Large eddy simulation of serration effects on an ultra-high-bypass-ratio engine exhaust jet

Serrated jet nozzles are considered to be an efficient and practical passive control approach for jet noise. However, some fundamental mechanisms of serration effects on jet noise are not fully understood, especially in terms of the sound source. In this paper, a high-fidelity simulation framework using large-eddy simulation (LES) is demonstrated to predict near-field turbulence and far-field acoustics from an ultra-high-bypass-ratio engine with round and serrated nozzles. Far-field sound is predicted using Ffowcs Willams–Hawkings (FWH) integration. The results show that the serrated nozzle increases mixing near the nozzle and hence the turbulence decay rate, reducing the turbulence level downstream. The serrations shift the energy from the low frequencies to the high frequencies and decrease overall sound pressure levels by about 3 dB over the low-frequency range. Sound sources are analysed based on fourth-order space–time correlations. There are six major source components ($R_{1111}$, $R_{2222}$, $R_{3333}$, $R_{1313}$, $R_{1212}$, and $R_{2323}$) inside the jet shear layers. The serrations are able to reduce the amplitude of these source terms, causing them to decay rapidly to a level below the round nozzle jet within 2D downstream of the nozzle.     

Infrared radiation characteristics of liquid oxygen/kerosene rocket engine plume with different number of nozzles

A method for calculating the liquid oxygen (LOX)/kerosene rocket engine plume infrared radiation characteristics was proposed, infrared radiation characteristics of the gas oxygen/kerosene model engine were studied by simulation and experiment, and the accuracy of the method was proved. On this basis, studies on infrared spectral radiation characteristics and infrared imaging characteristics of single- and double-nozzle 120?ton thrust LOX/kerosene engine plume were carried out. It is found that, the engine nozzle number can be determined according to infrared imaging characteristics of the engine plume at the appropriate detection angle. Compared to using 2–5?µm band, the infrared radiation imager with clearer plume structure can be obtained using the spectral band such as 2.7 and 4.3?µm bands. The change in the detection angle and the increase in the engine number will have a significant effect on the infrared radiation intensity of the LOX/kerosene engine plume, but they will not change the infrared spectral radiation characteristics.     

Impact of post-injection strategy on the physicochemical properties and reactivity of diesel in-cylinder soot

Post injection has significant benefit in the reduction of diesel soot emissions. Therefore, there is a need to understand the effect of post-injection strategy on soot physicochemical properties and reactivity because they play an important role in soot oxidation process that governs the final soot emissions. This work focuses on the impact of post injection on the physicochemical properties and reactivity of diesel in-cylinder soot using a main plus post injection (M*P) and a single injection (M) strategy. The soot was sampled by a developed total cylinder sampling system, and the dividing points of soot formation-dominant and oxidation-dominant phases were used for studying the impacts of post injection on the characteristics of in-cylinder soot. The physicochemical properties of the soot samples, including primary particle size, nanostructure, carbon chemical state and surface functional groups, were characterized. The soot reactivity was evaluated in terms of peak temperature, burnout temperature and apparent activation energy. In the oxidation-dominant phase, the M*P soot initially possesses smaller primary particle size, shorter fringe length, larger tortuosity, lower sp²/sp³ hybridization ratio of carbon atoms and higher content of aliphatic CH groups than the M soot. The beneficial influence of physicochemical properties on soot reactivity when using post injection is validated by the thermogravimetric data, which shows that the M*P soot is more reactive than the M soot at the onset of the oxidation-dominant phase. In the M*P case, the soot generated from the main-injection combustion has lower reactivity than the soot from the post-injection combustion after they experience the soot formation-dominant phase. The results indicate that the use of post injection leads to in-cylinder soot with physicochemical properties that favor reactivity. The enhancement of reactivity means that the soot will be more readily oxidized in the subsequent combustion process, and consequently contributes to a reduction in final soot emissions.     

Understanding in-cylinder soot reduction in the use of high pressure fuel injection in a small-bore diesel engine

This study shows how soot particles inside the cylinder of the engine are reduced due to high pressure fuel injection used in a light-duty single-cylinder optical diesel engine fuelled with methyl decanoate, a selected surrogate fuel for the diagnostics. For various injection pressures, planar laser induced incandescence (PLII) imaging and planar laser-induced fluorescence of hydroxyl (OH-PLIF) imaging were performed to understand the temporal and spatial development of soot and high-temperature flames. In addition, a thermophoresis-based particle sampling technique was used to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. The OH-PLIF images suggest that an increase in the injection pressure leads to wider distribution of high-temperature flames likely due to better mixing. The enhanced high-temperature reaction can promote soot formation evidenced by both a faster increase of LII signals and larger soot aggregates on the TEM images. However, the increased OH radicals at higher injection pressure accelerates the soot oxidation as shown in a higher decreasing rate of LII signals as well as dramatic reduction of the sampled soot aggregates at later crank angles. The analysis of nanoscale carbon layer fringe structures also shows a consistent trend that, at higher injection pressure, the soot particles are more oxidized to form more graphitic carbon layer structures. Therefore, it is concluded that the in-cylinder soot reduction at higher injection pressure conditions is due to enhanced soot oxidation despite increased soot formation.     

物理化学在线课程教学设计与实践——以热力学第二定律与卡诺热机为例*

提出在线课程应立足于对线下课堂教学内容的疏导、梳理、凝练和升华,并以“热力学第二定律与卡诺热机”在线课程为例进行介绍。采用问题链形式引导学生从卡诺热机推导热力学第二定律数学表达式,阐明了卡诺热机在热力学第二定律形成过程中的重要作用和地位,强调了卡诺对于热力学的重要贡献,培养了学生探究精神和学科素养。为在线课程提供了教学案例和设计思路。     

多循环脉冲爆震发动机流场数值研究

针对二维带有收敛扩张喷管的脉冲爆震发动机模型，采用带基元化学反应的Euler方程组和H2、空气的9组分20基元反应，对发动机在前六个工作循环的流场进行了数值模拟。通过对前几个循环流场进行比较，发现脉冲爆震发动机在第五个循环后流场就基本稳定，单循环得到的流场和多循环稳定后的流场有很大的差别，同时喷管对发动机内流场影响特别大。     

Detonability of simple and representative components of pyrolysis products of kerosene: pulsed detonation engine application

The use of liquid fuels such as kerosene is of interest for the Pulse Detonation Engine (PDE). In this context, a representative gaseous mixture of the lighter products resulting from the decomposition of a kerosene of type JP-10 was studied. The detonability limits of simple components (hydrogen, ethylene, propylene) and mixtures of these components were tested in a 50 mm diameter and 2.5 m long detonation tube. This dimension is compatible with the applications of the aircraft industry and more particularly the PDE. The influences of the nitrogen dilution, geometry of the DDT device (Schelkin spiral), ignition energy and initial pressure were investigated. This paper was based on work that was presented at the 19th International Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27 - August 1, 2003. Communicated by J.E. Shepherd     

气缸垫对机体受力和缸套变形影响的实验研究

某四缸柴油机在试制中出现机体裂纹现象，为保证内燃机正常工作，机体裂纹问题急待处理．应用力学理论对此进行分析，得出机体裂纹是由机体受力不均匀引起的结论．同时采用实验应力分析方法，对两种四缸柴油机机体进行了对比实验研究，在实验中发现了缸垫对机体受力和缸套变形的影响，从而提出了一种有效的改进方法，使机体的应力有明显下降，并不再出现裂纹，也未出现漏油、漏气和活塞拉缸等不良现象，大大提高了产品可靠性．     

Theory of Disappearance of the Electric Signal Induced by an Aircraft Jet Engine in the Afterburner Regime

Electrical aspects of aircraft jet engine operation are considered. A phenomenon previously observed experimentally, namely, the disappearance of the engine current and alternating electric signal induced by the engine jet in the afterburner regime, is explained. It is shown that this phenomenon results from the detachment of electrons from negative ions when the gas temperature in the afterburner increases. This leads to an increase in the effective conductivity of the gas. As a result, the engine current circuit is closed on the internal duct walls and engine charging becomes insignificant.A physico-mathematical model of the electrical processes in the afterburner is formulated and model problems are solved.     

内燃机活塞环－缸套摩擦功耗的设计计算方法之研究

以往在计算内燃机活塞环－缸套摩擦副的摩擦功耗时，只是计算活塞环与缸套之间流体润滑剂的粘性剪应力，这显然不能客观地反映出该摩擦副的润滑状态。实际上，活塞环－缸套间的摩擦力产生于两个方面，一是粘性流体的剪应力，二是摩擦界面相互接触峰元的剪切作用。然而截至目前，针对这两方面进行全面分析研究的报道却还很少，因此，为了给低摩擦功耗环组的设计提供科学依据，基于对活塞环在运动过程中润滑状态的分析，提出了一种适用于不同类型内燃机设计计算的活塞环－缸套摩擦功耗的计算方法，并以其对现有结构的Ｓ１９５柴油机活塞环－缸套摩擦副的摩擦功耗进行了计算，同时还对影响摩擦功耗的因素作了考察与讨论，利用台架试验测量油膜厚度的方法对这种算法进行试验验证的研究结果表明，理论值与实测值吻合得很好。