Experimental and numerical study of wall-induced granular convection

In this paper, wall-driven convective motion in granular systems under vibration is investigated. A pseudo-2D experimental set-up was constructed in which the convection was generated via a controlled roughness at the side walls. The optical measurement technique particle image velocimetry was used to obtain whole field particle velocity information. This allows for direct ‘one-to-one’ comparison with results obtained from numerical simulations. It was found that the model was capable of reproducing trends that were observed in our own experiments and those reported in literature. However, the convection strength was usually over-predicted in the simulations.     

Dynamics of a spherical particle in mixed convection flow field

Numerical and experimental study on steady mixed convection gas flow around a hot spherical particle is presented for small values of Reynolds and Grashof numbers and all mixed convection parameter range. The flow regimes considered are the axis-symmetric co-flow and opposed flow for the non-dimensional Boussinesq model. The sensitivity of the numerical computations with respect to the dimension of the computational domain is discussed. The opposed flow regime and the formation of re-circulation secondary flow is analyzed. A qualitative expression for the drag force induced on the particle by the flow is given for all range of the mixed convection parameter, the expression holds for the co-flow configuration but brakes up in the opposed flow regime when re-circulation secondary flow evolves. Experimental measurements of the drag force induced on a small spherical particle allows further examination of the numerical computations and theory.     

Heat/mass transfer from a neutrally buoyant sphere by mixed natural and forced convection in a simple shear flow

Building on the work of Yang et al. in 2011, the finite difference method and the Boussinesq approximation were applied to solve the time‐dependent Navier‐Stokes, convection diffusion and continuity equations in spherical coordinates. An idealized condition, the mass transfer from a neutrally buoyant sphere in a horizontal simple shear flow with natural convection was numerically simulated for the first time in this work. In the hybrid transfer case, the outwardly spiraling streamlines enhanced the transfer process, but the counter‐gravity spiraling streamlines near the sphere hindered the natural convection and the spatial dilution action weakened the natural convection transfer process. These competing effects led to nonmonotonic behavior of the Nusselt number with Reynolds number. Results from these previously undocumented cases were summarized into correlations for predicting Nusselt numbers at finite Reynolds numbers for various Grashof and Prandtl numbers. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2816–2827, 2018     

Experimental detection of bubble-wall interactions in a vertical gas-liquid flow

Bubble motions and bubble-wall interactions in stagnant liquid were experimentally investigated by high-speed CCD and PIV technique with the main feature parameters such as E(o)tv(o)s numbers Eo =0.98-1.10,Morton number Mo =3.21 × 10-9 and Reynolds numbers Re =180 ～ 190.The effect of bubble injecting frequency and the distance S between the gas injection nozzle and the wall on the statistical trajectory of bubbles,average velocity distribution of flow field and Reynolds shear stress were studied in detail.It was shown that the combination of bubble injecting frequency and the distance S caused different bubble motion forms and hydrodynamic characteristics.When the normalized initial distance was very little,like S* ≈ 1.2 (here S* =2S/de,and de is the bubble equivalent diameter),bubbles ascended in a zigzag trajectory with alternant structure of high and low speed flow field around the bubbles,and the distribution of positive and negative Reynolds shear stress looked like a blob.With the increase of distance S*,bubbles' trajectory would tend to be smooth and straight from the zigzag curve.Meanwhile,with the increase of bubble injecting frequency,the camber of bubble trajectory at 20 ＜ y ＜ 60 mm had a slight increase due to the inhibitory effect from the vertical wall.Under larger spacing,such as S* ≈ 3.6,the low-frequency bubbles gradually moved away from the vertical plane wall in a straight trajectory and the high-frequency bubbles gradually moved close to the vertical wall in a similar straight trajectory after an unstable camber motion.Under the circumstances,high-speed fluid was mainly distributed in the region between the wall and the bubbles,while the relative large Reynolds shear stress mainly existed in the region far away from the wall.     

Experimental study of O2 diffusion coefficient measurement at a planar gas–liquid interface by planar laser‐induced fluorescence with inhibition

A new method for determining the molecular diffusivity of oxygen in liquids is described. The technique was applied through a flat air–liquid interface in a Hele‐Shaw cell (5 × 5 × 0.2 cm³) and was based on planar laser‐induced fluorescence (PLIF) with inhibition. A ruthenium complex (C₇₂H₄₈N₈O₆Ru) was used as the fluorescent dye sensitive to oxygen. A mathematical analysis was developed to determine the molecular diffusivity of oxygen simply by localizing the gas diffusion front. The specificity of this mathematical analysis is that it does not require the properties of the fluids (such as the saturation concentration) to be considered, which is especially relevant for complex media that are sometimes difficult to characterize properly. This technique was applied to three different fluids (viscosities ranging from 1 to 2.4 mPa·s) corresponding to binary diffusion coefficients ranging from 9.5 × 10^?10 to 2 × 10^?9 m²/s. Experimental data were found with an uncertainty of about 5% and were in good agreement with the literature. Particle image velocimetry and numerical simulations were also carried out to determine the optimal gas flow rate (0.01 L/s) to reach purely diffusive transfer, and the corresponding hydrodynamic profiles of the two phases. © 2012 American Institute of Chemical Engineers AIChE J, 59: 325–333, 2013     

Experimental determination of the drag coefficient in a swarm of bubbles

Simultaneous measurements of liquid velocity by laser Doppler velocimetry and bubble velocity, diameter, and void fraction by a double optical probe are performed in a bubble column to study the influence of the void fraction on the relative velocity of a swarm of gas bubbles. Bubble diameters d_b vary from 2 to 10 mm and local void fractions α_loc can reach 35%. It is found that, for α_loc<15%, the relative bubble velocity is determined by the hindrance effect and consequently decreases with the void fraction. Beyond this critical value, the aspiration of bubbles in the wake of the leading ones dominates the hindrance effect and the relative velocity thus increases suddenly. The contribution of the bubble diameters to this evolution is also determined. Finally, a drag correlation, valid for the whole range of void fraction and for pure water-air systems, is proposed.     

Experimental and mathematical studies of natural convection for solid-salt dissolution in water

The phenomena of natural convection caused by a concentration gradient usually creates currents as a result of buoyancy forces. These forces are formed from the combined effect of a fluid density gradient and the body force (gravitational force). The above phenomenon was studied experimentally in a circular glass column of 0.08 m diameter and 1 m length. This column was erected vertically and filled with distilled water with a salt bag placed on top of it and partially immersed in water. The dissolution mechanism was then modelled and a finite difference method was used to solve the formulated equations by using the implicit scheme of MacCormack (Anderson et al., 1984). The experimental results and the numerical predictions are represented graphically and give compatible agreement.     

Experimental determination of axial dispersion coefficient in a bubble column

A new graphical method of finding axial dispersion coefficients in a bubble column is developed. This method requires only the area above an experimental curve, thus is simpler than the conventional method of curve fitting. The new method is used to find the dimensionless dispersion number in the bubble column and shows that the dispersion number decreases with column Reynolds number due to increasing coalescence of bubbles at higher gas flow-rates.     

过渡流下钝体绕流特性的可视化实验对比

为解明钝体绕流机理,设计了开式低速循环水槽,建立了钝体绕流实验平台。采用粒子图像测速法（PIV）和染色法,对钝体绕流特性进行实验研究。采用INSIGHT 4 G进行全面调控。通过检测水槽不同高度激光面上的流动状态验证实验台稳定性。将PIV及染色法所得数据与已有模拟及实验结果进行对比,验证了实验方法正确性。选取过渡流下特定钝体（圆柱）进行周期研究,探寻钝体绕流周期性。并分析圆柱、方柱绕流在不同Reynolds数下的流动特性异同。实验表明：Re=200时,在一个周期内,圆柱周围正涡、负涡周期性脱落,正涡逐渐形成、增强、消散,负涡也在对应的位置循环发展。通过圆、方柱流动实验对比可知,圆柱绕流分离点不固定而方柱绕流分离点固定,位于其前后锐边上。     

Experimental investigation of the flow dynamics of rheologically complex fluids in a Maxblend impeller system using PIV

The Maxblend^® mixer is used in processes involving Newtonian, shear-thinning or viscoelastic fluids and, as for many impellers, little is known regarding the relationship between the rheological behavior and the flow and mixing properties. An experimental study of the hydrodynamics in the Maxblend was carried out using Newtonian and non-Newtonian fluids in laminar and early transitional regimes. Flow fields were measured by two-dimensional particle image velocimetry (PIV) to evaluate the effect of highly shear-thinning and viscoelastic behavior on the performance of the mixer. The experimental setup consisted of a 35-L tank equipped with two baffles. A total of 100 measurements were proven to be sufficient in order to attain convergence of the velocity components for all studied regimes. Isolated zones in the highly shear-thinning case were observed by means of a decolorization technique. Elasticity in the laminar regime produced a reversal of the flow and a solid body rotation in the bottom region of the tank. Fluid velocity magnitude and spread were observed to be reduced by the presence of elastic forces.     

Impact of thixotropy on flow patterns induced in a stirred tank: Numerical and experimental studies

Agitation of a thixotropic shear-thinning fluid exhibiting a yield stress is investigated both experimentally and via simulations. Steady-state experiments are conducted at three impeller rotation rates (1, 2 and 8 s⁻¹) for a tank stirred with an axial-impeller and flow-field measurements are made using particle image velocimetry (PIV) measurements. Three-dimensional numerical simulations are also performed using the commercial CFD code ANSYS CFX10.0. The viscosity of the suspension is determined experimentally and is modelled using two shear-dependant laws, one of which takes into account the flow instabilities of such fluids at low shear rates. At the highest impeller speed, the flow exhibits the familiar outward pumping action associated with axial-flow impellers. However, as the impeller speed decreases, a cavern is formed around the impeller, the flow generated in the vicinity of the agitator reorganizes and its pumping capacity vanishes. An unusual flow pattern, where the radial velocity dominates, is observed experimentally at the lowest stirring speed. It is found to result from wall slip effects. Using blades with rough surfaces prevents this peculiar behaviour and mainly resolves the discrepancies between the experimental and computational results.     

Modelling natural convection at complex surfaces and solid bodies using electrochemical techniques and flow visualisation

Since the review by Wragg [Journal of Applied Electrochemistry, 21 (1991) 1047] of more than 15 years ago there has been a considerable research output involving the use of combined electrochemical and flow visualisation techniques to investigate complex flows in natural convection situations. This paper reviews recent work involving electrode geometries and orientations such as vertical cylindrical electrodes of varying aspect ratio, down pointing and up pointing pyramidal electrodes, down facing horizontal circular surfaces with different edge conditions, isosceles triangular surfaces of various inclinations, long narrow upward facing linear tracks, open upward facing cavity electrodes, upward and downward facing truncated cones, and inclined disks with either single or both sides active. This constitutes a wide range of experimentation in which new insights into data treatment and correlation have been obtained. We have used approaches taking account of the differing behaviours of the single sides of three dimensional objects and computing interference factors taking account of flow interactions. Some highly successful illustrations of data correlation for extremely complex situations are demonstrated. Flow visualisation for most of the above mentioned situations have been obtained using schlieren photography which has provided good illustrations of flow structure occurring at single faces of objects and in complexly interacting flows. In addition we have combined some flow visualisation sequences with monitoring of the current-time relationships at the onset of convection to illustrate flow development phenomena from the onset of convective instabilities to the achievement of steady state conditions.     

Visualization of room fire induced smoke movement and flow in a corridor

A study was conducted of the smoke and flow field in a corridor subject to a room fire. The study was conducted using a scale model of roughly 0.35 m in height. The effect of corridor-exit doorway width was recorded while the room doorway and fire-room temperature were maintained constant. Smoke was generated from cotton wads soaked with titanium tetrachloride which produces white particles of titanium dioxide. By this means, the smoke layer resulting from the room fire and the corridor flow characteristics were visualized. The results show the lowering interface of the corridor smoke layer with decreasing corridor-exit door width. Also a four-layer horizontal counter-current flow pattern was displayed and shown to result from a restriction (e.g. soffit) at the corridor exit. The mixing of the incoming cold flow and exiting hot flow at the corridor exit was observed to be shedding vortices swept into the cold floor jet. Results based on velocity measurements and smoke observations are presented for the corridor smoke layer height and doorway neutral-plane heights. The limitations of current predictive models are demonstrated for layer-heights and flow rates for the room and corridor experiments.     

Effect of the fluffiness of nonwoven materials on the surface heat-transfer coefficient in natural convection

The effect of the fluffiness of the surface of nonwoven material on its heat transfer is manifested by a small range of the difference in the surface and ambient temperatures (∼20°), in which the heat-shielding properties of clothing are used. When the difference in the temperature of the surface of the material and the ambient temperature is greater than 30°, the effect of the fluffiness on the surface heat transfer coefficient can not be considered. The thermophysical constants of the material can be determined in this temperature region with the cooling curve. __________ Translated from Khimicheskie Volokna, No. 5, pp. 37–41, September–October, 2007.     

Numerical predictions of flow patterns due to natural convection in a vertical slot

Natural convection in rectangular slots has vital applications in cooling of nuclear reactor insulation, chemical vapor deposition, insulation of double pane windows, environmental processes, and phase change processes. Flow patterns arising out of density differences depend on height to width ratio (aspect ratio (AR)) and temperature difference. These flow patterns have a significant effect on rate of heat transfer. Hence flow patterns in a slender vertical slot with large height to width ratio (AR=20) have been numerically studied for four fluids (mercury, air, and water and silicon oil) of varying Prandtl numbers. The circulation cell formation and merging has been seen as a quasi-periodic behavior for water and silicon oil at various Rayleigh numbers. The genesis of formation of circulation cells due to the pressure differentials (as a result of thermal gradients) has been satisfactorily explained for mercury, air, and water and silicon oil and a criterion for the same has been developed for the range of Prandtl number between 0.71 and 7 on the basis of numerical predictions.     

Experimental determination of the effective thermal conductivity of Vacuum Insulation Panels at fire temperatures

In this study, the effective thermal conductivity of a commercial Vacuum Insulation Panel (VIP) at temperatures up to 900 °C is experimentally determined. An experimental setup, based on the Heat Flow Meter Apparatus (HFMA) method, is designed and realized. Two commercially available VIPs (each 20 mm thick) are joined together to form a specimen, which is subjected to fire conditions from one side, while the other side is at ambient conditions. The temperatures on both sides of the specimen and the heat flux on the unexposed side are recorded. The experimental data are coupled with a numerical model, which takes into account the one dimensional steady state heat transfer through the thickness of the specimen and the detailed heat transfer mechanisms for the effective thermal conductivity of the VIP. Gas, solid and radiation conduction mechanisms are considered and their parameters are defined through an optimization technique. The defined optimized values are found to lie between the respective values reported in the literature. The contribution of each heat transfer mechanism to the overall effective thermal conductivity is also discussed. The paper provides a generalized methodology for the estimation of the effective thermal conductivity of VIPs from ambient to fire temperatures. Copyright © 2016 John Wiley & Sons, Ltd.     

Influence of multisource fire on temperature distribution and natural smoke exhaust effect in urban tunnels with a shaft

The combustion characteristics of multisource fire and single-source fire are quite different, and there is little research on the influence of multisource fire on the natural smoke extraction effect of shaft in urban tunnels. Therefore, in this article, the method of numerical simulation was used to study the influence of fire power and distance between two fire sources on the natural smoke extraction effect of shaft and the temperature distribution in tunnel in the case of multisource fire. Typical characteristics of smoke are analyzed, such as mass flow rate, temperature distribution, velocity vector, and CO concentration. The simulation results show that when there is a certain distance between the fire sources, the two flames are inclined and close to each other. The smoke temperature under the ceiling is higher under multiple fire sources than that under single fire source. In addition, when one of the fire sources is located at the downstream of the shaft, the smoke emission in the shaft is relatively high. As the distance between fire sources continues to increase, the smoke exhaust rate basically remains stable, and an empirical relationship between smoke exhaust rate and fire source location is established.     

Experimental and numerical investigation of liquid circulation induced by a bubble plume in a baffled tank

Bubble induced liquid circulation is important in applications such as bubble columns and air-lift reactors. In this work, we describe an experimental and numerical investigation of liquid circulation induced by a bubble plume in a tank partitioned by a baffle. The baffle divides the tank into two compartments. Liquid can flow from one compartment to the other through openings at the top and the bottom of the baffle. Gas (air) was injected in the riser section in the form of bubbles at one corner of the tank. The temporal and spatial variation of velocity field in the liquid as a function of the gas flow rate was measured using particle image velocimetry (PIV). At a constant gas flow rate, the liquid flow field is unsteady due to the interaction with the bubbles. The time scales associated with the velocity-time series and the bubble plume thickness variation were calculated. The time averaged-velocity field was used to quantify the variation of the liquid circulation rate with gas flow rate. The turbulence in the liquid was measured in terms of turbulent intensities. These were calculated from the experimental data and were observed to be less than 3 cm/s. A 2-d Euler-Euler two-fluid model with buoyancy and drag as the interaction terms was used to simulate the flow. The parameters chosen for the simulations were selected from literature. It is shown that inclusion of turbulence model such as k-ε is necessary to capture the overall flow behavior. Good agreement was observed between experimentally obtained velocity profiles and the recirculation rates with the simulation results.     

Experimental and mathematical modeling of three-dimensional natural convection in an enclosure

Three-dimensional natural convection patterns in an enclosure under conditions simulating flow in glass-making furnaces have been calculated using an approximate method in which solutions for the temperature and velocity fields in orthogonal two-dimensional planes are superposed. The computational method was tested by comparison with measurements of the temperatures and velocities in an enclosure heated and cooled from above and with a controlled heat loss from the sides. Good agreement was found between measurements and computations under conditions of interest in glass-making furnaces, wherein the motion in one of the two orthogonal planes is dominant.     

一种新型多节隔板-平板式光生物反应器的数值和实验研究

光是影响光生物反应器培养效率的最主要因素之一.而在一定的外部光强下,光生物反应器内部的混合状况对微藻细胞生长有重要影响.采用CFD(Computational Fluid Dynamics)模型对一种新型多节隔板-平板式光生物反应器在不同通气量下的流场分布进行了模拟并与PIV(Particle Image Veloeimetry)测量结果进行比较,结果表明CFD模型可用于光生物反应器流场的模拟;利用CFD模型对不同隔板节数的平板式光生物反应器的内部流场进行了模拟,对光照方向混合进行定量研究,并通过分析光照方向径向速度U、下降通道体积平均湍动能ADT、流体(藻液)绕隔板循环一周所用时间tc、下降通道(光区)停留时间占藻液绕隔板循环一周所用时间的比例ε四个参数来优化光生物反应器的隔板节数.理论分析表明,三节隔板-平板式光生物反应器为最佳选择,并通过球等鞭金藻3011培养实验对此进行了验证.