Combustion and emission characteristics of BD20 reformed by ultrasonic energy for different injection delay and EGR rate in a diesel engine

The purpose of this study is to understand the operational characteristics of a diesel engine that uses BD20 reformed by ultrasonic energy irradiation. In particular we study the effects of tuning injection delay and EGR rate. BD containing about 10% oxygen has attracted attention due to soaring crude oil prices and environmental pollution. This oxygen decreases soot by promoting combustion, but it also increases NOx. To solve this problem, injection timing may be delayed or an EGR system may be applied. These adjustments normally lower engine power and increase exhaust emission but, in using fuel reformed by ultrasonic energy irradiation (which is changed physically and chemically to promote combustion), we may hope to circumvent this problem. To control the duration of the ultrasonic energy irradiation, the capacity of the chamber in an ultrasonic energy fuel supply system was tested at 550cc and 1100cc capacities. As for the results of the experiment, we could identify the optimum EGR rate by investigating the engine performance and the characteristics of exhaust emissions according to the injection timing and the EGR rate while ultrasonically irradiated BD20 was fed to a commercial diesel engine. With UBD20 (at an injection timing of BTDC 16°), the optimum EGR rate, giving satisfactory engine performance and exhaust emissions characteristics, was in the range of 15∼20%.     

Response of carbon fiber trapezoidal corrugated sandwich structure under air explosion loading北大核心CSCD

The effects of the thickness of the face plate, angle of the wall plate and height of the core layer on the anti-explosion performance of carbon fiber reinforced composite trapezoidal corrugated sandwich structures were investigated. [Methods]First, based on the 3D Hashin failure criterion, a subroutine module of the damage evolution of fiber reinforced composites is developed using the VUMAT user subroutine interface in ABAQUS. Second, through comparison with experiments in the public literature, the effectiveness of the dynamic response simulation method of carbon fiber reinforced composites based on a development subroutine under explosion impact loading is verified. Finally, a parametric study on the explosion resistance of carbon fiber reinforced composite trapezoidal corrugated plates is carried out based on the numerical method. [Results]The results show that, compared with increasing the thickness of the blast face panel, increasing the thickness of the back blast face panel can improve the explosion resistance of the sandwich plate more obviously; when the folding angle of the core wall plate decreases from 45° to 30°, the explosion resistance increases by 1.3%; when it decreases from 60° to 45°, the explosion resistance increases by 6.3%; and whenthecoreheightincreasesfrom8mmto20mm, theexplosionresistanceincreasesby27.7%.[Conclusions]The results of this study can provide references for the explosion-proof design of carbon fiber reinforced composite sandwich structures. © 2023 Journal of Nanjing Agricultural University. All rights reserved.     

Experimental study of drillship resistance performance in open and closed state of moonpool北大核心CSCD

[Objective] This paper carries out an experimental study of a multi-function dirllship model with moonpool structure in towing tank, aiming at analyzing the effects of the moonpool structure on the ship resistance in open and closure condition.[Methods]Taking a dirllship as the research object, the ship motion response in regular and irregular waves is investigated. The resistance of the ship in hydrostatic water and waves is measured with tension sensors, and the acceleration characteristics of the bow, midship and stern are analyzed by acceleration sensors.[Results]The results indicate that hull resistance under light load conditions is greater when open moonpool, while hydrostatic water resistance with closed moonpool is greater under design load conditions. The closed moonpool in regular waves reduces stern acceleration by 58.2%, bow resistance by 46.7% and heave response by 41.8%. The peak of resistance at the bow in irregular waves is about ten times higher than that at the stern, and the peak of resistance occurs more often when the moonpool is open at the same time.[Conclusions]The experimental study shows that the difference in load conditions changes the waterplane area and affects the hull resistance in hyrostatic water. In contrast, the closure of the moonpool not only reduces acceleration and resistance, but also improves surge and heave. As such, this paper can provide references for the structural design of drillships with moonpools. © 2023 Journal of Nanjing Agricultural University. All rights reserved.     

Characteristics and calculation method of sound radiation of cylindrical shell with porous sound-absorbing material under acoustic excitation北大核心CSCD

［Objective］This paper aims to study the characteristics and calculation method of the vibration and sound radiation of single ring-stiffened cylindrical shells with porous fiber composite materials installed in the inner wall under acoustic excitation. ［Method ］ Based on the equivalent fluid theory model of Johnson–Champoux–Allard (JCA) and the transfer matrix of the multilayer medium, a theoretical formula of the sound absorption coefficient of multilayer sound absorption structures is derived. The three methods for calculating the vibration and sound radiation of a single ring-stiffened cylindrical shell with porous fiber materials under acoustic excitation, namely acoustic solid modeling of porous media, finite element model combined with theoretical formula and imposition of impedance boundary on sound absorption coefficient, are then verified and compared. Finally, the influences of sound-absorbing material's thickness, backed-air gap, static flow resistance, and material arrangement order on the acoustic absorption performance of the cylindrical shell are investigated. ［Results］The results show that laying porous fiber composite materials on the cylindrical shell internally can reduce the vibration and acoustic radiation of cylindrical shell structure. The sound absorption coefficient curve can quickly and effectively predict the resulting trend of the vibration and sound radiation of the cylindrical shell. ［Conclusion］The acoustic absorption performance of sound absorption structures can be effectively improved through the rational design of their properties and arrangement order of the sound-absorbing materials in order to achieve the purpose of vibration and noise reduction. © 2023 Chinese Journal of Ship Research. All rights reserved.     

A line impedance stabilization network for pulse current injection北大核心CSCD

［Objective］Aiming at the deficiency of the existing line impedance stability network (LISN) in the electromagnetic pulse protection capability, a LISN suitable for the pulse current injection (PCI) test of electrical and electronic equipment is proposed. ［Methods］Aiming at the characteristics of high peak value and fast rise of the pulse current in PCI testing, the circuit structure and physical structure of the LISN are improved on the existing basis through PSpice time-domain and frequency-domain simulation combined with engineering design requirements, thereby giving it good nanosecond pulse protection performance and impedance stability at the same time. Pulse current protection performance test and impedance curve test experiments are then designed and carried out.［Results］The experimental results show that the improved LISN can attenuate the injected pulse current by 60 times, while the error of its impedance curve is less than 5% compared with the Type 5 µH LISN in GJB 151B-2013. ［Conclusions］The proposed LISN has good impedance stability and decoupling ability, and can be used in the PCI testing of electrical and electronic equipment in order to protect the power supply and improve the repeatability of testing. © 2023 Chinese Journal of Ship Research. All rights reserved.     

Ballistic performance of ultra-high molecular weight polyethylene laminates against fragment-simulating projectiles北大核心CSCD

[目的]旨在探究破片侵彻作用下高强聚乙烯(UHMWPE)纤维增强层合板的毁伤响应过程、失效模式转变和能量吸收特性。[方法]采用有限元软件ANSYS/AUTODYN,建立UHMWPE层合板抗破片侵彻数值模型,分析UHMWPE层合板的失效模式转变和能量吸收特性。[结果]破片侵彻作用下UHMWPE层合板的动态响应过程大致可以分为剪切冲塞阶段和拉伸变形阶段。破片入射速度和靶板厚度会直接影响靶板的能量吸收特性。靶板厚度越大,剪切冲塞模式占比越大。在靶板厚度不变的情况下,随着破片侵彻速度的提高,剪切冲塞模式占比越来越大,最终趋于稳定。在破片弹道极限速度以上初始小范围内,靶板吸能随破片入射速度增大有所减小,随后破片速度继续增加会扩大靶板剪切冲塞破坏范围,导致靶板整体吸能量增加。[结论]基于所建立的数值模型能够较好地模拟破片侵彻作用下UHMWPE层合板的动态响应过程,可以为UHMWPE材料在弹道防护领域的应用提供参考。     

山区河流航道整治施工对水生态环境的影响

山区河流航道整治一般采取筑坝、疏浚、炸礁、护岸等工程，在施工中不可避免对现状水生态环境造成扰动。以岷江下游航道整治一期工程为例，通过对代表性河段施工前后的水质、浮游生物、底栖动物、鱼类等种类和资源量开展现场监测，研究发现：施工前后水体化学需氧量、五日生化需氧量、氨氮、总磷、石油类、悬浮物的浓度有所升高，但均未超过相关标准限定值；藻类、水生维管束植物、浮游动物、底栖动物、鱼类等施工前后无明显减少。因此通过合理的时间、空间的施工避让，航道整治施工对山区河流水生态环境的影响较小。     

大型水库群蓄水后下游河道枯水期流量增幅估算

随着流域上游大型水库群建设，由于“蓄丰补枯”的调节方式，下游河道枯水期流量增幅明显，对航道条件改善具有重要意义。在金沙江下游控制性水文站历年水文资料的基础上，对不同时期水库群建设后河道枯水期流量变化进行分析，提出一套基于水库群调节库容的下游河道枯水期不同保证率流量增幅的理论计算方法，并定量预测了乌东德、白鹤滩运行后向家坝站95%保证率流量为2 172 m³/s,与实际统计的2 007 m³/s较为接近。此理论计算方法可为其他河流梯级水库群建设后，下游航道枯水期流量的估算提供借鉴。     

Constructal design of tree-shaped microchannel disc heat sink with wavy walls北大核心CSCD

［Objective］To meet the efficient thermal management needs of electronic devices such as ships and underwater vehicles, this study focuses on the constructal design of a tree-shaped microchannel disc heat sink with wavy walls. ［Method］A design prototype of the heat sink with wavy walls is first proposed. Based on constructal theory, the amplitude and wavelength of the wavy walls are designed under the constraints of fixed heat sink volume and fixed microchannel volume by maximizing the comprehensive performance evaluation criteria (PEC) while considering both heat transfer and flow pressure drop. ［Results］The results show that the wavy walls increase the heat transfer surface areas and generate vortices in their cavities, effectively reducing the maximum temperature. When the inlet Reynolds number is fixed at 700, 900 or 1100 respectively, the maximum temperature is reduced by 13.5 K by increasing the amplitude of the wavy walls, while the pressure drop increases significantly; and the maximum temperature is reduced by 4.7 K by reducing the wavelength of the wavy walls, while the pressure drop increases slightly. There are optimal amplitudes that raise the comprehensive performance evaluation criteria to extreme values for given larger wavelengths, while the comprehensive performance evaluation criteria increase monotonously as the amplitude increases for given smaller wavelengths. ［Conclusion］Wavy walls can significantly improve the thermal performance of tree-shaped microchannel disc heat sinks, and the use of constructal design can realize optimal geometric constructs with optimal comprehensive performance evaluation criteria. © 2023 Authors. All rights reserved.     

Numerical analysis of impact resistance of honeycomb structures with different Poisson's ratios北大核心CSCD

［Objectives］This papers aims to analyze the impact resistance of honeycomb structure with different Poisson's ratio. ［Methods］Based on the explicit dynamic finite element method, this paper analyzes the dynamic mechanical properties of honeycomb structures with different Poisson's ratios under in-plane impact load, and explores the influence laws of Poisson's ratios on their impact resistance. Three typical honeycomb structures with negative/zero/positive Poisson's ratios (reentrant hexagon, hexagon and semi-reentrant hexagon) are selected, their geometric parameters are changed to give them the same relative density and different Poisson's ratios (−2.76 – +3.63), and their dynamic mechanical properties under low/medium/high-speed dynamic displacement loads are analyzed. ［Results ］ The results show that the zero Poisson's ratio semi-reentrant honeycomb structure has the best structural stability without transverse deformation under compression deformation; without structural instability, the platform stress has little correlation with the Poisson's ratio; and the compact strain and total energy absorbtion increases with the absolute value of the Poisson's ratio. Negative Poisson's ratio honeycomb structures with large t/l and small θ are suitable for applications with high platform stress (strong deformation resistance), and negative Poisson's ratio honeycomb structures with small t/l and small θ are suitable for high total energy absorbtion applications, while zero Poisson's ratio semi-reentrant honeycomb structures are suitable for applications with high platform stress (strong deformation resistance). ［Conclusions］This study can provide references for the type selection and geometric parameter design of side impact honeycomb structures. © 2023 Authors. All rights reserved.