混合层流场中涡结构对流速度的特性

基于大涡模拟和光线追踪方法, 对光线穿越流场后的光程分布与混合层流场中涡结构之间的关系进行了分析, 提出了一种基于涡核位置提取的涡结构瞬时对流速度定量计算方法, 并使用直接几何测量数据进行了验证. 通过对不同尺寸的涡结构、涡-涡配对及融合过程中的涡结构和强压缩性流场中涡结构瞬时对流速度的定量数值计算, 揭示了混合层流场中涡结构对流速度的特性: 对单个涡结构而言, 其瞬时对流速度具有脉动特性, 且脉动幅度随涡结构尺寸和流场压缩性而变化; 在涡-涡配对及融合过程中, 涡对中各个涡结构的瞬时对流速度都表现出类似正弦波动的特点. 针对混合层流场中涡结构对流速度的特性, 给出了其背后的物理原因.     

A method for estimating the convective radon transport velocity in soils

A method for estimating the convective radon transport velocity in soils is developed. The approach under review is based on measurements of the radon concentration in soil air. Mathematical models for describing the convective radon transport velocity are discussed. Data on the convective radon transport velocity in commonly encountered soil types are presented. The results obtained from a 2-month experiment aimed at investigating the effect of the atmospheric condition on the convective radon transport velocity are reported. The soil gas radon concentration at 30–70 cm depth was measured by means of passive track detectors (Type III-b SSNTDs) with 72–96 h exposure time.     

强化对流传热场协同唯象机制及其控制

根据对流传热强度与流体中存在的各种内场及其外场的关系,从唯象上阐明了降低温度边界层厚度、增加流体扰动和增加近壁面速度梯度的强化对流传热方法的物理机制,其本质是控制流体中内场及其外场之间的相互协同。给出了当流体中存在内场和外场时强化对流传热场协同控制方法,以此可指导发展强化对流传热单元的新方法。     

Onset and development of convective burning in highly porous charges of ammonium perchlorate and its mixtures with aluminum

The onset and development of convective burning in the charges with a high porosity prepared from the finely dispersed ammonium perchlorate and its mixtures with aluminum ASD-4 is studied. The experiments were carried out in a constant volume bomb with the record of the pressure-time history and in the confinement with a slit, which makes it possible to perform simultaneously the photographic and piezometric recording of the process. Special attention is given to the mixtures with the increased aluminum content. The minimum lengths of samples are determined at which convective burning or explosion occur. The dependence of these lengths on the aluminum concentration in the mixture is determined. The possibility of convective burning and low-velocity detonation in ammonium perchlorate without the combustible additive is shown. It is established that the introduction of aluminum causes the ignition of the dispersed suspension behind the front of convective burning with the formation of the brightly glowing high-pressure zone (the secondary wave), which intensively expands in both sides from the place of origin. When the secondary wave overtakes the front of convective burning, the low-velocity detonation appears. The obtained results are of interest for explosion safety of the mixtures of ammonium perchlorate with aluminum and for designing generators of high-temperature suspensions with aluminum particles.     

Derivation of the amplitude equation for a hydromagnetic convective problem

The amplitude equation for a convective system under a vertical magnetic field is derived. The coefficients in these equations have been numerically calculated for infinite Prandtl number fluids and for boundary conditions both free and rigid top and bottom. The results confirm that, for realistic parameter values only stationary convection can be present and the pattern is made by convective rolls.     

Derivation of the amplitude equation for a hydromagnetic convective problem

The amplitude equation for a convective system under a vertical magnetic field is derived. The coefficients in these equations have been numerically calculated for infinite Prandtl number fluids and for boundary conditions both free and rigid top and bottom. The results confirm that, for realistic parameter values only stationary convection can be present and the pattern is made by convective rolls.     

Digital speckle photography and speckle tomography in heat transfer studies

The line-of-sight speckle photography of transparent media is used for quantitative measurements of the instantaneous temperature fields in 3D unsteady flows. Both electronic and photographic methods are employed for specklegram recording. The subsequent specklegram treatment uses the Young's fringes method as well as cross-correlation analysis of small interrogation areas of the recordings. Experimental data for three different heat transfer configurations are obtained and discussed. The first one is natural convection over extended vertical heated plates with forward facing steps, the second is unsteady 3D convective flow around a suddenly heated vertical thin wire, and the third one is a convective plume above a multi-jet flame. Both local and global Nusselt numbers are determined via measuring local surface temperature gradients for these convective flows. The results are compared with Ostrach's theory for a single vertical plate and with the data obtained by Mach–Zehnder interferometry. The 3D temperature fields are reconstructed for axisymmetric convective flows around a suddenly heated vertical wire using quasi-double projection measurement and the Radon inversion. 3D temperature distributions above the combustion zone are reconstructed using multi-projection speckle photography measurements and computerised tomography.     

Convection in relatively thin layers of nematic liquid crystals

Laser-induced convective motion in thin layers (less than a millimeter) of nematic liquid crystals is investigated experimentally. It is shown that convection arising in the presence of the vertical gradient of temperature is toroidal. Velocities of convective motions in the liquid crystal are measured for two different diameters of the incident laser beam. Comparison is made of the threshold of the temperature gradient of convective motions obtained by theoretical estimations and with use of experimental parameters.     

自然循环加热段内摩擦阻力及对流传热的实验研究

以常压去离子水为工质,对自然循环工况下上升加热段内单相水的摩擦阻力及对流传热特性进行了实验研究.结果表明,自然循环工况下加热段内由浮升力引起的自由流动对摩擦阻力及对流传热特性有重要影响,自然循环与强制循环二种工况下加热段内的摩阻系数及对流换热系数存在明显差别;并且,自然循环工况下加热段内的摩擦阻力存在滞后现象.通过实验提出了计算自然循环工况下加热段内单相水的摩阻系数及对流换热系数的经验关系式.     

纳米流体对流换热的实验研究

建立了测量纳米流体对流换热系数的实验系统,测量了不同粒子体积份额的水-Cu纳米流体在层流与湍流状态下的管内对流换热系数,实验结果表明,在液体中添加纳米粒子增大了液体的管内对流换热系数,粒子的体积份额是影响纳米流体对流换热系数的因素之一。综合考虑影响纳米流体对流换热的多种因素,提出了计算纳米流体对流换热系数的关联式。     

Convective instabilities in films of binary mixtures

We present a model for the evolution of films of isothermal binary liquid mixtures with a free evolving surface. The model is based on model-H supplemented by appropriate boundary conditions at the free surface and the solid substrate. The equations account for the coupled transport of the concentration of a component (convective Cahn-Hilliard equation) and the momentum (Korteweg-Navier-Stokes equation). The inclusion of convective motion makes surface deflections possible, i.e., the model allows to study couplings between the decomposition of the mixture and the evolving surface corrugations. We present selected steady layered film states for representative polymer mixtures, and show that convective motion favors their destabilization and qualitatively changes the linear instability modes in experimentally accessible ranges of parameters.     

底部局部加热多孔介质自然对流传热的格子Boltzmann模拟

运用格子Boltzmann方法研究了底部局部加热多孔介质方腔的自然对流传热.方腔的上壁面为低温热源,下壁面为局部高温热源,左右壁面为绝热条件.重点分析了高温热源位置a及尺寸b对多孔介质方腔自然对流传热性能的影响,提出了平均Nusselt数Nu和位置a及尺寸b的拟合关系式.研究结果表明:高温热源位置及尺寸对多孔介质方腔内自然对流传热性质的影响很大,且存在最佳高温热源位置(a=4/16)和尺寸(b=0.75),以达到最强的对流换热强度(Nu_(max)≈10.35)和最大的对流换热量(Q_(max)≈5.69).     

流态化固体颗粒对对流沸腾传热的强化

在对流沸腾系统中引入固体颗粒,固体颗粒在液体中呈流态化,从而形成气液固三相对流沸腾过程。对气液固三相流对流沸腾过程的传热特性进行了实验研究,结果表明流态化固体颗粒对液体的对流沸腾传热具有显著的强化作用。基于固体颗粒撞击沸腾气泡时的受力分析,获得了固体颗粒穿透气泡并使气泡破碎的条件,分析了流态化固体颗粒强化沸腾传热的机理。实验结果与理论分析符合良好。     

Convective deformations of the matter tensor for the relativistic perfect magnetofluid

Maugin's Scheme for a relativistic perfect magnetofluid is used to study the convection-free stress and convective deformations of the matter tensor for the magnetofluid. It is proved that the convection-free stress of the magnetofluid implies the conservation of the pressure, the density, and the magnitude of the magnetic field along the flow vector. The relation between convective deformations of the matter tensor for the magnetofluid and deformation tensor is obtained.     

Convective amplification of gas turbine engine internal noise sources

The acoustic field of a noise source is altered when the source is in motion. The change in the acoustic field introduced by the source motion, caused by source alteration and propagation effects, is defined as convective amplification. Previous studies of this phenomenon have been based on analytical models that did not incorporate the physical features necessary for calculation of the convective amplification factor for the internal noise sources of a gas turbine engine, which is required to predict in-flight noise levels from static engine noise measurements. An improved theoretical model was developed. At low frequencies, this model resulted in a convective amplification factor of (1?M₀ cos θ_e)^?4, which is identical with the factor established in earlier studies. At high frequencies, however, convective amplification is a function of flight speed, radiation angle, and source geometry.     

Heat Transfer in Electric Arc Chamber of Linear Plasmatron

Classification and performance capability of electric are linear plasmatrons are presented. The results of investigation into convective and radiative heat transfer in the discharge channel of a plasmatron are reported. The radiative heat transfer dominates both in the structure of heat losses and in the formation of discharge volt-ampere characteristic at the initial region of the flow. Relations to calculate heat losses by radiation are given. Radiative convective interaction at the arc boundary is shown to play a substantial role for high-current discharges stabilized by axial vortex gas flow. It is shown that the calculation of convective heat transfer at the turbulent region of the flow may be carried out by flow averaged parameters.     

Convective heat transport in a fluid layer of infinite Prandtl number: upper bounds for the case of rigid lower boundary and stress-free upper boundary

We present the theory of the multi--solutions of the variational problem for the upper bounds on the convective heat transport in a heated from below horizontal fluid layer with rigid lower boundary and stress-free upper boundary. A sequence of upper bounds on the convective heat transport is obtained. The highest bound is between the bounds for the case of a fluid layer with two rigid boundaries and for the case of a fluid layer with two stress-free boundaries. As an additional result of the presented theory we obtain small corrections of the boundary layer thicknesses of the optimum fields for the case of fluid layer with two rigid boundaries. These corrections lead to systematically lower upper bounds on the convective heat transport in comparison to the bounds obtained in [5]. Received 29 September 1999     

亚临界与超临界压力水在管内的对流传热与传质

本文对含有金属腐蚀物杂质的亚临界与超临界压力水在竖直加热圆管内的受迫对流与混合对流传热与传质进行了数值模拟，分析了变物性、浮升力以及压力等因素对管内对流传热与传质的影响。结果表明：浮升力使自下而上流过竖直加热圆管的对流传热和传质增强；在不同的温度条件下，超临界压力水的热物性对传热传质的影响有很大不同。     

Absolute and convective instabilities of a viscous film flowing down a vertical fiber

The stability of a viscous film flowing down a vertical fiber under the action of gravity is analyzed both experimentally and theoretically. At large or small film thicknesses, the instability is convective, whereas an absolute instability mode is observed in an intermediate range of film thicknesses for fibers of small enough radius. The onset of the experimental irregular wavy regime corresponds precisely to the theoretical prediction of the threshold of the convective instability.     

Effect of nonstationary thermal gravitation-capillary convection on temperature distribution in a thin vertical wall

Time dependences of temperature distributions in a thin metal wall were studied experimentally under two conditions of convective heat transfer in a tank model. In the first case, the vertical working wall was heated from within due to a convective heat flux from the opposite wall heated monotonously, and it was cooled due to heat transfer to the ambient medium. Dependence of the temperature field on a thin wall at the stage of convective flow development was retraced with the help of the thermographic camera and thermocouple sensors. In the second case, the tank wall was heated uniformly by IR radiation from the outside, and non-stationary convective flow and volumetric liquid heating were formed inside. Time dependence of temperature distribution over the wall height is studied. It is shown that the flow structure and convective heat transfer in a fuel layer with free boundary are subjected not only to the buoyancy force, but also to the thermocapillary effect. The local features of the flow affect temperature distribution in a thin wall.