Lean Blowout Fuel-air Ratio Research Articles

Effect of the Swirl Intensity of Pilot Inner Swirler on the Combustion Stability of a Lean Staged Injector With a Prefilm Atomizer

Abstract Lean staged combustion can reduce the NOx emissions by prevaporizing and premixing fuel with air, which is considered the state-of-the-art solution strategy in achieving low emission in aeronautical combustors. However, lean premixed combustion is subjected to combustion stability problems, which restrict the ground and altitude operation limits of the commercial engine. In this work, the effect of the swirl intensity of pilot inner swirler on combustion stability of a lean staged injector is experimentally and numerically studied. The lean staged injector is piloted by a dual swirler prefilm atomizer. The swirl intensity of the pilot inner swirler is varied by parameterizing the vane angle as +20 deg, −20 deg, and −35 deg, with −20 deg selected as the baseline with a counterswirling design. A single sector model combustor is designed, and the nonreacting flow field and fuel concentration distributions are measured by particle image velocimetry (PIV) and kerosene planar laser induced fluorescence (kerosene-PLIF) techniques. The alteration of swirl direction from counterswirling to coswirling induces a negligible effect on flow structures, but the spray distribution changes from a solid pattern to a hollow pattern. The increase in the pilot inner swirl intensity causes a shrunk cyclone recirculation zone (CRZ) and a reduction of kerosene concentration in the central region. The influences of the pilot inner swirler angle on combustion stability are evaluated. The ignition and lean blow-out (LBO) results show that the baseline injector exhibits excellent combustion stability, while the coswirling design holds the highest ignition and LBO fuel–air ratio (FAR). In order to find out the physical mechanisms dominating the ignition and LBO processes, nonreacting numerical simulations are conducted to provide information regarding the flow structures and kerosene concentrations at ignition limits. Moreover, the ignition sequences are redefined as the radial flame propagation phase, the axial flame propagation phase, and the flame stabilization phase. The comparison of kerosene concentration along the radial and axial propagation routes concludes that the fuel enrichment in the two processes improves the ignition performance. On the other hand, the Karlovitz number of flame anchoring points in the flame rooting region is calculated to evaluate the flame stabilization characteristics. The results indicate that promoting the number of flame anchoring points and their radial range benefits the LBO performance.

Effect of Spray Cone Angle on Flame Stability in an Annular Gas Turbine Combustor

AbstractEffect of fuel spray cone angle in an aerogas turbine combustor has been studied using computational fluid dynamics (CFD) and full-scale combustor testing. For CFD analysis, a 22.5° sector of an annular combustor is modeled and the governing equations are solved using the eddy dissipation combustion model in ANSYS CFX computational package. The analysis has been carried out at 125 kPa and 303 K inlet conditions for spray cone angles from 60° to 140°. The lean blowout limits are established by studying the behavior of combustion zone during transient engine operation from an initial steady-state condition. The computational study has been followed by testing the practical full-scale annular combustor in an aerothermal test facility. The experimental result is in a good agreement with the computational predictions. The lean blowout fuel–air ratio increases as the spray cone angle is decreased at constant operating pressure and temperature. At higher spray cone angle, the flame and high-temperature zone moves upstream close to atomizer face and a uniform flame is sustained over a wide region causing better flame stability.

Preliminary design and experimental verification of a triple swirler combustor

A preliminary combustor design procedure based on triple swirler is presented, and then a triple swirler combustor with design fuel–air ratio of 0.033 is designed according to the design procedure and studied experimentally. The test results show that: the total pressure loss coefficient increases linearly from 0.021 to 0.04 as the inlet airflow velocity increases from 40 m/s to 70 m/s, while the flow drag coefficient decreases nonlinearly with the inlet airflow velocity increasing; there exists an optimal velocity, at which the ignition overall fuel–air ratio is minimal at each given inlet airflow temperature, and the increasing of inlet airflow temperature makes for combustor ignition; the lean blowout fuel–air ratio is no more than 0.005 for most cases at the inlet airflow temperature of 473 K; the combustion efficiency is high at low fuel–air ratio and increases with inlet airflow velocity and temperature increasing on the whole. Pattern factor (OTDF) ranges from 0.15 to 0.17 for all cases at the inlet airflow temperature of 473 K; the combustion performance validates that triple swirler is fit for the dome of high-temperature-rise combustor and the design rules of the triple swirler combustor mentioned in this paper are feasible.

Effect of different triple swirlers on the performance of a triple swirler combustor

Broadening the stable combustion range is particularly desirable for future aircraft engines. The triple swirler is considered to be a promising solution. Experiments were conducted to study different triple swirlers on the performance of a triple swirler combustor, which includes several technology innovations at different inlet airflow velocity (40–70 m/s), temperature (296 K, 373 K, and 473 K), and combustor overall fuel–air ratio with fixed atmospheric pressure. The total pressure loss coefficient increases linearly, while the flow drag coefficient decreases nonlinearly as the inlet airflow velocity increases from 40 m/s to 70 m/s. The flow drag of the combustor assembling counter-rotating swirlers for intermediate swirler and outer swirler is less than that of co-rotating swirlers at the same inlet airflow velocity. The ignition overall fuel–air ratio and lean blowout fuel–air ratio decrease along with inlet airflow velocity and temperature increasing on the whole. The triple swirler with swirl number combination labeling “1.5-1-0.8” has better combustion performance than the other one labeling “0.7-1-1.5”. At the temperature of 473 K, the lean blowout fuel–air ratio is almost below 0.005 for the triple swirler with swirl number combination labeling “1.5-1-0.8” at different inlet airflow velocity, and from this point, it has proved the feasibility of the design rules of triple swirler combustor in this paper.

CFD predictions of LBO limits for aero-engine combustors using fuel iterative approximation

Lean blow-out (LBO) is critical to operational performance of combustion systems in propulsion and power generation. Current predictive tools for LBO limits are based on decades-old empirical correlations that have limited applicability for modern combustor designs. According to the Lefebvre’s model for LBO and classical perfect stirred reactor (PSR) concept, a load parameter (LP) is proposed for LBO analysis of aero-engine combustors in this paper. The parameters contained in load parameter are all estimated from the non-reacting flow field of a combustor that is obtained by numerical simulation. Additionally, based on the load parameter, a method of fuel iterative approximation (FIA) is proposed to predict the LBO limit of the combustor. Compared with experimental data for 19 combustors, it is found that load parameter can represent the actual combustion load of the combustor near LBO and have good relativity with LBO fuel/air ratio (FAR). The LBO FAR obtained by FIA shows good agreement with experimental data, the maximum prediction uncertainty of FIA is about ±17.5%. Because only the non-reacting flow is simulated, the time cost of the LBO limit prediction using FIA is relatively low (about 6h for one combustor with computer equipment of CPU 2.66GHz × 4 and 4GB memory), showing that FIA is reliable and efficient to be used for practical applications.

Design and Performance Evaluation of a Two-Position Variable Geometry Turbofan Combustor

Variable geometry combustors offer the potential for reductions in ignition and lean blowout fuel-air ratios required for expanded operational envelopes, while maintaining acceptable levels of combustion efficiency and pattern factor at high-temperat ure-rise operating conditions. Following a screening procedure that evaluated simulated variable geometry configurations, a variable geometry mechanism was incorporated into the combustion system and test evaluated. The testing included ground level ignition, altitude ignition, stability, combustion efficiency, and discharge temperature quality as functions of three fuels: JP-4, JP-5R, and a blend of JP-5R and DF2. The test conditions were consistent with high-temperature-rise, advanced turbofan engine systems operating over a wide flight envelope. The test results showed a definite improvement in combustion performance and the variable geometry system functioned properly and sealed adequately. The lean blowout fuel-air ratio goal of 0.002 was achieved for the majority of test conditions with JP-4 fuel and the ignition fuel-air ratio goal of 0.008 was achieved for all conditions. The temperature spread factor and combustion efficiency as well as liner temperatures were within the normal operating limits for maximum power conditions. A slight degradation in combustor operating characteristics was noted after changing to JP-5R fuel and further degradation after changing to JP-5R/DF2.