The curvature and temperature dependency of the liquid-vapor surface tension has a significant influence on the accurate prediction of the nanobubble/nanodrop nucleation process. In this work, a mesoscopic approach combining the pseudo-potential multiphase lattice Boltzmann method (LBM), the principle of dynamic similarity, and the van der Waals theory of corresponding states is adopted to quantitatively investigate the curvature and temperature dependency of the surface tension and Tolman length for real fluids (water and R134a). By Tolman length, we mean the distance from the surface of tension to the equimolar surface, which measures the extent by which the surface tension of a nanodrop/nanobubble deviates from the corresponding flat interface limit. We show that the Tolman lengths for flat liquid-vapor interfaces (δF) increase with the increase of temperature and are proportional to (1−Tr)−1.044. Equations for predicting surface tensions of water and R134a with effects of temperature and curvature radius taken into consideration are proposed. We demonstrate that the surface tensions increase while the Tolman lengths (δB) decrease with the increase of curvature for nanobubbles. For nanodroplets, however, the surface tensions decrease while the Tolman lengths (δD) increase with the increase of curvature. Effects of the equation of state for real fluids, which determines the interparticle interaction force in the pseudo-potential LBM, are also discussed. This mesoscopic approach can quantify the curvature dependency of liquid-vapor surface tensions for various real fluids in a wide temperature range with low computation costs, providing a new avenue for the accurate prediction of nucleation processes in micro-/nanoscale phase change heat transfer with applications to boiling, evaporation, and condensation.

航天领域的快速发展对热管理材料提出了更高的要求,根据实际应用开发的不同热控产品除导热率外,还对材料的密度、膨胀系数、耐温范围、力学强度及可加工性能等有特殊要求.从高导热材料的导热机理出发,总结了国内外高导热材料发展现状,介绍了各类金属基复合材料、碳基复合材料及高导热功能材料等在航天热管理领域应用的现状.针对航天热控技术对高导热材料的需求,结合各种高导热材料的热物理特性和工艺特点,分析了其在航天领域的应用前景.

Numerical simulations of film boiling heat transfer on a horizontal surface are conducted in this paper using a modified pseudo-potential liquid-vapor phase change lattice Boltzmann model. A conjugate heat transfer problem, including heat conduction in the heater and its thermal responses during the film boiling process, is investigated. Unlike previous numerical studies which needed to initialize the shape of the liquid-vapor interface wave at the beginning of the computation, the computation domain for fluid region is occupied initially by saturated liquid in this paper. Taylor instability at the liquid-vapor interface is triggered by small temperature perturbations imposed at the bottom of the heater during a short initial period. Consequently, this paper represents a more direct and complete numerical simulation for film boiling heat transfer on a horizontal heater. The simulated time- and space-averaged Nusselt number is found in good agreement with a previous correlation equation. Temporal and spatial variations of the vapor film thickness are also investigated numerically and compared with existing correlation equations. It is demonstrated that the temperature at the top surface of the heart changes with position and time during the film boiling process. Although the transient film boiling patterns may depend on temperature perturbations imposed on he bottom of the heater during an initial period, the time- and space-averaged film boiling heat flux is independent of initial temperature perturbations.

Boiling heat transfer on rough surfaces with single cavity and multi-cavities is numerically investigated by Gong-Cheng lattice Boltzmann models for liquid-vapor phase change heat transfer. Boiling curves from natural convection regime to film boiling regime on surfaces with multi-cavities are numerically obtained. Simulation results suggest that hydrophilic and hydrophobic cavities have different bubble nucleation and growth characteristics. There exists a critical cavity depth, beyond which the bubble departure frequency jumps and therefore enhancing boiling heat flux. The three-phase contact line region exhibits the lowest local temperature and the highest local heat flux.

实验研究了改性 SiO2纳米流体液滴蒸发后的沉积图案,以及改性 SiO2纳米颗粒沸腾沉积层对沸腾换热的影响.液滴蒸发实验研究表明:改性官能团会影响改性 SiO2纳米颗粒是否吸附在液-气界面,从而推断出在沸腾过程中改性官能团对纳米颗粒沉积方式的影响.沸腾实验研究结果表明:用聚乙二醇基团改性的SiO2纳米颗粒沸腾沉积层使加热面的平均粗糙度从160 nm大幅增长到977 nm,且能增强纯水的沸腾传热系数;而用磺酸基团改性的SiO2纳米颗粒沸腾沉积层对加热面的平均粗糙度的改变不明显,只使其增大了60 nm,且恶化了纯水的沸腾传热系数.通过沸腾换热实验结果较好地验证了通过液滴蒸发实验推断出的沸腾过程中改性官能团对纳米颗粒沉积方式的影响.

Strategy and battle equipment maintenance measurement calculates the power and resources of equipment maintenance support before the war to provide guarantee for the commander's decision-making and guarantee the equipment support personnel complete the security tasks. But now the strategic battle equipment maintenance support calculation content and the relationship between each other, there is no unified system argument. Therefore, based on the study of the demand of equipment maintenance support, this paper determines the content of maintenance support of strategic battle equipment maintenance and studies the importance of different content in different levels which can provide the basis for strategic battle equipment maintenance.

基于航天器高热流密度散热的需求,综述了国内外近年单孔和阵列受限式浸没射流技术的研究进展.阐明了射流结构和试验工况等因素对单相/两相换热及流动性能的影响,列出了射流冷却实验研究的条件和结果,分析了换热机理和影响规律.给出了单/多微小通道、冲击表面处理等射流冷却强化结构的研究结果,概括了微通道、肋片和微纳涂层等表面结构对射流冲击换热的原理和强化效果.

为验证微细通道内流动沸腾散热技术在空间的适用性,用实验方法研究了平行微细通道内流动沸腾换热的重力无关性.搭建了两相流体回路系统,设计了水力直径0.91 mm、倒梯形截面的平行微细通道铜基热沉,通过电火花腐蚀技术在铜基加热器表面刻蚀制作微细通道,两者集成一体.由实验分析了 90°～90°不同重力倾角下微通道内的流动沸腾特性.结果发现:不同流量下不同重力倾角的微细通道的沸腾曲线基本吻合,临界热流值基本一致,表明微通道不仅强化换热能力,而且削弱了重力对沸腾换热的影响.实验结果处于文献中Bo准则和Fr准则的重力无关性区域内,验证了微细通道内流动沸腾换热具一定程度的重力无关特性,地面上测得的微通道内流动沸腾换热系数和临界热流等实验数据能安全有效用于空间环境的热控设计.

根据电子器件的高热流密度散热需求,设计了阵列式射流与微小通道热沉相结合的闭环回路冷却系统,并进行流动及换热性能测试.通过实验探索了系统运行工况参数如流量、背压、工质除气和入口过冷度对冷却性能的影响;比较了不同结构尺寸参数(射流孔形状、射流孔直径d、射流高度H)下热沉换热及流动性能的差异.实验数据与现有文献中经验关系式的预测结果相符.

Special operation has put forward new requirements for support guarantee including timely and reliable information assurance, pluralistic integration of support forces, accurate and sensitive equipment command, timely and efficient support operation and so on. So it is necessary to change the support concept, reform the support system, adjust the operating mechanism in order to realize timely, appropriate and adequate precise equipment support and thus meet the needs of special operation.

针对短脉冲高功率热流作用下的薄壁金属导热问题,基于傅里叶导热模型,采用固液耦合计算方法对金属瞬态温度特性进行了数值仿真,并分析了液体工质流速及固体材料物性参数对金属温度瞬态响应和分布的影响作用.分析结果表明:温度响应特性与时间尺度有关,在单次脉冲作用下,在ms量级内热量才能开始通过水侧对流散热散出,25 ms后金属内部温度渐趋平衡;在连续脉冲作用下,金属内部温度逐渐升高,一定时间后温度变化达到动态平衡,壁面温度在一定范围内波动;停止加热后,在2 s内温度逐渐降低至初始状态.提高水的流速和固体壁面热扩散系数均可降低壁面温度,且缩短温度趋衡所需时间.

A theoretical model is developed to predict the critical heat flux in pool boiling on a heated surface with micro/nano structures based on a force balance analysis, with effects of the capillary wicking force and modification of the critical instability wavelength taken into consideration. An analytical expression for CHF of a heated surface with micro/nano-size structures is obtained in terms of the surface roughness factor, the solid fraction as well as the contact angle explicitly. It is shown that geometric parameters of these micro/nano structures influence the ability of liquid spreading on a hydrophilic surface, effectively supplying liquid to the heated surface, and thus delaying the occurrence of the CHF. The micro/nano-sized cavities provide additional sites for vapor formation, altering the critical distance between vapor columns and thus reducing the critical instability wavelength. The critical wavelength for the occurrence of critical heat flux is obtained based on the change in surface free energy. The predicted values of CHF on heated surfaces with micro/nano-size structures in pool boiling obtained from the present model are found in agreement with existing experimental data.

Experiments on subcooled and saturated pool boiling of ethanol are conducted on horizontal heated surfaces with micro- and nano-sized structures. The effects of structure size on bubble nucleation and departure characteristics as well as the heat transfer coefficient are discussed. It is found that microstructures can enhance bubble nucleation by significantly increasing the active nucleation site density at low heat fluxes, thus reducing the wall superheat and enhancing heat flux; while nano-structures can accelerate bubble departure by decreasing bubble departure diameter and increasing departure frequency. On the other hand, nano-structures will delay bubble mergence and prevent the vapor film from spreading at high heat fluxes during boiling crisis.

This report investigates the impact of droplet temperature on the head-on collision of binary droplets on a superhydrophobic surface. Understanding droplet collision is critical to many fundamental processes and industrial applications. There are many factors, including collision speed, collision angle and droplet composition, that influence the outcome of the collision between binary droplets. This work provides the first experimental study of the influence of droplet temperature on the collision of binary droplets. As the droplet temperature increases, the possibility increases for the two droplets to coalesce after collision. The findings in this study can be extended to collision of droplets under other conditions where control of the droplet temperature is feasible. Such findings will also be beneficial to applications that involve droplet collision, such as in ink-jet printing, steam turbines, engine ignition and spraying cooling.

With the development of high technology and its application in the equipment, the traditional extensive maintenance mode is unable to meet the demand of equipment maintenance. There is an urgent need for precision equipment maintenance mode. CBM is one of the important development directions of equipment maintenance, and it is also a research hotspot in the areas of maintenance at home and abroad. It is the important way of the realization of precision equipment maintenance. This paper firstly analyzes the necessity of precision equipment maintenance; followed by the analysis of why CBM is the important way of the realization of precision equipment maintenance; finally analyzes the implementation process of precision equipment maintenance based on CBM, and discusses its developing trend.

The interaction of surface microstructures and wettability effects on heterogeneous nucleation in pool boiling is analyzed in this paper based on the changes of free energy and availability. It is shown that the bubble is most easily formed on a concave surface in comparison with a convex surface or a plane surface at the same wettability and the same wall temperature. It is found that the effect of microstructures greatly enhances nucleation of bubbles when the curvature radius of these microstructures is in the range of 5–100 times less than the bubble radius. Larger than this limit, the surface roughness effect is negligible and the wettability effect predominates. Closed form analytical solutions for the critical radius and change in availability are obtained for the special case of homogeneous nucleation where no wall temperature gradient exists on surfaces with microstructures. Under this simplified assumption, it is found that the microstructures have no effect on critical nucleation radius and their effect on the change in availability is underestimated.

Onset of heterogeneous nucleation is analyzed in this paper based on the criterion that change in the derivative of the availability function with respect to bubble radius is equal to zero. Comparisons of the predicted nucleation parameters obtained based on this criterion with those obtained previously based on the criterion that the change in Gibbs function equal to zero are made. It is found that the Gibbs function nucleation criterion is the necessary condition for onset of bubble nucleation while the availability nucleation criterion is the sufficient condition for onset of nucleation. Comparing with the Gibbs function criterion, the availability criterion predicts a larger nucleation radius at the same wall temperature and requires a higher nucleation temperature at the same heat flux although the differences in values are small.

In recent years, the computer simulation technology in military field is applied more and more widely. It has been developed from the traditional single and centralized simulation to distribute interactive simulation. The composition of the modeling and simulation common technical framework is discussed. Based on six-element abstract modeling method, equipment maintenance support simulation modeling process is divided into conceptual modeling, object modeling and extensive model design. The advantage of unified modeling process is analyzed, and the deficiencies needed be improved are pointed out.

Experiments have been carried out to determine annular condensation heat transfer coefficient of steam in two silicon microchannels having trapezoidal cross sections with the same aspect ratio of 3.15 at 54<G<559kg/m2s under 3-side cooling conditions. A semi-analytical method, based on turbulent flow boundary layer theory of liquid film with correlations of pressure drop and void fraction valid for microchannels, is used to derive the annular local condensation heat transfer coefficients. The predicted values based on the semi-analytical model are found within ±20% of 423 data points. It is shown that the annular condensation heat transfer coefficient in a microchannel increases with mass flux and quality and decreases with the hydraulic diameter.