Nonpremixed Reaction Zone Research Articles

Numerical Investigation of Partially Premixed Flames Under Transcritical Conditions

Understanding the characteristics of partially premixed flames (PPFs) under transcritical conditions is of critical importance for the development of fuel-rich staged rocket engines. While substantial progress has been achieved for transcritical non-premixed flames (NPFs), comparatively little effort has been made to investigate transcritical PPFs. To this end, a series of transcritical counterflow gaseous hydrogen/liquid oxygen (GH2/LOX) PPFs is simulated to investigate the thermodynamic structure of PPF and to examine the effects of molecular diffusion modeling, strain rate, and the equivalence ratio at the fuel-rich side on PPFs in physical space and mixture fraction space, as well as reduced temperature/reduced pressure space. The comparisons between the NPF and PPF demonstrate that the PPF exhibits a bimodal structure in physical space: a premixed reaction zone at the fuel inlet side and a non-premixed reaction zone at the oxidizer side. In mixture fraction space, the C-shaped structure of PPF is observed owing to the differential diffusion of species. It is found that the choice of molecular diffusion model has a significant impact on PPF structure. The presence of a loop at subcritical pressures in a reduced temperature/reduced pressure space is caused by the differential diffusion of species and the formation of [Formula: see text] in the non-premixed reaction zone. Furthermore, the results indicate that the premixed reaction zones in the PPFs are very sensitive to the change in strain rate and/or equivalence ratio of the premixed mixture at the fuel inlet side. For a given equivalence ratio, increasing strain rate can suppress the differential diffusion effect and the C-shaped structure, while it has a negligible impact on the non-premixed reaction and hence on loop formation.

Combustion characteristics of biodiesel with different dimethyl ether blending strategies

In this study, the effects of adding dimethyl ether (DME) on biodiesel (BD) combustion were studied through counterflow flame modeling. DME was added through two blending strategies, i.e., air-side and fuel-side blending, and methyl decanoate (MD) was used as the surrogate for BD. The profiles of the flame temperature, primarily species, and heat release rates (HRRs) after DME addition into MD were analyzed to explore the kinetic differences between pre-blending and post-blending combustion modes of BD and DME in engines. Results show that the air-side and the fuel-side additions of DME can slightly increase the peak temperature and widen the reaction zones of the flames. MD presents two distinct oxidation zones, while DME is primarily oxidized within an identified flame region with a higher temperature than MD. DME premixing on the fuel side can prompt MD dehydrogenation through enhancing H and OH formation. The flames enriched by air-side DME premixing present significant hybrid heat-release structure that depends on the fuel-side air entrainment as well. The heat-release characteristics of the non-premixed reaction zone (NPZ) of the counterflow flames with air-side DME additions suggest that the increased temperature is the main factor leading to the change of heat-release reaction routes of the flames.

Effects of methanol addition on the combustion characteristics of bio-diesel based counterflow nonpremixed flames

With the improvement of combustion technology and the prominence of emission-related environmental concerns, the combined application of macromolecular unconventional fuels (bio-diesel) and alcohol ether fuels in engines is being considered. In this study, the effect of methanol addition on the combustion characteristics of methyl decanoate (MD)/air counterflow nonpremixed flames were investigated using numerical simulation. Two addition strategies of methanol were employed: premixing methanol into MD on the fuel side, and partially premixing of methanol on the air side. The corresponding flames are respectively double-flames with a rich premixed reaction zone and a nonpremixed reaction zone, or triple-flames with an additional lean reaction zone close to the air side. Results showed that, with the increased methanol addition, the global parameters of the double-flames and triple-flames were altered to different degrees. The both strategies of methanol addition can widen the flame thickness, and comparatively, the broadened flames with air-side addition of methanol were more obvious. The introduction of methanol showed little direct effects on the oxidation and pyrolysis of MD, and the key kinetic roles on the production of the active intermediates, such as OH and H in the flame domain. For both premixing strategies, the integral production of OH and H in the flames were increased, and the flames with the air-side methanol addition present the more significant production.

Effects of air-side dimethyl ether premixing on combustion characteristics of nonpremixed biodiesel/air counterflow flames

Counterflow flame modeling was conducted to investigate the effects of air-side dimethyl ether premixing on the combustion of biodiesel/air non-premixed flames, using the detailed kinetic mechanisms of bio-diesel and dimethyl ether. Methyl decanoate was adopted as the bio-diesel surrogate component. Comparative analyses focused on heat release, methyl decanoate oxidation, and the soot precursor, i.e. benzene production in the flames. The complex triple-flame structures were observed in the flames with air-side addition of dimethyl ether. Due to the interactions in the hybrid reaction zones, the flames with air-side premixing had higher peak temperatures, broadened flame thickness, and greater production of the integrated OH radical, consequently resulting in more robust flame power. The reactions related to self-pyrolysis and H-abstraction are the top main routes for methyl decanoate oxidation. The thermal effect induced by the higher flame temperature enhanced the radial leakage of MD and other intermediates and lowered the benzene production rate. The non-premixed reaction zone was the most active region for benzene production. The air-side premixing could exert its chemical effect on benzene formation within this zone by changing propargyl and phenyl chemistry.

Characterization of multi-regime reaction zones in a piloted inhomogeneous jet flame with local extinction

Gradient free regime identification (GFRI) is applied to 1D Raman/Rayleigh/LIF measurements of temperature and major species from the intermediate velocity case of the Sydney piloted inhomogeneous jet flame series to better understand the structure of reaction zones and the downstream evolution of multi-regime characteristics. The GFRI approach allows local reaction zones to be detected and characterized as premixed, dominantly premixed, multi-regime, dominantly non-premixed, or non-premixed flame structures, based on flame markers (mixture fraction, chemical mode, and heat release rate) derived from the experimental data. The statistics of chemical mode zero-crossings, which mark premixed reaction zones, and the relative populations of flame structures are shown to be sensitive to the state of mixing in the near field of the flame and to the level of local extinction farther downstream. Multi-regime structures, where premixed and non-premixed reaction zones occur in close proximity and both contribute to overall heat release, account for nearly half the total population at streamwise locations within the first several jet diameters. There is a rapid transition within the near field whereby the relative population of non-premixed and dominantly non-premixed structures grows from 0.05 to nearly 0.5, and the population of premixed and dominantly premixed structures decreases correspondingly as fluid entering the reaction zone becomes progressively fuel-rich. Local extinction and re-ignition bring a resurgence in premixed-type structures, many of which occur at fuel-lean conditions. There are also modest populations of multi-regime structures, having chemical mode zero-crossings at lean conditions, which would not exist in a fully burning jet flame.

Derivation and analysis of two-dimensional composition space equations for multi-regime combustion using orthogonal coordinates

Interactions between premixed and non-premixed reaction zones can lead to complex mixed combustion regimes, here denoted as multi-regime combustion, which pose challenges to many conventional combustion modeling approaches. Such conditions occur in most practical combustors and can originate from partial premixing, mixture inhomogeneities/stratification, hot product recirculation, or local flame extinction and re-ignition. Therefore, novel equations are derived for modeling multi-regime combustion which are formulated with respect to a two-dimensional composition space spanned by mixture fraction and reaction progress variable. Contrary to previous works, the dependency of the progress variable on the mixture fraction is considered in the new model. This is achieved by splitting the progress variable gradient into an aligned and an orthogonal component with respect to the mixture fraction gradient and the latter is used to define the second coordinate. In the theory that follows, a balance equation for the progress variable on mixture fraction iso-surfaces is formulated. Using this balance equation together with the orthogonal coordinate system, the transformation of species and temperature equations to the 2D composition space yields a novel set of equation without so-called cross-terms. This is advantageous since cross-terms obtained with previous approaches lack a general closure and it is uncertain if it exists at all. Furthermore, the approach allows to naturally distinguish between non-premixed and premixed combustion regimes, auto ignition, and it covers multi-regime combustion characteristics. The theory is validated and discussed by means of a fully resolved solution of a laminar triple flame using detailed chemistry. At first, regions which exhibit premixed, non-premixed or multi-regime combustion characteristics are identified. The triple flame solution then serves as a database from which all relevant theoretical relations are post-processed and validated. In comparison to budgets of conventional 1D flamelet equations for premixed and non-premixed combustion it is shown that only the full set of transport terms considered in the 2D equations accurately balances chemical source terms everywhere in the triple flame, especially in regions where multi-regime combustion prevails.

Local flame structure analysis in turbulent CH4/air flames with multi-regime characteristics

In practical applications, partial premixing of fuel and oxidizer, as well as recirculation of combustion products, result in complex combustion scenarios where multi-regime effects arise and a numerical representation of local reaction zones by purely premixed or purely non-premixed flame structures may not hold. Here, a novel burner system is introduced to investigate the fundamental characteristics of multi-regime combustion and to provide a basis for validating numerical models. This multi-regime burner (MRB) is specifically designed to produce flames with multi-regime characteristics while maintaining well-defined boundary conditions. Thermochemical data from Raman/Rayleigh/CO-LIF scattering experiments are provided for two selected operating conditions. The experimental investigation focuses on the overall flame structure by examining radial profiles of temperature and mixture fraction, as well as scatter plots of temperature, CH4, and CO versus mixture fraction. In order to assess the relative importance of different flame regimes, the gradient-free regime identification (GFRI) approach is extended to allow for an automated classification of local reaction zone structures. Classification criteria are defined, based on the ratio of local heat release rate peaks associated with premixed and non-premixed reaction zones located in close spatial proximity, and an automated process is implemented to classify 1D Raman/Rayleigh sample lines as premixed, dominantly premixed, multi-regime, dominantly non-premixed, or non-premixed flame zones. The importance of different flame zones, indicated by their population fractions, are found to evolve with downstream distance and show distinct differences between the two selected flames. Further, a prior analysis is used to test the applicability of 1D flame structure assumptions for the underlying combustion regime.

Simultaneous imaging of soot volume fraction, PAH, and OH in a turbulent n-heptane spray flame

Soot formation and oxidation in a turbulent n-heptane jet spray flame are investigated through simultaneous imaging of quantitative soot volume fraction (fv), flame-front (OH), and soot-precursor marked by smaller (2–4 ring) polycyclic aromatic hydrocarbons (PAH). Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques are used. The spray flame is composed of a dual branch axisymmetric structure. The inner composite branch (B1) consists of fuel-lean and non-premixed reaction zones, whereas combustion in the outer branch (B2) occurs in a non-premixed mode. Frequent local flame extinctions occur along B1, while no extinctions are observed in B2. Consequently, the soot inception occurs near B2. In the onset of soot detection (fv ≥ 0.03 ppm) region, soot layers are thinner than those from downstream regions where soot occupies nearly the same area as of PAH. Soot generally oxidizes across the flame-front as evidenced from the absence of soot-OH overlap. The soot structures are intermittent with peak soot probability of 60%. The most probable peak fv measures 1 ppm, while the conditional mean is 0.42 ppm which is 2.5 times that of time-averaged fv. Conditional mean fv shows monotonically increasing trend with height. The flame lift-off height and local extinctions do not instantaneously influence the soot concentration, possibly due to larger convective and soot formation timescales. Soot-PAH correlations are explored. However, no strong instantaneous correlations are noted, most likely due to different chemical timescales of soot and the imaged smaller-PAH. Nevertheless, a few distinct trends are noted in soot-PAH correlations. fv weakly increases with the PAH-LIF intensity. The PAH intensity from soot onset to peak region decreases, while both soot and PAH are nearly consumed at a local flame-tip (oxidation) region. The soot-PAH overlap decreases consistently from soot onset to oxidation region. The reported database and deduced insights can aid in the development and validation of soot models in two-phase reacting flows.

Low-frequency combustion instabilities of an airblast swirl injector in a liquid-fuel combustor

ABSTRACT Low-frequency combustion dynamics of a prefilming airblast injector were experimentally investigated in a laboratory-scale liquid-fuel combustor operated with Jet A-1 fuel and air at atmospheric pressure and elevated temperatures. Our measurements reveal that multiple modes – ranging from 45 to 292 Hz – can be excited in the non-premixed combustion system, including the Helmholtz, longitudinal, and hydrodynamic instabilities. The system's propensity for mode selection depends strongly on combustor length and pilot equivalence ratio. A strong Helmholtz mode with a peak-to-peak amplitude of ∼14 kPa occurs, provided that the combustor length is relatively short and the pilot equivalence ratio is high. A high-amplitude, intermittent burst at approximately 12 Hz always develops when the system is on the verge of transition to the Helmholtz instability. With a longer combustor length, on the other hand, the system transitions to the ¾-wave longitudinal mode via a discontinuous mode-hopping process. The system undergoes well-defined, limit cycle oscillations with an extremely large pressure amplitude of ∼30 kPa, but the global OH* fluctuation amplitude is limited to merely 8.3%. Phase-resolved flame imaging measurements demonstrate that a ring-like partially-premixed flame emerges periodically in the dump region, when the non-premixed flame wrapping around the central recirculation zone is lifted off the fuel nozzle. The mutual interaction between the partially-premixed and non-premixed reaction zones is the dominant mechanism for intense sound generation. Under certain conditions, superposition of the Helmholtz and longitudinal modes occurs, leading to nonlinear interactions manifested by additional spectral peaks at the sum and difference of the two frequencies. In contrast to the other two cases, this superimposed tone is not characterized by limit cycle behavior, but by a noise-driven unsteadiness.

Assessing the relative importance of flame regimes in Raman/Rayleigh line measurements of turbulent lifted flames

Understanding and quantifying the relative importance of premixed and non-premixed reaction zones within turbulent partially premixed flames is an important issue for multi-regime combustion. In the present work, the recently-developed method of gradient-free regime identification (GFRI) is applied to instantaneous 1D Raman/Rayleigh measurements of temperature and major species from two turbulent lifted methane/air flames. Local premixed and non-premixed reaction zones are identified using criteria based on the mixture fraction, the chemical explosive mode, and the heat release rate, the latter two being calculated from an approximation of the full thermochemical state of each measured sample. A chemical mode (CM) zero-crossing is a previously documented marker for a premixed reaction zone. Results from the lifted flames show strong correlations among the mixture fraction at the CM zero-crossing, the magnitude of the change in CM at the zero-crossing, and the local heat release rate at the CM zero-crossing compared to the maximum heat release rate. The trends are confirmed through a comparable analysis of numerical simulations of two laminar triple flames. These newly documented trends are associated with the transition from dominantly premixed flame structures to dominantly non-premixed flames structures. The methods introduced for assessing the relative importance of local premixed and non-premixed reactions zones have potential for application to a broad range of turbulent flames.

Regime identification from Raman/Rayleigh line measurements in partially premixed flames

Current methods for combustion regime characterization, such as the flame index, rely on 3D gradient information that is not accessible with available experimental techniques. Here, a method is proposed for reaction zone detection and characterization, which can be applied to instantaneous 1D Raman/Rayleigh line measurements of major species and temperature as well as to results of laminar and turbulent flame simulations, without the need for 3D gradient information. Several derived flame markers, namely the mixture fraction, the heat release rate, and the chemical explosive mode, are combined to detect and characterize premixed versus non-premixed reaction zones. The methodology is developed and evaluated using fully resolved simulation data from laminar flames. The fully resolved 1D simulation data are spatially filtered to account for the difference in spatial resolution between experiment and simulation. Then, starting from just temperature and major species, a constrained homogeneous constant pressure, constant temperature reactor calculation gives an approximation of the full thermochemical state at each sample location along the line. Finally, the chemical explosive mode and the heat release rate are calculated from this approximated state and compared to those calculated directly from the simulation data. As a further test, experimental uncertainty is superimposed onto the filtered numerical data to produce a Raman/Rayleigh equivalent state before running the constrained homogeneous reactor, and results are again compared. After successful tests using the numerical data, the approach is applied to Raman/Rayleigh line measurements from laminar counterflow flames and a mildly turbulent lifted flame. The results confirm that the reaction zones can be reliably detected and characterized using experimental data. Furthermore, the relative importance of premixed and non-premixed reaction zones within the same flame can be qualitatively assessed as demonstrated in the results.

Nonpremixed and premixed flamelets LES of partially premixed spray flames using a two-phase transport equation of progress variable

Partially premixed spray flames are simulated with flamelet-based tabulated chemistry parameterized by the mixture fraction and progress variable. The transport equation of the reaction progress variable C is reconsidered, and its formulation for the reacting two-phase flows is derived and employed, which allows the inclusion of spray impacts through a new spray source term that is absent in its gaseous form. Both the nonpremixed and premixed flamelets assuming single reaction regime are implemented in LES, and their validities in spray flames and dependence on the evaporation effect when considering two-phase C equation are examined. The effect of spray, reaction and turbulence interaction is then investigated in comparison with experiments of Sydney reacting acetone sprays, covering the rich, lean and stoichiometric cases. The computed results generally follow the experimental data, but a disagreement between two flamelet simulations is observed especially in rich and lean flames. The premixed flamelets tend to capture the downstream jet spreading while overestimating the peak temperature compared to the nonpremixed chemistry. Flame index analysis indicates that in the present spray flames an evaporation-dominated regime exists inside the upstream core jet and it promotes the coexistence of subsequent interacting premixed and nonpremixed reaction zones, which impedes accurate flame prediction by the single regime flamelets. Furthermore, the spray source term appearing in the derived C equation is identified to act as scalar fluxes driven by sprays in flamelet structures. Including this new source term is found to be important to account for the dissipation effect induced by evaporation on the reaction zone in the flamelet simulation of turbulent spray flames.

Effects of Fuel Unsaturation on Transient Ignition and Flame Development in Sprays

ABSTRACTFundamental studies using homogeneous mixtures in shock tube and rapid compression machine (RCM) have shown that the fuel reactivity and ignition characteristics are significantly affected by the presence and number of double bonds in the fuel molecular structure. However, this phenomenon has not been investigated in turbulent reacting sprays under realistic conditions. In this article, we numerically examine the effects of fuel unsaturation and sensitivity on the transient ignition and flame evolution in n-heptane and 1-heptene sprays in the Sandia constant volume reactor. 3D simulations of two-phase reacting flows are performed using a commercial CFD code, CONVERGE. The ignition and combustion chemistry of n-heptane and 1-heptene is modeled using a reduced CRECK mechanism, which has been previously validated against the shock tube ignition delays and reacting spray measurements. Results indicate that, at intermediate temperatures, the ignition in both homogeneous mixtures and sprays is strongly influenced by fuel unsaturation. First of all, the ignition delay is much higher for 1-heptene compared to that for n-heptane, especially at low temperatures. Secondly, ignition in n-heptane sprays occurs in fuel rich mixtures, and is characterized by a two-stage ignition process. On the other hand, ignition in 1-heptene sprays occurs in lean mixtures. Results also indicate the existence of multiple ignition locations in sprays, which is in contrast to ignition in homogeneous mixtures. The flame evolution in sprays correlates strongly with the ignition kernel evolution. Consequently, the n-heptane spray flame contains two reaction zones, namely, a rich premixed zone (RPZ) and a non-premixed reaction zone (NPZ), while the 1-heptene flame is characterized by three reaction zones, i.e., a lean premixed zone in addition to NPZ and RPZ. The difference is a direct consequence of longer ignition delay in 1-heptene spray compared to that in n-heptane spray, which facilitates increased fuel-air mixing in the former. The flame lift-off location correlates with the ignition delay and ignition kernel location. Thus, at lower temperatures (1000 K and 1100 K), the lift-off length of 1-heptene flame is greater than that of n-heptane flame, which is due to longer ignition delays for 1-heptene sprays. In contrast, at higher temperatures (1200 K) the lift-off length of 1-heptene flame is smaller, which is due to the fact that the ignition kernel and, thus, lean premixed zone are located closer to the injector for 1-heptene spray compared to n-heptane spray.

Fuel unsaturation effects on NOx and PAH formation in spray flames

The effect of fuel unsaturation on NOx and PAH formation in spray flames is investigated at diesel engine conditions. The directed relation graph methodology is used to develop a reduced mechanism starting from the detailed CRECK mechanism (http://creckmodeling.chem.polimi.it/index.php/current-version-kinetic-mechanisms/low-and-high-temperature-complete-mechanism). The reduced mechanism and spray models are validated against the shock tube ignition data and high-fidelity, non-reacting and reacting spray data from the Engine Combustion Network (ECN). 3-D simulations are performed using the CONVERGE software to examine the structure and emission characteristics of n-heptane and 1-heptene spray flames in a constant-volume combustion vessel. Results indicate that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich premixed reaction zone (RPZ) near the flame stabilization region and a non-premixed reaction zone (NPZ) further downstream. Most of NOx is formed via thermal NO route in the NPZ, while PAH species are mainly formed in the RPZ. A small amount of NO is also formed via prompt route in the RPZ, and via N2O intermediate route in the region outside NPZ, and via NNH intermediate route in the region between RPZ and NPZ. The presence of a double bond leads to higher flame temperature and thus higher NO in 1-heptene flame than that in n-heptane flame. It also leads to the increased formation of PAH species, implying increased soot emission in 1-heptene flame than that in n-heptane flame. Reaction path analysis indicate that the increased formation of PAH species can be attributed to the significantly higher amounts of 1,3-butadiene and allene formed due to β scission reactions resulting from the presence of double bond in 1-heptene.

A Study on Flame Structures in CH<sub>4</sub> Counterflow Partially Premixed Flame

An experimental and simulation work had been conducted to study a one-dimensional partially premixed methane/air counterflow flame in this paper. Flame images are obtained through experiments and computations using GRIMech 3.00 chemistry were performed for the flames studied. The partially premixing effects upon the flame were revealed by comparing the flame structures and emissions with premixed flames at the same equivalence ratio. The results show the premixed flame only has a single flame structure. However, PPF has distinct double flame structures at present equivalence ratio. Temperature is relatively high in the whole combustion zone for premixed flame, while, for PPF, there are two temperature peaks in a rich premixed reaction zone on the fuel side and a nonpremixed reaction zone on the oxidizer side respectively. For PPF, NO concentration in the nonpremixed zone is much higher compared to that in the rich premixed zone because of higher OH concentration in the nonpremixed zone.

Effect of Fuel Molecular Structure and Premixing on Soot Emissions from n-Heptane and 1-Heptene Flames

Most liquid fuels contain compounds with one or more unsaturated C═C bonds. Previous studies have observed that the fuel reactivity and ignition behavior are strongly influenced by the presence and number of double bonds in the fuel molecular structure. Here, we report a numerical investigation on the effect of fuel unsaturation on PAHs and soot emissions in partially premixed flames (PPFs) burning n-heptane and 1-heptene fuels. A detailed soot and fuel oxidation model is validated against gaseous species measurements in n-heptane PPF and soot measurements in ethylene diffusion flames. Simulations are performed to examine the effects of double bonds on PAHs and soot emissions at different strain rates and levels of premixing. For both fuels, the global flame structure is characterized by a rich premixed reaction zone (RPZ) on the fuel side and a nonpremixed reaction zone (NPZ) on the oxidizer side. PAHs and soot are mainly formed in the region between the RPZ and stagnation plane. The presence of double b...

Effects of primary breakup modeling on spray and combustion characteristics of compression ignition engines

Abstract Injector flow dynamics and primary breakup processes are known to play a pivotal role in determining combustion and emissions in diesel engines. In the present study, we examine the effects of primary breakup modeling on the spray and combustion characteristics under diesel engine conditions. The commonly used KH model, which considers the aerodynamically induced breakup based on the Kelvin–Helmholtz instability, is modified to include the effects of cavitation and turbulence generated inside the injector. The KH model and the new (KH-ACT) model are extensively evaluated by performing 3-D time-dependent simulations with detailed chemistry under diesel engine conditions. Results indicate that the inclusion of cavitation and turbulence enhances primary breakup, leading to smaller droplet sizes, decrease in liquid penetration, and increase in the radial dispersion of spray. Predictions are compared with measurements for non-evaporating and evaporating sprays, as well as with flame measurements. While both the models are able to reproduce the experimentally observed global spray and combustion characteristics, predictions using the KH-ACT model exhibit closer agreement with measurements in terms of liquid penetration, cone angle, spray axial velocity, and liquid mass distribution for non-evaporating sprays. Similarly, the KH-ACT model leads to better agreement with respect to the liquid length and vapor penetration distance for evaporating sprays, and with respect to the flame lift-off location for combusting sprays. The improved agreement is attributed to the ability of the new model to account for the effects of turbulence and cavitation generated inside the injector, which enhance the primary breakup. Results further indicate that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich premixed reaction zone near the flame stabilization region and a non-premixed reaction zone further downstream. This flame structure is consistent with the Dec’s model for diesel engine combustion (Dec, 1997) [1] , and well captured by a newly developed flame index based on the scalar product of CO and O2 mass fraction gradients.

Structure of Unsteady Partially Premixed Flames and the Existence of State Relationships

In this study, we examine the structure and existence of state relationships in unsteady partially premixed flames (PPFs) subjected to buoyancy-induced and external perturbations. A detailed numerical model is employed to simulate the steady and unsteady two-dimensional PPFs established using a slot burner under normal and zero-gravity conditions. The coflow velocity is parametrically varied. The methane-air chemistry is modeled using a fairly detailed mechanism that contains 81 elementary reactions and 24 species. Validation of the computational model is provided through comparisons of predictions with nonintrusive measurements. The combustion proceeds in two reaction zones, one a rich premixed zone and the other a nonpremixed zone. These reaction zones are spatially separated, but involve strong interactions between them due to thermochemistry and scalar transport. The fuel is mostly consumed in the premixed zone to produce CO and H2, which are transported to and consumed in the nonpremixed zone. The nonpremixed zone in turn provides heat and H-atoms to the premixed zone. For the range of conditions investigated, the zero-g partially premixed flames exhibit a stable behavior and a remarkably strong resistance to perturbations. In contrast, the corresponding normal-gravity flames exhibit oscillatory behavior at low coflow velocities but a stable behavior at high coflow velocities, and the behavior can be explained in terms of a global and convective instabilities. The effects of coflow and gravity on the flames are characterized through a parameter VR, defined as the ratio of coflow velocity to jet velocity. For VR ≤ 1 (low coflow velocity regime), the structures of both 0- and 1-g flames are strongly sensitive to changes in VR, while they are only mildly affected by coflow in the high coflow velocity regime (VR > 1). In addition, the spatio-temporal characteristics of the 0- and 1-g flames are markedly different in the first regime, but are essentially similar in the second regime. A more significant difference in the first regime between these flames is the presence of a flow instability that manifests itself through the self-excited oscillations of the 1-g flame and the concomitant flickering of the nonpremixed reaction zone. For VR ≤ 1, as the coflow velocity is increased, the oscillation amplitude decreases and the oscillation frequency increases, both of which are in accord with previous experimental and computational results concerning 1-g jet diffusion flames. The modified conserved scalar approach is found to be effective in characterizing the flame structure and developing state relationships for both steady and unsteady partially premixed flames. This is demonstrated by the fact that the temperature as well as the major and minor species profiles follow similar state relationships in terms of the modified mixture fraction for the 0- and 1-g flames, even though these flames have markedly different spatio-temporal characteristics.

Analysis of entropy generation in hydrogen-enriched methane–air propagating triple flames

A theoretical–numerical analysis based on the second law of thermodynamics is used to examine the propagation of laminar H 2-enriched CH 4–air flames. The analysis is based on computing the various entropy generation terms in a transient reacting flow field. A comprehensive, time-dependent computational model, which employs a detailed description of chemistry and transport, is used to simulate the transient ignition and flame propagation in this reacting flow field. Flames are ignited in a jet-mixing layer far downstream of the burner. Following ignition, a well-defined triple flame is formed that propagates upstream with nearly constant flame displacement speed along the stoichiometric mixture fraction line. As the flame approaches the burner, it transitions to a double flame, and subsequently to a burner-stabilized nonpremixed flame. The triple point exhibits the maximum entropy generation, indicating that this point is characterized by high chemical reactivity, as well as large temperature and mass fraction gradients. The volumetric entropy generation is the highest in the two premixed reaction zones, and the lowest in the nonpremixed reaction zone. In the premixed zones, the volumetric entropy generation due to chemical reaction is the highest, followed by heat conduction, and then mixing. The converse is true for the nonpremixed zone. However, the integrated entropy generation rate indicates that heat conduction is the major contributor, followed by chemical reactivity, and then mixing. As H 2 addition to methane fuel is increased, the integrated entropy generation increases primarily due to enhanced heat conduction and chemical reactivity. However, the contributions of heat conduction, chemical reactivity, and mixing to total entropy generation weakly depend on the fuel being burned. While the flame propagates upstream entropy generation increases and reaches a maximum when the flame exhibits a well-defined triple flame structure, and then decreases as the flame approaches the burner. The second law efficiency of the system remains nearly constant with H 2 addition, since the increased irreversibilities due to H 2 addition are compensated by the increase in the flow availability in the fuel blend.

A numerical and experimental study of counterflow syngas flames at different pressures

Synthesis gas or “Syngas” is being recognized as a viable energy source worldwide, particularly for stationary power generation due to its wide availability as a product of bio and fossil fuel gasification. There are, however, gaps in the fundamental understanding of syngas combustion and emissions characteristics, especially at elevated pressures that are relevant to practical combustors. This paper presents a numerical and experimental investigation of the combustion and NO x characteristics of syngas fuel with varying composition, pressure and strain rate. Experiments were performed at atmospheric conditions, while the simulations considered different pressures. Both experiments and simulations indicate that stable non-premixed and partially premixed counterflow flames (PPFs) can be established for a wide range of syngas compositions and strain rates. Three chemical kinetic models, GRI 3.0, Davis et al., and Mueller et al. are examined. The Davis et al. mechanism is found to agree best with the experimental data, and hence used to simulate the PPF structure at different pressure and fuel composition. For the pressure range investigated, results indicate a typical double flame structure with a rich premixed reaction zone (RPZ) on the fuel side and a non-premixed reaction zone (NPZ) on the oxidizer side, with RPZ characterized by H 2 oxidation, and NPZ by both H 2 and CO oxidation. While thermal NO is found to be the dominant route for NO production, a reburn route, which consumes NO through NO + O + M→ NO 2 + M and H + NO + M → HNO + M reactions, becomes increasingly important at high pressures. The amount of NO formed in syngas PPFs first increases rapidly with pressure, but then levels off at higher pressures. At a given pressure, the peak NO mole fraction exhibits a non-monotonic variation with syngas composition, first decreasing to a minimum value, and then increasing as the amount of CO in syngas is increased. This implies the existence of an optimum syngas composition that yields the lowest amount of NO production in syngas PPFs, and can be attributed to the combined effects of thermal and reburn mechanisms.