Measurements of pollen grain dispersal in still air and stationary, near homogeneous, isotropic turbulence

The dispersal of ragweed, pine and corn pollen as well as polystyrene spheres in still air and stationary, near homogeneous, isotropic turbulence (HIT) was investigated using high-speed, digital inline holographic cinematography enabling Lagrangian tracking of the particles. Mean still air settling velocities were similar as reported literature values. Small discrepancies were most likely related to species/size differences and water content of the grains. Near-HIT was generated by loudspeakers mounted on the corners of a 40 cm³ chamber and the turbulent flow field at the center of the chamber was validated using stereoscopic Particle Image Velocimetry (PIV). Results showed near homogeneity and near isotropy with mean velocities 5–10 times smaller than the corresponding rms values of velocity fluctuations. The turbulent kinetic energy dissipation rate was determined from the PIV data sets and used to calculate the Kolmogorov scales and Taylor microscales. Experiments were carried out for two different loudspeaker amplifications corresponding to Taylor microscale Reynolds numbers, R_λ=144 and 162, respectively. The mean settling velocity in turbulent conditions was in all cases higher than the corresponding still air value, the difference becoming smaller as particle Stokes numbers increased. For the present conditions, the still air particle settling velocity was lower than the rms values of air fluctuating velocities. As a result, dispersion was dominated by inertia and for a given R_λ, particle fluctuating velocity autocorrelations fell more rapidly as the particle Stokes number decreased; corresponding particle diffusion coefficients also decreased. Transverse particle diffusion coefficients were lower than those in the direction of gravity in agreement with the continuity effect. Under the present range of experimental parameters, results showed that inertial particles (0.6<St<11) in highly turbulent conditions disperse more effectively than the air.     

Fine particle turbulence modulation

Streamwise turbulence intensities of fine particulate suspensions were studied in a 26 mm N.B. horizontal pipe loop. Colloidal silica spheres were prepared in 10^?4M and 1M KNO₃ solutions to control the degree of aggregate formation in the suspension. Using an ultrasonic Doppler velocity profiling sensor, the turbulence intensities of the fine particle suspensions were compared with those of a particle‐free flow over a range of Reynolds numbers. At low electrolyte concentration, the silica particles remain dispersed, with the turbulence intensity of the suspension flow comparable with that of the particle‐free flow. At high electrolyte concentration, increased particle‐particle interaction leads to the formation of particle aggregates which support turbulence augmentation over a critical Reynolds number range. The range of Reynolds numbers over which this turbulence enhancement is observed is limited by both fluid dynamic effects at low Reynolds numbers (Re ≈ 5500) and aggregate breakup at high Reynolds numbers (Re ≈ 8000). © 2010 American Institute of Chemical Engineers AIChE J, 2011     

管道矩形射流的时均特性

管道矩形射流在工程实际中,特别在燃烧装置上,应用普遍。因此了解和掌握它们的特性及其有关信息是很有价值的。本文通过作者的实际测量(工具为美国tsi公司的双通道热线风速仪)对小特征比的情况做了初步研究。文中给出了描述时均流场的部分结果,即轴线上时均速度的衰减、射流横截面上时均速度的变化、射流半宽度及主副轴的转换等特性。所得结果与前人完成的圆形射流、平面射流以及大特征比矩形射流的实验结果做对比并进行相应讨论。研究表明,矩形射流的时均特性与平面射流和圆形射流有较大区别,但小特征比与大特征比射流之间却存在不少相似之处。     

烟囱式格子体蓄热室气流分布规律及其底部流场的研究

本文为研究烟囱式格子体蓄室气流分布规律及其底部流场,采用空气冷态实体模型进行了烟囱式格子体蓄热室流体动力过程的相似模拟的研究。得出了烟囱式格子体蓄热室内气流分布是不均匀的,对造成这种分布下不均匀性的主要因素进行了讨论。当采用炉条(石玄)阶梯排列并在蓄热室底部加设挡板能够显著改善蓄热室格子体气流分布的均匀性。     

浅谈料垛码法对倒焰式燃气窑内气流分布及温度均匀性的影响

通过料垛码法形成的气流通道大小对倒焰式燃气窑内气流分布以及温度均匀性的影响进行了分析,认为在一定的条件下,通道大小与气体流动速度和流量成正比,根据这一结论,可对倒焰燃气窑内料垛码法起到一定的指导作用。     

Characteristic time scales for predicting the scalar flux at a free surface in turbulent open‐channel flows

The purpose of this study is to predict the turbulent scalar flux at a free surface subject to a fully developed turbulent flow based on a hydrodynamic analysis of turbulence in the region close to the free surface. The effect of the Reynolds number on turbulent scalar transfer mechanisms is extensively examined. A direct numerical simulation technique is applied to achieve the purpose. The surface‐renewal approximation is used to correlate the free‐surface hydrodynamics and scalar transport at the free surface. Two types of characteristic time scales have been examined for predicting turbulent scalar flux. One is the time scale derived from the characteristic length and velocity scale at the free surface. The other is the reciprocal of the root‐mean‐square surface divergence. The results of this study show that scalar transport at the free surface can be predicted successfully using these time scales based on the concept of the surface‐renewal approximation. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Numerical simulation of dilute turbulent gas‐particle flow with turbulence modulation

A numerical study of a dilute turbulent gas‐particle flow with inelastic collisions and turbulence modulation in an Eulerian framework is described. A new interpretation is provided for the interaction/coupling terms, based on a fluctuating energy transfer mechanism. This interpretation provides for a new robust closure model for the interaction terms with the ability to predict the turbulence dampening as well as the turbulence enhancement phenomenon. Further, the model developed herein is investigated along with a variety of other published closure models used for the interaction/coupling terms, particle drag, and solid stress. The models are evaluated against several sets of benchmark experiments for fully‐developed, turbulent gas‐solid flow in a vertical pipe. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Micro-particle transport and deposition in a human oral airway model

Laminar-to-turbulent air flow for typical inhalation modes as well as micro-particle transport and wall deposition in a representative human oral airway model have been simulated using a commercial finite-volume code with user-enhanced programs. The computer model has been validated with experimental airflow and particle deposition data sets. For the first time, accurate local and segmentally averaged particle deposition fractions have been computed under transitional and turbulent flow conditions. Specifically, turbulence that occurs after the constriction in the oral airways for moderate and high-level breathing can enhance particle deposition in the trachea near the larynx, but it is more likely to affect the deposition of smaller particles, say, St<0.05. Particles released around the top/bottom zone of the inlet plane more easily deposit on the curved oral airway surface. Although more complicated geometric features of the oral airway may have a measurable effect on particle deposition, the present simulations with a relatively simple geometry exhibit the main features of laminar-transitional-turbulent particle suspension flows in actual human oral airways. Hence, the present model may serve as the “entryway” for simulating and analyzing airflow and particle deposition in the lung.     

Influence of core sand properties on flow dynamics of core shooting process based on experiment and multiphase simulation

The influence of core sand properties on flow dynamics was investigated synchronously with various core sands, transparent core-box and high-speed camera. To confirm whether the core shooting process has significant turbulence, the flow pattern of sand particles in the shooting head and core box was reproduced with colored core sands. By incorporating the kinetic theory of granular flow(KTGF), kinetic-frictional constitutive correlation and turbulence model, a two-fluid model(TFM) was established to study the flow dynamics of the core shooting process. Two-fluid model(TFM) simulations were then performed and a areasonable agreement was achieved between the simulation and experimental results. Based on the experimental and simulation results, the effects of turbulence, sand density, sand diameter and binder ratio were analyzed in terms of filling process, sand volume fraction(αs) and sand velocity(Vs).