Two-dimensional Computational Fluid Dynamics Research Articles

Temperature-dependent gas accumulation in polymer electrolyte membrane electrolyzer porous transport layers

We investigate the temperature-dependent gas saturation in the porous transport layer (PTL), and the subsequent impact of this gas saturation on the mass transport overpotential of a polymer electrolyte membrane (PEM) electrolyzer. Via in operando neutron imaging of a PEM electrolyzer, we observe that increasing the operating temperature results in a) the reduction of gas saturation in the PTL (particularly near the catalyst layer (CL)-PTL interface) and b) the promotion of uniform gas distributions near the CL-PTL interface. Via two-dimensional computational fluid dynamics modelling of the PEM electrolyzer, we observe that increasing the operating temperature results in a) the reduction of dissolved oxygen concentrations and b) the promotion of uniform dissolved oxygen concentrations near the CL-PTL interface. Therefore, we attribute our observation of lower and more uniform gas saturation to correspondingly lower and more uniform gas evolution rates that would accompany the dissolved oxygen concentrations predicted by our model. Increasing the operating temperature leads to lower mass transport overpotentials, which we attribute to the combined effects of lower and more uniform gas saturations and lower dissolved oxygen concentrations near the CL-PTL interface. Based on this work, we recommend higher operating temperatures for optimal water and oxygen transport in the PTL.

Predictions of undesirable air–sand flow behaviors in a dual fluidized bed cold flow system via a CFD full-loop model

Abstract A two-dimensional computational fluid dynamics (CFD) multiphase flow model based on a laboratory-scale dual fluidized bed cold flow system has been developed to study the unsteady flow characteristics in that system. Different sizes of silica sand as multiple solid phases were simulated to thoroughly describe the actual sand flow, and hence enhance the accuracy of the simulation results. The fluidizing air velocity and sand bed height in the riser were found to have strong influences on the results of solid volume fraction, system pressure, and sand circulation rate. It was noteworthy that when the initial sand height and air inlet velocity in the riser were increased over their critical values, an undesirable reverse flow could occur in the system, causing pressure imbalance and unstable system operation. Therefore, the saturated carrying ability of the fluidizing air should be carefully considered to prevent those unexpected phenomena. The proposed CFD model could provide valuable predictions and insights into the sand flow behaviors for the optimization of the design and operation of practical systems.

Sand casting of sheet lead: numerical simulation of metal flow and solidification

Sandcast lead sheets are characterised by their superior aesthetic performance and mottled surface. Lead sheet casting is widely used in the construction industry for roofing and flashing applications, while the roots of this process can be tracked back to the Roman times. In this study, two-dimensional computational fluid dynamics (CFD) simulations have been performed to simulate the melt flow and solidification stages of the lead sandcasting process. The effects of process parameters such as pouring temperature, screed velocity and clearance between the screed and the sandbed on the final quality of the lead sheet are investigated. Lead sheet quality has been quantified by measuring the variance and the average value of the final sheet thickness over the sandbed length. The developed CFD model has been validated against experimental results by comparing the time evolution of the lead-sandbed interface temperature against data collected by thermocouples during the real-time process. The numerical results show that all of the aforementioned parameters affect the final quality of the cast product and suggest that superior quality lead sheets can be produced for a range of relatively low values of the pouring temperature and slow strickle motion.

Predicting and analyzing the performance of biomass-burning natural draft rocket cookstoves using computational fluid dynamics

Nearly three billion people in the world today rely on biomass for their cooking needs. Indoor cooking using biomass has been identified as a major cause of respiratory illnesses, resulting in four million premature deaths every year. Improved biomass cookstoves may help mitigate this challenge. This paper presents a two-dimensional axisymmetric steady-state computational fluid dynamics (CFD) model of a biomass burning, natural draft rocket cookstove. The CFD model includes coupled sub-models representing combustion, turbulence, and heat transfer. The model is validated against experimental data and used to predict temperatures and flow inside the cookstove, including the airflow rate through the cookstove and heat transfer to the cookpot. We find that the excess air is typically many times stoichiometric air during standard operating conditions and is sensitive to flow field obstructions. We analyze the effects of geometric and operational features such as the pot support height, secondary air entrainment, cone-deck shape, and baffle placement within the cookstove on the flow, airflow rate, mixing, and stove thermal efficiency. The model shows that secondary air entrainment, though ineffective by itself, increases turbulent mixing when used in conjunction with a central baffle but reduces thermal efficiencies due to enhanced heat transfer to the walls. We find that a lower pot support height decreases the airflow rate and increases thermal efficiency. We model thirty-six cone-deck configurations and find that the cone-deck shape primarily affects the airflow rate through the stove, with more constricted designs leading to higher thermal efficiencies.

Numerical Simulation of Uranium Extraction from Nitric Acid Medium Using Hollow-Fiber Contactor

ABSTRACTA two-dimensional axisymmetric computational fluid dynamics (CFD) model is presented to simulate uranium extraction from nitric acid medium using tri-n-butyl phosphate in n-dodecane in a hollow-fiber membrane contactor operated in non-dispersive solvent extraction mode. CFD model solves continuity and momentum-transport equations for the feed and shell sides and species transport equations for the feed side, shell side, as well as the membrane. Complex boundary conditions of flux continuity and concentration jump are implemented in the CFD model. The estimates of percentage of uranium extraction obtained from CFD simulations for different parametric conditions are compared with the experimental results, and a good agreement is observed. The validated CFD model is used to gain detailed insights into the hydrodynamics and mass transfer.Abbreviations CFD: Computational fluid dynamics; NDSX: Non-dispersive solvent extraction; TBP: Tri n-butyl phosphate

Mechanisms of power augmentation in two side-by-side vertical axis wind turbines

Using two-dimensional computational fluid dynamics (CFD) simulations, this study examines the physical mechanisms of performance improvement in a double rotor vertical axis wind turbine (VAWT) as compared to a single rotor VAWT. It was found that the primary mechanism for the approximately 10% improvement in power coefficient (CP) for the double rotor VAWT was a reduction in the bypass flow that passes around the outside of the single rotor VAWT. The single rotor VAWT is shown to cause greater than 54% of the incoming flow to bypass around the outside of the VAWT. By placing a second rotor in the system, the cross-stream flow that would normally exist to one side is suppressed due to the opposing cross-stream flow from the second rotor. The collision of cross-stream velocities results in a high pressure field upstream of the rotors. This high pressure field re-directs more flow through the rotors, reducing the total bypass down to below 51%. This reduction in total bypass is solely due to the reduction in bypass on the interior of the double rotor system as the exterior side actually experiences an increase in bypass of about 83%.

Measurements and Modeling of the Movement Behavior of a Radial Adaptive Seal

Abstract The radical change in the energy market with a raising share of renewable energy production leads to new operating cycles in turbo machinery. The machines must be optimized for frequent transient load cycles and part load utilization. New radial adaptive sealing concepts in the secondary air system can help to increase part load efficiency and achieve a long service life. These seals are pressure-activated and adapt their gap width during operation. The sealing concepts have to be analyzed in more detail to fully understand their operational behavior. In this paper, a model to understand and predict the movement of a radial adaptive seal called hydrostatic advanced low leakage (HALO) seal is presented. Experimental results of the gap width, leakage, and pressure distribution are used in combination with two-dimensional (2D) computational fluid dynamics (CFD) to obtain force distributions across the seal for static operation. These data are used to derive a model to predict the forces acting on the seal as well as the operational gap width, which is necessary for a reliable seal design.

On the Hydraulics of Downward Sloping Pipes With Entrapped Air Pockets

Abstract In this study, air–water flow in a downward sloping pipe subsequent to the entrapping of an air pocket is investigated both numerically and experimentally. A transient, two-dimensional computational fluid dynamics model is applied to study the different possible flow regimes and their associated phenomena. The numerical model is based on the Reynolds-averaged Navier–Stokes (RANS) equations and the volume of fluid (VOF) method. Both numerical and experimental investigations provide visualization for the hydraulic jump, the blowback regime, and the full gas transport regime. The numerical results predict that the flow structure in the pipe downstream the toe of the hydraulic jump is subdivided into three distinct regions including the jet layer, the shear zone, and the circulation region, which agrees qualitatively with the previous investigations of the hydraulic jump characteristics in open channel flow. Numerical results are in reasonable agreement with the experimental measurements of the circulation length and the hydraulic jump head loss.

Numerical Investigation on the Effect of Tube Geometry and Feeder Height on the Heat Transfer Performance of Horizontal Tube Falling Film Evaporation

Abstract This paper discusses about the effect of tube geometry and liquid feeder height on the heat transfer performance of falling film evaporation over the horizontal heated plain tubes. To investigate this, a two-dimensional computational fluid dynamics (CFD) model was developed, compared, and validated with published data available in the literature. A numerical simulation was carried out for varying liquid load, tube diameter, liquid feeder height, and corresponding changes in the heat transfer co-efficient (HTC), and mass transfer rate was recorded and analyzed. An attempt was also made to measure the thickness of the film around the tubes from the simulation model. Mechanisms that control the factors such as HTC, film thickness, and mass transfer were numerically investigated and discussed in this work. Numerical results indicated that low value of liquid film thickness appears approximately at the angular position of the range between 90 deg and 125 deg. Also the numerical investigation revealed that liquid film thickness decreases and HTC and mass transfer rate increases with the increase of feeder height. No remarkable change in film thickness was observed with increase in the tube diameter. This numerical study also proved that the prediction of thermally developed boundary region on the circumference of the tube could be possible in terms of mass transfer rate. It was also observed from the numerical study that the highest mass transfer rate takes place between the angle 135–165 deg near to the bottom of the tube.

Hydrodynamic Lubrication of Textured Journal Bearing Considering Slippage: Two-dimensional CFD Analysis Using Multiphase Cavitation Model

Partial texturing of the surface of journal bearings have been proven very beneficial in terms of friction coefficient. In the present work, the load support of the hydrodynamic textured journal bearing combined with artificial slippage is fully characterized by means of computational fluid dynamics (CFD) simulations based on the numerical solution of the Navier–Stokes equations for incompressible flow. In order to model slippage, the enhanced user-defined-function (UDF) code is developed. Realistic boundary condition is employed by implementing the mixture multiphase model to model a cavitation in the bearing. The numerical analysis is performed under the condition of different groove depths, eccentricity ratios and slippage placements along the textured area of bearing. The simulation results including hydrodynamic pressure and load support are gained and compared for conventional smooth parameters. A reference to determine optimal groove depths as well as best artificial slippage placement of textured bearing under different conditions of loading are proposed. Based on the present results, favorable slippagetextured journal bearing design can be assessed.

Impact of harmonic pitch functions on performance of Darrieus wind turbine

Abstract Wind power is known as a promising renewable energy source without any environmental pollution. Increasing efficiency of wind turbines plays a vital role in wind energy applications, approving their high efficiency and cost-effectiveness. Variable pitch angle approaches are one of the best strategies for improving the performance and self-start ability of Darrieus turbines and delaying the start of dynamic stall phenomena. However, although this method has been successfully used on several occasions, it has not yet been investigated for various complicated variable pitch functions in different amplitudes. Therefore, the present study analyzed the relationship between the angle of attack and self-starting performance of H-type vertical axis wind turbine. Four pitch functions (cases 1, 2, 3 and Sinusoidal pitch) with four amplitudes; 0°, 3°, 10°, and 20° were selected, and two-dimensional CFD (Computational Fluid Dynamics) modeling was developed to predict flow behavior around the turbine blades. The results showed that the best variable pitch function could reduce the blade angle of attack in the upstream phase while increases the angle of attack in the downstream stage. In all cases, the amplitude of 3 ° has an excellent performance compared to other amplitudes. Also, compared to other pitch angles, Case 1 could reduce the time required for starting and with an increase of 34% in power, so it is strongly recommended for implementing in conventional turbines. Simulations also indicated that in azimuth angles between 90° and 100°, and between 45° and 60° turbine has the best and worst self-starting performance, respectively.

Nondimensional Parameters’ Effects on Hybrid Darrieus–Savonius Wind Turbine Performance

Abstract Vertical axial wind turbines are the most commonly used turbines in residential and urban areas. This paper investigates the effect of combining Darrieus and Savonius wind turbines on power output and introduces a wind turbine with high starting torque addition to the wide working domain. A two-dimensional computational fluid dynamics transient simulation is developed, and a moving mesh is implemented for rotating moving parts. Comprehensive research has been carried out to investigate the effects of the initial overlap ratio (ɛ), arc angle Ø, and curvature (α) of Savonius blades on the performance of the turbine and 18 models are simulated at seven tip speed ratios. The results showed that combining the Darrieus turbine with the Savonius turbine has a favorable effect on self-starting performance. Also, it was observed that by changing each of the parameters, the primary model performance could be significantly improved. Finally, it is concluded that ɛ = 0.25, α = 0.25, and ∅ = 150 deg are the optimum values of the parameters which increase turbine power output compared to conventional vertical-axis turbines.

2D CFD Simulation of Water Injection Strategies in a Large Marine Engine

A two-dimensional computational fluid dynamics (2D CFD) simulation of a low-speed two-stroke marine engine simulation was performed in order to investigate the performance of 2D meshes that allow the use of more complex chemical schemes and pollutant formation analysis. Various mesh density simulations were compared with a 3D mesh simulation and with the experimentally obtained cylinder pressure. A heavy fuel model and a soot model were implemented in the software. Finally, the influences of three water injection strategies were simulated and evaluated in order to investigate the capability of the model and the influence of water injection on NOx formation, soot formation, and engine performance. We conclude that the direct water injection strategy reduces NOx emissions without adversely affecting the engine performance or soot emissions. The other two strategies—Intake air humidification and direct injection of fuel–water emulsion—reduced NOx emissions but at the cost of higher soot emissions or reduced engine performance.

Comparison of oscillating flow and slip velocity mass transfer enhancement in spacer-filled membrane channels: CFD analysis and validation

Unsteady shear methods have the potential to generate flow perturbations near the membrane surface, which play an important role in reducing concentration polarisation and fouling tendency. In general, there are two main approaches for generating time-varying flow perturbations: 1) generating oscillations in the bulk flow; or 2) forcing a slip velocity near the membrane surface. This paper presents a detailed comparison study of both approaches by means of two-dimensional computational fluid dynamics (CFD) simulations. The results show that both approaches result in significant increases in flux and maximum wall shear at the same disturbance resonant frequency and Reynolds number. This suggests that the mechanism by which the flow perturbations are generated is not as important as the perturbation frequency, in terms of increasing wall shear and permeate flux. However, it is more important to perturb flow near the membrane surface because it reduces energy consumption compared to oscillating flow approach. In addition, this paper confirms that a white noise perturbation can be used to simplify the approach for maximising vortex-shedding-induced mass transfer enhancement, without the need to identify the peak/resonant frequency for the flow in spacer-filled membrane channels at the expense of a higher pressure loss.

Multi-element ducts for ducted wind turbines: a numerical study

Abstract. Multi-element ducts are used to improve the aerodynamic performance of ducted wind turbines (DWTs). Steady-state, two-dimensional computational fluid dynamics (CFD) simulations are performed for a multi-element duct geometry consisting of a duct and a flap; the goal is to evaluate the effects on the aerodynamic performance of the radial gap length and the deflection angle of the flap. Solutions from inviscid and viscous flow calculations are compared. It is found that increasing the radial gap length results in an augmentation of the total thrust generated by the DWT, whereas a larger deflection angle has an opposite effect. Reasonable to good agreement is seen between the inviscid and viscous flow calculations, except for multi-element duct configurations characterized by large flap deflection angles. The viscous effects become stronger at large flap deflection angles, and the inviscid calculations are incapable of taking this phenomenon into account.

Performance evaluation of mass transport enhancement in novel dual-channel design of micro-reactors

Various design configurations of semi-T-shaped dual-channel micro-reactors were numerically examined for their laminar mass transport performance in heterogeneous catalytic combustion of methane and air. One single-channel and five dual-channel configurations (i.e., parallel, divergent, convergent, zig-zag, and curved configurations) were investigated with a two-dimensional computational fluid dynamics model. These innovative design configuration were compared in terms of CH4 utilization, pressure drop, CO/CO2 ratio, catalyst utilization, and a performance index at various Reynolds numbers. Dual-channel micro-reactors were found to enhance mass transport due to the well mixed flow and the increased reaction contact area. By suitably modifying the dual-channel layout angle and shape, recirculation zones can be formed within the reactor which increase CH4 utilization. However, the improved conversion rate is achieved at the cost of high pressure drop. The parallel dual-channel design provides the highest conversion per unit pressure drop over the range of the Reynolds numbers studied. For Reynolds numbers of 20 and 40, compared to the single-channel micro-reactor, divergent, convergent and curved channel designs yield higher conversion per unit pumping power. However, further increase of Reynolds number (i.e., 60, 80, and 100) deteriorates their performance due to the significantly increased pressure drop and shorter residence time.

CFD analysis and characterization of biochar produced via fixed-bed gasification of fallen leaf pellets

Biochar (BC) is a solid byproduct of gasification and could be valorized looking for contributing to the feasibility of gasification projects. In this study, the effect of airflow during fixed-bed gasification of pelletized fallen leaves to BC was evaluated at 0.075 and 0.150 kg/s/m2. The influence of glycerol in the leaf feedstock was also studied, and the gasification products were characterized. BC formation under gasification conditions was simulated using a two-dimensional (2D) unsteady computational fluid dynamics (CFD) model. When airflow was increased, convective heat transfer from the gas to the solid phase increased by 250%, and absorption of radiative heat transfer in the solid phase increased by 59%. This was ascribed to the increase in the process temperature, which also promoted the release of volatile matter. The H:C and O:C ratios in the feedstocks were reduced by 80% and 92%, respectively. The surface areas and pore volumes of the samples increased by up to 50%. The BC with the largest surface area (68.63 m2/g) also had the highest total organic carbon content (21.3%) and the highest water retention capacity (408.13%). The pH of the BCs obtained in the study ranged from 8.7 to 10, making them suitable for acid soils.

A Preliminary Computational Investigation Into the Flow of PEG in Rat Myocardial Tissue for Regenerative Therapy.

Myocardial infarction (MI), a type of cardiovascular disease, affects a significant proportion of people around the world. Traditionally, non-communicable chronic diseases were largely associated with aging populations in higher income countries. It is now evident that low- to middle-income countries are also affected and in these settings, younger individuals are at high risk. Currently, interventions for MI prolong the time to heart failure. Regenerative medicine and stem cell therapy have the potential to mitigate the effects of MI and to significantly improve the quality of life for patients. The main drawback with these therapies is that many of the injected cells are lost due to the vigorous motion of the heart. Great effort has been directed toward the development of scaffolds which can be injected alongside stem cells, in an attempt to improve retention and cell engraftment. In some cases, the scaffold alone has been seen to improve heart function. This study focuses on a synthetic polyethylene glycol (PEG) based hydrogel which is injected into the heart to improve left ventricular function following MI. Many studies in literature characterize PEG as a Newtonian fluid within a specified shear rate range, on the macroscale. The aim of the study is to characterize the flow of a 20 kDa PEG on the microscale, where the behavior is likely to deviate from macroscale flow patterns. Micro particle image velocimetry (μPIV) is used to observe flow behavior in microchannels, representing the gaps in myocardial tissue. The fluid exhibits non-Newtonian, shear-thinning behavior at this scale. Idealized two-dimensional computational fluid dynamics (CFD) models of PEG flow in microchannels are then developed and validated using the μPIV study. The validated computational model is applied to a realistic, microscopy-derived myocardial tissue model. From the realistic tissue reconstruction, it is evident that the myocardial flow region plays an important role in the distribution of PEG, and therefore, in the retention of material.

Pump Grooved Seals: A Computational Fluid Dynamics Approach to Improve Bulk-Flow Model Predictions

In multiple stage centrifugal pumps, balance pistons, often comprising a grooved annular seal, equilibrate the full pressure rise across the pump. Grooves in the stator break the evolution of fluid swirl and increase mechanical energy dissipation; hence, a grooved seal offers a lesser leakage and lower cross-coupled stiffness than a similar size uniform clearance seal. To date, bulk-flow modelbulk-flow models (BFMs) expediently predict leakage and rotor dynamic force coefficients of grooved seals; however, they lack accuracy for any other geometry besides rectangular. Note that scalloped and triangular (serrated) groove seals are not uncommon. In these cases, computational fluid dynamics (CFD) models seals of complex shape to produce leakage and force coefficients. Alas, CFD is not yet ready for routine engineer practice. Hence, an intermediate procedure presently takes an accurate two-dimensional (2D) CFD model of a smaller flow region, namely a single groove and adjacent land, to produce stator and rotor surface wall friction factors, expressed as functions of the Reynolds numbers, for integration into an existing BFM and ready prediction of seal leakage and force coefficients. The selected groove-land section is well within the seal length and far away from the effects of the inlet condition. The analysis takes three water lubricated seals with distinct groove shapes: rectangular, scalloped, and triangular. Each seal, with length/diameter L/D = 0.4, has 44 grooves of shallow depth dg ∼ clearance Cr and operates at a rotor speed equal to 5,588 rpm (78 m/s surface speed) and with a pressure drop of 14.9 MPa. The method validity is asserted when 2D (single groove-land) and three-dimensional (3D) (whole seal) predictions for pressure and velocity fields are compared against each other. The CFD predictions, 2D and 3D, show that the triangular groove seal has the largest leakage, 41% greater than the rectangular groove seal does, albeit producing the smallest cross-coupled stiffnesses and whirl frequency ratio (WFR). On the other hand, the triangular groove seal has the largest direct stiffness and damping coefficients. The scalloped groove seal shows similar rotordynamic force coefficients as the rectangular groove seal but leaks 13% more. For the three seal groove types, the modified BFM predicts leakage that is less than 6% away from that delivered by CFD, whereas the seal stiffnesses (both direct and cross-coupled) differ by 13%, the direct damping coefficients by 18%, and the added mass coefficients are within 30%. The procedure introduced extends the applicability of a BFM to predict the dynamic performance of grooved seals with distinctive shapes.

Interaction of two in-line bubbles of equal size rising in viscous liquid

Abstract The interaction of bubbles is the key to understand gas–liquid bubbling flow. Two-dimensional axis-symmetry computational fluid dynamics simulations on the interactive bubbles were performed with VOF method, which was validated by experimental work. It is testified that several different bubble interactive behaviors could be acquired under different conditions. Firstly, for large bubbles (d: 4, 6, 8, 10 mm), the trailing bubble rising velocity and aspect ratio have negative correlations with liquid viscosity and surface tension. The influences of viscosity and surface tension on leading bubble are negligible. Secondly, for smaller bubbles (d: 1, 2 mm), the results are complicated. The two bubbles tend to move together due to the attractive force by the wake and the potential repulsive force. Especially for high viscous or high surface tension liquid, the bubble pairs undergo several times acceleration and deceleration. In addition, bubble deformation plays an important role during bubble interaction which cannot be neglected.