oxidation combustion regime. A methodology for calculating the thermodynamic parameters was developed for adiabatic combustion with recirculation. This enables to find the oxygen concentration, temperatures and other principal combustion products at different operational combustion conditions and to obtain geometrical relationships of the combustor. Based on shock-tube tests and comparison with CHEMKIN code simulations, a methodology to estimate the heat density generation at the recirculation zone was elaborated. These results have enabled preliminary design of the combustor and two sector models (60 degrees each) were produced. The combustor sectors were tested and demonstrated very stable combustion operation over a wide range of equivalence ratios with low NOx emission.Copyright

Extension of the Combustion Stability Range in Dry Low NOx Lean Premixed Gas Turbine Combustor Using a Fuel Rich Annular Pilot Burner

The present work is concerned with improving combustion stability in lean premixed (LP) gas turbine combustors by injecting free radicals into the combustion zone. The work is a joint experimental and numerical effort aimed at investigating the feasibility of incorporating a circumferential pilot combustor, which operates under rich conditions and directs its radicals enriched exhaust gases into the main combustion zone as the means for stabilization. The investigation includes the development of a chemical reactors network (CRN) model that is based on perfectly stirred reactors modules and on preliminary CFD analysis as well as on testing the method on an experimental model under laboratory conditions. The study is based on the hypothesis that under lean combustion conditions, combustion instability is linked to local extinctions of the flame and consequently, there is a direct correlation between the limiting conditions affecting combustion instability and the lean blowout (LBO) limit of the flame. The experimental results demonstrated the potential reduction of the combustion chamber’s LBO limit while maintaining overall NOx emission concentration values within the typical range of low NOx burners and its delicate dependence on the equivalence ratio of the ring pilot flame. A similar result was revealed through the developed CHEMKIN-PRO CRN model that was applied to find the LBO limits of the combined pilot burner and main combustor system, while monitoring the associated emissions. Hence, both the CRN model, and the experimental results, indicate that the radicals enriched ring jet is effective at stabilizing the LP flame, while keeping the NOx emission level within the characteristic range of low NOx combustors. [DOI: 10.1115/1.4026186]

Design and Performance Analysis of a Gas Turbine Flameless Combustor Using CFD Simulations

This study investigates the performance and the conditions under which flameless oxidation can be achieved for a given annular adiabatic combustor. Numerical modelling of velocity, temperature and species fields are performed for different flow configurations of air and methane streams injected into a proposed design of a gas-turbine combustor. Parametric analysis was performed by systematically varying several parameters: radius of a recirculation zone, radius of the combustor, location of air and fuel ports, air and fuel velocities magnitudes and injection angles. The analysis was performed initially using a three-step global chemistry model to identify a design (geometry and operating conditions) that yield flameless combustion regime. The selected design was then modelled using a skeletal (46 reactions) and a detailed (309 reactions) chemical kinetics mechanism. The k–e turbulence model was used in the most calculations. Overall, similar qualitative flow, temperature, and species patterns were predicted by both kinetics models; however the detailed mechanism provides quantitatively more realistic predictions. An optimal flow configuration was achieved with exhaust NOx emissions of < 7.5 ppm, CO < 35ppm, and a pressure-drop < 5%, hence meeting the design criteria for gas turbine engines. This study demonstrates the feasibility of achieving ultra-low NOx and CO emissions utilising a flameless oxidation regime.Copyright

LOWERING EQUIVALENCE RATIO LIMIT FOR STABLE COMBUSTION IN GAS TURBINES BY INJECTION OF FREE RADICALS

Lean premixed combustion is one of the widely used methods for NOx reduction in gas turbines (GT). When this method is used combustion takes place under low Equivalence Ratio (ER) and at relatively low combustion temperature. While reducing temperature decreases NOx formation, lowering temperature reduces the reaction rate of the hydrocarbon–oxygen reactions and deteriorates combustion stability. The objective of the present work was to study the possibility to decrease the lower limit of the stable combustion regime by the injection of free radicals into the combustion zone. A lean premixed gaseous combustor was designed to include a circumferential concentric pilot flame. The pilot combustor operates under rich fuel to air ratio, therefore it generates a significant amount of reactive radicals. The experiments as well as CFD and CHEMKIN simulations showed that despite of the high temperatures obtained in the vicinity of the pilot ring, the radicals’ injection from the pilot combustor has the potential to lower the limit of the global ER (and temperatures) while maintaining stable combustion. Spectrometric measurements along the combustor showed that the fuel-rich pilot flame generates free radicals that augment combustion stability. In order to study the relevant mechanisms responsible for combustion stabilization, CHEMKIN simulations were performed. The developed chemical network model took into account some of the basic parameters of the combustion process: ER, residence time, and the distribution of the reactances along the combustor. The CHEMKIN simulations showed satisfactory agreement with experimental results.Copyright

The Use of Methanol as an Alternative Fuel: Droplet Formation and Evaporation

Methanol, produced from natural may be considered as an alternative fuel for fossil based liquid fueled gas turbines, especially for land based systems. In the present work, the effect of physical properties of methanol and kerosene on atomization and evaporation are compared. The spray’s liquid flux, droplet sizes and droplet velocities, cone angle were measured using Phase Doppler Particle Analyzer/Laser Doppler Velocimeter (TSI PDPA/LDV) system. Water, kerosene and ethanol (ethanol instead of methanol was used due to the toxicity of methanol) were used and tested at the same input liquid pressures. Analytical analysis of evaporation time for a single droplet of kerosene and methanol showed that the evaporation time is about the same for two fuels with the same droplet diameters. However, due the methanol’s lower calorific value and the fact that its volume flux must be more than twice as much (for similar thermal power), its corresponding evaporation time is longer than for kerosene. The evaporation time for kerosene and methanol, which took into accounts that more methanol should be evaporated, was simulated by CFD. The simulation results showed that methanol spray requires significantly longer distance than kerosene. Thus, the spray of methanol has larger droplet diameter than kerosene and prolonged evaporation time.Copyright

Increasing Operational Stability in Low NOX GT Combustor Using Fuel Rich Concentric Pilot Combustor

Abstract Lean combustion is a method in which combustion takes place under low equivalence ratio and relatively low combustion temperatures. As such, it has the potential to lower the effect of the relatively high activation energy nitrogen-oxygen reactions which are responsible for substantial NOX formation during combustion processes. However, lowering temperature reduces the reaction rate and deteriorates combustion stability. The objective of the present study is to reduce the lower equivalence ratio limit of the stable combustion operational boundary in lean Gas Turbine (GT) combustors while still maintaining combustion stability. A lean premixed gaseous combustor was equipped with a surrounding concentric pilot flame operating under rich conditions, thus generating a hot stream of combustion products with significant amount of reactive radicals. The main combustors fuel-air composition was varied from stoichiometric to lean mixtures. The pilots mixture composition was also varied by changing the air flow rate, within a limited rich mixtures range. The pilot fuel flow rate was always lower than five percent of the total fuel supply at the specific stage of the experiments.

Pressure Losses for Jet Array Impingement With Crossflow

Jet impingement is an efficient heat transfer method and has been used successfully in cooling of turbine blades in gas turbine engines. However the pressure losses encountered in such cooling arrays become important when applied to micro gas turbines, cooling solar panels, high density electronic chips, etc. Although many studies have been conducted in the past on the jet impingement array at high Reynolds number, there is a scarcity of information on laminar impinging arrays. Therefore the present work is dedicated to experimental and theoretical investigation of pressure losses in low Re impingement arrays, 200< Re <3000, i.e. significantly lower than what has been reported in the literature. Experiments were carried out on jet impingement array with an array dimension of 29x29 mm and with nozzle diameters of 0.2, 0.4, 0.6 and 0.8 mm. Numerical simulations were performed with available commercial CFD tools. The two equation turbulence models (k-e, kω) and laminar flow models were used in the CFD simulations to simulate the flow through the jet impingement array at various Reynolds number. Detailed flow structure: mass flow rate distribution, velocity profiles in the jets and the change of nozzle discharge coefficient along the array in the streamwise direction have been obtained. These simulations enhance the understanding of the physics within multiple jet impingement system. Reasonable comparisons between experimental results and numerical simulations have been obtained. Semi empirical and analytical methods are also used to develop a model for calculating the total pressure loss within such multi jet impingement systems. The methodology is validated by results obtained from experiments and from CFD simulations. The methodology developed for pressure drop calculation can be used to optimize the geometrical parameters of the jet impingement array with respect to the pneumatic power consumption of such systems. NOMENCLATURE