Increasing Equivalence Ratio Research Articles

Structures of Laminar Lean Premixed H2/CH4/Air Polyhedral Flames: Effects of Flow Velocity, H2 Content and Equivalence Ratio

Polyhedral Bunsen flames, induced by hydrodynamic and thermo-diffusive instabilities, are characterized by periodic trough and cusp cellular structures along the conical flame front. In this study, the effects of flow velocity, hydrogen content, and equivalence ratio on the internal cellular structure of premixed fuel-lean hydrogen/methane/air polyhedral flames are experimentally investigated. A high-spatial-resolution one-dimensional Raman/Rayleigh scattering system is employed to measure the internal scalar structures of polyhedral flames in troughs and cusps. Planar laser-induced fluorescence of hydroxyl radicals and chemiluminescence imaging measurements are used to quantify the flame front morphology. In the experiments, stationary polyhedral flames with varying flow velocities from 1.65 to 2.50 m/s, hydrogen contents from 50 to 83%, and equivalence ratios from 0.53 to 0.64 are selected and measured. The results indicate that the positively curved troughs exhibit significantly higher hydrogen mole fractions and local equivalence ratios compared to the negatively curved cusps, due to the respective focusing/defocusing effect of trough/cusp structure on highly diffusive hydrogen. The hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are first increased and then decreased with increasing measurement height from 5 to 13 mm, due to the three-dimensional effect of the flame front. With increasing flow velocity from 1.65 to 2.50 m/s, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps decrease, which is attributed to the overall decreasing curvatures in troughs and cusps due to the decreased residence time and increased velocity-induced strain. With increasing hydrogen content from 50 to 83%, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are amplified, due to the enhanced effects of the flame front curvature and the differential diffusion of hydrogen. With increasing equivalence ratio from 0.53 to 0.64, a clear increasing trend in hydrogen mole fraction and equivalence ratio differences between troughs and cusps is observed at constant flow velocity condition, which is a trade-off result between increasing effective Lewis number and increasing curvatures in troughs and cusps.

Experimental study on the combustion characteristics of hydrogen/ammonia blends in oxygen

Carbon-free ammonia and hydrogen have various disadvantages when used individually, which limits their application as clean energies. These disadvantages can be effectively overcome by combining them as ammonia/hydrogen mixtures in a rational manner. In this paper, the combustion of ammonia/hydrogen in oxygen (the basic form of combustion) is experimentally investigated at various experimental conditions. The results display that the laminar burning velocities of ammonia/hydrogen/oxygen mixtures increase with increasing fuel ratios. At P0 = 0.5 atm and φ = 1.0, the laminar burning velocity proliferated from 1.91 m/s to 4.7 m/s with the increase of fuel ratio from 0.5 to 2.0. As the equivalence ratio increases, the laminar burning velocity rises first and then decreases, with the highest point appearing at φ = 0.8. The initial pressure has minimal impact on the laminar burning velocity. At x = 0.5 and φ = 1.0, the laminar burning velocity decreased slightly from 3.16 m/s to 2.96 m/s with the initial pressure increased from 0.1 atm to 1.0 atm. The empirical correlation among the laminar burning velocity versus the equivalence ratio and hydrogen ratio is given, and the laminar burning velocity of ammonia/hydrogen/oxygen under varying hydrogen ratios (0 %–100 %) and equivalence ratios (0.5–1.5) at atmospheric pressure can be well predicted by the empirical equations. NO is the most emitted NOx substance from hydrogen/ammonia combustion. The amount of NO produced decreases gradually with the growing equivalence ratio. The results of this research can be utilized to renew the basic flame data of ammonia/hydrogen mixture and are important for the development and application of ammonia/hydrogen mixtures.

The Spatial Development Characteristics of High-Pressure Methane Jet Impinging on Methane Lean-Burn Premixed Flame

This work investigated the effect of high-pressure (10 MPa) methane gas jet impinging on the methane lean-burn premixed flame (equivalent ratio of 0.7) based on a three-dimensional numerical simulation by CONVERGE software. The results show that laminar premixed flame is accelerated to develop into a stable turbulent flame under the action of the methane jet, the whole process of flame front development is divided into three stages: laminar (Reynolds number maintains stable, 1–1.3 ms), transition (Reynolds number shows an increasing trend, 1.4–8 ms), and turbulent (Reynolds number tends to stabilize at a high value, 1.9–3 ms). The effect of high-pressure jet on flame development along jet direction (Z axis) is greater than that on vertical direction (Y axis). During turbulence stage, the momentum and kinetic energy of Z axis are 2.7 and 6.3 times greater than that of Y axis, respectively. The high-pressure methane jet causes a change in heat distribution, resulting in local flameout. The rate of change in the local flameout area is greater than that in the flame area, causing a temperature drop in Z axis. This temperature drop increases with the increase in equivalence ratio and with the decrease in distance between cross-section position and ignition center.

Effect of composition gradient on detonation initiation in fuel-rich hydrogen-air mixtures in an obstructed channel

Numerical simulations were performed to study the effect of composition gradient and obstacle arrangement on detonation initiation in fuel-rich hydrogen-air mixtures. A third-order WENO method with adaptive mesh refinement (AMR) was used to solve the unsteady, fully-compressible, reactive Navier-Stokes equations coupled to a calibrated chemical-diffusive model (CDM). An obstructed channel at a blockage ratio of 0.3 filled with non-uniform hydrogen-air mixtures of an average H2 concentration 50 vol% was considered. The results show that unilateral obstacle arrangement is more conducive to flame acceleration (FA) and DDT than bilateral obstacle arrangement for both homogeneous and inhomogeneous mixtures. For inhomogeneous mixtures, the flame accelerates faster, and the detonation onset time is shorter when obstacles are placed on the sidewall with a hydrogen concentration closer to the stoichiometric ratio than the those when obstacles are placed on the other sidewall. However, the placement of unilateral obstacles on either the upper or lower sidewall does not significantly affect the DDT run-up distance. Besides, for fuel-rich inhomogeneous hydrogen-air mixtures, detonation tends to be initiated in the regions of high H2 concentration. Idealized models were introduced to simplify the intricate interactions of the reaction waves and flow fields to better understand the connection between H2 concentration distribution and detonation initiation. The analysis suggests that the minimum shock strength required for detonation initiation decreases with increasing equivalence ratio. This is supported by a kinetics analysis with detailed reaction model, which shows that ignition delay increases with decreasing equivalence ratio when detonation is about to be initiated.

Flame characteristics and NO emission behaviors of methane/ammonia counterflow premixed flames with opposed N2 or air stream

Behaviors of methane/ammonia counterflow premixed flames and NO emission are numerically and experimentally investigated with opposed N2 or air stream by varying strain rate (ag) and equivalence ratio (ϕ). The premixed stream is 80% methane and 20% ammonia by volume mixed with air considering the utilization of ammonia by blending hydrocarbon fuels. The opposed stream is either N2 or air considering the potential application in gas turbines having single- or two-stage combustion. The simulations are conducted by increasing ag up to flame extinction for a specified ϕ and an experiment is conducted to evaluate various kinetic mechanisms. The extinction strain rate increases monotonically with ϕ with opposed air stream (AS), while increases and then decreases with opposed nitrogen stream (NS). The extinction mechanisms are explained by adopting the concepts of the local equilibrium temperature and heat loss ratio. For lean and rich mixtures with NS and lean mixtures with AS, the extinction occurs due to comparable effects of incomplete reaction and heat loss from the flame to the downstream burnt region. While for rich mixtures with AS, the extinction is attributed mainly to incomplete reaction. The contribution of thermal NO as compared with prompt NO in NO production is weak for all cases with NS and AS. The emission index (EINO) for lean and rich mixtures with NS and for lean mixtures with AS decreases monotonically with the increase of strain rate for all ϕ. Increasing equivalence ratio at a specified ag decreases NO production for lean and rich mixtures with NS and lean mixtures with AS. While for rich mixtures with AS, the EINO increases monotonically with ag. From the reaction pathway analysis, for lean mixtures, the effect of increasing ag on the reduction of NO production is attributed by weakening NH-related reaction paths, while NH2-related reaction paths change little. When ϕ increases for a specified ag with NS, NO production is reduced by weakening NH2-related reaction paths and strengthening NH-related reaction paths. For rich mixtures with AS, increasing ag increases NO production by strengthening NH2-related reaction paths.

Numerical investigation on flame propagation characteristics of non-premixed hydrogen and air in a curved micro-combustor

In the present study, propagation characteristics of non-premixed H2/air flames in a curved micro-combustor were numerically investigated. Five distinct flame propagation modes were identified, including the stable flames behind the separating plate and oscillating flames in the curved channel. Analysis showed that the flame sloping direction was determined by local distributions of both equivalence ratio and velocity. In addition, tip opening and tip intensified phenomena of V-shaped flames are attributed to the Lewis number effect. Flame splitting phenomenon occurs when ϕ ≤ 1.0, which is a result of the stretch effect. Moreover, the flame propagation time (i.e., the duration for the flame to reach the separating plate) first decreases and then increases with the increase of nominal equivalence ratio, and the shortest value appears at ϕ = 1.6 and 1.7. Furthermore, the flame propagation limit (i.e., maximum propagable velocity) varies non-monotonically with nominal equivalence ratio, and the peak value is obtained at ϕ = 1.3. In conclusion, the present study revealed more details of the propagation processes for non-premixed H2/air flames which cannot be fully detected through experiments.

Experimental and kinetic modeling study on auto-ignition of ammonia/n-heptane mixtures at intermediate temperatures

Ignition delay times (IDTs) were measured in a shock tube facility for NH3/n-heptane mixtures with NH3 concentrations in the blending fuel ranging from 0.3 to 0.95 by molar fraction. The measurements were conducted under low pressure of 2 atm and intermediate temperatures of 1350–1500 K at equivalence ratios of 0.5, 1, and 2. With the increase of n-heptane content or equivalence ratio, there is a decrease in the IDTs of NH3/n-heptane mixtures at intermediate temperatures. A detailed mechanism was updated in this study based on the mechanism of Dong et al. Subsequently, the proposed mechanism was compared to existing blending mechanisms of ammonia and n-heptane in terms of laminar burning velocities (LBVs), IDTs, and species profiles reported in literature. The present model improved the predictions in reproducing the performed experimental measurements compared to previous mechanisms. Finally, rate of production (ROP), sensitivity analysis, and instantaneous and cumulative reaction path analysis were performed to interpret the experiment observations and deepen the understanding of auto-ignition kinetics of ammonia and n-heptane. The results indicate that intermediate species, such as C2H4 and C3H6, characterized by long lifespans and high concentrations during n-heptane decomposition, play a crucial role at elevated temperatures, while the significance of n-heptane dehydrogenation by NH2 diminishes with increasing temperature.

An experimental study on gas-solid two-phase rotating detonation ramjet engine based on aluminum powder fuel

In order to explore the feasibility of solid powder detonation technology, the ground direct connection test of aluminum powder/hydrogen/air fuel detonation engine was carried out in this paper. The experimental results show that hydrogen is needed to achieve detonation combustion of aluminum powder with a diameter of 1 μm. A series of tests were carried out in the range of equivalence ratio of 0.8-1.2. With the increase of equivalence ratio, the mean value of steady pressure, average peak pressure, and average wave velocity of rotating detonation combustion increased first and then decreased gradually, and reached the maximum at the equivalence ratio of 0.9. The maximum propagation frequency of the detonation wave is 2653 Hz, and the average wave velocity is 1667 m/s.

An experimental study on the flame instability of NH3/DME blends with H2 addition

It is of significance to investigate the effect of H2 on the combustion of NH3/DME blends since NH3 can partially decompose to produce hydrogen (H2) in the combustion process. However, few studies were conducted on NH3/DME/H2 mixtures, especially for flame instability. This work presents an experimental study of the flame instability of NH3/DME blends with H2 addition. Experiments were conducted using the spherical constant-volume combustion method at φ = 0.7 – 1.4, Tu = 298 K. The influences of NH3/DME blend ratio (BNH3/DME = 80/60, 60/40, 40/60, and 20/80), initial pressure (Pu = 0.1 – 0.5 MPa), and H2 fraction (XH2 = 0%, 20%, and 40%) on the flame instability were examined in detail. The results show that increasing either DME fraction or initial pressure can promote the hydrodynamic instability of NH3/DME/air flames because of the decreased flame thickness. For both XH2 = 0% and 20%, the cellular flame instability with BNH3/DME = 40/60 and 20/80 increases with increasing equivalence ratio. For XH2 = 40%, the flame instability shows a similar trend with BNH3/DME = 20/80, but the most unstable flame appears around stoichiometric combustion with BNH3/DME = 40/60. This is due to the compound effects of the diffusive-thermal imbalance and hydrodynamic instability for lean or rich fuel, while the diffusive-thermal instability can be neglected for stoichiometric combustion. Therefore, an effective control of H2 or DME addition can help enhance the combustion performance of the fuel while reducing the occurrence of dangerous events caused by potential flame instabilities. In addition, critical parameters, i.e., critical flame radius rcr, critical Peclet number Pecr, and critical Karlovitz stretch factor Kacr were analyzed to quantify the onset of flame instability. This work provides insights into the flame instabilities of NH3/DME with H2 addition.

Experimental Study on Effects of Different Factors on Continuous Detonation of Superfine Lignite: Particle Size and Fuel Equivalent Ratio

ABSTRACT The Rotating Detonation Engine (RDE) is garnering attention for its high thermal cycle efficiency. In order to explore the detonation evolution of lignite/gas/oxygen gas-solid two-phase fuel in RDE, self-developed RDE(L:1000 mm) was used to study the detonation characteristics of lignite dust with ultra-fine particle size in methane/oxygen atmosphere, and the effects of ultra-fine particle size and equivalent ratio on detonation characteristics were explored, achieving stable propagation of rodtating detonation waves (RDW) with durations between 0.611 and 0.684s. The results indicate that the introduction of lignite dust significantly enhances the detonation effects compared to pure methane fuel. An increase in the overall equivalence ratio boosts the propagation velocity of RDW in methane-solid fuels, while a larger lignite dust particle size slows it down. Among ultrafine lignite powder sizes, a 1 μm particle size achieved the highest detonation wave speed. In terms of RDW propulsion, the impact of particle size and methane-solid fuel equivalence ratio on thrust mirrors their effect on detonation wave velocity. At a global equivalence ratio (GER) of 1.8, the specific impulse under different particle sizes was about 0.6 times that at a GER of 1.0, suggesting that an excess of fuel can reduce detonation effectiveness. The optimal specific impulse results were obtained with a lignite powder addition rate of 6 g/s, particle size of 1 μm, and GER of 1.0, reaching 4.6 kN·s·kg−1. The research results reveal the detonation process and detonation propagation law of lignite in methane/oxygen, especially for the future application of lignite in detonation energy release engineering.

Effect of hydrogen blending on ammonia/air explosion characteristics under wide equivalence ratio

In order to evaluate the effect of mixing hydrogen on the explosive characteristics of ammonia, this work explored the evolution of explosion overpressure and flame propagation characteristics of ammonia/hydrogen/air mixture under different hydrogen blending ratios in closed pipelines through experimental research. Experimental results show that as the hydrogen blending ratio increase, the instability of flame increases, and flame surface changes from smooth to wrinkled. Flame propagation undergoes the evolution of jellyfish-shaped/near-spherical, finger-shaped, plane-shaped, irregularly protruding flame and tulip flame. Flame surface area is constantly changing, accompanied by violent flame tip velocity pulsations. The maximum explosion pressure and the maximum explosion pressure rise rate are consistent with the evolution law of adiabatic flame temperature. They first increase and then decrease with increase of the equivalence ratio, and their peak value appears near the equivalence ratio of 1.1. On the contrary, the explosion duration and rapid combustion period first decrease and then increase with increase of equivalence ratio, and decrease monotonically with increase of hydrogen blending ratio. The conclusions obtained in this work will contribute to the structural design of flame arresters and the formulation of explosion venting strategies during the preparation, transportation and utilization of ammonia/hydrogen mixtures.

Stability analysis of premixed flames with downstream heat gain

Pulsating instability is a kind of combustion intrinsic instability, that can affect the flammable limit and may couple with other processes to generate more complex combustion instability phenomena. Pulsating instability was found in the premixed flame of the double-flame partially premixed flame (PPF), in which the flame patterns change from stable to pulsating unstable, and to stable again as the equivalence ratio or stretch rate increases. To further investigate this phenomenon, a simpler model, stretched premixed flames with downstream heat gain, is analyzed by the asymptotic expansion method in this paper. The downstream heat gain inhibits the steady extinction of stretched premixed flames with a large Zeldovich number. The stability of premixed flames changes in the zero-limit mode, two-limits mode and one-limit mode sequentially as the Zeldovich number increases. The zero-limit mode represents the premixed flame is stable for all stretch rates. The two-limit mode and one-limit represent the premixed flame changes in the pattern of stable-pulsating-stable and pulsating-stable as the stretch rate increases, respectively. The downstream heat gain suppresses the pulsating instability to yield the stable region at the large stretch rate and large Zeldovich number. The influence of other factors, including the Lewis number, the downstream temperature and the downstream boundary position are also discussed. The current theoretical method is also applied to the extracted parameters from numerical calculations of hydrogen/air double-flame PPF. A qualitative similarity is found between the theoretical results and numerical results.

Stable lean co-combustion of ammonia/methane with air in a porous burner

Ammonia is being considered as a promising zero carbon fuel due to its unique properties, while its combustion still poses challenges in term of instability and higher nitrogen oxides (NOx) emissions. In this study, the co-combustion stability of ammonia/methane with air under lean burn condition is experimentally investigated using a silicon carbide (SiC) ceramic foam burner with a two-layer structure stabilizing the flame. The influences of different operating parameters (methane fraction ω, equivalence ratio φ, and flow velocity V) on the co-combustion flame temperature and emissions are analyzed. The results indicate that the lean stable co-combustion of ammonia/methane with air can be achieved in two-layer ceramic foam burner, with its lower limit extending to φ = 0.5, even for the pure ammonia at specific operating conditions. Notably, the nitric oxide (NO) emissions decrease significantly as the equivalence ratio increases, particularly with a rise in methane fraction. The reduction can reach as low as 843 ppm with φ increasing to 0.9, further reducing to an ultra-low level of less than 150 ppm at φ = 1.3, significantly lower than those of traditional free flame burners. Carbon monoxide (CO) emissions remain at ultra-low levels, indicating the excellent co-combustion efficiency of ammonia/methane-air in the porous burner. An optimal co-combustion region with lower NO and CO emissions were conclusively identified. These findings demonstrate the effectiveness of porous media combustion in improving the co-combustion stability of ammonia/methane and reducing NO emissions, thereby helping to develop the porous burners of ammonia-based fuel and promoting its clean utilization for realizing carbon neutrality.

Effect of temporal increase in equivalence ratio on combustion instability of a lean-premixed swirling hydrogen flame

The effect of temporal increase in the equivalence ratio on the combustion instability of a lean-premixed low-swirl hydrogen jet flame in a low-swirl combustor (LSC) is investigated in detail using a high-fidelity Large-Eddy Simulation (LES). The equivalence ratio is linearly increased from 0.3 to 0.5 over a duration of 0.4 s. The results show that the pressure oscillation amplitude in the combustor increases significantly when the equivalence ratio at the combustor inlet (ERCI) exceeds 0.42, and the maximum pressure amplitude and the combustion instability mode exhibit trends consistent with those in a previous experiment and numerical simulation conducted with the same LSC setup at a fixed equivalence ratio of 0.39. Temporal variations in the equivalence ratio and consequently the temperature inside the combustor cause the drastic amplification of pressure oscillation (when the ERCI exceeds 0.42) whose amplitude is larger than that at the fixed equivalence ratio (= 0.39). Prior to the onset of this exceptionally strong combustion instability, a transient irregular oscillation phenomenon comprising instantaneous changes in the pressure oscillation frequency is observed. While the pressure oscillations in the combustor and in the injector channel are in phase after the onset of strong combustion instability, they are in opposite phases during the occurrence of the irregular oscillation phenomenon prior to the onset of strong combustion instability. This irregular oscillation phenomenon predicted by the LES may play a crucial role in the mechanism of transition from stable combustion to combustion instability.

Investigation on the intrinsic instabilities of ethyl methyl carbonate flames

The intrinsic instabilities of ethyl methyl carbonate flames are investigated at equivalence ratios (0.6–1.8) and initial pressures (70–160 kPa) by using a 36-liter constant volume combustion chamber. The unstable laminar premixed flame is evaluated by the laminar burning velocity, explosion characteristics, flame cellularity, and flame self-acceleration. The experimental laminar burning velocity is well reproduced by the chemical kinetic mechanism. The extent of cellularization characterized by the crack length and average cell area non-monotonically varies with increasing equivalence ratio but monotonically increases with increasing initial pressure. Meanwhile, the critical flame radius for the onset of cellularity exhibits non-monotonic variation with the equivalence ratio, while it decreases monotonically with increasing initial pressure. The experimental critical Peclet number and critical flame radius are well captured by flame stability theory. The linear correlation between the critical Peclet number and the Markstein number is Pecl = 26.46 Ma + 122.22. In addition, the dependence of the critical Karlovitz number on the Markstein number is predicted as Kacl = 0.0226e-0.0880Ma. The flame self-acceleration increases as the flame instabilities increase. The acceleration exponent and fractal dimension are smaller than 1.5 and 2.33, respectively, which are the suggested values for flame self-turbulization.

Effect of Surface Reaction on the Distribution Characteristics of Temperature and OH Radicals in Microchannel Combustion

Microchannel burners suffer from low combustion efficiency and poor stability in applications. In order to explore the effect of wall reaction on methane/air premixed combustion performances in the microchannel, the effects of wall activity, inlet velocity, pressure, and equivalence ratio on the temperature and radical distribution characteristics were studied by CFD computational simulations. It is found that as the reaction pressure increases, there are more free-radical collisions, causing the reaction temperature to rise. The OH radicals participate in the reaction at the active near wall so that the mass fraction of the OH radical on the active wall is lower than that on the inert wall. As the equivalence ratio increases from 0.6 to 1.2, the high-temperature regions increase but the maximum temperature decreases. The mass fraction of OH radical increases with the increase of the equivalence ratio, and the increase of OH radical near the inert wall is larger than that of the active wall. As the flow rate increases, the disturbance increases, and the combustion reaction becomes more intense, resulting in an increase in the temperature and the mass fraction of OH radicals. The mass fraction of H, O, OH, and CH3 radicals in the inert wall was slightly higher than that in the active wall, in which the peak mass fraction of CH3 radical appeared at the axial position closest to the entrance, while the other three radicals reached the peak at about the same axial position. This study provides a reference for combustion stability in microcombustors.

On the NTC behaviors in explosion limits of C1 to C3 n-alkane/air mixtures

In this study, the explosion limits of methane, ethane and propane with air under various equivalence ratios were investigated. For the methane-air mixture, a Negative Temperature Coefficient (NTC) response progressively appears with an increase in equivalence ratio. The NTC response for ethane-air mixture remains relatively constant, and the explosion boundary shifts to high pressure in the high-temperature region. In propane-air mixtures, both the explosion limit curve in the NTC response section and the high-temperature section shift to high pressure, causing a double NTC response under high equivalence ratios. Detailed reaction path analysis confirmed that the primary reaction path of methyl radicals shifted from oxygen addition reactions to recombination reactions, leading to the NTC response and reduced reactivity. The decomposition of C2H5O2H promotes the reactivity of the ethane-air mixture, thereby generating the NTC response. Under high equivalence ratios, both the reaction path of the C3H7 radical via secondary oxygenation at low temperatures and the oxygen addition reactions of small alkane at high temperatures are hindered, leading to the generation of a double NTC response. Temperature and species profiles are compared under the NTC region for these fuels. These findings provide valuable insights and guidance for applications involving fuel-rich hydrocarbon combustion processes.

Theoretical Evaluation of YSZ and Alumina-YSZ Thermal Barrier Coatings in a Hydrogen Enriched Combustion Environment

Abstract Yttria-stabilized zirconia (YSZ) ((ZrO2)0.93(Y2O3)0.07) and alumina-yttria-stabilized zirconia ((Al2O3)0.853 + (ZrO2)0.93(Y2O3)0.07) thermal barrier coatings (TBCs) were modeled in the presence of hydrogen-enriched combustion product gases to evaluate phase composition and thermal expansivity (coefficient of thermal expansion). Thermal equilibrium simulations for various equivalence ratios (0.5–0.75) and hydrogen enrichment percentages (0−50%) were conducted to determine the product gas composition for various combustor operating conditions. The obtained product gases were then used in a second thermal equilibrium simulation to demonstrate their effect on the defined thermal barrier coatings. The modeling predictions showed that hydrogen enrichment percentage and equivalence ratio were positively correlated to thermal expansivity for both the thermal barrier coatings examined. The alumina-YSZ composite coating exhibited a higher CTE, more closely matching the CTE of a metallic bond coat, for the studied conditions. This closer match of thermal expansivity results in less significant thermal stresses than the YSZ thermal barrier coating. An increase in hydrogen enrichment percentage and equivalence ratio yielded increased percentages of phase transitions from tetragonal zirconia (t-ZrO2) to cubic zirconia (c-ZrO2). The YSZ thermal barrier coating had a larger percentage of phase transitions throughout the operating range examined, which renders concerns for potential failure from thermal cycling and creep. Theoretical examination of the phase composition and thermal expansivity provided further insights into the fate and behavior of the thermal barrier coatings.

Analysis Study of Thermal and Exergy Efficiency in Double-Layers Porous Media Combustion Using Different Sizes of Burner: A Comparison

Experimental investigations are currently exploring the impact of adding porous layers within burner housing on thermal and exergy efficiency. Specifically, the focus is on understanding the significance of double layers on porous media combustion and how it can improve fuel mixing and flame stability. Premixed butane-air combustion in rich conditions was examined using three different sizes of burners (i.e., 23 mm, 31 mm, and 44 mm) porous media with equivalence ratios ranging from ф = 1.3 to 2.0. The experimental findings revealed a substantial improvement in performance efficiency (thermal and exergy) as the equivalence ratio increased. This study reveals that smaller burner diameters (ID, inner diameter = 23 mm) provide greater efficiency than larger ones (ID = 31 mm and 44 mm). The maximum flame temperature and porous wall temperature are found to decrease as the equivalence ratio increases. The highest temperature measured was 924.82°C for 23 mm, 910.23°C for 31 mm, and 850.76°C for 44 mm at ф = 1.3. Lastly, the thermal and exergy efficiency in a 23 mm porous media burner (PMB) is higher at ф = 2.0 at 84.30% and 83.47%, respectively. It can be concluded that the diameter size of the burner and equivalence ratio for double-layer porous material influence the performance (efficiency) of PMB.

Chemical kinetic study of gasoline surrogate with ammonia on combustion: Iso-octane modeling

To facilitate the transition towards a carbon–neutral society, it is necessary to undertake comprehensive research on the utilization of ammonia in gasoline engine applications. In the present work, a new chemical kinetic mechanism for iso-octane/ammonia (iC8H18/NH3) was proposed and widely validated with the experiment data of ignition delay time (IDT) and laminar burning velocity (SL). The iC8H18 model of Cai [Combust. Flame, 162, 2015] and NH3 model of Zhang [Combust. Flame, 234, 2021] was chosen as the based model, and they connected through the chemical coupling between the iC8H18 and the NH2 chemistry. In addition, the influence of C-N interaction on the prediction ability of iC8H18/NH3 model was also analyzed and found that the H-abstraction reaction between NH2 and iC8H18 is the key reaction affecting the IDT and SL of mixture. Present work was also focused on the chemistry kinetic of iC8H18/NH3 mixture. The reaction pathway analysis shows that the addition of NH3 slightly enhanced the H-abstraction reaction of iC8H18 in tertiary site (cC8H17) and slightly inhibited the H-abstraction reaction of primary site (aC8H17, bC8H17, and dC8H17). In the end, the mole fraction of NO in iC8H18/NH3 flames is analyzed. The results show that NH3 addition increased the concentration of NO, but with the gradual increase of equivalence ratio, the increasing trend of NO concentration gradually decreases. A higher equivalence ratio can be selected to effectively control the increase of NOx concentration. Through the analysis of NO formation and reduction reaction pathway, it was found that NH3 addition increases the mole fraction of NH2 radical, the NO formation pathway was stronger than NO reduction pathway due to the sufficient OH radical, and finally leading to the increase of NO concentration. However, with the increase of equivalence ratio, the concentration of NH2 radical exceeds OH, and the number of OH is insufficient in satisfying the NH3-related NO reaction pathway, resulting in a decrease of the influence of NH3 addition on NO concentration.