Blowout Limit Research Articles

Experimental study on the effect of liquid loading on n-heptane spray jet flame stability

Flame stability significantly influences the performance of a practical combustor. The mechanisms governing the stability limits of spray jet flames are examined here using an annular co-flow spray burner. This study focuses on investigating the impact of liquid loading on spray jet flame stability, represented by flame lift-off height (LOH) and lean blow-out (LBO) limit. This is achieved by utilizing a parametric experimental study in which co-flow temperatures (Tco-flow = 298 K, 350 K, 400 K, and 450 K), spray nozzle sizes (dorifice = 0.209 mm, 0.219 mm, 0.232 mm, and 0.237 mm), fuel/air flow rates, and velocities are independently varied. A single liquid fuel, n-heptane, is used so that the vapor/liquid fractions (droplet lifetime) can be easily predicted via simple vapor–liquid equilibrium calculations to assist in the explanation of the stability mechanisms involved. The results show that the spray flame LOH mechanisms are affected by both chemical flame speed and, to a lesser extent, vaporization. On the other hand, the LBO limit is found to be controlled by the amount of liquid fuel in the presence/entering the flame. When the co-flow temperature is increased, while keeping constant co-flow velocity, the flame stabilizes near the burner lip. However, an opposite trend is noticed for flame blow-out limits, with the flame being more difficult to blow out when lower co-flow temperatures are applied. The variation of nozzle sizes leads to similar conclusions, i.e., when larger nozzle sizes are used, leading to the formation of larger droplets, the flame stabilizes further downstream and becomes more difficult to reach blow-out. The interpretation of the results indicates that delaying the vaporization and generating local enrichment regions near the flame edge enhances the flame LBO performance. Such an approach may be used as an efficient passive control mean to enhance flame stabilization; however, secondary impacts on emissions must be considered.

Effect of hetero-/homogenous combustion on energy conversion performances and flame stability of methane/air-fueled micro-combustors with heat-recirculating structure and platinum-coated

In this work, the joint effect of applying Pt catalyst and heat-recirculating structure on the flame stability and thermal performance has been numerically investigated in methane-air-fueled micro-combustors by a detailed three-dimensional computational model. Numerical results indicate that such strategies contribute to improving outer wall temperature by 14.4% and extending the blowout limit by a factor of 0.6 compared to those in the straight non-catalytic combustor. Such enhancements are due to the intensified heat transfer, thus highlighting the contributions of heterogeneous combustion chemistry and heat-recirculating effect. Homogeneous chemistry in the presence of a Pt catalyst is shown to be self-sustained over wide-ranging conditions. Meanwhile, the effectiveness of heat recirculation structure on combustion performance is more significant as compared to that of the catalytic combustion effect, as evidenced by a 4% and 17.6% improvement in the wall mean temperature and blowout limit respectively. Further analysis of the heat recirculation catalytic combustors is conducted to obtain the optimal geometry. There exists a critical flow rate corresponding to the maximum outer wall temperature and radiation energy. In general, this work demonstrates the viability of applying a Pt catalyst and heat-recirculating structure to extend the flame stability limit and energy conversion performance in micro-power systems.

Experimental investigation on flame stability and emissions of lean premixed methane–air combustion in a developed divergent porous burner

Flame stability and pollutant emissions are issues of the greatest concern for low-calorific-value gas combustion. Although the conventional straight porous burner is conducive to improving the flame stability, it still cannot meet the industrial demand. To improve the flame stability and reduce the pollutant emissions during fuel-lean combustion, a divergent double-layer porous burner was creatively proposed in this study. The influences of the inlet velocity and equivalence ratio on the flame stability, temperature distributions, the emissions were experimentally investigated in detail. The results showed that the flame front gradually moved downstream, the preheating zone length increased as the inlet velocity increased. Compared with the traditional straight porous burner, the divergent porous burner extended the blowout limit by 300%. Since the burner was operated under fuel-lean conditions (the equivalence ratio range was 0.45–0.6), the CO emissions were relatively high when the inlet velocity was less than 0.4 m/s. When the inlet speed was equal to or greater than 0.4 m/s, the maximum CO emissions concentration was 132 ppm. The burner showed superior performances in terms of NO emissions, with emissions concentrations of less than 20 ppm under all test conditions. In summary, the great advantage of the divergent porous burner lies in extending the flame stability limit. For clean combustion it is recommended that the burner be operated at a higher velocity (greater than or equal to 0.4 m/s) to obtain lower CO and NO emission levels.

Effects of swirling motion on the cavity flow field and combustion performance

An investigation has been carried out through experimental and simulation analyses to evaluate the cold flow field and combustion performance of a novel trapped vortex combustor that incorporates swirling motion. The cavity is an annular region, and the swirling flow (swirl number is 0.0, 0.6, 0.8, 1.0, and 1.2) is in the central axis of the cavity region. The technique of particle image velocimetry (PIV) is employed to investigate the flow field, with a specific focus on the swirling flow configurations within cavities. Additionally, it serves as a reference point for verifying and enhancing simulation methods. Methane is being subjected to combustion experiments at atmospheric pressure, where the fuel is only introduced into the cavity region and the air used is at a temperature of 300 K. The results show that: the cavity vortices are squeezed due to the swirl jet dumping into the cavity region as a result of the overwhelmingly increased centrifugal forces with the strong swirling flow. Additionally, the vortices' tangential velocity within the cavity area undergoes a significant rise under the influence of swirling flow, ultimately leading to a prolonged residence time. A significant increase in turbulence intensity of cavity vortices is also achieved when the swirl number is increased. For combustion results, the average temperature of the cavity increases with an increase in the cavity rear flow equivalence ratio (Φr); as the Φr further increases, the temperature reduces. The maximum temperature is approximately 1600 K for the mainstream with a swirling flow, and it is about 1200 K for no swirling flow. Finally, the rich/lean blowout limit is extended when the swirl number increases.

Blowout dynamics and plasma-assisted stabilization of premixed swirl flames under fuel pulsations

The present work investigates blowout dynamics of premixed swirl flames under low-frequency high-amplitude flow pulsations from the fuel feedline, and applies microsecond repetitively pulsed discharges with extremely-low power consumption to precisely control the stability of pulsating flames. First, both positive and negative fuel pulsations deteriorate the flame stability, causing much-narrowed lean blowout regimes. The fuel pulsation causes the flame to remain at the fuel-lean condition for a long period of time, which is considered to be the main cause of flame blowout. The flame response can be further explained by the convection process of the equivalence ratio oscillation. Secondly, the microsecond pulsed discharge with the same repetition rate as the fuel pulsation is used to alter the lean blowout via thermal, kinetic, and hydrodynamics effects. More importantly, the time delay between the discharge pulse and the fuel pulsation determines whether the plasma has an enhancement effect or not. It is found that the discharge prior to the convection of the equivalence ratio pulsation reaches the optimal effectiveness for flame stabilization, extending the blowout limit by approximately 10%. Finally, the lean blowout limit can be continuously extended with the increasing discharge repetition rate until saturation.

Design and Reliability Analysis of Integrated BOP

This article in view of existing BOP seal leakage point, low service life, large volume,high weight, height and installation of time-consuming to faults, the annular BOP and ram preventer, one-piece BOP design scheme is put forward, and seal the mouth of the well on the whole, 21 mpa pressure blowout limit design conditions for structural reliability analysis of one-piece BOP.The analysis results show that because the integrated BOP uses the integrated shell, does not need to use the flange connection, less leakage points, can greatly improve the traditional BOP short life, easy to leak and other shortcomings, and the maximum stress and maximum deformation displacement meet the design requirements.

An FV-EE model to predict lean blowout limits for gas turbine combustors with different structures and sprays

The occurrence of Lean Blowout (LBO) is a disadvantage that endangers a stable operation of gas turbines. A determination of LBO limits is essential in the design of gas turbine combustors. A semiempirical model is one of the most widely used methods to predict LBO limits. Among the existing semiempirical models for predicting LBO limits, Lefebvre’s LBO model and the Flame Volume (FV) model are particularly suitable for gas turbine combustors. On the basis of Lefebvre’s and FV models, the concept of effective evaporation efficiency is introduced in this paper, and a Flame Volume-Evaporation Efficiency (FV-EE) model is derived and validated. LBO experiments are carried out in a model combustor with 23 different structures and 10 different sprays. The prediction uncertainty of the FV-EE model is less than ± 13% for all of these 33 structures and sprays, compared with ± 50% for the FV model and ± 60% for Lefebvre’s model. Furthermore, the prediction uncertainty of the FV-EE model is also less than ± 13% for other combustors from available literature.

Stability Characteristics of an Actively Valved Resonant Pulse Combustor

Abstract Resonant pulse combustors, one of the deflagration-based pressure gain combustion devices, can significantly increase thermal efficiency in gas turbine engines. This experimental study investigates the stability characteristics of a newly designed actively valved resonant pulse combustor, capable of sustained operation and meaningful stagnation pressure gain. The resonant pulse combustor was fired with liquid gasoline fuel while ion and pressure sensors captured the temporally resolved heat release and chamber pressure. First, experimental results were used to demonstrate the general operating principle of the combustor. Then, the stability characteristics of the device were investigated through frequency domain analysis of the ion probe and pressure signal traces. A low frequency oscillation (also observed in steady flames and passively valved resonant pulse combustors), was observed as the device was brought near to its blowout limit. Finally, an index was defined to predict the stability characteristics of the resonant pulse combustor by quantifying the competition between low frequency oscillations and combustion-driven resonance. Experimental results demonstrated the ability of this index to provide early prediction of a blowout event for this device.

Using debris disk observations to infer substellar companions orbiting within or outside a parent planetesimal belt

Context. Alongside a debris disk, substellar companions often exist in the same system. The companions influence the dust dynamics via their gravitational potential. Aims. We analyze whether the effects of secular perturbations, originating from a substellar companion, on the dust dynamics can be investigated with spatially resolved observations. Methods. We numerically simulated the collisional evolution of narrow and eccentric cold planetesimal belts around a star of spectral type A3 V that are secularly perturbed by a substellar companion that orbits either closer to or farther from the star than the belt. Our model requires a perturber on an eccentric orbit (e ≳ 0.3) that is both far from and more massive than the collisionally dominated belt around a luminous central star. Based on the resulting spatial dust distributions, we simulated spatially resolved maps of their surface brightness in the K, N, and Q bands and at wavelengths of 70 µm and 1300 µm. Results. Assuming a nearby debris disk seen face-on, we find that the surface brightness distribution varies significantly with observing wavelength, for example between the N and Q band. This can be explained by the varying relative contribution of the emission of the smallest grains near the blowout limit. The orbits of both the small grains that form the halo and the large grains close to the parent belt precess due to the secular perturbations induced by a substellar companion orbiting inward of the belt. The halo, being composed of older grains, trails the belt. The magnitude of the trailing decreases with increasing perturber mass and hence with increasing strength of the perturbations. We recovered this trend in synthetic maps of surface brightness by fitting ellipses to lines of constant brightness. Systems with an outer perturber do not show a uniform halo precession since the orbits of small grains are strongly altered. We identified features of the brightness distributions suitable for distinguishing between systems with a potentially detectable inner or outer perturber, especially with a combined observation with JWST/MIRI in the Q band tracing small grain emission and with ALMA at millimeter wavelengths tracing the position of the parent planetesimal belt.

Blowout limits of inclined nonpremixed turbulent jet flames

The blowout behavior of inclined nonpremixed turbulent jet flames is investigated by varying the jet inclined angle in the range of -90° to 90° The critical jet velocity at blow-out limit is quantified experimentally for various nozzle diameters, different fuels and inclined angles. Numerical simulations are performed to emphasize the flow field difference for the positive and negative inclined angles. Physical modeling is conducted to incorporate the effect of the inclined angle on blow-out behavior. Major findings include: (1) The negatively inclined jet flames show more intense yellow luminosity with larger sooting zones than the positively inclined jet flames; (2) The blowout limit decreases appreciably with the jet inclined angle for the negatively inclined flames, while for the positively inclined jet flames, this decrease is relatively small; (3) Physical analysis of the flow development of inclined jets is conducted, indicating the centerline velocity along the jet trajectory decreases faster for the flame with smaller inclined angle. And the decrease rate is relatively larger for the negatively inclined jet flames; (4) Based on the analysis of the flow development as well as the characteristic velocity with the inclined angle variation, a model based on the Damköhler number (Da) accounting for the effect of jet inclined angle is developed to characterize the blowout limits of inclined jet flames. The proposed model successfully correlates the experimental data. The present findings provide new data and a basic scaling law for the blowout limit of nonpremixed inclined turbulent jet flames, revealing the effect of the relative angle between the jet momentum and buoyancy.

Localized Combustion Phenomena of Inverse Nonpremixed Pure O2/CH4 Coaxial Jet Flames at Near-Limit Conditions

ABSTRACT The stability of inverse nonpremixed oxygen (O2)/methane (CH4) coaxial jet flames near blowout limits in a model combustor is studied by analyzing signals for their local combustion phenomena using high-speed imaging and dynamic pressure. The power spectral analysis and the statistical analysis are conducted to delineate the stability of the inverse nonpremixed flames. Depending on the O2 to CH4 momentum flux ratio ((O/F)mom), two distinguished combustion phenomena which lead to different liftoff mechanisms are observed in the same burner configuration: the partial liftoff related to the aerodynamic mechanism and the local extinction related to the flame–turbulence interaction. The results of the power spectral density (PSD) show that on approaching the near-blowout limits, the local extinction or the partial liftoff occurs not in a specific frequency, but in a time-varying random distribution. The normalized root mean square (NRMS) based on the frame-integrated visible light signal (S VL) is found to be effective for quantifying the local extinction and the partial liftoff and particularly delineating the stability of the present inverse nonpremixed flames with the partial liftoff.

Experimental investigation of the stability and emission characteristics of premixed formic acid-methane-air flames in a swirl combustor

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO2 and H2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid.

Limited driving characteristics of combustion instability in a swirling flame: An experimental study

Regarding combustion instability, heat-release pulsation leads to pressure pulsation. Conversely, the driving force of the pulsating flame restricts the oscillation equilibrium point of combustion instability. Thus, in this paper, we developed a flame limited driving model explaining the finiteness of the driving energy of flame pulsation. Our research indicates that the limit cycle state of combustion instability was affected by the limitations of heat-release pulsation and Rayleigh integration. The coupling intensity of the heat-release pulsation and oscillating pressure decreases gradually with the increase in the oscillating pressure amplitude. In addition, an increase in flame pulsation amplitude presents finiteness similarly. When the acoustic amplitude exceeds a certain threshold, the amplitude of heat-release pulsation stops increasing until the flame approaches the oscillation blow-out limit. The further strengthening of the oscillation intensity of combustion instability is simultaneously restricted by the limitations of heat-release pulsation and Rayleigh integration, and thus the combustion instability finally stabilizes at the oscillation equilibrium point.

Key Mechanisms of the Effect of Atomization on Lean Blow-Out Limits of Aero Engines

Key Mechanisms of the Effect of Atomization on Lean Blow-Out Limits of Aero Engines

Effect of multiple bluff bodies on hydrogen/air combustion characteristics and thermal properties in micro combustor

The bluff body is commonly used to improve micro combustion. The micro combustor with multiple rectangular bluff bodies in a single row was proposed. The effects of bluff bodies on H2/air combustion characteristics were numerically studied. The temperature distributions, ignition position, combustion efficiency and blow-out limit were investigated via changing the total width and number of bluff bodies. The results show that the combined use of multiple bluff bodies can further expand the blow-out limit of H2/Air. The effect of high temperature and viscous force on the flow velocity is main factors for the flame morphology. When the total width of bluff bodies is 2 mm, the blow-out limit decreases with the increase of bluff body number. When the total width of bluff bodies is 4 mm and 6 mm, the blow-out limit increases with the increase of the number of bluff bodies. With the increase of inlet velocity, the complete combustion efficiency decreases. The combustion efficiency in the combustor with wider blow-out limit decreases more slowly. It indicates that the combustor with multi-bluff bodies is more suitable for the operation conditions with high flow velocity.

Comparative Study on Combustion and Flame Characteristics of Laminar Methane/Air and N-Butane/Air Flames in a Micro-Slot Burner

Combustion and flame characteristics of laminar methane/air and n-butane/air flames in a 3D-printed micro-slot burner is compared and reported in this study. The stability limit, flame appearance, and emission performance are investigated experimentally. In addition, past research on conventional burners is compared with the results of this study throughout the paper. The construction of this micro-slot burner was met by selective laser melting (SLM) process. Flame characteristics such as lift-off height, length, visible area, maximum width, and neck width are obtained using an image processing algorithm and are examined at different fuel and airflow rates. The results show that the blow-out limits of methane/air and n-butane/air flames are almost the same when compared at the same volume flow rates, although the methane/air flames are more stable than n-butane/air flames at the same thermal input powers. A region of interesting rope-like oscillatory flames (that has never been seen before in conventional burners) is observed in a small portion of a stable region for n-butane with a period ranging from 75.0 to 210.0 m s . It is also observed that the fuel type and fuel and airflow rates affect the flame shape and appearance and the flames formed by heavier fuel (n-butane) have longer length, lift-off height, maximum width, and visible area and lower neck width. Furthermore, methane/air flames exhibit lower values of C O and higher values of N O x in the flue gas when compared to n-butane/air flames.

Inlet pulsation-induced extinction and plasma-assisted stabilization of premixed swirl flames

• The flame and flow dynamics under air flow pulsations are resolved by performing validated LES and simultaneous OH-PLIF/PIV. • Temporal variations of stretch rate and heat release rate can be used as a criterion for flame extinction/reignition. • A novel decoupled simulation which integrates the plasma source terms and the CFD model is developed. • Thermal and kinetic effects of microsecond repetitively pulsed discharge on combustion are quantitatively interpreted. This paper intensively investigates the inlet pulsation-induced extinction of a premixed swirl flame (PSF) and its plasma-assisted stabilization using the microsecond repetitively pulsed (MRP) discharge. A well-designed low-frequency flow pulsation is applied to the air feedline for the mimicking of transient operations/disturbances in practical engines, which exhibits significant deterioration on the lean blowout (LBO) limit. The MRP discharge is utilized to improve the stability of perturbed flames. It extends the LBO limit at a proper time delay between the air flow pulsation and the discharge, with discharge power less than 1% of the combustion power. Further, the validated large-eddy simulations (LES) and the simultaneous OH planar laser-induced fluorescence/particle imaging velocimetry (OH-PLIF/PIV) measurements are performed to capture the unsteady evolution of flames approaching lean blowout. The nonlinear flame response to the flow pulsation quantitatively reveals the phase difference between the maximum local stretch rates ( κ max ) and volumetric heat release rates ( q ̇ c ). A combined effect of the excessive stretch and the reduction of heat release due to flow pulsation can be used to interpret the flame extinction. Finally, a novel LES-ZDPlasKin combined approach, which decouples the discharge and combustion processes, is dexterously developed to simulate plasma-assisted combustion behaviors. The reignition and stabilization due to the plasma effects are well reproduced in the predictive model, pronouncing the indispensable and synergistic thermal and kinetic effects of the MRP discharge on flame stabilization.

Effect of fuel reactivity on flame properties of a low-swirl burner

The present paper examines the capability of low swirl flows in stabilizing low calorific value fuels. To such an aim, CO2 gas is used to adjust the reactivity of CH4/air mixtures. The results show that increases in CO2 percentage in the fresh mixture result in shrinking the burner stability map by moving the lean and rich blowout limits toward the stoichiometric condition. The effects of CO2 addition on rich blowout limits are more intense than the lean blowout limits. For instance, the equivalence ratio associated with the lean blowout increases by 0.23, when the discharge flow velocity from the burner rises from 1.5 to 5 m/s in the 40 % CO2 diluted case. Under a similar augmentation in the burner discharge velocity, the rich blowout limit reduces by more than 0.5 in the 40 % CO2 diluted case. Unlike the lean blowout, the rich blowout limit responds non-monotonically to the discharge flow velocity from the burner for both undiluted and diluted mixtures. For the 50 % CO2 diluted case, the low swirl burner operates efficiently at low discharge velocities (1.5 to 3.5 m/s), but it blows out at high discharge velocities (3.5 to 9 m/s). Furthermore, unsteady features of low swirl flame near blowout limits are studied in detail. Moreover, strong curvy structures appear at the flame boundary prior to the flame flashback. However, such structures diminish occasionally and specifically as the flame approaches the flashback. Some deterministic features are more obvious before the flame blowout than those before the flame flashback.

Co-firing hydrogen and dimethyl ether in a gas turbine model combustor: Influence of hydrogen content and comparison with methane

To promote the utilization of hydrogen (H2) in existing gas turbines, dimethyl ether (DME) was used to co-fire with H2 in a model combustor. The swirl combustion characteristics of DME/H2 mixtures were measured under the varying H2 content up to 0.7. The results show that the flow velocity elevates as the H2 content increases, which is associated with the increased flame temperature. The OH level firstly increases and subsequently keeps nearly unchanged as the H2 content increases. Meanwhile, the OH area nonlinearly increases with the increasing H2 content. Moreover, the increasing H2 content induces almost linearly decreased lean blowout limit (LBO), increased NO emission, and intensified combustion acoustics. Furthermore, the combustion characteristics of the 0.46DME/0.54H2 mixture and CH4 with the same volumetric heat value were compared. The 0.46DME/0.54H2 flame displays lower LBO and higher NO emission than the CH4 flame, which mainly results from the higher reactivity of 0.46DME/0.54H2 mixture.

Dynamics of Premixed Flames Near Lean and Rich Blowout

ABSTRACT Practical combustors in furnaces, industrial heaters, and gas turbine engines face a sudden loss of flame due to rich blowout (RBO) at an extremely rich fuel-air mixture or lean blowout (LBO) at an extremely lean fuel-air mixture. Thus, the stability limits of the combustion regime are governed by RBO and LBO. In the present study, we focus on the dynamics of swirl-stabilized premixed combustion near lower and higher flammability limits, where the possibilities of LBO and RBO, respectively, are observed. Near RBO and LBO, we employ metrics from statistics and dynamical systems theory to characterize the transition in flame dynamics. We observe that the range of frequencies obtained using Fourier Transform near RBO and LBO is alike. The emergence of dominantly low-frequency oscillation near those blowout limits is investigated using Hilbert Transform. The mean frequency of the combustion system gradually reduces near both blowout limits due to behavioral oscillations prominently observed. The scaling property of flame oscillation is examined using the Hurst exponent, and we observe a decreasing trend of the parameter as combustion approaches both LBO and RBO. Therefore, for the gradual reduction of the Hurst exponent near both flammability limits, we can use the parameter as a precursor for both RBO and LBO.