•The primary breakup of a liquid jet is analyzed numerically.•A recently developed VOF formulation is applied for the first time to this case.•The methodology showed a high accuracy.•The flow physics ...is discussed depending on the fluid properties and injection conditions.•Results are compared to data obtained through the One-Dimensional Turbulence model.
In this paper the characteristics of the primary breakup of a liquid jet is analyzed numerically. We applied the Volumes of Fluids (VOF) approach utilizing the Direction Averaged Curvature (DAC) model, to estimate the interface curvature, and the Direction Averaged Normal (DAN) model, to propagate the interface. While being used for the first time to predict liquid atomization, this methodology showed a high accuracy. The influence of varying the fluid properties, namely liquid-gas density and viscosity ratio, and injection conditions is discussed related to the required grid resolution. Resulting droplet sizes are compared to distributions obtained through the One-Dimensional Turbulence (ODT) model.
The impact of CO2 dilution on combustion of syngas (a mixture of H2, CO, and CH4) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution ...levels of CO2, corresponding to 15% and 34% of CO2 mole fraction in the syngas/CO2 mixtures, were experimentally investigated to evaluate the effects of CO2 dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO2 dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO2 dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO2 dilution, and (d) the NOx emission becomes significantly lower with increasing CO2 dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO2 dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures.
•We simulate pulsed sprays and jets.•The key results previous experimental and analytical works could be recovered.•The pulse tail-speed was analyzed depending on different gas and liquid ...parameters.•Increasing the injection frequency showed a significant improvement in the mixing.•The main mechanisms of enhanced mixing were identified and discussed.
This paper considers the effects of intermittent injection of a liquid jet or spray on the initial break-up and mixing of one fluid with the surrounding ambient fluid. The aim of the analysis is to describe the physical process and indicate the mechanisms that control the mixing under different flow conditions (time-dependent injection and its frequency relative to the time scales of the flow) and fluid properties (density ratio), Schmidt number for a single phase case which is studied for comparison, or the Weber number for the two-phase cases. The computations use Large Eddy Simulation (LES) to account for turbulence, and either Volume Of Fluid (VOF) for the initial break-up or Lagrangian Particle Tracking (LPT) with droplet break-up model in the case of liquid droplets injected into the ambient gas. The results show that, depending on the physical properties of the liquid and ambient gas, the initial break-up and turbulent mixing can be enhanced substantially with intermittent injection. The numerical modeling is validated by recovering key results of experimental and analytical works. It can be observed that a main effect during the mixing is the suction of ambient fluid at the tail of the injected liquid, which depends on the fluid properties. Increased injection frequency shows to increase the mixing significantly during the initial transient phase.
•We propose a comparison of 3D numerical and experimental spray data.•The comparison is based on the local extinction coefficient.•The comparison is feasible in dense situations, where PDA data is ...not accessible.•We show how to measure it experimentally and the procedure to derive it numerically.•We show how the method can further guide the model development.
In the recent years, large progresses in laser imaging techniques have allowed to extract spatially resolved 2D and 3D quantitative spray information even in optically dense situations. The main breakthrough of these techniques is the possibility of suppressing unwanted effects from multiple light scattering using Structured Illumination. Thanks to this new feature, effects due to light extinction can also be corrected allowing the measurement of the local extinction coefficient. These quantitative information which is available even in challenging conditions, where Phase Doppler does not work anymore, can be used for data comparison between experiment and simulation. The local extinction coefficient is particularly valuable for the description of the droplet field, defined as the “spray region”, as it contains information related to both droplets size and concentration. In this article we detail, then, the procedure enabling the modelers to obtain numerically this local extinction coefficient over the full 3D spray system. Following this procedure, results can now be adequately compared between simulation and experiment. The proposed comparison approach can better guide model adjustments in situation where the initial droplet size distribution is unknown or approximated and presents a step towards future validations of spray simulations, especially those based on Lagrangian Particle Tracking. The approach is exemplified here for the case of a Diesel-type spray. The results reveal at which specific spray locations discrepancies occur, and highlight the sensitivity of the initial droplet size distribution on the resulting extinction coefficient.
The impact of pilot flame operation on the combustion of pure methane and hydrogen-enriched methane (H2/CH4: 50/50 in vol%) fuels was investigated in a gas turbine model combustor under atmospheric ...conditions. The burner assembly was designed to mimic the geometry of an industrial burner, the Siemens DLE Burner, in which a concentric annular ring equipped with pilot flame burners is implemented in the dome of the combustor. Two pilot burner configurations have been investigated: a non-premixed and a partially premixed pilot arrangement. The performance of the pilot burners was evaluated for varying Reynolds number (Re) and H2 enrichment. High-speed OH∗ chemiluminescence imaging, as well as simultaneous planar laser-induced fluorescence measurements of the OH radicals and formaldehyde (CH2O) were used for evaluating the dynamics and structures of the flames for different conditions. Furthermore, emission measurements were carried out to determine the influence of hydrogen dilution on the NOx and CO emission levels. The main findings are (a) the effect of the pilot flame is sensitive to the Reynolds number of the main flame and the type of the pilot flame, (b) the stability range becomes narrower with increasing hydrogen ratio, due to the tendency to flashback, (c) non-premixed pilot flames lower the NOx and increase the CO emissions, albeit rather small differences in the emissions have been detected, and (d) the NOx and CO emissions become significantly lower with increasing hydrogen ratio.
•Optical measurements were used to study hydrogen-enriched methane/air flames in a lab-scale gas turbine combustor.•Hydrogen-enriched methane/air flames are shown to have a narrower operability range than methane/air flames.•Higher Reynolds number flames tend to be stabilized more upstream and have a wider operability range.•Hydrogen-enriched flames exhibit lower emissions of NOx and CO than methane/air flames.•Partially premixed pilot flame tends to lift off, while non-premixed pilot flames tend to attach to the burner.
Purpose
– One current trend in burner technology is to obtain high efficiency while keeping low levels of NOx emissions. A swirling flow in combustion ensures a fixed position of a compact flame. ...Therefore, it is necessary to design efficient swirlers. Flow patterns are simulated for the different swirl devices proposed in this work. Two axial-swirlers are studied: one based on curve-vanes consisting of a straight line with an arc of a circle as the trailing edge and the other is the common flat-vanes. The purpose of this paper is to assess the accuracy of different swirl generators using a well-known benchmark test case.
Design/methodology/approach
– This work deals with modelling the swirler using two approaches: the general purpose Computational fluid dynamics (CFD) solver Ansys-Fluent® and the suite of libraries OpenFOAM® to solve the Reynolds Averaged Navier Stokes equations, showing there is a slight deviation between both approaches. Their performance involves analyzing not only the Swirl number but also the size of the recirculation zones in the test chamber. A subsequent process on the flow patterns was carried out to establish the intensity of segregation which provides insight into the quality of mixing.
Findings
– CFD models are feasible tools to predict flow features. It was found that numerical results tend to reduce the inner recirculation zone (IRZ) radial size. Further, an increase of the swirl number involves larger IRZ and a smaller outer recirculation zone (ORZ). The curved swirler displays a better axi-symmetric behaviour than flat vanes. There is weak influence of the chord vanes on the swirl number. The number of vanes is a compromise of head loses and guidance of the flow.
Originality/value
– The paper offers two different approaches to solve turbulent swirling flows. One based in a general contrasted commercial tool and other using open source code. Both models show similar performance. An innovative set up for an axial swirler different from the conventional flat vanes was proposed.
The impact of CO2 dilution on combustion of syngas (a mixture of H2, CO, and CH4) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution ...levels of CO2, corresponding to 15% and 34% of CO2 mole fraction in the syngas/CO2 mixtures, were experimentally investigated to evaluate the effects of CO2 dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO2 dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO2 dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO2 dilution, and (d) the NOx emission becomes significantly lower with increasing CO2 dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO2 dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures.
This work investigates the impact of pilot flame and fuel composition on the structures and stabilization of swirling turbulent premixed methane/hydrogen/air flames in a lab-scale gas turbine model ...combustor. Simultaneous measurements of the velocity field and OH radicals distribution in the combustor were conducted using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) methods, respectively. Flames under stable and close to lean blow-off (LBO) conditions were studied for two fuel mixtures, with a hydrogen mole ratio of 0 and 50 % in the hydrogen/methane mixture, respectively. The studied flames were at a constant Reynolds number of 20,000 with different equivalence ratios. Two pilot-to-global fuel ratios were investigated (2 % and 6 %) while keeping the pilot-to-global air ratio constant at 2 %. Data for non-piloted flames were also acquired for comparison. The pilot flames were shown to extend the operability range. The LBO equivalence ratio of the main flame decreased with increasing fuel mass flow rate in the pilot flames due to the increased amount of hot gases with high concentrations of OH radicals in the outer recirculation zone (ORZ), which significantly enhanced the stabilization of the main flame. The stable flame reaction zone was in the high-speed shear layer between the ORZ and the inner recirculation zone (IRZ). When approaching LBO, the reaction zone was pushed downstream to the IRZ and subsequently decreased the size of IRZ, indicating a strong flow/flame interaction. Hydrogen enrichment was shown to reduce the LBO equivalence ratio. When close to LBO, the OH radicals in the hydrogen-enriched flames were observed in isolated pockets due to differential diffusion, which enhanced resilience to LBO. The flame front curvature, mean progress variable, and flame surface density were calculated from the acquired OH-PLIF data to quantify the impact of fuel composition and pilot flames on the flame structures.
In this paper the Triple Annular Research Swirler, a fuel injector characterized by complex design with three concentric air passages, has been studied numerically. A swirl-stabilized lean premixed ...flame has been simulated by means of Large Eddy Simulation. The computations characterize successfully the dynamics of the flame and their interactions with the complex swirling flow. The flame is stabilized upstream the fuel injector exit, and the dynamics are led by a Precessing Vortex Core which seems to originate in the inner air passage. The results obtained by Proper Orthogonal Decomposition analysis are in agreement with previous findings in the context of swirling flows/flames.
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