旋转方形截面螺旋管内流动与传热特性

利用数值计算方法研究了旋转矩形截面螺旋管内的粘性流动，分析了在离心力，科氏力共同作用下曲线管道中的二次流动结构、轴向流速分布、截面温度分布、摩擦系数比以及管道Nusselt数比随各参数的变化情况。计算结果表明：当旋转方向和主流方向相同时，旋转的作用与增大Dean数的作用相同，使得管道摩擦系数变大，管道换热效果增强，而当旋转方向和主流方向相反时，管道内流动结构变化十分明显，当F≈-1．2时(F为科氏力与离心力之比)，二次流出现类似于直扭管内的鞍状流动结构，轴向速度类似于静止直管内的流动结构，管道内的摩擦系数与静止直管内的摩擦系数大约相等，换热效果减至最弱；挠率对流动结构以及摩擦系数比和Nusselt系数比的影响效果与F有关。     

Heat transfer promotion with a cylinder array located near the wall

Heat transfer from a flat plate has been investigated when a cylinder array is located near the wall. Each cylinder in the cylinder array was positioned normal to the flow direction and parallel to the flat plate surface. Measurements of the heat transfer coefficient and the optimum value for the cylinder pitch and spacing between the cylinders and the flat plate surface were obtained. A comparison of the heat transfer mechanism in this flow system with that obtained previously for the case when a single cylinder is inserted in the boundary layer was made.     

Large-Scale Flows in a Turbulent Convective Layer with an Immersed Moving Thermal Insulator

Large-scale motions developing against the background of developed convective turbulence in a rectangular vessel heated from below, in which a horizontal disk floats freely at a fixed depth, are studied experimentally. It is found that regular or chaotic oscillations may develop in the vessel. Transitional regimes characterized by interruptions of the motion or frequency variations are also possible. A regime map in the Rayleigh number — relative disk depth plane is constructed. It is shown that in the regular-oscillation regime the oscillation frequency is a linear function of the Rayleigh number.     

Some Features of a Turbulent Separated Flow and Heat Transfer Behind a Step and a Rib. 2. Heat Transfer in a Separated Flow

Results of an experimental study of heat transfer in a separated flow behind a step and a rib are presented. The influence of the obstacle height (H = 6–30 mm) on heat and mass transfer and the structure of the thermal boundary layer is studied. The features of heat transfer in recirculation and relaxation zones of the separated flow are analyzed, and the effect of separation on intensification and suppression of turbulent heat transfer is determined.     

Application of a reflection-free DSM to turbulent flow and heat transfer in a square-sectioned U-bend

The paper reports the outcome of applying two differential second-moment (DSM) closures to resolve the complex three-dimensional motion that arises in turbulent flow in a square-sectioned duct passing around a 180°C bend. The initial results showed (in accord with a number of recent studies) that, with the same underlying closure hypotheses, a DSM scheme produces better agreement with experiment than does the corresponding algebraic second-moment (ASM) treatment, although the differences were acceptably small. Thereafter, applications are reported for a new type of DSM that employs no wall-reflection terms. This leads to markedly better predictions of the turbulence field and thus of the wall heat transfer than the conventionally adopted version.     

Turbulent heat transfer in a flat plate boundary layer disturbed by a cylinder

Augmentation of heat transfer from a flat plate using a turbulence promoter has been studied. A circular cylinder 8 mm in diameter was placed in the turbulent boundary layer detached from the flat plate. It was located parallel to the plate and perpendicular to the flow direction. Clearance, c, between the cylinder and the flat plate was varied in nine steps: c=0, 1, 2, 3, 4, 6, 11, 20 and 29.5 mm. Measurements were made of the local heat transfer coefficients, mean velocity profiles, turbulence intensity profiles, static pressure and skin friction. Experimental results showed that the heat transfer deterioration which occurs just downstream of the cylinder at c=0 mm can be removed by displacing the cylinder a small distance from the wall. The improvement in heat transfer is mainly due to the unsteadiness of the recirculating flow on the plate and the effect of intense turbulence arriving at the near wall region from the lower shear layer of the cylinder wake. Heat transfer augmentation is most effective when c=4 mm and becomes less effective when c is increased more than 6 mm. The enhancement disappears far downstream from the cylinder.     

Heat Transfer and Pressure loss of a very shallow fluidized-bed heat exchanger Part 1. Experiment with a single row of tubes

A novel gas fluidized-bed heat exchanger with a very small static bed height has been developed for a heat-exchanging system using a low-pressure fan. This fluidized bed is composed of a multislit distributor, a single row of 8 mm diameter tubes, and glass beads 48–195 μm in diameter. The measured performance of heat transfer is excellent and that of fluidization is satisfactory, in spite of the static bed height being as small as 13 mm. In the best case, the test fluidized bed exhibited a heat transfer performance comparable to that of a conventional fluidized bed with a perforated plate distributor and a static bed height of 150 mm, and showed one-fourteenth the pressure loss.     

Forced Convection Heat Transfer on a Flat Plate Embedded in Porous Media for Power-Law Fluids

The effect of the presence of an isotropic solid matrix on the forced convection heat transfer rate from a flat plate to power-law non- Newtonian fluid-saturated porous medium, has been investigated. Numerical results are presented for the distribution of velocity and temperature profiles within the boundary layer. The effects of the flow index, first-order and second-order resistance on the velocity, and temperature profiles are discussed. The missing wall values of the velocity and thermal functions are tabulated.     

Prediction of developing turbulent flow in 90°‐curved ducts using linear and non‐linear low‐Re k–ε models

This paper reports the outcome of applying two different low‐Reynolds‐number eddy‐viscosity models to resolve the complex three‐dimensional motion that arises in turbulent flows in ducts with 90^° bends. For the modelling of turbulence, the Launder and Sharma low‐Re k–ε model and a recently produced variant of the cubic non‐linear low‐Re k–ε model have been employed. In this paper, developing turbulent flow through two different 90^° bends is examined: a square bend, and a rectangular bend with an aspect ratio of 6. The numerical results indicate that for the bend of square cross‐section the curvature induces a strong secondary flow, while for the rectangular cross‐section the secondary motion is confined to the corner regions. For both curved ducts, the secondary motion persists downstream of the bend and eventually slowly disappears. For the bend of square cross‐section, comparisons indicate that both turbulence models can produce reasonable predictions. For the bend of rectangular cross‐section, for which a wider range of data is available, while both turbulence models produce satisfactory predictions of the mean flow field, the non‐linear k–ε model returns superior predictions of the turbulence field and also of the pressure and friction coefficients. Copyright © 2006 John Wiley & Sons, Ltd.     

网栅后湍流流动沿流程衰减特性的实验研究

应用ＬＤＶ测试技术对垂直方管内三种不同栅距的网所形成的湍流流动做了详细测量,网栅栅距Ｍ分别为１４ｍｍ,１０ｍｍ,６ｍｍ实验给出了管内不同位置湍流流动的各种参数沿流动方向的衰减规律,并就三种网栅进行比较分析,以探讨这种网栅流动所具有共同规律和在不同栅距对湍流结构及脉动大小的不同特点,从而对这种网栅所形成的湍流流结构较为清晰的了解。     

A Two-Equation Analysis of Convection Heat Transfer in Porous Media

This paper presents a two-equation analysis on the convection heat transfer in porous media based on the modeling developed by Carbonell and Whitaker (1984). The porous system under consideration is bounded by two parallel walls and heated uniformly from one side surface. The Darcy flow is imposed and the fully developed heat transfer is assumed. General solutions, which take into account the additional convective and conductive terms, are obtained for the temperature fields and the Nusselt number. The detailed studies are presented for the porous systems characterized by consolidated and unconsolidated circular unit cells. The results show that, for the consolidated unit cell case, a prediction without the additional convective term overestimates the heat transfer, while for the unconsolidated unit cell case, this effect is negligible. The additional conductive terms are also examined and found to act conventionally as part of the conductive terms.     

颗粒存在对网栅后气相流动湍流特性的影响

应用 L DV测试技术对方管内网栅后的气固两相流动 ,在颗粒平均粒径 Dp =0 .2 1 mm,颗粒质量浓度为 0 .2 4%、0 .36 % ;Dp =0 .35 mm,颗粒质量浓度为 0 .1 2 %、0 .2 1 %、0 .335 % ;Dp =0 .6 mm,颗粒质量浓度为 0 .1 6 %、0 .2 45 %、0 .34 5 % ;Dp =0 .9mm,颗粒质量浓度为 0 .2 0 5 %、0 .30 %多种工况下进行了测量 ,得出各种工况下气流脉动速度、湍动能沿流动方向的衰减规律 ,通过与纯气流条件下的实验结果比较 ,分析了颗粒浓度及颗粒尺寸对网栅后气相流动湍流特性的影响。根据测量结果 ,提出了一个在有固相颗粒存在时 ,关于湍流模型方程中模型常数 C2的修定方法。     

Intensification of Heat Transfer in a Downward Liquid-Film Flow

Heat transfer in a film flow of the FC-72 dielectric liquid down a vertical surface with an embedded 150×150 mm heater is experimentally examined in the range of Reynolds numbers Re = 5–375. A chart of liquid-film flow modes is constructed, and characteristic heat-transfer regions are identified. Data on the dependence of heater-wall temperature and local heat flux at the axis of symmetry of the heater on the longitudinal coordinate are obtained. Local and mean heat-transfer coefficients are calculated. It is shown that enhanced heat transfer is observed in the region where rivulets starts forming in the low-Reynolds-number liquid-film flow.     

Simulation of the Effect of the Freestream Turbulence Parameters on Heat Transfer in an Unsteady Boundary Layer

A variant of the two-parameter turbulence model which makes it possible continuously to calculate a flow region with laminar, transition and turbulent regimes is proposed for investigating the flow under conditions of high freestream turbulence intensity. It is shown that the properties of the thermal transition can be theoretically described using the quasi-steady turbulence model in the case of periodic freestream velocity distribution. The numerical results are compared with theoretical and experimental data. The approach proposed is developed for determining the combined effect of the parameters of harmonic fluctuations of the external velocity and freestream turbulence on the heat transfer characteristics on a flat plate with different boundary conditions for the enthalpy.     

井内流动与传热的三维耦合数值模型

钻井过程中,井内的压力、流场及温度分布是钻井作业必需的重要参数。本文提出了一个描述井内流动和传热的三维数学模型,用SIMPLER方法获得了该数学模型的数值解。该数值模型考虑了温度、压力对钻井液的流变性、密度以及各种介质的热物理参数的影响。现场试验数据与数值模拟结果的对比表明：本文提出的数学模型及采用的数值计算方法是可行的。     

Advanced Turbulence Modelling of Separated Flow in a Diffuser

The paper describes an investigation into the predictive performance of linear and non-linear eddy-viscosity models and differential stress-transport closures for separated flow in a nominally two-dimensional, asymmetric diffuser. The test case forms part of a broader collaborative exercise between academic and industrial partners. It is demonstrated that advanced turbulence models using strain-dependent coefficients and anisotropy-resolving closure offer tangible advantages in predictive capability, although the quality of their performance can vary significantly, depending on the details of closure approximations adopted. Certain features of the flow defy resolution by any of the closures investigated. In particular, no model resolves correctly the flow near the diffuser's inclined wall immediately downstream of the inlet corner, which may reflect the presence of a “flapping” motion associated with a highly-localised process of unsteady separation and reattachment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Laminar Heat Transfer of a Swirled Flow in a Conical Diffuser. Self-similar Solution

Heat transfer in a laminar swirled air flow in the divergent channel between a disk and a cone whose vertex touches the disk is studied. A self-similar solution of the Navier-Stokes and energy equations is derived using group analysis. An exact numerical solution of the problem is obtained for different radial-to-tangential velocity ratios at the channel inlet.     

反应堆三角形通道棒束间二次流动的数值研究

采用非线性Ｋ－ε湍流模式数值模拟三角形通道棒束中的二次流动，并考察其对流动和传热的影响。数值方法采用非正交曲线坐标系下求解控制方程的非交错网格方法。计算结果表明该模式能够较为有效地反映棒束中的二次流动，进一步分析表明二次流动有利于改善棒束中的流动和传热特性。     

Numerical Investigation of the Second Transition in the Problem of Plane Convective Flow through a Porous Medium

The problem of convective flow through a porous medium in a plane rectangular vessel with a linear temperature profile steadily maintained on the boundary is considered. Single-parameter families of steady-state regimes resulting from the existence of cosymmetry of the corresponding differential equations are investigated using the Galerkin method. The onset and development of instability on these families and the characteristics of convective regimes as functions of the seepage Rayleigh number and the rectangle side ratio are studied. It is shown that the number of regimes which lose stability, the instability type, the number of convective rollers developed, and the heat transfer depend significantly on the vessel geometry. Several bifurcations of single-parameter families of steady-state regimes are identified and investigated.     

Finite Element Modeling of Liquid Deuterium Flow and Heat Transfer in a Cold-neutron Source

A finite element simulation of flow and heat transfer in the moderator cell of a cold-neutron source (CNS), in which liquid deuterium subject to internal heat generation is flowing, is reported. The numerical scheme consists of a stabilized equal-order method. A time-accurate approach is adopted to resolve the large-scale eddies of the flow, with a Smagorinsky's model for the subgrid-scale effects. The thermal coupling follows a staggered strategy, with SUPG-type upwinding. A specific wall-law is developed that accounts for the correct partition of the heat deposited at the wall by radiation between the liquid deuterium and the helium gas flowing at the outer side of the wall. The average flow and thermal structure are presented. The turbulent fluctuations are both illustrated in physical space and decomposed into spectral components. The wavenumber spectrum suggests that adequate resolution of the large-scale eddies has been attained with just 200,000 nodes, while a DNS analysis would have required at least 1010 nodes. Usefulness of the approach in the design process of the CNS is highlighted.