Combustion of partially premixed spray jets

Gas turbines, liquid rocket motors, and oil-fired furnaces utilize the spray combustion of continuously injected liquid fuels. In most cases, the liquid spray is mixed with an oxidizer prior to combustion, and further oxidizer is supplied from the outside of the spray to complete diffusion combustion. This rich premixed spray is called “partially premixed spray.” Partially premixed sprays have not been studied systematically although they are of practical importance. In the present study, the burning behavior of partially premixed sprays was experimentally studied with a newly developed spray burner. A fuel spray and an oxidizer, diluted with nitrogen, was injected into the air. The overall equivalence ratio of the spray jet was set larger than unity to establish partially premixed spray combustion. In the present burner, the mean droplet diameter of the atomized liquid fuel could be varied without varying the overall equivalence ratio of the spray jet. Two combustion modes with and without an internal flame were observed. As the mean droplet diameter was increased or the overall equivalence ratio of the spray jet was decreased, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. The results suggest that the internal flame was supported by flammable mixture through the vaporization of fine droplets, and the passage of droplet clusters deformed the internal flame and caused internal flame oscillation. The existence of the internal premixed flame enhanced the vaporization of droplets in the post-premixed-flame zone within the external diffusion flame.     

Identification of combustion mode under MILD conditions using Chemical Explosive Mode Analysis

Direct Numerical Simulations (DNS) data of Moderate or Intense Low-oxygen Dilution (MILD) combustion are analysed to identify the contributions of the autoignition and flame modes. This is performed using an extended Chemical Explosive Mode Analysis (CEMA) which accounts for diffusion effects allowing it to discriminate between deflagration and autoignition. This analysis indicates that in premixed MILD combustion conditions, the main combustion mode is ignition for all dilution and turbulence levels and for the two reactant temperature conditions considered. In non-premixed conditions, the preponderance of the ignition mode was observed to depend on the axial location and mixture fraction stratification. With a large mixture fraction lengthscale, ignition is more preponderant in the early part of the domain while the deflagrative mode increases further downstream. On the other hand, when the mixture fraction lengthscale is small, sequential autoignition is observed. Finally, the various combustion modes are observed to correlate strongly with mixture fraction where lean mixtures are more likely to autoignite while stoichiometric and rich mixtures are more likely to react as deflagrative structures.     

Flame–wall interaction effects on the flame root stabilization mechanisms of a doubly-transcritical LO2/LCH4 cryogenic flame

High-fidelity numerical simulations are used to study flame root stabilization mechanisms of cryogenic flames, where both reactants (O₂ and CH₄) are injected in transcritical conditions in the geometry of the laboratory scale test rig Mascotte operated by ONERA (France). Simulations provide a detailed insight into flame root stabilization mechanisms for these diffusion flames: they show that the large wall heat losses at the lips of the coaxial injector are of primary importance, and require to solve for the fully coupled conjugate heat transfer problem. In order to account for flame–wall interaction (FWI) at the injector lip, detailed chemistry effects are also prevalent and a detailed kinetic mechanism for CH₄ oxycombustion at high pressure is derived and validated. This kinetic scheme is used in a real-gas fluid solver, coupled with a solid thermal solver in the splitter plate to calculate the unsteady temperature field in the lip. A simulation with adiabatic boundary conditions, an hypothesis that is often used in real-gas combustion, is also performed for comparison. It is found that adiabatic walls simulations lead to enhanced cryogenic reactants vaporization and mixing, and to a quasi-steady flame, which anchors within the oxidizer stream. On the other hand, FWI simulations produce self-sustained oscillations of both lip temperature and flame root location at similar frequencies: the flame root moves from the CH₄ to the O₂ streams at approximately 450 Hz, affecting the whole flame structure.     

Combustion rates of graphite rods in the forward stagnation field of the high-temperature, humid airflow

Relevant to High-Temperature Air Combustion, an experiment has been conducted to study carbon combustion in the high-temperature oxidizer-flow, by use of a graphite rod set in the forward stagnation field. Effects of humid air and O₂-reduced oxidizer have been examined, after re-confirming that the combustion rate in the high-temperature oxidizer-flow is enhanced, because of elevated transport properties when the mass flow rate of oxidizer is the same, and that it is suppressed, because of reduced mass transfer rates through the thickened boundary layer when the velocity gradient is the same. It is found that high H₂O mass-fraction is favorable for the enhancement of combustion rate at high surface temperatures (>2000 K), because of its participation in the surface C–H₂O reaction, while it is not the case at medium surface temperatures (1400–1700 K), because of the suppressing effect, caused by the establishment of CO flame. As for O₂ and CO₂ concentrations in the high-temperature oxidizer-flow, it is found that O₂ mass-fraction can be reduced without reducing combustion rate in the room-temperature airflow, and that it can further be reduced in existence of enough CO₂ that can be an oxidizer in carbon combustion. Theoretical works have also been conducted for the system with three surface reactions and two global gas-phase reactions. It is found that the Frozen mode without CO flame and the Flame-detached mode with infinitely fast gas-phase reaction can fairly represents combustion behavior before and after the establishment of CO flame, respectively, as far as the trend and the approximate magnitude are concerned. It is also shown that a new mode with suppressing H₂ ejection from the surface can fairly represent the combustion rate, experimentally obtained in humid airflow with relatively low velocity gradient when the surface temperature is high.     

Diffusion-controlled combustion of gases in the absence of forced convection

A mathematical model for the laminar diffusion combustion of gases in the absence of forced convection is developed. This combustion mode is realized near an orifice in the partition that separates the fuel and oxidizer. The stationary solution, size and shape of the flame, temperature distribution, and profiles of the concentrations of fuel, oxidizer, and combustion products are determined. It is shown that the limiting diameter of the diffusion flame is inversely proportional to the burning rate of an equivalent premixed mixture of the same fuel and oxidizer, whereas the flame length is proportional to the diameter of the orifice. The unsteady (quasi-stationary) solution to this problem for the case where the gas is confined in a finite-volume vessel is obtained. The time it takes the flame to approach the partition of the vessel with fuel and enter inside is determined. Experiments on studying the diffusion combustion of natural gas in the absence of forced convection near orifices and a slit in the partition separating the gaseous fuel and oxidizer in a finite-volume vessel are performed. The time of combustion is obtained. Depending on the orifice diameter and slit width, three modes of diffusion combustion were identified: combustion above the partition ending in flame extinction, combustion with a breakthrough, and combustion inside the vessel (submerged flame).     

Flow characterization and dilution effects of N_2 and CO_2 on premixed CH_4/air flames in a swirl-stabilized combustor

Numerically-aided experimental studies are conducted on a swirl-stabilized combustor to investigate the dilution effects on flame stability, flame structure, and pollutant emissions of premixed CH4/air flames. Our goal is to provide a systematic assessment on combustion characteristics in diluted regimes for its application to environmentally-friendly approaches such as biogas combustion and exhanst-gas recirculation technology. Two main diluting species, N2 and CO2, are tested at various dilution rates. The results obtained by means of optical diagnostics show that five main flame regimes can be observed for Nz-diluted flames by changing excess air and dilution rate. CO2-diluted flames follow the same pattern evolution except that all the domains are shifted to lower excess air. Both N2 and CO2 dilution affect the lean blow- out （LBO） limits negatively. This behavior can be counter-balanced by reactant preheating which is able to broaden the flammability domain of the diluted flames. Flame reactivity is degraded by increasing dilution rate. Meanwhile, flames are thickened in the presence of both diluting species. NOx emissions are significantly reduced with dilution and proved to be relevant to flame stability diagrams： slight augmentation in NOx emission profiles is related to transitional flame states where instability occurs. Although dilution results in increase in CO emissions at certain levels, optimal dilution rates can still be proposed to achieve an ideal compromise.     

An analytical study of droplet combustion under microgravity: Quasi-steady transient approach

An analytical model based on an assumption of combined quasi-steady and transient behavior of the process is presented to exemplify the unsteady, sphero-symmetric single droplet combustion under microgravity. The model used in the present study includes an alternative approach of describing the droplet combustion as a process where the diffusion of fuel vapor residing inside the region between the droplet surface and the flame interface experiences quasi-steadiness while the diffusion of oxidizer inside the region between the flame interface and the ambient surrounding experiences unsteadiness. The modeling approach especially focuses on predicting; the variations of droplet and flame diameters with burning time, the effect of vaporization enthalpy on burning behavior, the average burning rates and the effect of change in ambient oxygen concentration on flame structure. The modeling results are compared with a wide range of experimental data available in the literature. It is shown that this simplified quasi-steady transient approach towards droplet combustion yields behavior similar to the classical droplet theory.     

Combustion of a high-velocity hydrogen microjet effluxing in air

This study is devoted to experimental investigation of hydrogen-combustion modes and the structure of a diffusion flame formed at a high-velocity efflux of hydrogen in air through round apertures of various diameters. The efflux-velocity range of the hydrogen jet and the diameters of nozzle apertures at which the flame is divided in two zones with laminar and turbulent flow are found. The zone with the laminar flow is a stabilizer of combustion of the flame as a whole, and in the zone with the turbulent flow the intense mixing of fuel with an oxidizer takes place. Combustion in these two zones can occur independently from each other, but the steadiest mode is observed only at the existence of the flame in the laminar-flow zone. The knowledge obtained makes it possible to understand more deeply the features of modes of microjet combustion of hydrogen promising for various combustion devices.     

Numerical and experimental study on shear coaxial injectors with hot hydrogen-rich gas/oxygen-rich gas and GH2 /GO2

The influences of the shear coaxial injector parameters on the combustion performance and the heat load of a combustor are studied numerically and experimentally. The injector parameters, including the ratio of the oxidizer pressure drop to the combustor pressure (DP ), the velocity ratio of fuel to oxidizer (R V ), the thickness (WO ), and the recess (HO ) of the oxidizer injector post tip, the temperature of the hydrogen-rich gas (TH ) and the oxygen-rich gas (TO ), are integrated by the orthogonal experimental design method to investigate the performance of the shear coaxial injector. The gaseous hydrogen/oxygen at ambient temperature (GH2 /GO2 ), and the hot hydrogen-rich gas/oxygen-rich gas are used here. The length of the combustion (LC ), the average temperatures of the combustor wall (TW ), and the faceplate (TF ) are selected as the indicators. The tendencies of the influences of injector parameters on the combustion performance and the heat load of the combustor for the GH2 /GO2 case are similar to those in the hot propellants case. However, the combustion performance in the hot propellant case is better than that in the GH2/GO2 case, and the heat load of the combustor is also larger than that in the latter case.     

Transported PDF simulation of turbulent CH4/H2 flames under MILD conditions with particle-level sensitivity analysis

Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH₄/H₂ flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.     

燃油粒度对两相PDE爆震波速的影响

两相爆震燃烧的研究近来取得了较大的进步,但是仍有很多问题需要解决,诸如燃油的喷射、雾化和蒸发,燃油和氧化剂的混合,两相可爆混合物的短距离起爆等等．本文利用激光喷雾粒度分析仪分别就直射喷嘴与气动喷嘴研究了汽油的雾化情况,随着汽油流量的增加,两种喷嘴的雾化变化趋势相反．结合汽油、空气PDE模型机多循环爆震试验,发现汽油的粒度对模型机的爆震波速有较大的影响,粒度减小,波速增大,同时波速具有循环效应．     

A transported probability density function/photon Monte Carlo method for high-temperature oxy–natural gas combustion with spectral gas and wall radiation

A computational fluid dynamics model for high-temperature oxy–natural gas combustion is developed and exercised. The model features detailed gas-phase chemistry and radiation treatments (a photon Monte Carlo method with line-by-line spectral resolution for gas and wall radiation – PMC/LBL) and a transported probability density function (PDF) method to account for turbulent fluctuations in composition and temperature. The model is first validated for a 0.8 MW oxy–natural gas furnace, and the level of agreement between model and experiment is found to be at least as good as any that has been published earlier. Next, simulations are performed with systematic model variations to provide insight into the roles of individual physical processes and their interplay in high-temperature oxy–fuel combustion. This includes variations in the chemical mechanism and the radiation model, and comparisons of results obtained with versus without the PDF method to isolate and quantify the effects of turbulence–chemistry interactions and turbulence–radiation interactions. In this combustion environment, it is found to be important to account for the interconversion of CO and CO₂, and radiation plays a dominant role. The PMC/LBL model allows the effects of molecular gas radiation and wall radiation to be clearly separated and quantified. Radiation and chemistry are tightly coupled through the temperature, and correct temperature prediction is required for correct prediction of the CO/CO₂ ratio. Turbulence–chemistry interactions influence the computed flame structure and mean CO levels. Strong local effects of turbulence–radiation interactions are found in the flame, but the net influence of TRI on computed mean temperature and species profiles is small. The ultimate goal of this research is to simulate high-temperature oxy–coal combustion, where accurate treatments of chemistry, radiation and turbulence–chemistry–particle–radiation interactions will be even more important.     

Impact of co- and counter-swirl on flow recirculation and liftoff of non-premixed oxy-flames above coaxial injectors

Controlling the flame shape and its liftoff height is one of the main issues for oxy-flames to limit heat transfer to the solid components of the injector. An extensive experimental study is carried out to analyze the effects of co- and counter-swirl on the flow and flame patterns of non-premixed oxy-flames stabilized above a coaxial injector when both the inner fuel and the annular oxidizer streams are swirled. A swirl level greater than 0.6 in the annular oxidizer stream is shown to yield compact oxy-flames with a strong central recirculation zone that are attached to the rim of central fuel tube in absence of inner swirl. It is shown that counter-swirl in the fuel tube weakens this recirculation zone leading to more elongated flames, while co-swirl enhances it with more compact flames. These results obtained for high annular swirl levels contrast with previous observations made on gas turbine injectors operated at lower annular swirl levels in which central recirculation of the flow is mainly achieved with counter-rotating swirlers. Imparting a high inner swirl to the central fuel stream leads to lifted flames due to the partial blockage of the flow at the injector outlet by the central recirculation zone that causes high strain rates in the wake of the injector rim. This partial flow blockage is more influenced by the level of the inner swirl than its rotation direction. A global swirl number is then introduced to analyze the structure of the flow far from the burner outlet where swirl dissipation takes place when the jets mix. A model is derived for the global swirl number which well reproduces the evolution of the mass flow rate of recirculating gases measured in non-reacting conditions and the flame liftoff height when the inner and outer swirl levels and the momentum flux ratio between the two streams are varied.     

Simple model of inhibition of chain-branching combustion processes

A simple kinetic model has been suggested to describe the inhibition and extinction of flame propagation in reaction systems with chain-branching reactions typical for hydrocarbon systems. The model is based on the generalised model of the combustion process with chain-branching reaction combined with the one-stage reaction describing the thermal mode of flame propagation with the addition of inhibition reaction steps. Inhibitor addition suppresses the radical overshoot in flame and leads to the change of reaction mode from the chain-branching reaction to a thermal mode of flame propagation. With the increase of inhibitor the transition of chain-branching mode of reaction to the reaction with straight-chains (non-branching chain reaction) is observed. The inhibition part of the model includes a block of three reactions to describe the influence of the inhibitor. The heat losses are incorporated into the model via Newton cooling. The flame extinction is the result of the decreased heat release of inhibited reaction processes and the suppression of radical overshoot with the further decrease of the reaction rate due to the temperature decrease and mixture dilution. A comparison of the results of modelling laminar premixed methane/air flames inhibited by potassium bicarbonate (gas phase model, detailed kinetic model) with the results obtained using the suggested simple model is presented. The calculations with the detailed kinetic model demonstrate the following modes of combustion process: (1) flame propagation with chain-branching reaction (with radical overshoot, inhibitor addition decreases the radical overshoot down to the equilibrium level); (2) saturation of chemical influence of inhibitor, and (3) transition to thermal mode of flame propagation (non-branching chain mode of reaction). The suggested simple kinetic model qualitatively reproduces the modes of flame propagation with the addition of the inhibitor observed using detailed kinetic models.     

Head-on quenching of laminar premixed methane flames diluted with hot combustion products

Transient head-on quenching of laminar premixed methane flames diluted with hot combustion products is analyzed using full-chemistry 1D DNS. The impact of the dilution level, pressure and wall temperature on carbon monoxide (CO) emissions is investigated. Increasing dilution level and pressure reduce peak average near-wall CO concentrations, and reduce the near-wall CO reduction rate. However, the peak average near-wall CO and near-wall CO reduction rate increase with increasing wall temperature. Analysis of the species transport budget for CO near the wall before, during and after quenching indicates that there are conditions where diffusion is the dominant transport term. As a consequence, it may be possible to model the near-wall CO using only the integrated diffusion term within certain spatial regions. Dilution increases the size of these regions, whereas increasing pressure reduces this size.     

Effects of CO2 dilution on partially premixed CH4-air flames in swirl and bluff body stabilized combustor

The effect of CO₂ dilution on the flame characteristics and pollutant emission of a partially premixed CH₄-air flame in a confined bluff body and swirl influenced flowfield is investigated using optical and laser diagnostic methods. The non-premixed burner produced a converging-diverging flowfield at the burner exit and a lifted flame is produced at all test cases, with an upstream movement of the flame with decreasing global equivalence ratios (?_g). Based on variations in ?_g, two flame stabilization modes – bluff body influenced and swirl stabilized – with a transition mode in-between is observed for the cases with (flame FB) and without dilution (flame FM). The characteristics of the heat release zone are influenced by dilution, with the FB flames being longer and also less intense when compared to FM flames. Pollutant measurement at 30 mm downstream from the combustor exit highlighted the ultra-low NO_x capability of the IIST-GS2 burner. CO₂ dilution leads to a reduction in NO_x emission due to both thermal and chemical effects. For ?_g ≥ 0.7 extreme low levels of CO and unburned hydrocarbons (UHC) are observed for both cases. For ?_g ≤ 0.6 the dramatic increase of both CO and UHC maybe due to the lower flame temperatures and shorter flame zone residence times, respectively.     

Prediction of soot and thermal radiation in a model gas turbine combustor burning kerosene fuel spray at different swirl levels

Combustion of kerosene fuel spray has been numerically simulated in a laboratory scale combustor geometry to predict soot and the effects of thermal radiation at different swirl levels of primary air flow. The two-phase motion in the combustor is simulated using an Eulerian–Lagragian formulation considering the stochastic separated flow model. The Favre-averaged governing equations are solved for the gas phase with the turbulent quantities simulated by realisable k–? model. The injection of the fuel is considered through a pressure swirl atomiser and the combustion is simulated by a laminar flamelet model with detailed kinetics of kerosene combustion. Soot formation in the flame is predicted using an empirical model with the model parameters adjusted for kerosene fuel. Contributions of gas phase and soot towards thermal radiation have been considered to predict the incident heat flux on the combustor wall and fuel injector. Swirl in the primary flow significantly influences the flow and flame structures in the combustor. The stronger recirculation at high swirl draws more air into the flame region, reduces the flame length and peak flame temperature and also brings the soot laden zone closer to the inlet plane. As a result, the radiative heat flux on the peripheral wall decreases at high swirl and also shifts closer to the inlet plane. However, increased swirl increases the combustor wall temperature due to radial spreading of the flame. The high incident radiative heat flux and the high surface temperature make the fuel injector a critical item in the combustor. The injector peak temperature increases with the increase in swirl flow mainly because the flame is located closer to the inlet plane. On the other hand, a more uniform temperature distribution in the exhaust gas can be attained at the combustor exit at high swirl condition.     

Vaporization and combustion in three-dimensional droplet arrays

The quasi-steady vaporization and combustion of multiple-droplet arrays is studied numerically. Utilizing the Shvab–Zeldovich formulation, a transformation of the governing equations to a three-dimensional Laplace’s equation is performed, and the solution to Laplace’s equation is obtained numerically to find the effects of droplet interactions in symmetric, multiple-droplet arrays. Vaporization rates, flame surface shapes, and flame locations are found for different droplet array configurations and fuels. The number of droplets, the droplet arrangement within the arrays, and the droplet spacing within the arrays are varied to determine the effects of these parameters. Computations are performed for uniformly spaced three-dimensional arrays of up to 216 droplets, with center-to-center spacing ranging from 3 to 25 droplet radii. As a result of the droplet interactions, the number of droplets and relative droplet spacing significantly affect the vaporization rate of individual droplets within the array, and consequently the flame shape and location. For small droplet spacing, the individual droplet vaporization rate decreases below that obtained for an isolated droplet by several orders of magnitude. A similarity parameter which correlates vaporization rates with array size and spacing is identified. Individual droplet flames, internal group combustion, and external group combustion can be observed depending on the droplet geometry and boundary conditions.