HCCI燃烧过程化学发光光谱研究

在一台经改装单缸光学发动机机上,进行不同喷油策略和进气温度条件下均质压燃(HCCI)燃烧化学发光光谱实验研究。实验保证循环供油量一定,燃用正庚烷作为燃料,转速600 r.min-1,进气压力0.1MPa,控制2个不同的进气温度:95和125℃。化学发光光谱研究结果表明,低温反应阶段化学发光很弱,主要源于甲醛光谱;低温反应后期-负温度系数区-高温反应初始阶段主要发光来源还是甲醛光谱;高温反应阶段发光主要来源于CO—O*连续谱,同时在CO—O*连续谱上出现OH,HCO,CH,HCHO谱峰;高温反应后期化学发光明显减弱。与-30°ATDC喷油相比,-300°ATDC喷油时CO—O*连续谱发光强度更大,HCO和OH生成量更多,燃烧反应进行程度更深。较高进气温度下CO—O*连续谱发光强度更大,HCO和OH生成量更多。     

Laser-assisted homogeneous charge ignition in a constant volume combustion chamber

Homogeneous charge compression ignition (HCCI) is a very promising future combustion concept for internal combustion engines. There are several technical difficulties associated with this concept, and precisely controlling the start of auto-ignition is the most prominent of them. In this paper, a novel concept to control the start of auto-ignition is presented. The concept is based on the fact that most HCCI engines are operated with high exhaust gas recirculation (EGR) rates in order to slow-down the fast combustion processes. Recirculated exhaust gas contains combustion products including moisture, which has a relative peak of the absorption coefficient around 3 μm. These water molecules absorb the incident erbium laser radiations (λ=2.79 μm) and get heated up to expedite ignition. In the present experimental work, auto-ignition conditions are locally attained in an experimental constant volume combustion chamber under simulated EGR conditions. Taking advantage of this feature, the time when the mixture is thought to “auto-ignite” could be adjusted/controlled by the laser pulse width optimisation, followed by its resonant absorption by water molecules present in recirculated exhaust gas.     

Three-stage auto-ignition of n-heptane and methyl-cyclohexane mixtures at lean conditions in a flat piston rapid compression machine

One approach to enhancing the thermal efficiency of combustion systems is to burn fuels at ultra-lean conditions (equivalence ratio below 0.5). It has been recently reported that the auto-ignition of some hydrocarbon fuels, under specific temperature, pressure, and mixture conditions, releases heat in three distinctive stages. The three auto-ignition stages can be divided as a first low-temperature auto-ignition stage with conventional low temperature, and a high-temperature stage separated into two sub-stages. This study presents ignition delay time measurements of n-heptane and methyl-cyclohexane (MCH) mixtures in a flat piston rapid compression machine (RCM) under ultra-lean conditions. It provides experimental evidence of three-stage auto-ignition. This phenomenon of delayed high-temperature heat release is seldom reported in the literature and this is the first time to be reported for these types of fuels. The experiments cover two binary n-heptane/MCH mixtures of 15/85 and 70/30 by volume, pressures of 11 bar and 16 bar, temperature range of 700 to 900 K, and equivalence ratio of 0.4. The RCM optical access was utilized for high-speed chemiluminescence imaging. Detailed chemical kinetic simulations in a homogenous batch reactor with variable volume were conducted to further interrogate the three-stage auto-ignition phenomenon. Chemiluminescence shows that three-stage auto-ignition occurs in the adiabatically compressed end-gas, which indicates that this phenomenon is chemically-driven and is not induced by a thermal stratification in the RCM experiments. The model predicts the features of three-stage auto-ignition, which were experimentally observed at temperatures approximately below 750 K. As expected, significant discrepancies are observed in the ignition delays of experiment and simulation in the negative temperature coefficient (NTC) region. The simulation of the n-heptane/MCH 70/30 mixture shows better agreement with experiments in the Positive Temperature Coefficient (PTC) region compared to the 15/85 mixture.     

High-speed imaging of the ignition of ethanol at engine relevant conditions in a rapid compression machine

The rapid compression machine (RCM) is a great tool for investigating fuel properties under engine relevant conditions (high-pressures, low temperatures). The most common diagnostics is measuring the pressure over time and determining the ignition delay time (IDT). In this study, for the first time, the OH* luminescence of ethanol/air mixture is measured within an RCM experiment at 15 and 20?bar for Φ?=?0.5. Combining the common pressure measurements with the simultaneously recorded high-speed images (up to 74.5?kHz framerate) gives a first insight into understanding the ignition modes and the corresponding pressure traces. At 74.5?kHz, in contrast to findings in literature, the ethanol ignition did not show to be purely homogeneous. Four different propagating fronts of OH* luminescence have been recorded. Besides a flame kernel and a detonation-like ignition front two further fronts prior to main ignition have been observed. The propagating speeds of the fronts have been determined and depend on the overall IDT.     

Separation of a molecular mixture in a thermodiffusion column under supercritical conditions

The process of separation of carbon dioxide and hexafluorethane mixture in a thermodiffusion column under supercritical conditions is studied. It is shown experimentally that the separation efficiency of thermodiffusion column increases when passing from the gas phase to supercritical conditions.     

Numerical simulation of the ignition of liquid fuel with a limited-energy source under turbulent flow conditions

The ignition of a typical liquid fuel with a limited-energy source, a small metal particle heated to high temperature is numerically simulated with consideration given to the possible turbulization of the fuel vapor flow. The dependences of the integral ignition characteristics on the key parameters of the local heat source are established. The integral ignition characteristics, as well as the fields of fuel vapor concentrations and velocities predicted by models accounting for the laminar and turbulent modes of the vapor-oxidizer mixture flow are compared.     

Gasless combustion of a mixture with an extremely low caloricity under heat loss conditions

The gasless combustion of a mixture with an extremely low caloric value in a cylindrical channel is numerically simulated. Because of an intense heat loss, on the surface of the reaction front emerge waves, which lead to cyclic changes in the chemical reaction rate, which acquire axial symmetry in the steady-state regime. In the ignition stage, a ring of reaction sites arises at the lateral surface of the sample. In the stage of depression, combustion persists on circular protrusions of the front, whereas a site of products is formed is the central part of the sample.     

Effective gyromagnetic ratios in a mixture of noble gases with artificial nuclear magnetization

The precession of nuclear magnetic moments for a noble gas in an external magnetic field upon the laser pumping of nuclear magnetization is considered. A shift of the nuclear magnetic resonance for a mixture of noble gases with different gyromagnetic ratios of nuclei is observed.     

Vortex sink under conditions of a thermal choking with regard to the real properties of gases

Changes in the formulation of the problem for a vortex source and a vortex sink upon taking into account the change in the heat capacity and the adiabatic exponent for a diatomic gas (for the example of air) in response to an increase in the temperature from 300 K to a few thousands of Kelvin are discussed. A thermal choking is studied for a vortex sink, and critical values of the energy parameter are calculated. It is shown that the minimal radius of the vortex sink decreases upon a heat release. Similarity parameters including the dimensionless circulation (or mass flow), the energy parameter, and the position and thickness of the heat-release region are varied. Errors of the gas model that assumes constant heat capacities and a constant adiabatic exponent are estimated.     

Numerical study of the ignition behavior of a post-discharge kernel in a turbulent stratified crossflow

Ensuring robust ignition is critical for the operability of aeronautical gas-turbine combustors. For ignition to be successful, an important aspect is the ability of the hot gas generated by the spark discharge to initiate combustion reactions, leading to the formation of a self-sustained ignition kernel. This study focuses on this phenomena by performing simulations of kernel ignition in a crossflow configuration that was characterized experimentally. First, inert simulations are performed to identify numerical parameters correctly reproducing the kernel ejection from the ignition cavity, which is here modeled as a pulsed jet. In particular, the kernel diameter and the transit time of the kernel to the reacting mixture are matched with measurements. Considering stochastic perturbations of the ejection velocity of the ignition kernel, the variability of the kernel transit time is also reproduced by the simulations. Subsequently, simulations of a series of ignition sequences are performed with varying equivalence ratio of the fuel-air mixture in the crossflow. The numerical results are shown to reproduce the ignition failure that occurs for the leanest equivalence ratio ($?=0.6$). For higher equivalence ratios, the simulations are shown to capture the sensitivity of the ignition to the equivalence ratio, and the kernel successfully transitions into a propagating flame. Significant stochastic dispersion of the ignition strength is observed, which relates to the variability of the transit time of the kernel to the reactive mixture. An analysis of the structure of the ignition kernel also highlights the transition towards a self-propagating flame for successful ignition conditions.     

Simulation of the ignition of liquid fuel with a local source of heating under conditions of fuel burnout

The processes of heat and mass transfer with phase transitions and chemical reactions in the ignition of liquid fuel by a local source of heating, a hot metal particle, under conditions of fuel burnout are studied. The influence of liquid fuel burnout on the ignition characteristics is analyzed, and the results of investigation of the extent of influence of this factor for solid and liquid condensed materials under conditions of local heating are compared.     

Thermal explosion of a reactive mixture under conditions of oscillating ambient temperature

The problem of the thermal explosion of a finite volume of a reactive material in a medium with harmonically oscillating temperature is solved in the classical formulation. A kind of resonance is shown to arise when the oscillation period is commensurate with the adiabatic induction period of thermal explosion at the mean ambient temperature. At both high and very low oscillation frequencies, the critical condition parameter and induction period are only slightly affected by ambient temperature oscillations. By contrast, at moderately low frequencies, even small-amplitude oscillations of ambient temperature can strongly influence the critical condition and, especially, induction period of thermal explosion.     

Water tank studies of plume rise and diffusion in a stably stratified layer under calm conditions

Summary Water tank experiments were conducted to investigate plume rise and diffusion of gases discharged from stacks under calm conditions with field wind velocity less than 0.4 m/s with stable thermal stratification. Similarity rules of scaled model experiments were obtained from nondimensional analysis of the fundamental equation. The densimetric Froude number of a plume and the buoyancy ratio of the plume and the atmosphere were varied during the water tank experiments while plume rise height, plume thickness, etc. were measured. It was found that the experimental results could be expressed in terms of these two dimensionless parameters. Papper presented at the GNFAO/EURASAP Meeting, Turin, September 1989. To speed up publication, proofs were not sent to the authors and were supervised by the Scientific Committee.     

Onsager-Casimir reciprocity relations for a mixture of rarefied gases interacting with a laser radiation

The behavior of a mixture of optically excitable and inactive gases in the field of a laser radiation is considered from the viewpoint of nonequilibrium thermodynamics. The kinetic coefficients satisfying the Onsager-Casimir reciprocity relations are found from general properties of the Boltzmann equation, boundary condition, and terms describing the gas-radiation interaction. Various kinetic phenomena induced by the laser radiation are coupled with corresponding cross effects.     

Emission characteristics of a glow discharge in a mixture of heavy inert gases with iodine vapor

The results of a systematic investigation of the emission characteristics of a low-pressure UV excimer-halogen lamp pumped by a longitudinal dc glow discharge are presented. The discharge was initiated in mixtures of heavy inert gases with iodine vapor at a total pressure of 100–2000 Pa and a power deposited into the plasma of 10–100 W. Current-voltage characteristics of the glow discharge and emission spectra of the plasma in the region of 190–360 nm are studied. The radiation intensity at the resonance line of the iodine atom (206.2 nm) and the intensity at the peaks of the XeI(B-X) (253 nm) and I₂(B-X) (342 nm) emission bands are analyzed as functions of the pressure and partial composition of the mixtures of Ar, Kr, and Xe with iodine vapor, as well as the electric power of the glow discharge. The most efficient gas mixtures are determined for an electric-discharge UV iodine vapor lamp with continuous-wave emission and a long service life before a change of the mixture is required.     

Modeling and experimental investigation of methylcyclohexane ignition in a rapid compression machine

A new chemical kinetic reaction mechanism has been developed for the oxidation of methylcyclohexane (MCH), combining a new low temperature mechanism with a recently developed high temperature mechanism. Predictions from this kinetic model are compared with new experimentally measured ignition delay times from a rapid compression machine. Computed results were found to be particularly sensitive to isomerization rates of methylcyclohexylperoxy radicals. Three different methods were used to estimate rate constants for these isomerization reactions. Rate constants based on comparable alkylperoxy radical isomerizations corrected for the differences in the structure of MCH and the respective alkane, predicted ignition delay times in very poor agreement with the experimental results. The most significant drawback was the complete absence of a region of negative temperature coefficient (NTC) in the model results using this method, although a prominent NTC region was observed experimentally. Alternative estimates of the isomerization reaction rate constants, based on the results from previous experimental studies of low temperature cyclohexane oxidation, provided much better agreement with the present experiments, including the pronounced NTC behavior. The most important feature of the resulting methylcyclohexylperoxy radical isomerization reaction analysis was found to be the relative rates of isomerizations that proceed through 5-, 6-, and 7-membered transition state ring structures and their different impacts on the chain branching behavior of the overall mechanism. Theoretical implications of these results are discussed, with particular attention paid to how intramolecular H atom transfer reactions are influenced by the differences between linear alkane and cycloalkane structures.