Global Equivalence Ratio Research Articles

Pure ammonia flames with high thermal intensities through fuel and air staging under extreme rich-to-lean conditions

Ammonia combustion has gained significant attention due to its high hydrogen content and zero carbon emissions. It poses a challenge for stabilization due to their low energy content and limited flammability range, with the added concern of fuel NOx emissions. In the current study, a novel burner design equipped with four series of reactors is proposed to achieve stable pure ammonia-air flames with reduced NOx emissions. The impact of thermal intensity (∼0.7 MW/m3 to ∼4 MW/m3), number of stages, equivalence ratio (0.5–1.2), and fuel staging on flame stabilization and emissions were investigated in the proposed burner. Comprehensive emissions analysis is performed at various burner levels. Numerical simulations incorporating Large Eddy Simulation (LES) modeling are employed to enhance understanding of the impact of thermal intensity and equivalence ratios on ammonia dissociation, mixedness, and NO emissions. The results indicated that the present burner design improved reactant mixing, and flame stability, reduced NH3 emissions (∼ 0 PPM), and lowered NOx levels in non-premixed ammonia-air flames. The computational and experimental results demonstrated that the implementation of fuel staging is crucial for reducing NOx emissions for the flames with lean global equivalence ratios. Lower NO emissions were identified at a global equivalence ratio of 1.1 for all the considered flames in the range of 0.5–1.2. In the four-stage rich-lean combustion strategy employed in this study; it was observed that higher thermal intensity (4 MW/m3) with fuel staging resulted in lower NOx emissions per kW of energy input compared to lower thermal intensities 0.7 MW/m3. This finding underscores the significance of achieving uniform mixtures and ensuring local flame in rich conditions for achieving low NOx emissions in ammonia combustion. Exhaust gas analysis is conducted at three stages of the burner to enhance the understanding of emissions at various levels of the burner. The proposed combustor design has achieved a substantial reduction in NOx to ∼1 PPM/kW and ∼2.8 PPM/kW with and without fuel staging, respectively. This impressive outcome is attributed to the controlled ammonia consumption facilitated by uniform mixing generated through the use of tangential air inlets.

Impact of Preheating on Flame Stabilization and NOx Emissions From a Dual Swirl Hydrogen Injector

Abstract Flame stabilization, flame structure, and pollutant emissions are investigated experimentally on a swirled injection system operating with globally lean air/hydrogen mixtures at atmospheric conditions and moderate Reynolds numbers. This injector consists of two coaxial ducts with separate injection of hydrogen into a central channel and of air into an annular channel. Both streams are swirled. The resulting flames exhibit two stabilization modes. In one case, the flame takes an M-shape and is anchored to the hydrogen injector lips. In the second case, the flame is aerodynamically stabilized above the injector and takes a V-shape. Regions of existence of each stabilization mode are determined according to the operating conditions. For low air flow rates, the flame can be either anchored or lifted above the hydrogen injector lips depending on the path followed to reach the operating condition. At high air flow rates, the flame is always lifted regardless of the trajectory followed. The impact of air inlet temperature on these stabilization regimes is then evaluated from T= 300 K up to 770 K. Flame re-attachment is shown to be controlled by edge flame propagation and the impact of preheating is well reproduced by the model. Unburnt hydrogen and NOx emissions are finally evaluated. Unburnt hydrogen is only observed for global equivalence ratios below 0.4 and at ambient inlet temperature. NOx emissions decrease when the global equivalence ratio is reduced. Furthermore, at fixed global equivalence ratio, NOx emissions decrease as the thermal power increases, regardless of air preheating and the flame stabilization regime. At high power, NOx emissions reach an asymptotic value that is independent of the thermal power. The impact of flame shape, air preheating, and combustion chamber wall heat losses on NOx production is also evaluated. NOx emissions are shown to scale with the adiabatic flame temperature Tad at the global equivalence ratio and the residence time inside the combustor.

Effect of Fuel Reactivity and Operating Conditions on Flame Anchoring in the Premixing Zone of a Swirl Stabilized Gas Turbine Combustor

Abstract Flashback with subsequent flame anchoring (FA) is an inherent risk of lean premixed gas turbine combustors operated with highly reactive fuel. The present study has been performed to characterize flame stabilization in the premixing zone of a lean premixed swirl stabilized burner and to identify critical combustion characteristics. An optically accessible burner was used for experimental investigations under atmospheric pressure and elevated preheat temperatures. The air mass flowrate, global equivalence ratio and preheat temperature were systematically varied to identify critical operating parameters. Hydrogen-natural gas mixtures with hydrogen mass fractions from 15 to 100% were studied to evaluate the impact of fuel reactivity. The air-fuel mixture was ignited with a focused single laser pulse to trigger FA in the premixing zone during steady operation. High speed imaging with OH*-chemiluminescence were applied to observe flame characteristics and evaluate flame anchoring propensity. Flame anchoring limits (FAL) are reported in terms of the minimum global equivalence ratio at which the flame was blown out of the premixing zone within a critical time period. A comparison of characteristic time scales at FAL shows that the main impact during flame anchoring is given by the fuel reactivity and to some ex tent by preheat temperature. A Damköhler criterion is derived from the FAL that allows prediction of FA propensity based on operating conditions and 1-D reacting simulations.

Investigation of ignition and flame propagation in an axisymmetric supersonic combustor with laser-induced plasma

The ignition and flame propagation in an axisymmetric supersonic combustor were investigated. The laser-induced plasma was employed to ignite the supersonic inflow with a speed of Mach 2.5 and a total temperature of 1486 K. A direct-connect axisymmetric model scramjet with a fully transparent glass combustor was built, which enabled the circumferential and axial flame propagation in the cavity-based axisymmetric supersonic combustor to be visualized by the high-speed photography from the endoscopic and external views, respectively. An initial flame kernel is produced by the laser-induced plasma and propagates to the cavity leading edge along the axial direction. The establishment of the cavity shear-layer flame facilitates circumferential flame propagation. The circumferential flame propagation is coupled with the axial propagation, eventually generating a loop-shaped flame with a central-hole. Acceleration of the flame propagation can be observed, especially when the global equivalence ratio is increased. A plausible explanation for the flame propagation in the axisymmetric supersonic combustor was found using URANS numerical simulation. The axisymmetric cavity generates a low-speed loop-shaped recirculation region and thickened cavity shear-layer with an appropriate local equivalence ratio, resulting in the simultaneous axial and circumferential flame propagation. The increased temperature in the cavity and the thickened cavity shear-layer during the flame propagation produce a more intense heat release and mass transfer, leading to faster flame propagation.

Structure and NOx Emissions of Stratified Hydrogen-Air Flames Stabilized on a Coaxial Injector

Abstract In recent years, the need for low-carbon power has seen hydrogen emerge as a potential fuel to replace conventional hydrocarbons in combustion to limit CO2 emissions in several sectors, including aeronautics. The challenges posed by hydrogen combustion are similar to the issues of kerosene flames but more challenging, like nitrogen oxide (NOx) emissions and flame flashback. One potential solution to address these problems is to burn a rich mixture of hydrogen and air in globally lean conditions on a coaxial injector to obtain a stable and staged combustion and attempt to reduce emissions. In this article, the evolution of NOx production as more air is mixed into the fuel is studied, as well as the changes in flame size and structure. In particular, the appearance of a secondary flame front is observed and increasing the proportion of air in the fuel mixture both shortens the flame and reduces the NOx emission index. Additionally, the effect of the global equivalence ratio and flame thermal power is studied. Finally, existing models for NOx emission of hydrogen flames on a coaxial injector based on average flame residence time and strain rate are tested and shown to have promising results.

Intermittency of Flame Structure and Thermo-Acoustic Behavior in a Staged Multipoint Injector Using Liquid Fuel

Abstract This study investigates the impact of the staging factor, the ratio between the fuel injected through the pilot stage and the multipoint injection, on the flame dynamic. The BIMER combustor is an atmospheric pressure rig equipped with two corotating swirling air injections (a fixed amount of around 87% of the air goes inside the multipoint stage) and two fuel injection paths for staged combustion. Liquid dodecane is injected with air preheated at 437 K with a global equivalence ratio of 0.6 and a thermal power of around 72 kW. The change of the staging factor from 100% (pilot-only injection) toward 0% (multipoint-only injection) generates changes in the flame-shape which bifurcates from an anchored V-flame into a lifted flame. This flame shape bifurcation appears at a staging of factor around 25%. Around this staging factor, one can witness multistable flames where the flame structure transits randomly between five different states. Processing microphone signals recorded in the chamber provides an understanding of the flame dynamics. The attached flame presents limited pressure fluctuations level at 270 Hz, while the lifted flame features high-pressure fluctuations at 323 Hz. The intermittency between the five states (including the two stable states) is investigated.

Characteristics of multi-cycle two-phase pulse detonation waves traveling near the lean combustion limit

The need for high combustion efficiency in two-phase pulse detonation engines necessitates the implementation of a lean combustion concept. However, there have been no research initiatives attempting to conduct two-phase pulse detonation in a lean combustion environment due to the highly sensitive nature of the deflagration-to-detonation transition toward the reactivity of the reactant composition. The present study explores methods to realize lean combustion organization in two-phase pulse detonation through the incorporation of secondary air injection. Valveless pulse detonation operation based on gasoline was carried out, while the frequency varies from 20 to 100 Hz. The initiation and propagation characteristics of the pulse detonation wave are influenced first by the equivalence ratio of the detonation initiation section and then by the equivalence ratio of the detonation propagation section. Furthermore, secondary air injection enabled a reduction in the minimum global equivalence ratio for the stable operation of multi-cycle two-phase pulse detonation waves to 0.38, while maintaining an 80% detonation rate.

Effect of equivalence ratio on rotating detonation combustion with n-heptane sprays

Three-dimensional simulations are conducted for rotating detonative combustion fueled by n-heptane sprays without any fuel pre-vaporization. Parametric studies are performed to study the influences of equivalence ratio on the rotating detonation wave. The decoupled process of the detonation wave with lean equivalence ratio is also been analyzed. It is found that when the global equivalence ratio is lower than 0.6, a stable propagating detonation wave cannot be observed in the combustion chamber. With the gradual increase of the global equivalence ratio, the detonation wave can stably propagate in the combustion chamber after ignition. When the global equivalence ratio is 0.8–1.5, the propagation mode of the detonation wave in the combustion chamber is one-way propagation mode. When the global equivalence ratio gradually increase from 0.8 to 1.5, the formation time of the detonation wave first decreases and then increases with the increase of the equivalence ratio. Besides, the stability of the detonation wave first increases and then decreases with the increase of the equivalence ratio. Moreover, the thrust of the rotating detonation chamber fueled by n-heptane increases gradually with the increase of the equivalence ratio. The specific impulse of the combustion chamber first increases and then decreases with the increase of the equivalence ratio.

Towards the development of liquid ammonia/air spray combustion in a gas turbine-like combustor at moderately high pressure

This study is a numerical investigation of the potentials of liquid ammonia (LNH3) spray combustion in a gas turbine-like combustor at moderately high pressure. The energy costs for the vaporization of LNH3 and the compression of the fuel vapor in the gaseous ammonia (GNH3)/air combustion systems, as well as the capital cost for the equipment involved, can be mitigated by employing LNH3 spray combustion. Additionally, responsiveness to power demand can easily be controlled using direct liquid ammonia. In the first part of this study, the spray characteristics of pure LNH3 were extensively studied numerically and experimentally using a hollow cone nozzle to validate the numerical models and codes. In modeling LNH3 sprays at atmospheric pressure or moderately high ambient pressures, accounting for the flash (superheat) evaporation phenomenon in addition to equilibrium evaporation is very important. Measured and simulated data both show that LNH3 spray (both vapor and droplet phases) reached a minimum equilibrium temperature of 215 K (−60 °C) when LNH3 at a pressure of 0.9 MPa was injected into an environment at 0.1 MPa and 283 K. This minimum temperature is much lower than the saturation temperature of LNH3 of 240 K (−33 °C) at ambient pressure of 0.1 MPa due to occurrence of non-equilibrium flashing. The measured average LNH3 droplet size approximately varied from 15 ∼ 20 µm and the numerical results agreed with these measured values. In the second part of the study, numerical simulations of LNH3 spray combustion were carried out at a moderately high pressure of 0.3 MPa. First, the LNH3 spray was co-fired with gaseous hydrogen (GH2) while maintaining approximately 50 %, 60 %, and 70 % of ammonia in the fuel by energy fraction (ENH3) based on the lower heating value at a moderately high pressure of 0.3 MPa. Finally, 100 % LNH3 spray flame was simulated. Preheated swirling airflow at 600 K was used to enhance the stability of the LNH3 spray flames, enabling stable combustion over a wide range of global equivalence ratios from 0.99 through 1.43. Moreover, the simulated emission characteristics of LNH3/GH2/air flames at ENH3 = 60 % were compared to those of GNH3/GH2/air flames (ENH3 = 60 %) at the same equivalence ratios. The NO and unburnt emissions of LNH3/GH2/air (ENH3 = 50 % ∼ 100 %) flames and GNH3/GH2/air (ENH3 = 60 %) flames show a trade-off relation.

Acoustic and Optical Investigations of the Flame Dynamics of Rich and Lean Kerosene Flames in the Primary Zone of an Air-Staged Combustion Test-Rig

Abstract In this study, the thermoacoustic behavior of nonpremixed kerosene flames from rich to lean combustion conditions is investigated. Flame-transfer-functions (FTF) measured purely acoustically are compared with results based on flame chemiluminescence. OH*, CH*, C2*, and CO2* were selected as potential measures for representing steady and fluctuating heat release when burning nonpremixed kerosene. In addition, their ability for the quantification of equivalence ratio fluctuations will be highlighted. The measurements were performed in the primary zone of an atmospheric rich-quench-lean (RQL) combustion test-rig. The new experimental approach allows a characterization of the primary zone independent of the secondary zone. Rich and lean operating points were analyzed by fueling an aero-engine prototype injector with and without acoustic excitation. To improve the quality of the acoustic wavefield reconstruction a thermocouple correction method was implemented. The flame dynamics determined with the multimicrophone method (MMM) exhibit a frequency and equivalence ratio depending effect of rich combustion conditions. The results for the steady behavior of the chosen radicals by altering equivalence ratio and thermal power indicate proportionality of the chemiluminescence to thermal power. Furthermore, the CH*/C2* ratio is found to be a promising indicator for the global equivalence ratio in the combustion chamber. The flame-transfer-functions based on chemiluminescence show a good qualitative agreement with the multimicrophone method. Based on the experimental findings a calibration curve for the different radicals to obtain quantitatively correct flame-transfer-functions from chemiluminescence is presented.

Semi-confined layered kerosene/air two-phase detonations bounded by nitrogen gas

A two-dimensional numerical study is conducted on the semi-confined cellular detonations propagating through a layer of two-phase kerosene/air reactant bounded by an inert gas. By fixing a global equivalence ratio, a series of cases are simulated to investigate the effects of the background vapor/liquid droplet equivalence ratios (φg and φd) and droplet diameters. The results show that the propagation and failure of the layered detonation are strongly affected by the equivalence ratios and droplet diameters. When φg is 0.8, layered detonations can propagate at a droplet size of 5, 10, and 20 µm, while it fails when the droplet size is increased to 40 µm. When φg is less than 0.8, the self-sustained layered detonation can only be obtained with a small droplet size of 5 µm and a high φg above 0.5. The detonated fuel fraction and droplet contribution efficiency are found to be sensitive to the droplet diameter. It is found that the transverse detonation is a manifestation of the self-sustained ability of detonation waves. The loss of triple points leads to the appearance of transverse detonation whereas the transverse detonation can generate new triple points and stabilize the detonation front. The transverse detonation can significantly enhance the mass transfer and heat release even though it occurs behind the sonic line, which promotes the detonation re-initiation. Besides, it is found that h/λ=2.90 (h is reactant layer height and λ is cell width) is the critical value for the successful propagation of layered detonations in this study, which agrees well with the existing experiments. In addition, reflection efficiency is introduced and is effective in predicting the strength of the weak confinement.

Experimental investigation on the combustion characteristics and thermoacoustic instability of liquid spray flame with blended fuel

This study experimentally investigates the combustion characteristics and thermoacoustic instability of two fuels, ethanol and RP-3, at five different blending ratios. The flame images, root temperature of blended fuel flame and pollutant emissions were photographed and tested. The experimental results indicate that high-pressure pulsation amplitude and airflow disturbance can enhance droplets’ evaporation and combustion processes, resulting in a reduction in flame length. Lower evaporation temperature ensures a higher temperature of the flame root while increasing the blend ratio of RP-3 and increasing the global equivalence ratio can increase the temperature level inside the combustion chamber. The root temperature of ethanol flame can reach over 1400 K. Fuel characteristics and burner structure can influence the coupling between pressure pulsation and flame heat release pulsation, leading to differences in the amplitude of combustion instability. The shorter the outlet length of the burner, the wider the stability range of the flame. Moreover, the increase in total burner length results in a shift of the oscillation dominant frequency towards lower frequencies. Emission characteristic curves reveal that high-pressure pulsation amplitude can improve the combustion efficiency of blended RP-3 fuel but are unfavorable for the complete combustion of ethanol. Meanwhile, under the dilution effect of the airflow, the emissions of NOx and CO at the outlet gradually decrease as the airflow increases.

Effects of [formula omitted]-pentanol blending on soot formation in swirl-stabilized turbulent spray flames of Jet A-1 in a laboratory gas turbine combustor

Although the influence of alcohol supplement on soot in hydrocarbons fuel combustion in laminar flames and transportation engines is widely investigated, the information from well-controlled spray flames with clean boundary conditions are scarce, especially with kerosene type fuels doped with n-pentanol. Favorable physical properties of n-pentanol makes it an attractive oxygenated fuel extender for transportation fuels. We investigated the effects of adding 10% and 20% n-pentanol, by energy content, to a JetA-1 fuel on sooting characteristics of spray combustion in a swirl-stabilized model combustor. At an identical fuel feed rate, two different air flow rates were considered yielding global equivalence ratios of 0.64 and 0.69. Flow field generated by the combustion of swirling air and the injected fuel was measured using stereoscopic particle image velocimetry, and a diffraction based instrument was utilized to quantify the spray droplet size distribution. Laser-induced incandescence was used to measure the soot concentration distribution within the combustor and to estimate the primary soot particle sizes. Very small but measurable changes were observed in the spray droplet characteristics caused by the addition of n-pentanol to JetA-1. 10% replacement of JetA-1 by n-pentanol reduced the soot volume fractions and the total soot loading in the combustor significantly when the global equivalence ratio was 0.69, whereas with global equivalence ratio of 0.64, soot loading was almost zero. When the n-pentanol replacement increased to 20%, no soot was detected for both air flow rates, probably due to soot volume fractions well below the lower limits of laser-induced incandescence. The primary soot particle diameters covered a range from 25 to 50 nm mostly unaffected by the presence of n-pentanol in JetA-1.

Time Series and Spectral Analysis of Thermoacoustic Oscillations for Propane-Oxyfuel Combustion in a Swirl-Stabilized, Nonpremixed Combustor.

In this study, the time series data for sound pressure and heat release fluctuations were analyzed to establish the stability window of the propane-oxyflames under different operating conditions. The CO2 dilution level and the combustor power density were varied at a fixed global equivalence ratio to investigate the thermoacoustic instability of the flames. The phase difference between the fluctuations and the instantaneous Rayleigh index reveals whether the heat release and pressure fluctuations are coupled, which can result in the amplification of the sound pressure fluctuations. Results showed a negative Rayleigh index and uncoupled fluctuations at low (<40%) and high (>60%) CO2 dilution with coupled fluctuations and sound pressure amplification at intermediate CO2 dilution levels. A peak of varying magnitude appeared in the frequency domain at 465 Hz for both the heat release and pressure fluctuations in the coupled mode. The Strouhal number at different CO2 concentrations revealed a range of vortex-shedding frequencies (300-1000 Hz), suggesting that the coupled mode is vortex-induced. Phase space reconstruction for the sound pressure fluctuations was carried out and it is observed that although the pressure fluctuations are amplified in the coupled mode, limit cycle amplitudes have not been reached. The recorded coupling and uncoupling of the oscillations associated with flame-vortex interactions at certain CO2 concentrations provides a valuable insight on the combustor's dynamics and toward the development of nonpremixed-oxy-flames combustors.

Ethanol stratified ultra-lean combustion with hydrogen enrichment in a constant volume vessel

An experimental study was conducted to determine the effects of ignition timings, injection pressures, and hydrogen enrichment on ethanol stratified ultra-lean combustion. The combustion chamber was a constant volume vessel with an injection and ignition system. The ignition and combustion characteristics of ethanol spray with different ignition timings (0–50 ms after start of injection), injection pressures (5, 15, and 25 MPa), and hydrogen premixed equivalence ratios (0.05, 0.1, and 0.2) under ambient temperature 353 K and 1 atm. The direct injection of ethanol and hydrogen enrichment results in an increase in the stable ignition limit to an extremely lean condition: a global equivalence ratio of 0.15 (67% ethanol/33% H2). Furthermore, the probability of ignition first increased then decreased with the delay in ignition timings; rose with an increase in the injection pressure and hydrogen ratio. However, the delay in ignition timings and high injection pressures resulting in extremely lean zone in the constant volume vessel, which led to incomplete combustion. The addition of hydrogen not only stabilized ignition process but also improved the combustion characteristics. Stable ignition and combustion were achieved at early ignition timing (2–3 ms), moderate injection pressure (15 MPa), and a minimum global equivalence ratio of 0.2 (50% ethanol/ 50% H2).

Large eddy simulations of pilot-stage equivalence ratio effects on combustion instabilities in a coaxial staged model combustor

In this paper, the effects of pilot-stage equivalence ratio on combustion instabilities in a coaxial staged model combustor are investigated using the Wall-Modeled Large Eddy Simulation. The global equivalence ratio is maintained constant, and the Stratification Ratio of the first main-stage and the second main-stage is set to 1; the dynamic mode decomposition and system identification methods are employed to analyze the flame dynamics, velocity, heat release rate modes, and flame transfer function (FTF) of the model combustor under different pilot-stage equivalence ratios. The results show that when the pilot-stage equivalence ratio is 0.6, the oscillation amplitude of heat release rate (HRR) exceeds 7.5% of the global average HRR, and the velocity oscillation and the global HRR oscillation in the combustor are coupled. As the pilot-stage equivalence ratio increases to 0.8, the oscillation amplitude of HRR decreases to 2.5%, and the oscillation of velocity and global HRR in the combustor are decoupled. Furthermore, the maximum value of FTF decreases from 3.5 to below 1 with the increase in the pilot-stage equivalence ratio.

A comparison of carbon monoxide yields and particle formation at various global equivalence ratios in vitiated and under-ventilated conditions

There have been previous studies comparing experimental methods for the purpose of capturing gaseous yields at a range of global equivalence ratios. However, no work has investigated the capability of the open controlled atmosphere cone calorimeter for collecting such data where its two modes of operation are directly compared. The aim of this study is to compare carbon monoxide yields collected using vitiated and under-ventilated modes of atmospheric control in order to identify the preferable method of replicating carbon monoxide yields reported from larger scale enclosure fire experiments. Cone irradiances of 30, 50 and 65 kW/m2 were applied to PMMA and plywood samples. Vitiated tests were conducted using a mixed air/diluent gas, with an inflow rate of either 100, 150 or 180 L/min, resulting in a reduced oxygen concentration of 17.5 vol. %. Under-ventilated tests were conducted using flow rates of 5, 10 and 20 L/min in an air atmosphere. Particle formations and emissions were also measured using a particle analyser and have been reported herein. Results indicate that the under-ventilated mode of equivalence ratio control offers a more promising method of capturing species yields with favourable comparisons to other bench scale methods.

The effect of 1-octanol blending on the multi-stage autoignition of conventional diesel and HVO fuels

Alternative fuels can reduce the carbon intensity of diesel engine use, and a detailed understanding of the combustion process for these fuels can minimize resulting fuel consumption and emissions. 1-Octanol has been considered as an alternative diesel fuel, but little work evaluating its combustion process in detail exists. In this work, ignition of 1-octanol, both pure and in blends with conventional and renewable diesel base fuels, is analyzed in a constant volume combustion chamber and a modified Cooperative Fuels Research Octane Rating engine operating in homogeneous charge compression ignition combustion mode. 1-Octanol blending reduced the derived cetane number of either base fuel, but modest increases in chamber temperature at injection could compensate for even dramatic reductions in derived cetane number. It was further found that greater amounts of low-temperature heat release were required to reach main ignition as the 1-octanol blending volume fraction was increased in the constant volume combustion chamber. This finding is attributed to the changing local equivalence ratio, as the reacting spray diffuses further into the chamber and approaches the (lean) global equivalence ratio as the ignition delay grows. Finally, it was found that, despite lowering the derived cetane number, 1-octanol blending enhanced the reactivity of the conventional diesel fuel observed in the variable compression ratio engine. This finding is attributed to the higher rates of hydrogen abstraction from the weaker average C–H bonds in 1-octanol, as hydrogen abstraction governs intermediate temperature heat release. These findings clarify the effect of 1-octanol on physical processes and the various chemical regimes of heat release in compression ignition. The dependence of the effects of 1-octanol blending on the base fuel are emphasized, as these can inform the deployment of 1-octanol as a diesel blendstock. Application of these results can support the optimization of conventional combustion strategies for use with 1-octanol, as well as the development of 1-octanol-fueled low-temperature combustion strategies with the associated promise of very low criteria pollutant emissions.

Dynamic response characteristics of flame during condition transition in a cavity-based scramjet combustor

Experimental study is performed to investigate the dynamic response characteristics of the flame when global equivalence ratio increases suddenly during the condition transition in a cavity-based scramjet combustor. High-enthalpy supersonic flow enters the combustor on conditions that Mach number, stagnation temperature, and total pressure are 2.52, 1486 K, and 1.6 MPa, respectively. In this paper, more attention is paid to the characteristics of flame evolution after sudden increase of fuel injection pressure and combustion characteristic when the injection pressure gets stabilized again. Three flame modes after condition transition have been observed and analyzed according to the transient chemiluminescence image captured by high-speed photography. There are two kinds of stable flame modes after transition mission at a low injection pressure difference, which are mainly reflected in the distribution of flame and intensity of visible light. Whereas, once the injection pressure difference exceeds a critical value, intense periodic flame flashback phenomenon begins to appear in the combustor. It can clearly capture the back and forth motion of the flame in the combustor, along with the local blowout and reignition phenomenon. Furthermore, the dominant frequency of flame flashback decreases and contrast of combustion intensity becomes more obvious as the injection pressure difference increases.

Knock Mitigation and Power Enhancement of Hydrogen Spark-Ignition Engine through Ammonia Blending

Hydrogen and ammonia are primary carbon-free fuels that have massive production potential. In regard to their flame properties, these two fuels largely represent the two extremes among all fuels. The extremely fast flame speed of hydrogen can lead to an easy deflagration-to-detonation transition and cause detonation-type engine knock that limits the global equivalence ratio, and consequently the engine power. The very low flame speed and reactivity of ammonia can lead to a low heat release rate and cause difficulty in ignition and ammonia slip. Adding ammonia into hydrogen can effectively modulate flame speed and hence the heat release rate, which in turn mitigates engine knock and retains the zero-carbon nature of the system. However, a key issue that remains unclear is the blending ratio of NH3 that provides the desired heat release rate, emission level, and engine power. In the present work, a 3D computational combustion study is conducted to search for the optimal hydrogen/ammonia mixture that is knock-free and meanwhile allows sufficient power in a typical spark-ignition engine configuration. Parametric studies with varying global equivalence ratios and hydrogen/ammonia blends are conducted. The results show that with added ammonia, engine knock can be avoided, even under stoichiometric operating conditions. Due to the increased global equivalence ratio and added ammonia, the energy content of trapped charge as well as work output per cycle is increased. About 90% of the work output of a pure gasoline engine under the same conditions can be reached by hydrogen/ammonia blends. The work shows great potential of blended fuel or hydrogen/ammonia dual fuel in high-speed SI engines.