纳米组装相变储热材料的热设计前沿

本文简单回顾了固液相变储热材料发展历程,重点针对纳米多孔定形相变材料,从材料层面的研发设计,到热物理层面的微观限域空间负载、结晶、导热机理,乃至围绕异相/异质匹配提出的显著提升相变蓄传热性能的强化手段进行了总结.同时,指出了目前受制于单一尺度孔径无法兼顾储释热的密度和速率的现状,并探讨在此基础上借助新型多级尺度孔径的骨架材料以突破瓶颈的可能.最后,系统梳理了与之伴随的一系列亟待解决的科学问题、机遇和挑战.     

Heat Transfe for Power Law Non-Newtonian Fluids

We present a theoretical analysis for heat transfer in power law non-Newtonian fluid by assuming that the thermal diffusivity is a function of temperature gradient. The laminar boundary layer energy equation is considered as an example to illustrate the application. It is shown that the boundary layer energy equation subject to the corresponding boundary conditions can be transformed to a boundary value problem of a nonlinear ordinary differential equation when similarity variables are introduced. Numerical solutions of the similarity energy equation are presented.     

MHD Boundary Layer Flow of a Non-Newtonian Fluid on a Moving Surface with a Power-Law Velocity

A theoretical analysis for MHD boundary layer flow on a moving surface with the power-law velocity is presented. An accurate expression of the skin friction coefficient is derived. The analytical approximate solution is obtained by means of Adomian decomposition methods. The reliability and efficiency of the approximate solutions are verified by numerical ones in the literature.     

具有幂率速度移动表面边界层问题解析近似解

利用微分方程相似变换,摄动渐进展开和Padé逼近方法对幂率速度移动表面边界层问题进行了研究,得到了问题的解析近似解,对相应的流动特性进行了探讨.     

BIFURCATION SOLUTIONS TO A BOUNDARYLAYER PROBLEM ARISING IN THE THEORY OF POWER LAW FLUIDS

1IntroductionandMainResultsThesingularnonlineartwthpointboundaryproblemllasbeenstudiedbyCallegariandFriedll1an[2]forp=1alldbyNacl1lliallalldCallegari[:]forp>1-Theydemonstratedtl1eexistellce,uniq11elless,al1da11alyticityofthepositivesolutiontotheproblem.Recently,Soewono,Vajravelu,andMol1apatra[']havecollsideredthesingulaxnonlineartwo-poiutboundaryvalueproble1llwllereCisagivenrealnulltherandh(x)isass1llnedtobeacolltinuousandincreasingfullctiondefinedon[-k, oo)satisfyingforsomepositivec0l1sta…     

气凝胶及其复合绝热材料的导热系数测量

采用瞬态热带法在不同压力和不同温度下对气凝胶、硬硅钙石-气凝胶及陶瓷纤维-气凝胶复合绝热材料的导热系数进行了测量.结果表明,随着测试压力的降低,三种材料的导热系数都显著降低,在大约104 Pa时基本趋于常数;随着温度的升高,三种绝热材料的导热系数几乎与温度的立方成正比;当气凝胶的密度为145 kg/m3时,其导热系数具...     

CaO-Al2O3-SiO2熔渣结构和热导率的分子动力学模拟

利用分子动力学模拟研究CaO-Al₂O₃-SiO₂三元熔渣的微观结构和热导率。针对六组不同Al₂O₃含量的熔渣,基于BMH势函数分析了径向分布函数、键长、键角分布函数、氧类型和微结构单元Qⁿ等微观结构信息。结果表明硅和铝在熔渣中以四面体网络结构存在,硅氧四面体较铝氧四面体更为稳定。随着铝含量增加,熔体中的键长和四面体中心角无明显变化,Al-O-Al和Si-O-Al键角逐渐减小;网络中的桥氧(BO)和三配位氧(TO)含量增加,Q³和Q⁴含量增多,在高铝区出现较多的五配位铝(Al^V)。采用Muller-Plathe方法计算了熔渣的热导率,随着铝替代硅原子,网络的聚合程度增加,强化了熔体的热输运能力,热导率呈现增大趋势。     

掺杂石墨烯纳米片对硝酸钠相变特性的影响及机理

纳米增强剂通常被用来提升相变材料的导热性能,但这种方式通常伴随着复合后材料相变焓的降低.虽然这种降低难以避免,但其微观机理乃至影响规律却始终未能明晰.为深入探究纳米复合相变材料相变焓降低的机理,本文以熔融硝酸钠(太阳盐的重要组成成分)为相变材料,制备了石墨烯纳米片质量分数为0%,0.5%, 1%, 1.5%, 2%的复合相变材料.通过实验测量与分子动力学模拟的方法深入分析了石墨烯纳米片的掺杂导致熔融硝酸钠产生团簇以及复合材料熔点和相变焓非依数性降低的影响机理.结果表明,石墨烯纳米片质量分数为1.5%时,硝酸钠致密层和石墨烯纳米片间的质心等效距离最接近他们相互作用势的势阱位置,此时二者之间相互吸引作用最强,熔盐分子的运动受限最为严重,难以发生熔化,从而导致相变焓降低最为显著.为了最大限度地避免纳米复合相变材料相变焓的损失,应根据相变材料与纳米增强剂的类型及其相互作用类型,合理选择纳米增强剂的质量分数.在实际应用中,恰当的质量分数还将在一定程度上降低复合相变材料的制备成本.     

Approximate solutions to MHD Falkner-Skan flow over permeable wall

The magnetohydrodynamic (MHD) Falkner-Skan boundary layer flow over a permeable wall in the presence of a transverse magnetic field is examined. The approximate solutions and skin friction coefficients of the MHD boundary layer flow are obtained by using a method that couples the differential transform method (DTM) with the Padé approximation called DTM-Padé. The approximate solutions are expressed in the form of a power series that can be easily computed with an iterative procedure. The approximate solutions are tabulated, plotted for the values of different parameters and compared with the numerical ones obtained by employing the shooting technique. It is found that the approximate solution agrees very well with the numerical solution, showing the reliability and validity of the present work. Moreover, the effects of various physical parameters on the boundary layer flow are presented graphically and discussed.