Varying Equivalence Ratio Research Articles

OH-PLIF study on the mechanism regulating flame-wall interaction with catalytically active CeO2-ZrO2 coatings

Technically, using catalytically active coatings regulate flame-wall interactions to enhance flame stability in small-scale burners, such as the common ZrO2-based ceramics with doping CeO2 which benefit to improve the high-temperature mobility and reactivity of lattice oxygen as an oxygen source to promote gas phase combustion. Probing by OH-PLIF laser diagnosis, the aim of this study was to investigate the dopant content effect (0–20 wt.% CeO2) on the flame characteristics of methane-air mixtures of varying equivalence ratio (0.9, 1.0, and 1.2) in an adjustable-gap narrow channel (2–12 mm) at a wall temperature range of 373 K to 973 K, including the dynamic flame morphology, flame quenching performance, and near-wall spatial distribution of OH radicals. Results show that the flame-wall distances are strongly correlated with wall temperature, equivalence ratio, and channel scale. After doping CeO2, the measured quenching distances can significantly drop, but do not show a linear downward trend with increasing CeO2 content. The case of 5%CeO2-ZrO2 coating demonstrates a small critical quenching Peclet number at high wall temperatures. The maximum OH fluorescence intensity in flame core areas increases with CeO2 doping into CeO2-ZrO2 at 973 K when the channel clearance is less than 4 mm. However, the absolute concentration of near-wall surviving OH radicals in the presence of 20%CeO2-ZrO2 coating is substantially lower than in the 5%CeO2-ZrO2 case due to differences in the redox properties. In situ diffuse reflectance infrared Fourier transformations spectroscopy tests further indicate that the adsorption quantities of C2H4 and C2H6 species on the coating surface also show a non-monotonous CeO2 dependence, as they are important intermediates in the low-temperature C2 chain reaction of methane.

Combustion characteristics of a turbocharged direct-injection hydrogen engine

The direct injection hydrogen internal combustion engines avoid backfires and have a higher power density, especially when coupled with a turbocharger. However, while previous researches have focused on specific aspects of direct injection hydrogen internal combustion engines, their whole combustion characteristics remain largely unclear. Therefore, this study aims to comprehensively investigate the combustion characteristics of turbocharged direct injection hydrogen internal combustion engines across the entire operating map in order to optimize their performance. The combustion characteristics are hypothesized to be significantly influenced by operating parameters, including the equivalence ratio, spark timing, start of injection, and other relevant factors. To achieve this, the experiments were conducted on a 2.0 L turbocharged direct injection hydrogen internal combustion engine, covering a wide range of operating conditions. Specifically, the engine speeds varied from 1000 to 4000 rpm, loads ranged from 3.7 to 10.6 bar, equivalence ratios spanned from 0.33 to 0.71, spark timing was adjusted within a range of 21 degrees of crank angle, and start of injection varied within a range of 80 degrees of crank angle. The experimental results reveal notable findings regarding the turbocharged direct injection hydrogen internal combustion engine. For instance, the maximum pressure rise rate of the turbocharged direct injection hydrogen internal combustion engine (3.85 bar/°CA at 1500 rpm) at various engine speeds at wide open throttle are greater than those of a turbocharged port fuel injection hydrogen internal combustion engine and a gasoline engine. Furthermore, variations in equivalence ratio and spark timing cause significant increases in pressure rise rate by 328.4% and 233.3%, respectively. The peak pressure rise rate characteristics provide valuable insights for optimizing the noise, vibration and harshness of turbocharged direct injection hydrogen internal combustion engines. Additionally, the burning duration demonstrates an initial increase and subsequent decrease with changes in the start of injection, aiding in the determination of the optimal start of injection for achieving high brake thermal efficiency. Overall, this research provides valuable insights into the macroscopic combustion characteristics of a turbocharged direct injection hydrogen internal combustion engine across different operating conditions. These findings not only can be directly applied to hydrogen internal combustion engines of the same displacement and type, but also can be applied to the design and development of hydrogen internal combustion engines with different displacements and types.

Effects of ethylene addition and dilution on the explosion characteristics of ethane-ethylene mixtures

The properties of an ethane-ethylene mixture were analyzed under varying equivalence ratios (ranging from 0.58 to 1.4) and ethylene volume fractions (ranging from 0% to 100%) with varying dilution ratios, using a 20-liter apparatus. The study aimed to understand the explosion characteristics of the mixture and to identify the elementary reactions that have the most significant impact on the rise in pressure and ethylene consumption during the explosion process. The results showed that adding ethylene increases the maximum explosion pressure and broadens the flammability limit range. In addition, the work investigated the effects of diluting the mixture with either nitrogen or carbon dioxide. It was found that carbon dioxide has a higher specific heat capacity compared to nitrogen, which enhances the heat absorption capacity of the mixture. The collision of carbon dioxide molecules with the activated radicals in the reaction transfers energy from the radicals to carbon dioxide, reducing the radicals' activity and causing a more significant dilution effect than nitrogen. Finally, a chemical kinetics analysis of the premixed gas using the UC San Diego mechanism revealed that the production rate of key free radicals gradually increases with the addition of ethylene, enhancing the explosion reaction intensity.

Investigation of the applicability of conditioned particle image velocimetry under conditions close to lean blow-off

In recent years, conditioned particle image velocimetry (CPIV) has attracted much attention for flame front extraction. It is an economical and straightforward technique because the flame front can be obtained simply from Mie-scattering images. In the current work, Mie-scattering and hydroxyl planar laser-induced fluorescence (OH-PLIF) measurements were taken simultaneously to study the applicability of CPIV under conditions with varying equivalence ratios, and the reliable working range of the CPIV method and the source of bias were clarified quantitatively. The premixed dimethyl ether (DME)/air flames stabilized on a two-dimensional bluff body were tested. It is found that the accuracy of the CPIV method worsens as the equivalence ratio decreases. The bias of CPIV is supposed to be related to the flame structure and aerodynamics. The extraction deviation in the upstream region for the stable flames is more significant than that in the downstream area due to the intensified interaction between the shear layer and flame near the bluff body. However, for the flames approaching the lean blow-off (LBO), the bias in the upstream part is smaller than that in the downstream area, which is related to the “back-support” of the hot recirculation zone (RZ). In addition, the difference between the results obtained by CPIV and OH-PLIF is usually the preheat layer of flame and concave-wrinkled location of OH-PLIF filled with CH2O radicals, especially under conditions close to the LBO, which can be used to estimate the spatial distribution of CH2O.Graphical

Investigation on geometric throat matching characteristics of variable geometry rocket-based combined cycle combustor

One of the most effective ways to realize multi-mode and high-performance matching operation of rocket-based combined cycle (RBCC) engine is to adopt a variable geometry combustor. In this study, direct-connect experiments and three-dimensional numerical simulations were used to investigate the matching characteristics between the isolator and the geometric throat of the variable geometry rocket-based combined cycle combustor under Ma 6 condition. The influences of equivalence ratio and geometric throat height variations on the combustor matching characteristics were obtained. The study results show that: (1) The pressure in the combustor and the outlet pressure of the isolator increase gradually with the increase of equivalence ratio, which causes the pre-combustion shock train position in the isolator to move forward gradually. Meanwhile, the combined injection of secondary fuel with the isolator and the pylon is beneficial to the improvement of combustor pressure, thus further improving the engine performance. (2) Different geometric throat heights have a substantial impact on the isolator and combustor matching when the equivalent ratio of the combustor is fixed. Under the conditions of Ma 6 operating at an altitude of 24 km inflow, the starting position of the pre-combustion shock train in the isolator is precisely at the isolator entrance, and the corresponding geometric throat height is 2.10H. The isolator and the combustor can complete matching operation at this point which the geometric throat height is the minimal throat height for Equivalent ratio = 1.0. (3) The reduction in geometric throat height can result in an increase in combustor pressure but a loss in combustor heat release and combustion efficiency of fuel combustion, which are reduced by 21.1% and 7.2% respectively. (4) The heat release and combustion efficiency in the combustor will decrease with a decrease in equivalent ratio at the minimal throat height condition where the isolator and the combustor can complete matching operation. This happens when the equivalent ratio and the geometric throat height change synergistically.

Numerical and boundary condition effects on the prediction of detonation engine behavior using detailed numerical simulations

High-fidelity numerical simulations of an experimental rotating detonation engine with discrete fuel/air injection were conducted. A series of configurations with different feed-plenum pressures but with constant equivalence ratio were studied. Detailed chemical kinetics for the hydrogen/air system is used. A resolution study for the full rotating detonation engine (RDE) system simulation is also conducted. Two kinds of boundary conditions, a total pressure boundary and a constant mass flow rate boundary, are used to assess the effects of the inlet boundary. As mass flow rate is increased, the total pressure boundary causes more error in the axial pressure distribution while the constant mass flow rate gives a better solution for all cases ran. The simulations confirm experimental findings, and reproduce qualitative as well as some of the quantitative trends. These results demonstrate that a) fuel-air mixing is highly non-uniform within the detonation chamber, leading to variations in local equivalence ratio, b) the fuel and oxidizer injectors experience significant backflow as the detonation wave passes over, but recover at different rates which further augments the inefficiencies in mixing, and c) parasitic combustion in the mixing region makes the detonation wave weak by extending the reaction zone across the wave.

Parametric investigation on the performance-emissions trade-off and knocking occurrence of dual fuel engines using CFD

Dual fuel (DF) engines have been an attractive alternative of traditional diesel engines for reducing both the environmental impact and operating cost. The major challenge of DF engine design is to deal with the performance-emissions trade-off via operating settings optimisation. This study aims at parametrically investigating the engine settings optimisation for a simultaneous reduction of NOx emissions and brake specific fuel consumption (BSFC) of marine DF engines while avoiding knocking accurrence by using a computational fluid dynamics (CFD) model. The model is developed in the CONVERGE software and validated by employing the measured in-cylinder pressure and emissions. Subsequently, the model is used to parametrically investigate the engine operating settings (pilot injection timing, equivalence ratio and natural gas mass) that allow simultaneous reduction of NOx and BSFC emissions at three engine operating conditions (1800 r/min, 1629 r/min and 1457 r/min). Result shows that the optimal solution at 1800 r/min and 1629 r/min operation conditions can be achieved by controlling the pilot injection timing, equivalence ratio variation and NG mass variation within –5 to –7.5 °CA, –5% to +5 % and 0 % to +20 %, respectively. For the 1457 r/min operation condition, the appropriate ranges of pilot injection timing and the equivalence ratio variation are the same with those at 1800 r/min and 1629 r/min, whilst the NG mass variation range should be between –10 % and +10 %. The derived results in this study are expected to support the combustion analysis and enhancement of marine DF engines during the design phase, whilst the derived optimal solution is expected to provide guidelines of DF engine management for reducing operating cost and environmental footprint.

Swirl 타입 노즐 적용 수소 전소 연소기 연구

A hydrogen combustor for applying to 5MWe-class small gas turbines was designed and experimentally investigated in this study. A swirl type, not a multi-tube type, which is widely used in the recently developed hydrogen combustor, was applied to this hydrogen combustor. The test was conducted at the inlet temperature of 410℃ which is derived from cycle analysis and at atmospheric pressure (1.35 barA). First, the ignition test was performed while switching from NG flame to hydrogen flame. Flame structures (from direct photo and OHSUP*/SUP chemiluminescence), NOSUBX/SUB emission and dynamic pressure were observed with varying adiabatic flame temperatures and fuel splits of FC1 and FC2. Lean blowout and flashback characteristics were investigated with varying equivalence ratio and flow velocity. Flame was converted from V-shaped flame to M-shaped flame as equivalence ratio increases and M-shaped one has higher NOSUBX/SUB emissions. NOSUBX/SUB emission is under 3 ppm (@15% O₂) at 1860 K of flame temperature which is equivalent or even lower level of NG combustors. It was found that the operating range from lean blowout to flashback is very wide.

동일한 연소실 압력에서의 당량비 변화에 따른 기체메탄-액체산소 소형로켓엔진의 성능특성

동일한 연소실 압력에서의 당량비 변화에 따른 기체메탄-액체산소 소형로켓엔진의 성능특성

Deep learning-based denoising for fast time-resolved flame emission spectroscopy in high-pressure combustion environment

A deep learning strategy is developed for fast and accurate gas property measurements using flame emission spectroscopy (FES). Particularly, the short-gated fast FES is essential to resolve fast-evolving combustion behaviors. However, as the exposure time for capturing the flame emission spectrum gets shorter, the signal-to-noise ratio (SNR) decreases, and characteristic spectral features indicating the gas properties become relatively weaker. Then, the property estimation based on the short-gated spectrum is difficult and inaccurate. Denoising convolutional neural networks (CNN) can enhance the SNR of the short-gated spectrum. A new CNN architecture including a reversible down- and up-sampling (DU) operator and a loss function based on proper orthogonal decomposition (POD) coefficients is proposed. For training and testing the CNN, flame chemiluminescence spectra were captured from a stable methane-air flat flame using a portable spectrometer (spectral range: 250–850 nm, resolution: 0.5 nm) with varied equivalence ratio (0.8–1.2), pressure (1–10 bar), and exposure time (0.05, 0.2, 0.4, and 2 s). The long exposure (2 s) spectra were used as the ground truth when training the denoising CNN. A kriging model with POD is trained by the long-gated spectra for calibration, and then the prediction of the gas properties taking the denoised short-gated spectrum as the input: The property prediction errors of pressure and equivalence ratio were remarkably lowered in spite of the low SNR attendant with reduced exposure.

An experimental investigation of ammonia/landfill/air mixtures' pollutant emissions and temperature distribution under non-preheated moderate or intense low-oxygen dilution combustion.

This paper investigates the non-preheated MILD combustion behavior of landfill/ammonia mixtures. The experiments were carried out in a laboratory-scale furnace. The effects of equivalence ratio variation, CO2 dilution, and ammonia on the furnace temperature distribution and exhaust gas emissions were analyzed attentively. The experiment results indicate that a non-preheated MILD combustion establishment can provide a uniform temperature distribution and lower pollutant emissions while maintaining the same thermal efficiency. According to the results, equivalence ratio changes affect the MILD temperature structure sensibly. Variation of equivalence ratio is also effective on the pollutant emissions. Moreover, the rich burn condition increases the CO emissions by bringing the HCN and H radicals into the related reactions. Whereas in lean-burn states, little amounts of CO are generated, while the NO concentrations are higher due to the NH2 pathways. The results also indicate that CO2 dilution reduces the furnace's peak temperature. However, CO2 dilution may also increase the O and OH radicals, allowing HCN molecules to react and generate NO. Regarding the ammonia addition experiments, it was figured out that ammonia increases NOx emissions, while decreasing CO2 concentrations affect the flame temperature profile.

Equivalence ratio independence and dependence ranges of system responses for a nonlinear thermoacoustic oscillation in a Rijke tube

Thermoacoustic instabilities in a combustion system remains a challengeable problem in industry over decades. It usually involves with the nonlinear interaction between unsteady heat release and pressure fluctuations. The nonlinear oscillation excited by this interaction can be intense enough to result in combustion problems even severe damage of the engine. Therefore, it is important to understand the system response to various parameters. In this study, the system nonlinearity to equivalence ratio change was mainly focused on. A Rijke tube system with a premixed methane-air laminar flame was utilised to trigger the self-excited oscillations with varied equivalence ratios (Φ). The time-domain analysis methods including phase difference, Recurrence Plot (RP) and Recurrence quantification analysis (RQA) were applied to obtain the characteristics of system nonlinearities. Both Φ-independence and Φ-dependence ranges of system frequency responses have been discovered. The results of phase difference and recurrence analysis have also validated the finding of both ranges. The corresponding characteristics of system nonlinearity in both ranges have been investigated. The similarity among the systems in the Φ-independence range under the same burner position and methane flowrate can be highlighted from the system frequency response, phase difference and system dynamics. It is found that the phase difference analysis, RP and RQA utilised in this study are capable of investigating the nonlinearities of the self-excited thermoacoustic oscillation.

Small-Scale Toroidal Fluidized Bed Gasification System: Development and Experimental Aspects

Abstract This article deals with system development and experimental aspects of a small-scale toroidal fluidized bed gasifier (TFBG) using sawdust feedstock (carpentry waste) with average particle size of 4.18 mm. The reactor and its accessories are designed and constructed. The experiments have been performed to highlight the influence of equivalence ratio on performance parameters (i.e., reaction temperatures, gas yield, gas composition, heating value, and cold gasification efficiency). For variation of equivalence ratio from 0.181 to 0.273 (average basis), the maximum temperature increase from 1015 to 1036 K, combustibles components CO, H2, and CH4 decrease from 16% to 14.9%, 16.3% to 15.3%, and 3.5% to 2.3%, respectively, and consequently, lower heating value (LHV) decreases from 5.454 to 4.716 MJ kg−1. On qualitative basis, the maximum cold gasification efficiency is observed to be 74% at equivalence ratio of 0.273, which is considerably higher than fixed bed gasification for sawdust. The overall uncertainty of experimental data is worked out to be ±3.41% (i.e., ≤5% within 95% confidence level).

Impact of carbon dioxide and nitrogen addition on the global structure of hydrogen flames

Investigations into the combustion characteristics and flame structure of hydrogen/carbon dioxide/air flames have been carried out experimentally and numerically. The aim was to determine the variations of the combustion properties and flame shape for different carbon dioxide dilutions in the H2-air mixtures. For this purpose, premixed flames at various compositions of hydrogen-carbon dioxide mixtures are investigated experimentally in a heat flux burner configuration at varying equivalence ratios from 0.5 to 1.1, fresh gas temperatures and under atmospheric conditions. Optical investigations are performed to detect the number of cellular structures on the flame front. Furthermore, the laminar burning velocity and the influence of carbon dioxide addition were examined numerically. A detailed kinetic mechanism was implemented for the identification of the most representative intermediates via a reaction path analysis, and the most influential species and reactions are identified through sensitivity analyses at conditions relevant to the studied application. Considering the nature of the adopted mechanism, the presence of CO2 has the potential to shift the production/consumption rate of some hydrogen-containing radicals. Hence, numerical investigations employing an inert species having the same thermodynamic and transport properties as CO2 (referred to as fictitious CO2, FCO2) were compared and discussed in this work. To investigate the effect of CO2 on the dynamics of the hydrogen flames, one-dimensional and two-dimensional detailed simulations of the flame structure have been carried out. The addition of CO2 makes the flame more prone to thermo-diffusive instabilities through a decrease in the mixture’s thermal diffusivity. This results in a decrease of the Markstein number and an enhanced formation of characteristic cellular structures on spherically expanding flames. Overall, the comparison between the experimental and numerical investigations reveals similar conclusions.

Exergy Loss Characteristics of Laminar Premixed Flames for Hydrogen/Methane and Dimethyl Ether/Methane Fuel Blends

ABSTRACT To reduce fossil energy consumption and achieve global carbon neutrality, it is essential to develop alternative fuels such as hydrogen (H2) and dimethyl ether (DME). In order to analyze the energy-mass conversion behavior of alternative fuels for power units (including gas turbines) and to explore the principles of efficiency enhancement, the exergy loss characteristics of methane (CH4)/air laminar premixed flames blended with H2/DME were investigated. The effects of blending ratio, unburned gas temperature, operating pressure, and equivalence ratio on the exergy loss fraction were considered. The results indicated that DME and H2 had opposite effects on the total exergy loss fraction of the premixed flame. Elevating the operating pressure was beneficial to reducing the exergy loss caused by chemical reactions and exhaust gas, which more significantly enhanced the exergy efficiency of each fuel mixture compared to increasing the unburned gas temperature. The exergy loss fraction due to the exhaust gas was also lower in the lean flame, suggesting that DME could be a promising alternative to CH4 in this scenario. As the exergy loss fraction due to chemical reaction was minimized at an equivalence ratio of 1.2, a variable equivalence ratio operating strategy was expected to further optimize the exergy efficiency of each fuel mixture. Furthermore, response functions of the total exergy loss fraction, which considered the combined effects of multiple operating parameters, were constructed. These response functions allow the total exergy loss fractions of CH4 premixed flames with high DME blending ratios and moderate H2 blending ratios to be quantified. The results from this study enable a deeper understanding of the energy-mass conversion behavior of systems with added H2/DME during combustion and provide theoretical support for the application of alternative fuels to more complex configurations of industrial burners.

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < do < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < Tc < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.

Nitrogen oxide reduction in pressurized oxy-coal combustion

Pressurized oxy-coal combustion is one of the promising technologies for carbon capture in power plant. NOx control is one of the main challenges during solid fuel combustion, which has attracted more interest at higher pressure. Previous results reported that lower NOx emissions were tested during pressurized oxy-coal combustion, however, fundamental studies regarding the NO reduction by coal/char at elevated pressure are scarce. In this paper, NO reduction behaviors during coal combustion in pressurized oxy-environment were systemically studied. Experiments were carried out at pressures from 1 atm to 7 atm, and temperatures of 800 K, 1073 K, and 1273 K, respectively. Equivalence ratios in the range of 0.5 to 1.5 were arranged to evaluate the oxygen effect on the interaction of coal/char with NO. Results show that the high reduction capacity of combusting particles at higher pressure and temperature can significantly reduce the NO concentration in the furnace. A large difference in NO reduction efficiency is also found among coal types at elevated pressure. The influence of equivalence ratio shows marked differences between 1 atm and higher pressure. The reduction tests by char show that the NO reduction activity is notably lower than coal at elevated pressures and varied equivalence ratios, which is due to the much higher reduction efficiency by homogeneous reaction by volatile matter at higher pressure. Compared with the char produced in fixed bed, reduction efficiency of char samples produced in drop tube furnace is much active at 4 atm. ROP analysis shows that the dominant pathway of reducing NO by volatiles is NO+HO2O2+OH among cases, while the main reactions occurred in volatile-NO interaction are not changed, although the global kinetic rate is enhanced obviously at higher pressure.

Study of the heat release and exergy loss of iso-octane self-ignition under engine-like conditions

With the severe low-carbon policies, energy utilization from the maximum useful-work exergy becomes one of the breakthrough points towards engine high efficiency. Hence, for constraining the combustion which is the main exergy loss source, the energy analysis from second law is imperative for the further efficiency improving. In this paper, the influence of boundary conditions on the heat release and exergy loss of chemical reaction during the constant-volume combustion of iso-octane at engine-like conditions was studied. It was found that the boundary conditions: initial temperature of 700 K, 900 K and 1100 K, equivalence ratio of 0.5, 1.0 and 1.5, have the same trend effect on the heat release and exergy loss rates of the same chemical reaction. It is impossible to increase the heat release and reduce the exergy loss at the same time, but different equivalent ratios have different degree effects on the reaction. Hence, a parameter α, the ratio of exergy loss on heat release, which stands for the exergy loss of unit heat release was promoted to consider the cost feature of chemical reactions. The results show that enhancing the medium and high temperature reaction during the combustion process is beneficial to reduce the parameter α, and the lean mixture is beneficial to obtain a small α. From the perspective of reaction type, it showed that the α of Fuel Fragment series reaction is the largest and its α is controlled by both temperature and equivalent ratio, the rich mixture should be conducive to reduce its α. The α of H2O2 Loop, H-O and CO-CO2 series reaction are small, and they are mainly controlled by the equivalence ratio, and lean mixture is favourable for reducing their α. Therefore, under the constant equivalence ratio, the best way to optimize the heat release and exergy loss is to control the equivalence ratio as small as 1.0. Under the strategy of the variable equivalence ratio, it is the best to change the mixture from rich to lean in the combustion process, and the reactions would sequentially go through the best α region of Fuel Fragment, H-O, CO-CO2, H2O2 Loop reactions, which would keep the α of whole combustion process maintain at the lowest level.

Optical investigation and thermodynamic analysis of premixed ammonia dual-fuel combustion initiated by dodecane pilot fuel

In view of reducing greenhouse gas emissions the transition from fossils fuels to sustainable energy carriers is a prerequisite to keep global warming within tolerable limits. Since IC engines will continue to play a role in global energy strategies during a transitional phase, especially for large engine applications difficult to electrify, the use of ammonia as substitute fuel may be an approach for decarbonization. However, its utilization needs research since ignition concepts and combustion properties still pose considerable challenges in view of reliable and efficient operation. A new "optical engine" test facility ("Flex-OeCoS") has been successfully adapted enabling dodecane pilot fuel ignited premixed ammonia dual-fuel combustion investigations. It features IC engine relevant operation conditions such as pressures, temperatures, and flow (turbulence) conditions as well as adjustable mixture charge composition and pilot fuel injection settings. In parallel, thermodynamic heat release analysis in terms of ignition and combustion characteristics was performed. Simultaneously applied high-speed Schlieren/OH* chemiluminescence measurements supported the examination of the combustion process. Initially premixed ammonia dual fuel combustion has been compared to a representative methane combustion process in terms of different gas properties (lower heating value, air-fuel ratio) which illustrates its lower reactivity affecting heat release and flame propagation. Moreover, ignition delay, combustion transition, and turbulent flame propagation as well as heat release characteristics have been investigated for premixed ammonia dual-fuel combustion within variation of air-fuel equivalence ratio, start of pilot fuel injection, and pressure/temperature operation conditions. The results illustrate strong dependency on air-fuel equivalence ratio (energy content) and temperature conditions in terms of ignition delay, dual-fuel combustion transition, and corresponding heat release. The optical investigations confirm the thermodynamic analysis and promote assessment of pilot fuel evaporation, ignition, combustion transition, and flame propagation. Conclusions give extended insight into the thermo-chemical processes of ammonia pilot fuel ignited dual-fuel combustion. The acquired data may also support further development of numerical CRFD methods.

Development of laminar burning velocity measurement system in premixed flames with hydrogen-content syngas or strong oxidizer conditions in a slot burner

This study presents the experimental determination of laminar burning velocity for critical premixed flames with multiple fuels or strong oxidizers. Experiments were conducted with a slot burner under methane/air, methane/nitrous oxide, and syngas (CH4/CO/H2)/air premixed flames conditions with varying equivalence ratios from 0.8 to 1.4. With the flame surface area determined from the Schlieren measurement system and stretch effect corrected by Markstein length, unstretched laminar burning velocity can be garnered according to the conservation of mass. First, the experimental results of methane/air premixed flame velocities were validated by comparing with one-dimensional unstretched burning velocities through numerical simulations with GRI 3.0 mechanism. The experimental results in the equivalence ratio ranging from 0.85 to 1.2 demonstrated errors less than 3.8%. Then, the validated burning velocity measuring techniques were implemented under methane/nitrous oxide and syngas/air premixed flame conditions. Finally, appropriate chemical mechanisms, such as USM or UGM, can be validated for the numerical simulation of critical premixed flames via this proposed laminar burning velocity measuring technique.