Lean Blowout Equivalence Ratio Research Articles

Combustion and emission characteristics of a novel staged combustor for aero gas turbine engine

To simultaneously achieve high combustion efficiency, low pollutant emissions, and robust stability in aero gas turbine engines, a novel staged combustor concept combining the trapped-vortex and swirl combustion characteristics is proposed in this study. In addition, experimental studies were conducted under atmospheric pressure to obtain insights into the combustion efficiency, pollutant emissions, stability, and flame features of the combustor. The results indicate that improving the air temperature (T3) and equivalence ratio is helpful for achieving high combustion efficiency. Interestingly, the pressure drop has a significant influence on the combustion efficiency at T3=373 K, whereas it has little influence at T3=473 K. The NOx emissions of the combustor increased as the air temperature increased, whereas, it decreased as the pressure drop increased. The lean blowout equivalence ratio increased slightly as the pressure drop increased, and was below 0.074 at T3=473 K. Two separated flames were formed inside and downstream of the cavities under pilot-only fueling conditions. For the pilot-main fueling conditions, three separated flames were observed at relatively low-equivalence ratio regimes. With the increases of the equivalence ratio, the three separated flames merge into one flame.

Influence of strut on trapped vortex cavity flameholder: Ignition and flame propagation performances

In order to improve the performance of pilot flame propagation to the core flow, a novel strut design was proposed for the trapped vortex cavity flameholder. Experiments were conducted under high subsonic flow conditions to investigate the influences of strut on the ignition performance and flame propagation performance of trapped vortex cavity combustor. Furthermore, the flame development was acquired using a high-speed camera and the flame information and the flow field were combined to explicate the flame stabilization mechanisms of two flameholder layouts. The results showed that, both flameholder layouts achieved reliable ignition and flame stability under inlet flow velocity lower than Ma = 0.42. Cavity combined with strut weakened the ignition performance of the trapped vortex cavity-only flameholder based combustor, but improved the blowout performance and enhanced the flame stability. Compared with the cavity-only flameholder layout, the lean ignition equivalence ratio of the cavity-strut flameholder layout was increased of 14.6%, and the lean blowout equivalence ratio was decreased by 56.2%, under the inlet flow at 600 K. Strut changed the original flame propagation path of the cavity-only layout, and caused the flame stabilization mechanism change from cavity flame stabilization to strut flame stabilization. Although the change of flame stabilization mechanism prolongs the ignition delay time, it enhances the flame stability of the combustor. Moreover, the flame propagation ability to the radial direction behind the cavity combined with strut is enhanced, which is more suitable to the engineering combustor with higher requirements on circumferential flame propagation.

Flame stability optimization of cavity primary air-jet form in an augmentor

A trapped vortex cavity with a radial V-gutter flameholder is adopted to expand the flame stability of an augmentor. The improvement of flame stabilization limits is achieved experimentally by replacing the traditional slotted air-jet with the discrete-hole air-jet. The atomization characteristics of the air-assisted multi-point injector and the numerical fluid-structure are conducted to explain the results. Results indicate that a remarkable enhancement of the flame stability with discrete-hole air-jet is obtained in all conditions, while the slotted air-jet for the cavity leads to a failed ignition at 343 K except for the Mach number of 0.3. As the same passing area of the air-jet, the larger the aperture of the discrete hole, the better the flame stabilization performance. An increasing trend of lean blowout equivalence ratio is contributed by the increased Mach number. Whereas, the Mach number growth will lead to a reduction of lean ignition equivalence ratio at 343 K and 473 K, and a downward parabola is observed at 573 K. Eventually, the optimal flame stability performance is achieved in by the larger discrete space distance, which promotes the fuel/air mixture with the help of the recirculation zones between discrete holes.

A data-driven approach using machine learning for early detection of the lean blowout

A data-driven approach using machine learning is presented for the identification of the critical flame location for the early detection of an incipient lean blowout (LBO) in a realistic gas turbine engine combustor under engine-relevant conditions. This method is demonstrated by utilizing the temperature (T) and the hydroxyl radical mass fraction (YOH) data from high fidelity large eddy simulations (LES) of Jet-A combustion. The fuel flow rate is progressively reduced in numerical simulations with a fixed airflow rate to mimic experimental studies of LBO in the gas turbine combustor. These simulations are the first of their kind for a fully resolved realistic combustor geometry with adaptive mesh refinement and have accurately captured the dynamics of the LBO process and global lean blowout equivalence ratio. Time-series of T and YOH are extracted in the primary zone of the combustor, from stable flame condition to LBO condition, to train the machine learning model. A Support Vector Machine (SVM) model with radial basis function is successfully developed to identify the critical flame location for early detection of incipient LBO condition in a practical combustor for the first time. The performance of the SVM model is quantified using the F-score, and the critical flame location corresponds to the maximum value of the F-score. The critical flame location is found to be in the flame root region and is effective in the early detection of incipient LBO. The conventional statistical measures are compared with the results of the trained machine learning model to assess the feasibility of the latter for online flame health monitoring. The machine learning model successfully prognosticated the LBO approximately 20 ms before the event, and this study has shown significant promise for the use of the SVM model in engine prognostics and health management.

Spark ignition and stability limits of spray kerosene flames under subatmospheric pressure conditions

Flame stability limits and ignition delay time of the flameholder are experimentally studied in this work at the pressure of 0.03 – 0.06 MPa, Mach number of 0.1 – 0.3, and temperature of 320 K – 800 K. The results indicate that the reduction in equivalence ratios of lean ignition and blowout can be achieved by the increase in pressure or Mach number. Temperature rise will first increase and then decrease the lean ignition equivalence ratio. While increasing lean blowout equivalence ratio was observed with increasing temperature. The envelope of the stable combustion between the pressure and Mach number is measured, and the lowest temperature of successful ignition is 300 K, acquired at Mach 0.1 and 0.06 MPa. Besides, the correlation between the equivalence ratios of flame stability and inlet parameters, such as pressure, Mach number, temperature, is developed. The ignition process of the flameholder is imaged, and the ignition delay time decreases with increased pressure or temperature. The ignition delay time is shown first to decrease and then increase with the increase in Mach number.

Experimental and numerical prediction of lean blowout limits for micro gas turbine combustor

PurposeThe lean blowout (LBO) limit of the combustor is one of the important performance parameters for any gas turbine combustor design. This study aims to predict the LBO limits of an in-house designed swirl stabilized 3kW can-type micro gas turbine combustor.Design/methodology/approachThe experimental prediction of LBO limits was performed on 3kW swirl stabilized combustor fueled with methane for the combustor inlet velocity ranging from 1.70 m/s to 6.80 m/s. The numerical prediction of LBO limits of combustor was performed on two-dimensional axisymmetric model. The blowout limits of combustor were predicted through calculated average exit gas temperature (AEGT) method and compared with experimental predictions.FindingsThe results show that the predicted LBO equivalence ratio decreases gradually with an increase in combustor inlet velocity.Practical implicationsThis LBO limits predictions will use to fix the operating boundary conditions of 3kW can-type micro gas turbine combustor. This methodology will be used in design stage as well as in the testing stage of the combustor.Originality/valueThis is a first effort to predict the LBO limits on micro gas turbine combustor through AEGT method. The maximum uncertainty in LBO limit prediction with AEGT is 6 % in comparison with experimental results.

Experimental Prediction of Lean Blowout Limits for 3kW Micro Gas Turbine Combustor fuelled with LPG

The Lean Blowout Limit of the combustor is one of the important performance parameters for a gas turbine combustor design. This study aims to predict the total pressure loss and Lean Blowout (LBO) limits of an in-house designed swirl stabilized 3kW can-type micro gas turbine combustor. The experimental prediction of total pressure loss and LBO limits was performed on a designed combustor fuelled with Liquefied Petroleum Gas (LPG) for the combustor inlet velocity ranging from 1.70 m/s to 11 m/s. The results show that the predicted total pressure drop increases with increasing combustor inlet velocity, whereas the LBO equivalence ratio decreases gradually with an increase in combustor inlet velocity. The combustor total pressure drop was found to be negligible; being in the range of 0.002 % to 0.065 % for the measured inlet velocity conditions. These LBO limits predictions will be used to fix the operating boundary conditions of the gas turbine combustor.

Inlet Pressure Effects on Subatmospheric Flame Stabilization with an Optimum Size of a Cavity-Based Combustor

Experimental studies are conducted to find an optimum size of the cavity flameholder, which is a new combustion concept of a turbine-based combined-cycle (TBCC) engine with an excellent flame stabilization. Besides, the effect of inlet pressure on the subatmospheric performance is investigated. The experimental results indicate that the increase of the cavity length improves the flame stability with an enlarged fuel/air mixture residence time, which suggests that the big length-height ratio in a proper range of the cavity with a stable dual-vortex should be chosen when designing the cavity-based combustor. In addition, the decrease in lean ignition and the lean blowout equivalence ratios can be attributed by either increase in the inlet pressure and temperature or decrease in the Mach number. The increase in inlet pressure will lead to a linear decrease in the lean blowout equivalence ratio with a slope of 0.66 per 0.1 MPa, whereas the lean ignition equivalence ratio has a rapid drop with the increase of pressure from 0.06 MPa to 0.08 MPa and reduces slowly with the growth of pressure in the range of 0.08 MPa to 0.1 MPa. The detailed analysis of the flow field indicates that the characteristic time-scale theory can ideally explain and predict the change of flame stability in the trapped vortex cavity.

Study on the Effects of Dome Fuel Distribution Ratio on Lean Blowout of a Model Combustor

The experimental studies and numerical simulation were conducted on the effects of the dome fuel distribution ratio on the lean blowout of a model combustor. The experimental results indicate that as the key parameter, the dome fuel distribution ratio, increases from 2.06% to 16.67%, the lean blowout equivalence ratio declines obviously at the beginning, and then the decrease slows down, in addition, the amplitude of the pressure fluctuation in the combustor reduces significantly while the dominant frequency keeps basically constant. In order to analyze the experimental results, the numerical simulation is adopted. The temperature and local equivalence ratio distributions are employed to explain the reason why the lean blowout performance improves with the increase of the dome fuel distribution ratio.

Impact of gas composition variations on flame blowout and spectroscopic characteristics of lean premixed swirl flames

The injection of gases from different oil and gas fields and external sources such as liquefied natural gas increases operational risks for the relevant gas turbine power plant operators. In practice, the absence of interchangeability specifications in the gas supply network code has caused combustion blowout, wear and tear to occur due to combustion dynamics and diluent effects. Understanding the effects of diluents on natural gas combustion is essential to ensure the safe operation of existing facilities. The present work investigates the flame stability and spectroscopic characteristics of diluents-containing natural gas by using a swirl flame burner. Stable and continuous swirl flames were successfully established using different types of gas compositions, including those diluted with nitrogen and carbon dioxide. Diluting the modelled natural gas with CO2 and N2 results in higher blowout limit as compared to the baseline pure methane case. Preheating the burner and mixtures can extend the flame blowout limits, although the effect of CO2 on flame blowout is more pronounced than that of N2 due to its higher heat capacity. This work shows the effects of non-reactive diluents on gas turbine flame can be significant, particularly at high-level dilutions. Mitigation measures such as gas composition and flame spectroscopy monitoring can be deployed to ensure safe operation of the system. By using the statistical analysis technique of linear regression, the proportions of all the fuel mixture components of CH4, C2H6, CO2 and N2, alongside temperature were found to be significant factors in determining flame blowout limits. The developed predictor equations for OH intensity and lean blowout equivalence ratios show the predictive capability of >89% at 90–95% confidence level.

Preferential vaporization impacts on lean blow-out of liquid fueled combustors

Recent experimental works have shown that the global equivalence ratio defining lean blow-out (LBO) in model gas turbine combustors correlates with the derived cetane number (DCN) of the tested fuel, which represents the chemical reactivity potential of the fuel, but additional physical and kinetic parameters of the fuel also have influence. The current work explores the significance of preferential vaporization impacts on LBO behaviors; i.e., rather than parameterizing the fuel by overall averaged fuel properties, it looks at DCN correlations based upon distillation properties prior to full vaporization. Preferential vaporization potentials of six fuels are evaluated by measuring the DCN values of five distillation cuts (each of 20% liquid distillation volume recovered). In spite of relatively large disparities in total fuel DCN values (∼9.1), two petroleum-derived jet fuels are found to have nearly the same LBO equivalence ratios, which is attributed to the relatively indiscernible difference of DCN values (∼2) for the initial 20% distillation cut of each fuel. Trade-off impacts between fuel chemical and physical properties are demonstrated by comparing n-dodecane and Gevo-ATJ, which do not have preferential vaporization potential. LBO results suggest that fuel physical properties (particularly fuel boiling characteristics) predominantly control LBO behaviors at low air inlet temperature conditions, whereas fuel chemical properties appear to gain significance with increasing air inlet temperature. Further evidence of preferential vaporization effects on LBO is discussed with two surrogate mixtures formulated to emulate the fully pre-vaporized combustion behaviors of Jet-A, but having drastically different preferential vaporization potentials. Finally, the relationship between DCNs and LBO equivalence ratios is re-examined using the DCN values of initial 20% distillation cuts of all six fuels. The results display a significantly improved correlation, suggesting that the relevance of preferential vaporization on LBO can be significant for fuels that exhibit significant departure of the DCN for high volatile fractions (i.e., the initially vaporized constituents) in comparison to the overall fuel DCN.

Experimental investigation on alternative fuel combustion performance using a gas turbine combustor

In order to ensure reliable combustion performance and low emissions profiles for prospective alternative fuels, this study was conducted in order to evaluate the impact of fuel properties and their composition on lean blowout performance and emission characteristics of various fuels. Significant contributions to knowledge have been achieved using visual images of the flames at the occurrence of lean blowout (LBO) that were captured by a high-speed camera and analysed to develop a new model for the evaluation of the LBO of current and future alternative fuels.The results show that high-density and high aromatic content fuel has the potential to produce higher soot formation. In addition, a low lean blowout equivalence ratio and low soot propensity is unlikely to be achieved at the same time by simply altering the value of fuel properties. It was discovered that the Derived Cetane number was observed to have a considerable effect on improving the LBO performance without inducing heavier soot emissions. An analysis of the impact of the fuel aromatic content (type and proportion) presents a hypothesis that not all aromatics species produce the same levels of particulate matter emissions.This work provides a comprehensive data foundation, analytical concepts and potential research techniques for the application of expeditious fuel screening tools to assess the combustion behaviour of current fuels, and also facilitates the potential for further operational fuel development. The visualisation method used in this work could contribute towards developing applications for flame detection in fuel performance assessment.

Experimental study of lean ignition and lean blowout performance improvement using an evaporation flameholder

The hyperburner of a turbine-based combined cycle (TBCC) works at a relatively low temperature and high local velocity flow conditions compared with an afterburner or a ramjet combustor. Therefore, wider lean ignition and blowout limits are required in a TBCC. Both a circular and crescent-shaped evaporation tube flameholders were designed and numerically simulated. Moreover, experiments were performed to measure the lean ignition and blowout performance for the two flameholder types under conditions of T=450–650K and Ma=0.1–0.4. The results indicate that a crescent-shaped evaporation tube provides steady high temperature wake flow, thereby increasing the fuel evaporation rate, and that a crescent-shaped evaporation tube flameholder is superior to a circular evaporation tube flameholder; this is because the wake flow of a crescent-shaped evaporation tube is not affected by the Mach number of the flow. This is beneficial for both reliable lean ignition and wide lean blowout limits. For flows with high Mach numbers, when compared with a circular evaporation tube flameholder, the lean ignition and blowout performance is strongly enhanced for a crescent-shaped evaporation tube flameholder. The experimental results show that a crescent-shaped evaporation tube flameholder exhibits much better performance under flow conditions where T=450K and Ma=0.4. The lean ignition and lean blowout equivalence ratios of a crescent-shaped evaporation tube flameholder were only 31% and 47% that of a circular evaporation tube flameholder, respectively.

Experimental Investigations of the Lean Blowout Limit of Different Syngas Mixtures in an Atmospheric, Premixed, Variable-Swirl Burner

An atmospheric, variable-swirl combustor was used to study the influence of syngas composition on the lean blowout (LBO) limit under various flow conditions created at different swirl numbers. The fuels used in the experiments consisted of generic mixtures of CO, H2, and CH4. The results were compared to those for CH4 at the same swirl numbers. The effect of dilution on the LBO limit was studied by adding N2 to the syngas mixture. The swirl number was varied by changing the ratio of axial/tangential flow through the combustor inlet and was determined using laser Doppler anemometry (LDA). A perfectly stirred reactor (PSR) model was used to test whether the experimental results could be explained by changes in chemical kinetics. The experiments showed that increasing the swirl number reduced the LBO equivalence ratio for a given fuel composition. At a certain swirl number, increasing the H2/CO molar ratio of a binary mixture decreased the LBO equivalence ratio significantly. The addition of CH4 to a binary mixture shifted the LBO limit to higher equivalence ratios. N2 dilution increased the LBO equivalence ratio of the syngas mixture; however, the impact was relatively small. The PSR model was able to predict the effect of adding CH4 reasonably well, but it underestimated the effect of the H2/CO molar ratio on the LBO limit.

Development of a multiple opposing jets’ burner for premixed flames

A burner was developed to accommodate multiple opposing premixed flames, where the turbulence stimulated by impingement was found to extend the stability limits of methane/air mixtures. The opposing jets established closed loop flow currents in a number of sectors, which is twice that of the jets thus providing more eddies within a stronger turbulent flame structure. The stagnation zone for four opposing jets was testified to be larger than that for two jets such that it was able to sustain higher reactive jet velocities up to 14.2 m/s at larger flow shearing rates. Increasing the number of opposing jets and decreasing their separation distance provided higher rates of heat/mass transfer into the ports. As the flow deceleration zone was enlarged, the correspondingly higher burning capacities were correlated to the port diameter, the separation distance, the number of jets, Reynolds number, and the strain rate. Strain rates of growing magnitudes up to 13,080 s−1 were sustained at jet velocities as high as 21.8 m/s. At such velocities and upon increasing the ratio of the jet diameter to the separation distance, the turbulent kinetic energy was found to exhibit one local peak, as the strain rate due to impingement became enriched by the jet radial shearing effects. A lean blowout equivalence ratio of 0.59 was pronounced as a basis for acquiring low NO x emissions. The burner serves as a potential prototype for increasing the power-to-weight ratio of the combustion chambers like those utilized in furnaces, boilers, gas turbines, as well as jet-propelled vehicles with effectively minimized emissions of pollutants.

Experiment and simulation study on lean blow-out of trapped vortex combustor with various aspect ratios

This paper investigated the lean blow-out (LBO) performance of several combustors which utilized trapped vortex in the cavity to improve the flame stability. Experiments and simulation study had been performed for various height to length (Hf/L) ratios of the cavity. The experiment results showed that the minimum pressure drop across the combustor occurred when the Hf/L ratio was 0.8; and that very low overall LBO equivalence ratios were observed over a wide range of primary air flows at the same H/L ratio. In the simulations, the large eddy simulation (LES) was used to predicate the transient flow field in the cavity. Simulation results showed that the core of the trapped vortex was in the middle of the cavity at Hf/L=0.8, and the velocity profile of the vortex was better organized than those of other combustors with different Hf/L ratios. Moreover, the simulated period of vortex swirling was the shortest at Hf/L=0.8. Therefore, the experimental observations on the LBO performance of several combustors with different Hf/L ratios were well explained by the present simulation results. Based on the experimental data, a preliminary study of the empirical correlation of LBO was conducted on trapped vortex combustor (TVC). Compared with Lefebvreʼs empirical correlation, the proposed new empirical correlation has better agreement with the experimental data.

Experimental Blowout Limits and Computational Flow Field of Axial Single and Multijet Flames

Measurements of the lean blowout equivalence ratio (Φoverall,b) along with the numerical simulations of flame structure and dynamics of the flow field for coaxial burner configurations are reported. The burner comprises central mixture (air+liquefied petroleum gas) issuing either through six holes distributed radially each of 2mm diameter or through a circular single port of area equal to the total areas of the six holes. A bluff-body stabilizer is attached to provide recirculation of the coaxial air surrounding the central flame. The study covers the effect of the central injection configuration with emphasis on the multijet on the overall lean equivalence ratio at which flame is extinguished. The dynamics of the flow field for the multiflame configurations were identified and compared with the single flame, using the generalized finite-rate chemistry model of FLUENT 6.2 with the detailed chemical reaction mechanism defined by GRI-MECH 3.0 and other mechanisms for the higher carbon species. The computed flow field of the multijet flame provides an extra intermediate vortex in addition to the two counter-rotating vortices observed for cases of the single central stream configuration. Such a vortex is believed to enhance the stability characteristics for all the test flames in the form of reduced experimental Φoverall,b-values.

Active control of lean blowout in a swirl-stabilized combustor using a tunable diode laser

A novel tunable diode laser temperature sensor is used to prevent lean blowout (LBO) in a swirl-stabilized, partially premixed dump combustor which serves as a model of gas turbine combustors. The fast-response temperature sensor is based on rapid, repetitive (2 kHz) wavelength-scanned laser absorption of two vibrational overtone transitions of water vapor near 1.4 μm using a single fiber-coupled diode laser. Although the gas composition and temperature are not uniform along the line-of-sight, low-frequency temperature fluctuations are identified in the FFT power spectrum of the real-time temperature measurements at an optimized location. We found that the fraction of the FFT power at low-frequencies (0–50 Hz) increases exponentially as the flame approaches LBO. The intensity of these low-frequency temperature fluctuations is used to detect the proximity to LBO and as a control variable for the active LBO control system. Successful prevention of LBO during power reduction and transient fuel variation is demonstrated even without knowledge of the LBO fuel/air ratio limit for the specific operating conditions of the combustor. The active control system also can maintain the flame at very lean conditions (0.03 above the LBO equivalence ratio), and thus reduce the LBO margin. To our knowledge, this is the first application of laser-based sensors in active LBO control of swirl-stabilized flames. The results demonstrate the potential of tunable diode laser sensors for closed-loop combustion control applications.