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A recent rotational flamelet model (Sirignano [Three-dimensional, rotational flamelet closure model with two-way coupling, J. Fluid. Mech. 945 (2022), p. A21; Inward swirling flamelet model, Combust. Theory Model. 26 (2022),pp. 1014–1040; Stretched vortex layer flamelet, Combust. Flame. 244 (2022), p. 112276]) is developed and tested with an improved framework of detailed chemistry and transport. The rotational flamelet model incorporates the effects of shear strain and vorticity on local flame behaviour and is three-dimensional by nature. A similarity solution reduces the three-dimensional governing equations to ODEs involving a transformation to a non-Newtonian reference frame. A 9-species chemical kinetics model is used for $ \mathrm {H_2} $ H2- $ \mathrm {O_2} $ O2 combustion with non-reacting $ \mathrm {N_2} $ N2. In all non-premixed flame cases, the oxidiser is pure $ \mathrm {O_2} $ O2 while the fuel $ (\mathrm {H_2}) $ (H2) is diluted with $ \mathrm {N_2} $ N2. Multiple flamelet cases including non-premixed, premixed, and partially-premixed flames are performed. Across all cases, vorticity extends flammability limits by up to 30% in terms of the ambient extinction strain rate and modifies both local flame structure and mixture composition. For $ \mathrm {N_2} $ N2-diluted $ \mathrm {H_2} $ H2- $ \mathrm {O_2} $ O2 non-premixed flames, where the location of minimum density coincides with the location of peak temperature, the centrifugal force induced by vorticity reduces the mass flow rate through the flame, effectively lowering the local strain rate. This increases residence time, thus extending flammability limits and reducing burning rates. This analysis is done also for premixed and partially-premixed flames. For pure $ \mathrm {H_2} $ H2- $ \mathrm {O_2} $ O2 non-premixed flames, where minimum density lies between the flame zone and the fuel inlet boundary, centrifugal forces do not significantly modify flame behaviour. Stable and unstable branches of S-curves for non-premixed and partially-premixed flames and stable branches for premixed flames show extended flammability limits due to vorticity. The capabilities of the rotational flamelet model reveal that vital physics are currently missing from two-dimensional, irrotational, constant-density, flamelet models. Improvements of detailed chemical kinetics, transport formulation, and thermo-physical properties bring the new flamelet model to par in these areas with existing models, while adding new features in terms of physical emulation. [ABSTRACT FROM AUTHOR]

Rotating detonation engines (RDEs) have attracted significant attention in the aerospace field due to their high thermal cycle efficiency. This study explores the characteristics of rotating detonation in gas-solid mixed fuel within an air atmosphere. A theoretical detonation model for gas-solid mixed-phase fuel is developed in a cylindrical coordinate system. Using the three-dimensional conservation element and solution element (CE/SE) method, numerical simulations of the detonation process in a disc combustor are conducted in three dimensions. The flow field structure during the stable propagation of gas-solid mixed-phase rotating detonation waves is calculated. Subsequently, the analysis is conducted on the distribution characteristics of thermodynamic parameters in the detonation flow field, the distribution of chemical reaction zone, and the characteristics of wave system following the rotating detonation wave. The research results demonstrate that micron-sized coal powder can undergo a rapid reaction and support the stable propagation of rotating detonation waves with the assistance of hydrogen. However, due to the flat structural characteristics of the disc combustor and the non-premixed injection mode, the wave front of detonation wave appears as an irregular surface. The difference in the ability of coal powder and hydrogen jet to penetrate the air layer leads to the separation of the chemical reaction zone of the gas-solid mixed fuel, ultimately supporting the stable propagation of rotating detonation wave in the form of a dual reaction zone. The irregular surface structure of the rotating detonation wave leads to multiple reflections in the flat combustor, consequently causing the regular appearance of multiple reflected shock waves after the detonation wave.

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Numerical simulations for moderate and intense low oxygen dilution (MILD) combustion with essential solvers and detailed mechanisms involve more complications and computational time. Various advanced combustion modeling techniques have recently been developed to study MILD combustion characteristics. However, every combustion model has specific issues predicting the temperature and emissions of the MILD combustion flames. The diffusive nature of the MILD flame is considered, and individual Lewis numbers are investigated on a nonpremixed flame. The current study analyzes the methane/hydrogen flame propagation with different Lewis number combinations in a hot co-flow environment. Individual Lewis numbers for methane and hydrogen are investigated from stochiometric to ultra-rich mixtures in non-premixed flames. Several numerical simulations are performed in the OpenFOAM9 environment using a modified EDC model with tuned turbulence and combustion model constants. The numerical simulation results with hydrogen and methane Lewis numbers of 0.4 and 0.9, respectively, show promising agreement with the experimental findings of Dally1, et al. Various combustion parameters are studied with different CH4 and H2 Lewis number combinations. In addition, the unity Lewis number case is simulated and compared to the situations that are taken into consideration. [ABSTRACT FROM AUTHOR]

A numerical investigation is conducted using OpenFOAM to study the characteristics of ammonia flames enriched with methane and hydrogen, influenced by a diluted co-flow (exhaust gas). The study aims to explore various flame characteristics, including fuel consumption, ignition delay time, lift-off height and position, temperature, and CO and NO emissions while maintaining a constant energy input of 5 kW. Non-stabilized flames are identified by analyzing the OH peak in the computational domain among ammonia flames enriched with CH 4 /H 2 for different dilutions of O 2 (6 %, 9 %, and 12 %). The study is structured into three stages of numerical simulations corresponding to stabilized flames: (1) no reaction, (2) reaction in the absence of O 2 , and (3) reaction with O 2 to see the sole impact of the hot co-flow and local turbulence mixing on the primary cracking of ammonia. Furthermore, the rates of H 2 and N 2 formation resulting from the utilization of ammonia in the presence of hot co-flow are thoroughly examined. Additionally, the behavior of ammonia flames and CH 4 /H 2 -enriched ammonia flames under preheated air conditions (23 % O 2) is analysed. Stable flames are observed in the co-flow at 12 % O 2 , displaying characteristics of Moderate or Intense Low-oxygen Dilution (MILD) combustion. Conversely, ammonia flames under preheated air conditions exhibit conventional flame behavior. Pure ammonia flames exhibit stronger temperature gradients and higher levels of CO and NO emissions. As the enrichment of NH 3 with CH 4 /H 2 increases, peak temperatures decrease while lift-off heights increase, indicating a transition towards more distributed flame structures. Notably, the instance with 40 % H 2 enrichment exhibits lower NO emissions and a higher rate of ammonia consumption along with more distributive flame characteristics. • Thoroughly examined NH 3 flames in co-flow with varying levels of O 2 dilution. • The stability of ammonia flames is tested under various O 2 dilutions. • Primary cracking of NH 3 enhanced under the hot co-flow conditions. • Minimum NO emissions were observed under lower O 2 dilutions with H 2 enrichment. • MILD NH 3 flames are identified among the CH 4 /H 2 enriched NH 3 diluted flames. [ABSTRACT FROM AUTHOR]

The performance of a scaled industrial, non-premixed, low-swirl burner design was experimentally investigated for hydrogen addition to natural gas. Two strategies for introducing hydrogen are considered, namely, conserving (i) heat input and (ii) velocity/volumetric flow of the original fuel. This work characterises the effects on key performance metrics of the burner as hydrogen fraction is increased. Compared with natural gas, the results with hydrogen showed a 33 % reduction in the radiant fraction and up to a 380 % increase in NOx emissions. The lift-off height was reduced by a maximum of 23 % and 51 % for addition of 10 and 30 vol% hydrogen addition, respectively, with 100 % cases becoming completely attached to the burner. The influence of hydrogen-addition strategy and air adjustment was shown to be significant with respect to NOx emissions but less significant than the resulting changes in fuel composition and heat input with respect to flame appearance, stability and radiant heat transfer. • H 2 and air addition strategies less significant than heat input and fuel composition. • Radiant fraction reduced by 33 % by operating with pure H 2. • NOx emissions increased by 380 % by operating with pure H 2. • H 2 reduced lift-off height, becoming reattached at 100 vol% H 2. [ABSTRACT FROM AUTHOR]

In this study, numerical methods were used to assess the effects of adding hydrogen and varying the equivalence ratios on the combustion and emission properties of a non-premixed combustor. The results revealed that the additions of H 2 and varying Φs significantly affected the flame configuration and emission characteristics. An elevation in the H 2 vol. Promotes fuel combustion, yielding a higher flame temperature and a wider flammability region. Lower Φs and increased H 2 vol. Significantly decrease CO and CO 2 emissions by 87.48% and 96.63% respectively. Conversely, NOx emissions increase with higher H 2 vol. and Φs due to the catalytic effect of elevated flame temperature. At Φ = 0.2 and 50 vol% of H 2 , outlet temperature and pattern factor were enhanced by 61.66% and 30.16% respectively. Under these conditions, NOx emission was calculated by a factor of 74.14. Investigation emphasizes hydrogen-enriched fuels' potential for enhancing combustion and emissions control through vol. and equivalence ratios. • Hydrogen-enriched fuel mixture combustion is realized in a can-type combustor. • Effects of H 2 enrichment and equivalence ratios on combustion are investigated. • The addition of H 2 in the fuel mixture enhances combustion performance. • CO, CO 2 , and NO x pollutant emissions are numerically computed. [ABSTRACT FROM AUTHOR]

A simplified two-mixture-fraction-based flamelet model is proposed in this work for a non-premixed staged combustion mode in which three feeds are introduced into the flow field. As the third stream is injected downstream of the main port, the system can be considered a special case of the two-mixture-fraction-based systems. It is considered that if a well-mixing is achieved in the upper stream prior to the third-stream injection, the simplification of the two-mixture-fraction-based flamelet model is feasible. In this work, a simplified model is proposed which can greatly reduce the library size compared to the complete two-mixture-fraction-based flamelet library providing identical resolutions for the fuel stream mixture fraction and the progress variable, respectively. Analysis associated with the interpolation strategy has been implemented. Extension of the current model for the cases in which the well-mixing state is not attained is also analysed, as well as the consideration of heat loss. To validate the model, two adiabatic cases of two-dimensional (2D) direct numerical simulations (DNS) have been performed in this work. To describe the chemical events, one is using finite-rate chemistry (FRC), while the other one is realised by means of the current flamelet model (FLM). An a priori test case that directly looks up the libraries by using the tracking parameters obtained from the FRC case is also considered. It is observed that the interpolation along the primary oxidiser mixture fraction direction outperforms that along directions for both fuel and primary oxidiser mixture fractions. It is also found that three one-mixture-fraction-based flamelet libraries which form the current model are sufficient for simulation of the non-premixed staged combustion, while the extended one which composes five libraries is expected to gain higher accurateness. In the FLM case, although the distributions of tracking parameters deviate from the FRC case slightly, good agreements can be obtained in terms of temperature and species mass fractions. The a priori test shows that the current model can reproduce the reacting flow accurately when the tracking parameters are identical to the FRC case. It is confirmed that the current model can be used to predict the characteristics of the reacting flow in the non-premixed staged combustion. [ABSTRACT FROM AUTHOR]

Natural gas (NG) is an attractive fuel for heavy-duty internal combustion engines because of its potential for reduced CO2, particulate, and NOX emissions and lower cost of ownership. Pilot-ignited direct-injected NG (PIDING) combustion uses a small pilot injection of diesel to ignite a main direct injection of NG. Recent studies have demonstrated that increased NG premixing is a viable strategy to increase PIDING indicated efficiency and further reduce particulate and CO emissions while maintaining low CH4 emissions. However, it is unclear how the combustion strategies relate to one another, or where they fit within the continuum of NG stratification. The objective of this work is to present a systematic evaluation of pilot combustion, NG combustion, and emissions behavior of stratified-premixed PIDING combustion modes that span from fully-premixed to non-premixed conditions. A sweep of the relative injection timing, RIT , of NG and pilot diesel was performed in a heavy-duty PIDING engine with P inj = 140–220 bar, ϕ g = 0.47–0.71, and a constant NG energy fraction of 94%. Apparent heat release rate and emissions analyses identified interactions between the pilot fuel and NG, and qualitatively characterized the impact of NG stratification on combustion and emissions. Changes in the RIT resulted in six distinct PIDING combustion regimes, for all considered injection pressures and equivalence ratios: (i) RIT-insensitive premixed, (ii) stratified-premixed (early-cycle injection), (iii) NG jet impingement transition, (iv) stratified-premixed (late-cycle injection), (v) variable premixed fraction, and (vi) minimally-premixed. Parametric definitions for the bounds of each regime of combustion were valid for the wide range of P inj and ϕ g investigated, and are expected to be relevant for other PIDING engines, as previously identified regimes agree with those identified here. This conceptual framework encompasses and validates the findings of previous stratified PIDING investigations, including optimal ranges of operation that provide significantly increased efficiency and lower emissions of incomplete combustion products. [ABSTRACT FROM AUTHOR]

The modelling of soot formation is investigated for two turbulent flames, at atmospheric and 3 atm pressure conditions. For the first time, a semi-empirical soot formulation that accounts for soot inception, coagulation, surface growth, and oxidation processes is coupled with the turbulent combustion model, Conditional Source-term Estimation (CSE) using Reynolds-Averaged Navier–Stokes equations. Detailed chemistry is included and an optically thin radiation model is considered. Non-adiabatic chemistry tabulations are created. Good agreement with the experiments is found for turbulent mixing and temperature fields in both flames, with some discrepancies believed to be due to the turbulence modelling approach. At 1 atm, the soot volume fractions are in reasonable agreement with the experiments, but typically smaller than the measurements with the centerline peak locating closer to the fuel exit. At 3 atm, good agreement between the numerical predictions and experimental data is achieved for the soot volume fraction within the experimental error. The centerline peak location is observed slightly farther downstream. Possible sources of discrepancies are examined and comparison with previously published numerical results is also undertaken. Differential diffusion and modified soot chemistry constants may bring further improvement. Without any particular tuning of soot chemistry, soot modelling within CSE is shown to be a promising approach. [ABSTRACT FROM AUTHOR]

Identifying combustion regimes is important for understanding combustion phenomena and the structure of flames. This study proposes a combustion regime identification (CRI) method based on rotated principal component analysis (PCA), clustering analysis and the back-propagation neural network (BPNN) method. The methodology is tested with large-eddy simulation (LES) data of two turbulent non-premixed flames. The rotated PCA computes the principal components of instantaneous multivariate data obtained in LES, including temperature, and mass fractions of chemical species. The frame front results detected using the clustering analysis do not rely on any threshold, indicating the quantitative characteristic given by the unsupervised machine learning provides a perspective towards objective and reliable CRI. The training and the subsequent application of the BPNN rely on the clustering results. Five combustion regimes, including environmental air region, co-flow region, combustion zone, preheat zone and fuel stream are well detected by the BPNN, with an accuracy of more than 98% using 5 scalars as input data. Results showed the computational cost of the trained supervised machine learning was low, and the accuracy was quite satisfactory. For instance, even using the combined data of CH4-T, the method could achieve an accuracy of more than 95% for the entire flame. The methodology is a practical method to identify combustion regime, and can provide support for further analysis of the flame characteristics, e.g., flame lift-off height, flame thickness, etc. [ABSTRACT FROM AUTHOR]

• Investigation of heat release features of co/counter-swirl flames. • Noise shows intermittency, comprised of periodic and aperiodic regions. • Global fluctuations of HRR responsible for periodic regions. • Rotational motions coexist with global fluctuations. • Global fluctuations decrease, and rotation motions dominate as M increases. Understanding co/counter-swirl or twin-swirl flames remains challenging due to the complex interaction of two swirling streams. In the present study, we investigate the heat release features of non-premixed co/counter-swirl syngas/air flames and their' influence on combustor noise in a ∼20 kW combustor using simultaneous high-speed OH*-chemiluminescence (5 kHz) and microphone measurement (50 kHz) by varying the momentum ratio (M) from 0.4 to 0.95. Furthermore, the velocity field is examined using a low-speed two-dimensional particle image velocimetry (2D-PIV, frequency = 7 Hz). For all studied momentum ratios, the frequency spectra of noise measurements for both co/counter-swirl flames consistently exhibit a dominant frequency (∼285 Hz), close to the fundamental axial mode of the combustor. A further analysis using spectrograms, phase spaces, and recurrence plots reveals intermittent patterns in noise measurements, featuring periodic (P) and aperiodic regions (A). In periodic regions (P), noise synchronizes with the global fluctuation of the heat release rate as observed in chemiluminescence. Along with global fluctuation, the chemiluminescence also reveals a rotational component of heat release rate with distinct frequencies for co and counter-swirl configurations. This rotational motion possibly originated from a precessing vortex core (PVC) as indicated by the zig-zag arrangements of vortices in the inner shear layer. Furthermore, the impact of M on global fluctuation and rotational motion has been investigated using the frequency spectrum of OH*-intensity and the distribution of peaks in noise measurement. The global fluctuation is found to be suppressed when M increases while the rotational component becomes prominent at higher M. Therefore, the study elucidates the co-existence of global fluctuation and rotational motion and how these motions evolve with the varying momentum ratio (M), thus enhancing the understanding of combustion characteristics of the complex twin-swirl (co/counter-swirl) flames. [ABSTRACT FROM AUTHOR]

This work reports the development and validation of evaluation methods for measurements of 3D, instantaneous, local flame propagation speed on a turbulent methane jet diffusion flame. Despite its fundamental importance to turbulent flame research, the experimental measurements for 3D flame propagation speed are scarce due to the complexity of turbulent flame surfaces and the mathematical challenges. Existing evaluation methods based on surface fitting and normal vectors cannot be readily extended to a laboratory-scale, turbulent flame. Two new methods were therefore developed to address this issue. The methods were validated numerically with artificially created phantoms and experimentally with 3D flame structures obtained by tomographic chemiluminescence on a turbulent jet flame. The methods were demonstrated to be able to evaluate the 3D flame propagation speed of complex flame structures with significantly enhanced accuracy and robustness. Based on the evaluations, the relationship between flame propagation speed and curvature was studied on the presented flame. [ABSTRACT FROM AUTHOR]

Stable flame is still one of the challenging issues in the micro-combustors. Besides, the premixed flame may flash back under the large equivalence ratio, and the non-premixed combustion is an effective way to avoid the flame flash-back issue. In this work, a novel non-premixed CH 4 /air micro-combustor with the flame holder, the backward-facing steps and the separated preheating channels is developed to avoid the flame flash-back issue and extend the flame blow-off limits. This paper investigates the influence of two different CH 4 /air injection locations on the flame blow-off limits numerically and profoundly reveals the mechanisms in this micro-combustor. Results show that this novel configuration can considerably anchor the flame without the flame flash-back issue and significantly extend flammability owning to the flow & heat recirculations. Compared with the "Air (out) /CH 4(in) " injection location, the "Air (in) /CH 4(out) " injection location exhibits a comparatively larger size of recirculation, causing a more vital anchorage ability. In addition, after the preheating in the preheating channels, the CH 4 /air mixture of the "Air (in) /CH 4(out) " injection location presents a higher temperature level than that of the "Air (out) /CH 4(in) " injection location. Consequently, compared to the "Air (out) /CH 4(in) " injection location, the larger blow-off limits are attained for the "Air (in) /CH 4(out) " injection location. These results provide new insights into the stable non-premixed flame and give valuable guidance for designing the similar non-premixed micro-combustor. [Display omitted] • A novel CH 4 /air non-premixed micro-combustor is developed. • This novel configuration can anchor the flame without the flame flash-back issue. • Influence of CH 4 /air injection locations on flame blow-off limits is studied. • "Air (in) /CH 4(out) " presents larger recirculation & better heat recirculation. • "Air (in) /CH 4(out) " attains larger blow-off limits than "Air (out) /CH 4(in) ". [ABSTRACT FROM AUTHOR]

Measurements of 3D flame edge dynamics were made on a high-speed jet diffusion flame to assess the global/local hydrodynamic instability. The flame was generated by issuing high-speed ethylene (Uj = 170 m/s) into a low-speed vitiated hot coflow (Uc = 1.5 m/s), resulting in a hydrodynamic shear layer instability at the interface between combustion products and ambient flow. The measurements used a high-speed camera combined with nine-headed fiber endoscopes to simultaneously collect both the soot radiation and chemiluminescence projections of the flame from nine views, based on which 3D flame edges were obtained via computed tomography at 15 kHz. The measurements clearly capture the time-varying, 3D instantaneous flame edge structures with fine-scale corrugations, enabling the observation of small-scale vortices' evolution. The flame edge deformations induced by those vortices were calculated globally and locally to infer the relationship between global and local flame edge oscillations. Results show that various local oscillation frequencies exist at different locations along the flow direction for such a highly sheared flame. They are dominated by the periodical formation and motion of various localized, small-scale vortices. The local oscillations are much severer than the global oscillation, indicating a self-suppression of the instabilities between these local oscillations. The suppression mechanism is attributed to the constructive and destructive interference behavior of the local disturbances. The global oscillation of the flame edge turns out to be the linear superposition of local oscillations at different locations. [ABSTRACT FROM AUTHOR]

Alternative fuels and stocks like biomass or chemical and refinery waste, may potentially be used in gas turbines and industrial applications after gasification. Thus, understanding the role of hydrogen in these fuels is critical to the broader aim of utilising alternative fuels for power generation. In this work, the interaction between the flame and the flow field was studied in a quarl-stabilised swirl non-premixed flame burning CH 4 and H 2 –enriched CH 4. Simultaneous high-speed OH-PLIF/PIV imaging at 5 kHz was carried out on these flames to explore the flame-flow interaction. The instantaneous flow fields in the CH 4 or CH 4 +H 2 flames showed a small scale vortical structure near the shear layers, which were not apparent in the time-averaged flow fields. Increasing H 2 % in the fuel jet was observed to dampen the velocity fluctuations. The fuel composition affected the spatial location of the reaction zone; in the CH 4 flames, the axial position of the reaction zone is seen to track the relatively large-magnitude axial velocity fluctuations while remaining in locally low-speed regions of the flow. In contrast, in H 2 -enriched flames, where the flame is more robust, the reaction zone was able to survive longer, in terms of axial distance, in the vicinity of high swirling jet velocity, with less sensitivity to velocity fluctuations. With increasing the H 2 %, the reaction zone steadily leaves the IRZ towards the swirling jet flow and localised between its outer and inner vortices. This acts as a stabilisation factor where the internal vortices convect hot product towards the fresh mixture. Moreover, the flame curvatures, the vorticity and compressive strain fields interactions with the reaction zone are presented and discussed. This article outlines results that yield more in-depth insight into hydrogen-enriched hydrocarbon non-premixed swirling flames' combustion, which is essential to accelerate the fuel switching from hydrocarbons to hydrogen. [Display omitted] • Decarbonisation of the energy sector is a complex process that includes different approaches to a low carbon energy future. • Thus, there is a considerable promise of using hydrogen-blended fuels in different power generation systems. • Three swirling flames are studied in the current work, differing only in the H 2 % associated with the methane jet. • High-speed OH-PLIF/PIV imaging was carried out on these flames to explore the flame-flow interaction. • The reaction zone for the pure methane flames resides mainly in the regions of low-axial velocity fluid. • In the H 2 -enriched flames, the reaction zone is sustained in the vicinity of high swirling jet velocity. [ABSTRACT FROM AUTHOR]

Methane-air diffusion filtration combustion in a radiative round-jet burner was numerically investigated in this work. The purpose of this study was focused on the effects of porous media porosity and gas velocity on temperature distribution and CO and NOx emissions. Reduced chemical kinetics was used where air and methane were assumed to be at their stoichiometric ratio, while thermo-physical properties were varied per the solid matrix porosity variation. Combustion characteristics were evaluated based on conduction and radiation as the two primary heat transfer modes within the solid matrix. Numerical simulations were carried out based on a packed bed with 3 mm alumina pellets. Results show that combustion temperature increases while the temperature gradient decreases with the increase in porosity, yielding higher NOx, and lower CO emissions. Furthermore, the combustion temperature is the lowest and most uniformly distributed with 1 m/s and 3 m/s gas velocities, wherewith 3 m/s gas velocity, combustion occurs outside of the porous zone. The corresponding NOx and CO emissions are the lowest with 1 m/s gas velocity and increase with the increase in gas velocity from 1 m/s to 10 m/s. [ABSTRACT FROM AUTHOR]

The design of efficient and clean biomass combustion systems requires a good understanding of the combustion process. Computational Fluid Dynamics (CFD) can play a key role. However, the success of CFD simulations depends, to a great deal, on the chosen chemical kinetic reaction mechanism, among others. Due to the greater computational cost when using detailed chemical mechanisms, reduced or semi-reduced chemical mechanisms have always been the preferred option in the simulations of biomass combustion. This is mainly due to the limitations of the most widely used combustion model − the Eddy Dissipation Concept (EDC). A computationally inexpensive flamelet-based partially premixed combustion model has recently been proposed as an alternative combustion model for CFD studies of biomass combustion. To further improve the predictions of this combustion model, the present study introduces an improved detailed chemical mechanism. This is achieved via the addition of N O x steps to an existing detailed chemical mechanism. The computational performance of this combustion model, which adopts the improved chemical mechanism, is then evaluated by studying the thermal and velocity fields as well as emissions of a biomass combustion furnace. The results reveal that the improved chemical mechanism produces superior predictions compared to its original version. More importantly, the results show that the improved chemical mechanism can overcome the limitations of the partially premixed combustion model's inability to predict N O x emissions. • A new chemical mechanism for use in the gas-phase combustion of a biomass furnace. • The performance of a flamelet-based partially premixed combustion model is evaluated. • The new chemical mechanism produced superior results compared to the original mechanism. • The new chemical mechanism made it possible for the partially premixed combustion model to predict NOx emission. [ABSTRACT FROM AUTHOR]

This paper focuses on a detailed numerical investigation combined with experimental research for a non-premixed swirl combustor, which aims to analyze the effects of the blade angle of the outer swirler and equivalence ratio on flow and combustion characteristics. In the experiment, the temperature in the furnace was obtained with a thermocouple, while a realizable k-ε turbulence model and two-step reaction mechanism of methane and air are used in the numerical method. The calculation results are in good agreement with the experimental data. The results reveal that the air flow rate through the swirler accounts for a small amount of the total air due to the influence of the draft fan, and there is no central recirculation zone (CRZ) despite the presence of the swirler. It was also found that NO emissions gradually decrease as the blade angle of the outer swirler increases. It was also indicated that the average temperature is 100 K higher than the general combustor with a 58° blade angle in the furnace by increasing the equivalent ratio of the tertiary air area, and the NO emissions reduced by approximately 25%. This study can provide guidance for the operation and structural design of non-premixed swirl combustors.

In order to investigate the formation process and propagation characteristics of detonation wave, developing process of detonation wave from initiation to stable detonation formation under non-premixed conditions has been studied by experiments and numerical simulation. The results show that when mass flow rates of air and hydrogen are 158.957 g/s and 2.728 g/s respectively, stable detonation can be formed in the combustor. Due to the lower inlet pressure, there is an unstable stage in combustor before the stable detonation is formed. Reducing the air pressure will increase the lowest detonation limit of combustor and lead to flame-out and re-initiation in the combustor. The propagation direction of detonation wave may change after re-initiation. Non-premixed intake structure lead to the inconsistency of rotating detonation combustion fluid in the radial direction. Moreover, peak pressure appears near the outer wall, while peak temperature appears near the inner wall. • Hydrogen-air rotating detonations under low total air pressure were obtained. • Low total air pressure disturbs rotating detonation. • Re-initiations of RDWs were discovered. • The propagation direction of RDW changes randomly after re-initiations. • Differences of temperature and pressure on the inner and outer wall were discovered. [ABSTRACT FROM AUTHOR]

Abstract Studying the lift-off behavior of non-premixed jet flames is important to understanding the flame stabilization mechanisms in practical systems, including gas flares and combustors, and to improving the safety of pressurized fuel tanks in case of fuel leaks. This is typically done with the canonical configuration of an axisymmetric fuel jet issuing into a quiescent or co-flowing oxidizer and abundant data are available in the literature. However, most of these data were collected at normal or sub-atmospheric pressure and little data are available at elevated pressure and high Reynolds numbers, conditions relevant to practical configurations. The present study fills this gap by reporting lift-off height measurements of methane and ethane non-premixed jet flames for pressures up to 7 bar and Re = 57,500 in the presence of an air co-flow. Data are interpreted using Kalghatgi's model for the dimensionless lift-off height, which was previously proven successful for sub-atmospheric to normal pressures, as well as elevated pressure but only for low turbulent Reynolds numbers and propane. In this contribution, Kalghatgi's model has been shown to accurately predict the slope of the lift-off height vs. jet velocity curves at elevated pressure and large Reynolds number for methane and ethane. A new term, a function of the stoichiometric mixture fraction, the laminar burning velocity, and a turbulent Schmidt number, is also introduced to extend the predictive capabilities of Kalghatgi's model to configurations featuring a co-flow. [ABSTRACT FROM AUTHOR]

The extraordinary complexity of real Rotating Detonation Combustors (RDC) demands a deep knowledge of each phenomenon involved in the wave development and propagation. Since the reactants are typically injected separately, a key element for the combustor design optimization is understanding the refilling process and the reactants mixing. However, due to the harsh environment and high working frequencies of RDCs, the experimental diagnostics is usually limited, so high-fidelity simulations represent an essential tool to complement the measurements with detailed insights into the flow. In the present work, the non-premixed RDC installed at TU Berlin is simulated with the AVBP code by solving the fully-compressible, spatially-filtered reactive Navier-Stokes equations. The complete hydrogen and air injection system is included in the numerical model to accurately describe both the reactants mixing and the complex turbulent flow field in the resulting refill region. This study shows how both the injection system configuration and its transient interaction with the wave are fundamental for the reactants mixing, as they directly influence the refilled gas properties. Limiting the imbalances between the blockage dynamics of the fuel and oxidizer ducts and optimizing the fuel injectors can improve considerably the homogeneity of the fresh mixture, and consequently the leading shock strength and reasonably the pressure gain. In fact, the detailed analysis of the detonation front speed shown a higher instability near the chamber base for the periodic presence of unmixed reactants. Nevertheless, the unstable root of the front does not affect the whole wave speed, and remaining part propagates steadily thanks to the tangential mixture uniformity. Moreover, the local speed distribution does not appear directly related to the small-scale mixture properties, indicating a higher sensibility to the annulus curvature rather than to the local gas state. [ABSTRACT FROM AUTHOR]

In this study, the turbulent non-premixed methane-air flame is simulated to determine the effect of air velocity on the length of flame, temperature distribution and mole fraction of species. The computational fluid dynamics (CFD) technique is used to perform this simulation. To solve the turbulence flow, k-ε model is used. In contrast to the previous works, in this study, in each one of simulations the properties of materials are taken variable and then the results are compared. The results show that at a certain flow rate of fuel, by increasing the air velocity, similar to when the properties are constant, the width of the flame becomes thinner and the maximum temperature is higher; the penetration of oxygen into the fuel as well as fuel consumption is also increased. It is noteworthy that most of the pollutants produced are NOx, which are strongly temperature dependent. The amount of these pollutants rises when the temperature is increased. As a solution, decreasing the air velocity can decrease the amount of these pollutants. Finally, comparing the result of this study and the other work, which considers constant properties, shows that the variable properties assumption leads to obtaining more exact solution but the trends of both results are similar.

Graphical abstract Structure of diffusion counterflow combustion Highlights • Non-premixed combustion of dust particles in a counterflow system is studied. • Particles porosity and thermal radiation modeling are investigated. • Effects of fuel Lewis number, particle porosity on flame temperature are studied. • Effects of equivalence ratio and particles radius on flame temperature are studied. Abstract The paper deals with modeling counterflow, non-premixed combustion of porous fuel particles (lycopodium) with the consideration of thermal radiation effects. Assuming that the streams of fuel particles and air as oxidizer, move towards the stagnation plane from the two opposing nozzles in a counterflow configuration. It is presumed that particles first vaporize in order to yield a gaseous fuel, methane, which then reacts with the oxidizer which is air. In this research, conservation equations with certain boundary conditions are solved using mathematical methods with the consideration of radiation heat transfer in different regions and compared to cases in which radiation heat transfer is not considered. Furthermore, flame temperature and mass fraction profiles are presented in terms of oxidizer and fuel Lewis numbers. In addition, effects of particle porosity are investigated. As a result, with the increase of dust concentration and reduction of particles radius and porosity, it would lead to a rise in the flame temperature. [ABSTRACT FROM AUTHOR]

The accuracy of turbulent swirl-stabilized flame simulation strongly depends on the choice of turbulence model. In this study, four 3D unsteady turbulence closures, including large eddy simulation, scale-adaptive simulation, and two detached eddy simulation variants, along with four RANS models, including RNG k−ϵ, SST k−ω, transition SST, and RSM, are examined for moderate- and high-swirl case studies. It is observed that the scale-adaptive simulation provides the most accurate results for almost all variables and both swirl conditions in the reactive flow. Only the 3D unsteady models predict the vortex breakdown bubble and flame attachment state correctly. However, based on our error analysis, the flow and composition fields predicted by the RANS models are in acceptable agreement with the experimental fields, especially the ones of transition SST when higher swirl number cases or minor species concentration are of interest. Moreover, it is concluded that the viscosity ratio criterion is a better measure of the local LES quality than the turbulent kinetic energy ratio, and the accuracy of a hybrid simulation may be much more dependent on the ability of the model to operate close to the RANS mode where the grid resolution is not sufficient for a resolving simulation than the fraction of the resolved kinetic energy. Finally, the propriety of the base (RANS) model of a DES for the application of interest is important, such that DES with realizable k−ϵ outperforms the commonly used DES with SST k−ω model. [ABSTRACT FROM AUTHOR]

Four bluff-body cones with/without rifled v-grooves were installed behind a non-premixed traditional combustion nozzle to intensify the bluff-body effects and swirl flow. The spiral rifles transformed axial momentum (or axial velocity) into tangential momentum (or tangential velocity). The interaction between the fuel tangential component and axial air flow increased turbulence intensity ( T . I .). The Schlieren photography was utilized to visualize the flame structures and classify three flame patterns—jet flame, recirculation flame, and turbulence flame. The jet flame occurs when fuel-jet velocity is high and air-jet velocity ( u a ) is low. However, the turbulence flame exists at the high air-jet velocity. The flame lengths were measured using the direct photography scheme. The flame length at high u a is significantly shorter than that at low u a . Furthermore, the increase of rifle number ( i . e ., increasing T . I .) induces the high maximum temperature and low nitric-oxide concentration. [ABSTRACT FROM AUTHOR]

The non-premixed steady flamelet model is extended by two simple, robust and effective heat loss modelling approaches. The heat release damping (HRD) approach decreases the chemical source term in the energy equation by a constant factor, while the artificial radiation (AR) approach introduces an augmented temperature dependent radiative source term. The models are tested in a simulation of the 0.78 MWth IFRF (International Flame Research Foundation) pilot scale, non-premixed, natural gas/oxygen flame. Both approaches are applied in steady laminar flamelet calculations with detailed chemistry, tabulating the thermo-chemical state as a function of mixture fraction and normalised heat loss. The turbulence-chemistry interaction is modelled using the β -PDF approach. An enthalpy transport equation is solved to keep track of the heat loss, while radiative heat transfer is calculated by the P-1 model. We observe that the major species, temperature, velocities and velocity fluctuations show a good agreement with the available experimental data. The heat loss modelling yields a significant improvement over the adiabatic model. Interestingly, both heat loss models (HRD and AR) show negligible differences in the simulations of the turbulent flame and permit to apply the steady laminar flamelet model to oxy-fuel processes in a simple, robust and user friendly manner. [ABSTRACT FROM AUTHOR]

Soot prediction in a combustion system has become a subject of attention, as many factors influence its accuracy. An accurate temperature prediction will likely yield better soot predictions, since the inception, growth and destruction of the soot are affected by the temperature. This paper reported the study on the influences of turbulence closure and surface growth models on the prediction of soot levels in turbulent flames. The results demonstrated that a substantial distinction was observed in terms of temperature predictions derived using the k-ε and the Reynolds stress models, for the two ethylene flames studied here amongst the four types of surface growth rate model investigated, the assumption of the soot surface growth rate proportional to the particle number density, but independent on the surface area of soot particles, f ( A ) = ρ N , yields in closest agreement with the radial data. Without any adjustment to the constants in the surface growth term, other approaches where the surface growth directly proportional to the surface area and square root of surface area, f ( A ) = A and f ( A ) = √ A , result in an under- prediction of soot volume fraction. These results suggest that predictions of soot volume fraction are sensitive to the modelling of surface growth. [ABSTRACT FROM AUTHOR]

In this paper, the suppression effects of non-premixed inert gas (N 2 , CO 2) on hydrogen-air explosion behaviors are experimentally studied. The effects of N 2 and CO 2 addition on flame propagation, explosion pressure and flame length are analyzed. The results show that N 2 and CO 2 can effectively reduce the explosion hazard in non-premixed state. The mixing process of non-premixed gases leads to the development of turbulent flame, but the suppression of inert gas on explosion exceeds the promotion of turbulent flame. Under the action of N 2 and CO 2 , the energy of the flame decreases, and the front contour becomes blurred. When the fuel zone is only set at the ignition end, with the suppression of N 2 and CO 2 , the maximum explosion pressure is reduced by 35.1% and 50.6%, respectively. It is worth noting that the high-frequency periodic oscillation and pressure soaring appear in the pressure histories, which are attributed to the rapid phase transition of water. Furthermore, the two inert gases can significantly reduce the chemical reaction rate and heat release rate during the explosion, which is reflected in the decrease of the maximum flame length. It is found that N 2 and CO 2 have different explosion suppression effects and mechanisms, but CO 2 has better suppression performance on hydrogen-air explosion. [ABSTRACT FROM AUTHOR]

The stability of turbulent non-premixed biogas flame was studied experimentally by varying the fuel composition (i.e., changing the carbon dioxide content in the fuel, which is mainly composed of CH 4 and CO 2 ) and altering the fuel nozzle geometry. The fuel was discharged through a central nozzle surrounded by a co-axial airflow passing through a low-swirl (25°-angle vanes) generator. The results revealed that the biogas flame stability limits are highly sensitive to fuel composition in that a small increase in carbon dioxide content can lead to a significant shrinkage in the flame stability operating conditions. On the other hand, the results showed that the fuel jet momentum (fuel nozzle geometry) plays a key role in reducing the dependency of the stability of swirling non-premixed flame on fuel composition. The experimental data with varying both the fuel composition and fuel nozzle geometry was used to develop semi-empirical non-dimensional correlations capable of describing both the lower and upper flame stability limits of both biogas and methane fuel. The validity of these correlations was extended to include both non-premixed and premixed swirling flames. [ABSTRACT FROM AUTHOR]

An investigation into the response of non-premixed swirl-stabilized flames to acoustic excitation and jet in cross-flow (JICF) perturbations was carried out. The fluctuations of OH∗ chemiluminescence emission measured simultaneously with the pressure were used to determine flame describing function (FDF). It could be seen that the three specific conditions exhibited stronger heat release oscillations coupled with pressure fluctuations under acoustic forcing and JICF perturbations, which were expressed by the in-phase delay and positive flame response indices originated from Rayleigh criterion to conduct the stability analysis. Low acoustic excitation amplitude triggered more significant response in the flame heat release rate than high acoustic excitation amplitude at the particular forcing frequency, indicating that nonlinear saturation characteristic of the FDF is also emphasized in JICF. Poincaré map was adopted to analyze the lock-in amplitudes of the dynamics system and JICF effects on combustion instabilities under acoustic forcing frequencies nearby/far from the natural frequency. It was found that lock-in occurred more easily when the forcing frequency was greater than the natural frequency, showing that JICF did not affect the system dynamics due to their little impact on the vortex/flame interactions in this work studied. [ABSTRACT FROM AUTHOR]

Turbulent combustion modelling under high-pressure conditions is a key issue for the design of future aero or rocket engines. In the context of large-eddy simulations, the exact filtered equation of state (EoS) is generally approximated by an EoS directly computed from the Favre filtered quantities for species mass fractions and temperature. The soundness of this approximation is presently addressed through the analysis of laminar CH 4 -air and CH 4 -O 2 high-pressure premixed and non-premixed flames. Their computations were performed either with the ideal gas equation of state (EoS) or with a high-pressure package that gather a cubic EoS along with additional pressure-dependent transport parameters and thermodynamic relations. Various kinetic schemes are used for the computation of laminar premixed flames and a comparison with available experimental data is provided for flame speed ( S L ). Laminar premixed CH 4 -O 2 flames exhibit micro metric flame thickness for pressures above 2.0 MPa as well as a high flame temperature (>3000 K). These flamelets were then filtered either with a Gaussian filter or through a β -pdf technique. The resulting filtered profiles were used to approximate the filtered pressure, p ¯ . A negligible error is observed for CH 4 -air flames when comparing p ¯ with the exact filtered pressure. However, significant errors were found for high-pressure CH 4 -O 2 flames, that increase with the filter size. This behaviour is exacerbated for transcritical injections meaning that classical techniques that use tabulated thermochemistry methods in compressible codes must be revisited in that context. A correction to the tabulated chemistry approach coupled to a presumed subgrid pdf is finally proposed to comply with high-pressure considerations. [ABSTRACT FROM AUTHOR]

In small-scale combustors, the ratio of area to the combustor volume increases and hence heat loss from the combustor’s wall is significantly enhanced and flame quenching occurs. To solve this problem, non-premixed vortex flow is employed to stabilize flames in a meso-scale combustion chamber to generate small-scale power or thrust for propulsion systems. In this experimental investigation, the effects of thermal recuperation on the characteristics of asymmetric non-premixed vortex combustion are studied. The exhaust gases temperature, emissions and the combustor wall temperature are measured to evaluate thermal and emitter efficiencies. The results illustrate that in both combustors (with/without thermal recuperator), by increasing the combustion air mass flowrate, the wall temperature increases while the wall temperature of combustor with thermal recuperator is higher. The emitter efficiency calculated based on the combustor wall temperature is significantly increased by using thermal recuperator. Thermal efficiency of the combustion system increases up to 10% when thermal recuperator is employed especially in moderate Reynolds numbers (combustion air flow rate is 120 mg/s).

• The gas-phase combustion of a lab-scale biomass furnace is numerically studied. • Flamelet-based partially premixed combustion models are evaluated. • C-equation/UFM based partially premixed model is found to be the most optimum. Depending on the mass flow rates of fuel, the primary air (injected usually from underneath of the fuel bed) as well as the composition of volatile gases extracted from the bed, it is believed that the volatile gases taking part in the gas-phase combustion undergo a partially premixed process close to the bed section. Therefore, the aim of this study is to evaluate the prediction capability of flamelet-based partially premixed combustion models in simulating the gas-phase combustion process of a grate firing biomass furnace. Additionally, the effects of the adopted premixed models (i.e., C Equation based model and extended coherent flame model) and non-premixed models (i.e., steady diffusion flamelet (SFM) and unsteady diffusion flamelet (UFM)) on the overall prediction of partially premixed model are assessed. The predicted temperature field and species concentrations are compared with published experimental measurements and also with numerical simulations which use other combustion models (i.e., EDC/Flamelet hybrid model, SFM and UFM). The results of this study revealed that except for slow forming and chemically dominated species, partially premixed combustion models (both extended coherent flame model/SFM and C-equation/SFM based partially premixed model) are capable of reproducing the experimental temperature and major species with reasonable accuracy. For instance, the predicted temperature deviates from the experimental counterpart by a maximum of 9.01%, and the major species by 15.44% (CO 2) and 21.60% (O 2). In addition, C-equation/UFM-based partially premixed model is found to be the most optimum combination of the examined partially premixed models for overcoming the deficiency faced while predicting the slow-forming species. [ABSTRACT FROM AUTHOR]

The effect of fuel nozzle geometry on the stability of a low swirl non-premixed biogas flame has been examined experimentally. Fuel nozzle geometry was varied by changing its diameter or the number of peripheral holes/slots, while keeping constant the total cross-sectional area. In addition, the effect of the nozzle slot discharge angle was investigated. A low-swirl strength (25°-angle vanes) was used in the co-airflow, and the biogas fuel composition was kept constant (60% CH4and 40% CO2by volume). The results revealed that the swirl is essential for stabilizing a turbulent non-premixed biogas lifted flame. More importantly, fuel nozzle was found to influence drastically both the attached and lifted biogas flame stability limits. Finally, semi-empirical correlations have been proposed to describe the lifted biogas flame blowout limits. [ABSTRACT FROM AUTHOR]

The effect of electrostatic fields on the dilute plasma of chemi-ions generated by a non-premixed, N2-diluted, methane-oxygen flame was studied in an experimental burner whereby a counterflow flame was positioned between the parallel plates of a large-scale capacitor. It was shown that flame morphology and location could be controlled solely through electrostatics. The location of the flame could be controlled through applied voltage, virtually independently of overall mixture composition and strain rate applied on the flame. In fact, through electrostatic actuation, it was possible to push the flame without extinction literally at the fuel nozzle, where there is very little oxidizer. Also, a computational framework for the study of these flames was developed in a computer program and verified with previous computations of the same flame. In future work, this framework will be used in tandem with the electrostatics computations that the computer program enables in order to probe the underlying physics of the flame. [ABSTRACT FROM AUTHOR]

Autoignition of turbulent methane/air mixing layers, in which n-heptane droplets have been added, was investigated by DNS. This configuration is relevant to dual-fuel, pilot-ignited natural gas engines under direct injection conditions. Two passive scalars were introduced in order to describe the dual fuel combustion. It was shown that the pre-ignition phase is dominated by n-heptane oxidation while methane oxidation is less intense. During the pre-ignition phase, the methane/air mixing layer is distorted due to turbulence creating regions around the n-heptane droplets allowing the transport of intermediate species to the methane reaction zone. According to the passive scalars introduced, it was shown that ignition occurs at mixtures rich in n-heptane vapor. Subsequently, consumption of both n-heptane and methane is rapidly increased and promoted by the high temperatures achieved. The competition of the two fuels makes autoignition retarded relative to the pure n-heptane case, but accelerated relative to the pure methane case. [ABSTRACT FROM PUBLISHER]

In support of the development of robust combustion models, the present study describes experimental and computational results on the non-premixed counterflow ignition of all four butanol isomers against heated air for pressures of 1–4 atm, pressure-weighted strain rates of 200–400 s −1 , and fuel molar fractions in nitrogen-diluted mixtures of 0.05–0.25. Comparison of the parametric effects of varied pressure, strain rate, and fuel loading among the isomers facilitates a comprehensive evaluation of the effect of varied structural isomerism on transport-affected ignition. The experimental results are simulated using isomer-specific skeletal mechanisms developed from two comprehensive butanol models available in the literature, and are used to validate and assess the performance of these models. Comparison of the experimental and computational results reveal that while both models largely capture the trends in ignition temperature as functions of pressure-weighted strain rate, fuel loading, and pressure, for all isomers both models over-predict the experimental data to an appreciable extent. In addition, neither model captures the experimentally-observed ignition temperature rankings, with both models predicting a large spread among n -/ iso -/ sec -butanol which does not appear in the experimental results. Sensitivity and path analyses reveal that the butene isomers play a significant role in determining the ignition temperatures of the butanol isomers in both models, with the relative branching ratios likely accounting for the ignition temperature rankings observed using each model. It is observed that the reactivity of the butene isomers varies appreciably between the two butanol models, which may account for some of the variability in predictions between the two models. Furthermore, effects of transport properties and their uncertainties on ignition temperature predictions are discussed. [ABSTRACT FROM AUTHOR]

The present experimental work reports the first observations of primary and secondary transitions in the time-averaged flame topology in a non-premixed swirling flame as the geometric swirl number S G (a non-dimensional number used to quantify the intensity of imparted swirl) is varied from a magnitude of zero till flame blowout. First observations of two transition types viz. primary and secondary transitions are reported. The primary transition represents a transformation from yellow straight jet flame (at S G = 0) to lifted flame with blue base and finally to swirling seated (burner attached) yellow flame . Time-averaged streamline plot obtained from 2D PIV in mid-longitudinal plane shows a recirculation zone (RZ) at the immediate vicinity of burner exit. The lifted flame is stabilized along the vortex core of this RZ. Further, when S G ∼ 1.4–3, the first occurrence of vortex breakdown (VB) induced internal recirculation zone (IRZ) is witnessed. The flame now stabilizes at the upstream stagnation point of the VB-IRZ, which is attached to the burner lip. The secondary transition represents a transformation from a swirling seated flame to swirling flame with a conical tailpiece and finally to a highly-swirled near blowout oxidizer-rich flame . This transition is understood to be the result of transition in vortex breakdown modes of the swirling flow field from dual-ring VB bubble to central toroidal recirculation zone (CTRZ). The physics of transition is described on the basis of modified Rossby number ( Ro m ). Finally, when the swirl intensity is very high i.e. S G ∼ 10, the flame blows out due to excessive straining and due to entrainment of large amount of oxidizer due to partial premixing. The present investigation involving changes in flame topology is immensely important because any change in global flame structure causes oscillatory heat release that can couple with dynamic pressure and velocity fluctuations leading to unsteady combustion. In this light, understanding mechanisms of flame stabilization is essential to tackle the problem of thermo-acoustic instability. [ABSTRACT FROM AUTHOR]

A non-premixed hydrogen-oxygen flame is modeled, incorporating the reactions of intermediate radical species, by the use of a computational fluid dynamic modeling technique. The computed and experimentally measured mass fractions are shown to agree well overall. [ABSTRACT FROM AUTHOR]

The present experimental and computational study describes the non-premixed counterflow ignition of nitrogen-diluted pre-vaporized n -butanol/hydrogen mixtures impinging on a heated air stream for pressures of 1–4 atm and hydrogen molar percentages in the binary fuel blends ranging from ξ H = 0% (pure n -butanol) to ξ H = 100% (pure hydrogen). The experimental data show that hydrogen addition results in a non-linear decrease in ignition temperatures that can be broken into two regimes; a hydrogen-enhanced regime of ξ H = 0–40%, where the addition of more hydrogen significantly decreases ignition temperature, and a hydrogen-dominated regime in the range of ξ H = 40–100%, where ignition temperatures exhibit minimal sensitivity to further hydrogen addition. The experimental results are then simulated using n -butanol-specific skeletal mechanisms developed from two comprehensive butanol models available in the literature. The ability of these mechanisms to predict the variation of ignition temperatures associated with hydrogen addition to the n -butanol “base” fuel is assessed. Comparison between the experimental and computational results reveals that both chemical kinetic models capture the two-regime behavior associated with hydrogen addition observed in the present experiments, though both models over-predict experimental ignition temperatures. Further chemical kinetic analysis of the mechanisms reveals that the two-regime behavior is controlled by the production of hydroperoxyl radicals, with production via the reaction of formyl radicals and oxygen molecules dominating at low hydrogen addition levels, and production via the three-body H + O 2 +M = HO 2 + M reaction dominating at high hydrogen addition levels. [ABSTRACT FROM AUTHOR]

Critical to the development of predictive combustion models is a robust understanding of the coupled effects of chemical kinetics and convective–diffusive transport at both atmospheric and elevated pressures. The present study describes a new variable-pressure non-premixed counterflow ignition experiment designed to address the need for well-characterized reference data to validate such models under conditions sensitive to both chemical and transport processes. A comprehensive characterization of system behavior is provided to demonstrate boundary condition and ignition quality as well as adherence to the assumption of quasi-one-dimensionality, and suggest limitations and best practices for counterflow ignition experiments. This effort reveals that the counterflow ignition experiment requires special attention to ignition location in order to ensure that the assumption of quasi-one-dimensionality is valid, particularly at elevated pressures. This experimental tool is then applied to the investigation of n -butanol for pressures of 1–4 atm, pressure-weighted strain rates of 200–400 s −1 , and fuel mole fractions of 0.05–0.25. Results are simulated using two n -butanol models available in the literature and used to validate and assess model performance. Comparison of experimental and numerical ignition results for n -butanol demonstrates that while existing models largely capture the trends observed with varying pressure, strain rate, and fuel loading, the models universally over-predict experimental ignition temperatures. While several transport coefficients are found to exhibit order-of-magnitude or greater sensitivities relative to reaction rates, variation of transport parameters is not able to account for the large deviations observed between experimental and numerical results. Further comparison of ignition kernel structure and fuel breakdown pathways between two literature models suggests that an under-prediction in the radical pool growth with respect to temperature variation may be responsible for both the deviation from the experimental results and the discrepancy in ignition temperature results observed between models. [ABSTRACT FROM AUTHOR]