Increasing Blockage Ratio Research Articles

Twin Marine Hydrokinetic Cross-Flow Turbines in Counter Rotating Configurations: A Laboratory-Scaled Apparatus for Power Measurement

This article proposes an experimental apparatus design to measure the power of a cross-flow marine hydrokinetic turbine system operating in a laboratory water tunnel. Data, from one Hall sensor output signal, was processed to capture the three types of torque exerted on the turbines: mechanical loss, brake, and hydrodynamic torque. The method was then applied to compare the power of a twin turbine system in different counter-rotating configurations. Controlled by a hysteresis brake, the tip-speed-ratio was varied in a constant freestream velocity of 0.316 m/s. While the braking torque was independent of the speed, the mechanical loss was found to depend on the system rotational speed and the amount of mass mounted on the mechanical support. In a counter-rotating configuration, the turbines were synchronized through a pair of spur gears and timing pulleys. Operating at the average chord based Reynolds number of 8000, each turbine had three NACA0012 blades mounted at 15∘ pitch angle. The power coefficient results of 8 turbine configurations showed the tendency of power enhancement of counter-rotating configurations due to blade interaction and increase in blockage ratio. Comparison of the results suggested direct application in a river flow scenario and manipulation of the blade interaction for optimal power production.

Study of Scour Characteristics Downstream of Partially-Blocked Circular Culverts

Debris accumulations upstream and through crossing hydraulic structures such as culverts cause the upstream water level and the downstream scour depth to increase, which can lead to structure failure. This experimental study aimed to investigate the effects of various inlet blockage ratios on culvert efficiency and scour hole depth. In a non-blocked case, various submergence ratios (S = 1.06, 1.33, 1.60, and 1.90) were tested with different discharge rates. In a blocked case, the effects of inlet blockage with various blockage ratios (Ar = 10%, 20%, and 30%) were seen as sediment blockage on the pipe bed or floating debris upstream of the culvert. The results show that as the submergence ratio increases, the maximum scour depth decreases at the same discharge rate, and the relative energy loss also decreases in the non-blocked case. In the sediment blockage (Ar d) case, the relative maximum depth increases with increasing densimetric Froude number and with an increasing blockage ratio. An empirical equation was developed to predict the relative scour depth under the present study conditions.

Influence of volume blockage ratio on turbulent buoyant plume dispersion in mixed ventilated tunnel

The objective of this study is to numerically investigate the effects of volumetric blockage ratio on the thermal plume behavior in a mixed ventilated tunnel with heat source and vehicular blockage. The tunnel model consists of three naturally ventilated roof openings and longitudinal ventilation is provided by supplying forced air through the tunnel inlet port. The non-Boussinesq variable density approach is used to model the high thermo-buoyant flows with large eddy simulations (LES) turbulence model discretized by finite difference method (FDM). The investigation is performed by varying the Grashof number (Gr) in the range 108≤Gr≤ 1010 and the forced ventilation velocity (Uc) and volumetric blockage ratio (ξ) are varied between 0.05 ≤Uc≤ 0.5 and 0.012 ≤ ξ ≤ 0.05. The outcome of this study emphasizes that the longitudinal ventilation and volume blockage ratio significantly affects the thermal plume behavior and venting efficiency of the roof openings. It is found that inertial force from forced air increases the mass flow rate through downstream vent in comparison with the upstream vent. It is observed that the increase in Grashof number enhances the bidirectional exchange rate through roof vents and an increase in volume blockage ratio decreases the discharge rate through the roof openings.

Effect of blockage on critical ventilation velocity in longitudinally ventilated tunnel fires

Effect of blockage on critical ventilation velocity in longitudinally ventilated tunnel fires was investigated based on small-scale experiments and numerical simulations. Blockages were located right upstream of the fire or 3.5 m upstream of the fire. At each location, blockages have five blockage ratios and two orientations. Experimental data shows that the factors as blockage ratio, blockage-fire distance and blockage orientation could have coupling effect on the critical ventilation velocity. For blockage right upstream of the fire, the critical ventilation velocity decreases with the increasing blockage ratio when blockages are horizontally positioned, while there is first decrease and then increase in the critical ventilation velocity for blockage vertically positioned. A correlation was proposed to predict the critical ventilation velocity for blockages right upstream of the fire and horizontally positioned. For blockage 3.5 m upstream of the fire, the change in the critical ventilation velocity is very slight with the increasing blockage ratio for blockages vertically positioned, however, when blockages are horizontally positioned, there is an increase in the critical ventilation velocity for large blockage ratios. The change in the critical ventilation velocity with increasing blockage ratio is more obvious for blockages right upstream of the fire than that for blockages 3.5 m upstream of the fire. Numerical simulations were also conducted in the tunnel without fire. The altered tunnel flow field due to the obstruction of blockage was observed and its effect on the critical ventilation velocity was addressed. Comparison of the critical ventilation velocity between the experimental data and the results from previous studies showed that the critical ventilation velocity changed differently with the existence of blockage. The reason for this difference might be that the relative position between the blockage and the fire, and the fuel type could have coupling effect on the critical ventilation velocity.

Effects of unequal blockage ratio and obstacle spacing on wave speed and overpressure during flame propagation in stoichiometric H2/O2

Experiments on flame propagation and detonation onset behind two solid obstructions were carried out in premixed stoichiometric hydrogen–oxygen mixtures at 20 kPa in a closed-ended tube. Obstacles with three different blockage ratios (25%, 40%, and 80%) were used, and the arrangement between the obstacles was changed in terms of blockage distribution (increasing, decreasing, and equivalent); obstacle distance (38, 76, and 114 mm); and opening geometry. Changes in the obstacle pair characteristics resulted in shocks with distinct intensities and averaged Mach numbers, $$ \overline{{M_{\text{s}} }} $$ , that propagated into the undisturbed mixture; $$ \overline{{M_{\text{s}} }} $$ varied from 1.3 to 4.5 for the range of obstacle pairs tested. Four distinct deflagration-to-detonation transition (DDT) timescale groups were identified with average values varying between 0.7 and 12 ms. Obstacle pairs with increasing blockage ratio (25–80% and 40–80%) resulted in jet ignition downstream of the second obstacle, reducing the DDT length and timescale significantly when compared to the other obstruction combinations investigated. Soot foil records showed that detonation onset was via ignition from one or more hot spots near the flame front for all obstacle pairs that resulted in jet ignition. The results from this work suggest that channels with irregular obstacle patterns can experience faster detonation onset depending on the position of the obstructions relative to the ignition point.

A three-dimensional simulation of the effects of obstacle blockage ratio on the explosion wave in a tunnel

In this paper, a three-dimensional simulation has been performed to investigate the potential consequences of flammable vapor explosion in a tunnel with an obstacle, the obstacle with different area blockage ratios (10, 20, 30 and 40%). The blast wave shape, increase in the maximum explosion overpressure, and the arrival time of peak pressure in the vicinity of the obstacle with different blockage ratios have been characterized. The simulation results show that the configuration of the blast wave was remarkably compressed due to the effects of obstacle and the deformation process which can be divided into four stages. The influence of blockage ratio on the maximum overpressure and the arrival time of peak pressure were mainly reflected in the back of the obstacle 1 m from the ground, and the maximum overpressure for this location decreases as the blockage ratio increases. Meanwhile, the maximum overpressure of 10% blockage ratio is 57% higher than that of 40% blockage ratio for the location of the back of obstacle. The arrival time of peak pressures for the location of the back of obstacle is delayed as the blockage ratio increases. The maximum overpressure under the tunnel ceiling showed an opposite trend between the location of the front of the obstacle and the location behind the obstacle. In the front of the obstacle, the maximum overpressure under the tunnel ceiling increases as the blockage ratio increases; however, it decreases as the blockage ratio increases in the back of the obstacle. What’s more, the comparison of this paper’s data and the results from available are published. It was found that the simulation results in this article are consistent with the experimental results of studies.

An Aerothermal Study of Influence of Blockage Ratio on a Supersonic Tube Train System

Evacuated tube transportation is an important development direction for the high-speed transportation technology of the future. However, a train running at supersonic speed in a closed tube can create an unstable aerothermal phenomenon, causing the temperature to rise sharply inside the tube and endangering the safe operation of trains and equipment. The blockage ratio is one of the key factors affecting the aerodynamic characteristics in the tube. In this paper, a 2D axisymmetric model and Delayed Detached Eddy Simulation (DDES) based on the Shear Stress Transport (SST) κ-ω turbulence model are used to study the aerothermal environment in the tube. The calculation method used in this paper was verified by a wind tunnel experiment. The aerothermal phenomenon and distribution of the flow field in the tube with different blockage ratios were compared and analysed. The results show that the aerothermal environment is significantly affected by the blockage ratio. A choking limit formed in the flow field will aggravate the aerodynamic phenomenon as the blockage ratio increases, which further deteriorates the aerothermal environment of the tube. Moreover, the existence of the choking limit, shock wave, and Mach disk make the flow field in the tube more complicated.

Experimental studies on external pressures during vented lean hydrogen deflagrations in a 27 m3 chamber

A series of experiments were carried out in a 27m3 cubic chamber with different vent areas to study the external pressure during vented lean hydrogen deflagrations. Uniform hydrogen-air mixtures with concentrations ranging from 15 % to 21 % and stratified mixtures with maximum concentrations equalled to average values of uniform mixtures were tested. Different layouts of obstructions were used in the experiments to achieve different volumetric blockage ratios. Experimental data from vented deflagration tests were presented, and the effect of hydrogen concentration, vent area, non-homogeneous mixtures and obstacles on the external pressure during vented deflagrations was investigated. The results show that the external explosion has an effective influence on the internal pressure by several ways. The external pressure increases with the hydrogen concentration inside the chamber, and the maximum external overpressure with back ignition is higher than that with center ignition. Larger vent area leads to a higher external pressure measured at the position far from the vent, but an opposite trend is found at the position close to the vent due to the higher velocity of jet flame corresponded to the smaller vent. As the volumetric blockage ratio increases, the external overpressure increases accordingly, and the difference in magnitude between back ignition and center ignition becomes more pronounced. However, for the non-homogenous hydrogen deflagration, whose external pressure is mainly governed by the maximum hydrogen concentration inside the chamber, only when the volumetric blockage ratio inside the chamber is large enough, the obstruction can have a significant influence on the external pressures, but its effect is much stronger for the homogenous deflagration.

Investigation of the effect of blockage ratio on flow and heat transfer in the wake region of a cylinder embedded in a channel using whole field dynamic measurements

Non-intrusive field measurements in the wake region of a circular adiabatic cylinder fitted in a channel are reported. The top wall of the channel is subjected to constant heat flux while maintaining the bottom wall and the cylinder under adiabatic condition. The cylinder diameter has been varied to achieve three different blockage ratios (D/H) of 0.25, 0.38 and 0.5. Experiments are performed in the range of 63≤Re ≤ 165 with water as the coolant fluid under hydrodynamically fully developed and thermally developing conditions. Effect of blockage ratio on heat transfer enhancement capability, its capacity to alter the flow physics by modifying the size of vortices, the downstream distance they travel and the Reynolds number at which the vortices get initiated, is reported. Path-integrated temperature distribution in the channel has been mapped through the classical form of Mach Zehnder interferometer. The temperature distribution thus obtained has been used to determine the variations in local heat transfer coefficient along the length of the top horizontal surface of the channel. Schlieren deflectometry is used to quantify the vortex shedding frequency under three different blockage ratio regimes. The experimentally obtained results are benchmarked and validated by performing numerical simulations using FLUENT. It has been observed that a decrease in blockage ratio at a constant Re leads to alterations in the size of the generated vortices, which manifest as an increase in the vortex shedding frequency. Vortex shedding is seen to get initiated at relatively lower value of Re (= 65) when the blockage ratio is low (D/H = 0.25). However, when the wake transition to three-dimensionalities is considered, an increase in blockage ratio from D/H = 0.25 to 0.5 leads to an increase in the value of transition Re (165 for D/H = 0.5). It is observed that the vortices travel maximum downstream distance for blockage ratio of 0.38. For all blockage ratios, due to vortex shedding, the thermal boundary layer assumes a corrugated shape and aids in enhancing the heat transfer rates from the channel surface.

Effect of arc obstacles blockage ratio on detonation characteristics of hydrogen-air

In this paper, the influence of maximum blockage ratio of arc obstacles on hydrogen-air detonation characteristics is simulated and analyzed by using OpenFoam package. The results show that the arc obstacles can effectively reduce the deflagration-to-detonation transition distance and the optimal maximum blockage ratio is 0.7. As the maximum blockage ratio increases, the flame acceleration and initiation of detonation wave show 3 different regimes: (1) For small maximum blockage ratio, the flame is affected by Squish flow, resulting the velocity differences between the flame near the wall and in the center of the tube, and the tulip flame formation. The tulip flame will generate two local explosion points and the explosion points develop and merge to initiate detonation. (2) For intermediate maximum blockage ratio, under the interaction of the reflected shock wave and the obstacles, the flame is a narrow fingertip shape. The initiation of detonation wave is engendered by the development and fusion of multiple hot spots. (3) For large maximum blockage ratio, the flame is also a narrow fingertip-shaped. The detonation wave is initiated by the coupling of the flame front with the leading shock wave at the throat of the obstacle.

Moving model test of the smoke movement characteristics of an on-fire subway train running through a tunnel

A moving model test was carried out to investigate the associated smoke movement characteristics when a subway train on fire runs in a tunnel. Train models of the 1/10 and 1/15 scales were used. The spatial distributions of airflow velocity and smoke concentration were then analyzed, and the differences between moving fire sources and stationary fire sources were discussed. The results show that the smoke movement characteristics of a stationary fire source were greatly different from those of a moving one. Specifically, the smoke movement for the moving fire source was dominated by piston wind. Moreover, the process of the smoke spread could be divided into three stages, during which time the flow direction changed. The peak smoke concentration value occurred after the train tail passed by the measuring point. Besides, the impacts of train speed (60 km/h, 80 km/h, 100 km/h, and 120 km/h) and blockage ratio (0.19 and 0.43) on airflow velocity and smoke concentration were also investigated. With increasing train velocity, the airflow velocity increased, and the smoke concentration decreased. The maximum airflow velocity was approximately linear with the train velocity. Furthermore, the increasing blockage ratio enhanced the piston effect in the tunnel, thus increasing the airflow velocity and reducing the smoke concentration.

Computational investigation of wind tunnel wall effects on buffeting flow and lock-in for an airfoil at high angle of attack

Wind tunnel walls may significantly affect the aerodynamic characteristics of an airfoil if the distance between walls is not large enough and the experimental data may be influenced by the confinement effect. The purpose of this paper is to investigate the wall effects on a NACA 0012 airfoil at high angle of attack (the flow over the airfoil is assumed to be fully separated). The effects on both a static airfoil and also an oscillating airfoil (pitching motion) are studied. Reynolds number varies from 105 to 3×105. For a static airfoil, there is a dominant frequency of the flow oscillations called the Strouhal or buffet frequency. Strouhal number of an airfoil in confined flow is affected by blockage ratio and angle of attack. A new wall correction formula for the Strouhal number has been determined. For an oscillating airfoil, lock-in is determined by a combination of excitation frequency and amplitude of the oscillating airfoil. The reason of difference between computational and experimental lock-in regions are explained. Lock-in region decreases with increase of blockage ratio. Reynolds number has slight impact on lock-in region, both in unconfined flow and confined flow.

Effect of blockage ratio on the existence of multiple waves in rotating detonation engine

The effect of blockage ratio on the existence of multiple waves in rotating detonation engine was investigated through experimental approach. It has been shown experimentally that four typical detonation propagation modes (single wave, hybrid mode of single wave and double waves, double waves, longitudinal pulsed detonation) were obtained by varying blockage ratios and air mass flow rates at the global equivalence ratio of around 1.60. The results show that the increase of blockage ratio reduces the critical mass flow rate for the occurrence of multiple waves. The occurrence of multiple waves experiences coexistence of single wave and two co-rotating detonation waves, and steady two co-rotating detonation waves. The increase of blockage ratio intensifies the effect of reflected oblique shock waves on fresh reactants and elevates average pressure in the combustor, which play important role in the occurrence of multiple waves. Moreover, the increase of blockage ratio elevates the pre-combustion pressure. The high pre-combustion pressure resulting from high blockage ratio leads to the existence of volumetric explosion immediately after ignition, which extends the formation time of rotating detonation wave. Further increasing of blockage ratio triggers the longitudinal pulsed detonation (LPD), and the highly decreased injection pressure ratio is considered to be the main reason for the occurrence of LPD.

The influence of an orifice plate on the explosion characteristics of hydrogen-methane-air mixtures in a closed vessel

We reported an experimental study on the explosion characteristics of hydrogen-methane-air mixtures in a closed vessel at normal pressure and temperature. The explosion parameters, such as the maximum explosion pressure, the explosion time, the maximum pressure rise rate and the deflagration index were derived based on the pressure-time evolution. The effect of the blockage ratio as well as the position of the orifice plate on these explosion parameters were analyzed. The results show that the maximum explosion pressure was decreased in the presence of an orifice plate due to the heat transfer to the baffle. At the early stage of the flame propagation, the flame was suppressed. Subsequently, the flame speed was increased at the later stage in some cases. Combined with the various reactivities of the mixtures, the explosion times obtained in all cases were very close. For a given mixture and an orifice plate position, the maximum pressure rise rate and the deflagration index were increased with the increase of blockage ratio. Besides, the maximum pressure rise rate as well as the deflagration index were decreased when the orifice plate was close to the ignitor and oppositely, increased when the orifice plate was away from the ignitor.

Effects of blockage ratio and Reynolds number on particle deposition in duct airflow over a forward-facing step

Deposition processes of particles in duct airflow over a forward-facing step (FFS) were numerically studied by computational fluid dynamics method. Reynolds stress model with velocity fluctuation modification and discrete particle model with turbulent particle dispersion were developed to predict airflow fields and particle motions respectively. After grid independence study and numerical validation, the effects of different Reynolds numbers and blockage ratios on particle deposition characteristics were investigated. Particle deposition velocities, deposition efficiency and deposition mechanisms in duct with a forward-facing step were analyzed and discussed. The results showed that particle deposition velocity in FFS duct keeps monotonically increasing as particle diameter increases. Moreover, particle deposition profile in FFS duct increases as blockage ratio and Reynolds number increases. Deposition efficiency is higher for small particles (dp 10 µm) than uniform duct case. When air velocity is fixed as 5.5 m/s, the maximum deposition efficiency of FFS duct can reach 84 when λ = 0.25 and particle size is 1 µm. Moreover, when air velocity increases, the deposition efficiency will increase. Reduction of deposition distance, production of high TKE region and interception of windward surface by FFS are dominant mechanisms to increase particle deposition rates. Moreover, the “particle free zone” appears when particle diameter is more than 10 um and blockage ratio is 0.5.

Experimental Hydraulic Investigation of Angled Fish Protection Systems—Comparison of Circular Bars and Cables

The requirements for fish protection at hydro power plants have led to a significant decrease of the bar spacing at trash racks as well as the need of an inclined or angled design to improve the guidance effect (fish-friendly trash racks). The flexible fish fence (FFF) is a new developed fish protection and guidance system, created by horizontally arranged steel cables instead of bars. The presented study investigated experimentally the head loss coefficient of an angled horizontal trash rack with circular bars (CBTR) and the FFF with identical cross sections in a flume (scale 1:2). Nine configurations of different bar and cable spacing (blockage ratio) and rack angles were studied for CBTR and FFF considering six different stationary flow conditions. The results demonstrate that head loss coefficient is independent from the studied Bar–Reynolds number range and increases with increasing blockage ratio and angle. At an angle of 30 degrees, a direct comparison between the two different rack options was conducted to investigate the effect of cable vibrations. At the lowest blockage ratio, head loss for both options are in similar very low ranges, while the head loss coefficient of the FFF increases significantly compared to the CBTR with an increase of blockage. Further, the results indicate a moderate overestimation with the predicted head loss by common head loss equations developed for inclined vertical trash racks. Thus, an adaption of the design equation is proposed to improve the estimation of head loss on both rack options.

Sensitivity Analysis of Piston Wind in Hoistway of Super High-Speed Elevator

The piston wind will be produced when the car runs at high speed in the narrow and long hoistway, which will produce aerodynamic noise with the car enclosure and the hoistway walls, thus affecting ride comfort. In order to put forward better noise reduction measures, a piston wind model of super high-speed elevator with ventilation holes during the whole operation process was established firstly in this paper. Then, based on this theoretical model, the influence of the variation of ventilation-hole area, hoistway blockage ratio and car length on the piston wind was analysed separately. The analysis proved that the larger the area of ventilation holes, the more obvious the weakening of piston wind was when the building code permits. With the increase of blockage ratio, the intensity of piston wind increases obviously, but the duration of maximum piston wind does not change obviously. As the length of the car increases, there is no significant change in the movement of the piston wind, and this small change can be ignored in engineering applications.

Numerical study of the effect of a square block on the deformation of channel walls

ABSTRACT In this paper, the flow around a square block in a two-dimensional channel with flexible walls and steady laminar flow is analysed by the one-way fluid-structure interaction numerical method. Flow analysis is carried out by FLUENT and the wall deformations are studied with the assistance of finite element software ANSYS. The study aims to investigate the effects of Poisson’s ratio, blockage ratio, channel wall thickness, and elastic modulus on hydrodynamic parameters and ultimately the deformation of channel walls. The results show that an increase in the blockage ratio increases the wall displacement, the total drag coefficient and the friction coefficient on the block. In contrast, an increase in the elastic modulus or Poisson’s ratio decreases the displacement of the walls.

Effect of blade thickness on the hydraulic performance of a Francis hydro turbine model

Francis turbines are the most commonly used turbines for hydroelectric power generation. Preliminary studies to verify turbine designs are often performed with small-scale models; however, when the runner blade of a full-size turbine is geometrically scaled down to prepare a model for evaluating the design variables and performance characteristics, the blades become very thin and difficult to manufacture. Hence, the blockage effect of the runner blade should be considered to find a suitable blade thickness that satisfies the required hydraulic performance. Furthermore, a clear understanding of the blockage ratio at the highest efficiency point and off-design condition is required to investigate different blade thicknesses and performance characteristics. Here, the blockage effect of the runner blade on the hydraulic performance and internal flow characteristics of a 300-class Francis hydro turbine was investigated. Three-dimensional Reynolds-averaged Navier–Stokes calculations were performed with a shear stress transport turbulence model to analyze the internal flow characteristics near the runner blade and compare the blockage effect with various blade thicknesses on major performance parameters such as the hydraulic efficiency. Flow analyses for the off-design conditions were also performed with various blade thicknesses. The obtained results indicated that the power and efficiency gradually decreased with increasing blockage ratio. The runner head loss increased due to the mismatches between the flow angle and blade angle with changing the inlet velocity triangle components according to blockage ratio. Especially the efficiency of approximate 3.4% decreased as the blockage ratio increased with 12.5%, compared to the reference model. It was verified that the blockage effect significantly affects the design of Francis turbine models.

An experimental study on angled trash screen in open channels

Trash screens are widely employed at entrance of hydraulic structures (culverts, barrages…, etc.), water intakes, and pumping stations to trap flowing debris that could instigate downstream troubles. Traditional trash screen (perpendicular-vertical screen to the flow direction) can cause problems of increasing head loss through it, especially with the blockage ratio. This experimental study presents a concept of an adjusted screen with circular bars to find a solution for such problem. In this regard, angled screen models with different angles, blockage ratios, and flow discharges were investigated. The results indicated that the head loss coefficients reduced by reducing the screen angle, so angled screen (α < 90°) reduced the loss in comparison with the traditional trash screen (α = 90°). The values of head loss coefficient increased with increasing blockage ratio, and flow discharge. The screen losses increase strongly when the screen becomes blocked by 40% or more. Screen angle, blockage ratio, and discharge influence effectively the head loss of a screen. Furthermore, a new equation for the head loss, the key element in designing screens, is proposed based on the different tested parameters, and should be useful in designing trash screens.