Physicochemical effects of varying fuel composition on knock characteristics of natural gas mixtures

The physicochemical origins of how changes in fuel composition affect autoignition of the end gas, leading to engine knock, are analyzed for a natural gas engine. Experiments in a lean-burn, high-speed medium-BMEP gas engine are performed using a reference natural gas with systematically varied fractions of admixed ethane, propane and hydrogen. Thermodynamic analysis of the measured non-knocking pressure histories shows that, in addition to the expected changes arising from changes in the heat capacity of the mixture, changes in the combustion duration relative to the compression cycle (the combustion “phasing”) caused by variations in burning velocity dominate the effects of fuel composition on the temperature (and pressure) of the end gas. Thus, despite the increase in the heat capacity of the fuel–air mixture with addition of ethane and propane, the change in combustion phasing is actually seen to increase the maximum end-gas temperature slightly for these fuel components. By the same token, the substantial change in combustion duration upon hydrogen addition strongly increases the end-gas temperature, beyond that caused by the decrease in mixture heat capacity. The impact of these variations in in-cylinder conditions on the knock tendency of the fuel have been assessed using autoignition delay times computed using SENKIN and a detailed chemical mechanism for the end gas under the conditions extant in the engine. The results show that the ignition-promoting effect of hydrogen is mainly the result of the increase in end-gas temperature and pressure, while addition of ethane and propane promotes ignition primarily by changing the chemical autoignition behavior of the fuel itself. Comparison of the computed end-gas autoignition delay time, based on the complete measured pressure history of each gas, with the measured Knock-Limited Spark Timing shows that the computed delay time accurately reflects the measured knock tendency of the fuels.     

燃油温度的多变量模糊预测函数控制

燃油供给温度的精确控制是一个具有非线性特性的流体加热供给多变量控制问题,实际测试表明,现有的单回路PID控制很难实现对燃油供给温度的动态跟踪控制,影响燃油的充分喷射、雾化及其与空气的混合,使部分燃油得不到充分燃烧,造成了能源浪费和环境污染.基于粒子群优化模糊预测函数控制(PSO-F-MPFC)的油料燃烧供给温度多变量解耦控制方法,通过与PID控制方法的比较,以及在阳极焙烧炉重油燃烧供给温度的动态跟踪控制应用表明,该方法优于原有燃油燃烧系统的PID控制,实现了燃油供给温度的动态跟踪精确控制.     

Numerical simulation of proton exchange membrane fuel cells at high operating temperature

A three-dimensional, single-phase, non-isothermal numerical model for proton exchange membrane (PEM) fuel cell at high operating temperature (T ≥ 393 K) was developed and implemented into a computational fluid dynamic (CFD) code. The model accounts for convective and diffusive transport and allows predicting the concentration of species. The heat generated from electrochemical reactions, entropic heat and ohmic heat arising from the electrolyte ionic resistance were considered. The heat transport model was coupled with the electrochemical and mass transport models. The product water was assumed to be vaporous and treated as ideal gas. Water transportation across the membrane was ignored because of its low water electro-osmosis drag force in the polymer polybenzimidazole (PBI) membrane. The results show that the thermal effects strongly affect the fuel cell performance. The current density increases with the increasing of operating temperature. In addition, numerical prediction reveals that the width and distribution of gas channel and current collector land area are key optimization parameters for the cell performance improvement.     

Chip-embedded thin film current collector for microfluidic fuel cells

Microfluidic fuel cells are an attractive candidate for low-power applications and provide a unique advantage over traditional fuel cells by elimination of the membrane. More importantly, microfluidic fuel cells enable a simple single-layer structure similar to common lab-on-chip devices, which makes conventional microfabrication or micromachining techniques readily applicable. Microfabrication is a preferable fabrication tool for microscale devices due to the benefits of high precision and repeatability at relatively low cost. However, the performance of most microfluidic fuel cells reported to date was negatively influenced by intrinsic contact resistances arising due to the highly porous nature of the electrodes. In the present work, a chip-embedded thin film current collector for vanadium fueled microfluidic fuel cells is proposed, fabricated, and evaluated as a potential mitigation strategy. The micromachining based thin film process is compatible with the overall cell fabrication, comprising photolithography and soft lithography, and does not require a substantial modification of the original cell design. Cells with and without current collectors are directly compared experimentally: the cell with current collectors demonstrates a 79% increase in peak power density, indicating that the contact resistance is significantly reduced by this approach. A volume specific peak power density of 6.2 W cm⁻³ is achieved, which is significantly higher than for previously reported microfluidic fuel cells. Electrochemical impedance spectroscopy (EIS) analysis is carried out to measure the combined ohmic cell resistance and confirmed a 32% reduction using the current collectors, which shows a good agreement with slope decrements in the polarization curves.     

Accumulation of nuclear spent fuel from UK power stations

In the absence of a reprocessing industry able to deal with large quantities of irradiated nuclear fuel, it is expected that the bulk of the oxide spent fuel discharged from nuclear reactors will be stored for some decades. In this report the rate of accumulation of spent fuel in the UK and the proportion of its plutonium content is assessed. It is shown that the plutonium content of the metal spent fuel arising from Magnox stations alone should be sufficient to fuel a modest fast breeder programme of 1–2 GW_e well into the next century. As there is an established reprocessing industry for metal fuel, it is argued that reprocessing of oxide fuel need not take place until uncertainties over its cost and necessity are resolved.     

Microwave pyrolysis,a novel process for recycling waste automotive engine oil

Used automotive engine oil was treated using a microwave-induced pyrolysis process, with the intention of assessing the suitability of the process in recovering valuable products from this otherwise difficult to dispose of waste. The resulting pyrolysis gases were condensed into liquid oil; the yield and composition of the recovered oil and remaining incondensable gases were determined, and these were compared with those arising from fresh oil. Process temperature was shown to have a significant effect on the overall yield and formation of the recovered oils. The recovered liquid and gaseous pyrolysis products contained various light hydrocarbons which could be used as a valuable fuel and as an industrial feedstock. Our results indicate that microwave pyrolysis shows extreme promise as a means for disposing of problematic waste oil. The recovery of commercially valuable products shows advantage over traditional, more destructive disposal methods, and suggests excellent potential for scaling the process to the commercial level.     

柴油机燃油物性参数的研究

提出了燃油密度随压力和温度变化的综合经验公式,根据各参数间的严格关系推出了压缩率和音速的经验公式,并介绍了粘度的经验公式。试验结果表明,提出的经验公式具有较高的精度。     

Thermal-hydraulic analysis of UO2 and MOX fuel considering different cladding materials at various burnup levels in pressurized water reactor

Thermal analysis of fuel elements with UO₂ and mixed-oxide (MOX) fuels at different fuel burnup levels has been performed analytically and by simulation using ANSYS. Results showed that UO₂ incurred a lower fuel temperature than MOX under all conditions. Higher fuel element temperatures were obtained for higher levels of burnup for UO₂ fuel. For MOX fuel, higher temperatures were obtained for low and high burnup fuel. Radial temperature, thermal gradient, and thermal heat flux were determined across reactor pressure vessel (RPV), demonstrating the highest value at the center of the RPV. The maximum linear power density was determined for UO₂ and MOX, showing that using UO₂ fuel at 2 at% burnup rendered the highest allowable linear power density. Furthermore, the transient analysis showed that there was a small rise in fuel temperature for a decrease in mass flow rate from 100% to 60% followed by a rapid increase in temperature for further reduction in flow rate.     

试论石油替代燃料产业开发的特征要求

阐述了石油替代燃料开发必须考虑以下内容：①规模化生产,并能满足产品质量和数量方面的要求;②石油替代燃料的性能评价必须考虑其潜在能量;③必须满足产业链和供应链方面的要求;④少满足燃料动力性能和驾驶性能的要求。介绍了在进行石油替代燃料开发项目的可行性研究时,要注意经济效益分析结论的准确度、受原材料价格的影响．以及要注意过程的能源利用效率分析。强调了石油替代能源产业对环境的影响应包括：①产品的使用过程;②能源转化过程;③对大生态环境的影响。最后指出应该重视石油替代燃料开发过程中各种可能的产业风险的分析和规避。     

A single-phase, non-isothermal model for PEM fuel cells

A proton exchange membrane (PEM) fuel cell produces a similar amount of waste heat to its electric power output, and tolerates a small temperature deviation from its design point for best performance and durability. These stringent thermal requirements present a significant heat transfer problem. In this work, a three-dimensional, non-isothermal model is developed to account rigorously for various heat generation mechanisms, including irreversible heat due to electrochemical reactions, entropic heat, and Joule heating arising from the electrolyte ionic resistance. The thermal model is further coupled with the electrochemical and mass transport models, thus permitting a comprehensive study of thermal and water management in PEM fuel cells. Numerical simulations reveal that the thermal effect on PEM fuel cells becomes more critical at higher current density and/or lower gas diffusion layer thermal conductivity. This three-dimensional model for single cells forms a theoretical foundation for thermal analysis of multi-cell stacks where thermal management and stack cooling is a significant engineering challenge.     

熔融碳酸盐燃料电池动态性能数值模拟

建立了正确描述带有热量生成、质量迁移以及电化学反应特性的三维变参数燃料电池动态数学模型,并采用数值模拟方法,对燃料电池温度、速度分布等性能进行预报。通过实验研究,获取燃料电池发电系统输出性能以及温度分布等实验数据,并将数值计算结果与实验结果进行对比分析,验证了数值模拟的准确性,证明了采用的数学模型具有较高的可靠性。     

Implications of electronic short circuiting in plasma sprayed solid oxide fuel cells on electrode performance evaluation by electrochemical impedance spectroscopy

Electronic short circuiting of the electrolyte in a solid oxide fuel cell (SOFC) arising from flaws in the plasma spray fabrication process has been found to have a significant effect on the perceived performance of the electrodes, as evaluated by electrochemical impedance spectroscopy (EIS). The presence of a short circuit has been found to lead to the underestimation of the electrode polarization resistance (R_p) and hence an overestimation of electrode performance. The effect is particularly noticeable when electrolyte resistance is relatively high, for example during low to intermediate temperature operation, leading to an obvious deviation from the expected Arrhenius-type temperature dependence of R_p. A method is developed for determining the real electrode performance from measurements of various cell properties, and strategies for eliminating the occurrence of short circuiting in plasma sprayed cells are identified.     

Dehydrogenation of perhydro-N-ethylcarbazole under reduced total pressure

Liquid organic hydrogen carrier (LOHC) systems represent a promising storage option for hydrogen produced from renewable electricity by water electrolysis. Regarding the efficiency of the endothermal hydrogen release reaction, this technology greatly benefits from a direct heat integration with the waste heat of the energetic use of the released hydrogen, e. g. in a fuel cell. To enable such beneficial set-up, the reaction temperature of hydrogen release must be below the operation temperature of the applied fuel cell which calls for both low temperature dehydrogenation catalysis and high temperature fuel cell operation. This paper demonstrates that such combination may be suitable if reduced pressure dehydrogenation of perhydro-N-ethylcarbazole (H12-NEC) is combined with hydrogen electrification in a high temperature polymer electrolyte membrane fuel cell (HT-PEMFC). Dehydrogenation reactions of H12-NEC were carried out between 160 °C and 200 °C applying different hydrogen partial pressures in the dehydrogenation unit to mimic the effect of a sucking fuel cell operation mode, i.e. the reduction of hydrogen partial pressure in the dehydrogenation unit caused by the fuel cell operation. Our kinetic analysis reveals that a dehydrogenation temperature of 180 °C combined with 500 mbar hydrogen partial pressure represent, for example, a suitable parameter set for efficient hydrogen release.     

Effect of fuel mixture on moderate and intense low oxygen dilution combustion

The effects of fuel mixture on the establishment of moderate and intense low oxygen dilution (MILD) combustion in a recuperative furnace were investigated. Experimental as well as computational results are presented in this paper. Data from exhaust sampling of NO_x and thermocouple measurements of temperature are reported along with results from simultaneous measurement of temperature and OH using Rayleigh scattering and laser induced predissociation fluorescence, respectively. A variety of fuel mixtures using methane, ethylene, and propane were investigated. It was found that dilution of fuel with CO₂ or N₂ reduced the NO_x emission and made the flame inside the furnace invisible. This dilution caused the stoichiometric mixture fraction to shift toward the rich side where the scalar dissipation is highest. This implies that premixing of the fuel stream with circulated exhaust gases can have beneficial effects on the establishment of MILD combustion without the need for higher fuel jet momentum. The reaction zone was found to be characteristically broad and the temperature images were patchy in the lower part of the furnace. The probability density function of the temperature exhibited a bimodal behavior with a cross over temperature of 1300 K.     

气体燃料与气体发动机工作性能

气体燃料组分的变化,不但改变气体燃料与空气混合气热值,影响气体发动机输出功率和燃料经济性,而且可能会引起气体发动机爆震、空燃比改变等一系列液体燃料发动机所遭遇不到的问题。本文探讨如何处理这些问题。     

Performance and emission study of preheated Jatropha oil on medium capacity diesel engine

Diesel engines have proved its utility in transport, agriculture and power sector. Environmental norms and scared fossil fuel have attracted the attention to switch the energy demand to alternative energy source. Oil derived from Jatropha curcas plant has been considered as a sustainable substitute to diesel fuel. However, use of straight vegetable oil has encountered problem due to its high viscosity. The aim of present work is to reduce the viscosity of oil by heating from exhaust gases before fed to the engine, the study of effects of FIT (fuel inlet temperature) on engine performance and emissions using a dual fuel engine test rig with an appropriately designed shell and tube heat exchanger (with exhaust bypass arrangement). Heat exchanger was operated in such a way that it could give desired FIT. Results show that BTE (brake thermal efficiency) of engine was lower and BSEC (brake specific energy consumption) was higher when the engine was fueled with Jatropha oil as compared to diesel fuel. Increase in fuel inlet temperature resulted in increase of BTE and reduction in BSEC. Emissions of NO_x from Jatropha oil during the experimental range were lower than diesel fuel and it increases with increase in FIT. CO (carbon monoxide), HC (hydrocarbon), CO₂ (carbon dioxide) emissions from Jatropha oil were found higher than diesel fuel. However, with increase in FIT, a downward trend was observed. Thus, by using heat exchanger preheated Jatropha oil can be a good substitute fuel for diesel engine in the near future. Optimal fuel inlet temperature was found to be 80 °C considering the BTE, BSEC and gaseous emissions.     

Propagation and extinction of benzene and alkylated benzene flames

Laminar flame speeds and extinction strain rates of benzene, n-propylbenzene, toluene, o-, m-, and p-xylene, and 1,2,4- and 1,3,5-trimethylbenzene flames were studied experimentally in the counterflow configuration under atmospheric pressure and at the elevated temperature of 353 K for the unreacted fuel-containing stream. The experimental data revealed that the aromatic fuel structure plays a critical role on flame propagation, with the laminar flame speed decreasing with an increase in methylation of benzene. Numerical simulations suggest that the aromatics flames are highly sensitive to fuel-specific chemistry and more specifically to the reaction kinetics of the first few intermediates in the oxidation process following the fuel consumption, and that the different flame propagation speeds relate strongly to radical–radical termination facilitated by benzyl or benzyl-like intermediates. The tendencies of stretch-induced extinction of non-premixed flames was found to follow a trend that is identical to the laminar flame speed, but the extinction data revealed a more discriminative effect arising from fuel-structure differences. Comparisons between kinetic model predictions and experimental data showed that there exist significant discrepancies among these models and uncertainties in the oxidation and pyrolysis kinetics of one-ring aromatics.     

NUMERICAL SIMULATION OF THE GAS/DIESEL DUAL-FUEL ENGINE IN-CYLINDER COMBUSTION PROCESS

ABSTRACT

An alternative fuel such as natural gas may be used in a dual-fuel engine for both economic reasons and emission advantages. However, the performance at relatively light load and idling conditions has been quite poor, while at very high load, engine knock is often encountered. In this study, a three-dimensional, dual-fuel, in-cylinder model has now been developed. This is used to provide an improved understanding of the operational features arising from the interaction between the gaseous fuel and the pilot fuel, the preignition processes, and subsequent combustion of the pilot fuel and gas during the piston movement.     

Experimental study and empirical correlation development of fuel properties of waste cooking palm biodiesel and its diesel blends at elevated temperatures

In this experimental work, the density, dynamic viscosity and higher heating value of methyl ester based waste cooking palm-biodiesel oil (WMEPB) was investigated under varying temperature and blend ratio condition with No. 2 diesel fuel. The transesterified fatty acid methyl ester of palm vegetable oil collected from local food and beverage shops was used as neat biodiesel. Four different fuel blends (20%, 40%, 60% and 80% by volume mixing with base diesel) were studied along with base No. 2 diesel fuel and pure biodiesel. Tests for dynamic viscosity and density were performed in the temperature range 0–130 °C for each fuel sample whereas the higher heating values were determined at 25 °C room temperature condition. It is found that pure biodiesel has the highest density and dynamic viscosity at a given temperature whereas it exhibits lowest combustion heating value among the six fuels. Moreover, the density for each fuel sample decreases linearly with the increase in temperature. On the other hand, the dynamic viscosity decreases exponentially with the temperature for each fuel sample. In addition, based on the experimental results, regression correlations have been proposed for the density, dynamic viscosity, and higher heating value of the fuels. Subsequently, comprehensive error analyses of these proposed correlations were performed. In particular, the correlation for density and dynamic viscosity were respectively compared with Kay's mixing rule and Grunberg-Nissan mixing rule theory in order to validate their applicability. It is found that density correlations predicted within ±0.3% average error band. And, as high as 72.2% of the dynamic viscosity data were in the range of ±5% average error while the remaining data fell within ±10% error range. And finally, through a comparative study with the available fuel property results of fresh methyl ester palm biodiesel, it is found that available existing correlations derived from fresh palm biodiesel studies can not accurately predict the fuel properties of same waste biodiesel and its blends with diesel.     

燃气轮机燃油燃烧室改用双燃料流场数值分析

针对燃气轮机燃油燃烧室改成双燃料燃烧室对燃料喷嘴进行一体化概念设计,并采用CFD技术对其双燃料燃烧流场进行数值模拟。针对燃烧室燃用C7H16和裂解气燃料的不同情况,采用标准κ-ε湍流模型、化学平衡条件下的快速化学反应系统和简单概率密度函数（PDF）燃烧模型、液体燃料的喷雾模型以及SIMPLE算法。模拟并对比分析了两种燃料燃烧时的燃烧效率、出口温度均匀性、壁面最高温度以及速度分布等参数随工况变化的趋势,并得出结论：1）不同燃料燃烧时的流场特征基本保持一致;2）裂解气燃料燃烧时,其燃烧效率较高,但出口温度均匀性较差;3）在加入相同焓值的燃料进入燃烧室时,裂解气燃料燃烧得到的出口温度低于燃油的燃烧状态。