Narrow Results

In this study, we propose a configuration of partially blocked oxidant channel with baffle plate(s) transversely inserted in the cathode channel and their effect on the oxygen transport and fuel cell performance is investigated. In order to investigate the effects of the selected shape, size and the number of baffles on the oxygen transport in the gas diffusion layer (GDL) and fuel cell performance a numerical modeling is carried out in 27 cases. With the consideration of both maximum oxygen concentration in the GDL and reasonable pressure drop criterions, the results indicate that in all cases, an increase in baffle height is more effective than an increase in number of baffle plates. Also, installing many large rectangular baffles seem quite appropriate, but when there is restriction in securing pressure in fuel cell, installing the semicircle baffle is better than the rectangular one.

An aerofoil commonly used in aerospace engineering to produce lift is also employed in the motor sport industry to produce downforce for improving traction during cornering. This paper investigates aerofoil surface modification through ‘golf ball dimpling’, used to reduce flow separation behind a golf ball. The studies of other researchers have shown that this type of design can have a positive effect on improving aerofoil performance. However, no optimization information of dimple sizing is given in literature. Therefore, three types of dimpling sized at 5, 10 and 15 mm are applied to the surface of a NACA 6615 wing at 25% chord length from the leading edge in this study using Computational Fluid Dynamics (CFD) as an initial design process. Then a physical model, made through 3D printing additive manufacturing (AM), is tested at angles of attack (AoA) ranging from $0^{\circ }$ to $20^{\circ }$ and wind speed up to 30 m/s in a subsonic wind tunnel. Experimental and CFD results show that the smallest dimple size provides the most significant increase on lift to drag ratio at high AoA above $10^{\circ }$. This ratio increases further with the wind speed, indicating that a high AoA wing favors down force to improve drag reduction performance.

In high-density data center, energy consumption is increasing dramatically. For reducing the energy consumption, CFD software, Fluent 15.0, is used to simulate the flow and temperature field distribution with $k-𝜀$ turbulence model and fluid–solid coupling method. Fans on the back of racks are simplified as walls with a certain pressure jump. Severs are treated as solid heat sources and porous media. Simulation results reveal that the temperature distribution on the back of racks is not uniform when air conditioners are arranged face-to-face, and local high temperature points emerge near the side wall of air conditioners. Factors affecting cooling efficiency, such as location of air conditioners, speed of inlets, distance of racks, etc., need to be improved. Geometric model is optimized by using a diagonal rack arrangement and drilling holes on the side wall. Based on this, four different cases with various hot aisle distance are proposed. Single and double modular data center are both simulated. Results of new model are better than those of baseline model.

It is of great importance to reveal the refrigeration mechanism at low-temperature around 4 K in pulse tube for improvement of the cooling performance in 4 K pulse tube crycoolers (PTCs). In this paper, the thermo-physical process in 4 K pulse tube was studied analytically from microscopic viewpoint using CFD simulation. The thermodynamic cycles of different gas parcels throughout the pulse tube were investigated. It was found that the real gas characteristics of the helium at the pulse tube cold end manifest more apparently from the central part nearer to the wall outside the boundary layer. Furthermore, it was revealed that boundary layer has the weakened heat loss or helpful effect on refrigeration in phase-shifting 4 K pulse tube operating at low frequency. This research newly provides intensified understanding of the inherent characteristics of 4 K GM-type pulse tube.

In this research study, computational investigations are prepared to demonstrate the influence of the nonhomogeny magnetic source on the thermal efficiency of the convergent tube with the nanofluid flow. The major attention of our examination is to analyze the flow stream and temperature spreading on the average Nusselt number of the base fluid with Fe2O3 nanoparticles. The effects of inlet velocity and magnetic intensity on the thermal characteristics of nanofluid stream through the convergent tube are fully investigated. SIMPLEC algorithm is used for the imitation of the incompressible nanofluid flow inside the convergent tube. Our results indicate that growing Reynolds number from 50 to 100 surges heat rates up to 18% in the convergent tube because of the existence of the magnetic field in the vicinity of the tube.

The aim of this work is to examine numerically the effect of using a rotating cylinder and porous layers on the forced convection in a bifurcating grooved channel (BGC) filled with two types of nanofluids (MgO-water, SiO₂-water). The semi-implicit finite volumes method was used to solve the governing equations. The effects of Reynolds number, nanoparticles volume fraction, and cylinder rotation speed on hydro-thermal performances have been investigated. According to the obtained results, the rotation direction plays a significant role in the formation of vortices at the branching channel, such that when the cylinder rotates clockwise, the vortex occurs in the vertical channel, and it decreases with increasing Reynolds number. Besides, using BGC with a porous medium enhances the heat transfer rate by 52% and 49% at the vertical and horizontal walls of the porous layer, respectively. On the other hand, the heat transfer rate is improved by 2.6% when using MgO nanoparticles compared to SiO₂. Therefore, the use of bifurcating grooved channels can improve the thermal performance of various applications in thermal engineering, from fuel cells to electronic cooling.

A generic two-fluid (water/air) numerical model has been developed and applied for the simulation of the complex fluid flow around a wave driven rotating vane near a shoreline in the context of a novel wave energy device OWSC (Oscillating wave surge converter). The underlying scheme is based on the solution of the incompressible Euler equations for a variable density fluid system for automatically capturing the interface between water and air and the Cartesian cut cell method for tracking moving solid boundaries on a background stationary Cartesian grid. The results from the present study indicate that the method is an effective tool for modeling a wide range of free surface flow problems.

Ink-jet technology is a novel method for rapid deposition of accurately measured material with high precision. Consequently it has been used for applications such as, deposition of light emitting polymers and more recently for fabricating 3D objects and micro-mechanical structures. Ink-jet technology is also being applied to produce tactile maps for the visually impaired. The efficiency of the tactile maps, as outlined by psychophysical and cartographic studies of haptics, depends on its 3D features. To comprehend and control these features, detailed understanding of interaction amongst micro-drops, which are typically 50μm in diameter, is imperative. Multiphase interaction takes place between each liquid drop at impact with liquid or solid cured drops (deposited previously) and the solid substrate in an envelop of air. The behavior of micro-drops with regards to surface tension, drop coalescence among liquid and solid drops, drop impact kinetics, wettability, surface energy and drop spread has been analyzed using a computational model.

This paper will explain the structure of the flow induction in a non-steady supersonic fluid in which steam is the working fluid. The ratio of the throat diameter is varied and the analyses related to the induction processes are studied. This ejector is used for compression applications. The work to be presented herein is a Computational Fluid Dynamics investigation of the complex fluid mechanisms that occur inside a non-steady, three-dimensional, steam supersonic pressure exchange ejector, specifically with regard to the pressure exchange mechanisms and the induction processes between a primary fluid and a secondary fluid and how this is related to the shape of the aerodynamic shroud-diffuser surface. The results will show the correct throat diameter ratio that is capable of producing the desire affect of the flow induction in a three-dimensional supersonic, non-steady, viscous flow. The calculated throat diameter ration is about 2.90.

An application of CFD model for the simulation of a strongly swirling and high speed flow in the vortex tube is presented in this paper. A partly modified standard K-ε turbulent model has been used to investigate the flow characteristics and energy separation effect in the vortex tube. It is found that there is an obvious energy separation effect in the vortex tube and the numerical solutions of the flow and temperature fields agree well with the experiments. More detailed flow features are obtained by the CFD calculation. Based on the validated numerical model, the influence of the cold flow fraction on the energy separation effect is also investigated and compared with experimental results.

A parallel Navier-Stokes solver based on dynamic overset unstructured grids method is presented to simulate the unsteady turbulent flow field around helicopter in forward flight. The grid method has the advantages of unstructured grid and Chimera grid and is suitable to deal with multiple bodies in relatively moving. Unsteady Navier-Stokes equations are solved on overset unstructured grids by an explicit dual time-stepping, finite volume method. Preconditioning method applied to inner iteration of the dual-time stepping is used to speed up the convergence of numerical simulation. The Spalart-Allmaras one-equation turbulence model is used to evaluate the turbulent viscosity. Parallel computation is based on the dynamic domain decomposition method in overset unstructured grids system at each physical time step. A generic helicopter Robin with a four-blade rotor in forward flight is considered to validate the method presented in this paper. Numerical simulation results show that the parallel dynamic overset unstructured grids method is very efficient for the simulation of helicopter flow field and the results are reliable.

The thermal plasma conditions under a practical use could be affected by the insertion of a solid, cold probe severely. In this study, we calculated flowing thermal plasmas with two different sizes of a Langmuir probe into an argon free burning arc system and investigated the thermal and flow disturbances caused by the metallic probe. From the results, the severe disturbance of a large part of the thermal plasma, especially the axial velocity field, by the inserted probe has been found. Therefore, it might be a practical solution to use a Langmuir probe which has sufficiently small diameter for avoiding such a severe disturbance.

The flow field past the rotating blade of a horizontal axial wind turbine has been modeled with a full 3–D steady–RANS approach. Flow computations have been performed using the commercial finite–volume solver Fluent. The NREL phase VI wind turbine blade sections from the 3–D rotating geometry were chosen and the corresponding 2–D flow computations have been carried out for comparison with different angles of attack and in stalled conditions. The simulation results are analyzed. The main features of the boundary layer flow are described, for both the rotating blade and the corresponding 2–D profiles. Computed pressure distributions and aerodynamic coefficients show evidence of less lift losses after separation in the 3–D rotating case, mostly for the inward sections of the blade and the highest angles of attack, which is in agreement with the literature.

Solar panels mounted on the roof of a building or ground are often vulnerable to strong wind loads. This study aims to investigate wind loads on solar panels using computational fluid dynamic (CFD). The results show good agreement with wind tunnel data, e.g. the streamwise distribution of mean surface pressure coefficient of a solar panel. Wind uplift for solar panels with four aspect ratios is evaluated. The effect of inclined angle and clearance (or height) of a solar panel is addressed. It is found that wind uplift of a solar panel increases when there is an increase in inclined angle and the clearance above ground shows an opposite effect.

In this paper, the extreme waves were generated using the open source computational fluid dynamic (CFD) tools — OpenFOAM and Waves2FOAM — using linear and nonlinear NewWave input. They were used to conduct the numerical simulation of the wave impact process. Numerical tools based on first-order (with and without stretching) and second-order NewWave are investigated. The simulation to predict force loading for the offshore platform under the extreme weather condition is implemented and compared.

Modeling of unsteady cavitating flow is a critical issue in a lot of practical cases. The objective of this paper is to assess the practical applicability of three widely used mass transport cavitation models under RANS framework, including the Kubota model, Kunz model, and Singhal model, for predicting partial sheet cavitating flow around an axisymmetric body with hemispherical head and unsteady cloud cavitating flow around a Clark-Y hydrofoil. The results show that for the axisymmetric cylindrical body, all three cavitation models could generally predict the pressure distributions. The significant differences are found around the closure region of the attached cavity due to the magnitude and distribution of mass transfer rate. For the unsteady cavitating flow along the hydrofoil, the significant differences with different cavitation model are observed in time-averaged and time-dependent concerning the cavity shapes, multiphase structures and the cloud shedding dynamics. The Singhal model coupling the effect between the vorticity distribution and the cavity dynamics agrees best with the experimental measurements.

The objective of this paper is to numerically investigate the unsteady cavitating flow around a four-blade inducer, with focus on the cavitation instability and the flow-induced vibration characteristics. In the numerical simulation, the modified rotation/curvature correction turbulence model and the Zwart cavitation model are used for the simulation of the flow field. The tightly coupled algorithm is adopted for the precise prediction of the fluid-structure interaction, including the calculation of the hydrodynamic loads based on the multiphase fluid dynamics and the computation of the structural displacement via the Finite Element Method (FEM). The results showed that good agreement has been obtained between the experimental and numerical results. The fluctuation of cavity volume is the main cause of the change in the head of the inducer, and the backflow vortex cavitation has little effect on that at this flow condition. The backflow vortex cavity develops and rotates with the blades of the inducer, but with a much lower rotational velocity than that of the blades. The flow-induced vibration of the inducer caused by the unsteady cavitating flow mainly manifests as a first-order bending mode. The backflow vortex cavitation has a significant impact on the vibration of both the blades and the guide-water cone. Besides, a cavitation auto-oscillation at the inlet of the inducer has also been detected based on the phase correlation analysis.

The development of micropump is directly related to the popularization and application of microfluidic systems. Recently, microfluidic systems have more urgent requirements for stable operation and simple structure of micropumps. For these purposes, a novel micro steam jet pump which uses planar induction heating to evaporate the initial fluid is presented. This new micropump could generate continuous flow with merits of no moving parts, stable operation and easy integration. A negative pressure is formed flowed with high-speed flow of vapor in the chamber to pump and eject liquid, and then realize continuous pumping. The working chamber of the micropump is calculated with aerodynamic function, and the simulation analysis is carried on with COMSOL Multiphysics 5.5. By changing key working parameters, appropriate working conditions are adopted with simulation analysis. When the primary vapor temperature is 403.15 K, the inlet width of the tapered section of the mixing chamber is 0.4 mm and the outlet width of the diffuser is 0.5 mm, the mingling and enhancing pressure performance of the micro steam jet pump is better. The optimization of design and simulation involved in this paper provided a good guidance for the design and experimental study of the micro steam jet pump.

The steam condenser is a crucial component in power plants, playing a vital role in influencing the overall performance of steam power plants. This paper delves into a detailed assessment of the thermal aspects and evaluation of steam surface condensers. To conduct this rigorous evaluation, we meticulously crafted a three-dimensional (3D) model of a multi-tube single-pass counter-flow heat exchanger using ANSYS Design-Modeller. Within this model, a multiphase mixture model was harnessed to replicate the condensation process that takes place within the condenser. After the model’s development, a series of computational fluid dynamics (CFD) simulations were expertly executed utilizing ANSYS Fluent Workbench. The simulations encompassed diverse cooling water flow rates and steam inlet velocities. Notably, two sets of CFD simulations were carried out: the initial set featured a velocity inlet in the condenser, while the second set involved CFD simulations with a pressure steam inlet. The findings derived from these simulations unveiled a noteworthy correlation between the condensation rate within the shell and both the rate of circulating water flow and the operational pressure of the condenser. Additionally, it was discerned that the condensation rate could be further influenced by the specific geometry of the condenser. In sum, this study concludes that optimizing the geometrical configuration and baffle arrangements holds promise for increasing the condensation rate and overall performance of steam condensers employed within steam power plants.

Problem Statement: The hafnium particles are suspended through carrier fluid (Williamson fluid) to discuss the momentum analysis in multiphase flow in two different types of configurations.

Research gap: The analysis of the interaction of hafnium nanoparticles with the Williamson fluid model through the convergent and divergent conduits has not been discussed before.

Methodology: The equation of continuity and momentum equations are used for this analysis. The solution of both fluid and particle velocities is obtained through the perturbation analytical technique. The perturbation solution is also compared with the numerical solution.

Computational results: The Weissenberg number decays the velocity distribution. The suspension of hafnium particles updates the flow distribution through the conduits. The magnitude of the stream function decreases via the Weissenberg number.

Applications: This study can help develop a new approach to cancer therapy by using a high atomic number of nanoparticles.

Originality: This analysis is original and has neither been submitted nor published before.