Flow structure and distribution effects in gas-liquid mixture flows

Air-water mixtures which are assumed to flow homogeneously in a pipe are usually described by a one-dimensional momentum balance. This allows definition of a friction factor in a manner similar to single phase flows. By defining a momentum flux distribution parameter, the momentum balance has been modified to correctly include the etfects of phase and velocity distributions and the effect of these on calculated friction factors has been investigated. Resistivity probes were used to measure void fraction and gas phase velocity distributions for selected vertical and horizontal flow conditions, and these were combined with static pressure measurements to calculate friction factors. For bubbly flows, the inclusion of these distribution effects did not substantially alter friction factor estimates which are approximately 10% above single phase values (for Reynolds numbers based on liquid viscosity).

Friction factor values are shown to be related to flow development with higher values associated with deveioping flows. In particular, high friction factors are associated with the need to break-up bubbles to an “equilibrium” size. In order to experimentally simulate fully developed vertical flows, the highly turbulent nozzle mixer is most suitable while the less turbulent wall-injection type seems appropriate for horizontal flows.     

Energy-based decomposition of friction drag in turbulent square-duct flows

The generation of friction drag in turbulent duct flows has direct connection with statistical quantities and corresponding turbulence dynamics in the duct cross-section. In this study, we generalize the RD identity (Renard and Deck, 2016) to a ‘two-dimensional’ form which we exploit to decompose the mean friction drag in turbulent square-duct flows into contributions associated with viscosity, turbulence and cross-stream convection. The friction Reynolds number of the duct flows ranges from 220 to 2000. The scaling, spatial distribution and local normalization of the contributions to friction are investigated and compared with those in pipe and channel flows. As in other canonical flows, we find logarithmic growth of the turbulent contribution in contrast to the viscous one, the former thus becoming dominant at high enough Reynolds numbers. Whereas cross-stream convection has no net effect on friction, its contribution may be locally comparable to the other two, hence may be responsible for redistribution of friction along the duct perimeter.     

Polymer Induced Drag Reduction in a Turbulent Pipe Flow Subjected to a Coriolis Force

The skin friction factor f in a turbulent wall-bounded flow can be greatly reduced by using polymer solutions. In this paper we discuss experimental results on the effect of the Coriolis force on turbulent drag reduction. To study this, a horizontal smooth-walled pipe with internal diameter 25?mm is placed on a horizontal table rotating about its vertical axis. The rotation is made non-dimensional with friction velocity and pipe diameter, to form the Rotation number Ro. For a range of bulk Rotation number (Ro _b ) between 0 and 0.6 for two different Reynolds numbers (Re _b = 15 & 30 × 10³), the pressure drop is measured, from which the average friction factor f is obtained. Additionally the effect of four different polymer concentrations has been investigated. The single-phase results show that the friction factor increases monotonic but gradual with Rotation. With polymer additives a drag reduction is found that increases with concentration, but which is not affected by the rotation.     

Direct numerical simulation of turbulent flows through concentric annulus with circumferential oscillation of inner wall

Periodic wall oscillations in the spanwise or circumferential direction can greatly reduce the friction drag in turbulent channel and pipe flows. In a concentric annulus, the constant rotation of the inner cylinder can intensify turbulence fluctuations and enhance skin friction due to centrifugal instabilities. In the present study, the effects of the periodic oscillation of the inner wall on turbulent flows through concentric annulus are investigated by the direct numerical simulation (DNS). The radius ratio of the inner to the outer cylinders is 0.1, and the Reynolds number is 2 225 based on the bulk mean velocity U_m and the half annulus gap H. The influence of oscillation period is considered. It is found that for short-period oscillations, the Stokes layer formed by the circumferential wall movement can effectively inhibit the near-wall coherent motions and lead to skin friction reduction, while for long-period oscillations, the centrifugal instability has enough time to develop and generate new vortices, resulting in the enhancement of turbulence intensity and skin friction.     

Turbulent thermal convection in wide horizontal fluid layers

Turbulence in thermal convection is investigated for flows in which the production of turbulence energy is due solely to buoyancy, and the statistics of the flow are homogeneous in horizontal planes. New experimental results for high Rayleigh number unsteady turbulent convection in a horizontal layer heated from below and insulated from above are presented and compared to turbulent Rayleigh convection, convection in the planetary boundary layer, and laboratory penetrative convection. Mean temperature fields are correlated in terms of wall layer scales and convection scales. Joint statistics of turbulent temperature and horizontal velocity and vertical velocity through fourth order are presented for the core region of the convection layer.This paper was presented at the Ninth Symposium on Turbulence, University of Missouri-Rolla, October 1–3, 1984     

Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube

An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and helically dimpled tube under turbulent flow with constant heat flux is presented in this work using CuO/water nanofluid as working fluid. The effects of the dimples and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with a fully developed turbulent flow for the Reynolds number in the range between 2500 and 6000. The height of the dimple/protrusion was 0.6 mm. The effect of the inclusion of nanoparticles on heat transfer enhancement, thermal conductivity, viscosity, and pressure loss in the turbulent flow region were investigated. The experiments were performed using helically dimpled tube with CuO/water nanofluid having 0.1%, 0.2% and 0.3% volume concentrations of nanoparticles as working fluid. The experimental results reveal that the use of nanofluids in a helically dimpled tube increases the heat transfer rate with negligible increase in friction factor compared to plain tube. The experimental results showed that the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% (for 0.1%, 0.2% and 0.3% volume concentrations respectively) higher than the Nusselt number obtained with plain tube and water. The experimental results of isothermal pressure drop for turbulent flow showed that the dimpled tube friction factors were about 2-10% higher than the plain tube. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds number, pitch ratio and volume concentration fits with the experimental data within ±15%.     

气固两相流中颗粒弥散的拉格朗日模拟

本文提出了一种对于均匀,稳定及各向同性气固两相紊流场中圆形固体颗粒弥散的拉格朗日模拟计算方法,应用该方法对带有网栅的垂直与水平管道中均匀,稳定的气固两相流模拟计算结果与Snyder及Wells等人所做的相同情况下的试验结果进行了比较,以证明该模拟计算方法的有效性,。     

An analytical skin friction and heat transfer model for compressible, turbulent, internal flows

An analytical skin friction model for compressible, turbulent, internal, fully developed flow involving adiabatic and non-adiabatic, smooth and rough flows has been developed by extending the incompressible law-of-the-wall relation to compressible cases. The formula recovers Prandtl's incompressible law of friction for pipes (within 2%) for incompressible flow. The model also shows good correlation with available data for compressible, adiabatic flows and flows involving cold wall heat transfer (within 15%). Comparison with hot wall data is only moderate (15–30%). Finally, using Reynold's analogy, the Stanton number and Nusselt numbers may be estimated.     

Visualization and picture processing of turbulent flow

The tracer method was used to visualize the three-dimensional structure of turbulent open-channel flow. A horizontal cross-section of the flow was illuminated by light passing through a thin slit. The illuminated cross-section was shifted upward, and at the time, successive pictures of flow patterns were taken. The picture-taking system was then shifted in downstream direction to follow the flow structures. The pictures obtained were processed by computer. Various kinds of physical properties of the flow were quantitatively evaluated and displayed as graphical outputs. These results contribute to the elucidation of the three-dimensional structure of turbulent open-channel flows.     

Experimental investigation on flow characteristics in a narrow annulus

Experiments are carried out to investigate the flow characteristics with/without heat exchange in a narrow annulus. In the experiments, directions of flow include horizontal, upstream and downstream flow. Experimental results show that the flow characteristics of water through the narrow annulus are different from those in normal tubes. Flow directions have little influences on the flow friction for the fluid flow in the narrow annulus with/without heat exchange. The flow characteristics in the narrow annulus have relations to the liquid temperature difference at the inlet and outlet of the annulus. Their influences on the flow characteristics are relatively obvious in the laminar flow area. When the Reynolds number is larger than 10⁴, there are little differences between the flow friction factors with/without heat exchange. It is also found that the asymmetrical flow can make the friction factor increase, whereas the symmetrical flow can reduce the flow friction. In the experiments, the transition from laminar to turbulent flow is carefully observed. In the narrow annulus, the flow transition is initiated earlier than that in normal pipes at a Reynolds number range from 1,100 to 1,500, which is different from the heat transfer transition. The results are gained to provide bases for the further investigations on the two-phase flow in narrow annuli.     

A model for the flow of solid particles in gases

The flow of solid particles in air streams involves a great deal of variables and complex phenomena, difficult to analyse. In practice the flow quantities in gas-solid flows are predicted by the use of empirical correlations of data or semi-empirical methods. The predictive power of these methods varies substantially between different systems. This paper presents an analytical approach to the subject of gas-solid flows, based on a turbulent model. The mixture is modeled as a variable density fluid flowing in a duct; the equations for the Reynolds stress incorporate the variation of velocity and density together, and yield the velocity profile of the flow and average quantities of interest such as the mass flux, the friction factor, the average density and average areas occupied by each phase. The predicted values for the friction factor are compared with known correlations emanating from experimental data. It is found that there is a very good agreement between the predicted values and the experimental correlations.     

Remarks on the Rayleigh-Bénard Convection on Spherical Shells

The main objective of this article is to study the effect of spherical geometry on dynamic transitions and pattern formation for the Rayleigh-Bénard convection. The study is mainly motivated by the importance of spherical geometry and convection in geophysical flows. It is shown in particular that the system always undergoes a continuous (Type-I) transition to a 2l _c -dimensional sphere ${S^{2l_c}}$ , where l _c is the critical wave number corresponding to the critical Rayleigh number. Furthermore, it has shown in Ma and Wang (Physica D 239:3–4, 167–189, 2010) that it is critical to add nonisotropic turbulent friction terms in the momentum equation to capture the large-scale atmospheric and oceanic circulation patterns. We show in particular that the system with turbulent friction terms added undergoes the same type of dynamic transition, and obtain an explicit formula linking the critical wave number (pattern selection), the aspect ratio, and the ratio between the horizontal and vertical turbulent friction coefficients.     

壁湍流相干结构和减阻控制机理

剪切湍流中相干结构的发现是上世纪湍流研究的重大进展之一,这些大尺度的相干运动在湍流的动力学过程中起重要作用,也为湍流的控制指出了新的方向.壁湍流高摩擦阻力的产生与近壁区流动结构密切相关,基于近壁区湍流动力学过程的减阻控制方案可以有效降低湍流的摩擦阻力,但是随着雷诺数的升高, 这些控制方案的有效性逐渐降低.近年来研究发现, 在高雷诺数情况下外区存在大尺度的相干运动,这种大尺度运动对近壁区湍流和壁面摩擦阻力的产生有重要影响,为高雷诺数湍流减阻控制策略的设计提出了新的挑战.该文将对壁湍流相干结构的研究历史加以简单的回顾,重点介绍近壁区相干结构及其控制机理、近年来高雷诺数外区大尺度运动的研究进展,在此基础上提出高雷诺数减阻控制研究的关键科学问题.      

输气管道壁面涂料减阻机理的实验研究

用IFA-300热线风速仪以高于对应最小湍流时间尺度的分辨率精细测量了风洞中不同壁面涂料的管道湍流边界层不同法向位置流向速度分量的时间序列信号,利用湍流边界层近壁区域对数律平均速度剖面与壁面摩擦速度、流体黏性系数等内尺度物理量的关系和壁面摩擦速度与壁面摩擦切应力的关系,在准确测量湍流边界层近壁区域对数律平均速度剖面的基础上,间接测量湍流边界层的壁面摩擦阻力．对不同壁面涂料的壁湍流脉动速度信号用子波分析进行多尺度分解,用子波系数的瞬时强度因子和平坦因子检测管道湍流边界层中的多尺度相干结构,提取不同尺度相干结构的条件相位平均波形,对比研究输气管道壁面涂料的减阻机理．     

Analysis of polymer drag reduction mechanisms from energy budgets

The transfer of energy in drag reducing viscoelastic flows is analyzed through a sequence of energetic budgets that include the mean and turbulent kinetic energy, and the mean polymeric energy and mean elastic potential energy. Within the context of single-point statistics, this provides a complete picture of the energy exchange between the mean, turbulent and polymeric fields. The analysis utilizes direct simulation data of a fully developed channel flow at a moderately high friction Reynolds number of 1000 and at medium (30%) and high (58%) drag reduction levels using a FENE-P polymeric model.Results show that the primary effect of the interaction between the turbulent and polymeric fields is to transfer energy from the turbulence to the polymer, and that the magnitude of this transfer does not change between the low and high drag reduction flows. This one-way transfer, with an amplitude independent of the drag reduction regime, comes in contradiction with the purely elastic coupling which is implicit within the elastic theory of the polymer drag reduction phenomenon by Tabor and De Gennes (Europhys. Lett. 2, pp. 519–522, 1986).     

Turbulent Transition in the Inlet Region of a Circular Pipe

Calculated and experimental data on turbulent transition in a circular pipe are analyzed. The calculations were performed using the three-parameter turbulence model. The dependence of the distance from the inlet to the point of minimum friction during transition on the Reynolds number for fixed inlet conditions and the distribution of the turbulence parameters over the pipe length and radius are obtained. The dependence of the maximum (critical) Reynolds number, Re^*, for which there is no transition in the pipe, on the inlet intensity and scale of turbulence is found. It is suggested that Re^* depends on the inlet perturbation parameters up to Re^* = 1000, where the friction coefficients for laminar and turbulent flows coincide.     

An analysis on effect of transverse convex curvature on turbulent flow and heat transfer

An analysis based on a model of modified mixing length by Hornby, Mistry und Barrow [1] was made on the effect of transverse convex curvature in turbulent boundary layer for incompressible axial flows along circular cylinders. The deviation of various turbulent flow and heat transfer properties from those of flat plates is presented. The agreement between the analyses and the experimental results for skin friction and heat transfer rate is good. The study demonstrated that, for a given condition, both the friction coefficient and Stanton number increase with decreasing value of the cyclinder radius and that their values are always greater than those for the flow over a flat plate.     

Experimental and numerical study of turbulent flow and heat transfer inside hexagonal duct

Flow and heat transfer characteristics in transition and turbulent regions are studied experimentally and numerically in a horizontal smooth regular hexagonal duct under constant wall temperature boundary condition covering a range of Reynolds number from 2.3 × 10³ to 52 × 10³. Two types of k-omega (standard and shear stress transport (SST)) and three types of k-ε (standard, renormalization (RNG), and realizable) turbulence model are employed for transition and turbulent regions, respectively. Both average and fully developed Darcy friction factor and Nusselt number are presented as a function of Reynolds number. It is seen that k-omega SST and k-ε realizable turbulence models gave the best agreement with the experimental data in transition and turbulent regions, respectively. All the experimental results are correlated within an accuracy of ±13 % and ±7 % for Nusselt number and Darcy friction factor, respectively. Results obtained in this study are compared with circular duct results using hydraulic diameter.     

Evaluation of three techniques for wall-shear measurements in three-dimensional flows

Recent improvements in three techniques for measuring skin friction in two- and three-dimensional turbulent wall-bounded shear flows are presented. The techniques are: oil-film interferometry, hot wires mounted near the wall, and surface hot-film sensors based on MEMS technology. First, we demonstrate that the oil-film interferometry technique can be used to measure the skin-friction magnitude and its direction in two- and three-dimensional wall-bounded shear flows. Second, a simple method is outlined to measure the skin friction with a wall wire located outside of the viscous sublayer. Finally, a systematic study of the parameters influencing wall-friction measurements with MEMS sensors is presented. The results demonstrate that accurate measurements of the mean skin friction with MEMS sensors are possible in two- and three-dimensional wall flows. Measurements by the three techniques are compared to each other and to past measurements in the same facility.