Lower Equivalence Ratio Research Articles

Experimental study of kerosene supersonic combustion with pilot hydrogen and fuel additive under low flight mach conditions

Reliable ignition and efficient combustion are critical for low flight Mach starting stage of dual-mode scramjet. In this paper, liquid kerosene ignition and combustion with pilot hydrogen and fuel additive were experimentally studied in a model supersonic combustor with clean air inflow, under the simulated conditions of flight Mach 3.8–4.0. The selected composite additive is mainly composed of about 20%-vol low-carbon small-molecular unsaturated alkanes and about 80%-vol diethylmethoxyborane. It is found that, under the low-temperature supersonic conditions, either RP-3 kerosene or with additives can all be ignited by the pilot hydrogen flame of least ERH = 0.065–0.076, only fuel additive effects seem to be still not enough for auto-ignition. Self-sustaining flame stabilization and combustion were realized with the tandem dual-cavity flame-holders, an appropriate amount of hydrogen addition can significantly promote the liquid kerosene combustion heat release without inlet disturbing. Enhancement of kerosene supersonic combustion and faster/higher temperature rises with the fuel additive of certain concentration were experimentally validated, and the positive effects showed more significant at a lower equivalence ratio of ERK = 0.43, in which the combustion enhancement induced mode transition from transonic to subsonic is found due to the 20%-vol additive. The additive effects might be greatly depended on the concentration in a nonlinear way when reducing concentration from 20%-vol to 10%-vol, a small concentration of 10%-vol additive did not produce an obvious enhancement effect. A small amount of hydrogen addition or an appropriate proportion of additive can improve the combustor performance, while too high total equivalence ratios or large amount of hydrogen addition tend to lower combustion efficiency.

A numerical investigation of oblique detonation waves in hydrogen-air mixtures at low mach numbers

Oblique detonation waves (ODWs) have potential applications in hypersonic propulsion, but the boundary of steady ODWs has not yet been examined comprehensively. In this study, Euler equations coupled with detailed chemical reaction models are used to simulate ODWs in hydrogen-air mixtures with relatively low flight Mach numbers (from 7 to 8) and different flight altitudes (30 km and 25 km). Decreases in the flight Mach number and altitude are shown to result in unsteady ODWs in a stoichiometric hydrogen-air mixture. These ODWs can be re-stabilized by decreasing the fuel‒air equivalence ratio. Regardless of different parameters of Mach number and altitude, unsteady ODWs appear only when the velocity in the induction zone exceeds that of the corresponding Chapman-Jouguet detonation. A low equivalence ratio also induces a long initiation length, limiting the availability of decreases in the equivalence ratio to maintain a steady ODW in practical applications. A flow‒combustion criterion is proposed for the application of ODWs, based on the steadiness of ODWs and fast initiation.

The effects of addition of carbon dioxide and water vapor on the dynamic behavior of spherically expanding hydrogen/air premixed flames

Utilizing efficiently and securely hydrogen as clean energy source, it is required not only to analyze the experimental data under a certain condition but also to create the mathematical model for the prediction of flame propagation velocity under various conditions. Thus, it is significant to understand the characteristics of dynamic behavior of hydrogen/air premixed flames and to elucidate the effects of addition of inert gas, i.e. carbon dioxide CO2 and water vapor H2O. We performed the experiments of hydrogen explosion in two types of closed chambers to observe spherically expanding flames using Schlieren photography. Wrinkles on the flame surface were clearly observed in low equivalence ratios. Analyzing the Schlieren images, the flame propagation velocity depending on the flame radius was obtained. Increasing the addition of inert gas, the propagation velocity decreased, especially in the case of CO2 addition. The propagation velocity increased monotonically as the flame radius became larger. The appearance of flame acceleration was found, which was caused by the evolution of wrinkles on the flame surface. Moreover, the Markstein length decreased as the concentration of inert gas became higher, indicating that the addition of inert gas promoted the instability of hydrogen flames. Furthermore, the wrinkling factor, closely related with the increment in propagation velocity, decreased as the inert-gas concentration became higher. The wrinkling factor normalized by the propagation velocity of flat flame increased, on the other hand, under the conditions of high inert-gas concentration, except for near the quenching conditions. This indicated that the addition of CO2 or H2O promoted the unstable motion of hydrogen flames, which could be due to the enhancement of the diffusive-thermal effect. Based on the characteristics of dynamic behavior of hydrogen flames, the parameters used in the mathematical model on propagation velocity including flame acceleration was obtained, and then the flame propagation velocity under various conditions was predicted.

Numerical optimization of flame stability in a swirl combustion chamber with helical tapes

In this study, the effects of forced vortices on non-premixed flames' reaction are investigated using Computational Fluid Dynamics (CFD) in a cylindrical chamber with an internal helix. For this purpose, using the Response Surface Method (RSM), optimization has been performed to increase the flame's stability by considering two parameters of the geometric variable and the objective function of increasing its thermal efficiency. Two different geometrical parameters of the helix, i.e., width and pitch, are varied, and their effects on vortex generation and mixing inside the chamber are investigated for various Reynolds numbers. Results show that combustion's overall efficiency could be improved by carefully altering the helix's width and pitch. This study indicates that helical tapes can increase the combustion rate at lower equivalence ratios, which reduces the concentration of greenhouse gases and pollutants in combustion. By decreasing the width and the step rates of helical tape, the flame moves closer to the chamber's centerline, and the reaction zone elongates axially. Hence, the average temperature of the chamber is comparatively increased. Studies show that by doubling the Reynolds number, the reaction rate increases, and the chamber average temperature increases by about 28%. The best performance is achieved when helical tape with a small pitch and width is used.

Experimental investigation on the influence of central airflow on swirl combustion stability and flame shape

Central airflow has been widely used to improve the performance of swirl burners in engineering applications. This paper reports an experimental investigation on the effects of such airflow on the combustion stability and shape of swirl flames. The results show that, for a low equivalence ratio, central airflow changes the flame shape from an “inverted cone” to a “rectangle” and significantly increases the flame height. Raising the speed of the central airflow increases the maximum temperature on the central axis of the swirl flame because the airflow enhances the upward momentum of the fuel. By contrast, for a high equivalence ratio, the swirl flame is prone to liftoff owing to the influence of the central airflow on the axial momentum of the fuel. In this case, increasing the fuel flow causes the swirl-flame blowout limit to increase and then decrease. This limit for different equivalence ratios is well described by dimensionless function. These findings will provide an important reference for the design of safe and high-performance swirl burners.

Two-Dimensional Numerical Study of Methane-Air Combustion Within Catalytic and Non-catalytic Porous Medium.

This study numerically investigates a two-dimensional physical model of methane/air mixture combustion in catalytic and non-catalytic porous media. The temperature distribution and flame stability of combustion in inert alumina (Al2O3) pellets and platinum (Pt) catalyst-supported alumina (Al2O3) pellets, were studied by changing the burner structure, operating parameters, and physical properties of alumina pellets. The simulation results indicated that the gas temperature in the inert porous medium is higher than that in a catalytic porous medium, while the solid temperature in an inert porous medium is lower than that in a catalytic porous medium. The flame moved toward the burner exit with the increasing diameter of the packed pellets at a lower equivalence ratio and moved toward upstream with the increased thermal conductivity of packed pellets. The flame location of the catalytic porous burner was more sensitive to the flame velocity and insensitive to thermal conductivity compared to the inert porous burner. The distance of the flame location to the burner inlet is almost constant with the increasing length of the porous media for both the catalytic and inert porous burner, while the relative position of the flame location moved toward the upstream.

Engine efficiency enhancement and operation range extension by argon power cycle using natural gas

Argon power cycle engine has been proved as a promising technology to increase engine efficiency and to reduce engine out emissions. In this research work, experiments were conducted in a port injection single-cylinder engine with air and argon/oxygen as the working fluids under spark ignition, homogeneous charge compression ignition, and spark assisted compression ignition conditions. Natural gas was used as fuel. Experimental results indicate that argon power cycle increases thermal efficiency, extends lean burn limit, suppresses oxides of nitrogen and unburned hydrocarbon emissions, compared to conventional air cycle. The maximum thermal efficiency of argon power cycle reaches 41.5% under low equivalence ratios and high compression ratio conditions. Given a fixed oxygen content, the thermal efficiency increases with decreasing equivalence ratio until the lean burn limit. With the decrease of oxygen content from 21% to 7.5%, the thermal efficiency of argon power cycle first increases and then drops with the peak occurring at ~15%. In practical applications, the engine load of argon power cycle can be controlled by changing equivalence ratio, oxygen content, or both. With argon power cycle, the use of spark assisted compression ignition can further increase the combustion stability, thermal efficiency, and decrease unburned hydrocarbon emission. Combined with an optimal spark timing, the engine cycle’s coefficient of variation of the indicated mean effective pressure decreases up to 84% and the thermal efficiency increases up to 2%, compared to homogeneous charge compression ignition.

Numerical simulation on lean-burn characteristics of a naturally aspirated opposed rotary piston engine fuelled with hydrogen at wide open throttle conditions

Opposed rotary piston engines are characterized by high power density, which makes them as an ideal power source for hybrid vehicles and range extended electric vehicles. Hydrogen applications can fully exhibit the merits of opposed rotary piston engines, and achieve zero carbon dioxide emissions; however, the applications seriously worsen the nitrogen oxides emissions. In this investigation, lean-burn was adopted to achieve low nitrogen oxides emissions using a three dimensional numerical simulation approach. The results indicated that engine speed of 3000 r/min presented the highest in-cylinder pressure during combustion among the given scenarios, and the pressure over 3000 r/min depended more on the equivalence ratio than that of 1000 r/min and 2000 r/min. Heat release rates were very sensitive to low equivalence ratio. Combustion duration over the equivalence ratio of 0.8 was the shortest among 1000 r/min cases; however, it decreased with equivalence ratio for 2000 r/min and 3000 r/min. Heat loss rates through cylinder walls increased significantly with engine speed, meanwhile they were more dependent on the equivalence ratio over higher engine speed. Maximum nitrogen monoxide formation rates over 3000 r/min occurred slightly earlier than those of 1000 r/min and 2000 r/min. Equivalence ratio of 0.8 showed the highest indicated thermal efficiency over corresponding engine speed, and nitrogen dioxide emission factors were quite low over the equivalence ratio of 0.7 for the given engine speed.

Sensitizing gaseous detonations for hydrogen/ethylene-air mixtures using ozone and H2O2 as dopants for application in rotating detonation engines

The effect of ozone and hydrogen peroxide as dopants on hydrogen-air and ethylene-air detonations was investigated with one-dimensional ZND calculations. Also, the effects of dopants were studied numerically with argon and helium as diluents with an aim to reduce the temperature of detonation products while maintaining a detonation wave of sufficient strength such that its propagation is stable near its propagation limits. The primary goal of the present investigation is to isolate the chemical kinetic effects from fluid and gas dynamic effects by altering the ignition chemistry of an unburned mixture without significantly changing its thermodynamic and physical properties. The ZND calculations demonstrate that the addition of O3 and H2O2 in small quantities will substantially reduce the induction length (Δi) and time (τi), even with higher diluent percentages of argon and helium, making it a viable solution for reducing the operating temperatures of rotating detonation engines (RDEs). The effects of O3 and H2O2 are also studied numerically at lower equivalence ratios for H2/C2H4-air detonations with an aim to reduce the post-detonation temperatures below 2000 K for its application in practical engine cycles. Also, the efficacy of CF3I, as an ignition promoter at small quantities, is studied numerically for hydrogen-air detonations, and its performance is compared with O3 and H2O2.

Performance analysis of a biogas operated porous radiant burner for domestic cooking application.

In this paper, the combustion characteristics of biogas in a Porous Radiant Burner (PRBBG) designed for domestic cooking appliances are presented. Developed PRBBG consists of two layers of porous matrices viz., Silicon Carbide (SiC) and Alumina (Al2O3) and operates in the biogas flow range of 177 to 530 l/h with stable equivalence ratio (ϕ) range of 0.75 to 0.95. The effects of biogas flow rate and equivalence ratio on thermal efficiency (ηth) and emission characteristics of the burner are investigated and also compared with its conventional counterpart. Overall performance assessment shows that PRBBG operating at lower equivalence ratio and lower firing power has better thermal efficiency with lower CO and NOx emissions. The performance of the burner in terms of temperature mapping suggests that firing power variation is of higher importance than that of equivalence ratio. The maximum temperature difference between the center and the periphery of the burner surface is found to be ~ 83 °C at an equivalence ratio of 0.95. The thermal efficiency varies in the range of 51-62% and its maximum is at 0.75 equivalence ratio and 177 l/h flow rate of biogas. Within the range of studied biogas flow rate range, CO emission is in the range of 29-80 ppm and NOx concentration was lower than 4 ppm. Whereas, for Conventional Burner (CB), thermal efficiency, CO, and NOx emission are found in the range of 43-52%, 211-276 ppm, and 9-15 ppm, respectively. The overall performance showed that PRBBG is capable of burning a lean biogas-air mixture with better thermal efficiency and lower emissions.

Synergistic effects of co-gasification of municipal solid waste and biomass in fixed-bed gasifier

The co-gasification of refuse derived fuel (RDF) and biomass is a promising technology for environmentally friendly municipal solid waste (MSW) utilization. Herein, we investigated the co-gasification of RDF and straw mixtures. The development of syngas suggested that higher temperature and equivalence ratio (ER) values promote syngas yield during gasification and co-gasification. Temperature plays a positive key role in gas yield and carbon conversion efficiency once the ER is higher than 0.3 during co-gasification. Except for an ER of 0.3, synergistic enhancements in carbon conversion efficiency, gas yield, and cold gas efficiency were observed for co-gasification. The positive synergistic effects of co-gasification mainly appeared in lower ER (0.1−0.2) and higher temperatures (800−900°C); however, the negative synergistic effects more tend to appear in higher ER (0.3−0.4). From the development of bottom ash in co-gasification, at higher temperature and ER values, potassium in the bottom ash was favored to stay in sulfur-containing compounds; whereas at lower temperature and ER values, potassium preferred to exist in potassium chloride. A higher calcium content more easily enriched in small particles that were integrated into aluminosilicate led to the formation of sticking ash particles, which could promote significant melting agglomeration. However, these melting agglomerations could be suppressed by adding straw into the RDF. These results provide a feasible method for the resource utilization of MSW.

An experimental and numerical study of polyoxymethylene dimethyl ethers on a homogeneous charge compression ignition engine

ABSTRACT Polyoxymethylene Dimethyl Ethers (PODEn) are a kind of high-activity and promising soot-reduction additive fuel. The effects of enhanced intake charge temperature (Tin ) and low fuel-air equivalence ratio (Φ) on the combustion and emission characteristics of PODE2-8 were captured on a homogeneous charge compression ignition (HCCI) engine. Meanwhile, a numerical study using Chemkin was conducted to investigate the effects of Tin on the combustion characteristics of PODE3. The results indicate that with the increase of Tin , rate of production (ROP) of CH2O and OH radicals and rate of exothermic reactions related to H- and O- species are greatly promoted, which result in a highly reactive atmosphere, advanced ignition timing (IT) and increased heat release rate (HRR). Typical two-stage auto-ignition behavior is also observed in lean PODE2-8 HCCI at high Tin . The enhanced Tin can promote the rate of CO-to-CO2 and decrease combustion duration (CD), which eliminates the third HRR oxidation peak. At Φ = 0.143, the peak in-cylinder pressure has increased by about 40% and the combustion duration (CD) has decreased by nearly 25% when Tin is from 80°C to 140°C. Moreover, for all test cases, NOx, HC, and CO emissions are lower than 15 ppm, 100 ppm, and 1%, respectively. Φ has a slight effect on the ignition timing of low temperature (ITL ) but an obvious effect on the ignition timing of high temperature (ITH ). With an increase of Φ, CO emissions decrease, HC emissions decrease first and then increase, and NOx emissions slightly increase. This research reveals that the combustion and emission characteristics of PODEn are sensitive to Tin , PODEn with enhanced Tin can form a chemical atmosphere with high-activity radicals.

Operability of a premixed combustor holding hydrogen‐enriched oxy‐methane flames: An experimental and numerical study

This work presents a detailed investigation on the stability, flame macrostructure and combustion characteristics of oxy-methane/hydrogen (H2CH4O2CO2) flames in a premixed swirl-stabilized model gas turbine combustor. The effects of equivalence ratio, inlet Reynolds number and inlet swirl number on stability and combustion characteristics were examined experimentally and numerically. A base operation case was selected as a reference to study the effects of the different parameters with equivalence ratio of 1.0, bulk mean throat velocity of 5.2 m/s, and swirl number of 0.98. Fixed compositions of the oxidizer (30%O2/70%CO2) and fuel (50%H2/50%CH4) streams were considered throughout the study. Large eddy simulation (LES) technique was adopted to compute the premixed turbulent flame regime. Experimentally captured flame images were compared with the flame contours obtained from the numerical simulations, and good conformations were observed. The results showed that near-stoichiometry flames exhibit strong inner recirculation zone (IRZ) and are stabilized between two shear layers. At lower equivalence ratios, the flames are corner-stabilized, and the flow is dominated by a single primary eddy in the IRZ. The calculated strain rates predict flame quenching at equivalence ratio of 0.5, which was also confirmed experimentally. It was observed that the effect of Reynolds number on the flame shape is not significant; however, the flow is dominated by larger primary eddy with strong IRZ as the Reynolds number increases. The IRZ also gets stronger with the increase in the flow swirl number. At higher swirl numbers, the flame stabilization occurs at the inner shear layer and the outer recirculation zone (ORZ) diminishes. It was found that the chemical time scale for all cases under investigation is shorter than the Kolmogorov time scale, signifying that the chemical reactions are dominated by the laminar flame speed.

Effect of fuel ratio of coal on the turbulent flame speed of ammonia/coal particle cloud co-combustion at atmospheric pressure

This study aims to clarify the effect of fuel ratio of coal on the turbulent flame speed of ammonia/coal particle cloud co-combustion at atmospheric pressure under various turbulence intensities. High-fuel-ratio coals are not usually used in coal-fired thermal power plants because of their low flame stability. The expectation is that ammonia as a hydrogen-energy carrier would improve the ignition capability of coal particles in co-combustion. Experiments on spherical turbulent flame propagation of co-combustion were conducted for various coal types under various turbulence intensities, using the unique experimental apparatus developed for the co-combustion. Experimental results show that the flame speed of co-combustion with a low equivalence ratio of ammonia/oxidizer mixture for bituminous coal case was found to be three times faster than that of pure coal combustion and two times faster than that of pure ammonia combustion. On the other hand, the flame speed of co-combustion for the highest-fuel-ratio coal case is lower than that of the pure ammonia combustion case, although the flame propagation can be sustained due to the ammonia mixing. To explain the difference of tendencies depending on the fuel ratio of coal, a flame propagation mechanism of ammonia/coal particle cloud co-combustion was proposed. Two positive effects are the increases of local equivalence ratio and the increases of radiation heat flux, which increases the flame speed. In opposite, a negative effect is the heat sink effect that decreases the flame speed. The two positive effects on the flame speed of co-combustion overwhelm a negative effect for bituminous coal case, while the negative effect overcomes both positive effects for the highest-fuel-ratio coal case. The findings of the study can contribute to the reduction of solid fuel costs when the ammonia is introduced as CO2 free energy carrier and can improve the energy security through the utilization of high-fuel-ratio coals.

Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.

Investigation of kerosene supersonic combustion performance with hydrogen addition and fuel additive at low Mach inflow conditions

Stable and efficient combustion is critical for low Mach starting stage of Dual-Mode Scramjet. In this paper, basic experimental validation and three-dimensional simulation of hydrogen addition and fuel additive effects on kerosene supersonic combustion performance were conducted under flight Mach 3.8 inflow conditions. Hydrogen addition was injected independently and the additive effects were simulated by the method of reducing activation energy. The simulation results are well-agreed with the experimental measurements from hydrogen addition cases and 20%-vol additive cases, validating the numerical method and models employed. The relative activation energy needed for well simulating 20%-vol additive effects tends to be higher than the theoretical prediction and also different by equivalence ratios. The heat release is intensified by either hydrogen addition or fuel additive, resulting in higher temperature-rises and pressure-rises, and more OH radicals for combustion enhancements at low-temperature conditions. An appropriate small amount of hydrogen addition (ERH < 0.08) can promote kerosene pre-evaporation and combustion heat release to achieve higher combustion efficiency; a large amount of hydrogen addition exhibits a competitive mechanism with kerosene cracking and CO oxidation conversion, also reduces the degree of diffusive mixing. Either the same small amount of hydrogen addition or fuel additive, more significant enhancement effects can be observed at the lower equivalence ratio of kerosene, due to the increased heat release induced flame stabilization and combustion mode transition. Compared with hydrogen addition, combustion efficiencies with increasing additive effects are increased monotonously to a higher level when kerosene equivalence ratio of ERK = 0.53, whereas lower efficiencies when ERK = 0.43.

Carbon oxides methanation in equilibrium; a thermodynamic approach

Global warming and greenhouse gases as two main threat to human societies due to increasing carbon oxides, such as CO and CO2 and lack of energy storages results in challenges efforts to controlling these atmospheric pollutions in various ways and methods. Carbon oxides methanation was considered as chemical process to conversion carbon oxides to their products as syngas. Various parameters can be effective on this process such as temperature, pressure and equivalence ratio of feeding products specially H2/CO2 and H2/CO. In this study, three various equivalence ratio of feeding products were investigated against pressure and temperature in equilibrium condition to determine concentration of main products. Five various pressures applied to system of equilibrium, i.e. 1, 5, 10, 25, 50 atm beside temperature change from 200 K to 1500 K. Moreover, fugacity effects also were investigated in Soave–Redlich–Kwong equation of state in comparison with ideal gas. Results revealed that fugacity was completely changes the results especially for water production and hydrogen consumption. According to the results, carbon di and monoxides conversion were increased during pressure increasing where methane selectivity also increased. In maximum condition of coke formation there was 0.1 mol fraction of it in both CO and CO2 methanation. Although, higher equivalence ratio of each carbon oxides combination feeding products ascended CH4 selectivity and yield but in high equivalence ratio (ER = 6) CH4 yield decreased about 8% for both investigated methanation process. In lower equivalence ratio (lower than stoichiometric) condition, methane yield replaced with mainly carbon yield.

On the Effects of Injection Strategy, Exhaust Gas Recirculation, and Intake Boost on TSCI With Wet Ethanol

Abstract Using a split injection of wet ethanol, where a portion of the fuel is injected during the compression stroke, has been shown to be an effective way to enable thermally stratified compression ignition (TSCI), an advanced combustion mode that aims to control the heat release process by enhancing thermal stratification, thereby extending the load range of low temperature combustion (LTC). Wet ethanol is the ideal fuel candidate to enable TSCI because it has a high latent heat of vaporization and low equivalence ratio sensitivity. Previous work has shown “early” compression stroke injections (−150 to −100 deg aTDC) have the potential to control the start of combustion (SOC) while “mid” compression stroke injections (−90 to −30 deg aTDC) have the potential to control in-cylinder thermal stratification, thereby controlling the heat release rate. In this work, a mixture of 80% ethanol and 20% water by mass is used to further study the injection strategy of TSCI combustion. Additionally, the impact of external, cooled exhaust gas recirculation (EGR), and intake boost on the effectiveness of a TSCI with wet ethanol to control the heat release process are investigated. It was found that neither external, cooled EGR, nor intake boost level has any impact on the effectiveness of the compression stroke injection(s) at controlling the burn rate of TSCI. External, cooled EGR has the potential to increase the overall tailpipe combustion efficiency, while intake boost has the potential to decrease NOx emissions at the expense of combustion efficiency by lowering the global equivalence ratio.

Direct comparison of self-excited instabilities in mesoscale multinozzle flames and conventional large-scale swirl-stabilized flames

The present experimental investigation demonstrates important trends and offers physical insights into self-excited combustion instabilities in mesoscale multinozzle flames composed of sixty small injectors. Here we focus on the response of a prototypical micromixer-type injector assembly, fabricated using an additive manufacturing technique, in comparison with the behavior of conventional large-scale swirl-stabilized flames. Our results highlight that the development of self-excited instabilities in unconventional mesoscale flames is fundamentally different from that in large-scale swirl flames, in terms of the onset of instabilities, nonlinear modal dynamics, and amplitude/frequency of limit cycle oscillations under the same operating conditions. These differences are attributable to the alteration in local flow/flame structures and the resulting flame-to-flame/flame-wall interaction mechanisms. An integrated analysis of large datasets reveals that the two interacting swirl-stabilized flames tend to couple strongly with a low-frequency L1 mode at about 220 Hz, whereas the sixty-injector small-scale flames are capable of triggering multiple higher-frequency instabilities at ∼ 310, ∼ 470, and ∼ 600 Hz. That is, the use of the micromixer-type injector assembly in a lean-premixed system causes the occurrence of combustion instabilities to shift toward a higher equivalence ratio. However, due to the absence of a large recirculation zone near the primary reaction region, the combustion system equipped with the small-scale multinozzle injectors was found to suffer from lean blowoff phenomena at low equivalence ratio.

Tailoring thermal stratification to enable high load low temperature combustion with wet ethanol on a gasoline engine architecture

Wet ethanol is a biofuel with unique fuel properties: a high latent heat of vaporization and low equivalence ratio sensitivity. These two fuel properties enable control over the heat release process in low temperature combustion through the enhancement of in-cylinder thermal stratification. In this work, it is shown that the direct injection of wet ethanol 80 (80% ethanol, 20% water, by mass) during the intake stroke on a mid-compression ratio engine with a side-mounted gasoline direct injector does not fully premix with incoming air, leaving stratification in the cylinder which elongates the combustion process, enabling high-load low temperature combustion without engine-damaging knock. As a result, the load under naturally aspirated conditions was only limited by oxygen content as the global equivalence ratio approached stoichiometry. The results show that swirl can not only modulate the induced stratification effectively by increasing the amount of bulk motion in the cylinder but also cause a thermal efficiency penalty, since swirl increases heat transfer losses. Modulation over the induced stratification can be achieved without an efficiency penalty by employing either a dual injector system where a fraction of the total fuel is injected into the port or a split injection strategy where a fraction of the total fuel is injected during the compression stroke. This strategy enables high-load low temperature combustion, while reducing the number of regions that form NOx and avoiding excessively rich regions that contribute to combustion inefficiency.