Experimental study of flow structures in a large range downstream the spacer grid in a 5 × 5 rod bundle using TR-PIV

In this paper, the time-resolved particle image velocimetry (TR-PIV) and match index refractive (MIR) techniques were used to study the flow field in a large range (0 – 22 D_h) downstream a spacer grid (SG) in a 5 × 5 rod bundle channel at different Reynolds number. The sodium chloride solution (1%) is used as the working fluid to reduce the refractive index error of fluorinated ethylene propylene (FEP) and water. The proper orthogonal decomposition (POD) background removal technique was used to minimize the FEP reflection. These methods greatly reduced the interference of background noise and improved the accuracy of cross-correlation calculation. For TR-PIV velocity fields downstream of the mixing vanes, time-averaged, statistical, spectral, and cross-correlation analysis were performed for the instantaneous full-field experimental data. The transport characteristics of coherent structures in different subchannels of rod bundles are calculated and discussed. The results show that the SG caused a relatively large transverse velocity and reduces the axial velocity. With the increase of the Reynolds number, the SG promotes the generation of transverse flow and has a great resistance to the axial flow. There is relatively large turbulence intensity downstream of the SG due to the mixing effect. The attenuation of transverse turbulence intensity component is slower than the axial component. Moreover, spectrum analysis shows that cross-arranged mixing vanes will generate periodic vortices but single mixing vane will not. These periodic vortices gradually propagate downstream along the inner subchannel and dissipate in the gap subchannel due to the effect of viscosity. The cross-correlation analysis shows that the mixing effect of the SG will reduce the scale of the coherent structure, and increase the convection velocity. The results of current research are helpful for understanding the strong anisotropic turbulence in the rod bundle channel with SG. Finally, the experimental results can be utilized to benchmark the applicability of turbulence models under different Reynolds number and the performance of partially averaged Naiver–Stokes or multiple RANS algorithms downstream of the SG.     

Experimental investigation of the interaction of two flows on the axial fan hollow blades by flow visualization and hot-wire anemometry

Experimental investigation of the interaction of internal flow with external flow around hollow airfoil NACA series in a low-speed wind tunnel was conducted and is presented in the paper. The region near the trailing edge of the hollow airfoil was studied in detail and measurements of velocity and turbulence intensities were performed with hot-wire anemometry. Determination of flow structure on the hollow airfoil was performed with computer-aided visualization. It can be concluded from the measurement analysis that higher values of velocities, lower turbulence intensities and a significant decrease of circulation effects on the suction side of the hollow blade were achieved, due to the introduction of internal flow. The results obtained on the hollow airfoil were applied on the rotating axial fan. Influence of the internal flow of the hollow blade on the flow field of the axial fan was studied. With the introduction of the internal flow a reduction of circulation effects on the fan hollow blade was achieved. Aerodynamic characteristic of the axial fan reached higher degree of total pressure difference and normalized efficiency through the entire fan working conditions.     

A new cruciform burner and its turbulence measurements for premixed turbulent combustion study

A new turbulent flow system is proposed for the study of premixed turbulent combustion processes. This cruciform burner consists of two cylindrical vessels. The long vertical vessel can provide a stable downward propagating premixed-flame at one atmosphere. The horizontal vessel was equipped with a pair of motor-driven fans and perforated plates at each end. The fans can generate two intense counter-rotating large vortical streams with controllable fan frequency up to 7620 rpm. It was found that an approximately isotropic stationary turbulence with large turbulent intensities (greater than 450 cm/s) located in the core region between two perforated plates can be generated, as verified by extensive LDV measurements. In that mean velocities are nearly zero, turbulent intensities in all three directions are roughly equal, and the energy spectrum has a −5/3 slope, indicating that the turbulence has some properties of isotropic turbulence. Other parameters of interest, such as the autocorrelation, the integral length scale, and the experimental uncertainties are also reported for the first time. The present turbulence generator can be conveniently adopted for many experimental studies, such as gaseous premixed flames propagation and particle settling in nearly isotropic turbulence.     

Effects of 3-D channel blockage and turbulent wake mixing on the limit of power extraction by tidal turbines

Three-dimensional incompressible Reynolds-averaged Navier–Stokes (RANS) computations are performed for water flow past an actuator disk model (representing a tidal turbine) placed in a rectangular channel of various blockages and aspect ratios. The study focuses on the effects of turbulent mixing behind the disk, as well as on the effects of channel blockage and aspect ratio on the prediction of the hydrodynamic limit of power extraction. To qualitatively account for the effect of turbulence generated by the turbine (rather than by the shear flow behind the turbine), we propose a new approach, called a blade-induced turbulence model, which does not use any additional model coefficients other than those used in the original RANS turbulence model. Results demonstrate that the power removed from the mean flow by the disk increases as the strength of turbulent mixing behind the disk increases, being consistent with the turbulent shear stress on the interface between the bypass and core flow passages acting in such a way as to decelerate the bypass flow and accelerate the core flow. The channel aspect ratio also affects the flow downstream of the disk but has less influence upstream of the disk; hence its effect on the limit of power extraction is relatively minor compared to that of the channel blockage, which is shown to be significant but satisfactorily estimated using one-dimensional inviscid theory previously reported in the literature.     

Experiments on the Flow Field and Acoustic Properties of a Mach number 0·75 Turbulent Air Jet at a Low Reynolds Number

In this paper we present the experimental results of a detailed investigation of the flow and acoustic properties of a turbulent jet with Mach number 0·75 and Reynolds number 3·5 10³. We describe the methods and experimental procedures followed during the measurements, and subsequently present the flow field and acoustic field. The experiment presented here is designed to provide accurate and reliable data for validation of Direct Numerical Simulations of the same flow. Mean Mach number surveys provide detailed information on the centreline mean Mach number distribution, radial development of the mean Mach number and the evolution of the jet mixing layer thickness both downstream and in the early stages of jet development. Exit conditions are documented by measuring the mean Mach number profile immediately above the nozzle exit. The fluctuating flow field is characterised by means of a hot-wire, which produced radial profiles of axial turbulence at several stations along the jet axis and the development of flow fluctuations through the jet mixing layer. The axial growth rate of the jet instabilities are determined as function of Strouhal number, and the axial development of several spectral components is documented. The directivity of the overall sound pressure level and several spectral components were investigated. The spectral content of the acoustic far field is shown to be compatible with findings of hot-wire experiments in the mixing layer of the jet. In addition, the measured acoustic spectra agree with Tam’s large-scale similarity and fine-scale similarity spectra (Tam et al., AIAA Pap 96, 1996).     

Numerical simulation of the flow field in the vicinity of an axial flow fan

The main purpose of the current investigation is the development and evaluation of a numerical model used to simulate the effect of an axial flow fan on the velocity field in the vicinity of the fan blades. The axial flow fan is modeled as an actuator disc, where the actuator disc forces are calculated using blade element theory. The calculated disc forces are expressed as sources/sinks of momentum in the Navier–Stokes equations solved by a commercially available computational fluid dynamic (CFD) code, Flo⁺⁺. The model is used to determine the fan performance characteristics of an axial flow fan as well as the velocity fields directly up‐ and downstream of the fan blades. The results are compared with experimental data. In general, good agreement is obtained between the numerical results and experimental data, although the fan power consumption, as well as radial velocity downstream of the fan blades, is underpredicted by the fan model. Copyright © 2001 John Wiley & Sons, Ltd.     

Turbulent characteristics of shear-thinning fluids in recirculating flows

 A miniaturised fibre optic Laser-Doppler anemometer was used to carry out a detailed hydrodynamic investigation of the flow downstream of a sudden expansion with 0.1–0.2% by weight shear-thinning aqueous solutions of xanthan gum. Upstream of the sudden expansion the pipe flow was fully-developed and the xanthan gum solutions exhibited drag reduction with corresponding lower radial and tangential normal Reynolds stresses, but higher axial Reynolds stress near the wall and a flatter axial mean velocity profile in comparison with Newtonian flow. The recirculation bubble length was reduced by more than 20% relative to the high Reynolds number Newtonian flow, and this was attributed to the occurrence further upstream of high turbulence for the non-Newtonian solutions, because of advection of turbulence and earlier high turbulence production in the shear layer. Comparisons with the measurements of Escudier and Smith (1999) with similar fluids emphasized the dominating role of inlet turbulence. The present downstream turbulence field was less anisotropic, and had lower maximum axial Reynolds stresses (by 16%) but higher radial turbulence (20%) than theirs. They reported considerably longer recirculating bubble lengths than we do for similar non-Newtonian fluids and Reynolds numbers. Received: 23 February 1999/Accepted: 28 April 1999     

Computational Modelling of Turbulent Mixing in Confined Swirling Environment Under Constant and Variable Density Conditions

A high-intensity swirling flow in a model combustor subjected to large density variations has been examined computationally. The focus is on the Favre-averaged Navier–Stokes computations of the momentum and scalar transport employing turbulence models based on the differential second-moment closure (SMC) strategy. An updated version of the basic, high-Reynolds number SMC model accounting for a quadratic expansion of both the pressure–strain and dissipation tensors and a near-wall SMC model were used for predicting the mean velocity and turbulence fields. The accompanied mixing between the annular swirling air flow and the central non-swirling helium jet was studied by applying three scalar flux models differing mainly in the model formulation for the pressure-scalar gradient correlation. The computed axial and circumferential velocities agree fairly well with the reference experiment [So et al., NASA Contractor Report 3832, 1984; Ahmed and So, Exp. Fluids 4 (1986) 107], reproducing important features of such a weakly supercritical flow configuration (tendency of the flow core to separate). Although the length at which the mixing was completed was reproduced in reasonable agreement with the experimental results, the mixing activity in terms of the spreading rate of the shear/mixing layer, that is its thickness, was somewhat more intensive. Prior to these investigations, the model applied was validated by computing the transport of the passive scalar in the non-swirling (Johnson and Bennet, Report NASA CR-165574, UTRC Report R81-915540-9, 1981) and swirling (Roback and Johnson, NASA Contractor Report 168252, 1983) flow in a model combustor.     

Experiment and prediction of a cavity type separated flow

A detail study involving flow visualization, Laser Doppler Velocimeter (LDV) measurements and numerical prediction is presented. The visualization experiments revealed striking results of a pulsatile motion in the separated flow region associated with the formation and passage of large eddy structures. Measurements of mean velocities and turbulence intensity profiles across the separated flow field, provided information about the separated shear layer development and the recirculating flow pattern. The numerical predictions, obtained with a two-layer turbulence model in conjunction with the SIMPLE algorithm, failed to reproduce the coherent eddies and the pulsatile motion, but the mean velocities are reasonably reproduced.     

单柱单锥型液—液旋流分离管内流场的LDV诊断

应用二维激光多普勒仪（ＬＤＶ）对一种单柱单锥型液－液旋流分离管内流场进行了测量,考察了流量、溢流比、压力比和气芯等参数对流场的影响。测量结果表明：切向速度分布呈典型的Ｒａｎｋｉｎｅ涡结构,沿轴向衰减很少,表明所用锥角是合适的;因该旋流管的水力直径较大,切向速度的总体水平较低,由于对了离特性带来了不利影响。此外,没有观察到切向速度分布的的双峰分布现象。轴向速度的总体水平较低,尤其是在锥形管的上游更为     

High resolution PIV measurements around a model turbine blade trailing edge film-cooling breakout

Film cooling downstream of a model turbine blade trailing edge has been studied experimentally. High resolution particle image velocimetry was used to obtain spatially resolved mean velocity and turbulence measurements in the immediate vicinity of the trailing edge breakout. The mean velocity measurements imply the presence of a pair of counter-rotating longitudinal vortices shed from the sides of the breakout lands. The turbulent shear stress measurements above the breakout are significantly intensified as blowing ratio is increased. These results suggest that there is a strong mixing between the film cooling slot jets and the mainstream flow which degrades the film cooling effectiveness.     

Density effects on jet characteristics in confined swirling flow

Density effects on isothermal jet mixing in confined swirling flow are investigated. The experiment is carried out with helium/air as the jet fluid in the same facility as that used by So et al. (1984a, b) and the test conditions are chosen to be the same as before. Contrary to the homogeneous mixing results, the helium jet is preserved up to 40 jet diameters downstream. The behavior of the mean and turbulence field depends highly on the initial jet velocity. Since the jets are fully turbulent and the jet momentum fluxes for inhomogeneous mixing are less than those for homogeneous mixing, the cause of this difference in behavior is directly attributed to the combined action of density difference and swirl. In spite of this, near isotropy of the turbulence field is again observed at 40 jet diameters downstream.     

Modeling of Liquid Fuel Injection, Evaporation and Mixing in a Gas Turbine Burner Using Large Eddy Simulations

The interaction of turbulence, temperature fluctuation, liquid fuel transport, mixing and evaporation is studied by using Large Eddy Simulations (LES). To assess the accuracy of the different components of the methods we consider first isothermal, single phase flow in a straight duct. The results using different numerical methods incorporating dynamic Sub-Grid-Scale (SGS) models are compared with DNS and experimental data. The effects of the interactions among turbulence, temperature fluctuation, spray transport, evaporation and mixing of the gaseous fuel are studied by using different assumptions on the temperature field. It has been found that there are strong non-linear interactions among temperature-fluctuation, evaporation and turbulent mixing which require additional modeling if not full LES is used. The developed models and methods have been applied to a gas turbine burner into which liquid fuel is injected. The dispersion of the droplets in the burner is described. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental study of the mean wake of a tidal stream rotor in a shallow turbulent flow

The mean wake of a three-bladed horizontal axis tidal stream turbine operating at maximum power coefficient has been investigated experimentally in a wide flume with width 11 times the depth, providing minimal restriction to transverse wake development and behaviour of large-scale horizontal turbulence structures. This is an important first stage for understanding wake interaction in turbine arrays and hence large-scale power generation. The rotor diameter has a typical value of 60% of the depth and the thrust coefficient is representative of a full-scale turbine. The shear layers originating from the rotor tip circumference show classic linear expansion downstream, with the rate of a plane shear layer vertically and 1.5 times that horizontally. These shear layers merge by around 2.5 diameters downstream forming a self-similar two-dimensional wake beyond eight diameters downstream with a virtual origin at two diameters downstream of the rotor plane. The spreading rate is somewhat less than that for solid bodies. The detailed velocity measurements made in the near wake show rotation and vorticity similar to that measured previously for wind and marine turbines although with asymmetry associated with bed and surface proximity. The longitudinal circulation in a transverse plane is conserved at about 1% of the swept circulation from the blade tip within two diameters downstream, the extent of detailed measurement. Turbines are usually designed using blade element momentum theory in which velocities at the rotor plane are characterised by axial and tangential induction factors and it is now possible to see how this idealisation relates to actual velocities. The axial induction factor corresponds to velocity deficits at 0.4–0.8 radii from the rotor axis across the near wake while the tangential induction factor at the rotor plane corresponds to velocities at 0.4–0.6 radii between 1–2 diameters downstream, indicating some general correspondence. For the two-dimensional self-similar far wake the two parameters defining the centreline velocity deficit and the transverse velocity profiles are likely to be insensitive to Reynolds number in turbulent conditions.     

不同参数下环形通道内湍流旋流流动的模拟

应用一种合理考虑湍流一旋流相互作用及湍流脉动各向异性的新的代数ＲｅｙｎｏｌｄＳ应力模型,对环形通道内的湍流旋流流动进行了数值模拟．研究了旋流数、进口轴向速度和内外半径比等参数对环形通道内湍流旋流流动的影响,以及由此产生的流场变化对强化环形通道内传热的作用．     

Forced convective heat transfer from premixed flames: —Part 1: Flame structure

In this, the first part of a two-part study of convective heat transfer from impinging flames, the aerodynamic structure of four flames was studied. The flames examined were of stoichiometric mixtures of methane and air with a Reynolds number range extending from the laminar to fully turbulent flow regimes. Instantaneous Schlieren photographs revealed that with increasing Reynolds number the flame reaction zone extended further downstream and became thicker and more diffuse. Associated with this, measurements of mean and rms velocities and mean temperatures showed that the properties of the flames became drawn out in the downstream direction as Reynolds number increased. Schlierenstroboscopic techniques also revealed the existence of large scale vortex rings which originated in the shear layer of the flames, and which were found to cause low frequency oscillations in measured instantaneous velocities. These oscillations lead to misleadingly high levels of rms velocities downstream of the flame reaction zone which should not be interpreted as representing turbulence within the flame.     

Die Wirkung von Polymerzusätzen auf die Turbulenzstruktur in der ebenen Mischungsschicht zweier Ströme

The plane mixing layer formed between two parallel streams moving with different velocities is one of the simplest types of free turbulent boundary layers and has frequently been studied for Newtonian fluids. As a result of this and because of its good experimental accessibility this type of flow provides a good opportunity for obtaining information about the influence of drag reducing additives on the structure of free turbulence. This is all the more so because of the presence of a characteristic vortex structure which can be clearly distinguished from the overlying statistical fine turbulence. The turbulence field was investigated using an existent laser Doppler anemometer system that had been designed for space-time correlation measurements. This enabled measurements to be made of the mainstream velocity as well as of the longitudinal and transversal turbulent fluctuations and, after a simple modification, also of the Reynolds shear stresses and the cross correlation coefficients. The main result of the addition of 50 ppm of the polymer used (Separan AP30) was found to be an intensification of the Reynolds shear stresses. The resulting substantially more rapid increase (than in water) in the thickness of the shear layer can be explained theoretically; such behaviour has also been observed in free jets. On the other hand, the reduced thickness of the mixing layer in the initial region and the associated enhancement of the longitudinal fluctuations and damping of the transversal fluctuations indicate that the main shear flow induces a flow anisotropy by uncoiling and aligning the polymer molecules. The increase in the spreading angle suggests that the entrainment process at the edges of the mixing layer is intensified. This can be explained by the enhancement of the large energy carrying vortices in the turbulence spectrum. This is probably also the reason for the general increase in the correlation coefficients observed at all positions along the centreline of the flow field. However, a complete discussion of the energy transfer mechanism present here, in particular with inclusion of the fine turbulence responsible for dissipation, is only possible with the help of a detailed analysis of the vortex structure in the mixing layer. This is presented in a following paper. The relation between the degree of drag reduction and the intensity of the Reynolds shear stresses enables the direct influence of the rheological properties of the fluid on the turbulent momentum transfer to be clearly recognized.     

The low Reynolds number turbulent flow and mixing in a confined impinging jet reactor

Turbulent mixing takes an important role in chemical engineering, especially when the chemical reaction is fast compared to the mixing time. In this context a detailed knowledge of the flow field, the distribution of turbulent kinetic energy (TKE) and its dissipation rate is important, as these quantities are used for many mixing models. For this reason we conduct a direct numerical simulation (DNS) of a confined impinging jet reactor (CIJR) at Re = 500 and Sc = 1. The data is compared with particle image velocimetry (PIV) measurements and the basic flow features match between simulation and experiment. The DNS data is analysed and it is shown that the flow is dominated by a stable vortex in the main mixing duct. High intensities of turbulent kinetic energy and dissipation are found in the impingement zone which decrease rapidly towards the exit of the CIJR. In the whole CIJR the turbulence is not in equilibrium. The strong mixing in the impingement zone leads to a rapid development of a monomodal PDF. Due to the special properties of the flow field, a bimodal PDF is generated in cross-sections downstream the impingement zone, that slowly relaxes under relaminarising conditions. The time required for meso-mixing is dominating the overall mixing performance.     

Structure of turbulent bubbly jets—II. Phase property profiles

This is the second part of a two-part study reporting structure measurements in bubbly turbulent round jets in a still environment. Measurements are compared with three theoretical approaches: (1) locally homogeneous flow analysis, where velocity differences between the phases were neglected; (2) deterministic separated flow analysis, where relative velocity was considered but bubble/turbulence interactions were ignored; and (3) stochastic separated flow analysis, where both relative velocity and bubble/turbulence interactions were considered using random-walk methods. This part of the study considers measurements and predictions of mean and fluctuating phase velocities and mean bubble number intensities at several axial stations. Locally homogeneous flow analysis was not very satisfactory since effects of relative velocity were important for present test conditions. Deterministic separated flow analysis was also ineffective, since neglecting turbulent dispersion caused the width of the bubble-containing region to be underestimated. In contrast, the stochastic separated flow analysis yielded reasonably good predictions.