Impacts of a jet''s exit flow pattern on mixing and combustion performance

The influence of modifying a jet's exit flow pattern on both the near and far-field turbulent mixing processes and on the resulting combustion performance, is explored. This reveals that, in contradiction to some common assumptions, increasing the coherence of large-scale motions can decrease molecular mixing rates, and yet can still be beneficial in some applications.

Even relatively minor changes to the exit flow pattern of a non-reacting round jet, through changes to the nozzle profile are found to propagate downstream into the far field, apparently through the underlying turbulent structure. Importantly, while a jet from a smoothly contracting nozzle is found to have higher rates of entrainment, mean spread and mean decay of the scalar field than does a long pipe jet, it has a lower rate of molecular mixing. That is, increased large-scale mixing does not necessarily result in increased fine-scale mixing. A range of devices are reviewed which enhance, or stimulate the large-scale, coherent motions in an emerging jet using acoustic, mechanical or fluidic methods. The available evidence suggests that those methods which induce instantaneously asymmetric flow structure are more effective at increasing the near-field spreading than are those which induce instantaneously axisymmetric flow structure. Only limited data are available of the effects of such near-field changes on the far-field properties. Nevertheless, the available data reveal a clear trend that this near-field flow undergoes a transition to a far-field state whose spread and decay is comparable with that of a steady jet, albeit being indelibly altered by the near-field excitation. It also suggests that “self-exciting” devices (i.e. that are not externally forced), cause a net reduction in the total entrainment relative to the unexcited jet, due to the losses induced by the device itself. Nevertheless, the changes which they can impart to the flow, such as redistributing the turbulent energy from the fine to the larger scales, can be beneficial for combustion in applications where high radiant heat transfer is desirable.

Precessing and flapping jets are found to cause an increase in flame volume relative to an equivalent simple jet (SJ), implying lower molecular mixing rates. However, importantly, this decrease in mixing is achieved with no increase in the flame length. Rather the width to length ratio of these flames is increased significantly. This is of practical significance because the length of a flame is often the limiting dimension in industrial systems. The reduced strain-rates lead to an increased presence of soot within the flame, while not, in general, significantly influencing the emission of soot from the flame. The increased volume of soot leads to increased radiation, which in turn acts to reduce flame temperature, so lowering thermal NO_x emissions through a global residence time–temperature reduction. For example, in full-scale cement kilns these burner nozzles are found to reduce NO_x emissions by around 40–60% and increase fuel efficiency (or output) by around 5–10%.     

Interactions between electricity-saving measures and carbon emissions from power generation in England and Wales

The relationship between electricity demand reduction and the consequent change in carbon emissions is central to greenhouse gas emissions policy. This paper examines this relationship for the power system of England and Wales. Previous analysis showed that the commonly used conversion factor based on the system average emission factor significantly underestimates these savings (Hitchin and Pout, 2002. The carbon intensity of electricity: how many kgC per kWhe?. Building Serv. Eng. Res. Technol. 23(4)). Thus any policy analysis based on the system-average emission factor will under-estimate the potential for carbon savings from reductions in electricity demand. The present paper extends the previous analysis by using more detailed modelling to explore differences between demand reductions of differing load shape and magnitude; and the sensitivity of these figures to changes of the fuel mix of the generation system.     

Distributed grid-connected photovoltaic power system emission offset assessment: statistical test of simulated- and measured-based data

This study assesses the pollutant emission offset potential of distributed grid-connected photovoltaic (PV) power systems. Computer-simulated performance results were utilized for 214 PV systems located across the US. The PV systems’ monthly electrical energy outputs were based on a performance calculator called PVWATTS. Offset emissions of sulfur dioxide (SO₂), carbon dioxide (CO₂), and nitrogen oxides (NO_x) were determined from PV system outputs and average utility emissions data from each state. For validation, the simulated monthly results were statistically compared with measurement-based data (both production and corresponding emissions data) from 29 PV systems installed at different sites across the US.

While the data shows high (geographic) variability, the substantial number of measurements allows reliable statistical analysis. The methods are found to give consistent results in spite of the necessity to employ some even quite crude input approximations—such as the use of statewide rather than specific emissions data for the systems. No significant differences between simulated and measured monthly means for any of the pollutants were noted on the basis of individual monthly analyses, though the results for NO_x suggest the possible existence of some difference in that case. A more detailed statistical modeling using all monthly data in one combined analysis (allowing improved variability estimation) confirms these conclusions. Even the shorter confidence intervals for expected offsets obtained through the combined analysis show no significant differences between simulated and measured methods for SO₂ and CO₂. The differences for NO_x are statistically significant but consistent—suggesting useful prediction by the simulations via a constant correction factor. As expected, significant differences between months are evident for both simulated and measured offsets.     

广州市东濠涌流域低影响开发措施雨洪控制效应评估

随着我国城市化进程的快速推进和海绵城市理念的不断加深,低影响开发(LID)措施日益受到各地重视。为评估LID措施的雨洪控制效果,构建基于SWMM的东濠涌流域城市雨洪模型,分析选取适合该研究区域的LID措施,分别采用重现期为0.5、1、2、5、10年的设计降雨对LID措施的雨洪控制效应进行分析。研究结果表明:LID措施能够较为有效地减小流域洪峰流量、径流量及推迟峰值时间,对初期雨水径流污染负荷也具有较好的削减作用,且径流量和污染物削减率均随着重现期增大而减小。     

温室中农村废水再生滴灌过程中污染物的时空变化规律

在温室中采用土壤过滤系统处理后的农村废水进行滴灌,分析灌溉后土壤和渗漏水中化学需氧量COD_(Cr)、生化需氧量BOD_5、全氮TN、氨氮NH_3—N和全磷TP的时空变化规律。在试验区滴头正下方20 cm深度处埋设一组负压计,当负压计指示的土壤基质势低于-25 k Pa时进行灌溉,每次灌水量为5 mm。实验结果表明:不同季节土壤深层渗漏水中污染物浓度由高到低依次为夏季秋季春季冬季;污染物浓度随深度降低,1. 5 m深度处深层渗漏水中,发现COD_(Cr)、BOD_5、TN、NH_3—N和TP的最高截留率分别达到76. 3%～88. 1%、84. 2%～89. 2%、69. 3%～76. 8%、57. 3%～63. 1%和69. 8%～78. 6%;在滴灌区域外边缘,沿着水流的水平方向,各污染物浓度衰减率逐渐增大;采用农村废水再生滴灌,深层渗漏水中TN、TP的浓度较低,调节再生废水浓度,可以控制污染物流入浅层地下水,从而防止面源污染。     

Performance and exhaust emissions of a diesel engine using diesel nanoemulsions as alternative fuels

The aim of this paper is to investigate the effect of water-in-diesel fuel nanoemulsions on diesel engine performance and exhaust emissions of a single cylinder diesel engine at different engine loads. Emulsified diesel fuel was prepared by mixing diesel fuel with surfactant in the percentage of 4, 6, 8 and 10 wt% of the emulsion total weight. Emulsified diesel oils with varying content of water and surfactant concentrations were prepared via the batch method technique. Different concentrations of water as 5, 6 and 7 wt% was gradually added. Effect of water content and surfactant concentration on engine performance parameters and exhaust emissions were investigated. From the obtained results, specific fuel consumptions for water diesel emulsions were reduced by 8% compared to pure diesel fuel at 4 wt% surfactant concentration, 7 wt% water content and engine load of 1 kW. Furthermore, the lowest HC, CO and NOx emissions value of 66, 48 and 32%, respectively were obtained in case of using 6 wt% of surfactant concentration, 7% water content and engine load of 1 kW. The prepared emulsified diesel fuel achieved a higher engine performance and lower exhaust emissions compared to neat diesel fuel.     

柴油发动机颗粒排放物的组分研究

重点对柴油机的颗粒排放物进行分析研究,将柴油机颗粒排放中的碳和可溶性有机成分(SOF)分离开来,并对SOF的来源进行了进一步的分离.研究结果表明:润滑油对柴油机颗粒排放中SOF的贡献不容忽视,降低柴油机润滑油消耗量和改善润滑油品质是降低柴油机颗粒排放的重要技术措施之一.     

基于MF-DCCA的呼吸道疾病与大气污染物相关性分析

针对呼吸道系统疾病与大气PM2.5、SO2浓度序列的相关性特征,应用多重分形消除趋势波动分析法(MF-DCCA),对张家界市永定区呼吸道系统疾病患病人数与大气PM2.5、S02浓度序列进行了研究.结果发现该地区呼吸道系统疾病患病人数与大气PM2.5、So2浓度的相关性具有长期持续特征和多重分形特征.随后对它们相关性多重...     

污灌污染系统整体数学模型

依据污染物的迁移转化途径，建立了模拟预测迁移型污染物变化的整体数学模型，整体模型由表土层模型、下包气带模型、含水层模型组成，表土层模型为黑箱模型，下包气带模型为一维垂向迁移模型，含水层模型为二维平面迁移模型，子模型之间用流量和污染物浓度耦合·在河北省新河县污灌试验区进行的实例应用中，对求解复杂的含水层模型，采用美国IOWA大学教授陈景仁提出的有限分析法求解，经实测资料验证，整体模型是可靠的，并模拟预测了该区在现状污灌状态下，迁移型重金属As、Cr 6和可降解有机物BOD、COD的变化趋势，及其达标年限     

基于优势效应理论的双绞线简化建模

在对双绞线电磁干扰进行研究中,一些学者建立了精确的干扰模型,但并不适合工程应用.本文根据电磁兼容优势效应理论,推导出双绞线传导干扰主要由差模电流引起,辐射干扰主要由共模电流引起.分别利用广义二端口网络模型和电流元分析方法建立双绞线串扰和辐射的简化模型,从而有效估算双绞线串扰电压和对外辐射场的大小,为工程人员采取有效措施抑制互连设备电磁干扰提供了理论帮助.