Academic literature on the topic 'Counter-swirl flames'

The effect of radial distribution of swirl has been examined on the thermal behavior of two different premixed flames using a double concentric premixed swirl burner. The double concentric swirl burner allowed systematic variation in the radial distribution of swirl (both co- and counter-) between the inner and outer annulus of the burner. The burner had two annular jets and a central nozzle. Information on the thermal field in two flames formed by co- or counter-swirl in the outer annulus and co-swirl in the inner annulus has been examined. Specifically mean and fluctuating temperatures, integral and micro thermal time scales, and probability density distribution of temperatures have been determined at various spatial positions in the flames. The micro-thermocouple output was compensated to provide high-frequency (about 1 kHz) response of the thermocouple. Direct flame photographs were taken to provide information about the global features of flames and flame stability. The global and thermal characteristic data presented here provided a complete insight on the thermal behavior of co- and counter-swirl flames. The results show that the direction of swirl (co- or counter-) used to stabilize a flame from annular jets provides a great influence on flame symmetry. The simultaneous combination of co- and counter-swirl in the burner showed a very nonsymmetrical behavior of the flame. The global and thermal data presented here confirmed these findings. The results suggest significant effect of co- and counter-swirl distribution in flames on the NOx emission levels.

This paper presents 5 kHz stereo particle image velocimetry and OH planar laser induced fluorescence measurements of transversely forced swirl flames. The presence of transverse forcing on this naturally unstable flow both influences the natural instabilities, as well as amplifies disturbances that may not necessarily manifest themselves during natural oscillations. By manipulating the structure of the acoustic forcing field, both axisymmetric and helical modes are preferentially excited away from the frequency of natural instability. The paper presents a method for spatially interpolating the phase locked $r{-}z$ and $r{-}\unicode[STIX]{x1D703}$ planar velocity and flame position data, extracting the full three-dimensional structure of the helical disturbances. These helical disturbances are also decomposed into symmetric and anti-symmetric disturbances about the jet core, showing the subsequent axial evolution (in magnitude and phase) of each of these underlying disturbances. It is shown that out-of-phase acoustic forcing excites $m=\pm 1$ modes, but the flow field preferentially amplifies the counter-winding, co-rotating helical disturbance over the co-winding, counter-rotating helical disturbance. This causes the flow and flame to transition from a transverse flapping near the jet exit to a precessing motion further downstream. In contrast, in-phase forcing promotes axisymmetric $m=0$ disturbances which dominate the flow field over the entire axial domain. In both cases, the amplitudes of the anti-symmetric disturbances about the jet core grow with downstream distance before saturating and decaying, while the symmetric disturbances appear nearly negligible. It is suggested that this saturation and decay is due to linear effects (e.g. a negative spatial growth rate), rather than nonlinear interactions.

This paper focuses on investigating the flow structures in a multi-hole swirl burner. Using the Particle Image Velocimetry(PIV) technique, the experiment measured the velocity distributions of the swirling flame in a muti-hole burner. The experiments show that there is a central recirculation zone (CRZ) in the middle of the flow field, and two counter-rotating vortices exist along the centerline symmetrically. With fuel jet increase: the width of recirculating zone and axial mean velocity peaks changes little; length of recirculation zone and the biggest reverse flow velocity increases; the expansion angle of swirling jet increase at first, and then changes little; axial non-uniform coefficient of outlet reduces at first, and then increases. With airflow velocity increase: axial mean velocity peaks increase; the dimension of recirculating zone and the expansion angle of swirling jet are unchanged; axial non-uniform coefficient of outlet increases.The data from this experiment is helpful for optimization of the swirl burner design, and can be established as benchmarks for the development and validation of swirl combustion numerical simulations.

Abstract Radiative furnaces pose significant thermal inertia and single impinging flames have been observed to cause occurrence of hotspots on the target surface. Multiple burners arranged in suitable array configuration represent one of the plausible solutions for more uniform heat transfer. In this study, computational fluid dynamics (CFD) simulations have been carried out for multiple swirling impinging flames arranged in a hexagonal array configuration. The turbulence chemistry interactions in the flame field are solved numerically using renormalization group (RNG) based k–ε/eddy dissipation model (EDM) framework. Comparison of co-and-counter-swirling configurations has been studied for interactions and spent gas release mechanism. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZ) for all the surrounding except central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared to counter-swirl case. Results indicate that some flames in counter-swirl case are underutilized due to the fluid dynamics developed in the system and co-swirling hexagonal array configuration is a better arrangement for effective heating of target surface. Effect of interjet spacing (S/Dh = 5, 7, and 9) and separation distance (H/Dh = 3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing interjet spacing and separation distance. Central flame represented a region of low heat flux and this region has been noticed to expand in size for increasing interjet spacings. Suppression of central flame has been observed to be maximum for minimum separation distance.

The centrally staged combustor is an effective way to reduce NOx emissions from combustors. However, combustion instability caused by the mutual coupling between flames and acoustics during the combustion process is almost unavoidable. To better understand this problem, the effect of the swirl rotational direction is investigated in this paper using two different schemes with co-swirl and counter-swirl configurations. Pressure fluctuations and flame dynamics are investigated under self-excited combustion oscillation conditions. The CH* chemiluminescence distribution captured by a high-speed camera is utilized to characterize the flame macrostructure and heat release fluctuations. Furthermore, non-oscillating reaction velocity fields are acquired using particle image velocimetry (PIV) technology. The results indicate that the amplitude and frequency of the counter-swirl scheme are higher than those of the co-swirl scheme at varying main stage equivalence ratios. Combining the results from dynamic mode decomposition and the local Rayleigh index, it is found that the heat release regions of the counter-swirl scheme are mainly concentrated in the shear layer. Higher velocity gradients, vorticities, and strain rates in the inner shear layer (ISL) and outer shear layer (OSL) for the counter-swirl scheme are verified using PIV technology. The driving sources of thermoacoustic oscillations are located in the regions of the ISL, OSL, and the area where the flame impinges on the sidewall of the liner. Additionally, the counter-swirl scheme exhibits larger vorticities and strain rates in the ISL and OSL, facilitating the development of thermoacoustic oscillations.

In recent years, syngas has gained research interest as the focus shifts from fossil to renewable fuels. Syngas generated from biomass such as wood, agricultural residue, and paper is a promising fuel to achieve net-zero carbon emissions. It is mainly composed of CO, H2 and diluents such as CO2 and N2. The combustion characteristics of biomass-derived syngas significantly differ from the well-studied fuels in terms of calorific value, stoichiometric fuel-to-air ratio, ignition delay, and flammability limits. In the present study, a combination of catalytic and swirl combustion is explored using a two-stage combustor to achieve low emissions without losing combustion stability. The first stage is the catalytic stage, and the second stage is the swirl stage. A preheated and premixed syngas-air mixture (Фfirst = 4) is supplied to the first stage, where syngas is consumed partially. The remaining unburnt fuel is combusted using an oxidizer in the second stage. In this stage, the flame is stabilized by two co-centric swirling streams where the inner stream is the unburnt gases coming from the first stage, and the outer stream is the oxidizer. By changing the swirling direction of the streams, co and counter-swirl flames are obtained. In this thesis, the experimental results are described in six parts organized as follows. The first part of the study investigates the catalytic combustion of syngas in the first stage at an equivalence ratio of 4. The transient temperature measurements show three distinct regions of catalytic combustion: kinetically controlled, light-off, and steady-state regions. It is found that the start-up time depends on the conductivity of the monolith placed inside the first stage. The NOx emission is found to be low (< 1 ppm) at the exit. The second part of the study explores the lean unconfined co/counter-swirl flame stabilized in the second stage. Both flames can be stabilized at a higher flow rate, maintaining a constant overall equivalence ratio. Co-swirl creates a ‘U’-shaped flame, and counter-swirl creates a more distributed and compact flame. A low NOx concentration (< 20 ppm) is found in the flame; however, the CO emission is more than 10000 ppm. In the third part, the emission characteristics are studied, with the second stage being confined. The emission, temperature, flame shape and noise are investigated at three overall equivalence ratios (Фoverall). The CO emission reduces drastically in the presence of confinement, and the emission is found to be ~10 when Фoverall = 0.55. The NOx emission is below 2 ppm for all cases. The shape of co and counter-swirl flames behave differently when Фoverall changes. Planar laser-induced fluorescence (PLIF) shows the radicals are well distributed over the combustor for co/counter-swirl flames at a higher equivalence ratio, unlike existing studies. Particle image velocimetry (PIV) indicates that the flame shape is related to the shape of the inner recirculation zone. The sound pressure level (SPL) for co/counter-swirl flames is ~110 dB. The frequency domain contains a dominating peak at all Фoverall. The high-speed OH* chemiluminescence (5 kHz) confirms that the combustion is exciting the axial mode of the combustor, which is captured in the noise data. The sound pressure level can be reduced by changing the O2 concentration in the oxidiser stream. Thus, in the fourth part of the thesis, we investigate the effect of O2 concentration in the oxidizer stream on swirling flames, which is discussed in the fourth part. The OH* chemiluminescence intensity reduces, and flame height increases when O2 concentration decreases. The sound pressure level (SPL) also shows a decreasing trend with O2. The global luminosity calculated from high-speed chemiluminescence indicates that the peaks corresponding to axial mode and PVC are suppressed at low O2 concentrations. In terms of emissions, CO emission increases with a reduction in O2. The feasibility of achieving distributed combustion is assessed from emission, chemiluminescence intensity, SPL, and combustion stability. It is found that the co-swirl configuration is suitable for studying distributed combustion. In the fifth part, we investigated the combustion dynamics for co and counter-swirl flames. The noise emitted by the combustor shows signatures of intermittency. Pressure signals can be divided into periodic and aperiodic zones, and aperiodic epochs randomly appear between periodic epochs. The high-speed OH* chemiluminescence unravels two motions of the combustion. One oscillates in the axial direction, and the other represents the precessing vortex core (PVC) motion. Axial fluctuation of heat release decreases with an increase in momentum ratio; however, the PVC motion remains prevalent. Interestingly, both fluctuations are suppressed when the oxygen percentage is reduced to 13.13%, indicating that low-oxygen dilution can be an effective strategy to reduce combustion instability. Finally, in the last part of the study, the effect of momentum ratio (M), density ratio and heat-release rate on the flow field is studied in detail. For counter-swirl configuration, the flow field shows a drastic change at M = 1.7 as the vortex breakdown mode changes from bubble to conical form. The vortex breakdown mode influences the shape of the flame. In the bubble mode, the flame is distributed, whereas ‘V’-shaped for the conical mode. Thus, the current study has led to an improved understanding of fuel-rich catalytic combustion and shape, dynamics, and flow field of co/counter-swirl flames under various conditions. The present study also establishes that the two-stage rich-catalytic lean-swirl stabilized combustor is suitable for stationary power production applications with ultra-low NOx and low CO emissions.

This paper presents experimental study on self-excited combustion instability characteristics of premixed low-swirl flames in a multi-nozzle can combustor with counter-swirl and co-swirl arrays. Experiments were carried out over a wide range of inlet velocity from 4 m/s to 15.5 m/s and equivalence ratio from 0.5 to 0.85. Phase-locked OH planar laser-induced fluorescence was employed to measure flame shape and identify heat release rate. Four operation regions: stable combustion region, unstable combustion region, flashback region and extinguish region are observed for both array burners. The stable operating window for counter-swirl array is wider compared to the co-swirl array. Pressure fluctuation amplitude for co-swirl burner is larger than the counter-swirl arrangement at the same operating condition. In the unstable combustion region, the counter-swirl flames trigger the 2L mode of the combustion system while co-swirl flames incite three longitudinal modes with the highest amplitude near 3L. Rayleigh index distribution reveals neighboring flame interaction results in thermoacoustic coupling for multi-nozzle flames. Additionally, for counter-swirl array, thermoacoustic couplings in flame base region and shear region are also the main reasons for inducing self-excited combustion instabilities. For co-swirl array, the instability driving zones also locate at the lip region and the tail of center flame.

Three-dimensional (3-D) flowfield data has been obtained using Particle Image Velocimetry (PIV) for varying swirl distributions in the burner. The 3-D data also allows one determine the local swirl number of the resulting flow. Flow characteristics of the resulting flowfield, both without and with combustion, have been examined for the effect of co- and counter-swirl under lean direct injection conditions using unconfined and confined combustor geometry. Experimental results of the effect of swirl and combustion are presented to simulate the flow dynamics of Lean Direct Injection (LDI) configuration gas turbine combustion. The selected configuration is typical because it does not make use a premixing zone and relies totally on the swirl and the injector to accomplish rapid mixing. Specifically, the effect of radial distribution of combustion air and swirl in a burner are examined under non-burning and burning conditions using propane as the fuel. The present study explores single swirler interaction with the use of an experimental double concentric swirl burner that simulates one swirler of a practical gas turbine combustor. Results showed that both swirl and combustion has significant effect on the characteristics of the internal and external recirculation zones. The calculated local swirl number differs significantly form that estimated using geometrical relationship derived from the vane angle only. The effect of combustion for the confined and unconfined geometries was also been found to be different. In the confined geometry combustion decreases the size of the recirculation zone. This is in contrast to that found for the unconfined conditions. Combustion enhances the recirculation zone in the unconfined geometry. Combustion provides greater velocity magnitudes than their counter non-combustion conditions. The counter-swirl combination resulted in smaller and more well defined internal recirculation regions. The results provide the role of swirl and combustion on the mean and turbulence characteristics of flows over a range of swirl and shear conditions between the inner and outer flow of the burner. This data provides important insights on the flow dynamics in addition to providing data for model validation and model development.

This paper describes an experimental investigation of the role of swirl distribution in a co-annular swirl-stabilized spray combustor for passive control of flame structure using kerosene fuel. Three distinctly different flames have been examined. A low shear, a high shear, and a counter-rotating swirling flow flame have been examined. The droplet dynamics and flowfield associated with the low and high shear co-rotating swirl configurations under isothermal and combustion conditions have been examined using a phase Doppler interferometric techniques. The high-shear swirl configuration was found to decrease droplet diameter in the shear region, indicating secondary atomization of larger size droplets due to strong shear effects in the flow. Droplet mean and turbulence characteristics were obtained. In order to simulate oscillations in the flow pulsations were introduced into the fuel flow in order to excite instabilities in the burner. The role of swirl distribution was determined on the attenuation of this imposed instability. Swirl distribution was found to be effective in reducing the instability. Instabilities at the forcing and harmonic frequencies are found to be up to 5 dB more powerful in the counter-rotating flow, demonstrating the role of swirl on flame structure and the associated combustion instability in swirl-stabilized spray combustors. The airflow distribution in the burner was also found to play an important role on the alleviation of combustion instability.

Three-dimensional (3-D) flowfield data has been obtained using Particle Image Velocimetry (PIV) for varying swirl distributions in the burner. The 3-D data also allows one determine the local swirl number of the resulting flow. Flow characteristics of the resulting flowfield, both without and with combustion, have been examined for the effect of co- and counter-swirl under lean direct injection conditions using unconfined and confined combustor geometry. Experimental results of the effect of swirl and combustion are presented to simulate the flow dynamics of Lean Direct Injection (LDI) configuration gas turbine combustion. The selected configuration is typical because it does not make use a premixing zone and relies totally on the swirl and the injector to accomplish rapid mixing. Specifically, the effect of radial distribution of combustion air and swirl in a burner are examined under non-burning and burning conditions using propane as the fuel. The present study explores single swirler interaction with the use of an experimental double concentric swirl burner that simulates one swirler of a practical gas turbine combustor. Results showed that both swirl and combustion has significant effect on the characteristics of the internal and external recirculation zones. The calculated local swirl number differs significantly form that estimated using geometrical relationship derived from the vane angle only. The effect of combustion for the confined and unconfined geometries was also been found to be different. In the confined geometry combustion decreases the size of the recirculation zone. This is in contrast to that found for the unconfined conditions. Combustion enhances the recirculation zone in the unconfined geometry. Combustion provides greater velocity magnitudes than their counter non-combustion conditions. The counter-swirl combination resulted in smaller and more well defined internal recirculation regions. The results provide the role of swirl and combustion on the mean and turbulence characteristics of flows over a range of swirl and shear conditions between the inner and outer flow of the burner. This data provides important insights on the flow dynamics in addition to providing data for model validation and model development.

Abstract The present article experimentally investigates the influence of pilot swirling directions on low-frequency combustion instabilities of pilot diffusion flames in a laboratory-scale combustor with Jet A-1 fuel and air at atmospheric pressure. Airblast atomization nozzles with either counter-rotating (CTR) or co-rotating (COR) pilot swirl flows were examined using nonlinear time-series analyses and high-speed flame imaging measurements under idle and sub-idle operating conditions. We show that while the amplitude and frequency of limit cycle oscillations are observed to be similar for both cases, detailed examinations of measured experimental data reveal marked differences in stabilization mechanisms and pressure-heat release coupling processes. The spray flame dynamics subjected to counter-rotating swirl flows are governed by large-amplitude pressure oscillations, even under the influence of destructive pressure-heat release rate interference. The mechanism of destructive interference is closely related to the interactions between a spiral diffusion flame and a periodically-detached reaction zone. Non-premixed liquid-fueled flames involving co-rotating swirl, on the other hand, feature a more compact and intense reaction zone.

Abstract An experimental investigation has been carried out on four premixed flames using a double concentric swirl burner. The influence of radial distribution of swirl on the global flame behavior, thermal and emission characteristics have been determined. Temperature data was compensated with time constant data to generate mean and fluctuating temperature maps, probability density distributions of temperature, power spectra and thermal integral- and micro-time scales in the flames. Direct flame photographs were taken to archive flame shape and light intensity. These data provided valuable information for practical combustors on better swirl configurations. These configurations will depend heavily on the designed power settings. Contributed results directly from this investigation are for a lean premixed flame to achieve low emission and higher efficiency. The results reveal that in a counter-swirl configuration, the swirl strength in inner annulus should be greater than the outer annulus. However, for the co-swirl configuration it is more beneficial to have smaller swirl strength in the inner annulus than the outer annulus. It is found that premixed flames can possess significant circumferential non-uniformities at all flow and operational conditions. Detailed data shows that the flame thermal field and temperature distribution is strongly related to its emissions. High NOx emission is found in flames where the integral time scales and the mean temperature are high in the presumed lower recirculation region. High CO formation is found in flames that have high integral time scales in the lower recirculation region but have a low mean temperature in this same region. Thermal time scales provide important information on the thermal and emission characteristics of premixed flames.

This study provides the role of co- and counter swirl distribution in a experimental double concentric swirl burner that simulates the that simulates one swirl cup of a practical gas turbine combustor. Results of the effect of radial distribution of swirl in a burner under unconfined non-burning and combustion conditions are presented on the flow dynamics of a fuel-lean direct injection (LDI) configuration using propane as the fuel. Three-dimensional (3-D) flowfield data has been obtained immediately downstream of the burner exit to determine the detailed flow dynamics associated with the flow. The fuel was injected radially into the surrounding swirl flow. Flow characteristics, both without and with combustion, have been obtained for the co- and counter-swirl distributions to the combustion air flow under unconfined conditions. Flat vane swirlers have been used to induce swirl to the air flow. Both combustion and swirl distribution significantly influences the resulting flowfield. The resulting swirl number of the flow was calculated using the 3-D velocity data. Results show that swirl distribution in the burner and combustion provides significant effect on the characteristics of the internal and external recirculation zones. The heat release from combustion enhances the inner recirculation zone by increasing its width and length. Combustion causes significant increase to the velocity and vorticity magnitudes in the flow, and promotes flowfield symmetry. Combustion also affects the swirl number of the flow. The swirl number calculated from the geometrical relationships, derived from the swirl vane angle and swirler dimensions, is much different than that determined from the 3-D velocity field data. The entrained mass flow rate is larger for the co-swirl distribution case and this entrainment is further enhanced with combustion. The results provide the role of radial swirl distribution on the mean and turbulence characteristics of flows for the two different shear flow conditions between the inner and outer annulus of the burner.

Chemiluminescence and thermal imaging techniques have been used to examine the chemical and thermal behavior of turbulent flames with practical application to TBCC propulsion systems. The present study examines single swirler using an experimental double concentric swirl burner that simulates one swirler in a practical gas turbine combustor. The optical emission spectroscopy (OES) technique have been used to provide information on selected species in flames that mark the flame reaction zone and heat release rate. The instantaneous images are then integrated to obtain time-averaged information. Spatial distribution of OH, CH, C2 species from within the flames have been obtained at selected wavelengths using an ICCD camera and narrow band interference filters. The vibrational temperature distributions are obtained from the ratio of intensities of two discrete C2 bands of 470nm and 515nm. The time-averaged spatial distribution of flame generated radicals is processed using the Abel Inversion technique to project the initial 2-D image to represent the 3-D distribution of species and temperature in the flame. The results show that swirl distribution affects the shape of the spatial distribution by spreading the high intensity regions radially outwards with increase in swirl strength at inner regions of the fuel injector. Co-swirl distribution in the burner provided decrease in temperature and species intensity due to greater entrainment of the surrounding fluid. Calculated flame thermal strain rates were found to be significantly different for the co- and counter-swirl flames.

Three axial vane swirlers were investigated in a simulated sector of an annular combustor. The influence of co and counter rotating swirl on combustion emissions and flame stability are studied with kerosene as the fuel and inlet temperatures up to 800K. Comparison is made with one of the swirlers in a single swirler can combustor. The results show that the influence of the relative swirl direction does not influence the flame stability but does influence the combustion efficiency and NOx emissions. However, both systems are superior to a single swirler can combustor.

Three-dimensional (3-D) flowfield data has been obtained using Particle Image Velocimetry (PIV). Information on selected intermediate flame generated species that mark the flame front and heat release rate has been obtained using optical emission spectroscopy (OES). Instantaneous images on the spatial distribution of OH, CH, and C2 species from within the flames have been obtained using an ICCD camera and narrow band interference filters. The instantaneous images are then integrated to obtain time-averaged information. The PIV and OES diagnostics are employed for varying swirl distributions in the burner. The 3-D data also allows one determine the local swirl number of the resulting flow. Flow characteristics of the resulting flowfield, both without and with combustion, have been examined for the effect of co- and counter-swirl under lean direct injection conditions using unconfined and confined combustor geometry. The OES diagnostics provides information on the spatial distribution of desired species as affected by flame confinement and swirl distribution in flames. Experimental results of the effect of swirl and combustion are presented to simulate the flow dynamics of Lean Direct Injection (LDI) configuration gas turbine combustion. The selected configuration is typical because it does not make use a premixing zone and relies totally on the swirl and the injector to accomplish rapid mixing. Specifically, the effect of radial distribution of combustion air and swirl in a burner are examined under non-burning and burning conditions using propane as the fuel. The present study explores single swirler interaction with the use of an experimental double concentric swirl burner that simulates one swirler of a practical gas turbine combustor. Results showed that both swirl and combustion has significant effect on the characteristics of the internal and external recirculation zones. The calculated local swirl number differs significantly form that estimated using geometrical relationship derived from the vane angle only. The effect of combustion for the confined and unconfined geometries has also been found to be different. In the confined geometry combustion decreases the size of the recirculation zone. This is in contrast to that found for the unconfined conditions. Combustion enhances the recirculation zone in the unconfined geometry. Combustion provides greater velocity magnitudes than their counter noncombustion conditions. The counter-swirl combination resulted in smaller and better defined internal recirculation regions. OES results showed that swirl distribution affects the shape of the spatial distribution by spreading the high intensity regions radially outwards with the co-swirl configuration. Confinement increased the intensity and spatial distribution of the investigated species. The results provide the role of swirl, combustion and flame confinement on the mean and turbulence characteristics of flows over a range of swirl and shear conditions between the inner and outer flow of the burner. The results also provide important role on the flow and chemical behavior. This data provides important insights on the flow dynamics in addition to providing data for model validation and model development.