格子Boltzmann方法分析加热尺寸和瑞利数对可变形开口腔内自然对流的影响

采用格子Boltzmann方法对可变形腔体内自然对流问题进行数值研究，给出平均努赛尔数的经验关系式.腔体左壁加热长度分为左壁面的整个区域（H）和左壁面的中间区域（0.5H）两种情况，右壁向外界环境开放，上下边界绝热且可以上下移动，以此调节右出口尺寸.主要研究瑞利数（10⁴ ≤ Ra ≤ 10⁶），右出口尺寸（1.0H ≤ L ≤ 2.0H），左壁加热尺寸（L_h=0.5H或L_h=H）对腔体内等温线、流线、局部努塞尔数和平均努赛尔数的影响.结果表明：腔体内换热随着瑞利数的增大越来越强烈，表现为椭圆形准静止区域更加靠近上绝热壁，且热分层厚度逐渐变小，平均努赛尔数增加.而右出口尺寸的增加，对于两种加热尺寸下腔内的换热效果有不同程度影响，其中与加热尺寸为左壁面的全部区域L_h=H相比，加热尺寸为左壁面的中间情况L_h=0.5H时，右侧开口尺寸的增加对换热效果的影响不显著.此外，左壁加热尺寸为0.5H时显示出比加热尺寸为H时更高的平均传热效率.最后，针对不同的加热尺寸，提出加热面平均努赛尔数与Ra数及右壁面开口尺寸L^*之间函数关系的经验预测，拟合效果满足工程实践与设计需要.     

低Prandtl数水平流体层自然对流的振荡和分歧

本文用具有QUICK方案的有限差分法对底部加热的低Prandtl数水平流体层自然对流换热进行了数值计算,研究了这种问题中存在的振荡和分歧问题。结果显示,在Ra的一定取值区间,有4涡型流场和5涡型流场两个解的分支。但在这个区间以外,最终的结果没有出现分歧。在所发现的两个解的分支中,问题由稳态转变为非稳态的临界Racr是不同的。     

自然对流型PCR芯片的热分析与设计

在分析了现有自然对流型PCR芯片的热性能的基础上,提出了采用薄圆盘腔体内Rayleigh-Benard对流实现PCR过程,并通过数值模拟研究了几何尺寸对自然对流稳定性的影响以及腔体内不同位置处流体温度随时间的变化。模拟结果表明通过几何尺寸的优化能够实现稳定的Rayleigh-Benard对流和PCR过程。     

底部加热的低Prandtl数流体自然对流换热的分岔

本文用具有QUICK方案的SIMPLE算法对底部加热的低Prandtl数流体自然对流换热进行了数值计算,研究了这种流动与换热问题数值解的分岔问题。计算发现,在2200相似文献   

矩形腔内纳米流体自然对流的两相格子Boltzmann模拟

在考虑纳米颗粒和基液之间相互作用力的基础上,建立了两相格子Boltzmann模型,通过对比,结果表明本文建立的两相格子Boltzmann模型与相关文献吻合较好.本文利用该模型研究了矩形腔内纳米流体的流动和换热,讨论了瑞利数、纳米颗粒组分对换热的影响,并分析了纳米颗粒在流场中的分布.     

高Rayleigh数条件下竖圆环夹层内自然对流换热的实验研究

对内壁维持恒热流和外壁向环境冷却的大高宽比竖圆环夹层内自然对流换热进行了实验研究。实验装置高宽比分别为２３５和６６６７，半径比分别为２．０３和３．９２。实验数据整理考虑了热辐射影响以获得对流规律。由于已有工作均未考虑高Ｒａ数区域，首次得到Ｒａ数高达１０￣９的区域内平均Ｎｕ数的换热准则式。在低Ｒａ数区域，亦取得了与前人工作一致的结论。本文结果改进了高Ｒａ数区域换热规律的预测能力。     

微细金属丝在空气中的自然对流换热的实验研究

对直径分别39.9～350.1 μm的微细金属丝水平和垂直放置在空气中时的自然对流换热系数进行了实验测量.实验中对细丝采用焦耳加热.并通过标定金属丝电阻与温度的关系,精确测定了金属细丝的表面平均温度.将测得的实验值与经典关联式的预测值进行了比较,实验结果表明:随着细丝直径的减小,实验值与理论计算值的差别越来越大.这可能由于尺度的缩小,使得细丝的边缘效应加强,边界层变薄,从而强化了换热;水平放置的细丝表面的自然对流换热要强于垂直放置的细丝,这是由于竖丝表面受到流体的冲刷作用,边界层厚度大于横丝表面边界层厚度的缘故.     

水平空气层自然对流换热的分岔和振荡

本文用SIMPLE算法对底部加热的水平空气层的自然对流换热进行了数值计算,研究了这种空气层的流动与换热数值解的振荡和分岔问题。结果表明,对流与换热存在分岔情况。分岔存在一个临界Ra。分岔的临界值与Pr相关,随着Pr的增大,其相应的临界Ra也增大。但当Ra取到5×10~6,这种空气层的对流和换热没有发生振荡。     

几何位置对封闭方腔内水平孤立平板自然对流换热的影响

本文以冰箱蒸发器等实际问题为背景，以空气为介质用数值模拟方法研究了封闭方腔内水平孤立平板层流自然对流时Ｒａ数及平板位置变化对Ｎｕｍ的影响，得出了可供工程应用参考的结论。同时用烟可视化的实验验证了计算结果的正确性。     

高雷莱数条件下竖列管束的自然对流换热

1引言在工业应用中水平圆柱体自然对流散热，特别是二、三根竖列的水平圆柱体的自然对流散热，在许多场合涉及高雷莱数范围。文献研究却显示现有的研究未能提供此范围内的可靠实验数据和计算公式。因此有必要对此进行深入研究。前人对坚列管束的自然对流换热作过研究。1948年Eckert和Soehngen山曾对三根垂直布置的水平管作了研究，发现在相同加热条件下，壁温由顶管至底管减低，其实验管长径比为13，G，D为3人3X10‘。Lieberman和GebhartP］于1969年发表了若干水平竖列金属丝的自然对流换热的实验结果，D二0二127mm，G，D仅10－‘数量…     

Oscillations of a statistical scattering in the Rayleigh limit and the Rayleigh law violation

Oscillations of the Rayleigh wave statistical scattering in the Rayleigh limit are theoretically found. These oscillations are violation of the Rayleigh law of scattering. They are caused by the diffraction of a new form - the extraordinary scattering in the Rayleigh limit, which leads to the different frequency dependencies of the scattering coefficient and thus violates the Rayleigh law of scattering. The fundamental physical conception that a wave does not sense the structure of an irregularity in a long-wavelength scattering, when the wavelength is much greater than the character size of the irregularity, i.e. in the Rayleigh limit, is violated as well.     

Bound on vertical heat transport at large Prandtl number

We prove a new upper bound on the vertical heat transport in Rayleigh-Bénard convection of the form under the assumption that the ratio of Prandtl number over Rayleigh number satisfies where the non-dimensional constant c₀ depends on the aspect ratio of the domain only. This new rigorous bound agrees with the (optimal) bound (modulo logarithmic correction) on vertical heat transport for the infinite Prandtl number model for convection due to Constantin and Doering [P. Constantin, C.R. Doering, Infinite Prandtl number convection, J. Stat. Phys. 94 (1) (1999) 159-172] and Doering, Otto and Reznikoff [C.R. Doering, F. Otto, M.G. Reznikoff, Bounds on vertical heat transport for infinite Prandtl number Rayleigh-Bénard convection, J. Fluid Mech. 560 (2006) 229-241]. It also improves a uniform (in Prandtl number) bound for the Nusselt number [P. Constantin, C.R. Doering, Heat transfer in convective turbulence, Nonlinearity 9 (1996) 1049-1060] in the case of large Prandtl number.     

单面加热方管混合对流换热特性实验研究

本文对单壁面加热水平方管通道内混合对流换热进行了实验研究,其中加热面分别为底面、侧面、顶面,Re=1514～3028,Gr~*=2.06×10~6～8.22×10~6;通过对加热面温度进行曲线拟合减小壁面温度测量误差,得到考虑轴向导热的局部Nusselt数的变化规律。实验结果表明侧面加热和底面加热时,局部Nusselt数要明显高于顶面加热的情况;局部Nusselt数在管道前段受Re的影响较大,在后段受Gr~*的影响较大。     

Simulation of COPO II Lo experiments on natural convection of heat generating liquid at high Rayleigh numbers

The methodology is described and the results are presented concerning numerical modeling of COPO II Lo experiments on heat transfer in liquid with internal heat generation at very high internal Rayleigh numbers typical for natural convection in a core melt that can appear during progress of severe accident at a nuclear power plant (NPP). The work is keeping in the course of development of CFD-based tool for quantitative analysis of heat transfer in a stratified molten pool of different configurations possible in severe accident scenarios with melt retention in the reactor vessel or in the VVER core catcher. Such CFD methodology would be used for testing of simplified correlation models for simulation of the core melt interaction with NPP structures in system code SOCRAT. During verification the available experimental data on the core melt thermohydraulics were analyzed, and it was concluded that they are insufficient to measures of CFD quality. The data uncertainties, along with the complexities of convective flow, uncertainties of the reactor core melt conditions, limitations of experimental possibilities and of turbulence modeling, actually constrain the multivariate CFD simulations of natural convection at very high Rayleigh numbers. RANS turbulence models only can be efficiently applied here, and they are to be checked for such purposes. In a series of numerical modeling of COPO II Lo experiments and some others, availability of a k-? realizable model with included buoyancy effects was estimated, and the optimal set of CFD options was formed for minimizing numerical artifacts. It was demonstrated that in the investigated range of Rayleigh numbers the k-? model works qualitatively correctly, but is inclined to systematical deformation of the melt boundary heat transfer distribution. This allows one to use this model for qualitative multivariate CFD estimations but requires improvement of the model or finding of its efficient and more exact equivalent.     

自然对流情形下激光辐照液体贮箱的尺度律

根据质量守恒、动量守恒及能量守恒原理,建立了自然对流情形下激光辐照液体贮箱的理论模型。通过方程分析法,导出了该问题的尺度律,在此基础上给出了激光辐照液体贮箱的缩比方法,并对一组实例进行了数值计算,得到了缩比模型与原型结果完全相似的结论,模拟结果证明了该问题尺度律的成立。为验证理论模型与数值求解的正确性,本文还针对小尺度模型进行了实验研究,数值模拟结果与实验测量结果符合较好,表明理论模型可靠有效。     

大气压He-Ar射频容性放电Ar亚稳态粒子数密度

利用发射光谱法测量大气压He-Ar混合气体射频容性放电中的Ar亚稳态1s5(3s23p54s［3/2］2)粒子数密度。在不同的放电功率和气体组分下测量放电等离子体中的重要参数：气体转动温度、电子激发温度和Ar亚稳态1s5粒子数密度。结果表明：气体温度在不同放电功率及Ar气压在5103 Pa以内时变化不大,范围为300~350 K;电子激发温度随着放电功率的增加而增加,并且在Ar气压为4103 Pa时最大,在放电功率为70 W时达到0.58 eV;1s5粒子数密度随着放电功率以及电子激发温度的增加而增加,在放电功率为70 W、Ar气压为4103 Pa时达到1.53109 cm-3。     

A generic intermittency model and its 1-D meta-map: Power laws, invariants and the succession of laminar sequences

Intermittent behaviour has been found in many systems able to switch between two different dynamic states, e.g. between long laminar phases and short chaotic bursts. Despite the apparently high-dimensional complexity, certain one-dimensional (1-D) maps are known to mimic properties of such dynamics. To these belongs the iterative map x_n+1,i = (x_n,i + (x_n,i)^z + ε) mod 1, giving rise to long laminar lengths. The statistics of the laminar lengths are of special interest.

Starting from this map, we are interested in the values of x_0,i which arise after passing through the modulo operation. These determine the laminar lengths uniquely. A 1-D meta-map x_0,i = f(x_0,i−1) is derived heuristically. It is used to calculate statistical properties of the laminar phases. Our results show an improvement in the behaviour of short and very long laminar phases as compared to earlier analytical results. Introducing the concept of the generic starting value, we find laminar phases not to be strictly independent of their predecessors.     

Comparative characterization of thermal isomerization rates of 6‐phenyl‐1,5‐diazabicyclo[3.1.0]hexane at convection and microwave heating

Rate constants of thermal isomerization of 6‐phenyl‐1,5‐diazabicyclo[3.1.0]hexane into 1‐(benzyl)‐4,5‐dihydro‐1H‐pyrazole at convection and microwave heating in toluene and chlorobenzene (solvents) were determined within the temperature range 90°C to 120°С. These data were used for the calculation of activation parameters of isomerization. It is shown that microwave heating increases the rate constants at the same temperature by a factor of 2 to 2.5 as compared with those using convection heating. The reason is that the effective temperature of microwave heating exceeds that of convection heating by 6°C to 9°С in toluene and by 12°C to 20°С in chlorobenzene as solvent.     

Quantum size effects of Pb overlayers at high coverages

We have studied Pb thin films as a function of the thickness up to 60 monolayers (MLs) using ab initio first principles and model calculations. Magic heights corresponding to a modulated oscillatory pattern of the energy of Pb(1 1 1) films have been measured up to about 30 MLs. We demonstrate that this behaviour continues even for higher thickness due to an extra second modulation pattern in the energetics of the metal film as a function of the number of atomic layers. The origin of this second modulation is the nesting of two close values of the Fermi wavelength in the (1 1 1) direction.