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Experiments and numerical simulations were conducted to study the effects of obstacle shape and arrangement on the flame acceleration and deflagration-to-detonation transition (DDT) in an obstructed channel filled with a stoichiometric hydrogen–oxygen mixture. Two different shapes and arrangements of fence-type obstacles with the same blockage area were considered: continuous triangular and discrete rectangular obstacles. In the experiments, high-speed schlieren photography was used to record the flame acceleration and DDT process with a blockage ratio of 0.5. In the calculations with the different blockage ratios, a high-order numerical algorithm was used to solve the multidimensional, fully compressible, reactive Navier–Stokes equations coupled to a calibrated single-step chemical-diffusive model. The quantitative agreement between the calculations and experiments allows a detailed analysis of the DDT mechanism and the influence of obstacle shape/arrangement using the numerical results. The results show that the differences in the flame acceleration and distance to DDT between the triangular and rectangular obstacle arrays are closely related to the blockage ratio. For a low blockage ratio (b r < 0.5), flame accelerates faster and DDT occurs in a shorter time in the channel with the triangular obstacles because the inclined surfaces of the triangular obstacles allow stronger flame-vortex interactions and are more favorable to detonation survival once a detonation is initiated in the gap between obstacles. Nevertheless, for a high blockage ratio (b r > 0.5), the lateral triangle surfaces become steeper, and the advantages of triangular obstacles for promoting DDT become less effective. In addition, the triangles have a more significant weakening effect on the leading shock, while the rectangular obstacles allow the formation of Mach stems. These effects make the rectangular obstacles with a high blockage ratio more conducive to the occurrence of DDT than their triangular counterparts. [ABSTRACT FROM AUTHOR]

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The number of urban road tunnels which had effect ventilation to remove smoke from tunnel fire had increased in China. In this paper, a 3D numerical simulation model of urban road tunnel fire was developed to investigate the impact of block transportation on buoyancy flows. The heat release rate(HRR) is 10MW, 20mw, 40mw when the Fire Dynamic Simulation(FDS6.0) is used. The results of FDS simulations showed that the effect of longitudinal velocity on flame length(Lf) should not be ignored when the velocity is small in the models with small Br. Longitudinal ventilation promote combustion to some extent, when the velocity is less than 1.0m/s. Temperature of car behind fire source with a 10MW HRR is not too high to reach the ignition point of car when the distance between vehicles is 5m, but it is still too high for rescue. It is concluded that the temperature distribution is related to the longitudinal blockage. Further studies are needed to analysis the temperature of downstream car to avoid a second fire. [ABSTRACT FROM AUTHOR]

The aim of this study was to examine the propagation characteristics of liquified petroleum gas (LPG)/air explosion. As such, the flame propagation process and pressure parameters of LPG explosion with an equivalent ratio of 1 were obtained using horizontal, long straight pipes and a high-speed camera. The large eddy simulation and power-law flame model were used to reproduce the simulation results, and the morphological evolution of the flame front during the explosion process was analyzed. The results showed that as the blockage ratio gradient increased, the time required for the flame to propagate from the electrode to the end of the pipe increased. Meanwhile, a flocculent flame formed at the end of the pipeline with a blockage ratio of 91%. Additionally, the maximum flame propagation speed changed in the same way as the blockage ratio. The gradient change of the barrier blockage ratio considerably influenced the pressure kinetics characteristics of LPG explosion: the maximum explosion pressure increased first and then decreased, the time to reach the maximum explosion pressure increased, and the deflagration index increased first and then decreased. The chemical kinetics analysis of premixed gas showed that when propane, n-butane, and isobutane were present in the premixed system, some of the elemental reactions that promote n-butane consumption inhibited propane and isobutane consumption. Meanwhile, the formation and consumption rate of H*, O*, and OH* from LPG during the explosion process was less than that of single-component combustible gas except propane. [ABSTRACT FROM AUTHOR]

Experimental and numerical investigations were performed to determine the pressure distributions and the drag forces on a passenger car model. Experiments were carried out with l/5th scale model FIAT Lineafor 20% and ~1% blockage ratios in the Uludag University Wind Tunnel (UURT) and in the Ankara Wind Tunnel (ART), respectively. Computational fluid dynamics (CFD) analysis for 115th scale model with 0%, 5%, and 20% blockage ratios was performed to validate various blockage correction methods supplementary to the experimental results. Three-dimensional, incompressible, and steady governing equations were solved by STAR-CCM+ code with realizable k-e two-layer turbulence model. The calculated drag coefficients were in good agreement with the experimental results within 6%. Pressure coefficients on the model surfaces have shown similar trends in the experimental and numerical studies. Some of the existing blockage correction methods were successfully compared in this study and predicted drag coefficients were within ± 5%. The authors propose the continuity and the Sykes blockage correction methods for passenger car models because they are very simple and practical and they can be used economically for engineering applications. [ABSTRACT FROM AUTHOR]

This work lies within the scope of a study aiming the optimization of the parameters of operation of the industrial burners with non premixed turbulent flame configurations. The developed example is interested particularly in the study of the effect of the velocity ratio (Vfuel/ Vair) on the emissions of the pollutants specie NOx . The study is undertaken on a standard burner “bluff body” of selected sizeThe configuration is applied to methane/Air combustion tree variables of velocity ratio (VR) as 2.8, 1.4 and 0.84. The results obtained for various operating modes from the burner show that the structure of the formation of pollutants specie, the stability and the shape of the flame in the combustion chamber depend amongst other things on the speed ratios and the rate on blocking on the flame. [ABSTRACT FROM AUTHOR]

Flow separation is one of the major distinguishing phenomena between the potential flow and actual fluid flow and circular cylinder has long been considered as the simplest form of bluff body for analyzing flow separation characteristics. Here in this article, different flow separation phenomena such as onset of separation, angular position of separation point, size of separation bubble, drag due to the separation and effect of blockage ratio on position of separation point for steady flow over a smooth circular cylinder are studied numerically using finite volume method of solving the Navier-Stokes equations at very low Reynolds number up to 50. The trends of change of these phenomena with increasing Reynolds number are expressed in form of some empirical equations and compared with results of previously published literatures. The separation starts at Reynolds number as low as 2.0 and the starting-separation angle is 146.557 o from the upstream stagnation point and with increasing Reynolds number, the point of separation travels upstream while the separation bubble length increases with increasing Reynolds number. Profound effect of the blockage ratio on the location of separation point is evident in this numerical analysis. [ABSTRACT FROM AUTHOR]

Abstract: Based on the basic conservation equations, k − ɛ turbulence model, Zimont premixed combustion model and SIMPLE algorithm, the premixed methane-air flame propagating velocity and its pressure were numerical simulated in a half open square tube with three forms of solid obstacles. Simulation results showed that: (1) At the same blockage ratio of 40%, the triangular prism obstacle was beneficial to the flame and airflow accelerating processes, and its propagating velocity and deflagration pressure induced by obstacle were slightly larger than cuboid and cylinder obstacles. (2) The obstacles revealed the block effect which led to the reduced trend of flame speed before the flame front has arrived at the different obstacles, and the flame propagating velocity was enhanced rapidly in the process of the interaction between the flame and obstacles, and then the speed began to decline. But, the flame propagating pressure was increased quickly before the premixed flame interacts with the different obstacles, and then the pressure began to decline. (3) The time of the maximum premixed flame peak-pressure stayed prior to the time of the maximum flame propagating velocity under three solid obstacles. The numerical simulation study can be acted as a qualitative analysis for premixed methane-air flame propagation in the half open tube, and can provide the theoretical basis and technical guidance for preventing and controlling the gas explosion induced by obstacles in coal mines. [Copyright &y& Elsevier]

Flows past two tandem cylinders of different diameters placed centrally in a channel with fixed centre-to-centre spacing 6D and diameter ratio γ =1.5 are simulated based on the Lattice Boltzmann Method (LBM). In all the simulations, the diameter of the smaller cylinder is chosen as the characteristic length. The Reynolds number based on the average inflow velocity is 20-120 and studies are over the range of blockage ratio 2-8. In both Small-Big Arrangement (SBA) and Big-Small Arrangement (BSA), the effects of the channel width and Reynolds number on the flow structures and force coefficients are studied. Results show that the flows in BSA are more regular than those in SBA for the same flow fields. In BSA with 4D ≤ H ≤ 8D and Re =100, the force coefficients all fluctuate with constant amplitudes and a coupled frequency, the coupled frequency becomes small as the blockage ratio decreases and by an exact test we give out the relation of the blockage ratio and Strouhal number. As the blockage ratio decreases to 2, there exist pitchfork bifurcations in both SBA and BSA, and results show that the critical Reynolds numbers of pitchfork bifurcations for SBA and BSA are both between 60 and 80. In SBA with Re = 80, the flow structure has a static asymmetric mode. It is found that the channel width has also an effect on the critical spacing where the flow changes from single body mode into co-shedding mode. By an accurate survey on flows past two cylinders with equal diameters placed inside a channel with the width, the relation between channel width and the critical spacing is given and results show that the critical spacing increases as the channel width increases. [ABSTRACT FROM AUTHOR]

The flow around a square cylinder confined in a two-dimensional channel is investigated for Re = 100 using the lattice Boltzmann method (LBM). Three blockage ratios are considered. The effects of channel walls on drag and lift coefficients of cylinder, Strouhal number and vortex structures in the wake of the cylinder are examined. The variations of the coefficients and the Strouhal number with the blockage ratio are presented. It is found that both the drag coefficient and the Strouhal number increase with the blockage ratio while the lift coefficient decreases. The drag coefficient increases as much as 30% compared to that for an unconfined square cylinder. The results are compared with related experimental data in literature and a good agreement is achieved. The comparison also shows that the LBM has capability in simulating bluff body flows. [ABSTRACT FROM AUTHOR]

Abstract: The maximum water surface change just upstream of a bridge is termed the afflux in hydraulic engineering. An initial attempt at using the lattice Boltzmann method (LBM) to study afflux has been made in this paper. The LBM representing the 2D nonlinear shallow water equations (LABSWE) is applied to both single-span and multi-span bridges with a free water surface without introducing additional empirical coefficients. For a single-span bridge, three configuration parameters, blockage ratio, skewness and eccentricity, are investigated by the LBM; for a multi-span bridge, the influence of pier shape and skewness on afflux is also tested. The results agree closely with those of traditional methods, which illustrates that LABSWE is not only able to compute average afflux but also to show the afflux distribution across the section and the flow distribution around the obstruction. [ABSTRACT FROM AUTHOR]

Cross-ventilation is the most effective mode of wind-driven natural ventilation but completely depends on the external openings and internal layout. This study investigates how openings affect cross-ventilation of a generic four-wall space using Computational Fluid Dynamics (CFD) simulation. The building has one to four external openings of two different sizes, and the interior contains various configurations of vertical and horizontal walls with blockage ratios varying between 5% and 20%. Two-opening configurations proved to be tremendously efficient for cross-ventilation; for all similarly sized openings, the two-opening scenario doubles the ventilation rate of the single-opening scenario. Using a larger opening on the windward or leeward surface, the two-opening configuration achieved higher ventilation rates and better indoor wind circulation than both the three- and four-opening configurations. In turn, to enhance cross-ventilation in three- and four-opening configurations, more than one of the openings needs to be larger than the rest. The presence of internal walls always led to smaller ventilation rates than no internal walls; such reduction steadily increased with internal blockage ratio. The reduction in ventilation rates can be minimized by adopting several narrow internal walls instead of one wide vertical wall and positioning horizontally-oriented walls above the opening height. • Cross-ventilation of a building with various external and internal openings was modeled via CFD simulation. • The two-opening configuration and variants with a large opening are efficient for cross-ventilation. • Three- and four-opening configurations need more than one large opening to enhance cross-ventilation. • Cross-ventilation rate steadily decreases as internal blockage ratio increases. • Narrow vertical internal walls and horizontal ones above the opening height limit the decrease in ventilation rate. [ABSTRACT FROM AUTHOR]