Two-phase Numerical Simulation Research Articles

Single particle conversion of woody biomass using fully-resolved and Euler–Lagrange coarse-graining approaches

The conversion of woody biomass is studied by means of a layer-based model for thermally-thick biomass particles (Thunman et al. 2002, Ström et al. 2013). The model implementation is successfully validated against experiments that study particle conversion in a drop tube reactor. After this validation step, this work focuses on the well-known problem of grid dependence of two-phase numerical simulations using the standard Euler–Lagrange (EL) framework. This issue is addressed and quantified by comparing EL data that models the particle boundary layers to corresponding simulations which fully resolve these boundary layers (fully-resolved, FR, simulations). A comparison methodology for the conceptually different FR and EL approaches by extracting the heat transfer coefficient from the detailed FR simulations is proposed and confirms that the EL results are strongly grid-dependent. This issue is overcome by applying a set of coarse-graining methods for the EL framework. Two coarse-graining methods are evaluated, a previously suggested diffusion-based method (DBM) and a new approach based on moving averages referred to as MAM. It is shown that both DBM and MAM can successfully recover the detailed FR data for pure particle heating for a case where the grid size is half the particle diameter, i.e. when the standard EL method fails. Both coarse-graining methods also give improved results for an EL simulation that considers the more complex combined physics of particle heating, drying and devolatilisation, given that the CG model parameters that scale the corresponding CG interaction volumes are sufficiently large. Based on the available FR data, recommended model parameter ranges for DBM and MAM are provided as a function of normalised boundary layer thickness. The novel MAM approach is shown to be significantly more efficient than the DBM and therefore suitable for future EL simulations with multiple particles.

Two-Phase Numerical Simulation for the Heat and Mass Transfer Evaluation Across a Vertical Deformable Sheet with Significant Impact of Solar Radiation and Heat Source/Sink

Two-Phase Numerical Simulation for the Heat and Mass Transfer Evaluation Across a Vertical Deformable Sheet with Significant Impact of Solar Radiation and Heat Source/Sink

Using artificial intelligence and two-phase numerical simulation method for investigating the effect of the shape of pin fins on the entropy production of nanofluid flow in a heatsink at different flow rates

Numerical simulations of a heatsink (HTK) are performed in this article for cooling a heat source with multiple pin-fins (PFS). The HTK is placed in a channel with alumina/water nanofluid flow. The two-phase nanofluid is simulated using the finite element method (FEM). Artificial intelligence is also utilized for data analysis. The results of this study demonstrate that the enhancement of Re from 200 to 600 reduces the maximum temperature (T-mx) in the square and rhombus PFS by 6.8% and 7%, respectively. The increase of Re from 200 to 600 decreases the average temperature (T-av) of the HTK by 6.8% and 6.9% for square and rhombic PFS, respectively. An increment in Re diminishes the outlet temperature (T-ot) for both pin-fin shapes. As Re is enhanced, the amount of Δp is intensified. The use of square PFS results in more values of Δp in the HTK compared to the rhombus ones. A maximum enhancement in Δp, i.e. 17.9%, corresponds to Re = 600. The square shape of PFS generates more thermal, frictional, and total entropy than the rhombic shape of PFS. The increase of Re reduces the thermal ENG and enhances the frictional and total ENG for both shapes of PFS.

An Approach to Increase the Fusion Tritium Breeding Ratio Using Temperature Field Balance Based on Water-Cooled Ceramic Blanket

An advanced breeding blanket for excellent tritium breeding performance shall be designed to realize the operations of Chinese Fusion Engineering Testing Reactor (CFETR) at steady-state burning tritium fuel in the core plasma for fusion reactions. Tritium release from blanket interior effectively requires a temperature range above than 400 °C. This indicates that the blanket temperature field distribution and its evolution are related to the online tritium breeding ratio (TBR) assessment. In this article, water-cooled ceramic blanket (WCCB) models are built to perform the TBR estimation and the temperature field simulations. It is confirmed that the temperature field affects the TBR variation, which is linked to the blanket cooling pipes (CPs) layout. In addition, the effective volume fraction of tritium release of the blanket model is defined to evaluate the breeding regional proportion whose temperature is higher than 400 °C. The effective volume fraction of tritium release is analyzed for the inlet coolant temperature ranging from 0.6 to 5.0 m/s by an Eulerian-Eulerian two-phase numerical simulation method. The outer surface temperature of CPs can be raised toward 400 °C when inlet velocity is 1.1 m/s. This is a vital approach to reduce the deep temperature gradient in blanket interior, which is beneficial to the tritium release sufficiently. The estimation of critical heat flux (CHF) of first wall (FW) indicates that boiling crisis will not take place in FW. This work is helpful to the engineering design of fusion blanket considering the tritium self-sufficiency in the road toward fusion reactor.

Progress and challenges in exploration of powder fueled ramjets

• A detailed review of the research progress on powder fueled ramjets is presented. • The key technologies and scientific issues in powder fueled ramjets are proposed. • Some suggestions are proposed for the development of powder fueled ramjets. The powder fueled ramjet is a developing propulsion system, which makes use of metal powder as the propellant and presents noticeable advantages, for instance, high specific impulse and density-specific impulse, thrust adjustment, simple structure, and suitability for high Mach flight vehicles. In this paper, the basic work characteristics are introduced and the exploration progress is reviewed and summarized for powder fueled ramjets. Furthermore, several key technologies are proposed, which mainly include the powder feeding system, ignition and combustion processes of the powder fuel, combustion organization technology, multiple start, thrust adjustment, and thermal protection technology. The development of the powder fueled ramjet is in the proof-of-concept stage, and a large number of individual technology breakthroughs have been achieved in subsonic conditions. However, in both subsonic and supersonic conditions, the combustion mechanism of powder fuel needs to be investigated by high precision gas–solid two-phase numerical simulation methods, and ground tests, thus, to guide combustion chamber design. In order to achieve thrust regulation, coupling studies on powder fuel delivery and combustion are required. Besides, new thermal protection schemes based on the lattice structural and supercritical fluid active cooling have great potential to meet the demands of complicated two-phase combustion thermal environments.

Simulation of a turbulent annulus with interfacial waves in core-annular pipe flow

Interfacial waves in core-annular pipe flow are studied through two-phase numerical simulations. Here the water annulus is turbulent, whereas the oil core stays laminar. The low-Reynolds number Launder & Sharma k−ε model is applied. By extracting the moving wave shape from the two-phase results and imposing this as a solid boundary in a single-phase simulation for the water annulus gives single-phase results (for the pressure drop and holdup ratio) that are in close agreement with values obtained from the two-phase approach. The influence of wave amplitude and wave length on the pressure drop and hold up ratio is then studied by using the single-phase flow model. This gives insight in the appearance of core-annular flow, where the water-based Fanning wall-friction factor and the hold-up ratio are selected as the most important quantities. The effect of watercut and eccentricity on these quantities is also investigated.

The Influence of Pore Structure of the Core-Scale Fracture-Controlled Matrix Unit on Imbibition: Model Construction and Definition of the Fractal Coefficient

Abstract Imbibition oil recovery plays an important role in the development of low-permeability fractured reservoirs. However, the effects of the complex pore structure of the matrix on imbibition have not been considered comprehensively at different scales in the scientific literature. This paper reports a study of the mechanisms of influence of different matrix pores on imbibition recovery ratio and defines the concept of a fractal coefficient using the method of combining numerical simulation at the core scale with mathematical methods at a pore scale. A matrix model with different pore fractal dimensions and tortuosity of the capillary was established using Python language programming. Y-type and S-type were used to characterize the fractures, and a two-dimensional fracture-controlled matrix unit core-scale numerical model with complex pore structures was established. Based on phase field theory, an oil-water two-phase imbibition numerical simulation was conducted. Comparisons of numerical simulation results and microscopic analysis indicated that imbibition recovery ratio was 39.28% and 50.94%, respectively. The imbibition law revealed by the numerical simulation results is consistent with the results of the microscopic imbibition experiment, and the imbibition effect of bifurcated fractures was better than that of simple curved fractures. As the pore fractal dimension and tortuosity of the capillary increased, the imbibition recovery ratio decreased. A comparison of results of the mathematical model demonstrated that there was a difference between the pore scale and core scale because the pore fractal dimension and capillary tortuosity changed dynamically with pore structure at the core scale. Thus, a parameter that characterizes the relative change of pore fractal dimension and tortuosity was defined and called the fractal coefficient. When the skeleton particles are 200, 400, and 500, the fractal coefficient were calculated to be 0.625 and 0.6, respectively, indicating that when the pore structure changes, the imbibition recovery ratio should be dominated by capillary tortuosity.

Performance Comparison of a Microchannel Heat Sink Using Different Nano-Liquid Metal Fluid Coolant: A Numerical Study

Abstract This article presents performance comparison between different liquid metal-based nanofluids termed as nano-liquid metal fluids in a microchannel heat sink to achieve ultimate cooling solutions without sacrificing the compact structure and heavy computing speed. The hydraulic and thermal performance of nanofluids having five different liquid metals (Ga, GaIn, EGaIn, GaSn, and EGaInSn) as base fluid and four different nanoparticles (carbon nanotube (CNT), Al2O3, Cu, and diamond) as solute are evaluated comparing with water-based nanofluids. Three-dimensional flow inside miniaturized channels are predicted using single-phase and two-phase numerical simulations. Numerical models are validated against data obtained from experimental studies from the literature. Three different grids are developed, and several element sizes were compared to obtain the grid independence. Upon evaluation, the study can point out that liquid metal-based nanofluids can generate much superior heat transport characteristics with more than 3.41 times higher heat transfer coefficient compared to conventional water-based nanofluids. GaIn–CNT combination exhibits the best thermal solution possible with a heat transfer coefficient increment of 2.68%, 17.19%, 22.16%, and 2.62% over CNT particle-based EGaIn, EGaInSn, Ga, GaSn liquid metal, respectively, for Re = 750. Considering hydraulic performance, performance evaluation criterion (PEC) has been introduced and Ga-based nanofluids are found to be most effective in this perspective. The effect on overall cooling effectiveness has also been carried out with a detailed particle concentration study. This study paves the pathway of using these extraordinary coolants in mini/microchannel heat sinks.

Effect of cathode-side gas flow channels section geometry on the removal process of liquid slug in a proton exchange membrane fuel cell

In low operating temperatures, provided water of Proton-Exchange Membrane Fuel Cells (PEMFCs) is discharged in the state of tiny/large droplets, slugs, and semi-slugs, which strongly depends on the flow channels' characteristics. In this study, the discharge capabilities of channels with different height-to-width aspect ratios are inspected using a transient two-phase numerical simulation. The numerical model includes a segment of the gas channel on the cathode side, and the operating conditions are those related to an actual fuel cell application. Results showed that channels with minimum height could lift the initial slug to the upper part 2.16 times faster and immediately form film flow at the corners. For a fixed width, decreasing channel height from 1.5 mm to 0.5 mm, results in 38.3% faster discharge time and a 62.3% increment in the clearance rate of the gas-diffusion layer. For higher channels, the combined effect of shear stress, gravity, and the adhesive forces of hydrophilic walls cannot lift the initial slug continuously and causes it to rupture, distort, and partially spread over the Gas-diffusion layer (GDL) surface, which leads to mass transport limitation for the cell. Furthermore, when the channel width decreases from 1.5 mm to 0.5 mm for a fixed height, the initial slug leaves the computational domain 51.1% faster. Overall, it was found that the smaller dimensions for height and width show superior cell performance regarding faster water removal, clearer GDL surface, and more uniform flow distribution over the entire electrochemical active area. A channel with a cross-section dimension of 0.5 mm × 0.5 mm is suggested as the best channel design among the analyzed cases for maximum efficiency for the cathode-side flow fields of a PEMFC.

Research on the characteristics of fluid flow and heat transfer of Al2O3-water nanofluid in an L-shaped chaotic channel

PurposeThis paper aims to investigate the effects of different volume fractions of Al2O3-water nanofluid on flow and heat transfer under chaotic convection conditions in an L-shaped channel, comparing the difference of numerical simulation results between single-phase and Eulerian–Lagrangian models.Design/methodology/approachThe correctness and accuracy of the two calculation models were verified by comparing with the experimental values in literature. An experimental model of the L-shaped channel was processed, and the laser Doppler velocimeter was used to measure the velocities of special positions in the channel. The simulated values were compared with the experimental results, and the correctness and accuracy of the simulation method were verified.FindingsThe calculated results using the two models are basically consistent. Under the condition of Reynolds number is 500, when the volume fractions of nanofluid range from 1% to 4%, the heat transfer coefficients simulated by single-phase model are 1.49%–25.80% higher than that of pure water, and simulated by Eulerian–Lagrangian model are 3.19%–27.48% higher than that of pure water. Meanwhile, the friction coefficients are barely affected. Besides, there are obvious secondary flow caused by lateral oscillations on the cross sections, and the appearance of secondary flow makes the temperature distributions uniform on the cross section and takes more heat away, thus the heat transfer performance is enhanced.Originality/valueThe originality of this work is to reveal the differences between single-phase and two-phase numerical simulations under different flow states. The combination of chaotic convection and nanofluid indicates the direction for further improving the heat transfer threshold.

Modal decomposition analysis of unsteady viscous liquid sheet flows

The unsteady dynamics of a gravitational liquid sheet, driven by a continuous harmonic perturbation in the lateral velocity component applied at the inlet section, is analyzed. The topology and the dynamics of the relevant flow structures are characterized by applying POD (Proper Orthogonal Decomposition) and spectral POD (SPOD) modal decompositions on two-dimensional two-phase numerical simulation data obtained with the volume-of-fluid approach. The investigation is carried out by varying the Weber number, the forcing frequency (Strouhal number), and the Reynolds number. The supercritical regime (We > 1) features a traveling perturbation, exhibiting a spatial structure with leading sinuous modes. SPOD spectra confirm the occurrence of a discontinuity in frequency response between the supercritical and subcritical regimes. In the subcritical regime (We < 1), the investigation highlights the excitation of a combined sinuous–varicose motion when the system is driven at resonance frequency for a relatively high Reynolds number (approaching the inviscid limit). The emergence of varicose modes is favored by low Weber numbers. The excitation of these modes occurs when the Weber number is decreased from We = 0.90 down to 0.75, with a progressive shift of the varicose mode from higher harmonics toward the main frequency; it can be considered as a possible mechanism of breakup observed in experiments when the inlet flow rate is progressively reduced. The flow reconstruction based on both POD and SPOD confirms the good capability of SPOD modes to capture dynamically relevant features of the fluid motion in subcritical conditions.

A New Measurement of Anisotropic Relative Permeability and Its Application in Numerical Simulation

In this paper, we used a self-developed anisotropic cubic core holder to test anisotropic relative permeability by the unsteady-states method, and introduced the anisotropic relative permeability to the traditional numerical simulator. The oil–water two-phase governing equation considering the anisotropic relative permeability is established, and the difference discretization is carried out. We formed a new oil–water two-phase numerical simulation method. It is clear that in a heterogeneous rock with millimeter to centimeter scale laminae, relative permeability is an anisotropic tensor. When the displacement direction is parallel to the bedding, the residual oil saturation is high and the displacement efficiency is low. The greater the angle between the displacement direction and the bedding strike, the lower the residual oil saturation is, the higher the displacement efficiency is, and the relative permeability curve tends towards a rightward shift. The new simulator showed that the anisotropic relative permeability not only affects the breakthrough time and sweep range of water flooding, but also has a significant influence on the overall water cut. The new simulator is validated with the actual oilfield model. It could describe the law of oil–water seepage in an anisotropic reservoir, depict the law of remaining oil distribution of a typical fluvial reservoir, and provide technical support for reasonable injection-production directions.

Interactions of wakes and shock waves with two-phase air/mist cooling in a transonic gas turbine stage

A 2-D two-phase numerical simulation was conducted to investigate the effect of interactions of moving wakes and shock waves on mist cooling performance over airfoils in a transonic gas turbine stage. The two-phase flow consists of a continuous phase (i.e., air) and a dispersed phase (i.e., mist). The tracking and evaporation of the mist are simulated by using the discrete phase model (DPM). The interactions between the droplets themselves with respect to collision and breakup were simulated by using the breakup and coalescence sub-models. The motion of the rotor domain was simulated by using the linear sliding mesh technique to study the transient stator-rotor interaction. The results show that the passing unsteady wakes caused by the blade rotation press the mist toward the blade surface, providing more enhanced film cooling effectiveness. The oblique shock wave was observed downstream of the vanes passage. The lambda shock wave was observed downstream of the blades passage. Both shock waves did not exert noticeable effect on the air/mist cooling performance. Injecting a 10% mist ratio noticeably improved the cooling enhancement by obtaining a maximum temperature reduction of 200 K and an average temperature reduction of 51 K on the vane, and a maximum of 350 K reduction and an average of 24 K reduction on the blade surface under the influence of wakes and shocks. Injecting the tiny water droplets increased the total pressure losses by 0.1% on the vane and 4% on the blade compared to the air cooling case. Injecting mist has negligible effect on the formations of shocks/wakes and their corresponding behaviors.

Investigation of erosion influence in distribution system and nozzle structure of pelton turbine

Pelton turbine, with its unique advantages in high mountainous region especially in the southwest of China, has received more and more attention. This paper focuses on the erosion influence to the hydraulic characteristics by Euler particle tracking model. Two-phase numerical simulation results are compared with the model experiment results and are proved to be accurate. Solid particle is then added in the flow and three-phase flow is solved. Particle parameters (silt particle size, concentration) were considered to investigate the mechanism of silt erosion on Pelton turbine distribution system including distributor, bifurcation tubes, and nozzle structure. It proves that erosion destroys the vortex structure in bifurcation tubes. As the particle's diameter increases, discharge in different bifurcations appear more uniform. With the particle flow, a strong collision phenomenon appears at nozzle seat. As the particle's diameter increases, the erosion strengthens accordingly. This study reveals that the particle influence induces erosion which is important to be considered during the design and operating procedures in the whole life of Pelton turbine.

An investigation of the exergy and first and second laws by two-phase numerical simulation of various nanopowders with different diameter on the performance of zigzag-wall micro-heat sink (ZZW-MHS)

An investigation of the exergy and first and second laws by two-phase numerical simulation of various nanopowders with different diameter on the performance of zigzag-wall micro-heat sink (ZZW-MHS)

Wind-induced changes to surface gravity wave shape in deep to intermediate water

Abstract

Quantitative characterisation of gas loss and numerical simulations of underground gas storage based on gas displacement experiments performed with systems of small-core devices connected in series

In this study, an experimental system in which small core devices with different configurations are connected in series was designed, and high-intensity gas injection and production experiments (3 groups, 28 times) were conducted to quantify the per-cycle gas loss (difference between the amounts of injected and produced gas). These experiments reveal that the lost gas fraction is positively correlated with the porosity and permeability of sandstone but negatively related to the number of injection-production cycles. Based on the experimental results, a three-dimensional gas–water two-phase numerical simulation model for high-intensity injection and production of underground gas was established, and a numerical simulation study (10 injection-production cycles) was performed to clarify the macroscopic migration law of gas and water during injection-production. The simulation results show that the gas could cross the gas–water contact (GWC) into the lower water area at approximately 350 m in the injection phase, while the water intrudes into the gas area at approximately 150 m along the high-permeability channel in the production process. Owing to the complex interaction between the gas and water, a large amount of the lost gas that cannot be produced accumulates in the transition zone generated by the reciprocating motion of the gas and water. A certain amount of the lost gas is produced in every cycle; however, the lost gas fraction will decrease with an increasing number of injection and production cycles (a single cycle yields a reduction of more than 10%) and eventually stabilise at approximately 5%. Finally, based on our results, some suggestions were provided for avoiding gas loss while building underground gas storage systems.

Capillary rise in vuggy media

Carbonates can be highly heterogeneous formations with large variations in pore size distribution and pore space topology, which results in complex multiphase flow behavior. Here we investigate the spontaneous imbibition behavior of fluids in vuggy carbonates. Glass beads of 1.0 mm diameter, with dissolvable vug placeholders, are sintered to form multiple configurations of heterogeneous vuggy core with variations in matrix porosity, vug size, vug spatial location, and number of vugs. The core fabrication process is repeatable and allows the impact of vug textural properties to be investigated in a controlled manner.Capillary rise experiments are conducted in these proxy vuggy carbonate core and compared with the homogeneous non-vuggy core as reference. Continuous optical imaging is performed to track the position of the air-water interface in the cores. To understand the change in capillary height in the presence of a vug, a volume-of-fluid two-phase numerical simulation is performed in a parallel set of connected and disconnected tubes. Finally x-ray tomography scans are performed to identify the shape of the air-water interface in a select few cores.The results can be summarized as follows: disconnected vugs result in higher capillary rise compared to non-vuggy porous media. The vugs act as capillary barriers, diverting fluid flow to the adjacent connected channels, which ultimately results in a higher overall capillary rise. The results of this work highlight that radius of spontaneous invasion of aqueous phases, such as fracture fluid and hazardous wastes, are affected by vug porosity but not their distribution.

Local Capillary Pressure Estimation Based on Curvature of the Fluid Interface – Validation with Two-Phase Direct Numerical Simulations

With the advancement of high-resolution three-dimensional X-ray imaging, it is now possible to directly calculate the curvature of the interface of two phases extracted from segmented CT images during two-phase flow experiments to derive capillary pressure. However, there is an inherent difficulty of this image-based curvature measurement: the use of voxelized image data for the calculation of curvature can cause significant errors. To address this, we first perform two-phase direct numerical simulations to obtain the oil and water phase distribution, the exact location of the interface, and local fluid pressure. We then investigate a method to compute curvature on the oil/water interface. The interface is defined in two ways. In one case the simulated interface which has a sub-resolution smoothness is used, while the other is a smoothed interface which is extracted from synthetic segmented data based on the simulated phase distribution. Computed mean curvature on these surfaces are compared with that obtained from the fluid pressure computed directly in the simulation. We discuss the accuracy of image-based curvature measurements for the calculation of capillary pressure and propose the best way to extract an accurate curvature measurement, quantifying the likely uncertainties.

Research on Sand-dust Separation Technology Based on Gas-solid Two-phase Numerical Simulation

The separation technology in the large-scale sand-dust environment ground simulation test system applicable to the environmental adaptability and reliability verification of aerospace electromechanical products is studied. The gas-solid two-phase numerical simulation method is adopted, and the possible cyclone separation, inertial separation methods are used to study the separation efficiency and regularity technology, which provides a basis for the separation design and test of the sand-dust environment simulation of large electromechanical products.