Methanol is considered to be one of promising alternative fuels. For direct-injection (DI) methanol-gasoline fueled engines, the injection of fuels into the cylinder and the subsequent evaporation and mixing processes are of great significance. So far, quantitative measurements of fuel concentrations of both liquid and vapor phases in the evaporating spray of multi-component fuels are still difficult. In this study, a novel approach based on the laser induced breakdown spectroscopy (LIBS) is developed to quantitatively measure the fuel concentrations and evaporation ratio in DI methanol-gasoline blend sprays in a constant volume vessel. Three atomic lines of H656, N746 and O777 are employed to quantify the concentrations of methanol and n-hexane. The calibrations between two peak intensity ratios and corresponding atom number ratios are established for four kinds of blend fuels (M0, M15, M50, M100). The results show a good consistency of calibration curves for different fuels, suggesting that LIBS is independent of molecular species and capable of measurements of multi-component fuels. For the measurements of evaporation ratio, the LIBS and high-speed droplet microphotography were simultaneously conducted in the droplet flow environment to determine the intensity of C2 band and liquid fuel concentration, respectively. The high fuel-specific correlations are confirmed. The estimated measurement uncertainties of total fuel concentration are <6%, and the measurement uncertainties of evaporation ratio are reduced with the increase of the radial distance. Finally, the component concentration and evaporation ratio in sprays of methanol-n-hexane are quantitatively measured, and then the evaporation processes as well as mixing characteristics of different blend sprays are compared and analyzed. The results are valuable for building the relevant numerical models and developing the combustion systems of engines fueled with methanol-gasoline blend fuels.

A challenge of application of ammonia in combustion devices is due to its low chemical reactivity. Addition of hydrogen into ammonia can increase drastically the premixed laminar flame speed of the blends. In practical combustion devices, however, turbulent non-premixed jet flames are usually employed, while researches on ammonia jet flames are scarce. In this study, firstly, the premixed laminar flame speeds of CH4 and the 50%NH3+50%H2 blend in volumetric fraction are investigated by the Schlieren imaging. It is confirmed that the laminar flame speeds of the 50%NH3+50%H2 blend are remarkably faster than those of CH4 for equivalence ratios above 1.1, while they are similar for lower equivalence ratios. Then, the Schlieren imaging and planar laser induced fluorescence imaging of hydroxyl of the burning turbulent gas jets are conducted. It is found that the flame propagation speed and OH intensity in the CH4 jets are remarkably higher than those in the 50%NH3+50%H2 jet. To unveil the mechanism, the equivalence ratio and fuel mole fractions along the jet axis are investigated by the laser induced breakdown spectroscopy. The equivalence ratios of the 50%NH3+50%H2 blend are significantly lower than those of CH4. Compared to that of CH4, the volumetric flow rate of 50%NH3+50%H2 blend is higher for the choked flow of the under-expanded jets. Therefore, the lower equivalence ratios are attributed to the higher stoichiometric fuel-air ratio of the 50%NH3+50%H2 blend. Stratification of NH3 and H2 due to their distinct molecule transfer properties is experimentally observed, which probably lowers the flame propagation speed in the jet. These findings are expected to be a valuable reference for development of simulation models and design of practical combustion devices with zero-carbon fuels.

The ambient gas entrainment of the diesel spray affects the fuel evaporation and the subsequent ignition and pollutant formation processes. In this work, the gas entrainment process in the breakup length of the diesel spray is investigated by the high-speed micro-particle tracking velocimetry (micro-PTV) technique. With the high frequency of the micro-PTV technique of 28 kHz, the images of the tracer particle and the spray boundary are captured simultaneously in one shot. The experimental results show that during the quasi-steady and transient states, the gas entrainment velocity increases as the injection pressure rises. However, the ambient density has no impact on the entrainment velocity in the breakup length. During the quasi-steady state, the local gas entrainment rate shows a linear dependence on the spray axial distance (z), and the cumulative gas entrainment rate shows a z2 dependence. A theoretical zero-dimensional (0-D) model is proposed to estimate the cumulative gas entrainment rate during the quasi-steady state, and it is validated with the experimental results. The results suggest that the gas entrainment in the breakup length is probably dominated by the breakup process of the liquid core.

The direct-injection of gaseous ammonia is a possible way to fuel engines to meet the scenario of zero-carbon emission. In this study, for the first time, the injection characteristics and fuel-air mixing process of ammonia jets are investigated. A Schlieren system is employed to characterize the macroscopic behavior of ammonia jets including the tip penetration and jet angle. Besides, the fuel concentrations of ammonia jets are quantitatively measured by LIBS to investigate the fuel-air mixing processes. According to the time evolutions of jet tip penetration, the three-stage behavior, namely the t, t0.5 and (t-tau)0.25 dependence corresponding to the early stage of injection, quasi-steady stage and after two times of injection duration respectively, are proposed and the mechanisms are discussed in detail. The effects of both injection and ambient pressures on the tip penetration and jet angle, as well as the fuel concentration distributions are investigated. The increased injection pressure leads to increases in the tip penetration and fuel concentration but decreases in the jet angle. The effects of ambient pressure are opposite, indicating that the ammonia jet development is governed by the injection-toambient pressure ratio. The comparisons between ammonia and methane jets are conducted. The results show they have quite similar tip penetrations and jet angles, as well as the nearly equivalent fuel mole fractions at the jet axis. However, the equivalence ratios in ammonia jets are significantly lower than those in methane jets, leading to the distinct flammable mixture distributions in ammonia and methane jets.

Ammonia, as a zero-carbon fuel, is drawing more and more attention. The major challenge of using ammonia as a fuel for the combustion engines lies in its low chemical reactivity, and therefore more fundamental researches on the combustion characteristics of ammonia are required to explore effective ways to burn ammonia in engines. In this study, the laminar burning characteristics of the premixed ammonia/hydrogen/air mixtures are investigated. In the experiment, the laser ignition was used to achieve stable ignition of the ammonia/air mixtures with an equivalence ratio range from 0.7 to 1.4. The propagating flame was recorded with the high-speed shadowgraphy. Three different processing methods were introduced to calculate the laminar burning velocity with a consideration of the flame structure characteristics induced by the laser ignition. The effects of initial pressure (0.1 MPa-0.5 MPa), equivalence ratio (0.7-1.4), hydrogen fraction (0-20%) on the laminar burning velocity were investigated under the initial ambient temperature of 360 K. The state-of-the-art kinetic models were used to calculate the laminar burning velocities in the CHEMKIN-pro software. Both the simulation and experimental results show that the laminar burning velocity of the ammonia mixtures increases at first, reaches the peak around 0 of 1.1, and then decreases with the equivalence ratio increasing from 0.7 to 1.4. The peak laminar burning velocities of the ammonia mixture are lower than 9 cm/s and are remarkably lower than those of hydrocarbon fuels. The laminar burning velocity of the ammonia mixture decreases with the increase of the initial ambient pressure, and it can be drastically speeded up with the addition of hydrogen. While the models except for those by Miller and Bian can give reasonable predictions compared to the experimental results for the equivalence ratio from 0.7 to 1.1 in the ammonia (80%)/hydrogen (20%)/air mixtures, all the kinetic models overpredict the experiments for the richer mixtures, indicating further work necessary in this respect. (c) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Plasmas are widely used in engines as ignition sources and combustion assistances. Quantitative and qualitative analyses of the effects of plasmas on ignition are of great importance for improving engine performance, but with challenges. In this study, the vibrational and rotational temperatures are calculated based on the N-2 second positive molecular emission spectra, with different discharge powers, ambient pressures, gas compositions, and spark plug gap sizes. The spatial distribution of the rotational temperature is also investigated. With an increased discharge power, the vibrational and rotational temperatures increase, while the difference between the vibrational and rotational temperatures decreases. As the pressure ambient increases from 0.3 to 5.0 bar, the vibrational temperature decreases initially and increases subsequently. The rotational temperature increases with the increased pressure, while the temperature difference decreases. The gas composition and gap size greatly affect the vibrational and rotational temperatures. The rotational temperature increases with the enlarged gap size, and the difference between the vibrational and rotational temperatures decreases. For the spatial distribution of the rotational temperature in the spark gap, the highest rotational temperature occurs near the center of the spark gap. Meanwhile, the rotational temperature near the central electrode is higher than that near the ground electrode. (C) 2021 Elsevier Ltd. All rights reserved.

A novel ammonia/hydrogen dual-fuelled Linear Joule Engine Generator (LJEG) is developed for medium to large scale power generations and electrification of ship propulsion systems. The characteristics of premixed ammonia/hydrogen combustion of the LJEG are investigated through chemical kinetic modelling. Three representative mechanisms are compared based on their accuracy of reproducing experimental results. With robust combustion and low NOx emission as the primary targets, laminar burning velocity, ignition delay and flame species concentration are investigated over a wide range of equivalence ratio (0.8−1.6), hydrogen blending ratio (0.0−0.6), oxygen content (0.21−1.00), inlet temperature (300K−700K) and pressure (1bar−20bar). Rate of production (ROP) analysis is carried out to gain in-depth understanding of critical NO production and consumption pathways. The results indicate that an equivalence ratio around 1.1 is beneficial for both combustion robustness and NOx emission reduction. Both adding hydrogen in the fuel (40%Vol) and enriching oxygen in the oxidizer (60%Vol) promote burning velocity to the similar level of methane (37cm/s). Explicit reduction of NO emission is observed when pressure increases, which can be attributed to the combination of NHi radicals. The findings show the potential of the ammonia and hydrogen fuelled LJEG for ultra-low emission power generation.

Methanol is considered to be one of promising alternative fuels, and direct-injection (DI) of methanol-gasoline blends is attracting more and more interest due to the thermal efficiency advantages.However, quantitative measurements of component concentrations in DI methanol-gasoline blend sprays are still difficult.In this study, a novel approach based on the laser induced breakdown spectroscopy (LIBS) is developed to quantitatively and simultaneously measure the gas concentrations of n-hexane and methanol in methanolgasoline blend sprays in a constant volume vessel.Firstly, the calibrations between peak intensity ratios (PIR) of H656/N745 and atomic number ratios (ANR) of H/N, and PIR of O777/N746 and ANR of O/N are established respectively for three kinds of fuels (M0, M50, M100).The results show a good consistency of calibration curves for different fuels and different ambient pressures, suggesting the potentials of LIBS to measure multi-component fuels even at varied-pressure conditions.Then, the high-speed shadowgraph and diffused back-illumination method are employed to simultaneously image the entire spray and the liquid phase, and therefore determine the LIBS measuring positions.Finally, the fuel concentration measurements in M0 and M15 sprays are conducted and the radial distribution and the effects of ambient pressure are investigated.It is found that the blending fraction in M15 spray is increased with the radial distance increasing due to the lower boiling temperature and higher vapor diffusivity of methanol.The equivalence ratios in both M0 and M15 sprays are reduced with the increased ambient pressure, which can be attributed to the larger air density while nearly constant entrainment speed at higher ambient pressure.These results are believed to be valuable for development of numerical models and design of DI combustion systems.

Fuel concentration information is critical to the development of high-efficiency combustion system of direct-injection natural gas engines. In this study, a one-dimensional (1-D) measurement approach based on laser induced breakdown spectroscopy (LIBS) is developed to quantitatively measure 1-D fuel concentration distributions in gas jets at high ambient pressures. Except for the spectral dimension, another dimension of the intensified charge coupled device (ICCD) is employed for the spatial resolution of measurements. The effects of laser energy and ambient pressure on plasma dynamics are investigated to explore the feasibility of 1-D LIBS in concentration measurements at high ambient pressure. Since the peak intensity ratio (PIR) of H656/N746 shows significant spatial variations in the plasma volume, a 1-D calibration strategy is developed by establishing respective calibration curves at different spatial positions to quantify the fuel concentration distributions. The measurement uncertainties are evaluated, including both the systematic and random errors. The temporal variations of the 1-D fuel concentration distributions in methane jets are presented, and the shot-to-shot fluctuations are analyzed in detail. The results show that the coefficient of variation of equivalence ratio is lowest at the end of injection (EOI), suggesting that ignition at the EOI could enable a relatively stable flame kernel formation. Along the radial direction of the jet, a high ignitable mixture percentage appears at approximately 3.0 mm from the jet axis at the EOI, suggesting an optimal ignition position. The results reveal that the 1-D LIBS could be a powerful tool for calibration of numerical models and optimal designs of combustion engines taking advantage of high-pressure gas jets.

For spark-ignition (SI) engines, downsizing, exhaust gas recirculation and lean burn are promising methods for meeting more stringent emission regulations and lower fuel consumption requirements. With these technologies, high density and high flow velocity around the spark plug at spark timing bring severe challenges for stable ignition. The ignition processes could be affected by breakdown phases and restrikes under high-density conditions. A quantitative evaluation for the breakdown phase is of great significance especially for high power density engines. In this study, the experiments with pressure up to 40 bar and temperature up to 450 K are established to evaluate the breakdown voltage and current, with different spark plug gaps. The breakdown voltage is related to the ambient density, spark plug gap distance and gas composition. The coil parameters have little effects on the breakdown voltage. The breakdown current shows positive correlation with the breakdown voltage, and is also affected by the coil characteristics. The breakdown voltage and current show the nonlinear correlations with the pressure and gap distance. With the increase of pressure or gap distance, the increase rates of the breakdown voltage and current decrease. The fuel concentration and species have little effects on the breakdown voltage when the fuel concentration is applicable for engine operations. Finally, a novel expression of the breakdown voltage is constructed as a function of ambient pressure, temperature and spark plug gap distance. The newly developed expression is valuable for simulating spark ignition processes for a wide range of ambient density for SI engines.

In order to support the development of a novel linear engine generator (LEG), the characteristics of ammonia/hydrogen premixed combustion are studied by using a detailed chemical kinetics mechanism. The ammonia combustion mechanism is identified among several mechanisms and validated with published experimental data. A parametric analysis is carried out under LEG typical working conditions to study the effects of equivalence ratio (0.80–1.60), hydrogen blending ratio (0.0–0.6), initial temperature (300–700 K) and initial pressure (1–20 bar) on premixed laminar flame speed, ignition delay and key flame species concentrations. It is shown that an equivalence ratio of around 1.10–1.20 is beneficial to both ammonia flame stability and lower NOx emission. Ignition delay is reduced with the increase in hydrogen blending ratio, initial temperature and initial pressure. At a certain initial temperature and initial pressure, the effects of hydrogen blending ratio can be negligible for over 50% hydrogen in the fuel. Under higher pressure (>10 bar), the initial pressure has a minor influence on the ignition delay reduction. It is also found that the high-pressure high-temperature environment contributes to reducing NO emission considerably in ammonia/hydrogen combustion, which implies the potential of a low NOx LEG fuelled by ammonia/hydrogen.

The high-speed micro-particle tracking velocimetry (micro-PTV) technique was applied to investigate the near-nozzle air entrainment into the high-pressure diesel spray at 28 kHz by analyzing the 1024 x 768 pixel(2) image with the spatial resolution of 1.92 micrometer per pixel. The gas velocity component normal to a control surface along the spray boundaries was measured during the entire injection period, including both the start-of-injection (SOI) and end-of-injection (EOI) transients. Thus, the unique micro-PTV technique enables the instantaneous air entrainment rate to be evaluated as a function of time in one shot. The spray jets were injected by a single-hole nozzle with the ambient gas density of 18 kg/m(3) and the injection pressure of 120 MPa. The air entrainment and spray angle were simultaneously measured and their correlations were analyzed. The spray angle shows a double peak profile as a function of time, and it is remarkably larger during the SOI and EOI transients than during the quasi-steady state. The air entrainment velocity exhibits a close dependence upon the spray angle. It is the increased spray angle that leads to the enhanced air entrainment rate during the SOI and EOI transients.

The start of injection (SOI) transition processes present challenges for diesel spray combustion simulations by computational fluid dynamics (CFD). In the previous studies, claims are usually placed on the experimental uncertainties or numerical methods such as grid size selection and sub-model comparison. Until now, few studies have been attempted to revise the fuel injection rate (FIR) profile used as the fuel input boundary condition for numerical models. It this paper, considering the difference of in-nozzle flow between the Bosch long-tube method and actual injection, the ''Velocity Revised'' and ''Mass Revised'' FIR profiles are formulated to keep the fuel injection velocity and fuel injection mass consistent with the actual injection, respectively. After a brief description of the experimental and numerical methods, the effectiveness of the newly developed two FIR profiles and the uncorrected and the Engine Combustion Network (ECN) FIR profiles used as boundary conditions for CFD simulation is validated against the experimental data under both the non-reacting and burning conditions. The simulations with the uncorrected FIR profile show remarkably lower spray and flame penetrations at the initial stage of injection, while the simulations with the newly developed two FIR profiles exhibit almost identical spray and flame penetrations with the experimental data. The accuracy of the ECN FIR profile simulation is between the above two cases. These results are believed to be valuable reference for researchers to evaluate whether the potential shortcoming in diesel spray combustion simulations is from the variation in boundary condition or the code itself.

A novel ambient tracer LIF technique with addition of tracer into ambient gas is proposed in this study to quantitatively measure the two dimensional (2-D) fuel concentration distribution in high-pressure gas jets. Firstly, the fundamental principle of the ambient tracer-LIF technique is described, and the equation of fuel concentration in a jet is deduced from the ambient fluorescence intensity. In the ambient tracer-LIF technique, the on-site calibration can be performed, and much more degrees of freedom in selection of tracers or target fuels can be achieved in principle. Then, the experiments are carried out with the high pressure gas injected into the environment doped with acetone in an optical accessible constant volume vessel. The fluorescence in the ambient is induced by using the 266 nm laser, and is recorded by the ICCD camera. The dedicated image processing is implemented to reduce the uncertainty and obtain the quantitative fuel concentration distribution in the jet. The uncertainties of the measurement results are evaluated, including both the systematic and random errors. The uncertainty propagation analysis shows that the measurement uncertainty increases with the local fuel concentration decreasing, and at the boundary regions of the jet, where the mixture is over lean, the relative uncertainty goes up to 26%. Finally, the temporal variations of the fuel concentration distribution in the gas jet are presented, and the effects of injection pressure are discussed. These results are believed to be valuable for development of numerical models and combustion system designs. (C) 2019 Elsevier Ltd. All rights reserved.

Increasing discharge energies and employing advanced discharge strategies have been deemed to be effective methods for improving the ignition processes, especially for diluted or lean combustion. However, so far knowledge on the relevant mechanism is far from adequate. In particular, the effects of the plasma produced between the spark plug electrodes on spark ignition processes need to be further clarified. The plasma temperatures are important as they are closely related to the chemical reaction rate. In this study, the vibrational and rotational temperatures of the discharge plasma are quantitatively evaluated by a time series of spectral measurements with different discharge energies and strategies in air under atmospheric pressure, based on the N-2 second positive molecular emission spectra. The vibrational and rotational temperatures show a perfect consistent trend with the release rates of delivered energy to the spark plug gap. This indicates that the two temperatures can be enhanced by the higher energy release rates and can be effectively controlled by different discharge strategies. The vibrational and rotational temperatures measured in this study are in the range of 3700-4300 K and 1400-2600 K, respectively. The temperature differences between the vibrational and rotational temperatures exceed 1600 K, increasing with the energy release rate decreasing. This indicates that the spark discharge plasma is in a state of non-thermal equilibrium with the existence of the discharge energies under the non-flow conditions. These results would be a reference to further develop the advanced discharge strategies and improve the ignition stability.

Laser induced breakdown spectroscopy (LIBS) is an effective technique to obtain quantitative concentration information for the fuel injection and combustion processes. Practical applications of fuel injection and combustion usually involve a wide range of ambient pressures and temperatures, but understanding of the influence of these parameters on LIBS is far from adequate. The objective of this paper is therefore to investigate the pressure and temperature effects on LIBS over a wide range for gas concentration measurements, and further correct these effects to improve the measurement accuracy. The calibrations at various ambient pressures show that the peak intensity ratios (PIRs) exhibit better linear correlations with the equivalence ratios than the area intensity ratios (AIRs) for H656/N746. The intensities of H656 and N746 are remarkably influenced by the gas pressure due to the combined effects of varied gas density and self-absorption. While the PIR of H656/N746 depends greatly on gas pressures from 0.3 to 1.0 bar, the gas pressure effects on the PIR of H656/N746 are very slight at gas pressure above 1.0 bar. The effects of gas temperature on the PIR of H656/N746 are investigated at both constant gas pressure and constant gas density. The results show that the temperature differences between the calibrations and test conditions would significantly degrade the measurement accuracy, especially at low gas pressures. Then, a novel method to correct the gas temperature effects on LIBS for gas concentration measurements is proposed, based on a linear correlation between the PIR of H656/N746 and the gas temperature, with the gas temperature estimated by the full width at half maximum (FWHM) of H656.

Laser induced breakdown spectroscopy (LIBS) enables quantitative measurements of the equivalence ratio in flames, spray and gas jets. Although there are always considerable temperature differences between the calibrations and the LIBS measurements in flames and fuel jets, the temperature effects on LIBS were rarely mentioned and not corrected in the previous LIBS measurements. In this study, a novel method to simultaneously measure the equivalence ratio and the gas temperature by LIBS in a single laser shot is proposed. This method can correct the temperature effects based on the linear correlation between the peak intensity ratio (PIR) of atomic lines and the gas temperature. The PIR of H656/N746 and the full width at half maximum (FWHM) of H656 are employed to establish calibration curves with the equivalence ratio and the gas temperature, respectively, and both of calibrations show high coefficients of determination (R2). Then, the measurement accuracy and precision of the proposed method are discussed in detail. The total uncertainties in measurements of equivalence ratio and gas temperature are estimated by 5.6% and 5.8%, respectively. Finally, the simultaneous measurements of equivalence ratios and gas temperatures in methane jets at different positions and at different injection pressures are conducted. The comparison of equivalence ratios before and after correction of the temperature effects indicates that the accuracy of the fuel concentration measurements by LIBS would be remarkably degraded if the temperature variations were neglected.