Richtmyer-Meshkov不稳定性扰动增长率冲击模型的微尺度效应

本文通过考虑流体介质输运性质对激波理论的影响,针对微型激波管中的RichtmyerMeshkov(RM)不稳定性,分析了微尺度效应对三种冲击模型扰动增长率的影响。一维气体动力学计算结果表明:流动尺度对于Richtmyer模型、Meyer-Blewett(M-B)模型和Vandenboomgaerde-Mügler-Gauthier(V-M-G)模型中的线性扰动增长率有显著影响。当流动尺度由宏观状态逐渐减小至微尺度时,三种模型的扰动增长率均会经历从少量增长到明显下降,然后迅速上升的过程。微尺度条件下,V-M-G模型的扰动增长率相比宏观尺度有显著提高。与其他两种模型相比,修正后的V-M-G模型更合理地描述了微尺度效应对于线性扰动增长率的影响。此外,对于修正后的V-M-G模型,当入射激波马赫数较低时,扰动增长率受微尺度效应的影响更为明显。     

沸腾换热的分形分析

研究沸腾换热过程是安全、高效地利用能源的基础.简要评述了沸腾换热(池内沸腾、流动沸腾、临界热流密度和纳米流体沸腾换热)的研究进展;详细论述了采用分形理论和方法研究沸腾换热分析解的理论和方法;指出了采用分形理论和方法有可能解决其它尚未解决的有关沸腾换热的若干课题和方向.      

微尺度气体流动

了解微尺度气体流动特点是微机电系统设计和优化的基础.有关的研究可以上溯到20世纪初Knudsen的平面槽道流动质量流量的测量和Millikan的小球阻力系数的测量,实验结果揭示了稀薄气体效应即尺度效应对气体运动的重要影响.由于流动特征长度很小,微尺度气流经常处于滑流区甚至过渡领域,流动的相似参数为Knudsen数和Mach数.因此可以考虑利用相似准则,通过增大几何尺寸、减小压力的途径,解决微机电系统实验观测遇到的困难.为解决直接模拟Monte Carlo方法分析微机电系统中低速稀薄气流遇到的统计涨落困难,我们提出了信息保存法(IP),该方法能够有效克服统计散布,并已成功用于多种微尺度气流.      

微通道人工泡状流的Lattice Boltzmann数值模拟

采用高频电控热激发汽泡的方式构造微通道人工泡状流,可以有效抑制微通道沸腾流动的不稳定性和强化传热。本文基于Lattice Boltzmann大密度比多相流复合模型,数值研究了通道内人工泡状流的流动和传热,通过比较分析不同发泡频率的泡状流,量化分析了汽泡运动和增长对微通道流动与传热的相互影响。一方面着重分析了汽泡运动对微通道运动边界层以及汽泡相变增长对热边界层的影响,另一方面也研究了边界层对汽泡动力行为的影响,所得结论对研究抑制微通道沸腾流动不稳定性和强化传热有参考意义。     

微/纳尺度接触问题计算方法研究进展

接触问题广泛存在于现实生活的众多领域,近来随着微/纳米技术的不断发展,接触力学在基础理论和研究方法上面临许多新的挑战.本文在摩擦学的范畴内,对近年发展的若干求解微/纳尺度接触问题的计算方法及理论进行了综述.按发展先后及所解决问题的尺度范围划分,主要有3类评估微/纳尺度接触性能的计算方法:(1)连续介质力学方法;(2)分子动力学模拟; (3)多尺度方法.介绍了这3类计算方法的典型理论和主要数学描述,给出了这些方法对解决若干微/纳观接触问题如黏着效应、粗糙表面描述、表面摩擦及润滑、表面热效应、生物接触等的主要应用.最后, 探讨了微/纳尺度接触问题计算方法可能的发展方向及应用领域.      

基于应变梯度理论的微尺度蜂窝等效模量计算方法

本文提出了一种新的能够计及尺度效应的微纳米蜂窝等效模量的计算方法。将一种单参数应变梯度理论引入到本构方程当中,并基于能量等效原理推导了蜂窝面内等效模量地计算公式。算例分析表明,本文方法能够有效地计及尺度效应对蜂窝等效模量的影响。尺度效应与胞壁厚度和长度的值都有关,当胞壁厚度较小时,尺度效应显著,本文方法预测的模量会明显高于传统方法;而当胞壁厚度较大时,尺度效应变得微弱乃至可以忽略不计。但如果胞壁的长度/厚度比很大,则面内等效模量会趋近于0,此时是否考虑尺度效应意义不大。     

国际传热研究前沿──微细尺度传热

微细尺度传热问题的工程背景来自于８０年代高密度微电子器件的冷却和９０年代出现的微电子机械系统中的流动和传热问题．它的特点是,当空间和时间尺度微细化后,出现了很多与常规尺度下不同的物理现象,其原因可以分为两大类：一类是连续介质的假定不再适用,另一类则是各种作用力的相对重要性发生了变化．所需研究的挑战性问题有,导热系数的尺度效应、导热的波动现象,微小通道中流动和传热,流动压缩性和界面效应等的影响,微细尺度下的辐射和相变等．     

输液管振动与稳定性研究的新进展:从宏观尺度到微纳米尺度

本文围绕不同尺度级的输液管结构,针对液流引起的管道振动与稳定性综述了目前已有的几种物理和数学模型,详细介绍了梁模型输液管的各类振动控制方程,重点讨论了宏观尺度、微米尺度和纳米尺度下输液管振动方程的异同点.在此基础之上,进一步概述了近几年这些输液管振动与稳定性问题研究的现状和一些重要研究结果,其中也包括作者们近期的相关工作.最后对未来的研究趋势作了分析和预测.通过本文可以看到,输液管振动问题仍有不少难题尚未很好解决,特别是微纳米输液管的建模和流固耦振机理方面的研究亟需加强.     

微尺度气体滑动轴承的Monte Carlo模拟与性能分析

改进了直接模拟Monte Carlo (DSMC)方法并模拟研究了轴承构型、滑动速度、壁温及环境压力对微气体滑动轴承内部压力分布及承载能力的影响.结果表明:轴承的几何构型和滑动速度对轴承的性能影响很大,对于相同长度的轴承,气体的峰值压力与轴承的承载能力随轴承出口尺寸的减小和滑动速度的增大而增大;当轴承的几何形状和滑动速度固定时,通道壁温和环境压力是影响轴承性能的重要因素,壁温越高,轴承的承载能力越强;环境压力不同,轴承性能亦有所不同.     

考虑尺度效应的微梁刚柔耦合动力学分析

不少微观实验已经证实,微尺度领域材料的力学性能存在尺度效应.文章以转动刚体、柔性微梁组成的刚柔耦合系统为研究对象,采用偶应力理论(又称 Cosserat理论)研究微梁动力学特性的尺度效应,运用拉格朗日方程推导出系统考虑尺度效应的一次近似刚柔耦合动力学方程.仿真结果表明,较该文提出的一次近似耦合模型,传统的零次近似耦合模型在刚体作高速旋转时不能正确地描述微梁的动力学行为;尺度效应使微梁振动的振幅减小,频率增大.     

Recent achievements in the thermal hydraulics of high heat flux components in fusion reactors

Some components of fusion thermonuclear reactors, such as divertors, plasma limiters, or first-wall armor, are believed to be subjected to operating conditions characterized by extremely high thermal loads. It is therefore necessary to remove from the surface of these components very high heat fluxes, ranging from 2 to 60 MW/m². Water subcooled flow boiling, under conditions of high mass flux, high liquid subcooling, and small to intermediate channel diameter, can accomodate these very high heat fluxes. Further enhancement of the upper limit of cooling, the critical heat flux (CHF), can be obtained by making use of turbulence promoters such as twisted tapes and coiled wires even if coupled with a relevant increase in pressure drop. An overview is presented of recent achievements obtained in water subcooled flow boiling CHF under operating conditions of interest to the thermal hydraulic design of fusion reactors. Observed basic parametric trends—CHF as a function of mass flux, pressure, subcooling, and channel geometry—are outlined, together with findings on the use of CHF enhancement techniques. From experiments it was seen that water subcooled flow boiling allows CHF conditions as high as 228 MW/m² to be achieved under extreme geometric and thermal hydraulic conditions. On the other hand, design and engineering boundary conditions limit variation in these conditions, and a suitable compromise has not yet been reached. Predictive tools are presented for the evaluation of subcooled flow boiling CHF both in straight tubes and with twisted tapes, and are assessed with reference to recent available experimental data.

Although several indications for practical applications can be found in recent achievements, a full understanding of the basic mechanisms of heat transfer and CHF in subcooled flow boiling has not yet been achieved. Future research to overcome the present lack of knowledge in this field is suggested.     

Nucleate and transition boiling heat transfer under pool and external flow conditions

In this paper an overview of the boiling process, including recent advances made toward a mechanistic understanding of nucleate and transition boiling, is presented. Out of necessity, the review does not include boiling on enhanced surfaces or boiling of mixtures. Discussion of film boiling is also not included, as it is the subject of another review article. Only pool and external flow boiling of ordinary liquids are discussed. A few comments are made with respect to the theoretical and experimental studies that should be made in the future to further our understanding of the boiling process.     

Evaporator critical heat flux in the double-wall artery heat pipe

The fact that heat is transferred into a heat pipe through the liquid-saturated evaporator wick gives rise to the so-called boiling limit on the heat pipe capacity. The composite nature of the double-wall artery heat pipe (DWAHP) wick structure makes the prediction of the evaporator superheat (Δ T_crit) and the critical radial heat flux (q_r) very difficult. The effective thermal conductivity of the wick, the effective radius of critical nucleation cavity, and the nucleation superheat, which are important parameters for double-wall wick evaporator heat transfer, have been evaluated based on the available theoretical models. Empirical correlations are used to corroborate the experimental results of the 2 m DWAHP. A heat choke mounted on the evaporator made it possible to measure the evaporator external temperatures, which were not measured in the previous tests. The high values of the measured evaporator wall temperatures are explainable with the assumption of a thin layer of vapor blanket at the inner heating surface. It has been observed that partial saturation of the wick (lean evaporator) causes the capillary limit to drop even though it may be good for efficient convective heat transfer through the wick. The 2 m long copper-water heat pipe had a peak performance of 1850 W at 23 W/cm² with a horizontal orientation.     

Pool boiling performance of finned surfaces in R-113

Performance of horizontal copper heaters with a transverse fin structure was investigated for pool boiling heat transfer and critical heat flux limits. Data were obtained for 5.1 and 7.6 cm diameter structured cooper and brass heaters in saturated R-113 boiling at pressures ranging between 0.037 and 1 atm. The fin structure consisted of 0.16 cm×0.16 cm×0.32 cm high square fins with an interfin spacing of 0.16 cm. Following a similar methodology to Haley and Westwater¹, a numerical analysis of the heat transfer phenomenon was performed by solving the one-dimensional fin conduction equation with a non-linear heat transfer boundary condition obtained from the previously reported data for R-113 boiling on plain surfaces. The predictions agreed with the data at the 1 atm pressure levels but showed deviations at the low pressure levels. The results showed that, compared with plain surface heaters of the same diameters the finned structured surfaces investigated: (a) decreased the wall temperature differences for a given heat flux and saturated pool boiling conditions, thus improving the nucleate boiling heat transfer coefficients, and (b) increased the critical heat flux limits, calculated as the power input divided by the heater projected area, by a factor of 2–2.5.     

Low Thermal Capacity Heaters for Steady-State Transition Boiling Measurements

It is difficult to obtain steady-state transition boiling using electrical heating. A previous paper analyzed the control problems and concluded that it was important to use low thermal capacity heaters and have low thermal resistance between the heaters and the boiling heat transfer surface in addition to a steep temperature controller response. The present note describes an attempt to implement these recommendations. A compact, high-power, resistance wire heater was constructed and used to give pool boiling of methanol on a horizontal copper surface. Very good control of temperature is obtained, but the life of the heaters is short.     

Flow pattern recognition in heated vertical channels: Steady and transient conditions

Experimental work was carried out to determine the flow pattern map in vertical heated pipes under steady-state and transient conditions, using Freon 12 in forced convective flow as working fluid and optical probes for the measurements Existing maps are based on adiabatic tests, steady-state conditions, and fluids different from Freon 12. Signals from optical probes (whose response is based on the variations in fluid refractive index) are analyzed in terms oflocal void fraction, using either the probability density function (PDF) or the ratio between the average and maximum values of the signal. From the analysis of the experimental measurements the definition of a map for annular and intermittent flow regimes was achieved. The map turned out to be in good agreement with the Weisman and Kang map developed in adiabatic, steady-state conditions Qualitative results for the transient conditions are also presented.     

二维水平通道内流动沸腾换热的格子Boltzmann模拟

利用格子Boltzmann方法模拟二维水平通道内水的流动沸腾过程,获得不同壁面过热度下流型特点和不同因素对换热过程的影响规律。结果表明,随着壁面过热度升高,流道内流型依次经历从泡状流、弹状流到反环流的转变,平均热流密度和平均换热系数先增大后减小。入口流速降低会使流道内出现受限气泡流,核态沸腾受到抑制。提高入口流速能够有效促进气泡脱离,壁面平均换热系数随入口流速增大而增大,但增长速率有所减小。减小通道宽度有利于汽化现象发生,核态沸腾得到强化,壁面平均换热系数有所提高。     

Experimental study of critical heat flux enhancement during forced convective flow boiling of nanofluid on a short heated surface

Enhancements of nucleate boiling critical heat flux (CHF) using nanofluids in a pool boiling are well-known. Considering importance of flow boiling heat transfer in various practical applications, an experimental study on CHF enhancements of nanofluids under convective flow conditions was performed. A rectangular flow channel with 10-mm width and 5-mm height was used. A 10 mm-diameter disk-type copper surface, heated by conduction heat transfer, was placed at the bottom surface of the flow channel as a test heater. Aqueous nanofluids with alumina nanoparticles at the concentration of 0.01% by volume were investigated. The experimental results showed that the nanofluid flow boiling CHF was distinctly enhanced under the forced convective flow conditions compared to that in pure water. Subsequent to the boiling experiments, the heater surfaces were examined with scanning electron microscope and by measuring contact angle. The surface characterization results suggested that the flow boiling CHF enhancement in nanofluids is mostly caused by the nanoparticles deposition of the heater surface during vigorous boiling of nanofluids and the subsequent wettability enhancements.