Central Recirculation Zone Research Articles

Aerodynamic effect on atomization characteristics in a swirl cup airblast fuel injector

To investigate the influence of swirling air flow field on the spray characteristics and the droplet behaviors of the swirl cup airblast fuel injector, a single swirler cup and a double swirlers cup airblast injector were operated in a wide range of fuel mass flow rates (mf) and relative air pressure drops (ΔPa). Various laser based diagnostics, including planar Mie scattering, phase-Doppler particle analyzer, and high speed particle imaging velocimetry, were employed to provide the information of spray angle, droplet Sauter mean diameter (SMD), droplet velocity, and air flow velocity. The results show that the air flow field was characterized by central toroidal recirculation zone (CTRZ), the double swirlers cup airblast injector generated stronger CTRZ than single swirlers cup airblast injector. The spray characteristics and the droplet behaviors with different size classes were significantly affected by the air flow field. The larger droplets (25–100 μm) with higher initial momentum were likely to keep its velocity when passing through CTRZ, when the mf increased, more large fuel droplets could pass through the CTRZ and expanded to both sides, resulting an increase in a spray angle. On the other hand, smaller droplets (<25 μm) with lower initial momentum were trend to be enrolled by the CTRZ; therefore, the spray droplets at lower mf were confined in the CTRZ. These droplets trend to coalesce with each other resulting in an increase of SMD in this region.

The flame structure and combustion dynamics in a steam diluted H2/Air micromix flame

The OH* chemiluminescence is used to investigate experimentally the effects of nozzle diameter (d), steam dilution ratio (D) and equivalence ratio (φ) on flame structure and combustion dynamics in a steam diluted H2/air micromix burner. Two flame types, anchored and lifted, are found. The OH* signal for the anchored flame primarily presents in flame shear layer zone (SLZ), while that for the lifted flame mainly appears in the flame central recirculation zone (CRZ) and flame tail zone (FTZ). The increase of the velocity of hydrogen (as the D decreasing or the φ increasing) would change anchored flame to lifted flame. The Power Spectral Density (PSD) analysis shows that changing from anchored to lifted flame would result a high OH* fluctuation frequency, which shows the same trend as the decrease of d. The Dynamic Mode Decomposition (DMD) analysis shows that the distribution of dominant mode is similar to the distribution of OH* signal. The SLZ is the primary oscillation zone for anchored flame and the CRZ for lifted flame. The anchored flame is more stabler comparing to the lifted one, and the lifted flame should be avoided in practical burners. Besides, the high mixing intensity of fuel and oxidizer is beneficial to the stability of micromix flame.

Mean Flow Characteristics Downstream of a Vortex Tube Separator Array

Vortex tube separator (VTS) arrays are used to filter foreign particles from turboshaft engine intakes. Arrays may consist of hundreds of separators that share a common surrounding plenum, potentially allowing for significant interactions between the devices. However, most VTS studies have been limited to individual separators acting alone. The current study first involves an experimental investigation of the fluid flow emerging from the outlets of a VTS array installed in a wind tunnel. Particle image velocimetry is used to measure the flowfield at planes , , and downstream of the outlets, where is the inlet tube diameter of a separator. The results indicate that the emerging vortices quickly merge into a single vortex with high circumferential velocity concentrated near the edges of the duct, and that a central recirculation zone (CRZ) develops. This is in contrast to the flow from a single VTS, which does not have a clear CRZ. Comparisons to similar combustion swirler flows lead to the hypothesis that VTS arrays may uniquely generate fast-developing CRZs that produce strong axial flow gradients near or even within some VTS outlets. Such gradients will alter the filtration performance of affected VTSs, which is highly sensitive to flow rates. To explain the onset of recirculation that is exclusive to the array configuration, the framework of a momentum integral model has been developed and is presented here. Additionally, computer-aided design files of the experimental geometry along with inlet/outlet boundary conditions are being made available for any future computational validation studies of this complex flowfield.

Experimental study of gas-particle flow in the coal-fired industrial boiler applied radial air staging at variable over-fired air distributions

Radial air staging is an innovative method for reducing NOx emissions from coal-fired boilers compared to conventional axial air staging. However, an important issue in using this method is to set the over-fired air (OFA) distribution rationally to achieve good boiler performance. This paper aims to optimize the OFA distribution by evaluating important gas-particle (GP) parameters that influence the pulverized coal (PC) combustion performance. The GP flow experiment is conducted in a laboratory furnace model of a coal-fired boiler with different OFA distributions. The experimental results show that as RIO (Two-level OFA distribution ratio) changes from 0:1 to 1:0, the mixture of the OFA jet and airflow in the main combustion zone is advanced, which is conducive to PC burnout. Meanwhile, the size of the central recirculation zone decreases, resulting in poor PC ignition. It is worth noting that at RIO of 1:0, the central recirculation zone disappears at section y/d = 2.5, indicating that there is no high-temperature flue gas downstream to heat the fresh PC particles, which is extremely unfavorable for the stable combustion of the PC. In addition, at RIO of 0:1, compared to other cases, there are more particles in the central recirculation zone, which promotes the reduction reaction of N element released from PC particles, thus reducing NOx emission. According to the comprehensive evaluation, it is recommended that the RIO of the two-level OFA be set to 1:1 when using radial air staging in a coal-fired boiler. The results provide new insights into the application of the air staging method for efficient and clean PC utilization.

Effect of fuel temperature on the structure of a high-pressure liquid-fueled swirl flame

Experiments were conducted in a liquid-fueled, piloted swirl burner operating at high pressure (1.0 MPa and 2.0 MPa) to study the influence of fuel injection temperature on the coupled flow and combustion processes. Simultaneous 10 kHz stereoscopic particle image velocimetry and OH* chemiluminescence measurements were used to characterize the overall flow and flame structure over a range of equivalence ratios at three discrete fuel temperatures: 367, 486, and 577 K. The location of the pilot flame was found to be insensitive to the fuel temperature, while the main flame moved closer to the injector as the fuel temperature was increased. This was accompanied by an increase in the axial velocity in the region of the main flame and a downstream shift of the shear layer along the border of the central recirculation zone. Analysis of the mean velocity fields relative to the location of maximum OH* chemiluminescence intensity showed an increase in velocity normal to the mean flame position with higher fuel temperatures. This indicates the flame speed increases with fuel temperature, which, along with the changes to the fuel injection process, helps explain the observed changes to the flow and flame structure.

A Numerical Study of Aero Engine Sub-idle Operation: From a Realistic Representation of Spray Injection to Detailed Chemistry LES-CMC

High altitude relight is a matter of increasing importance for aero engine manufacturers, in which combustion plays literally a vital role. In this paper we want to evaluate the predictive capability of a combined Smoothed Particle Hydrodynamics (SPH) and Large Eddy Simulation with Conditional Moment Closure (LES-CMC) approach for a spray combustion process at these extreme conditions. The focus is on the SPH modelling of the kerosene primary atomization, the extraction of realistic spray boundary conditions for LES-CMC and the effect of the spray on combustion. Interestingly, it will be demonstrated that the fragment size distributions resulting from the airblast atomization are characterized by bimodal behaviour during the relight process and that small and large fragments differ significantly in their dynamical behavior. This is shown to affect the combustion in the Central Recirculation Zone (CRZ). Very large fragments are even able to supersede the flame from the CRZ, such that endothermic pyrolysis becomes dominant, but simultaneously essential to sustain and stabilize the remaining flame with reactive pyrolysis species. The study proves the ability of our methodology for extreme operating conditions, in which experimental insights are hardly possible.

Study on combustion performance of microgas turbine combustor with different fuels

AbstractA low‐pollution single‐tube microgas turbine combustor using different gas fuels was studied. The combustion performance of two kinds of landfill gas and natural gas was tested in a single‐head combustor. The effects of fuel nozzle position, and the fuel distribution ratio between pilot and main combustion stage on pollutant emission were investigated under rated load condition. The flow field, temperature field, and pollutant generation characteristics with different fuel nozzle diameters and nozzle positions were analyzed by numerical simulation. The results show that: the combustor can form an obvious central recirculation zone, which is convenient for ignition and stable flame propagation; in the case of limited premixed length, the influence of fuel nozzle position on fuel–air mixing uniformity is less obvious than that of fuel nozzle diameter. The fuel–air mixing uniformity plays a decisive role in the temperature field and the generation of pollutants in the combustor. The combustor has good pollutant emission characteristics, and the NOx emission can be reduced to about 10 ppmv (15% O2 concentration) under the appropriate combination of fuel nozzle diameter and position, which provides a reference for the further optimization of low‐emission combustor.

Absolute instabilities and dynamics of helical vortices in twin annular swirling jets

Modern low-emissions gas turbine combustors commonly employ a twin annular swirling flow configuration that consists of a central annular inner jet and a surrounding annular outer jet. This paper investigates the instability dynamics of helical vortices of such a flow configuration in non-reacting laminar setting with a varying outer jet swirling ratio S. The corresponding base flow features a centerbody wake (CBW), an outer recirculation zone, and a lip recirculation zone at low swirl ratios, whereas at high swirl ratios, the CBW is replaced by a central recirculation zone (CRZ). The azimuthal mode with wavenumber m=1 is found to be absolutely unstable in the CBW region at low swirl ratios (S<0.8), though not large enough to trigger global oscillations. With further increased swirl ratio (S≥0.8), the CBW is suppressed and the CRZ supports a large region of absolute instability for both m=1 and m=2 modes. A three-dimensional nonlinear time stepping performed at S=0.8 confirms that the absolute instability of m=1 mode near the nozzle exit leads to the formation of a single-helix vortex in the near-field. Downstream of the CRZ, the m=1 mode transits to convective instability, whereas the m=2 mode is absolutely unstable. The single-helix vortex is consistently found to disappear in the far-field, where the flow dynamics is dominated by a double-helix vortex counter-winding around the tail of the CRZ.

Effects of swirl intensity on flame stability and NO emission in swirl-stabilized ammonia/methane combustion

Ammonia is an attractive clean fuel for its potential to reduce CO2 emission and fossil fuel consumption. However, ammonia combustion has been mainly discouraged by the low flammability and the high fuel NOx emissions. This study numerically investigates the effects of swirl intensity on combustion characteristics in swirl-stabilized ammonia/methane combustion. Large eddy simulation was performed by using OpenFOAM, and the accuracy of numerical results was validated by the experiment. The results show that swirl intensity will significantly affects the flow field and flame structure, and flame stability decreases with the swirl number increasing. The high swirl intensity leads to extinguishing at the flame root, while the low swirl intensity cannot generate the central recirculation zone to stabilize the flame. Selective non-catalytic reduction of NO (SNCR) is the key to controlling NO emission, the decrease of temperature will promote the reduction of NO, while the decrease of residence time will inhibit the reduction of NO. Meanwhile, SNCR will weaken the correlation between NO and OH in ammonia/methane combustion. NO emission will decrease with the swirl intensity increasing. In a word, medium swirl number (S ≈ 0.76) is proper to control NO emission and ensure flame stability.

Numerical investigation on combustion flow characteristics of a micro gas turbine swirl combustor with different protruded bluff body structures

A micro gas turbine swirl combustor with protruded bluff body is proposed. Compared with no bluff body, the protruded bluff body can significantly improve the combustion performance. When the bluff body height is 40 mm and the length is 30 mm, the average outlet NO emission is reduced by 14.43%, the pressure loss is decreased by 5.96%, and the outlet temperature distribution factor (OTDF) is dropped by 31.93%. The influence of different bluff body lengths (15–30 mm) and heights (10–50 mm) on the combustion flow are numerically analyzed further. The results show that the change of bluff body structure will affect the performance of combustor. In the range of 15–25 mm, with the increase of the bluff body length, the scope of central recirculation zone becomes narrower, velocity near the inlet and pressure loss decrease, the OTDF and field synergy angle β both become larger. When the bluff body length grows from 15 mm to 30 mm, the average outlet NO emission increases by 36.74%. The range of the central recirculation zone is gradually widen, and the average reaction rate grows up with the increase of bluff body height. The synergistic effect becomes better, and the heat transfer capacity is enhanced. When the bluff body height grows from 30 mm to 50 mm, the average outlet NO emission reduces from 35.84 ppm to 25.65 ppm.

Numerical Simulation of NO x Formation in Non-Premixed Methane Combustion Using Internal Flue Gas Recirculation

ABSTRACT Internal flue gas recirculation (IFGR) is one of the most effective techniques for NO x reduction in boilers. Previous studies mainly focused on premixed combustion; however, the NO x formation principles in non-premixed methane/air flames are still unclear. This work aims to investigate the NO x formation mechanisms in non-premixed methane combustion at various IFGR rates. A series of three-dimensional computational fluid dynamics (CFD) simulations with detailed mechanisms was conducted. The results show that NO x is generated below 30 mg/m3 for IFGR rates higher than 17.7%, corresponding to the oxygen concentration in the oxidant mixture below 0.15. In high IFGR rates, the incompletely mixed oxidants and the presence of the central recirculation zones weakens the NO x reduction performance. Influenced by the decreased temperature by IFGR, the generation of thermal NO x is largely eliminated from 66 to 1 mg/m3. Prompt NO x contributes more than thermal NO x for the IFGR rates above 12.2%. The reaction range of thermal NO x is found moving upstream toward the nozzle with the axial distance of 0.1 m at most. IFGR gradually reduces prompt NO x but its spatial reaction ranges are hardly influenced. IFGR has no obvious effects both on the reaction rates and ranges on the minorities of the N2O and the NNH routes. This work connects the IFGR rates and the NO x formation pathways in non-premixed methane combustion, and highlights the importance of eliminating prompt NO x to achieve low-NO x combustion rather than sole NO x suppression method, which can provide foundation for designing low-NO x techniques.

Stability and structure of lean swirling spray flames with various degrees of prevaporization

Achieving full premixing and complete evaporation in lean prevaporized premixed combustors is challenging as it depends on the spray injector characteristics, prevaporisation strategy, and flow conditions. This article experimentally explores the stability and structure of a turbulent swirling n-heptane spray flame under various degrees of prevaporization. The results show that preheating the air to 343 K and 393 K has little effect on the lean blow-off velocity, while recessing the fuel injection significantly decreases the lean stability limit. To correlate these limits, various attempts to define a Damköhler number were made, but unlike previous studies with no prevaporisation, the difficulty in defining laminar flame speed in the present case does not allow a single correlation to work for all degrees of prevaporization. Four stable cases that differ in equivalence ratio, air preheat temperature, and fuel injection recess are investigated using one-dimensional PDA, OH* chemiluminescence and CH[Formula: see text]O-planar laser-induced fluorescence (PLIF). Cases without fuel injection recess or air preheat exhibit a conical-shaped heat release zone near the shear layers. Preheating the air to 393 K reduced the Sauter mean diameter, increased prevaporization, and enabled a second heat release zone downstream of the fuel injection. Recessing the fuel injection by 25 mm reduced droplet velocities and led to a semi-spherical instead of a conical heat release zone. The CH[Formula: see text]O-PLIF signal without injection recess was high along the central axis and its distribution resembled that observed for spray jet flames. In contrast, with recessed spray injection, CH[Formula: see text]O was mainly found outside the central recirculation zone and only appeared inside during lean blow-off; similar to previous work with premixed flames. These findings show that different methods of prevaporization, which only differ by subtle changes in droplet characteristics, strongly impact flame stability. The present data can be used for turbulent flame modelling focusing on sprays and finite-rate kinetics.

Numerical Study of Turbulent Swirling Diffusion Flame Under Lean and Rich Conditions Using Turbulence Realizable k-epsilon Model

ABSTRACT This work investigates numerically a non-premixed swirling flame in a coaxial burner with a radial fuel injection under lean and rich conditions of mixtures. The swirler is placed in the annular part of the burner. The fuel is injected radially through eight holes symmetrically distributed on the periphery of the central tube. All simulations are carried out using the ANSYS-Fluent CFD code. The turbulence is captured using the Reynolds Averaged Navier-Stokes approach. Chemistry/turbulence interaction is resolved using the Eddy Dissipation Model. Simulations are performed with a global equivalence ratio ranging from 0.5 to 1.3. Model validation is achieved by comparing computed results to our experimental data of Stereoscopic Particle Image Velocimetry obtained in the case of the stoichiometric regime. Good agreement between numerical results and experimental measurements is assigned. The central recirculation zone and the swirling jet region due to the presence of the swirl are well predicted by the simulations. The effect of the global equivalence ratio on the profile of axial velocity, temperature distributions and pollutant emissions (CO and NOx) is numerically studied. From dynamic point of view, the equivalence ratio modifies the mean axial velocity of the swirling diffusion flame. The increase of equivalence ratio destabilizes the flame by increasing the liftoff height. NO production decreases by the increasing of the equivalence ratio.

Effects of swirl numbers on the unsteady characteristics of pilot flame in a triple-swirler staged combustor

This paper studies the effects of the inner and outer pilot swirlers on the flame structures and thermo-acoustic instabilities in the triple-swirler staged combustors under various inner and outer pilot swirl numbers. We obtain energetic modes in the combustor's velocity and the OH radical mass fraction fields, utilizing the proper orthogonal decomposition (POD) technique. With the increment of the pilot inner swirl number, the vortex shedding mode and flame radial propagation mode for the flame fluctuation is enhanced, and the axial propagation mode is weakened. Nevertheless, the increased pilot outer swirl number primarily impacts the vortex shedding mode's enhancement while positively influencing the other flame fluctuation modes. Specifically, the vortex shedding in the inner shear layer is the leading factor affecting the thermo-acoustic instability in the combustion chamber. Nevertheless, coupling this mode and the pressure oscillation weakens the area downstream of the central recirculation zone. Thus, due to the comprehensive impacts of the vortex shedding mode, spatial distribution, and the premixed flame area proportion, the impact of this mode's energy ratio on the thermo-acoustic instability increases non-linearly with the increased pilot swirl number.

Experimental and numerical studies of the effects of the contraction ratios on the swirling flow characteristics of the model combustor outlet in lean gas turbines

Large Eddy Simulation (LES) is utilized in this work to study isothermal turbulent swirling flow characteristics for a lean partially premixed gas turbine model combustor. The goal is to investigate the influence of contraction ratio (CR) variation on the vortex breakdown phenomena features and the turbulence structure in these combustors. Particle image velocimetry (2D-PIV) measurements were carried out on a single-case model combustor using (air-air) phase to perform the mean flow behavior and the validation process under atmospheric conditions with CR equal to 0 (largest diameter, Dinlet = Doutlet), the equivalence ratio (φ) of 0.3, swirl number of (Sn) = 0.66 and Reynolds number of (Re) = 42,000. While five test cases with various CRs were applied in LES under the same conditions using (air-CH4) phase to predict the vortical structure and the precession vortex core (PVC). The CRs ranged as follows: 0, 50 %, 60 %, 70 % and 80 % (smallest diameter). Experimental and LES results show that there was a reasonable agreement between the PIV and LES results with maximum deviations of 2.8 % and 4.65 % for the normalized mean axial and radial velocities, respectively. As the CR was increased, the pressure drop and the mean axial velocity increased linearly at the contraction part. In addition, the downstream stagnation point and the size of the central toroidal recirculation zone (CTRZ) stabilized when CR = 60 %, 70 %, and 80 %. The swirl level was further enhanced by 40 % when CR = 60 % and 80 %. The presence of CR successfully suppressed the pressure and axial velocity fluctuations generated in the shear layer zones by 35 %. LES data was analyzed by Fast Fourier Transformation (FFT) and Proper Orthogonal Decomposition (POD) to investigate the PVC dynamics. The FFT showed four dominant frequencies as a result of the appearance of the PVC and vortex breakdown oscillation. The frequency of PVC (fpvc) decreased linearly with an increase in the CR where the fpvc was 20.8 Hz when CR = 0 and became 12.5 Hz at CR = 80 %. In contrast, the other frequencies were still constant even with increased CR at 187 Hz, 562 Hz, and 979 Hz. The POD analysis showed the first mode was more energetic and had 40 % of the total pressure fluctuations.

Numerical analysis on hydrogen swirl combustion and flow characteristics of a micro gas turbine combustor with axial air/fuel staged technology

• 1 Axial air staging, axial fuel staging and jet stabilized technology are combined with swirl combustion. • 2 Outlet NO emission decreases from 20.2 ppm to 2.28 ppm as secondary air ratio grows from 0% to 40%. • 3 Fuel staging has a more significant effect on reducing NO emission than air staging. To analyze the effects of the two staging techniques on combustion characteristics and emissions, the influence of different secondary oxygen ratios (0-40%) and secondary fuel ratios (0-10%) on the combustion flow, emission characteristics, field synergy and other parameters of the micro gas turbine combustor are investigated by numerical simulation based on computational fluid dynamics (CFD) method. The realizable k-epsilon model is adopted for turbulence simulation, the combustion model is general finite rate model. The results show that with the increase of secondary oxygen ratio from 0% to 40%, scope of central recirculation zone becomes smaller, outlet velocity grows, OTDF becomes larger and pressure loss rises, the average outlet NO emission decreases from 20.2 ppm to 2.28 ppm, synergy angle β of the central plane becomes larger, and the change mainly occurs in the recirculation zone and the rear of the combustion chamber. As the secondary fuel ratio grows, outlet velocity decreases, pressure loss drops, and the outlet temperature distribution becomes more and more uniform. The synergy between velocity and temperature gradient of the central plane becomes better. When the secondary fuel ratio is 10%, the maximum NO emission on the central axis is 9.27 ppm, which is only 8.51% of the fuel unstaged value. The outlet NO emission does not drop monotonously with more secondary fuel. Fuel staging has a more significant effect on reducing NO emission than air staging.

Experimental and Computational Investigation upon Combustion Characteristics of Liquid Fuel in a Novel Combustor with Hybrid Swirl and Recirculation Bowl.

In the present study, a novel hybrid swirl combustor is designed to reduce the size and complexity of a conventional gas turbine combustor. In this combustor, a dual-swirl pattern is adopted by providing the central vane swirler (45° vane angle) and circumferential tangential injection scheme to achieve higher recirculation of heat and combustion products inside the combustor. Numerical and experimental studies are carried out to understand the flow patterns and combustion characteristics in this high mass-heat interacting environment. Initially, computational studies were carried out to find the optimum geometry for greater recirculation and interaction among the reacting species inside the combustor. Liquid fuel (kerosene) is sprayed into the combustor for two thermal inputs of 25 and 50 kW. Three cases were studied to analyze the effect of bowl recirculation and tangential air inputs in addition to the swirlers. The hybrid swirl, formed by the counter-flow pattern, helps in achieving low and uniform temperature throughout and assists in flame anchoring. The tangential air flow provides a push to the combustion products from the downstream to the central recirculation zone of the combustor. The combined effect of central and tangential swirlers helps in attaining a more distributed combustion. The CO and NO emissions reduced with the use of hybrid swirl.

Computational Investigation on the Impact of Coal Feed Size in a Tangentially Fired Gasifier

ABSTRACT Using a validated comprehensive model, the effect of coal particle size on two-phase flow inside a tangentially fired, Mitsubishi Heavy Industry entrained-flow coal gasifier is investigated. It is observed that the length of the central recirculation zone (CRZ) formed in the gasifier flow increases with an increase in the mean size of the injected coal particles. The CRZ lengths are 5.6, 4.5 & 2.1 m, respectively, for the cases with particle size distributions (PSDs) having mass mean diameters of 80, 40 & 20 µm. The generic swirl number, , captures this impact of particle size on the CRZ length, whereas the conventional swirl number, , is unable to do so. The value of does not vary significantly over most of the gasifier’s constant diameter zone in each case simulated in the present work, thereby underlining the utility of this swirl number to characterize similarity in swirl dynamics. Further, the analysis of the impact of coal particle size on char conversion yields that the PSD with finer particles leads to higher conversion due to the better combustibility of finer particles.

Experimental investigation into the effect of main stage swirl on flow and spray characteristics inside a stratified partially premixed combustor

Lean combustion has been applied in gas turbine combustors to meet increasingly stringent regulations for pollutant emissions. As a representative low-emission technology, stratified partially premixed (SPP) combustion is capable of effectively reducing NOx emissions while still providing robust flames. In this work, the flow and spray processes inside an SPP combustor were experimentally investigated to further reveal the role of main stage swirl in flow development and fuel placement. Particle image velocimetry (PIV) and Planar Mie scattering (PMie) techniques were applied to provide the velocity distributions and droplet distributions under isothermal conditions with a relative pressure drop ranging from 1% to 5%. A spray post-processing algorithm was developed to acquire the characteristic parameters of the sprays without clear boundaries. Results show that the strong swirl from the main stage leads to the radial flow of pilot air streams and dominates the formation of central toroidal recirculation zone (CTRZ). With the addition of the main stage swirl, a proper flow structure including jet zones, recirculation zones and shear layers is formed, which promotes the widening of droplet radial distribution and the increase of the spray cone angle. The spray distribution therefore transforms from a bubble shape into a cone shape. The findings and conclusions could promote the understanding of interactions between pilot and main stage swirls for stage combustors and contribute to the optimization of such SPP injectors.

Lean blow-off of premixed swirl-stabilised flames with vapourised kerosene

Lean blow-off (LBO) conditions, duration of the extinction transient, and flame structure were investigated for swirl-stabilised, fully-premixed flames operated with methane and vapourised kerosene. High-speed (5 kHz) OH* chemiluminescence, OH-, and Fuel-PLIF imaging were used. Low-speed (10Hz) CH2O-PLIF imaging was also implemented to visualize the structure of CH2O. OH* chemiluminescence imaging of the LBO transient suggested that the primary reaction regions were confined within the central recirculation zone (CRZ). Sharp temporal increases in OH* signal were observed in the CRZ for both fuels, possibly due to flame re-ignition events during LBO transients. Both fuels showed a decrease in flame length and an increase in fragmentation in downstream regions. In the OH-PLIF images, pockets void of OH entered the CRZ from downstream. The CH2O- and Fuel-PLIF images suggest that these pockets contain either partially burned or fresh reactants. Furthermore, the CH2O- and Fuel-PLIF images indicated that CH2O and Fuel exist in the outer recirculation zone (ORZ) during the early stages of the LBO transient. The blow-off duration was calculated by integrating the OH* signal and for kerosene was found to be 1.5 times longer than for methane. In addition, the equivalence ratio at which kerosene blew-off was consistently lower than that of methane for a range of bulk flow velocities. These results indicate that fuel chemistry effects are important for the LBO condition and duration.