Experimental Study on the Skin Friction Reduction of Flat Plate by Formation of Air Cavity

The effects on the local skin friction of smooth flat plate by formation of air cavity are investigated experimentally,under the conditions of several variations of air injection angle,pore size,porous surface area and transverse step.The experimental results show that local skin friction of downstream of the porous section could be reduced at extent ranging from 50% to 90%,by injection air through pore or slot,with free stream velocities from 2 to 6 m/s.The pore size and area of air injection surface have small effect on skin friction reduction,step has significant effect on skin friction reduction.The mechanism of the skin friction reduction is due to the formation of air cavity,mixed with air and water,between the flat plate and its water boundary layer.

臂式离心机机室空气内摩擦功率及支座摩擦系数的实验反演分析

为了提高臂式离心机驱动功率的预估精度,对工程解析计算方法中几个前提参数(即机室空气内摩擦发热消耗功率与总风阻功率之比C_(1)、机室内空气从出风口自然流出的线速度与其环向线速度之比C_(2)、支座的摩擦系数f_(e))的确定方法进行了研究。首先,针对一台臂式离心机以四种不同转速稳定运行的状态,利用离心机控制系统,读取了四组驱动电枢电压U和电枢电流I数据,采用三杯仪测试了四个空气从出风口流出的线速度V_(0)数据;然后,根据离心机风阻功率、排风功率和支座摩擦功率的解析理论表达式、能量守恒原理,建立了含待定系数C_(1)、C_(2)、f_(e1)和f_(e2)的关于U、I和V_(0)的求解方程,并将四组实测数据U、I和V_(0)代入所建方程,得到了关于系数C_(1)、C_(2)、f_(e1)和f_(e2)的求解方程组;最后,将所得C_(1)、C_(2)、f_(e1)和f_(e2)的值用于计算另一台类似臂式离心机的驱动功率。研究结果表明:所得驱动功率的计算结果比实测结果大2.23%,而以前依据经验取定C_(1)、C_(2)和f_(e)时的计算结果比实测结果小6.30%。对应用实例的综合分析表明:所建方法可弥补这些参数在以前的工程计算中,其取值缺乏实验依据的不足,提高这些参数的取值准确性和臂式离心机驱动功率的预估精度。

超细晶层状结构304L不锈钢摩擦学行为研究

用MS-T3000型多功能摩擦磨损试验机评估了超细晶层状结构304L不锈钢在空气和PAO4润滑条件下的摩擦学性能,以传统粗晶态304L不锈钢为对比材料,分析磨损前后的材料结构、成分及磨痕表面形貌,揭示磨损机制。结果表明:在空气湿度为75%时,粗晶态304L不锈钢与超细晶层状结构304L不锈钢的平均摩擦因数分别约为0.44和0.33,粗晶态304L不锈钢表面发生严重的黏着磨损和氧化磨损,超细晶层状结构304L不锈钢表面发生较缓和的黏着磨损和微氧化磨损。在PAO4润滑条件下,粗晶态和超细晶层状结构304L不锈钢的摩擦因数和磨损率都显著降低,粗晶态样品为典型的微切削磨损机制,超细晶层状结构样品为更微弱的微切削磨损机制。

金属防腐用高分子封闭涂层的制备及性能

[目的]提出一种操作简单、防腐效率高、耐摩擦性能强的金属表面防腐涂层的制备方法。[方法]以环氧树脂、氨乙基氨丙基三甲氧基硅烷、微米级纤维素颗粒、纳米级二氧化硅颗粒、正硅酸四乙酯、丙烯酸六氟丁酯等原料制备了一种封闭剂,采用空气喷涂法在铜基体上得到封闭涂层。[结果]丙烯酸六氟丁酯单体和修饰剂三氟丙醇的引入使涂层拥有较低表面能的同时又具有优异的粘附性。涂有封闭剂的铜片经历6000 h的盐雾试验后质量仅减少了7.4%左右,在王水中被完全腐蚀掉的时间比空白铜片延后了至少350 min。[结论]该封闭涂层具有优异的耐腐蚀和耐摩擦性能。

Effect of Electrical Current on the Tribological Behavior of the Cu-WS_2-G Composites in Air and Vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.

Study of the Motion of a Weighted Cotton Ball in an Air Stream with a Known Impact Trajectory

In the cotton factories ginning process bales of raw cotton in cotton tube transporting through the air has written in the article. As a result, cotton with cotton in the air separated by a separator device with air traffic, has an important theoretical study. First of all, the air flow that affects the dynamic pressure and laws is based on the model established in this study and the results are obtained.

Liquid Circulation in a Multi-tube Air-lift Loop Reactor

A multi-tube air-lift loop reactor (MT-ALR) is presented in this paper. Based on the energy conservation, a mathematical model describing the liquid circulation flow rate was developed, which was determined by gas velocity, the cross areas of riser and downcomer, gas hold-up and the local frictional loss coefficient. The experimental data indicate that either increase of gas flow rate or reduction of the downcomer diameter contributes to higher liquid circulation rate. The correlation between total and the local frictional loss coefficients was also established.Effects of gas flowrate in two risers and diameter of downcomer on the liquid circulation rate were examined. The value of total frictional loss coefficient was measured as a function of the cross area of downcomer and independent of the gas flow rate. The calculated results of liquid circulation rates agreed well with the experimental data with an average relative error of 9.6%.

某空气舵系统非线性建模及参数识别

可靠的空气舵系统动力学模型对飞行控制系统设计非常重要。以某飞行器空气舵系统为对象,考虑运动副间隙和摩擦,研究系统非线性动力学建模问题。分别基于Coulomb摩擦模型和Stribeck摩擦模型建立了系统的四参数和六参数动力学模型;采用系统频响测试数据和参数辨识方法考察了模型的可靠性;改进Coulomb摩擦模型,提出系统的五参数非线性动力学模型。研究结果表明,用五参数模型拟合不同工况测试数据,获得的系统参数一致性优于四参数和六参数模型,可靠性更高。

Friction coefficient and error test via micro-rotation mechanics model

An air pressure-loading mode incorporated into the friction apparatus is firstly applied to coatings tribology involving large load, automation, stepless and continuous loading processes. A novel measurement principle is proposed and a micro-rotation mechanics model was developed for high precision measurement of friction coefficient. By properly designing and locating two sensors real-time monitoring the normal and friction forces, the troublesome influences in friction measurement is considerably relieved which come from surface characteristics of coatings of the samples in traditional friction test processes. By calculation and analysis, the max rotation angle 0max = 0.0018°is gained, which indicates that the measurement error of the apparatus is greatly reduced. The whole system error is about 1.15% given by finite element method and indication error of the least square fitting of measurements.

STUDY ON MECHANISM OF FIBRE DEPOSITION IN OE FRICTION SPINNING

This paper deals with the mechanism of fibre deposition in OE friction spinning.A fibremodel has been established to analize and evaluate fibre.straightness after deposition.Exper-iments have been done on the measurement of air flow speed at the entrance and exit of thefeed duct which is mounted in an experimental friction spinning unit.Conclusion has beendrawn:the weakness of friction spun yarn is mainly due to its poor fibre straigthness after dep-osition.

实现振动解耦的摩擦学行为模拟试验台设计方法

摩擦学行为研究对认识界面摩擦磨损特征,揭示摩擦自激振动产生机理及演变规律,保证摩擦系统的可靠运行具有重要意义.目前的摩擦学行为模拟试验装置由于刚性连接方式对界面摩擦振动响应的干扰,难以精确开展摩擦学行为研究和材料磨损性能评估,亟需隔离摩擦界面与机械连接部件之间的耦合振动.为此,基于气浮轴承设计了1种可实现振动解耦的摩擦学行为模拟试验台,通过锤击试验以及与非振动解耦摩擦试验机的对比试验,测定并验证了试验台振动解耦功能的有效性.锤击试验表明,未充气状态下在气浮轴承轴套处检测到多个频率,而充气状态下仅有较低的单一频率存在.摩擦学对比试验发现,随着法向载荷或往复滑移频率的增加,振动解耦试验台测得的振动加速度均方根变化率线性增加,而非振动解耦试验机的振动加速度均方根变化率呈先增后减的趋势,存在明显差异.因此,该摩擦学行为模拟试验台成功实现了摩擦界面与机械连接部件之间的振动解耦,为进一步精确探究界面摩擦学行为的影响因素和演变规律、揭示摩擦自激振动产生机理以及在对比评估不同材料摩擦磨损性能时排除摩擦学设备的影响方面提供了新的思路及有效的手段.

超高速空气静压轴承电主轴损耗及热特性分析

电主轴作为高度集成的机电一体化产品,相对于传统的机床主轴,其将主轴电机集成内置到主轴内部,具有结构紧凑、低惯性、高转速、低噪声、快速响应等优点,但由于电机集成于主轴内部,运转时产生的大量热量,会引起电主轴温度快速升高,使电主轴的热态特性和动态特性变差,进而影响电主轴的正常加工甚至出现运转过程中卡死。为定量计算整机的损耗及发热变形情况以解决此问题,通过建立超高速空气轴承电主轴热特性分析模型,采用解析法与有限元仿真相结合的方法,求解电主轴轴芯空气摩擦损耗、电机损耗,获得了电主轴温升等热态特性参数,并通过试验验证分析模型和方法的准确性,研究模型中轴芯空气摩擦损耗理论计算数值与实验数值偏差在2%以内,为电主轴设计提供了理论依据。

用阿特伍德机精确测量重力加速度

阿特伍德机是普通物理力学部分的一个常见模型,涉及刚体定轴转动过程中转动角速度、角加速度等物理量的相关计算。本文利用PASCO转动传感器搭建了一个简单的阿特伍德机装置,在考虑到滑轮转动惯量对实验影响的情况下测出重力加速度为g=9.618±0.051m/s^(2),之后本文进一步考虑转动轴承摩擦力矩的影响,使用改变装置释放条件的方式对摩擦力矩进行修正,测得重力加速度为g=9.791±0.028m/s^(2)。搭建的装置和实验方法具有通用性,可作为大学物理实验的内容提高学生实践动手能力和理实结合能力。

单巷长距离综掘工作面局部通风技术探讨

针对梅花井煤矿一水平11采区111801工作面回风巷单巷长距离综掘工作面压入式局部通风的技术难点,从风筒漏风量大、风筒摩擦阻力大、风机风压导致风筒脱节3个方面进行分析,通过合理选择局部通风机、减少漏风技术、降低通风阻力技术、强化保障管理等技术措施实现了3500 m以上通风距离,保证了单巷长距离综掘工作面快速掘进。

空气湿度对浸金属碳滑板磨耗的影响研究

冬季天气干燥时,浸金属碳滑板摩擦磨损情况往往进一步加剧。本文简要分析了空气湿度对浸金属碳滑板摩擦磨损性能的影响机理,并基于空气湿度可控的摩擦磨损等效试验装置进行了不同空气湿度下浸金属碳滑板载流摩擦磨损试验,通过分析试验数据得出空气湿度对于浸金属碳滑板摩擦磨损参数的影响规律。

空气流速对纤维素类材质文物霉菌病害的影响

霉斑是纤维素类材质文物中常见的一类病害。文章通过模拟实验研究纤维素类材质文物(纸质、棉质、竹木质)表面霉斑受空气流速与其表面摩擦系数的影响程度,单因素方差分析结果表明,空气流速对霉斑数量影响显著,且呈现线性递减关系;双因素方差分析结果表明,表面摩擦系数与空气流速有显著交互作用,共同影响霉斑数量,空气流速在0.25 m/s以上时,对霉斑数量有显著抑制效果,可作为文物存放位置的参考指标。

通量方差法估算海气界面动量通量的分析

利用南海北部博贺海洋气象观测平台涡动相关系统的长期湍流观测资料,分析并讨论了不同大气稳定度条件下的湍流通量方差相似性关系,提出了适用于小于18 m/s风速的归一化标准差与稳定度的参数化关系公式。应用通量方差法估算了海气界面的动量通量与摩擦速度,并与涡动相关系统的观测结果进行对比分析,对参数化公式进行了检验。结果表明:不同大气层结条件下的速度归一化标准差与大气稳定度满足1/3相似性关系,基于相似性函数的通量方差法估算结果与涡动相关系统测量结果较为一致,不稳定大气层结条件下的相关性高于稳定层结条件。应用通量方差法估算近海海气界面动量通量是可行的。

氦-空混合气体下热辅助磁存储头盘界面润滑剂迁移机理

为了提升硬盘读取数据的效果,进一步增加硬盘的存储量,对头盘界面空间散热的影响因素及其影响规律,以及氦-空混合气体环境下润滑剂的迁移规律进行研究,建立了磁头磁盘界面空间气浮轴承模型和热辅助下磁头磁盘界面空间润滑剂迁移的分子动力学模型,并以此为基础将气浮轴承模拟结果输入到模型中,探析氦气质量分数及其他影响参数对头盘界面空间散热的影响规律,以及激光热流及其他影响参数对氦-空混合气体环境下润滑剂迁移的规律。结果表明:氦气质量分数加剧了头盘界面空间的散热强度;与磁头飞行高度相比,润滑剂厚度、环境压强的增大更利于磁头磁盘界面空间的散热;润滑剂转移量随激光热流、磁盘速度的增大而增大;随着脉冲光斑尺寸的增加,润滑剂转移量先增加后减小,且光斑半径为20σ时润滑剂转移量达到最大值;Zdol2000,Zdol4000,Ztetraol2000及ZTMD等4种润滑剂中,Zdol4000造成的润滑剂转移量最大。研究获得的头盘界面空间散热因素及热辅助下影响润滑剂转移量影响因素的结果,能够为充氦热辅助硬盘的设计提供理论依据,也可为下一代磁存储记录技术的发展提供技术支持。

大比摩阻空调水系统可行性分析

本研究旨在探讨分析大比摩阻空调水系统的技术和经济可行性。以杭州市的一栋高层办公楼的冷水系统为例,比较了常规比摩阻和大比摩阻2种管道系统设计方案。通过技术分析,得到了冷水泵系统在供冷季运行期间的逐时扬程、流量和效率,进而得到供冷季的运行能耗。计算表明2种方案的耗电输冷比均满足规范要求。另外,分析比较了冷水泵供冷季的运行费用和管道系统的造价,通过财务净现值(FNPV)分析证明了采用大比摩阻管道在系统寿命周期内可节约成本。

商业建筑空调水管设计经济比摩阻选取

实际项目中,选用合适的管道比摩阻,可以兼顾管道总阻力、系统效率以及经济效益,使空调水系统设计更为合理。基于此,文章计算了单位长度水管阻力的耗电量及费用,匹配了对应单位长度管道初投资,对比了年平均综合使用费用,最终选取了最经济比摩阻。