Gas–solid flows are widely found in various industrial processes, e.g. chemical engineering and sand ingestion test for aero-engine; the interaction between continuum and discrete particles in such systems always leads to complex phase structures of which fundamental understandings are needed. Within the OpenFOAM, the present work uses the discrete element method combined with the computational fluid dynamics to investigate the gas–solid flow behaviors in a dense fluidized bed under various conditions. A drag law which is for polydisperse systems derived from lattice Boltzmann simulations is incorporated into the computational fluid dynamics-discrete element method framework and its suitability for different flow regimes is investigated. The regimes including, namely slugging bed, jet-in-fluidized bed, spout fluidization, and intermediate, are simulated and validated against experiments. The results show that the lattice Boltzmann drag relation performs well in capturing characteristics of different gas–solid flow regimes. Good agreements are also obtained quantitatively by comparisons of pressure drop fluctuation, and time-averaged gas velocity and particle flux.
When airflow fly over trees, a shelter area will be generated at downstream area, and wind speed in the region will be reduced correspondingly. In order to analyze the windbreak performance of tree canopy, porous media method and additional source term are explored to simulate by computational fluid dynamics. Based on the given method, windbreak performance of trees with different canopy porosity, canopy shapes and distances between trees is investigated in detail, and characteristics of wind flow on the downstream of tree are shown. Results showed that small canopy porosity does not always mean good performance of windbreak, especially for the lower part. Canopy shapes only affect wind speed in the wake range. In these regions, windbreak performance of tree depends on the thickness of canopy. When distance is beyond 3H (H is the height of tree), wind speed becomes uniform at the downstream of tree and spacing can be equivalent to the porosity. But, for the region close to tree, trees with spacing cannot provide effective windshield. These simulations of tree canopy windbreak performance are conducted. All the results of the new model show better canopy flow characteristics through the field.
The particle dynamics in an oscillating alveolus under tidal breathing can be dramatically different from those in a static alveolus. Despite its close relevance to pulmonary drug delivery and health risk from airborne exposure, quantifications of alveolar deposition are scarce due to its inaccessibility to in vivo measurement instruments, tiny size to replicate in vitro, and dynamic wall motions to model. The objective of this study is to introduce a numerical method to quantify alveolar deposition with continuous particle release in a rhythmically oscillating alveolus by integrating the deposition curves for bolus aerosols and use this method to develop correlations applicable in assessing alveolar drug delivery efficiency or dosimetry of inhaled toxicants. An idealized blind-end terminal alveolus model was developed with rhythmically moving alveolar boundary conditions in phase with tidal breathing. The dynamic wall expansion mode and magnitude were based on experimentally measured chest wall motions and tidal volumes. A well-validated Lagrangian tracking model was used to simulate the transport and deposition of inhaled micrometer particles. Large differences were observed between dynamic and static alveoli in particle motion, deposition onset, and final alveolar deposition fraction. Alveolar deposition of bolus aerosols is highly sensitive to breath-holding duration, particle release time, and alveolar dimension. For 1 µm particles, there exists a cut-off release time (zero bolus deposition), which decreases with alveolar size (i.e., 1.0 s in a 0.2-mm-diameter alveolus and 0.56 s in a 0.8-mm-diameter alveolus). The cumulative alveolar deposition was predicted to be 39% for a 0.2-mm-diameter alveolus, 22% for a 0.4-mm-diameter alveolus, and 10% for a 0.8-mm-diameter alveolus. A cumulative alveolar deposition correlation was developed for inhalation delivery with a prescribed period of drug release and the second correlation for the time variation of alveolar deposition of ambient aerosols, both of which captured the relative dependence of the particle release time and alveolar dimension.
A modified Fast Fourier Transform method based on the pulsation ratio preprocessing is carried out in this study. When the density wave instability occurs, the method is applied to capture the characteristic signals in the frequency domain. Thus, the stable boundary in two-phase flow can be recognized accurately. In this paper, experiments are conducted in a system based on a narrow annular channel. The method is verified through two groups of experimental data collected in different conditions. The results indicate that the modified method can avoid the problem of DC component spectrum leakage in traditional frequency-domain analysis with the false value interference eliminated. Accordingly, it can improve the accuracy of boundary identification effectively when the instability occurs.
Due to its advantages of low critical pressure and temperature, stability, non-toxic, abundant reserves and low cost, supercritical CO2 becomes one of the most common supercritical fluids in modern researches and industries. This paper presents an overview focusing on the researches of supercritical CO2 in nuclear engineering and prospects its applications in the field of nuclear industry. This review includes the recent progresses of supercritical CO2 research as: (1) energy conversion material in both recompression cycle and Brayton cycle and its applicability in Generation IV reactors; (2) reactor core coolant in the Echogen power system and reactors at MIT, Kaist and Japan, and other applications, e.g. hydrogen production. Based on the rapid progress of research, the supercritical CO2 is considered to be the most promising material in nuclear industries.
The moving particle semi-implicit method is a meshless particle method for incompressible fluid and has proven useful in a wide variety of engineering applications of free-surface flows. Despite its wide applicability, the moving particle semi-implicit method has the defects of spurious unphysical pressure oscillation. Three various divergence approximation formulas, including basic divergence approximation formula, difference divergence approximation formula, and symmetric divergence approximation formula are proposed in this paper. The proposed three divergence approximation formulas are then applied for discretization of source term in pressure Poisson equation. Two numerical tests, including hydrostatic pressure problem and dam-breaking problem, are carried out to assess the performance of different formulas in enhancing and stabilizing the pressure calculation. The results demonstrate that the pressure calculated by basic divergence approximation formula and difference divergence approximation formula fluctuates severely. However, application of symmetric divergence approximation formula can result in a more accurate and stabilized pressure.
Due to its advantages of low critical pressure and temperature, stability, non-toxic, abundant reserves and low cost, supercritical CO2 becomes one of the most common supercritical fluids in modern researches and industries. This paper presents an overview focusing on the researches of supercritical CO2 in nuclear engineering and prospects its applications in the field of nuclear industry. This review includes the recent progresses of supercritical CO2 research as: (1) energy conversion material in both recompression cycle and Brayton cycle and its applicability in Generation IV reactors; (2) reactor core coolant in the Echogen power system and reactors at MIT, Kaist and Japan, and other applications, e.g. hydrogen production. Based on the rapid progress of research, the supercritical CO2 is considered to be the most promising material in nuclear industries.
Fluid equations in Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) simulations solve the volume-averaged Navier–Stokes equations. Full coupling between the dispersed phase and continuous phase is made by the exchange of source terms as well as the void fraction. The void fraction is calculated from the presence of the particles in the computational fluid cells while the source terms are calculated from the point mass force models of the fluid–particle interaction forces. Dense particulate system with large spatiotemporal variations in the void fraction shows hard convergence behavior. This can impact the robustness of the solver during the time integration process. One option is to use partial coupling by neglecting the explicit effect of void fraction in the fluid momentum equations while retaining its effect on force models. Although the partial coupling is more stable and shows better convergence behavior, the mobility of the particles is found to be reduced as compared to the full-coupling approach. In the current work, we propose a revised partial coupling in which a modified fluid velocity is used in point mass force models to compensate for the omission of the void fraction in the fluid governing equations. The effectiveness of this method is demonstrated in a fluidized bed and in sediment transport simulations. In both cases it is shown that the use of the proposed method gives very good comparisons with the fully coupled simulations while reducing the fluid calculation time by factors ranging from 1.35 to 4.35 depending on the flow conditions. The revised partial coupling is not recommended as a substitute for full coupling in dense systems but as an alternate approach when full coupling leads to numerical difficulties.
This paper reviews the use of non-intrusive optical infrared sensing for gas–liquid flow characterisation in pipes. The application of signal analysis techniques to infrared-derived temporal signal outputs enables the objective determination of flow characteristics such as flow regimes, phase fractions and total pressure drops. Key considerations for improving the performance of infrared sensors are discussed. These include global and local measurements, ray divergence, effects of ambient light and temperature variations. Most experimental studies have reported consistent and excellent results for flow regime identifications and phase fraction estimation but with a few validating total pressures drop from correlations and direct pressure measurements. Other gaps in research were highlighted; these include the use of pipes sizes greater than 0.005 m for experimentation under high superficial velocities conditions greater than 10 m/s. The capabilities of infrared sensing as a standalone measurement for flow metering were considered a possibility via an inferential approach for phase volumetric rates. More so, the derived infrared sensing flow characteristics could be combined with available pressure–volume–temperature correlations in estimating mass flow rates of each phase. As a future development, a conceptual modification to surface installations using a transparent opaque coupling is suggested to overcome the accessibility limitation of infrared light penetration for opaque pipes.
In this paper, the mechanistic wall heat partitioning approach was used to capture the complex heat and mass transfer in sub-cooled boiling flows. In order to accommodate the changes of local variables to be relevant to the physical properties of sub-cooled fluids, the Wet-Steam (IAPWS-IF97) is used as the working fluid. Currently, the approach is evaluated based on the bubble sliding along the wall before lifting-off, which is usually found in the flow boiling situations. In the simulation, the closure mechanistic models, including the fractal analysis, the force balance and the mechanistic frequency, were coupled with the Eulerian–Eulerian two-fluid framework, while the Shear Stress Transport model was used as a turbulent modelling closure. The Multiple Size Group model was introduced to handle the bubble interactions and predict the bubble size distribution. Moreover, the effect of adopting the sub-cooled liquid properties into the modelling was investigated and compared with the experiments over a wide range of flow conditions. Specifically, the predicted void fraction and the sub-cooling temperature near the heated wall were precisely compared with the cases of using the constant-property liquid. Overall, the satisfactory agreements were found between the experiments and the predictions of the liquid temperature, void fraction, interfacial area concentration, Sauter mean diameter and bubble and liquid velocities with the exception of the case of high heat and mass fluxes. To enhance the current prediction accuracy for a situation of having a high superheating temperature, more bubble interactions on the boiling wall, such as merging of the bubbles while sliding, need to be considered. Furthermore, to assess the model capability, this mechanistic approach will be introduced to elucidate the sub-cooled boiling flow in situations of using different fluids in the near future.
The one-dimensional shallow water equations were modified for a Venturi contraction and expansion in a rectangular open channel to achieve more accurate results than with the conventional one-dimensional shallow water equations. The wall-reflection pressure–force coming from the contraction and the expansion walls was added as a new term into the conventional shallow water equations. In the contraction region, the wall-reflection pressure–force acts opposite to the flow direction; in the expansion region, it acts with the flow direction. The total variation diminishing scheme and the explicit Runge–Kutta fourth-order method were used for solving the modified shallow water equations. The wall-reflection pressure–force effect was counted in the pure advection term, and it was considered for the calculations in each discretized cell face. The conventional shallow water equations produced an artificial flux due to the bottom width variation in the contraction and expansion regions. The modified shallow water equations can be used for both prismatic and nonprismatic channels. When applied to a prismatic channel, the equations become the conventional shallow water equations. The other advantage of the modified shallow water equations is their simplicity. The simulated results were validated with experimental results and three-dimensional computational fluid dynamics result. The modified shallow water equations well matched the experimental results in both unsteady and steady state.
The transient performances of impeller machinery under typical mutational working conditions play an important role in the area of engineering and have attracted extensive attention. In order to fully understand transient behaviors of impeller machinery under mutational working conditions, it is very necessary to confirm an efficient and reliable solving method. In this paper, the dynamic mesh method and the sliding mesh method are respectively used to be solved the inner transient flow of a 2D centrifugal pump model during abrupt start-up period. The Renormalization-group (RNG) k– ε turbulence model in eddy viscosity models is chosen to close the Reynolds-average equation and to complete the turbulence calculation. Moreover, the SIMPLE algorithm is also adapted to accomplish the coupling calculation between velocity and pressure. The study shows that in theory, the most appropriate method to solve transient flow is the dynamic mesh method, but it has the shortage of low mesh quality during mesh reconstruction. The sliding mesh method with high mesh quality is an ideal solving method under mutational working conditions. In general, both of the methods would reflect the basic characteristics of inner flow, while the sliding mesh method has higher solution precision.
Aerosol transport and deposition in human lungs has attracted considerable attention in the past few years, as it has significant value to the study of toxicity consequence as well as therapeutic potential in occupational health and medical applications. In reproducing human tracheobronchial airways, two approaches were frequently taken: (1) anatomical realistic reconstruction through image scans (e.g. CT and MRI) or cadaver casts and (2) mathematical description using simplified models. Strengths and limitations are primarily focused on accuracy, resolution, repeatability, and computationalphysical expenses. While both approaches were reported in literature, detailed comparison of aerosol transport and deposition in the two representations were scarcely performed, largely due to the challenge to acquire comprehensive data from the irregular structured airway replicas (approach 1). To fill the gap, the current study performed a numerical comparison of nanoparticle transport and deposition in human upper ...
Bubbly flow in U-bend is widely encountered in two-phase flow systems because of its compactness and high heat transfer coefficient. The modeling of phase distributions, velocity fields, and interfacial area concentration in the U-bend is crucial for the analysis of mass, momentum, and energy transportation processes in the equipment. However, this subject has not received enough attention yet. In this paper, the combination of population balance model and two-fluid model was used in the simulation of air–water bubbly flow in a U-bend with 24 mm inner diameter and 96 mm curvature. The homogeneous multiple size group model was used to solve the population balance equation and reconstruct the bubble size distribution function. The phase distribution at 0°, 90°, and 180° was predicted and the results showed that the superficial velocities of gas and liquid phase were the control parameters. Under higher gas superficial velocity, the buoyant force is dominant and makes the bubbles concentrate on the outer side of the tube wall; while under lower gas superficial velocity, the centrifugal force is dominant and makes the bubbles concentrate on the inner side of the tube wall. These results met well with the experimental results of Usui.
Multiple interactions may occur when a poly-disperse spray is immersed in an acoustic field. In the context of spray combustion instabilities, acoustic agglomeration, the formation of a droplet number density wave and the modulation of the droplet size distribution are important effects. A droplet number density wave, i.e. preferential concentration of droplets in space, may result from sizedependent, one-way momentum coupling between the acoustic field and the spray. The modulation of the droplet size distribution, which has been evidenced in the experimental work of Gurubaran and Sujith (AIAA 2008-1046), is then a consequence of the droplet number density wave formation. In the present work the mechanisms that produce these effects, including acoustic agglomeration, are simulated and analyzed in depth by means of CFD. The spray is modeled with both Lagrangian (particles mass-point approach) and Eulerian (continuous phase approach) descriptions. The particular Eulerian method used is the presumed density function Method of Moments, which allows to account for the effects of poly-dispersity. Both the Lagrangian and Eulerian models are validated against experimental data for spray dynamics and spray response to an acoustic field.
Multi-phase flows, particularly two-phase flows, are widely used in the industries, hence in order to predict flow regime, pressure drop, heat transfer, and phase change, two-phase flows should be studied more precisely. In the petroleum industry, separation of phases such as water from petroleum is done using rotational flow and vortices; thus, the evolution of the vortex in two-phase flow should be considered. One method of separation requires the flow to enter a long tube in a free vortex. Investigating this requires sufficient knowledge of free vortex flow in a tube. The present study examined the evolution of tube-constrained two-phase free vortex using computational fluid dynamics. The discretized equations were solved using the SIMPLE method. It was determined that as the liquid flows down the length of the pipe, the free vortex evolves into combined forced and free vortices. The tangential velocity of the free and forced vortices also decreases in response to viscosity. It is shown that the concentration of the second discrete phase (oil) is greatest at the center of the pipe in the core of the vortex. This concentration is at a maximum at the outlet of the pipe.
In this paper, two-dimensional numerical simulations of the head-on and off-center binary collision of liquid drops are carried out using lattice Boltzmann method. The coalescence process of drops colliding is freely captured, regardless of tracking equations and drops model. Special attention is paid to investigate the effect of the Weber number, impact velocity, drop size ratio on the coalescence process. The research results demonstrate that numerical results of lattice Boltzmann method are in agreement with qualitatively experimental data in the same Weber number. It is also noted that the first oscillation period is invariably shorter than the second period, regardless of the Weber number. It is further found that variation of the first period of oscillation in off-center collision of two equal-size drops as function of Weber number are higher than the variation of the first period of oscillation in head-on collision of two equal-size drops in the same Weber number.
In the high-temperature gas-cooled reactor pebble-bed module, the helium bypass flow among graphite blocks cannot be ignored due to its effect on the temperature distribution as well as the maximum temperature in the reactor core. Bypass flow was previously analyzed in the discharging tube, in vertical gaps between graphite reflectors, and in control rod channels. The focus of this study is on the bypass flow that connects the small absorber sphere channels. Different from bypass flow connecting the control rod channels, there was no evident inlet or outlet flow paths into or out of the small absorber sphere channels at the top or bottom of the reactor core. Therefore, the bypass flow connecting the pebble bed with the small absorber sphere channels was mainly caused by the horizontal gaps, in which those gaps would also be irregular due to installation, thermal expansion, or irradiation of the graphite reflectors. After clarifying the resistant coefficients of those gaps by computational fluid dynamic to...
High-temperature gas-cooled reactor is a kind of advanced nuclear reactor in which the core is packed with spherical fuel elements. In high-temperature gas-cooled reactors, the operating temperature is higher than that in ordinary light water reactors. In an attempt to analyze the flow pattern and heat transfer situation to provide reference for the safe operation of the pebble bed reactors, a segment of simplified high-temperature gas-cooled reactor core is simulated with computational fluid dynamics method. Four kinds of arrangement, including simple cubic, body-centered cubic, face-centered cubic, and a combination structure of body-centered cubic and face-centered cubic, are studied, respectively. Based on the simulation results, higher heat transfer capability and lower pebble temperature are obtained in the case with the most compact arrangement. The drag coefficient ( Cd) for four arrangements with different inlet Reynolds number (Re) is obtained and relationship between Re and Cd is analyzed. In addition, a simulation with a broken fuel element in the body-centered cubic fluid domain has been performed. The results show that the presence of broken fuel may result in uneven flow, which will change the heat transfer condition. So it is better to avoid broken fuel element in a high-temperature gas-cooled reactor.
