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Improvement on Ignition Performance for a Lean Staged Low NOx Combustor
Abstract
KHI (Kawasaki heavy industries Ltd, Japan) and JAXA (Japan Aerospace Exploration Agency) have been working together since 2004 to improve lean staged concentric fuel injector technologies. One of the weak points of a lean staged fuel injector is said to be ignition / light around performance. Ignition characteristics were assessed on several fuel injector configurations in burner tests. Laser diagnosis, CFD analysis and high-speed video camera recording were used to understand the effect of fuel injector geometry on fuel spray distribution and ignition characteristics. They showed a clear relationship between the burner geometry and ignition characteristics. Light around characteristics was evaluated with the burner configuration optimized in burner tests. Light around performance deteriorated in multi sector unit compared to that in burner test. CFD analysis and some ignition tests with different configuration of combustor gave a clue to restore the light around characteristics deteriorated in multi sector unit.
... Recently, Meier et al. [8] studied the effect of air/fuel ratio on soot formation during pilot-only operation by planar laser-induced incandescence and it was found that soot is formed predominantly in the upstream directed part of the shaped pilot flow near the interface to the outer main flow. Kobayashi et al. [9] investigated the effects of swirler angle and swirler lip configurations on ignition performance, and the ignition performance was improved by extending heat shield to suppress the interaction of pilot flame and the main air stream, but other important performances of combustion were not evaluated. Dhanuka et al. [10] quantify sources of unsteadiness of lean premixed prevaporized gas turbine combustors; the time-averaged velocity fields indicate that the heat release greatly distorts the shape of the recirculation zone and the velocity field and there is a positive velocity on the centerline at the reacting case, while the nonreacting case displays a negative velocity on the centerline. ...
... Only pilot burner is fueled at the low power settings, such as idle and approach conditions, and the pilot and main burner are both fueled at high power settings, such as climb and take-off conditions. Compared with fuel injection devices in the previous work [4,5,8,9], the pilot swirl cup injector optimizes both the pressure swirl atomizer and swirler to improve the spray quality in terms of fuel/air mixing and spatial distribution by adjusting characteristics of primary spray, structure of venturi prefilmer, and air mass flow ratio of two swirlers, such as enlargement of spray cone for enhancing interactions between primary spray and swirling air, and augmentation of the operating range of pressure swirl atomizer in pilot stage for ensuring spray quality both at the low and high power settings. The right view of details of the main swirler is seen in Fig. 2. The variation of main swirl angles is 40 deg, 45 deg, and 50 deg, respectively. ...
... Although the flow-flow interactions between the pilot and main air streams enhance when increasing the main swirl angle from 40 deg to 50 deg, the entrainment variation of recirculating main air streams on droplets of pilot spray is slight because of collapse of pilot spray cone. Moreover, the concentration of fuel near the centerline increases in the process of altering the main swirl angle from 40 deg to 50 deg, which is beneficial to combustion stability performances (such as phase 2 in ignition process [1] and flame anchoring near LBO condition) basing on the conclusion in previous studies of Kobayashi et al. [9] and Liu et al. [15]. The verification experiments of ignition and LBO combustion performances were carried out. ...
In order to reduce NOx emissions, modern gas turbines are often equipped with lean-burn combustion systems, where the high-velocity fuel-lean conditions that limit NOx formation in combustors also inhibit the ability of ignition, high altitude relight, and lean combustion stability. To face these issues, internally staged scheme of fuel injection is proposed. Primary and main fuel staging enable fuel distribution control, and multi-injections of main fuel lead to a fast and efficient mixing. A fuel-staged low emission combustor in the framework of lean-burn combustion is developed in the present study, i.e., the central pilot stage for low power conditions is swirl-cup prefilming atomization, the main stage is jet-in-crossflow multi-injection, and a combination of primary and main stage injection is provided for higher power output conditions. In lean-burn combustors, the swirling main air naturally tends to entrain the pilot flame and quench it at low power conditions, which is adverse to the operability specifications, such as ignition, lean blow-out (LBO), and high-altitude relight. In order to investigate the effects of the main swirl angle on combustion performances, the ignition and LBO performances were evaluated in a single dome rectangular combustor. Furthermore, the spray patterns and flow field are characterized by kerosene-planar laser induced fluorescence and particle image velocimetry (PIV) to provide insight into spray and combustion performances. Flow-flow interactions between pilot and main air streams, spray-flow interactions between pilot spray and main air streams, and flame-flow interactions between pilot flame and main air streams are comprehensively analyzed. The entrainment of recirculating main air streams on pilot air streams enhances with the increase of main swirl angle, because of the upward motion and increasing width of main recirculation zone. A small part of droplets are entrained by the recirculating main air streams at periphery of combustor and a majority of droplets concentrate near the centerline of combustor, making that entrainment of recirculating main air streams on pilot spray and quenching effects of recirculating main air streams on pilot flame are slight, and the extinguishing effects can be ignored. The contributions of main swirl strength to improvement of ignition and LBO performances are due to enhancement of air/fuel mixing by strengthening turbulence level in pilot zone.
... Representative low emissions combustion technologies include Rich-burn/Quench/Lean-burn (RQL), [3][4][5] Lean Direct Injection (LDI) 6,7 and Lean Premixed Prevaporized (LPP). [8][9][10][11][12] In order to achieve better combustion stability and lower emission performance, LPP combustor can adopt separated stratified swirling spray flame mode, such as Twin Annular Premixing Swirler (TAPS) of General Electric (GE), 8 Lean-Burn of Rolls-Royce (R-R), 13 Lean stage fuel injector of Japan Aerospace Exploration Agency (JAXA) 14 and the Technology of Low Emissions of Stirred Swirl (TeLESS) of Beihang University. 15,16 These low emissions combustors generally adopt a centrally staged structure which is comprised of the pilot and main stages. ...
... The definition of NO x emission index of the main stage EI NOx;m is shown in Eq. (14). ...
Experimental investigations on NOx emissions of a single-cup, Lean Premixed Prevaporized (LPP), module combustor were carried out at elevated inlet temperature and pressure up to 810 K and 2.0 MPa, close to the real operating conditions of aero-engine combustors. This LPP combustor adopts centrally staged fuel injections which could produce separated stratified swirling spray flame. In the NOx emissions measurements, the ranges of dome equivalence ratio and fuel stage ratio were from 0.55 to 0.58 and 8% to 24%, respectively. The optical diagnosis on separated stratified swirling spray flame were carried out with fuel stage ratio changing from 15% to 30%. Therefore, NO* and OH* chemiluminescence images were obtained. The results show that NOx emissions increase with the increase of the fuel stage ratio. And from the chemiluminescence images, the main flame and pilot flame are found weakly coupled. The pilot flame plays a significant role in NOx emission production because of its higher adiabatic flame temperature. Based on the results of chemiluminescence optical tests, a new NOx emission prediction model is proposed based on the Lefebvre’s single flame model. The estimate of local equivalence ratio of the pilot stage’s non-premixed flame is modified considering the characteristics of spray combustion, and a “PLUS” emission prediction model suitable for separated stratified swirling spray flame is obtained. Compared to the experimental data, the “PLUS” model exhibits a good prediction in a range of ±13% of deviation. Keywords: Combustors, Lean staged combustion, Low emissions, NOx correlation, Optical diagnosis
... Phase 1 is the sufficient energy deposition of sparks to form a self-sustaining kernel. The flame propagation from the kernel within the injector is Phase 2. Then the stabilization of a single injector flame is Phase 3. The previous studies have been investigated experimentally and with LES in kernel growth (Chaudhuri et al., 2013;Haq, 2005;Law et al., 2005;Wu et al., 2014)and flame propagation through various single burner geometries (Ahmed et al., 2007;Ahmed and Mastorakos, 2006;Ahmed and Mastorakos, 2016;Letty et al., 2012;Marchione et al., 2009;Neophytou et al., 2012;Denton et al., 2018;Foust et al., 2012;Kobayashi et al., 2011;Lazik et al., 2008;Read, 2008;Suo et al., 2017;Wang et al., 2016;Yan et al., 2015). In addition to the ignition of a single burner, the combustion chamber ignition also includes the flame propagation from injector to injector in the annulus that corresponds to the Phase 4 called light-round, which has not been intensively studied in the laboratory. ...
In this paper, the injector-injector flow field, spray and flame patterns are investigated experimentally in an annular combustor under various mair and mfuel. The results show that the adjacent swirling jets have different velocity mangnitude on the central annular plane due to the curvature of the annular combustor. It can lead to a “deflection jet” instability and subquently switch on a periodic “wide-narrow-wide” CTRZ flow that does not evolve in time. Different bulk velocity airflow will not change the periodic CTRZ arrangement but the vortex structure inside the CTRZ. The spray and flame patterns are significantly impacted by the injector-injector flow field. The “rich fuel” region is consistent with the flame morphology. Both of their sizes rapidly decrease as the bulk velocity increases. Periodicity did not appear in spray and flame due to the weak recirculation and dispersed fuel downstream in low bulk velocity conditions. Under high bulk velocity, stronger recirculation air makes the shape of the spray and flame consistent with CTRZ. The effect of the periodic flow field on spray and flame becomes more apparent.
... A successful ignition process of full annular combustor features four phases according to the studies by Mastorakos [2,3]. Phase 1 is the sufficient energy deposition of sparks to form a self-sustaining kernel; The flame propagation from the kernel within the injector is Phase 2; Then the stabilization of a single injector flame is Phase 3. The previous studies have been investigated experimentally and with LES in kernel growth [4][5][6][7] and flame propagation through various single burner geometries [8][9][10][11][12][13][14][15][16][17][18][19][20][21], corresponding to the first three phases of the ignition process. In addition to that, the combustion chamber ignition also includes the flame propagation from injector to injector in the annulus that corresponding to the Phase 4 called lightround, which has not been extensively studied in the laboratory. ...
In this paper, the ignition transient process is experimentally investigated on a staged partially premixed annular combustor. This work focuses on the impact of bulk velocity and equivalence ratio on light-round speed and characteristic of flame propagation during light-round process. The set-up consists of two concentric transparent torus quartz walls and it is equipped with an air-curtain anti-pollution reflector above the combustor tilts at a 45° angle from the vertical, which allows visualization of the flame from both the side and the top of the combustor. High-speed images of the spontaneous flame emission with 5 kHz from the top view and 10 kHz from the side view are recorded. Image edge detection technology is used to analyze the flame front and leading point dynamics. Radial and azimuthal oscillations of the leading point trajectory indicate that there are the wrinkled turbulent flame fronts. The large span of flame fragment azimuthal propagation is due to swirling air in the combustor. It is found that the light-round process can be decomposed into three different distinct phases according to the light-round speed. The figures of light-round time and speed against equivalence ratio illustrate that bulk velocity and equivalence ratio play a positive role in light-round process under certain conditions (stochastic zone, linear descent zone). It is also confirmed that the ignition process is significantly impacted by the spray characteristics. The results presented in this paper bring new insights into the ignition of realistic gas turbine with centrally-staged combustor.
The ignition reliability of combustor is related to the safety of the whole machine, especially the high-altitude ignition and combustion stability, which are the common requirements of the combustor. In this paper, the simulated altitude test facility was introduced. And the ignition performances of a linear combustor with five burners were carried out under the simulated altitude of 0–8 km, including the ignition boundary and flame propagation between burners. The process of flame propagation was recorded by a high-speed camera. The results indicate that the fuel-air ratio increases with the increase of altitude at the same pressure drop, and it decreases with the increase of pressure drop. For the period of ignition delay, the evaporation of droplets and the chemical delay process are mainly affected with the increase of altitude. At the altitude of 6 km to 8 km, flame propagates from burner to burner in an axial pattern, and the influence of altitude on flame propagation is mainly manifested in ignition delay and initial flame propagation. The key to improve the reliability of high-altitude ignition is to improve the fuel atomization and evaporation characteristics. The contribution of this article is to explore the mechanism of altitude on ignition characteristics and propose two methods to improve the high-altitude ignition characteristics.
Lean Blow Out (LBO) poses a significant safety hazard when occurring in aero-engines. Understanding the lower stability limits of gas turbine combustors and the characteristics of spray flame close to LBO are imperative for safe operation. The objective of this work is to evaluate the effects of fuel decreasing rates and pressure drops of the injector on LBO performances in a multi-swirl staged combustor equipped with an airblast injector. A set of hardware and control system was developed to realize a user-defined fuel supply law. High-speed imaging was applied to record complete LBO processes under the conditions of linear fuel reduction and stable airflow. Partical Image Velocimetry (PIV) and Planar Mie (PMie) scattering were used to acquire the flow fields and spray fields under non-reacting conditions. Experimental results have shown that LBO limits extend to leaner conditions as the pressure drop of the injector increases. With an increase of the fuel decreasing rate, the exhaust temperature before flame extinction increases, and the LBO Fuel-to-Air-Ratio (FAR) decreases. The time evolution of the integral CH* intensity conforms to a linear function during the LBO process. Proper Orthogonal Decomposition (POD) was used to analyze the dynamic characteristics of lean-burn flames. Under different fuel decreasing rates and pressure drops of the injector, flames close to LBO present similar modal spatial distributions, alternately appearing axial, radial, high-order axial, and high-order radial oscillations.
Ground ignition and high-altitude ignition are important performance indexes of a combustor. In this study, an internally-staged combustor and five pilot-stage centrifugal atomizers with engineering scale were developed. Flow characteristics of these atomizers were studied under the atomizer pressure drop of 0.3 MPa-3.50 MPa, and the flow number of atomizers was obtained. Under the combustor relative air pressure drop of 1.5%-5.5%, the ignition performance of the internally-staged combustor was studied at normal inlet temperature with both normal and negative inlet pressures. In the ignition experiment, only the pilot-stage atomizer worked in the fuel system. The results indicated that, at normal inlet temperature and normal inlet pressure, the lean ignition fuel/air ratio kept increasing as the combustor relative air pressure drop increased from 1.5% to 5.5%. With the same relative air pressure drop, the ignition envelope could be widened by reducing the pilot-stage flow number. On the other hand, under normal inlet temperature and negative inlet pressure, only the pilot-stage flow numbers of 5.0 and 7.3 at the relative air pressure drop of 1.5% showed successful ignition, while ignition was not successful in other conditions or with other pilot-stage flow numbers. Based on the Lefebvre ignition model, an empirical correlation with prediction errors less than 5% was proposed for lean ignition fuel/air ratio of various pilot-stage flow numbers in the internally-staged combustor at normal inlet temperature and normal inlet pressure.
In order to establish the coupling connection of the ignition characteristics between the multi-injector burner and the annular combustor. The ignition process is investigated experimentally with/without the end-wall under the same multi-injector combustor rig. The spontaneous flame emission is recorded by an intensified high-speed camera on the side and top view. The image edge detection technology is used to obtain the leading point (LP) trajectories of the flame and the subsequent quantitative information (ignition delay time, light-round speed et al.). The results illustrate that a successful ignition of the multi-sector combustor can be decomposed into three flame propagation phases (I, II, III). The flame propagation speed can be improved by increasing air velocity and equivalent ratio. This effect will be obvious when it is close to the light-round limits. The light-round limit of the annular combustor is slightly lower than that of the multi-sector combustor. The flame fronts propagate symmetrically on both sides and tend to move along the radial centerline in the multi-sector confinement. While they propagate asymmetrically on both sides in the annulus confinement and are close to the inner or outer wall. The flame propagates slower in the multi-sector combustor than that in the annular combustor due to the restrictive effect of the end-wall on the expansion of the hot gas. This gap becomes greater when the flame approaches the end-wall. Higher bulk velocity conditions will also lead to a greater gap in light-round speed between annular and multi-sector combustors. This paper measures and summarizes the differences in ignition characteristics between the annular and multi-sector combustors. It is critical for the transition of the ignition prediction from the multi-injectors combustor to the annular combustor.
Dome spacing is one of the most critical factors to affect the ignition performance of the aerospace engine. In this paper, a three-dome model combustor is designed to investigate the effect of dome spacing on ignition performance with RP-3 liquid aviation fuel. The dome spacing considered are 48 mm, 60 mm, 72 mm, respectively, and the effect on ignition performance including ignition and propagation limits are revealed by a combustion experiments under ambient temperature and pressure, and the reference velocity is from 3 m/s to 8 m/s. The ignition performance is evaluated in terms of the fuel/air ratio, at which ignition occurs. The results show that the best ignition performance is achieved when the dome spacing is 60 mm. Furthermore, numerical simulations with validated methodology of non reacting flows inside the combustor have been conducted. The results show that when the dome spacing is 60 mm, the best mutual support between adjacent domes is attained according to the maximum value of mergin rate and dimensionless width of central toroidal recirculation zone. Besides, the restrictive effect of the primary hole jet on the central toroidal recirculation zone is found to be increased with the decrease of dome spacing.
Lean combustion has been applied in aero gas turbine engines in response to rising concerns regarding the effects of pollutant emissions on the environment. As one of low emissions combustion technologies, stratified partially premixed combustion is capable of reducing NOx emissions while still providing robust pilot flames. In this work, the processes of liquid fuel atomization, fuel–air mixing and spark ignition in a multi-swirl staged combustor with airblast atomizer are experimentally investigated to further illustrate the effects of flow field features on flame propagation and the underlying reasons leading to successful and failed ignitions (misfires). A trajectory processing algorithm is developed to identify the detailed flame propagation behaviors in successful and misfire scenarios. PIV, PMie and High-speed imaging technologies are applied to provide the velocity distribution, droplets distribution and time-resolved flame images, respectively. Evolution of projected flames shows that the mechanism of flame motions in the early phase following spark can be explained by non-reacting flow structures and spray distributions. It is demonstrated that the early phase propagating trajectories play an important role in determining overall successful ignition. The variation regarding the flow field, droplets distribution, and flame evolution under pressure drops varying from 1% to 6% are analyzed. The ignition and combustion reach a relatively stable state when the pressure drop of injector exceeds 4%, which reflects in the minimum LLO FAR varies around 0.021 ± 0.001 and the time of flame development stabilizes at 40 ± 5 ms.
The ignition of a lean staged injector aimed at aeronautical application is a transient and complex phenomenon, which involves fluid dynamics, turbulent mixing, chemical kinetics, as well as their mutual interactions. In the present research, a staged injector, designed based on stratified partially premixed combustion concept, is introduced. The ignition performance of stratified partially premixed injectors with different air split ratios between pilot swirlers are experimentally acquired, which exhibits apparent distinctions. In order to make quantitative analyses, the classical physical ignition model is improved, in which the flame propagation process is further divided into the axial and radial propagation sub-processes. Nonreacting flow field and discrete phase simulations, validated by experiment results, are utilized to obtain the velocity and spray distributions. Physical parameters characterizing the ignition sub-processes are defined and calculated based on the numerical simulation results. Conclusions are made by comparing the physical parameters of the ignition sub-processes. The radial propagation of the ignition kernel is responsible for the ignition performance difference between the two injectors with different pilot air split ratios (PASR) in that the average equivalence ratio along the radial propagation route of PASR = 7:3 is one order richer than that of PASR = 2:8. The present ignition analysis and model can be further extended and developed for the optimization of ignition performance of lean staged injector.
The lean blow-out performance of the pilot flame in a stratified partially premixed injector is studied in this work considering the influence of the main stage stratifier length. Particle image velocimetry and kerosene planar laser-induced fluorescence experiments at nonreacting conditions were carried out to give an explanation of the combustion performance. The evolutions of spray and velocity in space are discussed in detail to find out the inherent correlations of combustion stability, spray distribution, and flow structures. Moreover, special attentions are paid to the variation of primary recirculation zone with the increase of the stratifier length and its driving factors. Specifically, by lengthening the stratifier of the main stage, the primary recirculation zone is enlarged both axially and radially, and this creates an advantageous flow topology for the anchoring of the pilot flame. Finally, the dominating mechanisms for the primary recirculation zone that are responsible for the alteration of the primary recirculation zone size are pointed out with the assistance of computational fluid dynamics.
In order to balance the low emission and wide stabilization for lean premixed prevaporized combustion, the centrally staged layout is preferred in advanced aero-engine combustors. However, it is more difficult for the centrally staged combustor to light up as the main stage air layer will prevent the pilot fuel droplets arriving at igniter tip. The goal of the present paper is to study the effect of the main stage air on the ignition of the centrally staged combustor. Two cases of the main swirler vane angle of the TeLESS-II combustor, 20 degree and 30 degree are researched. The ignition results at room temperature and pressure show that the ignition performance of the 30 degree vane angle case is better than that of the 20 degree vane angle case. High speed camera, PLIF and CFD are used to better understand the ignition results. The high-speed camera has recorded the ignition process indicating that the initial kernel propagates along the radial direction to the combustor center. The CFD results show that the concentration of kerosene vapor of the 30 degree vane angle case is much larger than that of the 20 degree vane angle case close to the igniter tip and along the propagation route of the kernel, therefore, the 30 degree vane angle case has a better ignition performance. For the consideration of the ignition performance, a larger main swirler vane angle of 30 degree is suggested for the better fuel distribution when designing a centrally staged combustor.
The staged injector has exhibited great potential to achieve low emissions and is becoming the preferable choice of many civil airplanes. Moreover, it is promising to employ this injector design in military engine, which requires most of the combustion air enters the combustor through injector to reduce smoke emission. However, lean staged injector is prone to combustion instability and extinction in low load operation, so techniques for broadening its stable operation ranges are crucial for its application in real engine.
In this work, the LBO performance of a staged injector is assessed and analyzed on a single sector test section. The experiment was done in atmospheric environment with optical access. Kerosene-PLIF technique was used to visualize the spray distribution and common camera was used to record the flame patterns. Emphasis is put on the influence of pilot burner on LBO performance. The fuel to air ratios at LBO of six injectors with different pilot swirler vane angle were evaluated and the obtained LBO data was converted into data at idle condition. Results show that the increase of pilot swirler vane angle could promote the air assisted atomization, which in turn improves the LBO performance slightly. Flame patterns typical in the process of LBO are analyzed and attempts are made to find out the main factors which govern the extinction process with the assistance of spray distribution and numerical flow field results. It can be learned that the flame patterns are mainly influenced by structure of the flow field just behind the pilot burner when the fuel mass flow rate is high; with the reduction of fuel, atomization quality become more and more important and is the main contributing factor of LBO. In the end of the paper, conclusions are drawn and suggestions are made for the optimization of the present staged injector.
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