L.P.H. de Goey

Survivability of thermographic phosphors (YAG:Dy) in a combustion environment

The feasibility of applying laser-induced phosphorescence in a combustion environment was shown by testing the consistency of the emission‐temperature relations of thermographic phosphor particles (YAG:Dy). The relations were calibrated before and after the phosphor particles had passed a flame front. The calibrations were performed in air and in pure oxygen. The emission‐temperature relation prevails from around 300 K to 1300 K. The difference in emission‐temperature relation for the two different cases is less than the experimental precision (3%).

The heat flux method for producing burner stabilized adiabatic flames: an evaluation with cars thermometry

Flat adiabatic stretchless methane/air flames at atmospheric pressure are investigated with CARS thermometry in a folded BOXCARS configuration. The flames are stabilized on a perforated-plate burner by adjusting the flow until zero net heat flux to the burner is created. Vertical temperature profiles are measured in flames with equivalence ratios of 0·80, 0·90, 1·00, and 1·10, up to a height of 35·0  mm above the burner. One horizontal profile is measured in the flame with π = 0·80. The measurements are compared with calculations of free flames, based on the GRI 2·11 reaction mechanism. A simple model is

Comparison of Conventional and Low-Dimensional Manifold Methods to Reduce Reaction Mechanisms

The application of detailed reaction mechanisms for modelling laminar flames takes an extensive computational effort. In turbulent flames the interaction between turbulence and fast chemistry leads to additional modelling problems. Reduced reaction mechanisms are therefore needed for modelling laminar and turbulent flames. Two methods to reduce a H 2 and CO/H 2 reaction mechanism are compared. The recently developed ILDM method (Maas and Pope, 1992) is compared with the more conventional method of applying steady-state assumptions for intermediate species (Peters and Rogg, 1993). The results of both reduction methods agree well with detailed computations of adiabatic one-dimensional flames. In the composition space the differences between the adiabatic one-dimensional flame and perfectly-stirred reactor results are small. This indicates that reduced mechanisms will be appropriate for modelling turbulent flames as well.

Modeling of burner-stabilized hydrogen/air flames using mathematically reduced reaction schemes

ABSTRACT A mathematical technique is used to reduce several hydrogen/air reaction systems to one-and two-step schemes. The reduction technique is based on the use of intrinsic low-dimensional manifolds in composition space as introduced by Maas and Pope (1992). In this method it is assumed that the fastest reaction groups of the chemical source term are in steady-state For a reaction mechanism that does not include HO2, a one-step reduced scheme is used for burner-stabilized hydrogen/air flame calculations. It appears that the one-step reduced scheme predicts the flame structure quite well for several values of the equivalence ratio and mass flow rates. The differences in flame temperature between the reduced scheme and full scheme calculations are less than 50K A one-step reduced scheme is also used for the reaction scheme including HO2. For this scheme, however, only low mass flow rates can be used, otherwise the flame will blow off. This is caused by the fact that the one-step scheme underestimates the...

LES and RANS of Premixed Combustion in a Gas-Turbine Like Combustor Using the Flamelet Generated Manifold Approach

Dry-low NOx gas turbine technology relies on lean premixed combustion of fuel. Additionally the accurate prediction of turbulent premixed combustion is still very difficult. In the present paper the calculation of reduced chemistry is assessed efficiently through the use of the flamelet generated manifold (FGM), which is used in conjunction with a CFD code in a RANS as well as in an LES context. In order to predict the combustion phenomena in a high swirl and high Reynolds number flow (the SimVal setup, at atmospheric pressure with elevated temperature), the present model is used concomitantly with a pre-assumed PDF for which fluctuations are completely determined in terms of an algebraic model. The mixing model for the variance has an arbitrary model constant, and the results show that the flame stabilization is not very sensitive to the model parameter present in the model. Stabilization of the combustion occurs at a location comparable to that found in experiments. In order to investigate the effects of this parameter on the numerical solutions, first RANS simulations were addressed considering arbitrary values for this parameter, defined within a certain range, and in a next step the grid resolution was changed. LES calculations were also performed showing similar features predicted in RANS. It is found that with the use of FGM combustion features in gas turbine conditions can be reproduced in a robust way.Copyright

A 5-D implementation of FGM for the large eddy simulation of a stratified swirled flame with heat loss in a gas turbine combustor

Numerical simulations are foreseen to provide a tremendous increase in gas-turbine burners efficiency in the near future. Modern developments in numerical schemes, turbulence models and the consistent increase of computing power allow Large Eddy Simulation (LES) to be applied to real cold flow industrial applications. However, the detailed simulation of the gas-turbine combustion process remains still prohibited because of its enormous computational cost. Several numerical models have been developed in order to reduce the costs of flame simulations for engineering applications. In this paper, the Flamelet-Generated Manifold (FGM) chemistry reduction technique is implemented and progressively extended for the inclusion of all the combustion features that are typically observed in stationary gas-turbine combustion. These consist of stratification effects, heat loss and turbulence. Three control variables are included for the chemistry representation: the reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the stratification effect is expressed by the mixture fraction. The interaction between chemistry and turbulence is considered through a presumed beta-shaped probability density function (PDF) approach, which is considered for progress variable and mixture fraction, finally attaining a 5-D manifold. The application of FGM in combination with heat loss, fuel stratification and turbulence has never been studied in literature. To this aim, a highly turbulent and swirling flame in a gas turbine combustor is computed by means of the present 5-D FGM implementation coupled to an LES turbulence model, and the results are compared with experimental data. In general, the model gives a rather good agreement with experimental data. It is shown that the inclusion of heat loss strongly enhances the temperature predictions in the whole burner and leads to greatly improved NO predictions. The use of FGM as a combustion model shows that combustion features at gas turbine conditions can be satisfactorily reproduced with a reasonable computational effort. The implemented combustion model retains most of the physical accuracy of a detailed simulation while drastically reducing its computational time, paving the way for new developments of alternative fuel usage in a cleaner and more efficient combustion.

Steady large-scale modulation of a moderately turbulent co-flow jet

The effects of a spatial modulation acting at the inflow of a moderately turbulent planar jet surrounded by a faster co-flow are investigated using direct numerical simulation of the Navier–Stokes equations. We adopt a superposition of spatially filtered small-scale random perturbations and a structured large-scale flow modulation. The large-scale modulation is characterised in terms of a Beltrami flow, specified by a wavenumber K. These large-scale modulations are steady and spatially periodic, while the random small-scale perturbations fluctuate in time and in space. The flow configuration studied in this paper is agitated by this combined large- and small-scale agitation at the inflow plane of a rectangular domain of size L × L × 2L in the x-, y- and streamwise z-directions. The inflow perturbation is focused on a strip of size L × D in the x- and y-directions. A parametric variation is carried out considering different choices for the wavenumber of the large-scale modulation. We focus on effects that the inflow modulation has on global characteristics of the flow, e.g. the width of the mixing region formed between the two streams and the dissipation rate, ϵ. Results show that the width of the mixing region increases faster compared to the case without the large-scale perturbation, when the flow is agitated by structures of size comparable to the integral scales of the flow. For the dissipation rate, results show the presence of a maximum response at a certain wavenumber K in case we apply a large-scale modulation. This maximum is attained at modulation scales that vary locally with respect to the distance from the inflow plane. Close to the inflow, the maximum response occurs at small modulation scales, while further into the domain a maximum response is present at comparably large modulation scales.

Correlating Flame Location and Ignition Delay in Partially Premixed Combustion

Controlling ignition delay is the key to successfully enable partially premixed combustion in diesel engines. This paper presents experimental results of partially premixed combustion in an optically accessible engine, using primary reference fuels in combination with artificial exhaust gas recirculation. By changing the fuel composition and oxygen concentration, the ignition delay is changed. To determine the position of the flame front, high-speed visualization of OH-chemiluminescence is used, enabling a cycle resolved analysis of OH formation. A clear correlation is observed between ignition delay and flame location. The mixing of fuel and air during the ignition delay period defines the local equivalence ratio, which is estimated based on a spherical combustion volume for each spray. The corresponding emission measurements using fast-response analyzers of CO, HC and NOX confirm the decrease in local equivalence ratio as a function of ignition delay. Furthermore multiple injection strategies are investigated, applying pilot as well as post injections, in combination with a main injection at constant load. From these results it is concluded that both pilot and post injections result in an increase of unburned hydrocarbon and CO emission and a slight decrease of nitric oxide emissions.

NOVEL BURNER CONCEPT FOR PREMIXED SURFACE-STABILIZED COMBUSTION

Surface-stabilized combustion is credited with high burning rates, extended lean flammability limits, wide modulation range and other advantages. This makes it an attractive technology for compact low-emission combustors. The experimental gas turbine surface burners reported to this date are produced from compressed and sintered Fe-Cr-Al fiber mats. The authors have developed a new concept of surface burner fabricated by braiding ceramic cords around a ceramic frame. This simple method produces a basket-type surface suitable for stabilizing lean premixed flames over a broad range of operating conditions. The use of ceramics extends possibilities for operation at very high inlet temperatures with reduced risks of material sintering and oxidation. This paper presents test results with an experimental burner on a pressurized combustion rig with optical access. The experiments were performed under the following conditions: inlet temperatures of 22-740 C, pressures of 1-3 bar, thermal power between 4 kWTh and 32 kWTh and equivalence ratios of 0.28-0.95. Measurements of flue gas composition and pressure drop are also reported in the paper. The operating window for low-NOx and low-CO combustion is analyzed. With the demonstrated performance, the burner could cover the operating envelope of a 3 kWe recuperated micro turbine [1]-[2] with no pilot and no staging. This would also limit NOx to <40 ppm @ 0% O 2 within the micro turbine load range of 100% to 50%. NOMENCLATURE

Porous fuel air mixing enhancing nozzle (PFAMEN)

One of the challenges with conventional diesel engines is the emission of soot. To reduce soot emission whilst maintaining fuel efficiency, an important pathway is to improve the fuel-air mixing process. This can be achieved by creating small droplets in order to enhance evaporation. Furthermore, the distribution of the droplets in the combustion chamber should be optimized, making optimal use of in-cylinder air. To deal with these requirements a new type of injector is proposed, which has a porous nozzle tip with pore diameters between 1 and 50 µm. First, because of the small pore diameters the droplets will also be small. From literature it is known that (almost) no soot is formed when orifice diameters are smaller than 50 µm. Second, the configuration of the nozzle can be chosen such that the whole cylinder can be filled with fine droplets (i.e., spray angle nearly 180°). However, injecting through a porous nozzle is not the same as an infinite number of very small holes, due to the difference in nozzle internal flow. Therefore, the nozzle tip is modeled in COMSOL Multiphysics in order to predict the outflow direction and velocity of the fuel. The Darcy-Forchheimer equation, which follows from the Navier-Stokes equation, is used for this purpose. To validate the model, experiments have been performed in the Eindhoven High Pressure Cell (EHPC) where (for vaporizing sprays) the spray is visually analyzed and (for reacting sprays) the ignition delay has been measured.