V. Sherbaum

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.

Preliminary Analysis of a New Methodology for Flameless Combustion in Gas Turbine Combustors

Flameless combustion is one of the most promising technologies that can meet the stringent demands of reduced pollution and increased reliability in future gas turbine engines. Although this new combustion technology has been successfully applied to industrial furnaces, there are inherent problems that prevent application of this promising technology in a gas turbine combustor. One of the main problems is the need for recirculating large amount of burnt gases with low oxygen content, within limited volume, and over a wide range of operating conditions. In the present paper, thermodynamic analysis of a novel combustion methodology operating in the flameless combustion regime for a gas turbine combustor is carried out from the first principles, with an objective to reduce oxygen concentration and temperature in the primary combustion zone. The present analysis shows that unlike in the conventional gas turbine combustor, transferring heat from primary combustion zone to secondary (annulus) cooling air can substantially reduce oxygen concentration in reactants and the combustion temperature, thus reducing NOx formation by a large margin. In addition, to reduce the peak temperature, the proposed methodology is conceptualised / designed such that energy from fuel is released in two steps, hence reducing the peak flame temperature substantially. The new proposed methodology with internal conjugate heat transfer is compared vis-a-vis to other existing schemes and the benefits are brought out explicitly. It is found that transferring heat from the combustion zone reduces oxygen concentration and increases carbon-dioxide concentration in the combustor, thus creating an environment conducive for flameless combustion. In addition, a schematic of a practical engineering design working on the new proposed methodology is presented. This new methodology, which calls for transfer of heat from the primary combustion zone to alternative air streams, is expected to change the way gas turbine combustors will be designed in the future.Copyright

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]

Chemical Kinetic and Thermodynamics of Flameless Combustion Methodology for Gas Turbine Combustors

The paper is concened with a novel combustion methodology based on the flameless combustion phenomenon. Flameless combustion is a newly discovered combustion regime and is one of the most promising technologies that can meet the stringent demands of reduced pollution and increased reliability in future gas turbine engines. In the present paper, chemical kinetics of a novel combustion methodology is carried out using the Chemical Reactor Modelling (CRM) approach. The proposed combustion scheme is designed with an objective of reducing oxygen concentration and temperature in the primary combustion zone, thus creating an environment where flameless combustion can be sustained. In addition, the fuel is injected in the recirculation zone where the oxygen concentration is low and the temperature is above the auto-ignition temperature. The thermodynamic and CRM analysis of the combustion methodology shows that transferring heat from primary combustion zone to the secondary (annulus) cooling air can reduce combustion temperature and oxygen concentration in the reactants, thus reducing NOx formation by a large margin. One more novelty of the proposed methodology is that the energy from fuel is released in two steps with intermediate cooling to reduce peak flame temperature within the combustor. The new proposed methodology with its internal conjugate heat transfer is compared vis-a-vis to the conventional methodology and the benefits are brought out explicitly. The NOx and CO emission of this new combustion methodology is found to be quite less as compared to the conventional gas turbine combustor. For the proposed combustion methodology it is found that unlike in the conventional combustors, the NOx and CO emission behave similarly with various design parameters, thus providing the designer with new opportunities to reduce emissions from gas turbine combustors. Further, schematic of a practical engineering design working on the new proposed methodology is presented. This new approach of gas turbine combustor design is expected to change the way in which gas turbine combustors will be designed in the future.

Modified Vaporizer for Improved Ignition in Small Jet Engine

The modification of a fuel vaporizing system, typically used in small jet engines with gross thrust under 1000 N, is discussed. The modification focused on an alternative design of the fuel injection system within the vaporizer to improve atomization and, consequently, vaporization and engine ignition. Operating conditions and all external dimensions of the vaporizer remained unchanged. Three types of impact atomizer were investigated: the existing atomizer, an atomizer with additional air blowing, and an atomizer with an inclined and concave impactor. Droplet size and global spray characteristics under various nozzle pressures and air velocities were measured with phase Doppler particle anemometry and monitored by photography. All modified systems demonstrated better atomization characteristics than the existing design. Droplet Sauter mean diameter, liquid flux, and spray cone angle were investigated using water as the working fluid. An atomization model was developed for the specific atomizer/vaporizer configuration under two operating regimes: 1) liquid breakup due to hydrodynamic instability at low airflow velocities, as in a standard orifice atomizer, and 2) aerodynamic breakup of the large droplets by shear flow of the surrounding air during high airflow velocities as in an air blast atomizer. The characteristics of the modified vaporizers with the new impact atomizers were tested in a jet engine combustor model. Successful spark ignition was demonstrated under operating conditions that were previously outside the ignition envelope.

A Numerical Investigation of Mixing Processes in a Novel Combustor Application

A mixing process in a staggered toothed-indented shaped channel was investigated. It was studied in two steps: (1) numerical simulations for different sizes of the boundary-contour were performed by using a CFD code; (2) these results were used for simulation-data modeling for prediction of mixing performances across the whole field of changing geometric and the aerodynamic stream parameters. Support vector machine (SVM) technique, known as a new type of self learning machine, was selected to carry out this stage. The suitability of this application method was demonstrated in comparison with a neural network (NN) method. The established modeling system was then applied to some further studies of the prototype mixer including observations of the mixing performance in three special cases and performing optimizations of the mixing processes for two conflicting objectives and hereby obtaining the Pareto optimum sets.

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