Competing flow and collision effects in a monodispersed liquid–solid fluidized bed at a moderate Archimedes number

We study the effects of fluid–particle and particle–particle interactions in a three-dimensional monodispersed reactor with unstable fluidization. Simulations were conducted using the immersed boundary method for particle Reynolds numbers of 20–70 with an Archimedes number of 23 600. Two different flow regimes were identified as a function of the particle Reynolds number. For low particle Reynolds numbers ( $20 < Re_p < 40$ ), the porosity is relatively low and the particle dynamics are dominated by interparticle collisions that produce anisotropic particle velocity fluctuations. The relative importance of hydrodynamic effects increases with increasing particle Reynolds number, leading to a minimized anisotropy in the particle velocity fluctuations at an intermediate particle Reynolds number. For high particle Reynolds numbers ( $Re_p > 40$ ), the particle dynamics are dominated by hydrodynamic effects, leading to decreasing and more anisotropic particle velocity fluctuations. A sharp increase in the anisotropy occurs when the particle Reynolds number increases from 40 to 50, corresponding to a transition from a regime in which collision and hydrodynamic effects are equally important (regime 1) to a hydrodynamic-dominated regime (regime 2). The results imply an optimum particle Reynolds number of roughly 40 for the investigated Archimedes number of 23 600 at which mixing in the reactor is expected to peak, which is consistent with reactor studies showing peak performance at a similar particle Reynolds number and with a similar Archimedes number. Results also show that maximum effective collisions are attained at intermediate particle Reynolds number. Future work is required to relate optimum particle Reynolds number to Archimedes number.

Effects of Particle Properties on Visualizing Flows in a Two- Stage Electrostatic Precipitator Using Particle Image Velocimetry

The amount of particulate matter (PM) in the environment has been confirmed to be health risks on human bodies[1, 2], and therefore removing suspended particles has become the research goal of many studies. Electrostatic precipitator (ESP) is one of the high-efficiency particle collection technologies[3-7]. Particle Image Velocimetry (PIV) has been an effective tool for visualizing the flow patterns in experimental fluid mechanics, and many studies adopted this technique to study flows in ESP[8-10]. However, particles charged by the electric field can cause deviation in measurement results since it does not follow the ionized air flow which can be charged differently from the tracer particles. In this study, the observation of the effects of different particle properties on flow field in a two-stage ESP is the objectives of this study. A two-stage ESP was built and four different seeding particles, aluminum oxide (Al2O3) particle, oil droplet particle, sodium chloride (NaCl) particle, and titanium dioxide (TiO2) particle, are tested in the current study. In this study, the streamwise velocity of the flows ranges from 2.36 m/s to 4.18 m/s, the voltage of the corona electrode varies from 8 kV to 12 kV with a positive polarity, and the voltage of the collector electrode is fixed at 16 kV. To investigate the 3-D flow patterns inside the channel, data at different planes were taken for comparison. The results show that by increasing charge voltage from 8 kV to 12 kV with a streamwise flow velocity the 2.36 m/s, the y-component velocity for Al2O3 particle, oil droplet particle, NaCl particle and TiO2 particle increased by 50.6%, 76.0%, 33.5% and 51.9%, respectively. Moreover, for the case of the 4.18 m/s primary flow, the y-component velocity for Al2O3 particle, oil droplet particle, NaCl particle and TiO2 particle increase by 52.7%, 59.2%, 59.4% and 65.9% after the voltages increase from 8 kV to 12 kV. PIV results for oil droplet particle shows slower y-component velocities, which can be due to the lower Archimedes number of 3.12E-06 and the mobility number that is larger than 3. On the contrary, in most of results from TiO2 particles show high y-component velocity, which is due to the highest Archimedes number of 1.15E-03 of the seeding particles tested in this study. This result shows that the particle is less affected by buoyancy effect. The PIV results of the middle plane also shows that the ycomponent of velocity from -2.6 m/s to -0.5 m/s, in contrast to -1.0 m/s to 1.0 m/s from the near wall observation plane. These results are consistent to simulation results of the electric field distribution, whichshows unequal electric field strengths between the middle and near wall regions of the test section. Only half of the cage shape distribution of the electric field can be observed, and primary flow influences the ionic wind to move to the downstream area. Based on the results, the oil droplet and TiO2 particles are more suitable for the role of tracer particles compared to aluminum oxide and sodium chloride particles.

A Theoretical Analysis for Mixed Convection Flow of Maxwell Fluid between Two Infinite Isothermal Stretching Disks with Heat Source/Sink

The aim of this current contribution is to examine the rheological significance of Maxwell fluid configured between two isothermal stretching disks. The energy equation is also extended by evaluating the heat source and sink features. The governing partial differential equations (PDEs) are converted into the ordinary differential equations (ODEs) by using appropriate variables. An analytically-based technique is adopted to compute the series solution of the dimensionless flow problem. The convergence of this series solution is carefully ensured. The physical interpretation of important physical parameters like the Hartmann number, Prandtl number, Archimedes number, Eckert number, heat source/sink parameter and the activation energy parameter are presented for velocity, pressure and temperature profiles. The numerical values of different involved parameters for skin friction coefficient and local Nusselt number are expressed in tabular and graphical forms. Moreover, the significance of an important parameter, namely Frank-Kamenetskii, is presented both in tabular and graphical form. This particular study reveals that both axial and radial velocity components decrease by increasing the Frank–Kamenetskii number and stretching the ratio parameter. The pressure distribution is enhanced with an increasing Frank–Kamenetskii number and stretching ratio parameter. It is also observed that thetemperature distribution increases with the increasing Hartmann number, Eckert number and Archimedes number.

Inertial torques and a symmetry breaking orientational transition in the sedimentation of slender fibres

Experimental measurements of the force and torque on freely settling fibres are compared with predictions of the slender-body theory of Khayat & Cox (J. Fluid Mech., vol. 209, 1989, pp. 435–462). Although the flow is viscous dominated at the scale of the fibre diameter, fluid inertia is important on the scale of the fibre length, leading to inertial torques which tend to rotate symmetric fibres toward horizontal orientations. Experimentally, the torque on symmetric fibres is inferred from the measured rate of rotation of the fibres using a quasi-steady torque balance. It is shown theoretically that fibres with an asymmetric radius or mass density distribution undergo a supercritical pitch-fork bifurcation from vertical to oblique settling with increasing Archimedes number, increasing Reynolds number or decreasing asymmetry. This transition is observed in experiments with asymmetric mass density and we find good agreement with the predicted symmetry breaking transition. In these experiments, the steady orientation of the oblique settling fibres provides a means to measure the inertial torque in the absence of transient effects since it is balanced by the known gravitational torque.

Film Size During Bubble Collision With a Solid Surface

The impact and bounce of a bubble with a solid surface is of significant interest to many industrial processes such as froth flotation and biomedical engineering. During the impact, a liquid film becomes trapped between the bubble and the solid surface. The pressure buildup in this film leads to the generation of a film force. The drainage rate of this film plays a crucial role in dictating the bouncing process and is known to be a function of the radial film size. However, radial film size is not an easily attained experimental measurement and requires advanced instrumentation to capture. The bouncing process has been characterized using nondimensional numbers that are representative of the bubble collision and film drainage phenomena. These are: Bond number (Bo), Archimedes number (Ar), Froude number (Fr), and the ratio of film force to buoyancy force (FF/FB). These numbers are used to define a predictive function for film radius. Experimentally validated numerical modeling has been implemented to determine the relationship between the four nondimensional numbers, and a quasi-static model is employed to relate the film force to the radial film size. Comparison of our experimental results is in agreement with the predicted film size within ±20%. From these results, the radial film size during bubble impact with a solid surface may be predicted using the easily measurable experimental parameters of bubble size, bubble impact velocity, and the liquid properties.

Use of Air Infiltration in Swine Housing Ventilation System Design

The objective of this research was to develop and analyze the procedure for using recent air infiltration (AI) data collected from commercial swine finishing rooms (SFRs) in the design of negative pressure mechanical ventilation systems (VSs). Air infiltration is an integral part of any ventilation process. Infiltration reduces the pressure differential across planned inlets and at very low pressure differences, cold air jets may drop directly on the animals causing significant discomfort. In this article, a design procedure is proposed for swine housing ventilation systems with the influence of air infiltration included. The method was used on one SFR for which air infiltration data was collected by in-field testing. The air-jet throw, jet momentum number, a newly developed coverage factor, and Archimedes number were used to assess the influence of infiltration on predicted air-jet and fresh-air distribution and to help guide the design of planned inlets in SFR VSs with known infiltration. The analysis completed quantifies the severity of AI on air-jet and air distribution performance, and suggests that for the analysis room to ventilate properly requires a 50% reduction in AI levels beyond field measured curtain and fan infiltration. The analysis completed suggests a method for systematically planning three-dimensional ceiling inlet placement and operation and provides design guidance for new ceiling inlets suitable for SFR VSs. Keywords: Air distribution, Air-jets, Archimedes number, Infiltration, Jet Momentum Number.

Modeling the effect of shrinkage on fluidized bed drying of orthodox broken type tea

Fermented tea particles (dhool) are a polydisperse system subject to shrinkage during fluidized bed drying, which is an important process in the production of orthodox broken type tea. The effect of shrinkage on the physical properties and the minimum fluidization velocity were studied. Five different moisture contents of dhool particles were chosen in the range of 3-106 mass% (dry basis) and the changes in particle diameters and particle densities were measured. For each of the moisture contents, the minimum fluidization velocity was found for three different bed loadings using ambient air at 25?C in a fluidized bed with an area of 351?345 mm2. Since the conventional industrial type fluidized bed dryers operate at 124?C, the new correlations among the Archimedes number, Reynolds number at minimum fluidization and dimensionless moisture content were developed using air properties at 124?C. The results were validated for orthodox broken type tea, drying at 124?C, in a fluidized bed dryer with bed loadings in the range of 44.5 to 50.5 kg/m2. The predicted fluidization velocity was found to be in good agreement with the experimental data and the difference was below 10% for most cases.

Performance analysis of an active diffuser in mixing ventilation for cell office by using numerical approach

Energy efficiency in buildings is nowadays considered as an essential step to reduce CO2 emissions and energy utilization. At the same time, new technologies such as building space heating using active air heating has simplified the heating system without any need for backup heating system. This study investigated the thermal stratification of mixing ventilation system equipped with a radial active supply diffuser for space heating of an office room designed according to the Norwegian passive house standard by using Star-CCM+. Simulations were performed for different supply airflow rates with corresponding slot openings of the active diffuser at different outdoor conditions for winter season. The combined effect of the supply airflow rate and the outdoor air temperature were also described in the form of Archimedes number (Ar). The results showed that adopting active diffuser could avoid the temperature stratification for all the simulated cases by preserving the throw length of supply jet. In addition, the lowest temperature effectiveness of 38% occurred when a high supply temperature was used in the coldest day. Furthermore, with the supply temperature 24 °C, the airflow rate 49.4 l/s at the outdoor temperature -15 °C would result in a favorable average of PMV ⁓ 0.497.