Numerical Simulations of Isothermal Flow in a Swirl Burner

In this paper; the non-reacting flow in a swirl burner is studied using Large Eddy Simulation. The configuration consists of two unconfined co-annular jets at a Reynolds number of 81500. The flow is characterized by a Swirl number of 0.93. Two cases are studied in the paper differing with respect to the axial location of the inner pilot jet. It was observed in a companion experiment (Bender and Bu¨chner, 2005) that when the inner jet is retracted the flow oscillations are considerably amplified. This is also found in the present simulations. Large-scale coherent structures rotating at a constant rate are observed when the inner jet is retracted. The rotation of the structures leads to vigorous oscillations in the velocity and pressure time signals recorded at selected points in the flow. In addition, the mean velocities, the turbulent fluctuations and the frequency of the oscillations are in good agreement with the experiments. A conditional averaging procedure is used to perform a detailed analysis of the physics leading to the low-frequency oscillations.

Correlation of Landfill and Digester Gas Composition With Gas Turbine Pollutant Emissions

The deployment of small gas turbines at landfills and wastewater treatment plants is attractive due to the availability of waste fuel gases generated at these sites and the need for onsite power and/or heat. The fuel gases produced by these applications typically contain 35 to 75% of the heating value of natural gas and contain methane (CH4) diluted primarily with carbon dioxide (CO2) and sometimes nitrogen (N2). Demonstrations of 30 to 250 kW gas turbines operating on these waste fuels are underway, but little detailed information on the systematic effect of the gas composition on performance is available. Growth in the use of small gas turbines for these applications will likely require that they meet increasingly stringent emission regulations, creating a need to better understand and to further optimize emissions performance for these gases. The current study characterizes a modified commercial natural gas fired 60 kW gas turbine operated on simluated gases of specified composition and establishes a quantitative relationship between fuel composition, engine load, and emissions performance. The results can be used to determine the expected impact of gas composition on emissions performance.

CFD Based Analysis of Burner Fuel Air Mixing Over a Range of Air Inlet and Fuel Pre-Heat Temperatures for a Siemens V94.3A Gas Turbine Burner

The homogeneity of the fuel/air mix entering the combustion chamber of a gas turbine is known to be a factor in both the emissions performance (with poor mixing resulting in local hotspots and the formation of thermal NOx) and the generation of acoustic vibrations (humming). Obviously it is desirable to reduce both pollutants and unwanted acoustics as far as possible. The aim of this paper is to study the relationship between the local inlet conditions and the mixing of the fuel and air, specifically looking at the effects of fuel gas preheating and inlet air temperature on mixedness at the combustor inlet. A CFD model of the lean pre-mixed combustor for a Siemens v94.3A gas turbine was used to analyse the problem. The 3-dimensional model employs a structured mesh scheme and uses the symmetry of the burner to reduce computational effort. The model was solved using a 2nd order discretisation of the momentum and continuity equations along with the RNG k-ε turbulence model to provide closure. The boundary conditions for the model were taken from data obtained from in service measurements. Several runs were made using air inlet temperatures varying from −10°C to 30°C and gas inlet temperatures from 10°C to 450°C. The data obtained from the CFD simulations was processed to give an indication of the quality of the fuel/air mixing for each set of inlet conditions. This was then used to create a tool which can be used to determine the amount of gas pre-heat required to achieve the best possible mixing for a given set of ambient conditions. An estimation of the NOx produced at different conditions was derived from the mixing data. Analysis of the results showed that increasing the gas preheat produces an improvement in the mixing of the fuel and air in the burner. This improvement in mixing also resulted in a reduction in the estimated amount of NOx produced.

Preliminary Study of a Low Power Plasma Radical Jet Generator for Combustion Systems

A novel radical jet generator (RJG) was developed, whose purpose it is to supply concentrated, relatively low temperature radicals that penetrate into a flammable stream of reactants and trigger or modify a combustion process. The RJG is driven by a plasma whose power is only a fraction of a percent of the total power released in the combustor. In this approach, the plasma induces an incomplete combustion process in a small duct carrying a rich mixture of fuel and air. Results obtained using the developed RJG show that a jet, which consists of partially burnt reactants, some products and is, apparently, rich in radicals produced by the incomplete combustion process triggers extremely steady combustion in a fast moving combustible mixture whose flow rate far exceeds that of the RJG. Importantly, the results show that the jet, rich with radicals, that emerges from the RJG cavity at a temperature well below traditional ignition can ignite a fast moving stream of combustible mixture. Moreover, when injected normal to the main flow, this jet ignites the main stream at a location relatively far from the entrance point of the jet. This makes it possible to keep the combustion process away from solid walls while at the same time eliminating the need for solid flame holders. This in turn, provides an augmenter with reduced I.R signature. Finally, the results show a drastic effect of the RJG upon the flame dynamics in general and combustion instabilities in particular. Flames which displayed large, periodic pressure oscillations became completely stable when the plasma in the RJG was turned on. This suggests a novel use of the RJG to inhibit instabilities in combustors.

Towards the Multiobjective Optimisation of Gas Turbine Combustors

Gas turbine combustor design entails multiple, and often contradictory, requirements for the designer to consider. Multiobjective optimisation on a low-fidelity linear-network-based code is suggested as a way of investigating the design space. The ability of the Tabu Search optimiser to minimise NOx and CO, as well as several acoustic objective functions, is investigated, and the resulting “good” design vectors presented. An analysis of the importance of the flame transfer function in the model is also given. The mass flow and the combustion chamber width and area are shown to be very important. The length of the plenum and the widths of the plenum exit and combustor exit also influence the design space.

Spray and Emission Characteristics Near Lean Blow Out in a Counter-Swirl Stabilized Gas Turbine Combustor

An experimental study of a single, swirl cup burner is carried out to improve understanding of the lean reacting flow field near idle conditions for an annular spray combustor. The counter-swirler is mounted horizontally in a trapezoidal cross-section combustor with quartz plate walls. Liquid fuel, Jet-A, is initially atomized using a simplex nozzle, and then a designed re-atomization occurs from the swirler hardware. Measurements of non-reacting and reacting gas phase velocities enable the direct comparison of critical flow features at various power settings. Droplet diameter and exhaust composition measurements confirm that the initial droplet size is a key factor in emission levels. Smaller droplets in the spray periphery tend to evaporate and burn premixed, while larger droplets in the spray core convect downstream and burn with a sheath-type, non-premixed flame. The presence of small fuel droplets in the spray may ensure more complete combustion and improve combustor stability at lean, low power settings.

Combustion Optimization for the ALSTOM GT13E2 Gas Turbine

The NOx emissions of low NOx premix combustors are not only determined by the burner design, but also by the multi burner interaction and the related distribution of air and fuel flows to the individual burners. Often the factors that have a positive impact on NOx emission have a negative impact on the flame stability, so the main challenge is to find an optimum point with the lowest achievable NOx while maintaining good flame stability. The hottest flame zones are where most of the NOx is formed. Avoiding such zones in the combustor (by homogenization of the flame temperature) reduces NOx emissions significantly. Improving the flame stability and the combustion control allows the combustor to operate at a lower average flame temperature and NOx emissions. ALSTOM developed a combustion optimization package for the GT13E2. The optimization package development focused on three major issues: • Flame stability; • Homogenization of flame temperature distribution in the combustor; • Combustion control logic. The solution introduced consists of: • The reduction of cooling air entrainment in the primary flame zone for improved flame stability; • The optical measurement of the individual burner flame temperatures and their homogenization by burner tuning valves; • Closed loop control logic to control the combustion dependent on the pulsation signal. This paper shows how fundamental combustion research methods were applied to derive effective optimization measures. The flame temperature measurement technique will be presented along with results of the measurement and their application in homogenization of the combustor temperature distribution in an engine equipped with measures to improve flame stabilization. The main results achieved are: • Widening of the main burner group operation range; • Improved use of the low NOx operation range; • NOx reduction at the combustor pulsation limit and hence, large margins to the European emission limit (50 mg/m3 @ 15%O2).

Fuel Flexibility Influences on Premixed Combustor Blowout, Flashback, Autoignition and Instability

This paper addresses the impact of fuel composition on the operability of lean premixed gas turbine combustors. This is an issue of current importance due to variability in the composition of natural gas fuel supplies and interest in the use of syngas fuels. Of particular concern is the effect of fuel composition on combustor blowout, flashback, dynamic stability, and autoignition. This paper reviews available results and current understanding of the effects of fuel composition on the operability of lean premixed combustors. It summarizes the underlying processes that must be considered when evaluating how a given combustor’s operability will be affected as fuel composition is varied.

Chemical Reactor Network Application to Emissions Prediction for Industial DLE Gas Turbine

A chemical reactor network (CRN) is developed and applied to a dry low emissions (DLE) industrial gas turbine combustor with the purpose of predicting exhaust emissions. The development of the CRN model is guided by reacting flow computational fluid dynamics (CFD) using the University of Washington (UW) eight-step global mechanism. The network consists of 31 chemical reactor elements representing the different flow and reaction zones of the combustor. The CRN is exercised for full load operating conditions with variable pilot flows ranging from 35% to 200% of the neutral pilot. The NOpilot. The NOx and the CO emissions are predicted using the full GRI 3.0 chemical kinetic mechanism in the CRN. The CRN results closely match the actual engine test rig emissions output. Additional work is ongoing and the results from this ongoing research will be presented in future publications.

Thermo Acoustic Flame Transfer Function Prediction for Turbulent Non-Premixed Syngas Flames

The thermo-acoustic behaviour of non-premixed turbulent syngas flames is investigated by means of transient RaNS Computational Fluid Dynamics simulations. Three cases with two different fuel compositions are considered. Both fuels are combusted in a turbulent non-premixed swirl stabilised mode, and are mixtures of hydrogen, carbon monoxide and nitrogen. One fuel contains methane in addition. The flame transfer function considered here, describes the relation between a perturbation of the fuel mass flow rate and the rate of heat release in the flame. The fuel mass flow is perturbed by an impulse excitation. The investigated geometry is a laboratory scale burner that is designed in the framework of the European Union sponsored HEGSA project. Experimental data are generated in tests at DLR (chemiluminescence and LIF). The CFD results show that methane addition to syngas has a significant influence on the flame transfer function. The addition of methane to syngas induces thermoacoustic damping for higher frequency (¿400 Hz) regions and increases amplification for low frequencies (¡200 Hz). The time delay of the transfer function is affected by the addition of methane due to both calorific value and chemical time scale effects. A decrease in inlet temperature also affects the flame transfer function. This is due to the slower chemistry and lower velocity.