Rectangular Obstacles Research Articles

Study on the hydrogen-air premixed flame propagation characteristics in semi-open space with obstacle

In semi-open space, obstacles have the potential to accelerate flame propagation and increase hydrogen-air deflagration pressure. Therefore, this paper is dedicated to exploring the influence of obstacles on the premixed hydrogen deflagration, which is crucial for enhancing the safety of industrial production and energy utilization. By using Large Eddy Simulation (LES) model in OpenFOAM, this study investigates the deflagration characteristics of premixed hydrogen in the presence of three different shaped obstacles. The analysis results reveal that under obstacle conditions, the flame shape can be categorized into four phases: the hemispherical phase, finger-shaped phase, jet phase, and vortex phase. The velocity of the flame front is nearly same for elliptical and rectangular obstacle condition, but it is 36% higher compared to triangular obstacle condition. The impact of triangular and rectangular obstacles on explosion overpressure is less than that of triangular obstacles on explosion overpressure, but it is 16% higher than that of elliptical obstacle. Analyzing the vorticity generated by different obstacle reveals that the vorticity produced by rectangular obstacle is twice as much as that produced by elliptical obstacle, whereas the vorticity produced by triangular obstacle is 2.4 times greater than that produced by elliptical obstacle. The acceleration of hydrogen-air explosion process occurs due to the narrow space created by obstacle and pipeline walls, and the shape of obstacle significantly influences this acceleration.

A novel design of inlet/outlet header structure for improving flow uniformity and performance in planar solid oxide fuel cell

The poor flow uniformity of parallel flow field in Solid oxide fuel cell limits its performance. Based on computational fluid dynamics, a three-dimensional numerical model of a solid oxide fuel cell was established to improve performance by improving flow uniformity. The influence of different number, shape and arrangement of obstacles on fuel cell is studied, and optimization is also made according to flow distribution. Results show that flow uniformity can be improved by using rectangular obstacles and arranging them at the rear of inlet header. Compared with structure without obstacles, the structure with four rectangular obstacles in inlet header can reduce flow non-uniformity coefficient by 55.8%. Optimized structure uses obstacles with different heights, and two obstacles are also set in outlet header. Compared with basic structure, the flow non-uniformity coefficient of optimized structure is reduced by 87%, while current density and net power density are increased by 10.1% and 24.2% respectively.

Simulation of permeable surfaces using the pressure–velocity jump approach: A lamellar screen upstream of a ground-mounted obstacle

Permeable surfaces are nowadays widely adopted in the construction industry, with applications ranging from wind shields for bridge decks to the external layer of permeable double skin facades. However, due to the large scale separation between the overall structure dimension and the size of the pores, their modelling in CFD simulations is still extremely challenging and over-simplified homogenized models are often used in practice. Inspired by previous studies, the authors recently proposed a generalization of the well-known pressure-jump approach which accounts for flow deflections, denoted as pressure–velocity jump, PVJ. In this study, the derivation of the PVJ approach is briefly recalled and contextualized in the existing literature. Then, we use PVJ to study the influence of a lamellar screen positioned upstream of a ground-mounted obstacle using 2D URANS. In particular, simulations are performed using the proposed PVJ approach and Explicit Models, EM, in which lamellae are explicitly modelled, for square and a rectangular obstacles. Results show a good agreement between EM and PVJ based models, confirming the high potential of the proposed technique.

Influence of location of twin-adiabatic blocks on magnetohydrodynamics-based double-diffusive convection and entropy generation for a liquid metal

This research deals with a rectangular cavity encompassing two adiabatic rectangular and impermeable obstacles at various positions. This study serves as a platform to explore the interplay between diverse flow-governing parameters, such as the buoyancy ratio (N = −1, 0, and +1), Hartmann number (Ha = 0, 50, and 100), Lewis number (Le = 1, 5, and 10), Rayleigh number (Ra = 103 and 104), and geometric arrangements of twin-blocks (C1, C2, and C3) to help in developing insights into such complex transport phenomenon driven under the influence of buoyancy and concentration. The arrangements are chosen such that C1 and C3 represent the off-center position of the first obstacle, while C2 represents the in-line position with the second obstacle. The influence of liquid sodium–potassium alloy (Pr = 0.054) on fluid flow, heat, and mass transfer, and entropy generation characteristics due to double-diffusive natural convection in the twin obstacle-filled rectangular enclosure are observed using the lattice Boltzmann method. The results reveal that the maximum amount of heat and mass transfer occurs at the C2 position, making it the most efficient for heat and mass transfer among all. In contrast, the C2 configuration is a thermodynamically inefficient arrangement as entropy generation is maximum, while the C3 configuration is obtained to be more efficient thermodynamically. Furthermore, the results reveal that the average total entropy generation is directly related to the Lewis number, while it has an inverse relation with the Hartmann number.

Experimental and numerical study of droplet generation in the normal and modified cross-junction

In this work, a visualization experimental combined with numerical approach was used to analyze the process intensification achieved by incorporating a rectangular obstacle into the downstream section of a cross-junction channel for droplet generation. A comparative experimental study was conducted to evaluate the performance of the droplet generation in the modified and normal cross-junction channel. Five typical flow regimes were observed, including squeezing, dripping, jetting, tubing and a first described tip-multi-breaking which differs from those previously reported in capillary devices. among these regimes, squeezing and dripping were found to be the preferred flow regime to generate monodisperse droplets with a variation coefficient of the droplet size less than 3 %. The presence of the rectangular obstacle in the modified channel increases the fluid resistance within the main channel, leading to a higher pressure drop before and on the obstacle, which induces the earlier appearance of necking stage and more prompt pinching-off of the thread in the modified channel. As a result, smaller monodisperse droplets with higher generation frequency were observed within a much wider distribution of squeezing and dripping regimes in the modified channel.

The Particle-Based Method Utilization on The Various Obstacles Effect to The Two-Fluids Dam Break Simulation

The Molten Salt Fast Reactor (MSFR), a prominent Generation IV reactor design, offers improved safety by eliminating graphite as a moderator and incorporating a passive safety system. Understanding the fuel salt relocation mechanism is crucial in ensuring the reactor’s safety and performance. This study presents a particle-based approach using the Semi-Implicit Moving Particle (MPS) method, which accurately represents fluid as particles carrying essential physical properties. We validate the MPS method as an alternative tool for investigating fuel salt relocation in the MSFR, comparing its results with the well-established δ-SPH method for verification. The simulations encompass scenarios with and without obstacles, involving water-PEO fluids known for their distinct densities and propensity to induce splashing phenomena. The obstacles used are made in several size variations, such as semicircular, rectangular, and triangular obstacles. The excellent alignment of MPS method results with δ-SPH method outcomes confirms its effectiveness in predicting freeze plug melting and fuel salt relocation in the MSFR, providing valuable insights into the passive safety mechanisms. This research contributes to enhancing the safety and efficiency of advanced molten salt reactor designs. Where it can be concluded that the simulation results of the MPS method are almost the same as the simulation results of the δ-SPH method.

Lattice Boltzmann method based feedback control approach for pinned spiral waves

Spiral waves are common in nature and have received a lot of attention. Spiral wave is the source of ventricular tachycardia and fibrillation, and pinned spiral wave is less likely to be eliminated than free spiral wave. Therefore, it is important to find an effective method to control the pinned spiral wave. In this work, we investigate the feedback control approach to eliminating pinned spiral wave based on the lattice Boltzmann method, by using the FitzHugh-Nagumo model as an object. The numerical results show that the feedback control method has a good control effect on the pinned spiral wave no matter whether it is pinned on a circular or rectangular obstacle. In addition, the excitability coefficient, amplitude of feedback control, recording feedback signal time and obstacle size are systematically investigated by numerical simulation. The study shows that there are three cases of pinned spiral wave cancellation. Firstly, the amplitude of feedback control and excitability coefficient are related to the time required to eliminate the pinned spiral wave, and the larger the amplitude of feedback control signal or the smaller the excitability coefficient, the faster the cancellation of the pinned spiral waveis. Secondly, the size of the obstacle and the excitability coefficient affect the time interval between the time of recording the feedback signal and the time of adding the feedback control that can successfully control the pinned spiral wave. Finally, the recorded feedback signal time affects the minimum amplitude of feedback control required to successfully eliminate the pinned spiral wave, while the added feedback control time is constant. According to the discussion in this paper, it can be seen that the feedback control method has a good control effect on the pinned spiral wave.

Investigating the temperature distribution behavior and flow parameters of argon fluid in a nanochannel with changing dimensions of the obstacle using the molecular dynamics (MD) method

This article, examines the flow of argon inside a nanochannel with respect to the molecular dynamics (MD) in the free molecular flow regime using LAMMPS software. The nanochannel is made of copper featuring a square cross-section and obstacles of varying dimensions and values. In this study, the flow of argon fluid is three-dimensional. To gain a deeper understanding of the effect of solid walls within the nanochannel and their influence on flow behavior, the research is simulated in a nanochannel with all side walls for the 3D model and without side walls for the 2D model. This research assesses the effect of the obstacles’ dimensions and values on the nanochannel wall surface and areas above the wall surface. The total dimensions of all simulated two- and three-dimensional atomic structures with a square cross-section are assumed to be 60 × 60 × 100 Å3. and the presence of square obstacles (with dimensions of 8 × 8 × 8 Å3) and rectangular obstacles (with dimensions of 8 × 18 × 8 Å3) is examined. This study seeks to understand the influence on flow behavior, temperature distribution, density, heat flux, velocity, and thermal conductivity coefficient. This study is simulated using a time step of 1 fs for 10,000 time steps, involving approximately 10,000–15,000 argon and copper atoms. The results of this research indicate that obstacles with structures of P and R and larger dimensions increase the number of solid atoms exhibiting stronger attractive forces. Compared to a smooth nanochannel, the thermal exchange between fluid and solid atoms results in a density increase of 17.5 % and 17.3 %, respectively. On the other hand, in the 3D nanochannel, the sidewalls of the nanochannel have reduced the effect of the presence of R and P obstacles with larger dimensions, which comparing to a smooth nanochannel, have increased the density by 8.21 % and 7.53 %, respectively. The obstacles with different spatial positions (P and R structures) in the two-dimensional nanochannel cause a rise in the thermal conductivity coefficient. The P structure obstacles have a better effect on the thermal conductivity coefficient in the 2D nanochannel compared to the R structure. In the three-dimensional nanochannel, utilizing smaller obstacles proves to be more effective because it results in better atom distribution or temperature distribution due to increased atomic collisions in the central region compared to the wall regions.

Hydraulic Behavior of Weir-Gate Structure with Rectangular Side Obstacle under Free and Submerged Flow Conditions

Hydraulic Behavior of Weir-Gate Structure with Rectangular Side Obstacle under Free and Submerged Flow Conditions

The Effect of Obstacle Number, Shape and Blockage Degree in Flow Field of PEMFC on its Performance

<p>Proton exchange membrane fuel cell (PEMFC) has received extensive attention as it is the most common hydrogen energy utilization device. This research not only investigated the effect of obstacle number and shape on PEMFC performance, but also studied the effect of the blockage degree in the channel of PEMFC on its performance. It was found that compared with traditional scheme, longitudinally distributed obstacles scheme can significantly promote reactants transfer to catalyst layer, and the blockage degree in the channel effect PEMFC performance most. The scheme with 10 rectangular obstacles in single channel and 60% channel blockage had the best output performance and the most uniform distribution of reactants and products. Obstacle height distribution can significantly affect PEMFC performance, the blockage degree in the whole basin was large, particularly as the channel was blocked to higher degree in region 2 and region 3, higher net power density and better mass transfer effect can be obtained. Among them, the fuel cell with the blockage degree of 40%, 60% and 60% in region 1, region 2 and region 3 have the best PEMFC output performance and mass transfer, the net power density was 29.8% higher than that of traditional scheme.</p>

A study on the effect of non-uniform spacing on the performance of forced convection cooling of discrete heaters: A numerical investigation

A numerical simulation of forced convection conjugate heat transfer has been performed for flow over discrete, volumetrically heated rectangular obstacles mounted on the lower wall of a horizontal channel. Non-uniform spacing between the heaters is investigated with the help of isotherms, streamlines, heatlines and entropy generation along with cup-mixing temperature by fixing the geometric parameters for flow Reynolds number between 100 and 1500 (100≤ReL≤1500) and a fixed thermal conductivity ratio of 10 (ks/kf). To quantify heat transfer, local and average Nusselt number have been used. Pressure drop is calculated with the help of the apparent friction factor. Together, these two parameters provide the quality factor, which measures the performance of the discrete heaters. It has been found that the effect of spacing variation is not limited to the proximal heaters. Irregular behaviour in heat transfer has also been noticed in the results. As we move downstream of the flow, the heat transfer will decrease; however, there are a few particular cases where it was noticed that the performance of the last discrete heater is better compared to the heater just upstream of it.

Attractors for a fluid-structure interaction problem in a time-dependent phase space

This paper is concerned with the long-time dynamics of a fluid-structure interaction problem describing a Poiseuille inflow through a 2D channel containing a rectangular obstacle. Physically, this models the interaction between the wind and the deck of a bridge in a wind tunnel experiment, as time goes to infinity. Due to this interaction, the fluid domain depends on time in an unknown fashion and the problem needs a delicate functional analytic setting. As a result, the solution operator associated to the system acts on a time-dependent phase space, and it cannot be described in terms of a semigroup nor of a process. Nonetheless, we are able to extend the notion of global attractor to this particular setting, and prove its existence and regularity. This provides a strong characterization of the asymptotic behavior of the problem. Moreover, when the inflow is sufficiently small, the attractor reduces to the unique stationary solution of the system, corresponding to a perfectly symmetric configuration.

Subgrid-Scale Models for Predicting Premixed Methane–Air Flame Propagating in a Chamber with a Rectangular Obstacle

Subgrid-Scale Models for Predicting Premixed Methane–Air Flame Propagating in a Chamber with a Rectangular Obstacle

Numerical Investigation of the Effects of Rectangular Obstacles on the Thermal-Hydraulic Properties of a Circular Corrugated Channel

Bu çalışmada dairesel oluklu bir kanalın alt yüzeyine farklı açılar ve yüksekliklerde yerleştirilen dikdörtgensel engellerin kanalın ısıl-hidrolik özelliklerine olan etkisi sayısal olarak incelenmiştir. Analizler için 500x10 mm boyutlarında üç bölümden oluşan (yukarı akış, oluklu ve aşağı akış) bir kanal kullanılmıştır. Engeller kanal yüzeyine üç farklı açı (β=45°, 90° ve 135°) ve üç farklı yükseklikte (h/H=0,1, 0,25 ve 0,5) yerleştirilmiştir. Süreklilik, momentum ve enerji denklemlerinin çözümleri k-ε türbülans modeli kullanılarak Ansys-Fluent sonlu hacimler yöntemi ile gerçekleştirilmiştir. Akış alanına ait türbülans kinetik enerji (TKE) konturları ile ortalama Nusselt sayısı (Nu), sürtünme faktörü (f), basınç düşüşü (ΔP) ve performans değerlendirme kıstas sayısı (PEC) değerleri Reynolds sayısının (Re) 5000-20000 aralığı için elde edilmiştir. En yüksek termal-hidrolik performans h/H=0,1, β=45° kanal modeli için elde edilmiştir. Bu durum PEC değerlerine göre en düşük performansa sahip h/H=0,5, β=135° modele kıyasla %26,19 daha fazladır.

Визуализация тепловых потоков в высокоскоростном течении за ударной волной

The paper presents the thermographic studies of unsteady heat fluxes behind a plane shock wave in the rectangular 24x48 mm shock tube test section. Consecutive panoramic visualization of the heat fluxes plots on streamlined walls after the plane shock wave interaction with a rectangular obstacle fixed on the channel wall are obtained. The duration of the recorded thermal processes is up to 40 milliseconds after the shock wave passage. The heating and cooling of the test chamber walls streamlined by supersonic flow are visualized using the Telops FAST M200 high-speed infrared camera (operating range 1.5 – 5.1 microns) through the quartz windows transparent to infrared radiation. Visualization of the thermal fields were compared with the shadow images and results of 2D numerical simulation of a nonstationary gas dynamic process after the diffraction of a shock wave with Mach numbers M=2.0-4.5.

Water wave scattering by pair of porous barriers in the presence of a bottom-standing rectangular obstacle

ABSTRACT Oblique wave scattering by a pair of partially immersed porous barriers, placed (i) on one side of (Position I), (ii) just above (Position II) and (iii) on the two sides (Position III) of a thick rectangular bottom-standing rigid obstacle, is studied here. The mathematical formulation and solution procedure of the corresponding boundary value problem are discussed using Havelock's expansion and Galerkin's approximation technique. During the solution procedure, a set of first-kind Fredholm-type integral equations is solved approximately by choosing suitable basis functions (Chebyshev and Gegenbauer polynomials). To address the singularities of order half and one-third that appear at the submerged edges of the thin barriers and the corners of the thick obstacle, appropriate basis polynomials with suitable weight functions are chosen in Galerkin's approximation. The reflection and transmission coefficients, dissipating wave energy, and dimensionless wave force are computed for various values of different parameters and depicted graphically in quite a number of figures. The numerical results for some special cases are compared with those available in the literature and excellent agreement has seen to have been achieved thus validating the correctness of the numerical method presented here. Based on numerical results it is observed that Position III of the barriers is more effective than Position I and Position II in resisting wave loads on marine areas and attenuating wave energy.

Shape of an obstacle affects the mediolateral trajectory of the lower limb during the crossing process.

In previous studies involving obstacle crossing, vertical foot clearance has been used as an indicator of the risk of contact. Under normal circumstances, individuals do not always cross over obstacles with the same height on both sides, and depending on the shape of the obstacle, the risk of contact may differ depending on the foot elevation position. Therefore, we investigated whether task-related control of the mediolateral foot position is adapted to the shape of the obstacle. Sixteen healthy young adults performed a task in which they crossed over two obstacles with different shapes while walking: a trapezoidal obstacle and a rectangular obstacle, as viewed from the frontal plane. It was shown that when crossing over a trapezoidal obstacle, the participants maintained foot clearance by controlling the mediolateral direction, which chose the height that needed to be cleared. The results of this study suggest that the lower limb movements that occur during obstacle crossing are controlled not only in the vertical direction but also in the mediolateral direction by adjusting the foot trajectory to reduce the risk of contact. It was demonstrated that control was not only based on the height of the obstacle directly under the foot but also in the foot mediolateral direction, considering the shape of the entire obstacle, including the opposite limb.

Thermalhydraulic assessment and design optimization of incorporating flow obstructors in a supercritical minichannel heat sink

Modulation of the thermalhydraulic characteristics of the miniaturized heat sink by geometric alteration is a less explored topic, particularly in conjunction with the supercritical fluids, which can experience both enhancement and deterioration in performance based on the conditions. The present study appraises the response of a square minichannel by incorporating rectangular obstructions, with the primary aim of identifying an optimized orientation and design. A performance evaluation criterion, defined as a combined change in both heat transfer coefficient and pressure drop because of geometric modification, is selected as the primary variable. Installation of the baffles substantially augments the turbulent mixing among the fluid layers, realizing a lower wall-to-fluid temperature differential and enhanced heat transfer coefficient. A relatively lower level of temperature is maintained within the baffled section, thereby delaying the appearance of heat transfer deterioration. Obstructions, however, considerably increases the pressure losses, necessitating careful selection of the dimensions. The use of three pairs of baffles has been identified to be the most feasible option. Desirable values of the height, thickness, and inclination angle for individual plates in this orientation have also been identified, along with a detailed thermalhydraulic assessment about the role of the operating variables with the optimum design.

Lattice Boltzmann Computation of Steady Cross-Flow Across a Rectangular Obstacle with Different Aspect Ratio: Effect of Blockage Ratio

This work presents a two dimensional lattice Boltzmann analysis of steady and cross-flow of New tonian fluid across a built-in rectangular cylinder. In particular, the effects of the blockage ratio and aspect ratio of rectangular cylinder (width/height) on the momentum characteristics have been explored for range of flow governing parameters such as, blockage ratio (β = 1/8, 1/12, 1/16), aspect ratio of rectangular cylinder (1 ≤ a_r ≤ 6) at constant Reynolds number of Re = 40 corresponding to the laminar range. The physical insight of system is gained by evaluation of stream-function, vorticity and pressure coefficient variation, etc. Further, the engineering gross parameter, such as drag coefficient is determined for possible use in engineering design purpose. It is observed that the increase in blockage ratio drag coefficient values decreases and drag values show proportional variation with aspect ratio. Finally, a closure relationship is developed between drag coefficient, blockage ratio and aspect ratio of rectangular cylinder for possible use in engineering/scientific practices

Tsallis Entropy applied to microfluidic channels analysis

This work investigated the possibility to describe the fluid flow in a microchannel from a thermodynamic point of view, exploring the possibility to evaluate the presence of obstacles (or, more in general, geometry imperfection) and their influence on the fluid. Tsallis entropy concept was employed. This form of entropy was introduced in 1988 by Costantino Tsallis as a basis for generalizing the standard statistical mechanics and as a generalization of the standard Boltzmann-Gibbs entropy. Inspired by nature, where storing information is an intrinsic ability of natural systems, here we investigate the capability of interacting systems to transport/store the information generated/exchanged in the interaction process in the form of energy or matter, preserving it over time. In detail, here we test the possibility to consider a fluid as a carrier of information, speculating about how to use such information. The final goal is to demonstrate that information theory can be used to illuminate physical observations, even in those cases where the equations describing the phenomenon under investigation are intractable, are affected by a budget of uncertainty that makes their solution not affordable or may not even be known. In this exploratory work, an information theory-based approach is applied to microfluidic data. In detail, the classical study of the fluid flow in a microchannel with an obstacle of a specific geometry is faced by integrating fluid mechanics theory with Tsallis entropy. Technically, computational fluid dynamics simulations at Reynolds’ numbers (Re) equal to 1 were carried out in fluidic channels presenting a rectangular obstacle and on the simulated flow fields.