Rich Equivalence Ratios Research Articles

A Parametric Study on NOx Emissions From Ammonia Containing Product Gas in Rich–Quench–Lean Combustion

Abstract Due to climate change, there has been an increasing demand for fuels that can accelerate the transition away from fossil fuels to clean energy. Humidified product gas obtained from gasifying biomass is emerging as a promising candidate to replace natural gas, as it is composed of a gaseous mixture of hydrogen, steam, carbon monoxide, and methane. However, the gasification process releases ammonia and other nitrogen bearing compounds into the product gas, resulting in substantial increases in nitric oxides, NOx, in the exhaust. As such, there has been a recent push to understand the underlying chemical kinetics that drive NOx formation in order to optimize gas turbines to mitigate emissions at the source. In this study, a simplified chemical reactor network (CRN) model for a gas turbine rich–quench–lean (RQL) combustor was developed in cantera. The following parameters were investigated in this study: equivalence ratio of the primary section, overall equivalence ratio, steam dilution, postflame residence time, and recirculation from the postquench region to the primary section. Additionally, a benchmark CRN representing a lean burner (LB) is also developed. Results of the CRN model suggest that, when comparing to LB, a RQL type combustor delivers up to a 75% reduction in emissions. Additionally, it was found that, for both the LB and RQL combustors, an overall lean to stoichiometric equivalence ratio is well suited to reduce emissions in highly humidified fuels, while for moderately humidified fuels it is preferable to operate in an overall slightly rich equivalence ratio. The difference observed is mainly due to the fact that, at high humidification and lean conditions, the temperature is favorable for the conversion of ammonia to nitrogen, while, at moderate humidification and rich conditions, NO reacts with ammonia in the reburn process. Finally, it is suggested that the incorporation of recirculation from the secondary section to the primary section of the RQL burner results in a broader low emission region, due to more favorable conditions for ammonia conversion to nitrogen in the primary section.

The Effect of Combustion Phase According to the Premixed Ethanol Ratio Based on the Same Total Lower Heating Value on the Formation and Oxidation of Exhaust Emissions in a Reactivity-Controlled Compression Ignition Engine

A compression ignition engine generates power by using the auto-ignition characteristics of fuel injected into the cylinder. Although it has high fuel efficiency, it discharges a lot of exhaust emissions such as NOX and PM. Therefore, there is much ongoing research aiming to reduce the exhaust emissions by using the technologies applied in this regard, such as PCCI, HCCI, etc. However, these methods still discharge large exhaust emissions. The RCCI method, which combines the spark ignition method and compression ignition method, is attracting attention. So, in this work, the objective of this study is to numerically investigate the effect of combustion phase according to the premixed ethanol ratio based on the same total heating value in-cylinder by changing the initial air composition on the formation and oxidation of exhaust emissions in the RCCI engine. The heating value of the premixed ethanol ratio varied from 0% to 40% based on the same total lower heating value in-cylinder in steps of 10%. It was assumed that the ethanol introduced into the cylinder through the premixing chamber was evaporated, and the initial air composition in the cylinder was changed and set. It was revealed that when the premixed ratio based on the same total lower heating value was increased, the introduced fuel amount into the crevice volume with advancing the start of energizing timing was decreased, which increased the peak cylinder pressure. In addition, the ignition delay was also longer due to the low cylinder temperature by the evaporation latent heat of the ethanol, which reduced the compression loss, so the IMEP value was increased. The rich equivalence ratio had a narrow distribution in the cylinder, which caused a reduction in cylinder temperature, so the NO formation amount was reduced. The ISCO value increased the increase in premixed ethanol ratio based on the same total lower heating value in-cylinder because the flame propagation of ethanol by combustion of diesel did not work well, and the CO formed by combustion was slowly oxidized due to the cylinder’s low temperature as a result of the evaporation latent heat of ethanol. From these results, the optimal operating conditions for simultaneously reducing the exhaust emissions and improving the combustion performance were judged such that the start of energizing timing was BTDC 23 deg, and the premixed ethanol ratio based on the same total lower heating value in-cylinder was 40%.

Experimental and numerical study of combustion characteristics of ammonia–hydrogen–air swirling flame with/without secondary air injection

Ammonia (NH3) is currently considered to be a potential carbon-free alternative fuel, and its large-scale use as such would certainly decrease greenhouse gas emissions and meet increasingly stringent emission requirements. Although the low flame propagation speed and high NO production of NH3 hinder its direct application as a renewable fuel, co-combustion of NH3–H2 is an effective way to overcome these challenges. In this study, the combustion characteristics of NH3–H2 swirling flames under different equivalence ratios and H2 blending ratios conditions are both numerically and experimentally investigated. Numerically, the One-Dimensional (1D) laminar flame computation presents a comparison base and the Three-Dimensional (3D) numerical simulation yields detailed flame property distributions. Experimentally, the high-speed camera takes instantaneous swirl flame images and the gas analyzer measures the NO emission at the exit plane of the flame chamber. Qualitative and quantitative analysis is performed on the flame structure and NO emission for a series of NH3–H2 swirl flames. The variation trends of the NO emission calculated using different techniques agree very well. The quantitative results show that the NO emissions are much higher at lean equivalence ratios than those at rich equivalence ratios, and such difference is closely related to the combustion flame structure. Moreover, it is shown that the utilization of secondary air injection can achieve a significant reduction in NO emissions at the exit of the combustion chamber at equivalence ratios less than or equal to 0.9.

Ignition Delay Times and Chemical Kinetic Model Validation for Hydrogen and Ammonia Blending With Natural Gas at Gas Turbine Relevant Conditions

Abstract Ignition delay times from undiluted mixtures of natural gas (NG)/H2/Air and NG/NH3/Air were measured using a high-pressure shock tube at the University of Central Florida. The combustion temperatures were experimentally tested between 1000 and 1500 K near a constant pressure of 25 bar. As mentioned, mixtures were kept undiluted to replicate the same chemistry pathways seen in gas turbine combustion chambers. Recorded combustion pressures exceeded 200 bar due to the large energy release, hence why these were performed at the high-pressure shock tube facility. The data are compared to the predictions of the NUIGMech 1.1 mechanism for chemical kinetic model validation and refinement. An exceptional agreement was shown for stoichiometric conditions in all cases but strayed at lean and rich equivalence ratios, especially in the lower temperature regime of H2 addition and all temperature ranges of the baseline NG mixture. Hydrogen addition also decreased ignition delay times by nearly 90%, while NH3 fuel addition made no noticeable difference in ignition time. NG/NH3 exhibited similar chemistry to pure NG under the same conditions, which is shown in a sensitivity analysis. The reaction CH3 + O2 = CH3O + O is identified and suggested as a possible modification target to improve model performance. Increasing the robustness of chemical kinetic models via experimental validation will directly aid in designing next-generation combustion chambers for use in gas turbines, which in turn will greatly lower global emissions and reduce greenhouse effects.

Dynamics of mono-size aerosolized liquid fuel detonations

Detonation-based propulsion has been an area of interest in the scientific community in terms of detonative combustion providing higher efficiency. However, most detonation research has focused on gaseous products though liquid fuel propellants are of interest for propulsion applications due to their high energy density. This research explores the use of an aerosolized liquid fuel as the combustion reactant. Jet-A is aerosolized into 5 μm droplets to fill the experimental facility, which has been pre-filled with oxidizer, and ignited. The reaction travels through a series of perforated plates that induce turbulence into the flow, and the ensuing detonation is imaged through the test section windows. Evidence of the Jet-A detonation is captured though high-speed shadowgraph, CH* chemiluminescence, and Mie scattering. Pressure measurements were also taken along the test section. This paper explores the dynamics of an aerosolized Jet-A driven detonation and discusses the droplet burning behavior. Several varying amounts of nitrogen dilution and both a stoichiometric and rich equivalence ratio was tested at each dilution level, giving insights into how these affect the detonation characteristics.

Effect of the variation of oxygen concentration on the laminar burning velocities of hydrogen-enriched methane flames

The combustion properties of hydrogen-containing fuel mixtures and the effect of the variation of the oxygen content of the oxidizer are at the center of recent research interest. Laminar burning velocity measurements with varied oxygen content can help in the validation of reaction mechanisms for better simulations of combustion systems using exhaust gas recirculation (EGR) or oxygen-enriched atmosphere. Such measurements were carried out in hydrogen-enriched methane−air flames using the heat flux method with higher accuracy and a wider range of initial oxygen and hydrogen concentrations compared to the similar studies in the literature. The mole fractions of the hydrogen and oxygen contents of the initial fuel and oxidizer mixtures were varied between xH2 = 0 and 0.20, and xO2 = 0.14 and 0.23, respectively. The initial gas temperature and pressure were 298 K and 1 bar, respectively. It is demonstrated that the increase of combustion rate by the hydrogen enrichment can be compensated with the decrease of the oxygen content. This compensating effect was investigated in detail in a wide range of equivalence ratio (φ). The experimental data were simulated with 11 widely used methane combustion reaction mechanisms. The prediction accuracies of the mechanisms at lean and rich equivalence ratios were significantly different and the important reaction steps were identified using sensitivity analysis for three mechanisms. Mechanisms POLIMI-2014 and Caltech-2015 gave the best overall predictions.

Hydrogen production in ammonia-fueled spark ignition engines

Anhydrous ammonia (NH3) is a promising carbon-free fuel for internal combustion engines (ICEs) that can be generated from renewable energy sources and stored at high energy density. Ammonia is typically mixed with a small fraction of hydrogen (H2), which serves as combustion promoter to achieve stable combustion over a large range of engine operation. To overcome the difficulties of storing hydrogen, ammonia can be converted to hydrogen onboard by means of a catalytic reactor fed by waste thermal energy from the engine exhaust or by partial oxidation. This work explores an alternative method of producing hydrogen by partial oxidation of fuel-rich ammonia–air mixtures to hydrogen in an engine cylinder while generating useful work. Such an approach eliminates the need for costly catalyst materials and poses the possibility of a dedicated reforming cylinder to provide hydrogen for other cylinders in a multi-cylinder engine. Experiments were conducted in a single-cylinder spark-ignition (SI) engine with parametric sweeps of intake temperature, intake pressure, equivalence ratio, and spark timing. Experimental results show that 6% exhaust gas hydrogen concentration is attainable and 40% of the unburned fuel energy can take the form of hydrogen. Rich equivalence ratios produced higher concentrations of hydrogen and elevated intake temperatures extended the rich stable-operation limit of the engine. Low order chemical modeling yields insights into hydrogen production throughout the engine cycle, revealing that initial consumption of ammonia is advanced of hydrogen production and that competing elementary reactions for the formation and destruction of hydrogen drive its net quantity in the exhaust.

Behavior of Premixed Sooting Flame in a High-Pressure Burner

The second-order factor effect of burner optical ports and edge inter-matrices (EIM) and the first-order factor of pressure on the soot formation process and behavior of premixed sooting flames in a high-pressure burner are numerically investigated here. Three-dimensional computational fluid dynamics (CFD) simulations of a premixed flame C2H4/air at p = 1.01 and 10 bar using a one-step chemistry approach are first performed to justify the satisfied predictability of the prospective axisymmetric two-dimensional (2D) and one-dimensional (1D) simulations. The justified 2D simulation approach shows the generation of an axial vorticity around the EIM and axial multi-vorticities due to the high expansion rate of burnt gases at the high pressure of 10 bar. This leads to the development of axial multi-sooting zones, which are manifested experimentally by visible luminous soot streaks, and to the boosting of soot formation conditions of a relatively low-temperature field, <1800 K, and a high mixing rate of gases in combustion around and above the EIM location. Nevertheless, a tolerable effect on the centerline soot volume fraction (fV) profile, fV < 3%, is manifested only at high heights above the burner of the atmospheric sooting flame C2H4/air ϕ = 2.1, and early at the high pressure of 10 bar of this flame, fV < 10%. Enhancing the combustion process reactivity by decreasing the rich equivalence ratio of the fuel/air mixture and/or rising the pressure results in the prior formation of soot precursors, which shifts the sooting zone upstream.

Numerical investigation of a first-stage stator turbine blade subjected to NH3–H2/air combustion flue gases

Blending ammonia with hydrogen has the potential to replace conventional hydrocarbon fuels of jet engines and gas turbines to reduce carbon emissions. Previous research on the 70% NH3–30% H2 (vol%) fuel blend characterized its cycle efficiency and emissions, however, the thermal and aerodynamic effects of the NH3–H2/air combustion flue gases on the turbine blades were not identified. Therefore, the novelty of the analysis presented herein appears in characterizing such effects of the NH3–H2/air combustion flue gases on a generic turbine blade model using CFD simulation for lean (Φ = 0.75), stoichiometric (Φ = 1.00), and rich (Φ = 1.25) equivalence ratios, which are compared to a CH4/air combustion flue at Φ = 0.75, 1.00 and 1.25, respectively. Based on the obtained results, a cooling channel's ability to reduce the blade's temperature was negligible based on the temperature difference between the leading edge of the turbine blade and the temperature of the combustion flue gas at the inlet. As the combustion equivalence ratio was increased from 0.75 to 1.25, a second shockwave forms at the leading-edge surface, projecting across the blade's lower edge. The formation of the second shockwave was found to be increasingly significant for the NH3–H2/air mixture on the downstream flow when compared to a CH4/air flue gas. Furthermore, increasing the NH3–H2/air equivalence ratio improved the blade's ability to increase the average overall outlet Mach number compared to the inlet flow. Flow separation near the trailing edge remained relatively unaffected with increasing equivalence ratio. However, separation near the leading edge at the blade's lower edge becomes more significant for the NH3–H2/air combustion flue gases compared to CH4/air combustion, causing a large circulation zone under the blade's lower surface due to the higher kinematic viscosity for the NH3–H2 fuel to the CH4 fuel (i.e., 2.06 ×10−5cm2/s and 1.75 ×10−5 m2/s, respectively). The circulation induces a higher viscous force and fluid inertia in the boundary layer, causing higher levels of separation. Turbulence intensity was also found to be significantly increased for the NH3–H2/air flow to that of the CH4/air combustion flue gases with increasing equivalence ratio.

Experimental and numerical investigation of the transition from non sooting to sooting premixed n-butane flames, encompassing the nucleation flame conditions

In a small range of rich equivalence ratios, premixed flat flames, so-called nucleation flames, were shown to produce very small-size (2–4 nm) soot particles, which undergo negligible growth with the residence time. In such flames, the contribution of the soot nucleation step with respect to the soot growth process is larger than in standard sooting flames, making nucleation flames ideal target flames to better understand the inception process. In order to scrutinize the transition from non sooting to sooting flames, three premixed atmospheric n-butane/oxygen/nitrogen flames surrounding the nucleation flame conditions were investigated: the flame at ϕ=1.6 is a fuel-rich non-sooting flame, the flame at ϕ=1.75 is a nucleation flame and the flame at ϕ=1.95 is a lightly sooting flame. The soot volume faction (SVF) profiles were previously measured by laser-induced incandescence as well as the soot size distributions. In the present study, mole fractions of stable species up to benzene were measured by gas chromatography while those of naphthalene and pyrene were obtained by jet-cooled laser-induced fluorescence. It is found that acetylene, propyne, benzene, naphthalene and pyrene species show the highest sensitivity with the equivalence ratio, while a two orders of magnitude increase of SVF is observed. The chemical flame structure of these flames was modeled using three kinetic mechanisms of the literature to which a recently developed soot code, based on a sectional approach, was coupled. The chosen model is representative of the current state-of-the-art of models describing all steps of soot formation and growth and fully coupled with the gas phase. The ability of such a model to predict soot volume fraction with reasonable predictions of gas-phase species is validated here based on the new experimental database presented in this work. It also highlights the improvement required on particle inception modeling to correctly predict particle size distributions.

Shock Wave Propagation and Flame Kernel Morphology in Laser-Induced Plasma Ignition of CH4/O2/N2 Mixture

The application of laser ignition in the aerospace field has promising prospects. Based on the constant volume combustion chamber, the laser ignition of CH4/O2/N2 mixture with different initial pressure, different laser energy, different equivalence ratio and different oxygen content has been carried out. The development characteristics of the flame kernel and shock wave under different conditions are analyzed. In addition, the Taylor model and Jones model are also used to simulate the development process of the shock wave, and a new modified model is proposed based on the Jones model. The experimental results show that under pure oxygen conditions, the chemical reaction rate of the mixture is too fast, which makes it difficult for the flame kernel to form the ring and third-lobe structure. However, the ring structure is easier to form with the pressure and laser energy degraded; the flame kernel morphology is easier to maintain at a rich equivalence ratio, which is caused by the influence of the movement of hot air flow and a clearer boundary between the ring and the third-lobe. The decrease of the initial pressure or the increase of the laser energy leads to the increase in shock wave velocity, while the change of the equivalence ratio and oxygen content has less influence on the shock wave.

Reaction Model Development of Selected Aromatics as Relevant Molecules of a Kerosene Surrogate—The Importance of m-Xylene Within the Combustion of 1,3,5-Trimethylbenzene

Abstract The development of advanced reaction models to predict pollutant emissions in aero-engine combustors usually relies on surrogate formulations of a specific jet fuel for mimicking its chemical composition. 1,3,5-trimethylbenzene is one of the suitable components to represent aromatics species in those surrogates. However, a comprehensive reaction model for 1,3,5-trimethylbenzene combustion requires a mechanism to describe the m-xylene oxidation. In this work, the development of a chemical kinetic mechanism for describing the m-xylene combustion in a wide parameter range (i.e., temperature, pressure, and fuel equivalence ratios) is presented. The m-xylene reaction submodel was developed based on existing reaction mechanisms of similar species such as toluene and reaction pathways adapted from literature. The submodel was integrated into an existing detailed mechanism that contains the kinetics of a wide range of n-paraffins, isoparaffins, cycloparaffins, and aromatics. Simulation results for m-xylene were validated against experimental data available in literature. Results show that the presented m-xylene mechanism correctly predicts ignition delay times at different pressures and temperatures as well as laminar burning velocities at atmospheric pressure and various fuel equivalence ratios. At high pressure, some deviations of the calculated laminar burning velocity and the measured values are obtained at stoichiometric to rich equivalence ratios. Additionally, the model predicts reasonably well concentration profiles of major and intermediate species at different temperatures and atmospheric pressure.

A comprehensive combustion chemistry study of n-propylcyclohexane

Alkylated cycloalkanes are vital components in gasoline, aviation, and diesel fuels; however, their combustion chemistry has been less investigated compared to other hydrocarbon classes. In this work, the combustion kinetics of n-propylcyclohexane (n-Pch) was studied across a range of experiments including pressurized flow reactor (PFR), jet stirred reactor (JSR), shock tube (ST), and rapid compression machine (RCM). These experiments cover a wide range of conditions spanning low to intermediate to high temperatures, low to high pressures at lean to rich equivalence ratios. Stable intermediate species were measured in PFR over a temperature range of 550–850 K, pressure of 8.0 bar, equivalence ratio (φ) of 0.27, and constant residence time of 120 ms. The JSR was utilized to measure the speciation during oxidation of n-Pch at φ of 0.5–2.0, at atmospheric pressure, and across temperature range of 550–800 K. Ignition delay times (IDTs) for n-Pch were measured in the RCM and ST at temperatures ranging from 650 to 1200 K, at pressures of 20 and 40 bar, at φ=0.5,1.0. In addition, a comprehensive detailed chemical kinetic model was developed and validated against the measured experimental data. The new kinetic model, coupled with the breadth of data from various experiments, provides an improved understanding of n-Pch combustion.

Stability limits and NO emissions of premixed swirl ammonia-air flames enriched with hydrogen or methane at elevated pressures

This study reports measurements of stability limits and exhaust NO mole fractions of technically-premixed swirl ammonia-air flames enriched with either methane or hydrogen. Experiments were conducted at different pressures from atmospheric to 5 bar, representative of commercial micro gas turbines. The full range of ammonia fractions in the fuel blend, xNH3, was considered, from 0 (pure methane or hydrogen) to 1 (pure ammonia), covering very lean (φ = 0.25) to rich (φ = 1.60) equivalence ratios. Results show that increasing pressure widens the range of stable equivalence ratios for pure ammonia-air flames. Regardless of pressure, there is a critical ammonia fraction above which the range of stable equivalence ratios suddenly widens. This is because flashback does not occur anymore when the equivalence ratio is progressively increased towards stoichiometric and rich blowout occurs instead. This critical ammonia fraction increases with pressure and is larger for ammonia-hydrogen than for ammonia-methane. Provided that enough hydrogen is blended with ammonia (xNH3 < 0.9), flames with very lean equivalence ratios (φ < 0.7) can be stabilized and these yield competitively low NO emissions (<200 ppm), regardless of pressure. For this reason, very lean swirl ammonia-hydrogen-air flames are promising candidates for micro gas turbines. However, N2O emissions have the potential to be unacceptably large for these operating conditions if heat loss is too large or residence time is too short. As a consequence, the post flame region must be considered carefully. Due to the lower reactivity of methane compared to that of hydrogen, very lean swirl ammonia-methane-air flames could not be stabilized and good NO performance is limited to rich equivalence ratios for ammonia-methane fuel blends. The equivalence ratio above which good NO performance depends on pressure and bulk velocity.

Low NOx Annular Type Combustor for 20 kW Gas Turbine Engine

The present combustor addresses NOx reduction for mobile power application which can be easily carried to different locations. The Annular type combustor is developed for 20KW thermal capacity gas turbine engine, using hydrogen as fuel. For low NOx emission, the conventional annular liner is redesigned to incorporate Rich Burn, Quick Mix, and Lean Burn Combustor (RQL). The combustor is tested in high pressure test rig at design and off-design conditions. The performance parameters under consideration are temperature profiles, NOx emissions, pressure drop and pattern factor. These parameters are studied with respect to rich zone and lean zone equivalence ratio. The rich zone equivalence ratio is varied from 1.1 to 1.81, while the lean zone equivalence ratio is varied from 0.32 to 0.59. The experimental results suggest that rich zone equivalence is the main factor in controlling NOx emissions, while pattern factor (radial and circumferential) is dependent on lean zone equivalence ratio.

Stability limits and NO emissions of technically-premixed ammonia-hydrogen-nitrogen-air swirl flames

Hydrogen is a promising carbon-free fuel for power generation in gas turbines. However, this raises some challenges associated with the storage and distribution of pure hydrogen. Storing hydrogen chemically in the form of ammonia is a safe and efficient alternative. However, ammonia as a fuel features a low chemical reactivity compared to hydrogen and natural gas and, as a consequence, stabilizing turbulent ammonia-air flames is challenging. Offsetting this low reactivity by enriching ammonia with some amount of hydrogen, which is much more reactive, is a promising strategy. In this study, the stability limits of technically-premixed ammonia-hydrogen-air flames are measured in a laboratory-scale swirl combustor for a wide range of ammonia fractions in the ammonia-hydrogen fuel blend. Results are compared to that obtained in the same combustor for reference methane-hydrogen-air mixtures. Data show that increasing the ammonia fraction in the fuel blend promotes lean blowout but reduces the flames’ propensity to flashback. The latter effect is even more pronounced if the volume fraction of ammonia in the fuel blend exceeds 0.7. In that case, increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blowout occurs instead, yielding a much wider range of stable equivalence ratios. This study also reports exhaust NO mole fractions, measured for large ranges of equivalence ratio and ammonia fraction in the fuel blend. Regardless of the ammonia fraction, data show that competitively low NO emissions occur for slightly rich equivalence ratios of φ ≥ 1.05, which is consistent with earlier studies. Stable flames and good NO performance are also found for very lean ammonia-hydrogen-air mixtures with φ ≤ 0.50, demonstrating the strong potential of fueling gas turbines with ammonia-hydrogen blends.

Improving combustion performance of swirling double-concentric jets flames with rich equivalence ratios

The acoustic excitation method was used to improve the combustion performance of rich-combustion flames of swirling dual-disk double-concentric jets. The unexcited and excited conditions were compared. The central jet was propane gas and the swirling annular jet was air. The central fuel jet was acoustically excited by a loudspeaker that was installed by the “downstream longitudinal irradiation” method. A home-made, one-component hot-wire anemometer was used to measure the central jet velocities to reveal the jet pulsation behavior. Long and short exposure flame images were captured for identification of flame behavior. A home-made L-shaped fine-wire thermocouple was used to measure the flame temperatures. A gas analyzer was used to measure the concentrations of main combustion products, including carbon dioxide (CO2), unburned hydrocarbons (UHCs), carbon monoxide (CO) and nitric oxide (NO). The process parameters (Rec, Rea, Stexc, and Ip, identified in the nomenclature) able to provide higher combustion efficiency were introduced. The best operating zones of the unexcited and excited flames were observed. At large central fuel jet Reynolds numbers, the unexcited rich-combustion flames exhibited a long and soot-radiating yellow flame that had low combustion performance. By applying appropriate acoustic excitation, the rich-combustion flames presenting a soot-radiating yellow appearance could be changed to short, blue, high-turbulence, high-stability lifted flames and the combustion performance was improved. The acoustically excited lifted flame had a higher temperature and lower toxic emissions when compared with the unexcited flame. According to the presented results, two strategies of operating the burners of swirling dual-disk double-concentric jets at low and high central fuel jet Reynolds numbers were suggested.

Effect of Water Vapor Injection on the Distributions of Equivalence Ratio and the NO Emission Reduction in a CI Engine

The objective of this study was to investigate the influence of a water vapor injection into the intake port of a small compression ignition engine and analyze the effect of the collisions between the water particles and the injected fuel on combustion and exhaust emission performances. To simulate the water vapor by the ultrasonic humidifier in the numerical analysis, the water particles were introduced into the cylinder through an intake port during the intake process, and the amount was varied from 10% to 30% of the injected fuel mass per stroke. When the water vapor was injected into the intake port, the rich equivalence ratio region was distributed in the center of cylinder. In addition, the ISNO (indicated specific nitrogen oxide) values decreased up to 46% more than the values for the condition without the water-vapor-injection. However, the ISSoot (indicated specific soot) exhibited similar values in any conditions. For starts of energizing timing that were BTDC (before top dead center) 25 deg and 15 deg, the ISFC (indicated fuel consumption) values decreased with increased portions of water vapor. However, in the case of BTDC 05deg, the ignition delay was too long, which deteriorated combustion performance.

The effect of equivalence ratio, temperature and pressure on the combustion characteristics of hydrogen-air pre-mixture with turbulent jet induced by pre-chamber sparkplug

The pre-chamber sparkplug mode can increase the combustion velocity because it can induce the turbulent jet into the cylinder. Higher combustion velocity can increase the brake thermal efficiency and decrease the knock tendency for hydrogen engines. To explore the effect of pre-chamber sparkplug mode on the combustion characteristics of the hydrogen-air mixture, different equivalence ratios, initial pressures and temperatures were selected to study in a constant volume combustion chamber working with pre-chamber sparkplug mode and normal sparkplug mode. The results showed that the pre-chamber sparkplug mode can accelerate the combustion velocity, increase maximum combustion pressure and decrease the combustion duration at all initial conditions. The maximum combustion pressure of pre-chamber sparkplug mode occurred at the equivalence ratio of 1.0 while it occurred at the equivalence ratio of 1.2 with normal sparkplug mode, which means pre-chamber sparkplug mode can increase the higher brake thermal efficiency and power. The combustion intensity of pre-chamber sparkplug mode was bigger than 1 and the biggest value occurred at the equivalence ratio of 0.6. Moreover, the combustion intensity of pre-chamber sparkplug mode was higher with lean equivalence ratios than that of rich equivalence ratios. Increasing the initial pressure can increase maximum combustion pressure and combustion velocity obviously for pre-chamber sparkplug mode, which was different from the normal sparkplug mode. The initial temperatures had little impact on the combustion intensity. These results showed the pre-chamber sparkplug mode was more suitable to be used in the boosting hydrogen engines to improve the performance.

Numerical simulation on combustion process of a hydrogen direct-injection stratified gasoline Wankel engine by synchronous and asynchronous ignition modes

A Wankel engine with hydrogen direct-injection enrichment is recognized as an attractive method to enhance combustion efficiency. In this paper, on the basis of the chemical kinetic mechanisms, a three-dimensional simulation model was established and validated by the measured results. The ignition and combustion processes in a gasoline Wankel engine with hydrogen direct-injection enrichment were implemented for numerical simulation. The influence of twin-spark timing was firstly analyzed by synchronous ignition. Further investigation was then conducted to simulate how to effect the combustion by asynchronous ignition based on the favorable synchronous ignition timing. Results showed that, the average flow velocities were 18.8, 20.7, 23.6, and 25.4 m/s for the spark timings (ST) of 45, 35, 25, and 15°CA BTDC respectively. With retarded ST, the rich equivalence ratio approached to twin-spark plug continually. For synchronous ignition, at a larger advance of ST, the duration between the timing of the vortex dissipation and ST was longer, and the ignition delay was more prolonged. As the ST advanced, the combustion rate was enhanced with the increment in chamber temperature, the reactants (C8H18, C7H16, and H2) consumption and intermediates (H2O2, OH, and CH2O) generation were accelerated, the maximum H2O2 and CH2O decreased whereas the maximum OH increased, and nitrogen oxides (NOx) and carbon monoxide (CO) increased sequentially. For asynchronous ignition, the accelerated flame front was in accordance with the rotating direction of the rotor while the opposite direction was inhibited during the combustion process. An earlier ignition of leading-spark plug (L-plug) or trailing-spark plug (T-plug) provided the higher flame speed and faster H2 consumption, which contributed to the higher in-cylinder pressure, combustion temperature, and NOx and CO production. The preferable ignition and combustion characteristics was realized when the L-plug angle was 25°CA BTDC, and the T-plug angle was 35°CA BTDC. Compared with the favorable synchronous ignition timing, the flame propagation and H2 consumption were expedited, the peak pressure increased by 2.8%, the corresponding crank position advanced by 3°CA and there was no increase in NOx and CO generation. Therefore, it was recommended in engineering applications that the ST of T-plug advanced appropriately while the ST of L-plug remained constant.