Friction factor in micropipe gas flow under laminar,transition and turbulent flow regime

The present work deals with the compressible flow of nitrogen gas inside microtubes ranging from 30 to 500 μm and with different values of the surface roughness (<1%), for different flow regimes. The first part of the work is devoted to a benchmark of friction factor data obtained at DIENCA (University of Bologna) and the ENEA laboratories, using fused silica pipes of 50 and 100 μm. Data overlapping is excellent thus evidencing how the agreement of the experimental data with the classic theory is independent of the measurement system. The second part of the work demonstrates that classic correlations can predict friction factor in laminar flow without revealing any evident influence of the surface roughness. The laminar-to-turbulent transition starts for Reynolds number not lower than 2000 for smooth pipes, while tending to larger values (3200–4500) for rough pipes. Anyway, contrarily to other available results, no dependence of the critical Reynolds number on the L/D has been observed. Changes in the flow regime have been found of the sharp and smooth type, like for larger pipes; smooth transition looks typical of smooth pipes while the sharp transition in the flow pattern is associated with rough pipes. In the fully developed turbulent regime, obtained for both smooth and rough pipes, an agreement between experimental data and the Blasius correlation has been verified for smooth pipes, while for rough pipes the agreement with predictions given by the Colebrook equation is rather modest.     

高速轧机工作界面非稳态混合润滑过程多因素耦合辊缝力学特性研究

综合运用摩擦润滑理论、流体力学理论、轧制理论,建立了考虑动态辊缝非稳态润滑过程多因素耦合轧制界面力学模型.该模型综合运用了基于非稳态润滑过程的界面摩擦模型、工作辊运动模型、辊缝应力分布模型构成的界面薄膜约束多因素耦合模型.系统分析了非稳态混合润滑状态下,当压下率和表面粗糙度为定值时不同时刻工作区压应力和摩擦应力的分布情况,以及压下率为定值时工作区压应力、摩擦应力随表面粗糙度和后张应力变化的情况.结果表明:在压下率和表面粗糙度为定值时,当具有较大的后张应力时,工作区压应力和摩擦应力却较低,同时压应力梯度和摩擦应力梯度也相当小,摩擦应力在入口和出口边缘处达到最大,当后张应力减小时,辊缝间压应力和摩擦应力增加,压应力梯度和摩擦应力梯度较大;在压下率为定值而表面粗糙度为变量时,当后张应力较大时,表面粗糙度对工作区中的压应力和摩擦应力影响非常大,辊缝间压应力和摩擦应力随着表面粗糙度的增加而增加,当后张应力较小时,表面粗糙度对工作区中的压应力和摩擦应力影响不太明显.     

轮轨试样表面粗糙度取向对油润滑下摩擦系数的影响

黏着力是列车安全与平稳运行的关键因素之一.最大黏着力与摩擦力有关,摩擦力的减小会导致黏着力的降低.表面粗糙度及其取向是影响摩擦系数的重要因素,然而,有关表面粗糙度取向对于混合润滑状态下摩擦系数的影响的研究结论似乎是矛盾的.用激光离散改性技术将车轮试样表面制备成具有菱形、纵纹、横纹3种典型的形貌,并且与不作激光离散改性处理的车轮试样作对比,用基于确定性模型的统一雷诺方程数值分析法和小比例尺度的轮轨试样摩擦学实验,得到的结论是:在油润滑状态下,激光表面形貌大幅提高摩擦系数,其中菱形对应的摩擦系数最大,纵纹与横纹的摩擦系数相差不大,摩擦系数的大小主要取决于由表面粗糙度取向决定的接触区内粗糙峰接触压力与总压力之比,侧流效应也是影响摩擦系数的重要因素,它主要取决于接触区内表面粗糙度的取向.     

Direct numerical simulation of turbulent heat transfer on the Reynolds analogy over irregular rough surfaces

The effect of rough surface topography on heat and momentum transfer is studied by direct numerical simulations of turbulent heat transfer over uniformly heated three-dimensional irregular rough surfaces, where the effective slope and skewness values are systematically varied while maintaining a fixed root-mean-square roughness. The friction Reynolds number is fixed at 450, and the temperature is treated as a passive scalar with a Prandtl number of unity. Both the skin friction coefficient and Stanton number are enhanced by the wall roughness. However, the Reynolds analogy factor for the rough surface is lower than that for the smooth surface. The semi-analytical expression for the Reynolds analogy factor suggests that the Reynolds analogy factor is related to the skin friction coefficient and the difference between the temperature and velocity roughness functions, and the Reynolds analogy factor for the present rough surfaces is found to be predicted solely based on the equivalent sand-grain roughness. This suggests that the relationship between the Reynolds analogy factor and the equivalent sand-grain roughness is not affected by the effective slope and skewness values. Analysis of the heat and momentum transfer mechanisms based on the spatial- and time-averaged equations suggests that two factors decrease the Reynolds analogy factor. One is the increased effective Prandtl number within the rough surface in which the momentum diffusivity due to the combined effects of turbulence and dispersion is larger than the corresponding thermal diffusivity. The other is the significant increase in the pressure drag force term above the mean roughness height.     

基于表面粗糙度影响的摩擦声发射特性研究

接触表面粗糙度是影响摩擦的重要因素之一,而摩擦为复杂的动态过程,在线测量非常困难.为了研究表面粗糙度对摩擦的影响,利用声发射技术对不同粗糙度表面的摩擦过程进行了检测.研究结果表明：摩擦系数随着表面粗糙度的增大而增大,但当粗糙度增至一定程度时,摩擦系数随着表面粗糙度的增大而减小;声发射信号各参数（振铃计数、振幅、能量）随表面粗糙度的变化规律与摩擦系数随粗糙度的变化规律一致;声发射信号各参数与表面粗糙度存在密切的对应关系,因此,用声发射技术实时监测摩擦过程是可行的.     

A friction contact stiffness model of fractal geometry in forced response analysis of a shrouded blade

To investigate the nonlinear vibration behavior of a shrouded blade with friction dynamic contact interface, a friction contact stiffness model is proposed to describe the friction force at different rough interfaces and different normal loads. In the proposed model, the friction contact interface is discretized to a series of friction contact pairs and each of them can experience stick, slip, or separate states. Fractal geometry is used to simulate the topography of contact surfaces. The contact stiffness is calculated using the Hertz contact theory and fractal geometry, which is related to contact interfaces parameters including normal load, roughness, Young??s modulus, and Poisson??s ratio. The trajectory tracking method is used to predict the friction force and it is not necessary to judge the transition condition among stick, slip, and separate states. It is suitable for complicated periodic motion of the contact interfaces. The forced response of a real shrouded blade is predicted using the proposed model and the multi-harmonic balance method. The effect of surface roughness, initial normal load, and contact area on the forced response of a shrouded blade is studied. It is shown that contact stiffness increases with normal load and fractal dimension. The resonant amplitude is sensitive to the initial normal load and contact surface roughness. The response can be influenced by the contact area, which is an important parameter for blade designers.     

机械密封织构化表面粗糙度效应的有限元模型与摩擦学特性分析

为研究粗糙度效应对机械密封织构化端面承载能力、摩擦学特性以及密封性能的影响,基于有限元法建立了机械密封的混合润滑模型,针对确定性非高斯随机分布粗糙表面,考虑润滑液膜的宏微观空化作用和粗糙峰的接触,研究了圆孔型织构化机械密封的摩擦学性能和密封性能. 结果表明:平衡状态下处于全膜润滑区间的非高斯型粗糙织构表面,在研究范围内均方根值越大的表面其减摩效果越好,且泄漏率越大,从混合润滑区到全膜润滑区转变的速度也随之增大;非高斯表面的偏态值和峰态值对密封表面的承载能力,润滑性能以及密封性能有影响,但无明显规律;在混合润滑区,圆孔织构具有增摩效果,而在全膜润滑区表现出减摩效果.      

表面处理工艺对长石质牙科陶瓷摩擦学特性的影响

通过体外模拟口腔环境。采用往复摩擦磨损试验机考察了经自身上釉、离子交换及机械打磨抛光等3种不同表面处理方式的长石质牙科陶瓷材料在人工唾液润滑下同纯钛对摩时的摩擦学特性。探讨了表面处理对牙科陶瓷材料显微硬度、断裂韧性、表面形貌和元素组成的影响．结果表明：牙科临床常用的表面处理工艺对牙科陶瓷摩擦系数的影响较小。经3种不同表面处理的陶瓷试样的摩擦系数仅在滑动初期存在一定差异；经离子交换处理的陶瓷试样的抗磨性能较优；长石质牙科陶瓷同纯钛对摩时。偶件钛向陶瓷试样磨损表面发生转移；牙科陶瓷材料的耐磨性能同力学性能的相关性不强，原因在于耐磨性能同微观结构等密切相关．     

用侧向力显微镜对不同非晶态GeSbTe薄膜的摩擦性能研究

采用侧向力显微镜研究了磁控溅射方法制备的GeSbTe薄膜在大气环境中的纳米级摩擦性能,考虑了相对湿度、扫描速度及表面粗糙度对其摩擦性能的影响,对比不同成分的GeSbTe薄膜的摩擦特性.结果表明:在相对湿度较大时,扫描速度对针尖和GeSbTe薄膜之间的摩擦力影响很大;在其它条件相同、外加载荷较大时,同一载荷下的摩擦力与表面粗糙度呈线性关系,但在外加载荷较小的情况下,二者呈现非线性变化规律;相对湿度对Ge2Sb2Te5薄膜和针尖的粘附力影响较GeSb2Te4薄膜弱,且粘附力使得摩擦系数减小;在同一相对湿度下,由于薄膜成分的变化导致硬度不同,其对薄膜的摩擦性能也有一定影响.     

45钢/PA66配副干滑动摩擦磨损性能研究

采用45钢销和尼龙PA66盘,运用正交试验法在MMW-1A万能摩擦磨损试验机上研究干滑动摩擦条件下速度、载荷和金属销表面粗糙R_a对45钢/PA66配副摩擦学性能的影响. 通过极差分析与方差分析发现:载荷、粗糙度对摩擦系数与磨损量有显著影响,而速度影响相对较小. 当载荷为50 N,速度为11.25 m/s,R_a为0.60 μm时,摩擦系数与磨损量最小. 基于正交试验的最优结果,开展控制变量试验,试验结果表明:载荷小于90 N时,PA66以黏着磨损为主;载荷为90 N时,PA66磨损形式为犁削磨损和黏着磨损. 载荷为140 N时,PA66的磨损形式为黏着磨损并伴有胶合现象的产生. R_a小于0.46 μm时,PA66以黏着磨损为主;R_a为0.46 μm时,PA66的磨损形式为黏着磨损和犁削磨损且在对偶金属销表面上形成连续的转移膜;R_a大于0.46 μm时,PA66以犁削磨损为主.      

Three-dimensional separation in the neighborhood of roughness on the surface of an axisymmetric body

Steady-state viscous incompressible fluid flow past an axisymmetric slender body is considered at high Reynolds numbers in the regime with vanishing surface friction in a certain cross-section. In a small neighborhood of this cross-section interaction between the boundary layer flow and the external irrotational stream develops. In order to study the structure of the three-dimensional flow with local separation zones it is assumed that there is three-dimensional roughness on the surface of the body with the scale of the interaction zone. For this zone a numerical solution of the problem is obtained and its nonuniqueness is established. The surface friction line (limiting streamline) patterns with their inherent features are constructed.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 67–79, May–June, 1995.Thus, on the basis of the asymptotic marginal separation theory it is possible to obtain fairly simple solutions describing flows with a complex surface friction line structure.     

表面形貌对滑动接触界面摩擦行为的影响

为了研究表面形貌对拉延形成的滑动接触界面摩擦行为的影响,设计了一种新型的摩擦试验装置.在油润滑条件下,针对具有单向沟槽、规则圆形凹坑和随机表面的铝合金试样,以不同滑动速度与接触压力进行一系列摩擦试验.利用非接触式三维轮廓仪测量出试验前后试样的三维表面形貌参数,并选取表面高度算术平均偏差Sa,表面支承指数Sbi,中心区空体体积Vvc和谷区空体体积Vvv来分析滑动接触界面表面形貌的变化规律.结果表明:规则圆形凹坑表面比单向沟槽表面和随机表面具有较低的摩擦系数;在相对低的接触压力下,3种表面的摩擦系数随着接触压力的增大而减小,但在高的接触压力下,3种表面的摩擦系数随着接触压力的增大而增大;在接触压力一定的情况下,3种表面的摩擦系数对滑动速度有显著依赖性;表面形貌、滑动速度和接触压力是影响滑动接触界面摩擦行为的重要因素.     

Heat transfer and fluid flow analysis of artificially roughened ducts having rib and groove roughness

Artificially roughness is one of the well known methods of enhancing heat transfer from the heat transfer surface in the form of repeated ribs, grooves or combination of ribs and groove (compound turbulators). The artificial roughness produced on the heat transferring surface is used in cooling of gas turbine blades, nuclear reactor, solar air heating systems etc. Solar air heaters have wide applications in low to moderate temperature range, namely, drying of foods, agricultural crops, seasoning of wood and space heating etc. Solar air heaters have low value of convective heat transfer coefficient between the working fluid (air) and the heat transferring surface, due to the formation of thin laminar viscous sub-layer on its surface. The heat transfer from the surface can be increased by breaking this laminar viscous sub layer. Hence, in the present work compound turbulators in the form of integral wedge shaped ribs with grooves are used on the heat transfer surface, to study its effect on the heat transfer coefficient (Nusselt number) and friction factor in the range of Reynolds number 3,000–18,000. The roughness produced on the absorber plate forms the wetted side of upper broad wall of the rectangular duct of solar air heater. The relative groove position (g/p) was varied from 0.4 to 0.8 and the wedge angle (Φ) was varied from 10° to 25°, relative roughness pitch (p/e) and relative roughness height (e/D) was maintained as 8.0 and 0.033 respectively. The aspect ratio of the rectangular duct was maintained as 8. The Nusselt number and friction factor of the artificially roughened ducts were determined experimentally and the corresponding values were compared with that of smooth surface duct. It is observed that wedge-groove roughened surface shows more enhancement in heat transfer compared to only rib roughened surface arrangement. The investigation revealed that Nusselt number increases 1.5–3 times, while the friction factor increases two to three folds that of the smooth surface duct in the range of operating parameters. It is also observed that in rib–groove roughness arrangement with relative groove position of 0.65 shows the maximum enhancement in the heat transfer compared to the other rib-groove roughness arrangements. Statistical correlations for the Nusselt number and friction factor have been developed by the regression method in terms of the operating and roughness parameters. A program was also developed in MATLAB for the calculation of thermal efficiency and thermal effectiveness. It was observed that the thermal efficiency is more for wedge angle of 15° and relative groove position of 0.65 and its value ranges from 42 to 73 %. The uncertainties in the measurements due to various instruments for the Reynolds number, Nusselt number, and friction factor have been estimated as ±3.8, ±4.54 and ±7.6 % respectively in the range of investigation made.     

Frictional pressure drop during vapour–liquid flow in minichannels: Modelling and experimental evaluation

Condensation in minichannels is widely used in air-cooled condensers for the automotive and air-conditioning industry, in heat pipes and other applications for system thermal control. The knowledge of pressure drops in such small channels is important in order to optimize heat transfer surfaces. This paper presents a model for calculation of the frictional pressure gradient during condensation or adiabatic liquid–gas flow inside minichannels with different surface roughness. In order to account for the effects of surface roughness, new experimental frictional pressure gradient data associated to single-phase flow and adiabatic two-phase flow of R134a inside a single horizontal mini tube with rough wall has been used in the modelling. It is a Friedel (1979) [Friedel, L., 1979. Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow. In: Proceedings of the European Two-Phase Flow Group Meeting, Ispra, Paper E2] based model and it takes into account mass velocity, vapor quality, fluid properties, reduced pressure, tube diameter, entrainment ratio and surface roughness. With respect to the flow pattern prediction capability, it has been built for shear dominated flow regimes inside pipes, thus, annular, annular-mist and mist flow are here predicted. However, the suggested procedure is extended to the intermittent flow in minichannels and it is also applied with success to horizontal macro tubes.     

不同表面粗糙度的摩擦副润滑状态的 Stribeck曲线研究

利用Stribeck润滑曲线理论分析和薄膜润滑试验考察了不同表面粗糙度的钢球与圆盘点接触及钢滑块与圆盘面接触摩擦副的润滑状态，通过改变圆盘转速获得了包含薄膜润滑状态的Stribeck曲线．结果表明：在Stribeck曲线上可以划分出薄膜润滑状态，其位于摩擦系数谷底附近；薄膜润滑的产生及其区间大小同圆盘表面粗糙度密切相关；圆盘表面粗糙度较小时更易形成薄膜润滑状态，而圆盘表面粗糙度较大时薄膜润滑状态不明显；光滑表面对应的Stribeck曲线谷底较宽，相应的薄膜润滑区间亦较宽．     

偶件表面粗糙度对碳纤维增强尼龙复合材料摩擦学性能的影响

在栓-盘摩擦磨损试验机上考察了干摩擦条件下偶件表面粗糙度对碳纤维增强尼龙（PA1010）复合材料摩擦学性能的影响,采用不迩显微镜观察分析了偶件表面转移膜的形貌。结果表明,碳纤维能够明显提高PA1010的耐磨性能,当碳纤维增强相的质量分数为10%和20%时,增强PA1010复合材料的磨损率比非增强PA1010的降低3～6倍。这是由于碳纤维起到了承载作用并具有较强的抗犁削能力所致,磨损表面形貌光学显微分析表明：磨损前后偶件表面形貌发生了明显的变化;当偶件表面粗糙度Ra处于0.11～0.13um范围内时,复合材料的摩损率最低;随Ra值的增大或减小微切削和转移膜疲劳脱落加剧致使复合材料的磨损率快速增大。     

钢轨-磨石相互作用的摩擦学模拟试验研究

设计了新型钢轨打磨摩擦试验机,实现了钢轨-磨石相互作用的摩擦学模拟试验,研究了不同打磨参数下的钢轨-磨石界面的摩擦系数、表面粗糙度、磨损量及硬度等变化情况.结果表明:随磨石粒度增加,打磨钢轨表面粗糙度和磨损量呈减小趋势;增加磨石转速导致摩擦系数和表面粗糙度减小,打磨钢轨磨损量和表面硬度增加;随打磨压力增加,摩擦系数和表面粗糙度呈减小趋势,但磨损量和表面硬度增加;打磨钢轨表面存在明显的犁沟,随磨石粒度减小犁沟变宽;所设计钢轨打磨摩擦试验机可用来评价不同参数下钢轨-磨石界面相互作用的摩擦学行为.     

点接触润滑粗糙表面滑动摩擦力的预测研究

在整个润滑区域内基于统一Reynolds方程的混合润滑模型,根据流变模型计算流体摩擦力,根据边界膜极限剪应力模型计算微突体接触摩擦力,二者相加得到混合润滑摩擦力.分析了粗糙度幅值和纹理对摩擦系数的影响以及非牛顿流变模型对流体摩擦系数的影响.模拟跨越整个润滑区,即弹流润滑、混合润滑和边界润滑,得到完整的Stribeck曲线.结果表明,表面越粗糙,混合润滑的摩擦系数越大,弹流润滑和边界润滑时粗糙度幅值影响很小.交叉斜纹的润滑效果优于横向纹理.不同极限剪应力流变模型计算的摩擦系数相差不大.     

钛珠涂层和羟基磷灰石涂层与皮质骨界面的切向微动损伤研究

通过体外模拟切向微动行为,探究两种生物固定材料-钛珠涂层和羟基磷灰石涂层与皮质骨界面之间的微动摩擦磨损机理,建立钛珠涂层和羟基磷灰石涂层与骨组织生物固定界面的微动-松动-骨损伤的关系.结果表明,界面处于部分滑移区时,弹塑性协调作用起主导,固定界面不易产生松动,皮质骨表面损伤以剥落为主,表面损伤较小.增大微动位移幅值,界面摩擦系数增大,皮质骨的损伤增大.同时,划分出了钛珠涂层和羟基磷灰石涂层部分滑移区的工况分界线.通过对比两种材料的摩擦磨损机理,钛珠涂层与皮质骨界面固定效果较好,羟基磷灰石涂层与皮质骨界面损伤较小.结果对人工关节生物固定起到一定的参考作用.     

Determination of turbulent pressure loss of non-newtonian oil flow in rough pipes

Summary A detailed theoretical analysis is carried out of the turbulent flow of non-newtonian fluids in rough pipes in which approximate assumptions are of negligible importance. A new friction-factor equation (BNS-equation) is obtained which is valid for turbulent flow of power-law type non-newtonian fluids in the transition region between smooth and wholly rough wall turbulence.The accuracy of the deduced formula was checked by in situ measurements on a crude oil pipeline having a diameter of 305 mm and a length of 161 km. For this pipeline the Reynolds numbers ranged between 10⁴ and 10⁵. The computed and the measured friction factors showed good agreement. The mean of the absolute values of the relative error was 4.1%, while the standard deviation was 0.81%, these results appear to give better fitting than other similar equations published earlier.The BNS-equation is the first analytically deduced relationship for non-newtonian crude oils which includes the effects of both Reynolds number and relative roughness of the pipe.With 6 figures and 2 tables