柴油机连杆螺栓预紧力的数值分析

连杆螺栓预紧力对连杆盖和连杆体的接合状态影响较大,为了保证有较好的接触压力,对连杆组件进行有限元数值分析。利用接触算法,重点分析从28～60kN范围内的13种不同的螺栓预紧力对连杆接触面、轴瓦接触面等的平均应力、应变值的变化情况。综合考虑各种情况,最终确定一个合适的螺栓预紧力,为设计人员提供了理论依据。     

螺栓装配连接工艺对气缸密封的影响

在考虑气缸垫材料非线性及螺栓弹性相互作用条件下,设计了缸盖螺栓顺序预紧与交叉预紧两种装配连接工艺,采用非线性有限元计算方法建立了机体、气缸盖、缸套、螺栓、缸套支撑及气缸垫组合结构有限元模型,研究了螺栓预紧顺序与气缸垫接触压力之间的关系,得出了满足组合结构气缸密封特性要求的较好装配连接工艺.对比了螺栓残余预紧力的计算值与试验值.结果表明:采用非线性有限单元方法可以实现对机体、气缸盖、缸套、螺栓、缸套支撑及气缸垫组合结构螺栓预紧装配连接工艺的较精确仿真计算;顺序预紧时,螺栓平均残余预紧力及残余预紧力标准差都大于交叉预紧;气缸密封最差区域出现在气缸垫排气侧两螺栓中间位置,两种装配连接工艺都能满足气缸密封性能要求,顺序预紧气缸密封最差区域密封效果更好.     

考虑非线性因素的燃烧室密封特性分析

以某V型高强化柴油机为研究对象,考虑气缸垫中基材的弹塑性变形特性、密封橡胶的超弹性特性及燃烧室各部件间的接触非线性因素,建立了燃烧室气缸垫密封特性有限元分析模型。借助有限元非线性求解方法,计算气缸垫密封凸纹和橡胶密封圈表面的接触压力,以此评价发动机动力强化后燃烧室的密封效果,同时探讨了气缸盖螺栓预紧力与燃烧室密封特性的关联性,确定发动机动力强化后缸盖螺栓预紧力的优选范围。分析结果表明:强化后热-机耦合工况下,气缸垫密封凸纹上最小接触压力比强化前下降了13%,燃烧室密封效果降低;将缸盖螺栓预紧力控制在88~92kN范围内,可使燃烧室的密封性能达到强化前状态。     

高排温发动机EGR法兰面密封仿真及结构优化

某国六天然气发动机在进行台架试验时,排气歧管废气再循环(EGR)取气管面处发生漏气现象,影响发动机工作稳定性和可靠性,无法满足工作要求。数值仿真分析发现,原设计方案EGR法兰面最小接触压力较小,无法满足排气歧管与EGR取气管在高温恶劣条件下的密封要求。通过将EGR取气管中置、优化EGR密封垫片结构及法兰面螺栓布置方式等措施,提出了2种结构优化方案,并进行了密封性能仿真分析及试验验证。仿真结果表明,方案二最小接触压力较原失效方案增加63.9%,且螺栓轴力一致性及接触面密封均匀性优于方案一。后续试验结果也表明,采用第二种优化方案,EGR取气管法兰面漏气问题得到有效解决。     

6110型柴油机气缸套变形的有限元计算与分析

非工作状态下6110型柴油机气缸套的变形测试结果表明,六个气缸套在测试的三个截面上均发生失圆现象,且其长轴的方向有一定的规律性。机体顶平面与气缸盖之间的接触压力试验证明了这一部位的接触压力不均匀。通过有限元计算分析,发现缸盖螺栓预紧力的大小对缸套的变形有显著影响,气缸套的受力位置对缸套的变形也有较显著的影响。     

增压器放气阀物理模型建模方法研究

为了改善因排气波动造成放气阀不正常开启引起的天然气发动机在标定功率工况时出现严重的转矩波动情况,建立了增压器放气阀的瞬态模拟模型,对增压器放气阀受力情况进行理论分析,并利用GT-Power软件中的力学模块建立了增压器放气阀的压力-弹簧力平衡模型,应用模型对影响转矩波动的放气阀弹簧预紧力、刚度和系统阻尼进行分析,确定弹簧刚度是主要影响因素,并应用模型对弹簧刚度进行优化,给出具体的改进方案。应用改进后的放气阀在发动机台架上进行了验证试验,转矩波动由原来的70N.m降为19N.m,解决了发动机转矩波动问题。研究结果表明:增压器放气阀的瞬态模拟很好地预测出改进方案及效果,证明了瞬态模拟方法是可行的。     

涡轮增压汽油机排气歧管螺母松动仿真与试验研究

针对涡轮增压汽油机冷热循环试验中排气歧管螺母松动进行仿真分析,计算出不同工况下的变形和排气歧管垫片密封情况。结果表明排气歧管膨胀后与螺母产生相对变形,相对变形导致排气歧管螺母安装面受力不均匀。排气歧管螺母安装面在高温条件下,局部受力过大产生塑性变形,造成排气歧管螺母与排气歧管接触面产生间隙(接触面张开量),加上排气歧管冷热变化条件下会与排气歧管螺母产生相对滑动,进而导致螺母松动问题出现。本文还对比了不同螺柱轴力下排气歧管螺母安装面的塑性变形量和接触面的张开量,在减小螺柱轴力后,排气歧管螺母安装面的塑性变形量和接触面的张开量明显减小。为了验证分析结果,在保持螺母拧紧力矩不变的条件下通过改变螺母和螺柱表面处理方式来减小螺柱轴力,经过试验验证,没有再出现螺母松动问题。     

缸盖螺柱预紧力控制和把紧方法浅析

气缸盖螺柱的预紧力直接影响到柴油机工作的可靠性。根据气缸盖工作状态及螺柱的结构,计算出螺柱的预紧力,在保证螺柱工作可靠性的前提下,可适当降低螺柱刚度,提高螺柱的承载能力。螺柱预紧力确定后,如何准确的按照预紧力把紧螺柱成为很关键的步骤,螺柱的把紧方法有多种,根据操作的难易程度及精确度,选择一种最适合气缸盖螺柱把紧的方法,完成气缸盖螺柱的把紧工作。     

排气歧管螺栓预紧力衰减研究

由于排气歧管螺栓受到高低温的循环载荷,螺栓预紧力发生衰减,导致在排气歧管与法兰之间发生漏气的现象。而排气歧管螺栓在高温下的预紧力衰减情况用经典公式很难计算,本文使用有限元的方法,基于非线性弹塑性材料参数,计算在全速全负荷、倒拖以及怠速工况循环作用下的排气歧管螺栓的衰减情况,结果表明此方法可以有效的预测螺栓预紧力的衰减,判断排气歧管法兰是否存在漏气现象。     

海上风电螺纹连接的拧紧力矩计算方法分析

1拧紧螺栓的受力分析在连接螺栓的拧紧过程中,受螺纹连接面升角的作用,螺栓产生轴向拉伸变形,对螺栓夹层形成预紧力P0,在螺纹接触面形成摩擦力F1,在螺母接触面形成摩擦力F2,如     

Finite element evaluation of clearance effect on tube-to-tubesheet joint strength

Tube-to-tubesheet joint strength is measured in terms of residual contact pressure between the tube's outer surface and the tubesheet hole surfaces. The joint integrity is affected by several design parameters, including the type of material and the initial radial clearance.

The present work complements an experimental program on the effect of over-tolerance on heat exchanger tube-to-tubesheet joint strength. Finite element analyses address the effect of initial clearance on contact pressure and percent tube wall reduction. Results show that for low strain hardening materials the initial clearance effect is negligible. However, higher levels of strain hardening have a significant effect on residual stress and percent wall reduction. For low clearances, the calculated residual contact pressure compares well with an analytical result and with that inferred from the experimentally measured pull out force. The variation of the percent wall reduction with initial clearance is found to be similar to that measured.     

Study on shape error effect of metallic bipolar plate on the GDL contact pressure distribution in proton exchange membrane fuel cell

Thin metallic bipolar plate (BPP), due to mechanical strength, thermal conductivity, high power density, and relatively low cost, is considered to be an alternative to graphite BPP in proton exchange membrane (PEM) fuel cell. However, shape error of thin metallic BPPs is not avoidable due to its flexibility and springback in stamping process, as well as deformation resulted from thermal stress in welding process. In this study, fluctuation analysis is conducted and response surface methodology (RSM) is adopted to establish the relationship between shape error and contact pressure distribution on gas diffusion layer (GDL). Thin metallic BPPs made of stainless steel (SS) 304 sheets are fabricated and shape error is defined. Two types of specimens are selected and assembled with GDL. Effects of assembly force, BPP size and shape error are systematically investigated and a response surface model is developed to predict the effect on contact pressure distribution resulted from the shape error of BPP. The methodology in this study is beneficial to understand the effect of the shape error and predict the acceptable shape error. Based on the model, tolerance of the shape error of BPP is given to guide the manufacturing process of the thin metallic BPP.     

THE INFLUENCE OF CRACK-FACE FLUID PRESSURE ON THE FATIGUE CRACK PROPAGATION DUE TO ROLLING CONTACT WITH FRICTIONAL HEAT

A three-dimensional surface planar crack problem in a half-space is considered under rolling/sliding contact with frictional heat and hydraulic pressure by the entrapped fluid within the crack. Rolling contact is simulated as a line load with both normal and shear components, moving with constant velocity over the surface of the half-space. The body force method for three-dimensional fracture mechanics is utilized to determine the three modes of stress intensity factors along the crack contour. To account for mixed-mode propagation, the modified Paris power law is used. Numerical results for the stress intensity factors and the simulations of fatigue crack propagation are given for 30-degree inclined planar surface cracks of semicircular shape. The effects of the frictional coefficient, sliding/rolling ratio, and the crack-face fluid pressure on the crack propagation life are considered for a high carbon-chromium bearing steel.     

Effect of manufacturing processes on contact resistance characteristics of metallic bipolar plates in PEM fuel cells

In this study, metallic bipolar plate (BPP) samples manufactured with stamping and hydroforming under different process conditions were tested for their electrical contact resistance characteristics to reveal the effect of manufacturing type and conditions. Punch speed and force in stamping, and pressure and pressure rate in hydroforming were selected as variable process parameters. In addition, two different channel sizes were tested to expose the effect of BPP micro-channel geometry and its consequences on the contact resistance. As a general conclusion, stamped BPPs showed higher contact conductivity than the hydroformed BPPs. Moreover, pressure in hydroforming and geometry had significant effects on the contact resistance behavior of BPPs. Short term corrosion exposure was found to decrease the contact resistance of bipolar plates. Results also indicated that contact resistance values of uncoated stainless steel BPPs are significantly higher than the respective target set by U.S. Department of Energy. Conforming to literature, proper coating or surface treatments are necessary to satisfy the requirements.     

碳纤维复合芯导线股线应力有限元分析

考虑碳纤维复合芯导线的结构、材料特性,针对JRLX/T-315/40碳纤维导线进行了静力线形分析,并利用SOLIDWORKS软件建立导线实体模型,模拟分析了导线在实际运行过程中受到拉力、剪力和弯矩以及线夹的挤压力时外层股线与线夹接触部位的应力分布情况。结果表明,导线夹持区的应力集中部位主要在U型螺栓出口处和悬垂线夹出口处;在线夹出口处,T_0=0.15Tp时导线与悬垂线夹接触位置的外层中间一根股线的应力损伤最严重;在弯矩和拉力共同的作用下,碳纤维复合芯棒承担导线63%左右的应力,是碳纤维导线的主要承力部件。     

Contact resistance prediction and structure optimization of bipolar plates

The objective of this work is to investigate the effect of clamping force on the interfacial contact resistance and the porosity of the gas diffusion layer (GDL) in a proton exchange membrane fuel cell (PEMFC). An optimal rib shape for the bipolar plate is developed to analyze the electrical contact resistance. We found that the electrical contact resistance is determined by both the clamping force and the contact pressure distribution. A minimum contact resistance can be obtained in the case of a constant contact pressure distribution. The porosity of the GDLs underneath the rib of the bipolar plate decreases with increasing the clamping force, and the void volume is changed with the deformation of the GDLs. It is found that there exists an optimal rib width of the bipolar plates to obtain a reasonable combination of low interfacial contact resistance and good porosity for the GDL.     

A two-fluid model for two-phase flow in PEMFCs

In this study, a two-fluid (TF) model is developed for two-phase flows in proton exchange membrane fuel cells (PEMFCs). The drag force and lift force between gas and liquid phase are considered in N-S equations. In addition, a simplified model is introduced to obtain the liquid water droplet detachment diameter on the gas diffusion layer (GDL)/channel interface which involves the properties of the GDL/channel interface (contact angle and surface tension). The TF model and the simplified model for the prediction of water droplet detachment diameter on GDL/channel interface are validated by the comparison between the experimental data and the model results, respectively. The effect of the properties of GDL/channel interface (contact angle and surface tension) on two-phase behavior in PEMFCs is investigated, The results show that a high contact angle and a low surface tension are advantageous for liquid water removal in the gas channel and the GDL even though a low surface tension will lead to a low capillary force in the GDL.     

The use of a novel water porosimeter to predict the water handling behaviour of gas diffusion media used in polymer electrolyte fuel cells

A novel water porosimeter and its use in determining the capillarity of gas diffusion layers are described. It is found that, in accordance with the Washburn equation, the pressure required to force water into the gas diffusion layer depends on the cosine of the contact angle of water with the surface of the pore. Negative pressure is required to withdraw water from the gas diffusion layer, even when the surface is hydrophobic. The negative pressure required is found to be independent of surface contact angle. It is shown that the performance of gas diffusion layers in an operating fuel cell can be qualitatively predicted from the capillary pressure curves obtained. The advantages of the use of water porosimetry over the use of either mercury porosimetry or porosimetry using wetting fluids are discussed.     

Effects of functional surface on performance of a micro heat pipe

The effect of a functional surface with the axial ladder contact angle distribution on the thermal performance of a triangular micro heat pipe has been analyzed based on a one-dimensional steady-state model. Compared with the traditional micro heat pipe (MHP) with a uniform contact angle distribution on its surface, the simulation results show that a MHP with a functional surface can remove a greater amount of heat under the same condition. The increase in thermal performance is more obvious with the increase in the ladder difference of the contact angles between the adjacent sections of the MHP. The increased thermal performance associated with the functional surface can be attributed to the increase of the liquid capillary force as well as the no obvious increase of the liquid shearing force provided by the functional surface, which also brings about the increase in condensate mass flow rate through the adiabatic section–evaporation section interface. It is also found that for the traditional MHP with uniform contact angle surface, there is an optimal contact angle leading to the maximum heat input. The deviation of the optimal value will decrease the capillary force and thermal performance of the MHP.     

Motion of droplets induced by the Marangoni force on a wall with a temperature gradient

Motion of silicone oil and water droplets induced by the Marangoni force was numerically simulated by using two‐ and three‐dimensional second‐order finite difference methods with the CIP and the level set methods. The surface tension was introduced by the continuum surface force (CSF) method. The results clearly showed the flow induced by the Marangoni force and the dependence of droplet velocity on droplet size, contact angle, temperature gradient, and fluid properties. The Marangoni force balanced with the viscous force in the small contact angle case; on the other hand, in the large contact angle case, it balanced with the normal component of surface tension. As for the effect of fluid properties on droplet motion, the temperature coefficient of surface tension had a much larger effect than did viscosity, thermal diffusivity, or surface tension. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 81–93, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20004