Effect of the Reynolds number and the basic design parameters on the isothermal flow field of low-swirl combustors (original) (raw)
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International Journal of Energy Technology
Swirl stabilised combustion is one of the most widely used techniques for flame stabilisation in gas turbine combustors. Lean premixed combustion systems allow the reduction of NOx coupled with fair flame stability. The swirl mechanism produces an aerodynamic region known as central recirculation zone (CRZ) providing a low velocity region where the flame speed matches the flow velocity, thus anchoring the flame whilst serving to recycle heat and active chemical species to the root of the former. Another beneficial feature of the CRZ is the enhancement of the mixing in and around this region. However, the mixing and stabilisation processes inside of this zone have shown to be extremely complex. The level of swirl, burner outlet configuration and combustor expansion are very important variables that define the features of the CRZ. Therefore, in this paper swirling flame dynamics are investigated using computational fluid dynamics (CFD) with commercial software (ANSYS). A new generic s...
Investigation of aerodynamic structure of isothermal swirl flow in a two-stage burner
Journal of Physics: Conference Series, 2017
The work is devoted to experimental and nu merical study of aerodynamic structure of a swirl flow in isothermal model of a vortex burner device that is characterized by the fluid flow supply via two sequentially-mounted tangential swirlers. Depending on the way of the flow supply into the second-stage swirler, either co-swirl or counter-swirl of two flows can be realized. The effect of the second-stage supply direction on the resulting aerodynamic structure has been investigated. Using LDA measurement system the profiles of averaged axial and tangential velocity co mponents were obtained. Experiments have sh own that in the co-swirl case the flow inside the vortex burner model is characterized by strong non-uniformity, wh ile in the counter-swirl regime a rapid mixing of the flows from the first and second stages occurs, resulting in a uniform distribution of the flo w across the chamber section. Nu merical simu lation of 3D isothermal turbulent flow has been performed for the counter-swirl regime using the differential Reynolds stress model in the time-dependent formu lation. Using the Qcriterion for the identificat ion of vortices in numerical data arrays, the evolution of large-scale vortex structures of the swirl flo w inside the vortex chamber has been visualized, indicating the presence of two spiral-shape vortex filaments in the vortex chamber. The periodic character of dynamics of these vortex structures has been revealed.
Investigations of swirl flames in a gas turbine model combustor
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A gas turbine model combustor for swirling CH 4 /air diffusion flames at atmospheric pressure with good optical access for detailed laser measurements is discussed. Three flames with thermal powers between 7.6 and 34.9 kW and overall equivalence ratios between 0.55 and 0.75 were investigated. These behave differently with respect to combustion instabilities: Flame A burned stably, flame B exhibited pronounced thermoacoustic oscillations, and flame C, operated near the lean extinction limit, was subject to sudden liftoff with partial extinction and reanchoring. One aim of the studies was a detailed experimental characterization of flame behavior to better understand the underlying physical and chemical processes leading to instabilities. The second goal of the work was the establishment of a comprehensive database that can be used for validation and improvement of numerical combustion models. The flow field was measured by laser Doppler velocimetry, the flame structures were visualized by planar laser-induced fluorescence (PLIF) of OH and CH radicals, and the major species concentrations, temperature, and mixture fraction were determined by laser Raman scattering. The flow fields of the three flames were quite similar, with high velocities in the region of the injected gases, a pronounced inner recirculation zone, and an outer recirculation zone with low velocities. The flames were not attached to the fuel nozzle and thus were partially premixed before ignition. The near field of the flames was characterized by fast mixing and considerable finite-rate chemistry effects. CH PLIF images revealed that the reaction zones were thin (0.5 mm) and strongly corrugated and that the flame zones were short (h 50 mm). Despite the similar flow fields of the three flames, the oscillating flame B was flatter and opened more widely than the others. In the current article, the flow field, structures, and mean and rms values of the temperature, mixture fraction, and species concentrations are discussed. Turbulence intensities, mixing, heat release, and reaction progress are addressed. In a second article, the turbulence-chemistry interactions in the three flames are treated.
Computational Analysis of Isothermal Flow in a Test Swirl Combustor
Isothermal flow in an idealized swirl combustor is analyzed numerically and experimentally. The Reynolds number based on combustor inlet diameter and mean axial velocity is 4600. Measurements of time-averaged swirl and axial velocity components and corresponding rms turbulence intensities are performed by Laser Doppler Anemometry, along radial traverses at different axial locations. In three-dimensional, transient computations, Large Eddy Simulations (LES) and Unsteady Reynolds Averaged Numerical Simulations (URANS) are employed for modeling the turbulent flows. For LES, the Smagorinsky model is used to model the subgrid scale turbulence. For URANS, the Reynolds Stress Model (RSM) is employed as the statistical turbulence model. The URANS-RSM approach is observed to perform rather poorly. This is assumed to be due to the fact that the present flow has a rather low Reynolds number, and the applied RSM, being a high Reynolds number model, is not able to model low Reynolds number effec...
EFFECT OF COOLING AIR ON SWIRL COMBUSTOR
2007
This paper presents the numerical study of the turbulent swirling flow in a combustor and the effect of cooling air, which has practical applications in industrial furnaces and jet engines. Cooling air is used to protect the combustor wall from burnout, while allowing the combustion to occur at higher temperature. The governing differential equations using kε turbulence model closure are solved by a control-volume based iterative finite difference technique. Computations are done for constant vane angle type swirl generation at inlet. Different swirl numbers up to 1.5 are considered. To study the effect of cooling air on the combustor performance, calculations are repeated for two different velocities of the cooling air jet. The predicted distribution of the mean axial and tangential velocities, turbulence kinetic energy and streamline plots are discussed in the article. With the increase of swirl strength, secondary on-axis recirculation due to swirl is observed. The swirl produces larger turbulence kinetic energy and enhances mixing rate, thus require shorter combustor length. The interaction between the non-swirling cooling air and swirling core flow also increases the generation of turbulence kinetic energy and mixing rate. The capability of the computational model for predicting recirculating flows is tested by comparing the results with available experimental data and found to have reasonable matching.
Isothermal modeling of aerodynamic structure of the swirling flow in a two-stage burner
EPJ Web of Conferences, 2017
The work deals with the experimental study of the aerodynamic structure of a swirling flow in the isothermal model of two-stage vortex combustion chamber. The main attention is focused on the process of flow mixing of two successively connected tangential swirlers of the first and second stages of the working section. Data on flow visualization are presented for two patterns of flow swirling. Time-averaged profiles of the axial and tangential velocity components are obtained with the help of laser-Doppler anemometer. In the case of flow co-swirling between two stages of the working section, instability of a secondary flow in the form of precessing vortex was distinguished. For the regime with counter flow swirling, effective mixing of the swirl flows was found; this was reflected by formation of the flow with uniform distribution of axial velocity over the cross-section.
Computational analysis of incompressible turbulent flow in an idealised swirl combustor
Progress in Computational Fluid Dynamics, An International Journal, 2011
Isothermal flow in an idealized swirl combustor is analyzed numerically and experimentally. The Reynolds number based on combustor inlet diameter and mean axial velocity is 4600. Measurements of time-averaged swirl and axial velocity components and corresponding rms turbulence intensities are performed by Laser Doppler Anemometry, along radial traverses at different axial locations. In three-dimensional, transient computations, Large Eddy Simulations (LES) and Unsteady Reynolds Averaged Numerical Simulations (URANS) are employed for modeling the turbulent flows. For LES, the Smagorinsky model is used to model the subgrid scale turbulence. For URANS, the Reynolds Stress Model (RSM) is employed as the statistical turbulence model. The URANS-RSM approach is observed to perform rather poorly. This is assumed to be due to the fact that the present flow has a rather low Reynolds number, and the applied RSM, being a high Reynolds number model, is not able to model low Reynolds number effects. On the other hand, a very good agreement of the LES predictions with the measurements is observed.
Recirculation phenomena in a natural gas swirl combustor
Experimental Thermal and Fluid Science, 2004
This paper presents the experimental results obtained in a natural gas swirl combustor (input thermal power ¼ 17 kW) through different techniques (laser Doppler Anemometry for flow field characterisation, temperature measurements by thin thermocouples, emission spectroscopy of the flame front and pollutant emissions analysis at the exhaust). The main aim of the performed research was to investigate the recirculation phenomena induced by the swirl motion imparted to the air stream (swirl number S ¼ 0:82) inside the combustor: in fact, different recirculating regions (central and corner) have been observed and, by integration of the velocity profile measured by LDV, the corresponding flow rate has been estimated. Particularly, it has been found that flame confinement in the presence of intense swirl generates a wide central recirculation zone and a large corner vortex. The hot reverse stream propagating on the burner axis prevents penetration of the fuel jet, induces a rapid mixing and burning and provides flame stabilisation. The corner recirculation of hot burned gases favours entrainment in the outflowing reactants mixture contributing to their progressive pre-heating and leaning, thus influencing combustion process development and pollutants formation (especially thermal NO x ).
2014
Swirling jet is used as a means of controlling flames in combustion chamber. Swirl flow offer an interesting field of study for aerospace & mechanical engineers in general and for combustion engineers in particular since it involves complex interaction of recirculation & turbulent mixing, which aid flame stabilization in combustion system. Swirling flow in both reacting & non-reacting conditions occur in wide range of application such as gas turbines, marine combustor burner, chemical processing plants, rotary kilns & spray dryers. Stabilization of flame can be achieved by various techniques. The most common techniques used in modern gas turbine combustors is swirl stabilization in which swirl velocity is imparted to inlet air using vane Swirler. Swirl can reduce combustion length by producing higher rates of entrainment of ambient fluid and fast mixing close to exit nozzle & on boundaries of recirculation zone in strongly swirling zones. Experimental studies show that swirl has lar...