Low Swirl Numbers Research Articles

Experimental Study on Liquid Jet Trajectory in Cross Flow of Swirling Air at Elevated Pressure Condition

Abstract Motivated by fuel atomization applications in gas turbine combustors, this paper presents an experimental examination of liquid jet trajectory and spray dynamics in crossflow of swirling air, at elevated pressure conditions. The study was conducted within an annular passage and was focused on momentum flux ratio and swirl number as the main parameters. The momentum flux ratio was varied from 2 to 25 through incremental adjustments to the liquid injection velocity and the swirl number values of 0.42 and 0.74 were used, representing typical gas turbine fuel atomization conditions. Jet trajectories were captured by imaging the three-dimensional helical path traced by the liquid jet using two mutually perpendicular optical windows. Radial penetration was quantified by solving the equations of a helix. The key findings revealed that radial penetration of the liquid jet is larger for higher momentum flux ratios and is influenced by the helical arc length. Notably, the radial penetration observed at low momentum flux ratios was larger for crossflow with lower swirl number and the radial penetration observed at high momentum flux ratios was more for crossflows with higher swirl number. In comparison to the spray characteristics in swirling crossflows at atmospheric pressure, condition of elevated gaseous pressure resulted in lower radial penetration, jet spread and jet area. Jet trajectory correlations for elevated pressure conditions are additionally presented, developed using curve fitting to the experimental data, which will be useful to the industry for estimation of spray trajectory in swirling crossflows at high pressure conditions.

Swirl-induced hysteresis in a sudden expansion flow

Swirling sudden expansion flows are complex flow fields containing several coherent structures that depend on the swirl number and can exhibit hysteresis behavior between increasing and subsequently decreasing swirl levels. While these flows have extensively been studied in simple geometries, results involving special designed nozzles are scarce. Therefore, this paper aims to provide insights into a more complex geometry, specifically a two-step conical expansion with a converging outlet. Experimental data is acquired for changing swirl numbers at a Reynolds number in the range of 35, 000. Stereoscopic Particle Image Velocimetry is employed to characterize downstream flow structures, while Laser Doppler Velocimetry is used to characterize upstream structures and to determine the inlet swirl number. Several distinct flow patterns are found as a function of the swirl number and the identified flow patterns include, in order of increasing swirl, a Closed Jet Flow, an Open Jet Flow (OJF), and a Coandã Jet Flow (CoJF). A central positive axial velocity is noted for both OJF and CoJF downstream of the expansion due to the converging outlet geometry. At higher swirl numbers, Vortex Breakdown moves upstream into the nozzle until a negative axial velocity is noted in the inlet tube. For these higher swirl numbers, no hysteresis is observed in the inlet tube between increasing and subsequently decreasing swirl. However, downstream of the nozzle, it is observed that the CoJF detaches at a lower swirl number than the swirl number required for attachment, indicating a hysteresis effect between in- and decreasing swirl.

Experimental Investigation of a H2O2 Hybrid Rocket with Different Swirl Injections and Fuels

Hybrid rockets have very interesting characteristics like simplicity, reliability, safety, thrust modulation, environmental friendliness and lower costs, which make them very attractive for several applications like sounding rockets, small launch vehicles, upper stages, hypersonic test-beds and planetary landers. In recent years, advancements have been made to increase hybrid motor performance, and two of the most promising solutions are vortex injection and paraffin-based fuels. Moreover, both technologies can be also used to tailor the fuel regression rate, in the first case varying the swirl intensity, and in the second case with the amount and type of additives. In this way, it is possible not only to design high-performing hybrid motors, but also to adjust their grain and chamber geometries to different mission requirements, particularly regarding thrust and burning time. In this paper, the knowledge about these two technical solutions and their coupling is extended. Three sets of experimental campaigns were performed in the frame of the Italian Space Agency-sponsored PHAEDRA program. The first one investigated a reference paraffin fuel with axial and standard vortex injection. The second campaign tested vortex injection with low values of swirl numbers down to 0.5 with a conventional plastic fuel, namely polyethylene. Finally, the last campaign tested another, lower regressing, paraffin-based fuel with the same low swirl numbers as the second campaign.

Effects of Swirl Number and Central Rod on Flow in a Lean Premixed Swirl Combustor

ABSTRACT In most modern-day lean gas turbine combustors, flame is stabilized by swirl flows. A central toroidal recirculation zone (CTRZ), developed by these swirl flows, helps to stabilize the flame. The current study is focused on the effects of swirl number and a central rod on the flow behavior in a lean premixed swirl-stabilized combustor. Turbulence is modeled using the two-equation, realizable k-ϵ model of Reynolds-Averaged Navier–Stokes (RANS), and flamelet generated manifold (FGM) technique is used to model combustion. Both swirl number and central rod have a very significant effect on the flow and the flame in the combustor. The CTRZ moves upstream along with the flame, creating a flashback, with an increase in the swirl number. Also, the presence of a central rod accelerates the flame flashback at high swirl numbers. Higher turbulence is observed in inner shear layers, especially at high swirl numbers. emissions are higher in the regions of high temperature, especially in the CTRZ. These emissions decrease with the increase in the swirl intensities. At low swirl numbers, the case with the central rod exhibits lower production, and at high swirl numbers, the case without the central rod yields better results in the mitigation.

Azimuthal Instabilities of an Annular Combustor With Different Swirling Injectors

Abstract Experiments are carried out on the laboratory-scale MICCA-Spray annular combustor to examine the effects of swirlers on combustion instabilities. This system comprises 16 spray-swirl injectors and gives rise to instabilities coupled by azimuthal modes. Five types of swirlers producing clockwise rotation and varying in swirl numbers and pressure drops are considered. These swirlers can be broadly categorized into two groups: lower-swirl and higher-swirl groups, based on their swirl numbers. An arrangement where clockwise and counterclockwise swirlers alternate is also studied. Experiments are performed systematically with liquid heptane at five levels of thermal power and six equivalence ratios. Results reveal that none of the swirlers in the lower-swirl category exhibit instability in the operating region considered, whereas the higher-swirl units feature strong azimuthal instabilities that trace an overall limit cycle envelope with a few short and random bursts. Among the higher-swirl group, a higher pressure drop swirler is associated with a broader instability map. This shows that the transition to instability mainly depends on the swirl number through its effect on the flame structure and that the pressure drop adds to further variations in amplitude and frequency of oscillation. The spin ratio time series indicate that the modes are of mixed type and that their distribution depends on the operating condition. On specifically comparing the spin ratio distribution between a full set of clockwise rotating (CR) swirlers and a configuration where clockwise and counterclockwise rotating swirlers (CCR) are alternatively placed, it is found that there is no definite statistical preference for spin ratio linked to the effect of bulk swirl. In some cases, however, the CCR configuration promotes a broader distribution of spin ratios centered around the standing mode (sr = 0) while the CR setup favors azimuthal modes spinning in the counterclockwise direction. An attempt is made to interpret the occurrence of instabilities by making use of flame describing functions (FDFs) measured in a single-injector combustor. It is found that the FDFs corresponding to the two swirler categories (lower-swirl and higher-swirl) are relatively distinct. The observed behavior is tentatively interpreted using an instability analysis in which the injector and upstream plenum are represented by an impedance that shifts the band of instability. The unstable behavior is then linked to the relative position of the FDF phase with respect to the instability band in the frequency range corresponding to the expected azimuthal mode frequency. The phase and gain of the FDF notably depend on the swirl number, and it is possible to distinguish, for the present configuration, a category of low swirl number injectors inducing stable operation and another category of high swirl number units leading to oscillations.

NOx emissions in a swirled-stabilized magnesium flame

Alternative sources of clean energy with reduced greenhouse gas emissions have to be considered. Among them, the combustion of metal fuels such as aluminum or magnesium is promising. Nitrogen oxides (NOx) are the only undesirable gases produced during such metal combustion. The present study analyzes the amounts of NOx (NO and NO2) produced by a swirled stabilized magnesium flame. Two geometric swirl numbers are considered: 0.7 (low swirl number) and 7.3 (high swirl number). Different air–fuel ratios – equivalence ratio ranging from 0.17 to 1 – are considered, being obtained by changing the mass flow rate of the injected fuel which is a pulverized fuel of pure magnesium particles with a size fraction 50–70 µm. Thermocouples and a bichromatic pyrometer are implemented to monitor the gas temperature and the flame temperature, respectively. Oxygen (O2) is measured using an in-line paramagnetic analyzers and NOx are measured using an IR-UV analyzer.Whatever the swirl conditions and the air–fuel ratio tested, a stabilized flame of Mg is obtained. The metal combustion exhibits very stable performances. NOx emissions versus equivalent ratio follow a bell-shape curve. Under the tested experimental conditions, the amount of NOx never exceeds 7 gNOx/kWh, which is lower than the maximal value of NOx emission from a gasoline engine. The NOx mole fraction increases when the air–fuel ratio increases, as the power dissipated by the flame increases. The temperature of MgO particles in the flame reaches 2250 °C and the gas temperature never exceeds 1400 °C. The thermal NOx formation is estimated applying Zeldovich’s model. The model indicates that NOx can be produced in the gas surrounding the hot MgO particles. For the highest equivalence ratio, even if the gas temperature is still increasing, the NOx production is disfavored in the rich mixture. Based on these results, a global scheme for NOx formation is proposed which includes four successive steps.

Thermoacoustic instability drivers and mode transitions in a lean premixed methane-air combustor at various swirl intensities

In this study, an in-house developed fully compressible solver utilizing high-order numerical discretization schemes is used to study the effects of swirl intensity on thermoacoustic instabilities in a lean premixed swirl stabilized combustor. Turbulent combustion modeling is achieved using large eddy simulations along with dynamically thickened flame combustion model. Obtained results reveal that there are two critical burner swirl numbers in which the combustor dynamics change drastically. Results show that the combustor is stable at low swirl numbers (S ≤ 0.5). However, as the swirl intensity increases, the combustor experiences thermoacoustic instabilities. The first critical swirl number lies within the range of 0.5–0.6, in which a transition occurs in the combustor dynamics from a state of stable operation to thermoacoustic instabilities. The second critical swirl number (S = 0.9) corresponds to the transition of different thermoacoustic instability modes. The combustor experiences limit cycle instabilities coupled with the first tangential acoustic mode of the combustor for 0.6 ≤ S < 0.9, while for S > 0.9 the instabilities are burst like and coupled with the second tangential acoustic mode of the combustor. The combustor experiences instabilities coupled with both the first and the second tangential modes, when the burner swirl number is 0.9. In order to evaluate key parameters inducing the thermoacoustic instabilities, heat release fluctuations are calculated through hydrodynamic flow features (central and side recirculation zones, coherent structures, and precessing vortex core). Investigations show that all the above features contribute almost equally in inducing heat release fluctuations in-phase with the acoustic perturbations at low swirl numbers. However, as the swirl number increases, side recirculation zone and coherent structures play the key role in producing heat release fluctuations in-phase with the acoustic perturbations, while central recirculation zone has the least effects on driving the instabilities.

Experimental investigation of the oxygen enrichment in synthetic gases flames

Today, there are many different studies on increasing combustion efficiency and reducing flue gas emission values. In this study, H2 and CO have been added to 30% methane gas in a premixed laboratory scale model burner with high H2 / CO ratio (3) and medium (1.5). Swirl numbers (s = 0.6 and 1) have been changed by keeping the equivalent ratios constant. The experiments have been carried out at a constant burner power (3 kW), regardless of the fuel composition. The effect of the swirl number on the temperature distribution and emission values in the combustion chamber has been investigated radially and axially. At the low swirl number, CO emissions tend to increase with the decrease of H2/CO ratio. On the other hand, at the high swirl number, it has been observed that the decrease in CO emissions with the decrease of H2/CO ratio. Because of the high heating value of the hydrogen, NO emissions increased with the high H2/CO ratio, regardless of the swirl number. As a result of the experiments, it has been concluded that the swirl number and the rate of H2/CO has a great effect on combustion characteristics and emissions of flames. In addition, oxygen enriched combustion has been performed at constant (1) swirl number at different H2/CO ratios. Equivalence ratio was kept constant (0.7) for all experiments. It has been observed that the oxygen-enriched combustion has been caused the low CO and high NO emission at both mixtures. The experiments also showed that the increase of O2 ratio in the air for high H2/CO ratio (3) could be causing the increase in temperature.

A numerical investigation of CO2 dilution on the thermochemical characteristics of a swirl stabilized diffusion flame

The turbulent combustion flow modeling is performed to study the effects of CO2 addition to the fuel and oxidizer streams on the thermochemical characteristics of a swirl stabilized diffusion flame. A flamelet approach along with three well-known turbulence models is utilized to model the turbulent combustion flow field. The k-ω shear stress transport (SST) model shows the best agreement with the experimental measurements compared with other models. Therefore, the k-ω SST model is used to study the effects of CO2 dilution on the flame structure and strength, temperature distribution, and CO concentration. To determine the chemical effects of CO2 dilution, a fictitious species is replaced with the regular CO2 in both the fuel stream and the oxidizer stream. The results indicate that the flame temperature decreases when CO2 is added to either the fuel or the oxidizer stream. The flame length reduction is observed at all levels of CO2 dilution. The H radical concentration indicating the flame strength decreases, following by the thermochemical effects of CO2 dilution processes. In comparison with the fictitious species dilution, the chemical effects of CO2 addition enhance the CO mass fraction. The numerical simulations show that when the dilution level is higher, the rate of the flame length reduction is more significant at low swirl numbers.

Effect of Swirl Number on the Combustion Characteristic of Syngas using FGM Model Simulation

Synthesis gas (syngas) is one of the potential alternative clean fuel energy in gas-fired boilers swirl burners system. Computational Fluid Dynamic (CFD) simulations were conducted to study the effect of different swirl number on the combustion characteristics of CO-rich syngas in premixed swirl mode using a model boiler swirl burner. The composition of CO-rich syngas is 67.5 % CO, 22.5 % H2, 5 % CO2 and 5 % CH4. Two types of combustion model for simulation analysis were used in this study namely flamelet generated manifold (FGM) and chemical equilibrium (CE) approaches. The CFD results were validated with actual experimental data with the same swirl number. The FGM method shows better agreement with experimental result, hence adopted to model and predict the CO-rich syngas flame characteristic in the reaction zones. The CO-rich syngas was then numerically tested on different type of swirl number including 0.39, 0.59, 0.84, 1.20 and 1.80. Result shows that the angle of flame distribution was apparently equivalence with the vane angle swirler. Swirler with a higher swirl number (1.20 and 1.80) showed lower NO (57%) and lower CO (98%) species compared to the one with a lower swirler number (0.39 – 0.84). Flame with lower swirl number (0.39) exhibited higher temperature, OH and O radical content. Thus, it contribute to the high production of NO and CO species.

Flame–cooling air interaction in an effusion-cooled model gas turbine combustor at elevated pressure

The focus of this work is the investigation of flame–cooling air interaction by means of wall temperature (2D themographic phosphor thermometry), gas phase temperature (coherent anti-Stokes Raman spectroscopy), flame structure (planar laser-induced fluorescence of OH) and flow field (particle image velocimetry) measurements in an effusion-cooled single sector model gas turbine combustor. The rig is operated under close-to-reality boundary conditions, i.e., elevated combustor inlet temperature and increased system pressure. Parametric variations of important parameters, namely swirl, staging and effusion cooling air mass flow are conducted to investigate their effect on total film cooling effectiveness. Furthermore, we derive a dimensionless flame–cooling air interaction intensity parameter on the basis of OH-PLIF data. The latter revealed a higher interaction intensity for fully premixed operation at a low swirl number—a trend that is reversed for higher swirl numbers, where a partially premixed combustion mode leads to increased intensities. This trend is reflected in wall temperature as well as resulting film cooling effectiveness. Peak wall temperature varies up to $$150\,{\text {K}}$$ across all parametric variations and up to $$200\,{\text {K}}$$ in axial direction on the effusion-cooled liner for a given condition. This corresponds to a variation of total film cooling effectiveness between 0.45–0.57 and 0.45–0.62, respectively.

Swirling flow in a hydraulic turbine discharge cone at different speeds and discharge conditions

An experimental investigation of the cavitating turbulent flow in a simplified hydraulic turbine was carried out to examine flow features in a wide range of speed and discharge conditions. A swirling device that allows reproducing the speed distribution behind the runner of a real turbine was manufactured using a rapid 3D prototyping technology. Laser Doppler velocimetry was used to measure both axial and tangential velocity components at the runner outlet for 96 operating regimes. The limitations of the swirl number used for swirling flow characterization through swirl free speed and discharge conditions were investigated. Particular attention was paid to the instability of the precessing vortex rope in the transition regime at a low swirl number. The boundaries of the regime with an aperiodic pressure surge in terms of the swirl number were defined, contributing to insight into the stability of swirling flow in hydraulic turbines.

Effect of fuel nozzle geometry and airflow swirl on the coherent structures of partially premixed methane flame under flashback conditions

The effect of fuel nozzle geometry and swirling airflow on the flashback and its relationship with the coherent structures of partially premixed methane flame is investigated experimentally. Particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are used to document flow characteristics, and Schlieren imaging technique is used to study flame appearance and vortex shedding frequency downstream of the burner exit. Proper orthogonal decomposition (POD) technique is applied to capture the coherent structures, along with phase averaging of the linear superposition of the first four POD modes. Seven different fuel nozzle geometries and two swirl number (S = 0.79 and 1.15) are tested. The nozzles are categorized into three groups, with each has similar equivalent diameter; namely, a symmetric nozzle (used as a reference), nozzles with polygonal orifices (group A), and angled multi-orifice nozzles (group B). The results of the flow field inside the mixing tube show that the strength of coherent structures and flashback propensity increase with the swirling airflow Reynolds number, swirl number, nozzle bluff body area, and the number of the peripheral angled orifices of the fuel (central) nozzle. On the other hand, the results of flame appearance outside of the mixing tube indicate that methane flame experiences symmetric vortex shedding at high swirl number and low Reynolds number, while it experiences PVC near blowout conditions at low swirl number and high Reynolds number. Furthermore, the frequency of coherent structures is found to depend on the swirling airflow Reynolds number, swirl number, and fuel nozzle geometry. Additionally, the flashback’s mean region inside the mixing tube is found directly proportional to the strength and frequency of the coherent structures.

Aperiodic pressure pulsation under non optimal hydraulic turbine regimes at low swirl number

Off-design operating conditions of hydraulic turbines is hindered by pressure fluctuations in the draft tube of the turbine. A precessing helical vortex rope develops, which imperils the mechanical structure and limits the operation flexibility of hydropower station. Understanding of the underlying instabilities of precessing vortex rope at low swirl number is incomplete. In this paper flow regimes with different residual swirl is analysed, particular attention is paid to the regime with a small swirl parameter. Study defines upper and low boundaries of regime where aperiodic pressure surge is observed. Flow field at the runner exit is investigated by Laser Doppler Velocimetry and high-speed visualizations, which are complemented draft tube wall pressure measurements.

Scalar dissipation rate statistics in turbulent swirling jets

The scalar dissipation rate statistics were measured in an isothermal flow formed by discharging a central jet in an annular stream of swirling air flow. This is a typical geometry used in swirl-stabilised burners, where the central jet is the fuel. The flow Reynolds number was 29 000, based on the area-averaged velocity of 8.46 m/s at the exit and the diameter of 50.8 mm. The scalar dissipation rate and its statistics were computed from two-dimensional imaging of the mixture fraction fields obtained with planar laser induced fluorescence of acetone. Three swirl numbers, S, of 0.3, 0.58, and 1.07 of the annular swirling stream were considered. The influence of the swirl number on scalar mixing, unconditional, and conditional scalar dissipation rate statistics were quantified. A procedure, based on a Wiener filter approach, was used to de-noise the raw mixture fraction images. The filtering errors on the scalar dissipation rate measurements were up to 15%, depending on downstream positions from the burner exit. The maximum of instantaneous scalar dissipation rate was found to be up to 35 s−1, while the mean dissipation rate was 10 times smaller. The probability density functions of the logarithm of the scalar dissipation rate fluctuations were found to be slightly negatively skewed at low swirl numbers and almost symmetrical when the swirl number increased. The assumption of statistical independence between the scalar and its dissipation rate was valid for higher swirl numbers at locations with low scalar fluctuations and less valid for low swirl numbers. The deviations from the assumption of statistical independence were quantified. The conditional mean of the scalar dissipation rate, the standard deviation of the scalar dissipation rate fluctuations, the weighted probability of occurrence of the mean conditional scalar dissipation rate, and the conditional probability are reported.

Effect of Nozzle Spacing on Nitrogen-Oxide Emissions and Lean Operability

Nitrous-oxide emissions and the equivalence ratio resulting in lean extinction of two adjacent gas turbine fuel/air nozzles are measured as the spacing between the two nozzles is varied. Two swirl-stabilized nozzle types are tested: a high-stability pilot design, and a low-emission design with a much lower swirl number. Jet A is used as the fuel. The lean blowout point is tested for both nozzle types at each spacing. Increased spacing results in blowout at a lower equivalence ratio, which is likely due to increased air velocity between the nozzles that occurs at closer spacings. The spacing has a dramatic effect on the nitrous-oxide emissions index (grams of nitrous-oxide produced per kilogram of fuel burned) for the three possible nozzle combinations. The nitrous-oxide index emissions are greatest when two nozzles are placed close, and they decrease as the internozzle distance grows. The excited hydroxyl radical (OH)* chemiluminescence imaging shows how the OH* intensity is greater on the side of the nozzles toward the neighboring flame. This is an indication that the heat release rate and/or temperature is raised in part of the flame due to the adjacent nozzle. The magnitude of the effects of nozzle spacing, both on emissions and lean blowout, are dependent on the nozzle design. Asymmetries in the flow and flame structure of the two nozzles appear when using two high-swirl pilot nozzles at larger nozzle spacings. This disturbs the emissions trend and causes an increase in nitrous-oxide production for this case.

Combustion Dynamics of Swirling Flame at Thermochemical Conversion of Biomass

Development of the swirling flame dynamics at thermo chemical conversion of biomass (wood pellets) has been investigated experimentally at a low swirl number of the swirling flame flow (S 1) is used to provide mixing of the axial flow of volatiles with an air swirl and to support the combustion of volatiles with the formation of a downstream reaction zone. It is shown that the development of combustion dynamics is influenced by the competitive processes of endothermic thermal decomposition of biomass and exothermal combustion of volatiles. The enhanced thermal decomposition of biomass by increasing the primary air supply and the axial flow of volatiles results in an intensive heat energy consumption with a correlating increase of the air excess ratio at the bottom of the combustor and in ignition delay. This determines the incomplete combustion of the volatiles at the primary stage of flame formation and restricts the development of combustion dynamics. The combustion conditions can be improved by decreasing the secondary air supply and air excess ratio at the bottom of the combustor, which leads to enhanced ignition with a faster and more complete combustion of volatiles.

Impingement pressure characteristics of swirling and non-swirling turbulent jets

This paper experimentally investigates the effects of swirl on the impingement surface pressure for an incompressible, turbulent, swirling impinging air jet. The swirl flow is generated aerodynamically, where the nozzle can achieve a seamless progression from non-swirling to high swirling flows. Hotwire anemometer is used to measure velocity components. A digital micromanometer with flush-mounted pressure taps on the impingement plate is used to measure static pressures on the impingement surface. The effect of swirl number (S), nozzle-to-plate distance (H) and Reynolds number (Re) on the pressure distribution is examined for S=0–1.05, H=1D–6D and Re=11,600, 24,600 and 35,000.For low swirl flows, the coefficient of pressure (Cp) shows a non-swirl like behaviour with maxima at the stagnation point. For medium-to-high swirls, maximum Cp shifts radially outward from the stagnation point and becomes relatively flat with increasing S. The stagnation pressure reduces nonlinearly with increasing swirl intensity and follows a quadratic relationship for a given Re. For any S, the pressure distribution is found to be independent of Re for low swirl numbers (up to S=0.3), but it varies up to r/D=2 for larger swirl numbers. A negative Cp (flow separation) occurs near the stagnation region for H=1D, however, vanishes at larger H. For very high swirl number (S=1.05) and at H=1D, three different regions are recognised on the impingement surface from the stagnation point: a rotating, reversed inward flow at r/D⩽0.5, a transition and less stable region at 0.5<r/D⩽0.75 and an outward flow with stronger tangential component at r/D⩾1.0. These surface pressure results may significantly affect the heat transfer characteristics as well as wall shear stresses for future impinging jet studies.

Shaping Air Flow Characteristics of a High-Speed Rotary-Bell Sprayer for Automotive Painting Processes

Electrostatic rotary bell sprayers (ERBS) are widely used in automotive painting applications. These processes involve complex airflows to shape paint sprays and transport droplets toward automotive parts to be coated. Despite the importance of shaping airflow on global spray characteristics, a detailed characterization of this aerodynamic flow is still missing. For this purpose, an experimental study was conducted on the influence of some ERBS operating parameters on the development and characteristics of shaping airflow. Results show that, for low swirl numbers, the flow behavior is close to that of annular swirling jets and a good agreement is found between ERBS flow characteristics and data available in literature. When rotational speed of the bell cup is sufficiently fast, a change of regime is observed with a shift in the longitudinal flow development and an increase of recirculation zone length. This change of regime is attributed to vortex breakdown instability, known to occur when high swirl strength is beyond a critical value. Experimental results obtained in this study put forward a clear link between the shaping air flow rate and the rotation frequency on the aerodynamics and also provide valuable leads to design shaping air flow in modern ERBS.

Visualization of Different Flashback Mechanisms for H2/CH4 Mixtures in a Variable-Swirl Burner

Flame flashback from the combustion chamber to the premixing section is a major operability issue when using high H2 content fuels in lean premixed combustors. Depending on the flow-field in the combustor, flashback can be triggered by different mechanisms. In this work, three flashback mechanisms of H2/CH4 mixtures were visualized in an atmospheric variable-swirl burner using high speed OH* chemiluminescence imaging. The H2 mole fraction of the tested fuel mixtures varied between 0.1 and 0.9. The flow-field in the combustor was varied by changing the swirl number from 0.0 to 0.66 and the total air mass-flow rate from 75 to 200 SLPM (standard liters per minute). The following three types of flashback mechanism were observed: Flashback caused by combustion induced vortex breakdown (CIVB) occurred at swirl numbers ≥0.53 for all of the tested fuel mixtures. Flashback in the boundary layer (BL) and flame propagation in the premixing tube caused by auto-ignition were observed at low swirl numbers and low total air mass-flow rates. The temporal and spatial propagation of the flame in the optical section of the premixing tube during flashback was studied and flashback speed for different mechanisms was estimated. The flame propagation speed during flashback was significantly different for the different mechanisms.