Blockage Ratio Research Articles

Flame acceleration process of premixed hydrogen in confined space with different obstacle shapes

Obstacles can greatly stimulate the detonation power of gas, and have great research value in industrial safety and energy utilization. Thus, in this work, the combustion characteristics of hydrogen under different shape obstacles are investigated. The important parameters for the evaluation of the gas combustion characteristics are analyzed, including the flame structure, flame propagation velocity and explosion overpressure. The results show that it is difficult for the flame front to recover to the regular front after passing through grid-type obstacles with many voids. To further understand the influence of the obstacle shape on the flame structure evolution, the change in the flow field before and after the flame approaches the obstacle is analyzed. The results show that the flow field formed by the combustion products can be seriously unstable after the flame front passes through grid-type obstacles, resulting in a severe distortion of the flame front. Moreover, the influence of the obstacle shape on the flame propagation velocity under different blockage ratios is compared. The results show that the single-type obstacle has a better effect on the acceleration of the flame, and it is more sensitive to the increase in the blockage ratio. The grid-type obstacle can improve the sensitivity to the blocking rate by reducing the void area. Furthermore, by analyzing the pressure growth rate, it is found that the influence of the obstacle shape on the explosion overpressure under different blockage ratios is the same as that of the flame propagation velocity. This work can provide some guidance for industrial explosion prevention and control and energy power system design.

CFD Simulation of Heat Transfer Characteristics in a Rectangular Duct with Trapezoidal Turbulence Promoters

The heat transfer rate from the hot surface to the coolant flow increases by placing the ribs on the surface of the coolant flow passage. The effect of turbulence promoters with different angle of attack and with a fixed tapered angle on the heat transfer characteristics in a rectangular stationary duct is considered for this study. Here turbulence promoters increase the heat transfer in the duct. The examination has been conducted on the rectangular smooth duct and rectangular duct with inverted V-shaped ribs of a different angle of attack. The rib angle attack of 45º, 60º, 90º and tapered angle 20º (fixed) on the top surface of the rib are considered. The rib blockage ratio and pitch ratio are 0.0553 and 10 respectively. The heat transfer characteristics are compared with Reynolds number of 6000. The heat transfer and computational flow analysis are conducted by k-epsilon turbulence model in ANSYS FLUENT. The tapered angled cross-sectional inverted V-shaped ribs of 60º shown high average heat transfer coefficients among the 45º, 60º and 90º smooth duct.

Optimization of Screen-Hole-Clearing Devices for Mechanized Residual Film–Impurity Separation

The airflow velocity in some nozzles is low, and the clearing of the nozzle is ineffective because of unreasonable airflow pipe arrangements and the distance from the nozzle to the screen surface of screen-hole-clearing devices for trommel-sieve-type residual film–impurity wind separators. In the present study, the main structure and working parameters affecting the screen hole clogging situation were determined through theoretical analysis and computational fluid dynamics simulations. In addition, a three-factor, three-level quadratic regression orthogonal center of rotation combination test was performed. The distance from the nozzle to the screen surface, fan wind speed, and the number of airflow pipes were selected as test factors, and the ratio of impurities in the residual film and the blockage ratio of the screen holes were selected as the evaluation indexes. The results indicated that the ratio of impurities in the residual film was reduced by 2.42% and the blockage ratio of the screen holes was reduced by 1.92% at a nozzle-to-screen distance of 102 mm, a fan wind speed of 24 m/s, and with four air pipes. The resulting impurity ratio in the film was 5.86%, and the blockage ratio of screen pores was 5.41%. The minimum airflow velocity of 15.8 m/s at each nozzle position of the optimized screen-hole-clearing device satisfied the requirements of screen hole clearing and blockage. Furthermore, the ratio of impurities in the residual film and the blockage ratio of the screen holes remained unchanged during the continuous operation of the device. This indicated that the optimized screen-hole-clearing device had a stable working performance. This study may provide a theoretical framework for the future development of screen-hole--clearing devices.

Harvesting airflow energy from circular cylinder wake via a thin polyvinylidene fluoride film

To improve the characteristics of airflow energy harvesting, this paper presents a new piezoelectric wind energy harvester (PWEH), in which a polyvinylidene fluoride (PVDF) film was directly located in the wake of the circular cylinder. Four important parameters were independently varied for detailed experimental investigations: wind speed, spacing ratio for the distance between the center of the circular cylinder and the tip of the PVDF film to the circular cylinder diameter, diameter of the circular cylinder, and load resistance. The results indicated that the designed PWEH performed optimally with the occurrence of resonance, maintaining a relatively high output when significantly exceeding the resonance wind speed. The PVDF film inhibited the vortex shedding of the circular cylinder, which weakened with increasing spacing ratio. The optimum value of the spacing ratio should be approximately 2.5, which yields an effective coupling between the vortex and structure. The diameter of the circular cylinder played an important role in determining the output power. The match between the load resistance and PWEH also contributed to the high-level performance. Moreover, with an increase in the diameter of the circular cylinder, the increasing blockage ratio unexpectedly enhanced the vortex shedding frequency and decreased the theoretical resonance wind speed, which provided a new orientation for improving the performance of such PWEHs.

CFD analysis of turbulent forced convection over a spinning cylinder in an enclosed channel

• More practical scenario of speed ratio has been used in this study • Blockage ratio affects both the flow and thermal fields • Rotational velocity and direction do not affect the flow or thermal field • Bulk flow Reynolds number influences both thermal and flow fields • Pressure drag dominates over viscous drag at higher Reynolds number A CFD approach has been used to analyze fully developed turbulent flow and heat transport over a spinning hot circular solid cylinder inside an enclosed channel. The finite element approach is used to solve the Reynolds-Averaged Navier-Stokes and energy equations and the shear stress transport model. The flow is explored for various Reynolds numbers ranging from 3 × 10 3 to 10 7 with the blockage ratios of 0.05, 0.1, and 0.15 along with the variation of speed ratios of the rotating cylinder. This study found that the direction and speed of the cylinder have an insignificant effect on thermo-fluid characteristics. On the other hand, the drag coefficient decreases while the average Nusselt number increases to a specific value of Reynolds number and then becomes almost constant. Effectiveness increases for a lower Reynolds number and then becomes stable. The heat performance of the rotating cylinder for the stationary one and the contributions of pressure drag are presented here.

Numerical Investigation on The Effect of Grille Blockage Ratio on Air Flow Characteristics of Air Vents

Vents with louvers are an important component in the indoor heating, ventilation, and air conditioning (HVAC) system in providing a degree of freedom to the occupants to direct the air flow and prevent foreign objects from entering or exiting the air ducts. As a result, whether designing air ventilation or any duct design that involves louvers, the influence of louver design cannot be ignored. For vent design, aside from louver angle, blockage ratio is an important factor in air distribution due to its effect on pressure drop and flow distribution to indoor space. The blockage ratio is often described as the ratio of the projected area of the structure in flow direction to the cross sectional area of the domain around the structure. The purpose of this study is to investigate the effect of varying blockage ratios on the air flow characteristics of air vents including velocity and pressure drop. The chosen air vent model for this study is the Proton Wira air vent. Results are obtained using computational fluid dynamics (CFD) analysis and by utilizing the free and easily available open source software via Code-Saturne. The parameters were set according to available literature. The study finds that the pressure drop increases with the increasing blockage ratio. The velocity drops almost 7% for blockage ratio greater than 3 and the pressure drop increases more than 4%.

Investigation on the characteristics of fire burning and smoke spreading in longitudinal-ventilated tunnels with blockages

The present study investigated the ventilation and blockage effect on fuel burning rate, the maximum temperature, critical velocity, and smoke back-layering length in longitudinally ventilated tunnels. Quantitative analysis was carried out by analyzing massive experimental measurements from the published literature. Three typical blockage modes were extracted to illustrate the blockage effect. Results indicated that fuel burning rate exhibited different responses towards the longitudinal ventilation, wherein burning rate of acetone, gasoline, ethanol, heptane, and wood crib increased when the wind velocity increased from 0.0 m/s to 3.0 m/s while burning rate of methanol pool fire firstly decreased and then increased. Prediction of the maximum temperature in tunnels without blockage by using previous correlations agreed well with the literature data. However, the maximum temperature was poorly estimated when the blockage effect was introduced. Modified correlations were thus established considering different blockage ratios. Meanwhile, empirical formulae to calculate the critical velocity and smoke-back-layering length in tunnels with and without blockages were also proposed, presenting good agreement with the measurements from previous literature.

Flame acceleration and DDT in a channel with fence-type obstacles: Effect of obstacle shape and arrangement

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 (br<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 (br>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.

Laboratory Research on Hydraulic Losses on SHP Inlet Channel Trash Racks

There is currently a growing trend towards renewable energy sources, which are characterised by a guaranteed power supply and low failure rate. Hydropower plants (small or large) are an example of such a source. They supply a total of 16% of the world’s electricity. The advantages of a small hydropower plant include the relatively simple construction process and the lack of need for upstream water storage. SHPs are one of the most cost-effective and environmentally friendly energy technologies, which is why they are steadily increasing in popularity. One of the important components of SHPs are the trash racks in the inlet channels. Their main purpose is to catch debris and other elements carried downstream and to prevent these pollutants from reaching the turbine units. They can also protect migrating ichthyofauna such as larger fish. If trash racks are installed in the inlet channel, hydraulic losses are to be expected due to the reduction in the flow cross-section through the racks (bars) themselves and through the accumulation of debris and various types of trash on these racks. Energy losses on the trash racks affect the financial aspect of SHP investments. This paper presents the results of laboratory tests on trash racks for SHPs by taking into account the different shapes of the bars used, their number and spacing, and the angles of the trash racks to estimate the hydraulic losses on the trash racks. The measured values of hydraulic losses Δh on the trash racks varied according to the type of trash racks, the density of the bars in the cross-section, and the angle of the trash racks from the horizontal, reaching the highest values on the trash racks with angle bars (AB). They were almost eight times greater than those recorded on cylindrical-bar (CB) trash racks, although they involved different angles. It was shown that the discrepancy in the magnitude of losses on trash racks can be large, even for the same type of trash racks. It depends significantly on the design (shape and bar spacing) of the trash racks and the way the trash racks are installed. Depending on the inclination angle, the increase in energy losses reached 70% for angle bars, 60% for flat-bar trash racks, and almost 40% for cylindrical bars. The values of energy loss as well as the loss coefficient β varied non-linearly for the different bar types depending on the angle of inclination of the gratings, and the degree of this non-linearity depended on the type of bars and the blockage ratio of the section. The presented research results can be useful both during the design and the operation of an SHP.

Influence of passive wake control on thermal-hydraulic performance of a cylinder confined in wavy channel under high blockage ratios

Influence of passive wake control on thermal-hydraulic performance of a cylinder confined in wavy channel under high blockage ratios

Flame acceleration and onset of detonation in inhomogeneous mixture of hydrogen-air in an obstructed channel

Flame acceleration (FA) and deflagration-to-detonation transition (DDT) in combustible premixed gases is a complex process involving the flow and combustion phenomena. Practically, the combustible premixed gases are often inhomogeneous. The numerical study aims to explore the effects of different solid obstacle distribution patterns on the FA and DDT process in the inhomogeneous concentration field. Results show that the shape of detonation wave is arc-shaped due to the inhomogeneous hydrogen concentration, and the detonation initiation process can be classified into three types: i) detonation induced by the coupling of the flame surface and the high-pressure zone; ii) detonation induced by the interaction between flame surface and reflected shock wave; iii) detonation induced by the interaction between the oblique shock wave or Mach stem and obstacles in the flow field. Also, this study finds that the flame evolutions have experienced four velocity augmentation regions before the detonation initiation. Further, the rich hydrogen content in the aperture and the higher unilateral blockage ratio can both promote flame acceleration, and further shorten the run-up time of the DDT.

Spatial effect of a vertical multi-faceted rib on thermal performance and flow characteristics in a square channel

A multi-faceted vertical rib has been numerically investigated using the internal flow of rib in a rectangular channel method under turbulent flow for 17,000 &lt; Re &lt; 27,000. The vertical rib that can be encountered inside porous media design often creates a persisting issue of high-pressure drop. The challenge frequently intensifies, mainly when a two-phase state heat transfer is introduced inside the porous media. The vertical multi-faceted rib models the various surfaces often encountered inside the porous media. The effect of blockage ratio on the flow characteristics and thermal performance is observed. The results are compared to the more conventional circular rib, equal in surface area to the multi-faceted rib. The multi-faceted rib yields a higher average Nusselt number of approximately 33% and contributes to the higher average Nusselt number of the channel. The multi-faceted rib channel has a better thermal performance factor, in average of approximately 13% higher and lower pressure drop than the circular rib channel. The advantage of the multi-faceted rib is attributed to, surprisingly, the poor flow attachment on the rib surface, which results in a low Cf/ f ratio. Consequently, the multi-faceted rib suffers from very low velocity on the downstream rib surface. Ultimately, the spatial effect of the rib inside the square channel can be evaluated using the Cf/ f ratio, which takes into account the rib surface and channel walls. In the quasi two-phase state, the circular rib is observed to provide a more favorable condition for latent heat transfer under extreme condition investigated.

Influence of Dynamic Woody Debris Jam on Single Bridge Pier Scour and Induced Hydraulic Head

A woody debris jam around a bridge pier causes a change in flow structure and results in additional scour and an increase in the hydraulic head upstream of the pier, threatening its stability and safety. In the present paper, the spatio-temporal formation of a dynamic woody debris jam formed piece by piece of debris wood was used to investigate the influence of woody debris jams from a life-cycle perspective which included the processes of its formation, growth, failure, and rebirth. Several debris jams were formed in sequence during each experimental test. The results showed that the additional scour generated by the first woody debris jam compared with the scour depth without debris was a function of blockage ratio of the first debris jam, while the influence of the subsequent woody debris jams depended on their dimensions compared with the previous jam. When the subsequent debris jam’s dimensions were larger than the previous one, the scour further increased; otherwise, the scour remained constant and equal to the previous one. In addition, the debris-induced hydraulic head was analyzed and found to be correlated with the Froude number and the debris jam dimensions.

Flame–turbulence interactions during flame acceleration using solid and fluid obstacles

A combination of solid and transverse jet obstacles is proposed to trigger flame acceleration and deflagration-to-detonation transition (DDT). A numerical study of this approach is performed by solving the reactive Navier–Stokes equations deploying an adaptive mesh refinement technique. A detailed hydrogen–air reaction mechanism with 12 species and 42 steps is employed. The efficiency and mechanisms of the combined obstacles on the flame acceleration are investigated comprehensively. The effects of multiple jets, jet start time, and jet stagnation pressure on the DDT process are studied. Results show that there is a 22.26% improvement in the DDT run-up time and a 33.36% reduction in the DDT run-up distance for the combined obstacles compared to that having only solid obstacles. The jet acts as an obstruction by producing a suitable blockage ratio and introducing an intense turbulent region due to the Kelvin–Helmholtz instability. This leads to dramatic flame–turbulence interactions, increasing the flame surface area dramatically. The dual jet produces mushroom-like vortices, leading to a significantly stretched flame front and intensive Kelvin–Helmholtz instabilities, and therefore, these features produce a high flame acceleration. As the jet operation time decreases, the jet obstacle almost changes its role from both physical blockage ratio and turbulence and vorticity generator to a physical blockage ratio. There is a moderate jet stagnation pressure that reduces the run-up time to detonation and run-up distance to detonation in the obstacle-laden chamber. While further increasing the jet stagnation pressure, it does not have a positive effect on shortening the detonation transition.

Migration and rheotaxis of elliptical squirmers in a Poiseuille flow

The migration and rheotaxis of elliptical squirmers (a swimmer self-propels by imposing a given tangential velocity at its surface) in a Poiseuille flow are simulated numerically. The phase diagrams are employed to illustrate the effect of the aspect ratio (AR=0.2–1.0) and the Reynolds number of the squirmer (Rep=0.05–4.0), the self-propelling strength (β=−11 to 9), and the blockage ratio (κ=0.09–0.25) on the stable movement and orientation evolution of the elliptical squirmers. Five typical migration modes (including the stable sliding, periodic tumbling, damped swinging, periodic swimming, and chaotic migrating modes) and three rheotaxis states (including the stable, sub-stable, and unstable states) are identified. This pattern also exists for the locomotion of a pair of squirmers. It is found that, with increasing |β| and β≥5 or β≤−11 and κ≥0.21, the squirmers migrate in the more stable modes and rheotaxis states. With increasing Rep (Rep≥2.5), this pattern can also be found when the locomotion of two squirmers is considered, but it shows the opposite effect for an individual squirmer. In addition, a squirmer with a smaller AR is more easily to be trapped by the sidewall with yeq/H=0.18, θeq/π=1.5 because it is difficult to orient. Accordingly, a larger AR yields a migration, which is more easily along the centerline of the flow with yeq/H=0.5, θeq/π=1.0. It is interesting that the squirmers with AR = 0.2 almost maintain upstream oriented as they are usually attracted by the sidewall.

A comprehensive study of the onset of boundary layer separation in the unbounded flow around a circular cylinder

The onset of boundary layer separation in the unbounded flow around a circular cylinder is determined directly with a numerical experiment with accuracy to the second decimal point of the Reynolds number, Re. The governing equations are the steady state, two dimensional Navier–Stokes equations, which are solved with Galerkin finite elements. A new criterion, the entrance ratio E=D/L with D the diameter of the cylinder and L the entrance length of the domain, is introduced in addition to the traditional blockage ratio; the aim is to establish conditions for unbounded flow in both flow directions. A hypothesis is formulated and verified for the onset of separation leading to the conclusion that this flow undergoes a supercritical pitchfork bifurcation with respect to fixed points on the boundary. Results are presented for various blockage ratios, B, in the range 1.2238·10−8≤B≤0.005 and constant entrance ratio E=2.4472·10−9 until asymptotic behavior is observed for the onset of separation in the Reynolds number, in the angle and the length of the smallest eddy. Onset of separation occurs in the region 6.36≤Re&amp;lt;6.37. The bifurcation diagram of the flow is calculated. Streamlines of the flow domain before and after the bifurcation point are shown, including images of the smallest possible eddies. In addition, coefficients of base suction and total drag are computed and shown in the regime 0.1≤Re≤40. The results are compared with available laboratory and numerical measurements.

Cross-sectional blockage effect on the drag force of a scaled round-ended pier model under directional flow

Flume testing of a scaled model under steady flow is an essential reference for accessing the flooding-induced drag force on a prototype pier, but is highly susceptible to the cross-sectional blockage effect. Round-ended bridge piers have a non-axisymmetric cross-section, of which the drag force varies significantly with inflow direction. A flume testing program was carried out to illustrate the cross-sectional blockage effect under different inflow directions. CFD numerical simulations were then conducted to understand the blockage effect on the drag force, flow field and hydrodynamic pressure of the round-ended pier with different blockage ratios and inflow directions. The major findings include: 1) the blockage effect affects the drag force of the scaled model and changes with inflow direction; and 2) the limit of occurrence of the cross-sectional blockage effect is defined by the critical blockage ratio and given as a function of inflow direction. An empirical approach was finally developed to correct the cross-sectional blockage effect on the drag force of the round-ended pier model under directional flow. The influences of the blockage ratio and inflow direction were included in the empirical calculation of the drag force on the prototype from the scaled model test.

Blast wave interaction with structures – An overview

Blast–obstacle interaction is a complex, multi-faceted problem. Whilst engineering-level tools exist for predicting blast parameters (e.g. peak pressure, impulse and loading duration) in geometrically simple settings, a blast wave is fundamentally altered upon interaction with an object in its path, and hence, the loading parameters are themselves affected. This article presents a comprehensive review of key research in this area. The review is formed of five main parts, each describing: the direct loading of a blast wave on the surface of a finite-sized structure; the modified pressure of the blast wave in the wake region of three main obstacle types – blast walls, obstacles, wall/obstacle hybrids; and finally, a brief description of some methods for predicting loading parameters in such blast–obstacle interaction settings. Key findings relate to the mechanisms governing blast attenuation, for example, diffraction, reflection (diverting away from the target structure), expansion/volume increase, vortex creation/growth, as well as obstacle properties influencing these, such as porosity (blockage ratio), obstacle shape, number of obstacles/rows, arrangement and surface roughness.

Experimental investigation on interaction modes and geometric features of two ventilated cavities

In this study, the interaction modes and geometric features of two cavities are explored in a closed-loop water tunnel at Harbin Institute of Technology. Firstly, it was observed that there are four interaction modes under different cavitator diameter dn, Froude number Fr and distance between two cavitators d, namely, two basically independent cavities (TIC), two coupled-deformed cavities (TCDC), two partially fused cavities (TPFC) and two fully fused cavities (TFFC) respectively. In addition, an expression to describe the effect of different parameters on the interaction modes was established and obtained the critical value between the mode transitions. Subsequently, compared with the formulas of unbounded flows in the literature, it was found that the average value of the maximum diameters of the two cavities are close to those of the single cavitator at the same cavitation number. Moreover, a method used to quickly estimate the blockage ratio for two cavitators configuration was obtained, namely, the equivalent blockage ratio of the two cavities can be calculated from the equivalent diameter of the dn × 2dn rectangle. Finally, the estimation method was validated by Brennen's prediction.

Thermal-hydraulic performance of solar receiver duct with inclined punched-ribs and grooves

The article presents an experimental study on turbulent airflow friction and thermal behaviors in a solar receiver heat exchanger duct mounted with combined inclined chamfered-groove and turbulators. The experimental work was conducted for Reynolds numbers from 5,300 to 24,000, based on the hydraulic duct diameter. The 45°-inclined punched-ribs and grooves were placed periodically on the absorber plate. The punched-rib parameters were four inclination angles (β = 0°, 45°, 90° and 135°) of the punched holes and three relative rib pitches (PR = 1, 1.5 and 2) whereas only a rib blockage ratio (BR = 0.5) and an angle of attack (α) of 45° were fixed. Similarly, the parameters of the grooves included only three groove-pitch ratios (PR), similar to the rib pitches and one groove blockage ratio (BR = 0.2). The experimental result has revealed that the combination of rib-groove turbulators at β = 0° (solid rib) and PR = 1 gives the maximum heat transfer rate and friction loss while the greatest thermal enhancement factor of 2.1 was found at β = 45°, PR = 1. Moreover, the friction loss and heat transfer correlations for this thermal system were determined.