Three-dimensional Computational Fluid Dynamics Simulations Research Articles

Parametric study of flow field and mixing characteristics of TiCl4 jet injected into O2 crossflow in oxidation reactor for titanium pigment production by chloride process

A parametric study of the flow field and mixing characteristics of a TiCl4 jet injected into an O2 crossflow in an oxidation reactor for titanium pigment production is numerically investigated. A three-dimensional computational fluid dynamics (CFD) simulation of the turbulent gas mixing model is performed. The effect of geometrical (reactor diameter, jet nozzle number n, jet nozzle diameter, jet nozzle spacing S) and flow parameters (momentum flux ratio J) on the penetration depth (h/R) and mixing quality of the gases is examined. The results are validated with available experimental data and a good agreement is obtained. We show that three stages: under-, optimum, and over-penetration, occur sequentially in the oxidation reactor with increasing J. The kidney-shaped structure, characteristic of a jet-in-crossflow is formed, which is blurred when the mixture of TiCl4 and O2 moves into the downstream fluid. The h/R value at a minimum temperature difference is 0.683 and 0.604 for n = 32 and 16, respectively, which are within industrial production data range of 0.56–0.72. The optimum range of S is between 3.25 and 7.35. h/R strongly depends on the only dimensionless parameter J/n2 expressed in terms of the geometrical and flow parameters via the relation: h/R = 0.7274 + 0.20228 ln (J/n2+0.04587). The TiCl4 concentration profile changes from a quasi-sine to quasi-cosine distribution with increasing J/n2. Both the mixing non-uniformity and the time to attain the optimum mixing quality of TiCl4 and O2 decrease first and then increase with increasing J/n2.

Numerical prediction of a single phase Pressurized Thermal Shock scenario for crack assessment in an Reactor Pressure Vessel wall

A pressurized thermal shock (PTS) in a reactor pressure vessel (RPV) wall of a Pressurized Water Reactor (PWR) could eventually lead to reduced integrity of the RPV. A PTS scenario may be initiated by a Loss-of-Coolant Accident (LOCA) in which the Emergency Core Cooling (ECC) injection causes a thermal shock in the vessel wall while the RPV is still partially pressurized. The traditional Thermal-Hydraulic system codes fail to reliably predict the complex three-dimensional thermal mixing phenomena in the downcomer occurring during the ECC injection. Therefore, a three-dimensional computational fluid dynamics (CFD) simulation of the transient is required to provide an accurate temperature distribution for reliable probabilistic analyses. The assessment of the failure probability due to a single phase PTS initiated by postulated defects in the vessel wall requires the computation of a long transient solution. Since the CFD calculation is computationally expensive, the symmetry of a four-leg PWR geometry is utilized to reduce the computational domain.In the present work, several Unsteady Reynolds Averaged Navier-Stokes (URANS) CFD simulations are performed to select an optimal computational domain that is representative for a typical PWR. The computed temperature distribution is provided as input for the structural analyses to calculate the Stress Intensity Factor (SIF) along several postulated defects. The SIF is then used to evaluate the potential of crack propagation initiation of the defects.

Experimental approach for the analysis of the flow behaviour in the stator of a real centripetal turbine

During normal operation, radial turbines may work in off-design conditions. Off-design conditions may be characterised by very low expansion ratios, very high expansion ratios, very low rotational speeds or very high rotational speeds. All of these cases are difficult to characterise experimentally due to high experimental uncertainties or a lack of capabilities in the system feeding pressurised air to the turbine. Also, there are two- and three-dimensional computational fluid dynamics simulations at these operating points but could not be accurate enough due to high turbulence effects, flow detachment and shock wave generation. With a lack of high-quality data, experimental or computational, to fit the reduced-order turbine models used in zero- and one-dimensional engine simulations, there are large uncertainties associated to their results in off-design conditions. This work develops an experimental facility able to characterise the internal flow of radial turbine stators in terms of pressure and velocity fields at off-design and regular working conditions. The facility consists of an upscaled model of a radial turbine volute and stator fed with air in pressure- and temperature-controlled conditions, so different sensors can be used inside it with the least amount of flow disturbance. The different restrictions considered in the design of the upscaled model are presented, and their effects in the final experimental apparatus capabilities are discussed. A preliminary comparison between computational fluid dynamics simulations and experimental data shows encouraging results.

A Comparative Study on the Performance of a Horizontal Axis Ocean Current Turbine Considering Deflector and Operating Depths

Several different designs and prototypes of ocean current turbines have been tested over recent years. For every design test, emphasis is given to achieving an optimum power output from the flow. In this study, the performance of a Horizontal Axis Ocean Current Turbine (HAOCT) has been investigated using three-dimensional Computational Fluid Dynamics (CFD) simulations for three cases, namely, (1) a turbine without a deflector, (2) a turbine with a deflector, and (3) a turbine with a deflector operating at a higher fluid depth. The turbine design was modeled in DesignModeler software and simulations were carried out in commercial CFD software Flow-3D. The Torque Coefficient (Cm) and Power Coefficient (Cp) for the turbine have been investigated for a certain range of Tip-Speed Ratios (TSRs) in a flow velocity of 0.7 m/s. Furthermore, comparisons have been made to demonstrate the effect of the deflector on the performance of the turbine and the influence of a higher fluid pressure on the same. The results from the simulations indicate that the higher value of Cp was achieved for Case 2 as compared to the other two cases. The findings from the study indicate that the use of the deflector enhances the performance of the turbine. Furthermore, a higher fluid pressure acting on the turbine has a significant effect on its performance.

Numerical study on the hydrodynamics of agglomerates at intermediate Reynolds numbers

Abstract The flow pattern and hydrodynamics of a heterogeneous permeable agglomerate in a uniform upward flow at intermediate Reynolds numbers (1–40) are analyzed from three-dimensional (3D) computational fluid dynamics simulations. Different from the homogeneous or stepwise-varying permeability models used in previous papers, a continuously radially varying permeability model is used in the present study. The effects of two dimensionless parameters, the Reynolds number and the permeability ratio, on the flow field and the hydrodynamics were investigated in detail. The results reveal that unlike the solid sphere, a small recirculating wake initially forms inside the agglomerate. The critical Reynolds number for the formation of the recirculating wake is lower than that of the solid sphere and it decreases with the increase of permeability ratio. A correlation of drag coefficient as a function of the Reynolds number and permeability ratio is proposed. Comparisons of drag coefficients obtained by different permeability models show that at intermediate Reynolds numbers (1–40), the effect of radially varying permeability on the drag coefficient must be considered.

Numerical study on the gas-solid hydrodynamics and heat transfer in a rotating fluidized bed with static geometry dryer

In the present study, a three-dimensional computational fluid dynamics (CFD) simulation is carried out to investigate the gas-solid fluidization behavior and heat transfer characteristics in a rotating solid-bed with static geometry (RFB-SG) without slits. With a static geometry adit as the research objective, this paper makes use of the Eulerian–Eulerian approach to carry out numerical simulations in ANSYS FLUENT 14.5 software. The prediction of particle fluidization capacity and its behavior in the developed reactor has been analyzed, using appropriate parameters and simulation analysis. The results illustrated that the main factors, such as air inlet velocity, inlet pressure, temperature, and heat transfer coefficient, affected the reactor capacity. It is observed that for a solid inventory of 300 g, a well-stabled solid-bed has been obtained at an air velocity of 22 m s−1. On enhancing the inlet air velocity up to 33 m s-1, not only the fluidization capacity of the reactor is increased by 500 g but also the drying efficiency is found to be increased. On validating numerical results with experimental findings, the maximum error of temperature is found to be 6.98%.

CFD-Based Metamodeling of the Propagation Distribution of Styrene Spilled from a Ship

The present study aimed to numerically establish a new metamodel for predicting the propagation distribution of styrene, which is one of the hazardous and noxious substances (HNSs) spilled from ships. Three-dimensional computational fluid dynamics (CFD) simulations were conducted for 80 different scenarios to gather large amounts of data on the spatial distribution of the change in concentration over time. We used the commercial code of ANSYS Fluent (V.17.2) to solve the Reynolds-averaged Navier–Stokes equations, together with the scalar transport equation. Based on the CFD results, we adopted the well-known kriging model to create a metamodel that estimated the propagation velocity and spatial distributions by considering the effect of the current surface velocity, deep current velocity, surface layer depth, and crack position. The results show that the metamodel accurately predicted the changes in the local distribution of styrene over time. This model was also evaluated using the hidden-point test.

Performance enhancement of finned annulus using surface interruptions in double-pipe heat exchangers

The present study investigates the thermo-fluid characteristics of a double pipe heat exchanger (DPHX) with split longitudinal fins (SLF) on the annulus side. The studied tubes with SLF are a modification of the conventional longitudinal finned tubes (LF) that allows for consecutive surface interruptions to break the boundary layer and provide an interrupted fluid passage along the flow length. The study is conducted using a high Prandtl number fluid (engine oil) with variable thermo-physical properties. Three-dimensional computational fluid dynamics (CFD) simulations are carried out under laminar flow conditions for configurations with a fin split interval between 0.333 and 0.166 m. A comparative analysis of the SLF configurations against the reference LF configuration is conducted based on fluid flow, heat transfer rate, and required pumping power. Furthermore, by evaluating integrated quantities such as the axially local heat transfer coefficient, total pressure, and bulk fluid temperature, it is noted that the main advantage of SLF is to repeat an entrance region-like effect along the flow length. The results demonstrate that the heat transfer rates in annulus equipped with SLF are higher than those with conventional LF by 31%–48% for the same pumping power and unit weight.

Experimental and numerical investigation of twin vertical axis wind turbines with a deflector

In this paper we report on the design and testing of a novel vertical axis wind turbine (VAWT) system. The system consists of two counter-rotating VAWT rotors and a deflector that is placed between the two rotors. The system’s performance is first quantified in wind tunnel experiments and subsequently, three-dimensional computational fluid dynamics (CFD) simulations are performed to complement the experiments and to provide the fluid mechanical details. It is found that the deflector has a significant effect (power-increasing) on the power output of the two rotors, which is related to the rotational direction of the two rotors. With the addition of the deflector, the twin VAWT is able to harness up to 38.6% more wind energy compared with the system without deflector. The analysis of the CFD-produced flow field showed that the increased power output is the result of local acceleration of the fluid and local blockage effect of the deflector. The use of a deflector is found to be a simple and practical approach to improve the performance of twin VAWT system.

Numerical and experimental study on optimization of paddy field blade used in initial mud-cutting process

This study analyses the efficiency of a specially designed blade set used in the initial mud-cutting process that can mechanize the action of placing mud into the seedling trays to accelerate rice production as a whole. In this study, experiments were undertaken with specially designed blades, which consist of five optimized blades. Three-dimensional computational fluid dynamics simulations were adopted to optimize the blades for cutting, smashing, and spraying mud into the seedling trays. The effects of blade geometric parameters on efficiency were investigated based on mud splash volume in the first second and power consumption. Results of the analysis of variance (ANOVA) showed that for machine efficiency (ηm), bending angle (X2) is the most dominant input process parameter, followed by curved blade area (X1) and curved blade angle (X3). X3 is a significant factor for energy consumption. The results also indicated that the optimal specifications of the blade are X1 = 3150 mm2, X2 = 115°, and X3 = 41°. With the optimal blades, the machine has a mud-cutting efficiency of 21.32 L/kJ with a relative error of 4.44% compared with the numerical value obtained by means of regression. The mean relative errors of power consumption and mud splash volume in the first second are 8.46% and 5.89%, respectively, and the efficiency between the experimental tests and simulation tests are 12.99%, indicating that the simulation model used in this study is reliable for the initial mud-cutting process.

Effect of piston shape design on the scavenging performance and mixture preparation in a two-stroke boosted uniflow scavenged direct injection gasoline engine

Compared to a four-stroke engine, the two-stroke engine doubles firing frequency and has favourable power-to-weight or power-to-volume ratio as well as engine downsizing to improve the overall powertrain fuel economy. In order to overcome the shortcomings of the conventional cross-flow or loop scavenged two-stroke engines, a two-stroke boosted uniflow scavenged direct injection gasoline engine was designed and its performance was analysed. In this study, three-dimensional computational fluid dynamics simulations were performed to understand the impact of the piston shape design on the scavenging process, in-cylinder flow formation, turbulence level and subsequent fuel/air mixing process in the boosted uniflow scavenged direct injection gasoline engine. Both single injection and split injection strategies were investigated to study the interactions between piston designs and fuel injection strategies to achieve stoichiometric mixture around the spark plug. The results show that the optimised piston with the same opening timing for all scavenge ports could achieve much better scavenging performance than the baseline piston design. In particular, the shallow pistons, that is, Piston #1 and Piston #4, could produce stoichiometric mixture around the spark plug with relatively lower inhomogeneity and higher turbulence kinetic energy around top dead centre when implementing the split injection strategy with start of injection timing at 250/310 °CA.

Improving outdoor air quality based on building morphology: Numerical investigation

Due to rapid urbanization around the world, high concentrations of vehicular pollutants have deteriorated the outdoor air quality, which can affect the physical and psychological well-being of humans. Numerous strategies have been proposed to overcome these harmful impacts by improving the dispersion of air pollutants. Consequently, a question arises regarding the potential effects of building morphology on the dispersion of pollutants. Subsequently, transient three-dimensional Computational Fluid Dynamics (CFD) simulations are performed to examine the effect of building morphology on PM10 dispersion. Eleven cases with various prototypes and morphological methods are compared with a simple building form to identify the patterns of PM10 dispersion within a given time sequence under a prevailing inflow condition. The results indicate that the different designs of building morphology with varying Relative compactness (RC) indicator highlight the importance of considering morphological factors to improve outdoor air quality. In addition, the proposed prototypes can reduce PM10 concentrations by approximately 30%–90% at specific points in the studied time sequence. In particular, the vertical, horizontal, and grid folded prototypes can be considered more effective as an approximate decrease between 70% and 90% in PM10 concentrations is observed, which reflects the influence of building morphology on improving outdoor air quality.

Rapid production of biodiesel in a microchannel reactor at room temperature by enhancement of mixing behaviour in methanol phase using volume of fluid model

The demand for biodiesel to reduce consumption of fossil fuels has motivated engineers to design a rapid and safe production process. However, the slow mass transfer of multiphase transesterification reaction hinders the overall reaction time. A continuous process using a microchannel reactor was proven to achieve complete reaction time in 40 s with high oil conversion of 98.6% due to the enhancement of mixing inside the methanol slug phase. The reaction was successfully performed at ambient room environment, without heating element in the reactor that reduces the design complexity and makes the operation energy efficient and safer. The enhancement of reaction was investigated based on the hydrodynamic factors such as interfacial area and mixing. Three-dimensional Computational Fluid Dynamics (CFD) simulation shows a torus-shaped recirculation structure inside methanol slug that enhances mixing. Microchannel reactor has been proven to enhance the mixing and capable in having faster, highly efficient, and safer process.

A real gas-based throughflow method for the analysis of SCO2 centrifugal compressors

The supercritical carbon dioxide Brayton cycle is recognized as a promising candidate for the next generation of nuclear power and energy system. Among all the components in the cycle, the centrifugal compressor is one of the most important ones. This paper presents a streamline curvature throughflow method based on real gas properties and capable of dealing with condensation flows in the supercritical carbon dioxide compressors. A fluid thermodynamic property calculation method based on look-up tables is adopted to account for the real gas effects and fluid condensation and to reduce the computational time. For extending the simulation capability to the region below the saturation curve to assess the condensation possibility, the homogeneous equilibrium model is adopted. Finally, the real gas-based streamline curvature method is applied in the analysis of a supercritical carbon dioxide centrifugal compressor working near the critical point. Then, computational fluid dynamics calculations are performed to validate the method in detail. The results of the computational validation indicate that the real gas-based streamline curvature method presented in the paper can obtain an accurate enough flow field as that obtained by three-dimensional computational fluid dynamics simulations considering the coarse grid and the much less calculation time.

Computational Fluid Dynamics (CFD) Modelling to Estimate Fluvial Bank Erosion—A Case Study

River bank erosion models are an important prerequisite for understanding the development of river meanders and for estimating likely land-loss and potential danger to floodplain infrastructure. Although bank erosion models have been developed that consider large-scale mass failure, the contribution of fluvial erosion (the process of particle-by-particle erosion due to the shearing action of the river flow) to bank retreat has not received as much consideration. In principle, such fluvial bank erosion rates can be quantified using excess shear stress formulations, but in practice, it has proven difficult to estimate the parameters involved. In this study, a series of three-dimensional Computational Fluid Dynamics (CFD) simulations for a meander loop on the River Asker (200 m long) at Bridport in southern England were undertaken to elucidate the overall flow structures and in particular to provide estimates of the applied fluid shear stress exerted on the riverbanks. The CFD models, which simulated relatively low and relatively high flow conditions, were established using Fluent 6.2 software. The modelling outcomes show that the key qualitative features of the flow endure even as flow discharge varies. At bank full, the degrees of velocity and simulated shear stresses within the inner bank separation zones are shown to be higher than those observed under low flow conditions, and that these elevated shear stresses may be sufficient to result in the removal of accumulated sediments into the main downstream flow.

A novel printing channel design for multi-material extrusion additive manufacturing

Additive manufacturing has a great potential in terms of its capability to produce components with complex geometries and to make multi-material and composite products by combining different materials in a single manufacturing platform. Current trends for the multi-material extrusion additive manufacturing process are categorized by multi-nozzle systems and multi-material inlet systems. In the case of multiple nozzle system, materials are deposited from different nozzles in sequence. On the other hand, in the case of multi-material inlet system, different materials are sent into a mixing tube and deposited as a mixture of materials. In this case, functionally graded parts can be fabricated by changing the volume fraction of two or more materials. Hence, the fabrication of parts with a continuous material supply by varying ratios for the extrusion technologies requires the development of printing heads with suitable printing channels, capable of varying the mixing ratio of different materials. To evaluate the effect of different printing channel designs on the material’s flow pattern and the functionally graded material printability, this paper presents a three-dimensional transient computational fluid dynamics (CFD) simulation of the two miscible liquid-liquid system in a printing channel. Different geometries and materials are considered

Potential improvement in combustion behavior of a downsized rotary engine by intake oxygen enrichment

The attractiveness of oxygen-enriched combustion advantages are making a comeback in the implementation of the rotary engine concept. In this connection, a numerical investigation on influences of intake oxygen enrichment combined with excess air ratio on combustion behavior of the rotary engine needs to be clarified. A three-dimensional computational fluid dynamics simulations coupling with chemical kinetic mechanisms was performed in this study, and the numerical results had been validated with existing experimental data. The species evolution, combustion characteristics, and pollutant formation were monitored to assess the oxygen-enriched combustion of the rotary engine. Results showed that there was a significant influence of intake oxygen enrichment for improving flame development and burned volume due to the reasonable species concentration and intense turbulence intensity. Increasing oxygen enrichment led to the enhanced peak pressure and advanced corresponding position, resulted in a rapid combustion period for improving heat-release efficiency and combustion efficiency, meanwhile made a higher indicated mean effective pressure at the price of a smaller increment of nitrogen oxide formation. The substantial reductions of carbon monoxide, unburned hydrocarbon, and soot were recorded, which is attributed to the higher-oxygen mixture with suitable lean operations. Considering combustion characteristics and emissions performance, the operation of intake oxygen volume of 30% accompanying the excess air ratio of 1.1 is the best choice for the engine used in the current study.

Application of agent model in the optimal design of AIG for SCR-DeNOx system

Purpose Ammonia injection grid (AIG) is used as an input device for ammonia which reacts with NOx in the selective catalytic reduction (SCR) reactor. However, non-uniform concentration distribution of ammonia could produce partially poisoning or deposits of the catalyst. In this work, for making ammonia widely distributed throughout the flue gas and fully mixed, an optimization method of AIG is proposed. Design/methodology/approach Depending on the complexity of fluid flow, the relation between the concentration distributions of ammonia and the geometric parameters of AIG is nonlinear. Based on a certain amount of AIG samples, the computational fluid dynamics (CFD) simulations are applied to propose the agent model which describes the functional relation of the deviation of ammonia concentration and the geometric parameters of AIG. The optimization model of AIG based on the agent model is established. The optimized AIG based on the agent model can be used to produce uniform concentration distributions of ammonia, especially in the case that velocity distribution of flue gas is non-uniform. Findings For qualitatively confirming this optimization method, the three-dimensional CFD simulation of the optimized AIG is carried out. The results reveal that the diffusion process of ammonia gas is consistent with the development of the local vortices, which have a certain relation with the velocity distribution of the flue gas. The unequal ammonia injection designed by the optimization based on the agent model promotes a better mixing of ammonia and flue gas. Originality/value In this work, first, the method for optimizing AIG based on the agent model is proposed. Second, the three-dimensional CFD modeling and simulation of the optimized AIG is carried out, and the mixing effects of ammonia and flue gas are presented.

Numerical study of desalination by vacuum membrane distillation – Transient three-dimensional analysis

The performance of vacuum membrane distillation (VMD) modules can be optimized through careful selection of design parameters. The present study examines how the addition of cylindrical filaments in the feed channel increases momentum mixing and the overall performance of VMD modules under different operating inlet conditions. Three-dimensional transient Computational Fluid Dynamics (CFD) simulations are conducted using Wall-Adapting Local Eddy-Viscosity (WALE) subgrid-scale Large Eddy Simulation (LES) turbulence model. Local concentration, temperature, and flux are coupled at the membrane surface to predict the rate of water vapor diffused through the membrane by Knudsen and viscous diffusion mechanisms. The predicted and measured vapor flux agrees reasonably well; validating the employed model. The small-scale eddies induced by the presence of spacer filaments promote mixing in the module, thus the temperature and concentration polarization is alleviated and the water vapor flux is immensely improved. The insertions of filaments in the feed channel increase the water permeate rate by more than 50% at higher feed flow rates and inlet temperatures. The pressure drop by the spacer reduces the allowable module length by one order of magnitude, but the module length increases two folds at feed temperature 80°C. Even though the power consumption of the module containing the filaments is increased, the addition of filaments is strongly recommended since the required power for the process could be supplied from readily available low-grade heat source.

Numerical modeling of thermophoretic deposition on cylinder liner of a diesel engine using a sectional soot model

Soot deposition on combustion chamber walls of a diesel engine is one of the key concerns among diesel engine manufacturers as it affects the durability and wear of the power cylinder components. The deposition of soot on the liner also gets scraped to the crank case and leads to soot in the lubrication oil. In this work, a modeling framework is presented to estimate the deposition of soot on the liner walls. Three-dimensional computational fluid dynamic combustion simulations are performed at two operating loads for a direct injection heavy duty diesel engine. A sectional soot model coupled with gas phase chemical kinetics is used to model soot in the combustion chamber. The combustion and soot models are validated by comparing in-cylinder pressure, heat release rate and soot mass from simulations with the experimental data. A thermophoretic soot deposition model is presented which transports soot from combustion to the liner walls. Detailed in-cylinder soot mass fraction contours are used to explain the deposition phenomenon. The deposition of soot on the liner is qualitatively compared to measured soot in the lubrication oil and a good agreement is observed. The proposed modeling framework shall be helpful for optimizing combustion systems to reduce degradation of lubrication oil.