CFD investigation of sub-cooled boiling flow using a mechanistic wall heat partitioning approach with Wet-Steam properties

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.

Population balance modeling for air–water bubbly flow in a vertical U-bend

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.

Study on divergence approximation formula for pressure calculation in particle method

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.

Study on windbreak performance of tree canopy by numerical simulation method

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.

Revised partial coupling in fluid–particulate systems

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.

A solution method for one-dimensional shallow water equations using flux limiter centered scheme for open Venturi channels

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.

Deposition of bolus and continuously inhaled aerosols in rhythmically moving terminal alveoli

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.

The application of supercritical CO2 in nuclear engineering: A review

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 application of pulsation ratio preprocessing method in researching density wave instability

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.

Simulation of different gas–solid flow regimes using a drag law derived from lattice Boltzmann simulations

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.