Modeling of turbulent spray combustion with application to diesel like experiment

Turbulent combustion within sprays has been studied. Experiment and theory predict that the presence of fuel droplets leads to an increase in the fluctuations of temperature and equivalence ratio, and that these fluctuations must be considered to correctly estimate the mean reaction rate. A convenient way to describe the fluctuations of the medium is to compute the probability density function of all the fluctuating variables for the gas and liquid phases. The method used to derive the probability density function (PDF) equations in a pure gaseous medium was extended to two-phase flows by using the local equations written in terms of distribution functions. The PDF for the liquid phase contained complex unclosed terms representing dispersion and atomization. Instead of modeling these terms, we have used a stochastic Lagrangian simulation of the liquid droplets which employs classical atomization models and assumes spherical droplets. This method is used to close the vaporization terms which appear in the PDF equation for the gas. The PDF equation for the gas-phase could be solved by using a Monte-Carlo method but here we have used a more tractable model based on a presumed PDF shape for the mixture fraction. The effects of finite rate chemistry were taken into account by using the MIL model. Finally, the complete model was used to simulate auto-ignition and the establishment of a flame in a diesel type experiment. The auto-ignition delay and the shape of the flame were well simulated. The importance of taking into account the influence of the droplets on the fluctuations is demonstrated by testing the model with and without the corresponding terms.     

Application of spray combustion simulation in DI diesel engine

This work presents the development and implementation of combustion model for DI diesel engines by using the PDF-Chemical Equilibrium combustion model. The key concept of this approach is to predict the thermochemical variables (e.g., temperature, species mass fractions) and then the average scalars of these variables are evaluated by a probability density function (PDF) averaging approach. To realize flame propagation, the reaction time scale is employed to relax the infinitely fast chemistry of chemical equilibrium. The PDF-Eddy Break Up ignition model is adopted in the auto-ignition calculation. With regard to the comparison results, the simulation results are in good agreement with the experimental results in both ignition and combustion modes. In addition, the predicted lift-off length also corresponds to a power-law scaling of Siebers et al.     

Conceptual models for partially premixed low-temperature diesel combustion

Based on recent research within optically accessible engines and combustion chambers, conceptual models for low-temperature combustion (LTC) diesel engines are proposed. To provide a reference to which the LTC conceptual models may be compared, an established conceptual model framework for conventional diesel combustion is first reviewed and updated. Then, based on multiple optical diagnostic observations and homogeneous reactor simulations using detailed chemical kinetic mechanisms, extensions to the existing conceptual model are proposed. The LTC conceptual models are not intended to describe all LTC strategies, but rather a common subset of low-load, single-injection, partially premixed compression ignition conditions that are diluted by exhaust-gas recirculation to oxygen concentrations in the range of 10–15%. The models describe the spray formation, vaporization, mixing, ignition, and pollutant formation and destruction mechanisms that are consistent with experimental observations and modeling predictions for LTC diesel engines. Two separate subcategories are offered for either heavy-duty, large-bore or for light-duty, small-bore engines. Relative to the existing conventional diesel conceptual model, the features of the LTC conceptual models include longer liquid-fuel penetration, an extended ignition delay that allows more premixing of fuel, a more distinct and temporally extended two-stage ignition, more spatially uniform second-stage ignition, reduced and altered soot formation regions, and increased overmixing leading to incomplete combustion.     

A phenomenological model for the prediction of soot formation in diesel spray combustion

A phenomenological soot model coupled with complex chemistry mechanism for the prediction of soot formation in diesel spray combustion is presented. The prototype of the model is one proposed by Leung and Lindstedt in which soot formation is treated by four global stages: particle nucleation, surface growth, surface oxidation, and particle coagulation, each of which is represented by only a few reaction steps. In the present study, the model is modified according to recent literature data. The formation of soot particles is linked with gas-phase chemistry via diacetylene and naphthalene, which are presumed to be indicative species of particle inception/nucleation. The soot surface growth is described using Frenklach et al.'s active site model, and the oxidation mechanism includes both Nagle and Strickland-Constable's O₂ oxidation and Neoh et al.'s OH oxidation models. The soot model integrated with the gas-phase kinetics is then applied in multidimensional spray simulations. The KIVA3 code that is widely used in diesel combustion studies is modified and employed for the simulations. The turbulent flow is predicted using the compressible k-ε model, and the turbulence-chemistry interaction handled by a partially stirred reactor model. The IDEA experimental data for n-heptane sprays in diesel-like conditions (800 K and 50 bar) are used for evaluation of the model. Some reaction rate constants are adjusted to achieve better agreement with the measurements. Further, sensitivity studies have been carried out and the effects of some parameters that affect the predictions are discussed. The results indicate that the model, if applied together with other models that properly describe sprays and turbulent flow, can be used for qualitative and even quantitative prediction of soot formation in diesel combustion.     

Gel polymer electrolyte lithium-ion cells with improved low temperature performance

For a number of NASA's future planetary and terrestrial applications, high energy density rechargeable lithium batteries that can operate at very low temperature are desired. In the pursuit of developing Li-ion batteries with improved low temperature performance, we have also focused on assessing the viability of using gel polymer systems, due to their desirable form factor and enhanced safety characteristics. In the present study we have evaluated three classes of promising liquid low-temperature electrolytes that have been impregnated into gel polymer electrolyte carbon-LiMn₂O₄-based Li-ion cells (manufactured by LG Chem. Inc.), consisting of: (a) binary EC + EMC mixtures with very low EC-content (10%), (b) quaternary carbonate mixtures with low EC-content (16–20%), and (c) ternary electrolytes with very low EC-content (10%) and high proportions of ester co-solvents (i.e., 80%). These electrolytes have been compared with a baseline formulation (i.e., 1.0 M LiPF₆ in EC + DEC + DMC (1:1:1%, v/v/v), where EC, ethylene carbonate, DEC, diethyl carbonate, and DMC, dimethyl carbonate). We have performed a number of characterization tests on these cells, including: determining the rate capacity as a function of temperature (with preceding charge at room temperature and also at low temperature), the cycle life performance (both 100% DOD and 30% DOD low earth orbit cycling), the pulse capability, and the impedance characteristics at different temperatures. We have obtained excellent performance at low temperatures with ester-based electrolytes, including the demonstration of >80% of the room temperature capacity at −60 °C using a C/20 discharge rate with cells containing 1.0 M LiPF₆ in EC + EMC + MB (1:1:8%, v/v/v) (MB, methyl butyrate) and 1.0 M LiPF₆ in EC + EMC + EB (1:1:8%, v/v/v) (EB, ethyl butyrate) electrolytes. In addition, cells containing the ester-based electrolytes were observed to support 5C pulses at −40 °C, while still maintaining a voltage >2.5 V at 100 and 80% state-of-charge (SOC).     

Hydrogen assisted diesel combustion

Hydrogen assisted diesel combustion was investigated on a DDC/VM Motori 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel engine, with a focus on exhaust emissions. Hydrogen was substituted for diesel fuel on an energy basis of 0%, 2.5%, 5%, 7.5%, 10% and 15% by aspiration of hydrogen into the engine's intake air. Four speed and load conditions were investigated (1800 rpm at 25% and 75% of maximum output and 3600 rpm at 25% and 75% of maximum output). A significant retarding of injection timing by the engine's electronic control unit (ECU) was observed during the increased aspiration of hydrogen. The retarding of injection timing resulted in significant NO_X emission reductions, however, the same emission reductions were achieved without aspirated hydrogen by manually retarding the injection timing. Subsequently, hydrogen assisted diesel combustion was examined, with the pilot and main injection timings locked, to study the effects caused directly by hydrogen addition. Hydrogen assisted diesel combustion resulted in a modest increase of NO_X emissions and a shift in NO/NO₂ ratio in which NO emissions decreased and NO₂ emissions increased, with NO₂ becoming the dominant NO_X component in some combustion modes. Computational fluid dynamics analysis (CFD) of the hydrogen assisted diesel combustion process captured this trend and reproduced the experimentally observed trends of hydrogen's effect on the composition of NO_X for some operating conditions. A model that explicitly accounts for turbulence–chemistry interactions using a transported probability density function (PDF) method was better able to reproduce the experimental trends, compared to a model that ignores the influence of turbulent fluctuations on mean chemical production rates, although the importance of the fluctuations is not as strong as has been reported in some other recent modeling studies. The CFD results confirm that temperature changes alone are not sufficient to explain the observed reduction in NO and increase in NO₂ with increasing H₂. The CFD results are consistent with the hypothesis that in-cylinder HO₂ levels increase with increasing hydrogen, and that the increase in HO₂ enhances the conversion of NO to NO₂. Increased aspiration of hydrogen resulted in PM, and HC emissions which were combustion mode dependent. Predominantly, CO and CO₂ decreased with the increase of hydrogen. The aspiration of hydrogen into the engine modestly decreased fuel economy due to reduced volumetric efficiency from the displacement of air in the cylinder by hydrogen.     

装载机用柴油机低温起动研究

装载机经常在低温环境下启动然后开始工作,起动性能是保证正常工作的主要因素之一,因此研究装载机用柴油机低温起动性能是很有必要的.通过理论计算、预热参数设计、试验验证的方法对装载机用柴油机低温起动性能进行了研究,试验结果表明,进气预热和冷却液预热两种不同的预热方式对柴油机低温起动性能影响不同.因此,装载机用柴油机为了保证不同低温环境的起动性能,需要根据不同的使用环境温度,选配合理的预热方式.     

Impacts of low temperature combustion and diesel post injection on the in-cylinder production of hydrogen in a lean-burn compression ignition engine

Strategies were investigated for increased in-cylinder formation of hydrogen. The use of low intake oxygen with a post injection was proposed. An intake oxygen sweep was conducted on a lean-burn compression ignition engine by adjusting of the exhaust gas recirculation rate. The results revealed that the yield of hydrogen increased exponentially when the intake oxygen was reduced to achieve low temperature combustion. Further tests showed that low temperature combustion operation consistently produced more hydrogen than high temperature combustion for similar air-to-fuel ratios.To increase the hydrogen yield further, a post injection timing sweep was carried out with low temperature combustion operation. Increased yields of hydrogen were obtained, up to 0.76% by volume, when then the post injection timing was advanced from 70 to 20° crank angle after top dead centre. At the same time, the indicated NO_X emissions reduced to 0.013 g/kW·hr and the smoke emissions were 0.14 FSN. Thus, the tests established that the combination of low temperature combustion, low intake oxygen, and an early post injection produced a high yield of hydrogen with simultaneously ultra-low NO_X and smoke emissions. The main drawback of this strategy was the increased formation of methane, up to 3015 ppm by volume. However, further analysis showed that the hydrogen to methane ratio actually increased under low temperature combustion operation.     

Study of the characteristic of diesel spray combustion and soot formation using laser-induced incandescence (LII)

In this paper, the planar images of diesel spray combustion flame and soot formation were measured and analyzed by using LII, in a constant volume combustion vessel. The effects of combustion flame and fuel–air mixing characteristics on soot formation and distribution of soot concentration were studied at different conditions. The result indicates that, with increase in ambient temperature and pressure, the ignition delay of diesel fuel is shorter. The increase of ambient temperature and pressure and the reduction of injection pressure shorten the diesel flame lift-off length. The lower the ambient temperature and pressure, the weaker LII signal intensity. At the same ambient temperature and pressure condition, the higher the diesel injection pressure, the smaller the soot production in diesel jet spray, and soot particles are primarily produced in the relative fuel-rich region, which is encompassed by the flame surface front at the downstream of the diesel jet.     

Assessing LES models based on tabulated chemistry for the simulation of Diesel spray combustion

In the context of large-eddy simulation (LES) of Diesel engine combustion, two LES combustion models are proposed. Their ability to predict autoignition delays and heat release of an autoigniting liquid α-methylnaphthalene/n-decane jet injected into a constant-volume chamber under Diesel-like conditions is assessed. These models retain the tabulation of a complex chemistry scheme using autoigniting homogeneous reactors (HR) at constant pressure. This allows accounting for the chemical complexity of heavy hydrocarbon fuels over the wide range of conditions representative for Diesel engines, at comparatively low CPU time overhead. The tabulated homogeneous reactor (THR) approach assumes the local structure of the reaction zone to be that of an HR, while the approximated diffusion flame (ADF) approach is based on autoigniting strained diffusion flames. Two variants of each approach are considered, either neglecting sub-grid-scale mixture fraction variance (THR and ADF models), or accounting for it via a presumed β-PDF (THR-pdf and ADF–PCM models). LES results indicate that the ADF model assuming diffusion flame structures tends to predict faster propagation of the combustion toward less reactive mixture fractions then the THR model. Moreover, neglecting the mixture fraction fluctuations strongly overestimates initial experimental heat release rates after autoignition. Comparison between models shows that this assumption yields higher reaction rates and temperature levels close to the stoichiometric mixture fraction zones. Predictions in terms of autoignition are remarkably close with all models, and exhibit very few variations from one realization to the other. Variations in global heat release rate become more apparent for different realizations at later instants, in relation to the interaction of large flow scales with combustion.     

Autoignition and front propagation in low temperature combustion engine environments

In this paper, we computationally investigate the fundamental aspects of autoignition and subsequent combustion phenomena in low temperature combustion (LTC) engine environments using direct numerical simulations (DNS). In particular, the effects of thermal and equivalence ratio stratification on the autoignition and subsequent front propagation in high pressure and stratified hydrogen-air turbulent mixtures are studied using detailed chemistry. Depending on fuel injection timing, exhaust gas recirculation, and wall heat loss, different correlations between temperature (T) – equivalence ratio (?) fields can exist prior to the major heat release event. Here, we investigate three cases with different initial T–? correlations: (A) a baseline case of a uniform composition with temperature inhomogeneities only, (B) uncorrelated T–? fields, and (C) negatively-correlated T–? fields. Numerical diagnostics are developed based on an appropriately defined Damköhler number to distinguish different modes of heat release. It is found that the majority of heat release in the baseline case and the uncorrelated case occurs during the front propagation in the form of both spontaneous ignition fronts as well as deflagration waves, whereas the negatively correlated case ignites predominantly homogeneously.     

高温低氧燃烧的发展及其原理

介绍高温蓄热燃烧和低氧燃烧这两种燃烧方式的优缺点,解释了值得 优点产生的高湿低氧燃烧方式的基本原则,即高温蓄热燃烧能够充分回收余热,低氧燃烧方式可以控制NOx的排放量和噪声水平,并同时给出理论依据。     

某型号柴油机低温起动性能优化

基于缸内燃烧理论对某型号柴油机低温起动困难问题进行分析,提出提高压缩比、采取不同进气加热方式、优化气门间隙3种措施提升缸内温度,并对改进后柴油机开展低温起动性能对比试验.结果 表明:采用高压缩比可明显提升缸内压缩温度,利于改善低温起动性能;分缸加热可明显提升进气温度,相同加热时间下,分缸加热比格栅加热进气温度提升约53...     

Energy efficiency improvement strategies for a diesel engine in low‐temperature combustion

The lowered combustion temperature in diesel engines is capable of reducing nitrogen oxides and soot simultaneously, which can be implemented by the heavy use of exhaust gas recirculation (EGR) or the homogeneous charge compression ignition (HCCI) type of combustion. However, the fuel efficiency of the low‐temperature combustion (LTC) cycles is commonly compromised by the high levels of hydrocarbon and carbon monoxide emissions. More seriously, the scheduling of fuel delivery in HCCI engines has lesser leverage on the exact timing of auto‐ignition that may even occur before the compression stroke is completed, which may cause excessive efficiency reduction and combustion roughness. New LTC control strategies have been explored experimentally to achieve ultralow emissions under independently controlled EGR, intake boost, exhaust backpressure, and multi‐event fuel‐injection events. Empirical comparisons have been made between the fuel efficiencies of LTC and conventional diesel cycles. Preliminary adaptive control strategies based on cylinder pressure characteristics have been implemented to enable and stabilize the LTC when heavy EGR is applied. The impact of heat‐release phasing, duration, shaping, and splitting on the thermal efficiency has also been analyzed with engine cycle simulations. This research intends to identify the major parameters that affect diesel LTC engine thermal efficiency. Copyright © 2008 John Wiley & Sons, Ltd.     

Detailed chemical kinetic models for the low-temperature combustion of hydrocarbons with application to gasoline and diesel fuel surrogates

This paper presents a review of gas-phase detailed kinetic models developed to simulate the low-temperature oxidation and autoignition of gasoline and diesel fuel components (alkanes, ethers, esters, alkenes, cycloalkanes, aromatics, including from four atoms of carbon) and of mixtures of several of them, which have been proposed as surrogates. The recently proposed models are summarized, as well as the experimental results available for their validation. A comparison between the major models in terms of considered elementary steps and associated rate constants is also proposed.     

Hydrogen combustion under diesel engine conditions

The autoignition and combustion of hydrogen were investigated in a constant-volume combustion vessel under simulated direct-injection (DI) diesel engine conditions. The parameters varied in the investigation included: the injection pressure and temperature, the orifice diameter, and the ambient gas pressure, temperature and composition. The results show that the ignition delay of hydrogen under DI diesel conditions has a strong, Arrhenius dependence on temperature; however, the dependence on the other parameters examined is small. For gas densities typical of top-dead-center (TDC) in diesel engines, ignition delays of less than 1.0 ms were obtained for gas temperatures greater than 1120 K with oxygen concentrations as low as 5% (by volume). These data confirm that compression ignition of hydrogen is possible in a diesel engine at reasonable TDC conditions. In addition, the results show that DI hydrogen combustion rates are insensitive to reduced oxygen concentrations. The insensitivity of ignition delay and combustion rate to reduced oxygen concentration is significant because it offers the potential for a dramatic reduction in the emission of nitric oxides from a compression-ignited DI hydrogen engine through use of exhaust-gas-recirculation.     

内燃机车喷漆室的设计

指出为了减轻喷漆对环境的污染,改善工人的劳动环境,在我国各铁路机车车辆工厂普遍采用喷漆室,机务段、车辆段也陆续新增此设备。详细介绍了内燃机车喷漆室的结构及组成。     

An experimental study on high temperature and low oxygen air combustion

ho~cuon,Recently, much attenhon has been paid to uhliZinghidly Preheated air up to l,(XX)"C through waste gas inindustrial furnaces, in which about 15% of totalnational energy in KOrea were consumed, because ofhigh efficiency of energy savings. Moreover, one ofthree major issues in the fiscal 1996," UnderstandingEnhancement of ugh TemP~ Air COmbushon" hasbeen stUdied as successive subject in the Japanesenational Project tO reduce CO, for Protechon of earth.IntrDduction of high regenera…     

Effect of distributions of fuel concentration and temperature on ignition processes in diesel PCCI combustion

The distributions of fuel concentration and temperature have significant effect on the ignition processes of diesel premixed charge compression ignition (PCCI) combustion. It was found in this study that the ignition process of PCCI combustion organized by multi-pulse injection was strongly inuenced by conditions of fuel stratification. The start of low temperature reactions occurred in the leaner area of the combustion chamber in the test engine because the temperature here first reached the point of low temperature reactions. Ignition always occurred in the position where the mixture featured with equivalence ratios close to the mean equivalence ratio of the overall mixture, while the neighboring area of the initial ignition area accumulate heat with a finite speed until finally autoigniting. Moreover, the appearance of highest combustion temperature occurred in the same area at the combustion chamber. For more homogeneous mixture, a higher amount of mixture reached ignition simultaneously, resulting in a larger initial ignition area and a higher temperature at the ignition area. Furthermore, V-type distribution of equivalence ratio was found to be beneficial to retarding high temperature reaction.