Effect of Side Blowing on Fluid Flow and Mixing Phenomenon in Gas-Stirred Ladle

To investigate the gas agitation characteristics of side blowing, the fluid flow and mixing phenomenon in a 1:3 scale model ladle of a 150 t industrial gas-stirred ladle with bottom and side plugs were studied by using physical and numerical modelings together. Side blowing enhanced the horizontal flow of water in the model ladle. Compared with bottom blowing, side blowing that is close to the ladle bottom with more than two plugs increases the average velocity of water, which represents the agitation power, improves the uniformity of water velocity distribution, reduces the stagnant region rate, and shortens the mixing time. The mixing time of dual bottom plugs is almost 1.5 times of that of four side plugs at 116 mm under the same flow rate. The mixing time is not only influenced by the agitation power but also by the uniformity of water velocity distribution. Although the agitation power of four side plugs at 450 mm under the flow rate of 1.8 m3/h is about 1.5 times of that at 116 mm with 0.6 m3/h. The mixing time of the 1.8 m3/h flow rate is about 1.2 times of that of the 0.6 m3/h because of the different water velocity distributions.

Analysis of critical negative rake angle and friction characteristics in orthogonal cutting of AL1060 and T2

The stagnant region often appears in front of the tool cutting edge, which is caused by mechanical inlay and excessive pressing in plastic metal cutting with large negative rake angle tools at a low speed. It results in the change of the effective negative rake angle which can affect the flow characteristics of material, the quality of machined surface and the abrasion loss of cutting tools. However, the critical negative rake angle model based on the existence of the stagnant region has not been reported yet. Therefore, in order to investigate the critical negative rake angle value considering the stagnant region, a critical negative rake angle model based on the principle of minimum required energy is established, and the correctness of the theoretical model is verified by orthogonal cutting experiments. At the same time, the influence of the critical value of the large negative rake angle tool on the machined surface quality is studied through different cutting experiments. These experimental results show that the deviations of both experimental and theoretical critical negative rake angle are less than 5% during the orthogonally cutting of the aluminium (AL1060) and copper (T2) materials by the negative rake angle tool. Meanwhile, the critical negative rake angle is related to the adhesive friction coefficient of tool–workpiece contact surface. The analysis of friction characteristics shows that the deviation values of both theoretical and experimental critical negative rake angle are proportional to the coefficient of adhesive friction and the thickness of the stagnant region. Critical negative rake angle has a significant effect on roughness and residual stress of the machined surface.

The Influence of Heat Source Orientation on PCM Flow Behavior During Phase Transition

The present study reports the characterization of the transient flow behavior in the liquid PCM domain during the melting process, initiated by a planer heat source. Experiments were conducted at three angles of inclination of the heat source (0, 8 and 18-degrees) from the vertical. Paraffin wax was used as the PCM and was enclosed in an insulated, optically clear, thin rectangular chamber. Particle image velocimetry (PIV) was used during the melting process to measure the instantaneous velocity of the PCM and obtain two-dimensional velocity fields within the liquid domain. Results clearly show the presence of a dominant recirculation zone occurring at all angles. This recirculation zone was enhanced by an increase in the inclination of the heat source, supporting a direct correlation between the tilt angle and the bulk fluid flow in the melted region. The melting rate decreased with the progression of time for all investigated cases. A growing region of stagnant fluid flow within the center of the recirculation zone contributed to the reduction in the melt rate. The size of this stagnant region decreased with increases in the tilt angle due to the enhancement of natural convection. The data demonstrates significant changes in transient flow behavior with orientation in the liquid PCM domain. These results further our understanding of the phase change and associated heat transfer processes in PCM and have wide applicability to PCM-based thermal energy storage.

The Influences of CO2 Injection Pressure on CO2 Dispersion and the Mechanism of CO2–CH4 Displacement in Shale

The effects of CO2 injection pressure (PCO2) on CO2 dispersion and the mechanism of CO2–CH4 displacement in a shale sampled from Changning of China were studied. Results indicated that Coats–Smith dispersion–capacitance model gave a reasonable simulated result to the breakthrough curves of CO2 under different injection pressures. The shapes of CO2 breakthrough curves became more asymmetrical with the increase of CO2 injection pressure. A higher CO2 injection pressure caused early CO2 breakthrough and reduced the recovery of CH4 at CO2 breakthrough (Rpipeline-CH4), but improved the ultimate displaced CH4 amount (Rultimate-CH4). With the increase of CO2 injection pressure, dispersion coefficient (Kd) increased nearly exponentially. A larger Kd led to a lower Rpipeline-CH4 and a longer transition zone. With the increase of CO2 injection pressure, the flowing fraction (F) in pore space decreased nearly linearly and more CO2 diffused into stagnant region to replace adsorbed CH4 in a shale, which resulted in a larger Rultimate-CH4. The mass transfer coefficient (Km) between the flowing and stagnant regions increased with the increase of CO2 injection pressure, which led to a smaller F and larger Rultimate-CH4. CO2 diffusion provided major contribution to CO2 dispersion at lower injection pressure, and mechanical mixing of CO2–CH4 offered predominant contribution to CO2 dispersion at higher injection pressure. Larger mechanical mixing accelerated the mixing of CO2–CH4, which was unfavorable for Rpipeline-CH4. Lower CO2 injection pressure was conductive to gain higher Rpipeline-CH4.

Computational fluid dynamics study of human-induced wake and particle dispersion in indoor environment

The impact of human-induced wake flow and particle re-dispersion from floors in an indoor environment was investigated by performing computational fluid dynamics simulations with dynamic mesh of a moving manikin model in a confined room. The manikin motion was achieved by a dynamic layering mesh method to update new grids with each time step. Particle transport from the floors and its re-dispersion was tracked by a Lagrangian approach. A series of numerical simulations of three walking speeds were performed to compare the flow disturbance induced by the walking motion. The significant airflow patterns included: an upward-directed flow in front of the body combined with a high velocity downward-directed flow at the rear of the body; a stagnant region behind the gap between the legs and counter-rotating vortices in the wake region. The airflow momentum induced by the moving body disturbed PM2.5 particles that were initially at rest on the floor to lift and become re-suspended due to its interaction with the trailing wake. The residual flow disturbances after the manikin stopped moving continued to induce the particle to spread and deposit over time. The spatial and temporal characteristics of the particle dispersion and concentration showed that higher walking speed was conducive to reducing human's exposure to contaminants in breathing region.

Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part II: Control of the Vortex Rope

Numerical simulations and investigation of a method for controlling the vortex rope formation in draft tubes are carried out in this paper, which is the second part of a two-paper series. As shown in the companion paper, formation of the vortex rope is associated with a large stagnant region at the center of the draft tube. Therefore, it is concluded that a successful control technique should focus on the elimination of this region. In practice, this can be performed by axially injecting a small fraction (a few percent of the total flow rate) of water into the draft tube. Water jet is supplied from the high-pressure flow upstream of the turbine spiral case by a bypass line; thus, no extra pump is needed in this method. It is shown that this method is very effective in elimination of the stagnant region in a simplified draft tube operating at two part-load conditions, i.e., at 91% and 70% of the best efficiency point (BEP) flow rate. This results in improvement of the draft tube performance and reduction of hydraulic losses. The loss coefficient is reduced by as much as 50% for the case with 91% of BEP flow rate and 14% for the case with 70% of BEP flow rate. Unsteady, three-dimensional simulations show that the jet increases the axial momentum of flow at the center of the draft tube and decreases the wake of the crown cone and thereby decreases the shear at the interface of the stagnant region and high velocity outer flow, which ultimately results in elimination of the vortex rope. Furthermore, reduction (by about 1/3 in the case with 70% of BEP flow rate) of strong pressure fluctuations leads to reliable operation of the turbine.

Experimental Research and DEM Simulations on Stagnant Region in Pebble Bed Reactor

In pebble bed reactor, pebbles flow very slowly in the stagnant region, which is defined according to the burn-up level of fuel pebbles. It is not allowed to exist in real reactor, since the stay time of fuel pebbles in these regions goes beyond the burn-up level, which increases the risk of leakage of radiation. This research shows that the stagnant region is related to the geometric parameters of the core and the physical properties of pebbles. Experimental setup has been designed to observe the phenomenon of stagnant region, and analysis based on a phenomenological method has been carried out. The phenomenological method is an approach to study the dense pebble flow by means of investigating the interface features of different areas composed of differently colored pebbles. In addition, additional simulations by the DEM model are in good agreement with the experimental results, which successfully verify the availability of the discrete element method. On the basis of these researches, several key parameters have been investigated through DEM simulations, including height of the experimental setup, friction coefficient between pebbles and base cone angle. It is proved that, the stagnant region existing in the pebble bed can be eliminated by improving the design of pebble bed and the physical properties of fuel pebbles. All of these are very helpful to guide the design of pebble-bed reactor.

Visualization of the Ludford column

When a liquid metal flows around a truncated cylinder in the presence of a magnetic field which is parallel to the axis of the cylinder, a stagnant region develops above the cylinder. We call this region a Ludford column. The Ludford column represents the magnetohydrodynamics (MHD) analogue of the well-known Taylor columns in rotating flows. Whereas Taylor columns can be easily visualized using dye, the visualization of Ludford columns has remained elusive up to now because liquid metals are opaque. We demonstrate that this fundamental limitation of experimental MHD can be overcome by using a superconducting 5 T magnet. This facility permits us to perform MHD experiments in which the opaque liquid metals are replaced with a transparent electrolyte while maintaining the key MHD effects. We report results of a series of flow experiments in which an aqueous solution of sulphuric acid flows around a bar with square cross-section (which for simplicity shall be referred to as a cylinder). We vary the Reynolds number in the range $5\lt Re\lt 100$ and the Hartmann number in the range $0\lt Ha\lt 14$. The experimental procedure involves flow visualizations using tracer particles as well as velocity measurements using particle image velocimetry (PIV). Our experiments provide direct access to the Ludford column for the first time and reveal the spatial structure of this basic feature of MHD flows.