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This study conducted various explosion experiments under different porous obstacle conditions of blockage ratio (BR) and void fraction (Φ f). The impact of porous obstacle on flame propagation velocity (v) and peak overpressure (P op) were analyzed. The results indicate that an increase in the BR enhances explosion pressure near the blockage area for both porous and non-porous obstacles. However, it also increases the downstream pressure decline gradient. An increase in the Φ f weakens the hindrance of porous obstacle on the flame propagation, allowing more flame to pass through the internal void channels of obstacles, thus delaying the downstream pressure decay. Compared to non-porous obstacles, gas explosions disturbed by porous obstacles with both high BR (BR＞75%) and high Φ f (Φ f ＞0.55) exhibit a lower downstream pressure decline gradient and a longer positive pressure duration. As a result, the explosion can produce a broader range of destruction. • The impact of porous and non-porous obstacles on flames was compared. • The propagation stability of flame gradually reduces with an increasing BR. • Increasing BR results in the forward shift of the maximum P op point. • Increasing V f tends to delay downstream explosion pressure decline. • Porous obstacles can amplify the destructive range of explosion. [ABSTRACT FROM AUTHOR]

Experiments have been carried out to investigate the over expanded free jet at Mach number 2.0 without and with tabs of different cross-sectional shapes, tab length penetration into the jet core defined as blockage ratio, and symmetrical and asymmetrical placement of tabs at the jet exit. The cross-sectional shapes such as triangular, square, and circular have been studied with the penetration depth of 3 %, 7 %, and 11 % blockage. The Pitot tube and Schlieren flow visualization were carried out in experiments. The basic features of the jet with Mach disk, shock cell, and its distortion with the adoption of tabs at various nozzle pressure ratios indicate a definite influence that helps in the reduction of the core jet length. This characteristic is also influenced by changes in tab cross-section and length of penetration. Since the basic jet has three-dimensional structures, the asymmetric placement of tabs is beneficial compared to the symmetric orientation of tabs. The jet width increases with adoption of these tabs and different shapes. Whereas, a maximum core jet length reduction of the order of 60 % could be achieved using the present techniques. [ABSTRACT FROM AUTHOR]

Settling non-spherical and spherical particles in the annular region is common in many industrial applications. This work shows the intensification of the performance of settling particles in an annular channel by reducing the drag coefficient. The disk/cylinder particles settle in annular channels filled with Newtonian and non-Newtonian fluids. An aqueous solution of different concentrations of glycerin and Carboxymethylcellulose (CMC) was considered as the Newtonian and non-Newtonian fluids. The experimental results reported in the present work covered the 0.17 ≤ blockage ratio ≤0.55, 0.14 ≤ k (flow consistency index) ≤1.81 (Pa.sn), 0.64 ≤ n (flow behavior index) ≤1, and annular channel Reynolds number 0.40 ≤ Re ≤63.44 and 0.05 ≤ Re ≤43.02 in Newtonian and non-Newtonian fluids respectively. The wall factor, f, was increased with the increase in Re and the decrease in blockage ratio. At higher Reynolds numbers (Re), the particle's wall factor was only influenced by the blockage ratio. The wall factor was reduced due to the particle's shape change from cylinder to disk, i.e., decreasing the particle's sphericity. The wall factor of particles is observed to be higher in non-Newtonian than in Newtonian fluids. The critical Reynolds number (Rec) decreased with the H/d (Height/diameter of disk) ratio and sphericity of the studied particles. Both the drag coefficients, in the absence ( C D ∞ ) and presence ( C D ) of the wall effect, declined with the Reynolds number. Suitable correlations were developed for CD with low values of RMSLD (root mean square logarithmic deviation). The experimentally obtained drag coefficient data were also analyzed successfully using the laminar model available in Ansys (Fluent) −18.1. The computed velocity contours analyzed the wall effect and CD variations successfully. The use of the annular channel as a drag reduction device is encouraged due to the observed higher wall factor and decreased drag coefficient for particle settling in comparison to the non-annular channel. [ABSTRACT FROM AUTHOR]

Traditional exhaust-gas turbocharging exhibits hysteresis under variable working conditions. To achieve rapid-intake supercharging, this study investigates the synergistic coupling process between the detonation and diesel cycles using gasoline as fuel. A numerical simulation model is constructed to analyze the detonation characteristics of a pulse-detonation combustor (PDC), followed by experimental verification. The comprehensive process of the flame's deflagration-to-detonation transition (DDT) and the formation of the detonation wave are discussed in detail. The airflow velocity, DDT time, and peak pressure of detonation tubes with five different blockage ratios (BR) are analyzed, with the results imported into a one-dimensional GT-POWER engine model. The results indicate that the generation of detonation waves is influenced by flame and compression wave interactions. Increasing the airflow does not shorten the DDT time, whereas increasing the BR causes the DDT time to decrease and then increase. Large BRs affect the initiation speed of detonation in the tube, while small BRs impact the DDT distance and peak pressure. Upon connection to the PDC, the transient response rate of the engine is slightly improved. These results can provide useful guidance for improving the transient response characteristics of engines. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Zhejiang University: Science A is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Thermoacoustic engines (TAE) offer a promising avenue for converting low-grade heat into useable energy. Despite its simplicity in fabrication, designing thermoacoustic engines face efficiency challenges hindering their widespread adoption. One key obstacle faced by current thermoacoustic devices is their lack of efficiency. The core components of TAE are the stack, Cold Heat Exchanger (CHX), Hot Heat Exchanger (CHX) and the resonator. These components are crucial in advancing the performance of thermoacoustic engines. The CHX keeps the working gas cool, which is essential for the engine to function efficiently and create the desired acoustic wave. However, many existing efforts in this field have not taken into consideration the effect of the CHX design which may have effect on the operating temperature and the engines performance. This study examines the influence of different CHX blockage ratios (28 %, 36 %, 42 %, 47 %, and 59 %), along with the addition of an external CHX, on performance metrics. Measurements were conducted using air at room temperature and atmospheric pressure to assess temperature difference, startup time, volumetric velocity, and sound pressure levels. Key findings indicate that Efficiency directly correlates with volumetric velocity and inversely with onset temperature. Even though decreasing the blockage ratio increases operating temperature, it lengthens startup and reduces volumetric velocity, suggesting increased axial conduction. In addition, External CHX lowers operating temperature, reducing volumetric velocity and sound pressure. Interestingly, resonance frequency remains unaffected by CHX blockage ratio changes. An optimal configuration with a 42–47 % blockage ratio achieves 5.92 m/s velocity, 136 s startup time, and 77 °C onset temperature. These findings offer valuable insights for optimizing thermoacoustic engine efficiency and advancing their potential as sustainable energy solutions.

Using experimental and LES methods, this paper investigates the effects of equivalence ratio and blockage ratio on hydrogen flame acceleration, explosion overpressure characteristics, and flow field properties in a closed obstructed duct. The study indicates that there is a positive correspondence between the flame acceleration and pressure rise in a closed duct. The continuous flame acceleration is driven by flow compression and turbulent combustion propulsion, corresponding to a rapid pressure rising. After the quasi-plane flame, the explosion overpressure begins to oscillate at a high frequency. As the blockage ratio increases, the turbulent combustion is intensified, consequently leading to a noticeable increase in flame velocity and pressure rising rate. While high obstacles exert an obstructive effect on the initial flame propagation and the pressure oscillations. The vorticity is mainly distributed in the shear flow region and the burned region, and its magnitude and distribution area increase with a higher obstacle ratio. • Correspondence between flame acceleration and overpressure characteristics is revealed. • The adiabatic and isothermal compression are analyzed in a closed obstructed duct. • Vector field and vorticity field at different blockage ratios are investigated. [ABSTRACT FROM AUTHOR]

This study employed FLUENT to analyse smoke movement and temperature distribution in an evacuation corridor with varying blockage ratios, focusing on the subway tunnel section of Fuzhou Metro Line 4. The simulation results revealed that under natural ventilation condition, the smoke spread area in the evacuation corridor is significantly greater for the high blockage ratio tunnel than for the low blockage ratio tunnel in the tunnel's length direction, and the entire temperature distribution in the tunnel's height direction is also high. Following the introduction of longitudinal ventilation, smoke spread in the evacuation corridor and the tunnel ceiling upstream of the fire source are effectively controlled, with smoke suppression movement in the evacuation corridor being significantly faster than that near the tunnel ceiling. As ventilation time increases, the back-layering length of smoke in the evacuation corridor gradually shortens. Within 150 s of ventilation, the critical safety distance below the safety temperature for a low blockage ratio is shorter than that for a high blockage ratio tunnel. In conclusion, longitudinal ventilation increases the cooling rate of high-temperature smoke in a high blockage ratio tunnel, but the influence of high ventilation velocity on evacuation cannot be ignored. Practical application: This study provides recommendations for the evacuation plan and procedures under longitudinal ventilation. It is advisable to consider lowering the height of the evacuation corridor in the tunnel from the rail surface, thereby creating a more extensive safety space for personnel evacuation. Additionally, the implementation of prominent marks and voice prompts in the upstream area of the fire outbreak is crucial. This ensures that personnel are directed to evacuate from the upstream section during emergency situations. [ABSTRACT FROM AUTHOR]

The flame evolution, flame front speed and pressure dynamics are investigated through experimental and simulation analysis. The effects of the obstacle on velocity vector field and vorticity field are analyzed. The results indicate that a rise in blockage ratio causes a rapid rise in flame front speed. The turbulent combustion, flame backflow, and overpressure are significantly enhanced as obstacle distance and blockage ratio increase. The jet flame results in a surge in flame front speed, and a momentary increase in pressure rise. The flame-vortex interaction causes strong turbulent combustion and a rapid overpressure increase. The overpressure displays regular periodic fluctuations and high-frequency oscillations with high amplitudes, which can be attributed to the dynamic and impulsive responses. The vortex results in the formation of a curled flame with high localized vorticity. The backflow flame, exhibiting an extremely high vorticity magnitude, is induced by the overall pressure gradient and flow field recirculation. • Hydrogen-air flame evolution and pressure dynamics are investigated. • LES method is developed to predict hydrogen explosion in an obstructed chamber. • Effects of the obstacle on velocity vector field and vorticity field are analyzed. [ABSTRACT FROM AUTHOR]

Traditional exhaust-gas turbocharging exhibits hysteresis under variable working conditions. To achieve rapid-intake supercharging, this study investigates the synergistic coupling process between the detonation and diesel cycles using gasoline as fuel. A numerical simulation model is constructed to analyze the detonation characteristics of a pulse-detonation combustor (PDC), followed by experimental verification. The comprehensive process of the flame’s deflagration-to-detonation transition (DDT) and the formation of the detonation wave are discussed in detail. The airflow velocity, DDT time, and peak pressure of detonation tubes with five different blockage ratios (BR) are analyzed, with the results imported into a one-dimensional GT-POWER engine model. The results indicate that the generation of detonation waves is influenced by flame and compression wave interactions. Increasing the airflow does not shorten the DDT time, whereas increasing the BR causes the DDT time to decrease and then increase. Large BRs affect the initiation speed of detonation in the tube, while small BRs impact the DDT distance and peak pressure. Upon connection to the PDC, the transient response rate of the engine is slightly improved. These results can provide useful guidance for improving the transient response characteristics of engines.

This experimental study aims to characterize the behavior of pipe and box coverages concerning their size, form, and upstream blockage ratio. In an artificial trapezoidal cross-section, five experimental cases were carried out: Case 1 involved an artificial canal without coverage or blockage; Cases 2 and 3 concerned pipe coverages with circular cross-sections (Pipe 1 and 2); Cases 4 and 5 involved box coverages with square cross-sections (Box 1 and 2). While the area of pipe 1 and box 1 are equal, the area of pipe 2 and box 2 is the same and greater than the area of pipe 1 and box 1. Three blockage ratios and three water flow rates were used in the experimental study. Each case included an investigation of the hydraulic performance of the open channel and the scouring pattern downstream of the coverage. In comparison to the case when there was no coverage present at the same condition, the presence of coverage in the open channel and the significant increase in flow rate, blocking ratio, and decreasing inlet area of coverage increased the heading up, head losses, scour depth, and scour length. In the same area and condition, the pipe coverage achieves greater scour depth and length than the box coverage. The box coverage is better for the open channels' performance than the pipe coverage. The research recommended using the box coverage rather than the pipe coverage and checking the maintenance processes to avoid the negative effect on the open channels' performance.

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]

This experimental study aims to characterize the behaviour of pipe and box coverages concerning their size, form, and upstream blockage ratio. In an artificial trapezoidal cross-section, five experimental cases were carried out: Case 1 involved an artificial canal without coverage or blockage; Cases 2 and 3 concerned pipe coverages with circular cross-sections (Pipe 1 and 2); Cases 4 and 5 involved box coverages with square cross-sections (Box 1 and 2). While the area of pipe 1 and box 1 are equal, the area of pipe 2 and box 2 is the same and greater than the area of pipe 1 and box 1. Three blockage ratios and three water flow rates were used in the experimental study. Each case included an investigation of the hydraulic performance of the open channel and the scouring pattern downstream of the coverage. In comparison to the case when there was no coverage present at the same condition, the presence of coverage in the open channel and the significant increase in flow rate, blocking ratio, and decreasing inlet area of coverage increased the heading up, head losses, scour depth, and scour length. In the same area and condition, the pipe coverage achieves greater scour depth and length than the box coverage. The box coverage is better for the open channels' performance than the pipe coverage. The research recommended using the box coverage rather than the pipe coverage and checking the maintenance processes to avoid the negative effect on the open channels' performance. [ABSTRACT FROM AUTHOR]

The design of hydrokinetic plants in hydropower canals involves the choice of the array layout, rotor geometry, turbine spacing, and array spacing, and necessitates the control of the resultant backwater to avoid upstream flooding hazards. Several works in the literature have shown that array power optimization is feasible with small spacings between the arrays, disregarding the limitation in the power output induced by backwater upstream. In this study, a 1-D channel model with a Double Multiple Streamtube code and wake sub-models are integrated to predict an array layout that will maximize the array power. The outputs of the conducted sensitivity analysis confirm that this design enabled improved power conversion with closely spaced turbines and largely spaced arrays, thus allowing for a partial recovery of the total head variation for a new array deployed upstream. In addition to the quantitative assessment of the mechanical power converted, the tool enables depth control from the downstream undisturbed flow station to the backwater exhaustion far upstream, thereby increasing its flexibility. Furthermore, it overcomes the limitations of actuator disc models by considering rotor's fluid dynamic losses. The results show that power output linearly scales for a limited number of arrays (≤5), whilst the variation in water depth variation follows a power law from the most downstream array towards upstream, regardless of the plant size. Finally, the maximum upstream inflow depth is demonstrated to become asymptotic for large multi-array plants under ideal conditions. [ABSTRACT FROM AUTHOR]

The model-scale tunnel is used in this investigation to analyze the maximum smoke temperature rise of the interconnected tunnel for various longitudinal ventilation velocities, blockage ratios, and heat release rates where the fire is at the confluence of the underground interconnected tunnel. The results showed that the longitudinal ventilation velocities of both the ramp upstream of the fire source and the adjacent ramp influenced the maximum temperature rise under the underground interconnected tunnel, and the ventilation of both ramps jointly affected the maximum temperature rise. The change in the maximum temperature rise depends on who is more affected by the longitudinal ventilation velocity or the vehicle blockage ratio. As the longitudinal ventilation velocity in the interconnected tunnel increases, the convective heat transfer near the fire source increases, resulting in a decrease in the maximum temperature rise, and the effect of the blockage ratio on the maximum temperature rise is reduced. In this paper, a maximum temperature rise prediction model suitable for the case of blockage in the interconnected tunnel is proposed. [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.

As a consequence of research on developing a speedy transportation system, Hyperloop is one of the best solutions now as smaller resistant forces are developed on the capsule body compared to conventional ground transportation systems due to movement in a vacuum and no contact with the ground. In this study, a capsule of an elliptical-shaped head and semicircularshaped rear was chosen for analysis. Aerodynamic drags were calculated at different evacuated tunnel pressures. The computational regime was a 360 meters long tunnel. The inlet and outlet were pressure far-field boundaries while the wall was moving, with a Blockage Ratio (BR) of 0.36. Characteristics of different regions were identified in choked conditions. The drag was found to be lesser than the capsule of semicircular ends at different speeds. The pressure drag and friction drag were increased with the increase in velocity in the same tunnel pressure. By investigating different flow regions, it was found that a series of rhomboidal-shaped shock waves appear at high speeds. The formation and nature of this shock wave were also investigated, and found that it is caused due to shock wave and expansion wave interaction that results in the fall of pressure and temperature in the wake region. [ABSTRACT FROM AUTHOR]

The novelty of this work is a new type of stack, namely, the parallel wave stack (PWS). In this paper, we study the performance of an air-based standing wave thermoacoustic refrigerator using the PWS. We use small heat pipes with a heat transfer rate of approximately 30 W as heat exchangers for the system. We investigate the effect of the ratio of stack length (Ls) to the distance between stack center and reflector (Xs) on the performance of the refrigerator parallel wave stacks with a blockage ratio (BR) of 67 %, 71 %, and 74 %, and with Ls/Xs ratios of 0.74, 0.96, and 1.20. We tested and compared these under a cooling load (Qc) of 0.5 W to 2 W and a frequency of 150 Hz. The results show that a BR of 71% with an Ls/Xs ratio of 0.96 provides a minimum cold-side temperature (Tc), and a maximum temperature difference (ΔTm), a maximum coefficient of performance, and a maximum relative coefficient of performance. This new type of stack can provide a higher performance. The structure of the stack is also stronger than others. This stack is an alternative and can be used in a thermoacoustic refrigerator. The results obtained from the present study will be very useful for improving the performance of the thermoacoustic refrigerator in the future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The aerodynamic characteristics of the vertical tail of an aircraft are directly related to the size and weight of the vertical tail. Increasing the lift force of the vertical tail using flow control can further reduce the size of the vertical tail,which is of practical significance for reducing the weight of the structure. The computational fluid dynamics(CFD)method is used to perform the numerical simulation for the active flow control of vertical tail using the co-flow jet technique. The influence law of jet momentum coefficient on continuous co-flow jet and the influence law of jet momentum coefficient and the paramaters of jet momentum coefficient,blockage ratio,injection hole number on discrete co-flow jet are studied. The results show that for the continuous co-flow jet,with the increase of jet momentum coefficient,the effectiveness of lift enhancement,drag reduction and separation suppression becomes more significant. For the discrete co-flow jet,the effectiveness of lift enhancement gradually decreases with the increase of the blockage ratio,and the power consumption also decreases at the same time. With the increase of the discrete hole number,the lift enhancement effect first increases and then decreases,and the comprehensive aerodynamic efficiency considering energy consumption is improved.

Previously, research on blockages was primarily focused on ordinary single tunnels. This paper investigated the temperature decay and smoke propagation characteristics in underground interconnected tunnel with blockages. Considering various blockage ratios, ventilation velocities, and heat release rates (HRRs), 120 experiments were conducted on a model-scale underground interconnected tunnel. It was found that the ventilation schemes and HRR minimally impact the downstream temperature decay pattern. Nevertheless, as the blockage ratio increased, the temperature decay coefficient K initially increased and then decreased. Smoke entry into the connected tunnel was evaluated by the Froude number, and as the blockage ratio increased, the critical Froude number gradually decreased. Examining smoke backflow in underground interconnected tunnel under diverse conditions revealed that the smoke back-layering length was primarily influenced by D-ramp velocity. A predictive model for smoke back-layering length was proposed, considering blockage ratios. The outcomes of this research could help control smoke within underground interconnected tunnels with blockages. Fig. 1. The graphical abstract of current work. [Display omitted] • Experiments are conducted considering blockage ratios in underground interconnected tunnel. • Froude number is employed to estimate whether smoke entered the connected tunnel. • Prediction model for smoke back-layering length in underground interconnected tunnel with blockages is proposed. [ABSTRACT FROM AUTHOR]

Fundamental understanding of turbulent flow and heat transfer in composite porous-fluid systems, which consists of a fluid-saturated stochastic open-cell metal foam and a flow passing over it, is crucial for fostering technological development in numerous applications such as transpiration cooling in aerospace, packed-bed thermal storage and thermal management of electronic devices. In this work, conjugate heat transfer simulations were adopted to explore the turbulent flow and heat transfer features in a composite porous-fluid system at the pore scale. Simulations were performed to account for the influence of the blockage ratios (i.e., BR = 0.5, 0.8 and 1.0) on pressure drop and heat transfer rate by introducing a new concept called penetration cooling length. Furthermore, the effect of Reynolds numbers (i.e., Re = 1800, 3600 and 7200) at different blockage ratios was investigated in terms of pressure drop, fluid and solid temperatures, interstitial heat transfer coefficient, and flow leakage. Results indicate that for a fixed blockage ratio, as the Reynolds number increases by a factor of 3.0, there is a 14.9-fold increase in the pressure drop and a 2.9-fold increase in the interstitial heat transfer coefficient. Additionally, for a fixed Reynolds number, when the blockage ratio increases by a factor of 2.0, there is a 6.8-fold increase in the pressure drop and a 1.8-fold increase in the interstitial heat transfer coefficient. Flow visualization indicated that the penetration cooling length is influenced by flow leakage from the porous-fluid interface. Results show at small blockage ratios and low Reynolds numbers, a significant portion of the flow from the porous region leaves it to the clear region on top of the porous block. While, at high Reynolds numbers and large blockage ratios, the flow leakage is reduced. Additionally, for a low blockage ratio (BR < 0.5), the amount of flow leakage depends on the Reynolds number, while it is independent of the Reynolds number for BR > 0.8. Finally, this study introduces a novel correlation for the interstitial heat transfer coefficient derived from conjugate heat transfer pore-scale simulations. The correlation is established on the basis of Reynolds number, blockage ratio, and development length of the metal foam, offering valuable insights into heat transfer phenomena in porous media. Modelling conjugate heat transfer in a composite porous-fluid system with stochastic open-cell metal foam. Simultaneous effect of blockage ratio (B R) and Reynolds number (R e) on interstitial heat transfer coefficient contour; First row: B R = 0.5; Second row: B R = 0.8; Third row: B R = 1.0. [Display omitted] • Turbulent heat transfer of flow in composite porous-fluid systems is studied. • Pore-scale conjugate heat transfer analysis is performed. • A new concept termed penetration cooling length (PCL) is introduced. • The dependency of PCL on Reynolds number (Re) and blockage ratio (BR) is studied. • For a fixed BR, if Re increases by 3.0 times, the pressure drop rises by 14.9-fold. [ABSTRACT FROM AUTHOR]

The present study focuses on the mitigation of shock wave using novel geometric passages in the flow field. The strategy is to produce multiple shock reflections and diffractions in the passage with minimum flow obstruction, which in turn is expected to reduce the shock wave strength at the target location. In the present study the interaction of a plane shock front (generated from a shock tube) with various geometric designs such as, 1) zig-zag geometric passage, 2) staggered cylindrical obstructions and 3) zig-zag passage with cylindrical obstructions have been investigated using computational technique. It is seen from the numerical simulation that, among the various designs, the maximum shock attenuation is produced by the zig-zag passage with cylindrical obstructions which is then followed by zig-zag passage and staggered cylindrical obstructions. A comprehensive investigation on the shock wave reflection and diffraction phenomena happening in the proposed complex passages have also been carried out. In the new zig-zag design, the initial shock wave undergoes shock wave reflection and diffraction process which swaps alternatively as the shock front moves from one turn to the other turn. This cyclic shock reflection and diffraction process helps in diffusing the shock wave energy with practically no obstruction to the flow field. It is found that by combining the shock attenuation ability of zig-zag passage (using shock reflection and diffraction) with the shock attenuation ability of cylindrical blocks (by flow obstruction), a drastic attenuation in shock strength can be achieved with moderate level of flow blocking.

The numerical study of pressure drop on a tandem cylinder with the addition of a splitter plate and a vortex generator with the effect of a blockage ratio has been completed. The cross-sectional height and diameter of the cylinder in this study used H= 125 mm and D= 37.5 mm, respectively. The blockage ratio is 30%. The Reynolds number (Re) is 52100 ≤ Re ≤ 156000. The distance between cylinders is 5 to 8, where “s” is the distance from cylinder one to cylinder two. The dimensions of the splitter plate are L=D, L=1,5D, and L=2D where "L" is the length of the splitter plate, while the thickness in this study is 1, 75mm. The dimensions of the vortex generator in this study are used those of Hu, et al. [6]. The angle of the vortex generator is = 350 while the length of the vortex generator is H = 3 mm. All variations of this numerical study were carried out using the URANS (Unsteady Reynold Average Navier Stoke) method with a Reynolds number (Re) 52,100 Re 156,000. The smallest pressure drop value is obtained at the Reynolds number 52.100 for all variations, while the highest Reynolds number is obtained at Re 156.000. the addition of a splitter plate and a vortex generator, gives a higher pressure drop when compared to a circular cylinder.

Copyright of Journal of Central South University is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Heat exchange devices involving confined heated cylinders in crossflow require wake stability to minimize hydraulic losses, which are typically accompanied by a considerable loss in heat transfer. To address this problem, passive wake control for laminar flow past a cylinder is introduced in the form of wavy channel walls around the cylinder. The resultant effects on heat transfer and drag are investigated in detail for a range of Reynolds numbers (Re = = 20–200), blockage ratios (β = 0.5–0.9) and Prandtl numbers (Pr = 0.25–100). For the given range of parameters, it is shown that the wavy channel reduces cylinder drag by a minimum of ∼ 36 % and a maximum of ∼ 95 %, at the expense of ∼ 8 % and ∼ 40 % loss in heat transfer, respectively, compared to the plane channel. Thermal-hydraulic performance of the wavy channel compared to the plane channel for Pr = 0.744, is improved by ∼ 14 % for β = 0.5 and ∼ 160 % for β = 0.9. Therefore, the performance enhancement brought about by the wavy channel provides an appropriate trade-off between drag reduction and heat transfer loss. [ABSTRACT FROM AUTHOR]

order to study the characteristics and rules of deflagration to detonation transition (DDT) in linked vessels, two typical linked vessels were selected in this paper. The research was focused on the effects of pipe length, obstacle number, obstacle position and obstacle blockage ratio on DDT of methane air mixture in linked vessels. The experimental results showed that the pressure peak and the speed of flame propagation were enhanced by the obstacles. The precursory shock waves and the chemical reaction zone were coupled to propagate and caused the occurrence of DDT. When the obstacles were placed in the linked vessels, the distance of DDT occurrence moved forward. Moreover, with the increase of obstacle number, the induced distance of DDT was reduced. The induced distance of DDT was relatively short when the obstacle position was near the location of the explosion vessel. [ABSTRACT FROM AUTHOR]

In order to study the effect of thermal barrier coating deposition inside the film holes of turbine guide vanes on film cooling performance, film effectiveness on the suction side is measured by infrared thermal imaging technology. Film effectiveness is obtained at blockage ratios of 0, 0.2, 0.5, and 0.8, and blowing ratios of 0.7, 1.05, and 1.4. Film effectiveness decreases and the spanwise inhomogeneity becomes evident with an increase of the blockage ratio. When blowing ratios increase from 0.7 to 1.4, the surface averaged film effectiveness decreases by 55–60% at a large blockage ratio of 0.8, 21–27% at the middle blockage ratio of 0.5 and by no more than 11% at a small blockage ratio of 0.2. The rounded corners formed by blockage enhance the adhesion of the film at the small blockage ratio of 0.2, thereby improving the film cooling performance near the hole exit. There is a maximum increase of 0.2 in film effectiveness within four hole diameters. [ABSTRACT FROM AUTHOR]

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) k-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. [ABSTRACT FROM AUTHOR]

• Variations in the flame morphology, speed, and peak explosion overpressure in opposite flame directions are analyzed in this article. • The relationship between peak explosion overpressures in the left and right directions depended on the blockage ratio in each direction. • Accumulated obstacles can significantly increase the flame propagation speed and the peak overpressure in gas explosions. Coal mine gas explosions often occur in confined areas with accumulated obstacles. To investigate the effect of the blockage ratio (BR) of accumulated obstacles on the bidirectional propagation characteristics of gas explosions, this study conducted experiments using different BRs of accumulated obstacles in opposite flame propagation directions in a channel. Variations in the flame morphology, speed, and peak explosion overpressure in both directions were analyzed. Results indicate when the BR in either direction reaching 60% or above, a reverse propagation flame formed in the direction with a larger BR. With one direction completely blocked (BR = 100%) and the BR in the other direction lower than 60%, a Helmholtz cloud-like flame formed in the completely blocked direction. Effect of the accumulated obstacles on the flame speed in either direction shifted from suppression to acceleration as the BR in the other direction increased. When the BR in either direction was 80%, the effect of the accumulated obstacles on the flame speed in the same direction changed from suppression to acceleration as the BR in the opposite direction increased. The relationship between peak explosion overpressures in both directions depends on the BR in each direction. When the BRs in both directions did not exceed 60%, the higher peak overpressure appeared in the direction with a large BR. However, once the BR in either direction exceeded 60%, the peak overpressure was lower in the direction with a larger BR. [ABSTRACT FROM AUTHOR]

• Turbulent models were validated based on experimental data in entrance blocked rectangular channel. • Anisotropic turbulence in entrance blocked rectangular channel was studied based on validated turbulent model. Numerical study of turbulent flow in a narrow rectangular channel with side entrance blockage ratio from 0.25 to 0.9 was conducted under Reynolds numbers (Re) from 2,180 to 100,900. The Pearson similarity criterion was evaluated for mesh size sensitivity and turbulent models based on experimental data. Based on the validated turbulent model, predicted turbulent flow was studied. The entrance blocked channel forms jet-flow at inlet, shear-flow layer and recirculation vortex downstream of the blockage, with high Reynolds stress in the shear-flow layer. Then the streamwise flow reattaches the side channel wall. With blockage ratio increasing, the time-averaged velocity and Reynolds stress increase. With Re increasing, the time-averaged velocity and Reynolds stress decrease. The reattachment length increases in exponential law with blockage ratio and in power function with Re. The strong anisotropic turbulent flow in entrance-blocked narrow rectangular channel was evaluated by the Lumley triangle. In the streamwise plane, the anisotropic states in the recirculation vortex spread in the Lumley triangle, and they are two-dimensional wall turbulence in shear-flow layer and streamwise flow. Generally, the anisotropic states are dominated by two-dimensional wall turbulence and show tendency toward isotropic turbulence with Re increasing. The anisotropic states become towards isotropic in the recirculation vortex, stable in two-dimensional wall turbulence in shear-flow layer and one-dimensional turbulence in streamwise flow with blockage ratio increasing. These findings improve understandings of turbulent flow in the entrance blocked narrow channel. [ABSTRACT FROM AUTHOR]

The design of hydrokinetic plants in hydropower canals involves the choice of the array layout, rotor geometry, turbine spacing, and array spacing, and necessitates the control of the resultant backwater to avoid upstream flooding hazards. Several works in the literature have shown that array power optimization is feasible with small spacings between the arrays, disregarding the limitation in the power output induced by backwater upstream. In this study, a 1-D channel model with a Double Multiple Streamtube code and wake sub-models are integrated to predict an array layout that will maximize the array power. The outputs of the conducted sensitivity analysis confirm that this design enabled improved power conversion with closely spaced turbines and largely spaced arrays, thus allowing for a partial recovery of the total head variation for a new array deployed upstream. In addition to the quantitative assessment of the mechanical power converted, the tool enables depth control from the downstream undisturbed flow station to the backwater exhaustion far upstream, thereby increasing its flexibility. Furthermore, it overcomes the limitations of actuator disc models by considering rotor’s fluid dynamic losses. The results show that power output linearly scales for a limited number of arrays (≤5), whilst the variation in water depth variation follows a power law from the most downstream array towards upstream, regardless of the plant size. Finally, the maximum upstream inflow depth is demonstrated to become asymptotic for large multi-array plants under ideal conditions.

The model-scale tunnel is used in this investigation to analyze the maximum smoke temperature rise of the interconnected tunnel for various longitudinal ventilation velocities, blockage ratios, and heat release rates where the fire is at the confluence of the underground interconnected tunnel. The results showed that the longitudinal ventilation velocities of both the ramp upstream of the fire source and the adjacent ramp influenced the maximum temperature rise under the underground interconnected tunnel, and the ventilation of both ramps jointly affected the maximum temperature rise. The change in the maximum temperature rise depends on who is more affected by the longitudinal ventilation velocity or the vehicle blockage ratio. As the longitudinal ventilation velocity in the interconnected tunnel increases, the convective heat transfer near the fire source increases, resulting in a decrease in the maximum temperature rise, and the effect of the blockage ratio on the maximum temperature rise is reduced. In this paper, a maximum temperature rise prediction model suitable for the case of blockage in the interconnected tunnel is proposed.

The present study focuses on the mitigation of shock wave using novel geometric passages in the flow field. The strategy is to produce multiple shock reflections and diffractions in the passage with minimum flow obstruction, which in turn is expected to reduce the shock wave strength at the target location. In the present study the interaction of a plane shock front (generated from a shock tube) with various geometric designs such as, 1) zig-zag geometric passage, 2) staggered cylindrical obstructions and 3) zigzag passage with cylindrical obstructions have been investigated using computational technique. It is seen from the numerical simulation that, among the various designs, the maximum shock attenuation is produced by the zig-zag passage with cylindrical obstructions which is then followed by zig-zag passage and staggered cylindrical obstructions. A comprehensive investigation on the shock wave reflection and diffraction phenomena happening in the proposed complex passages have also been carried out. In the new zig-zag design, the initial shock wave undergoes shock wave reflection and diffraction process which swaps alternatively as the shock front moves from one turn to the other turn. This cyclic shock reflection and diffraction process helps in diffusing the shock wave energy with practically no obstruction to the flow field. It is found that by combining the shock attenuation ability of zig-zag passage (using shock reflection and diffraction) with the shock attenuation ability of cylindrical blocks (by flow obstruction), a drastic attenuation in shock strength can be achieved with moderate level of flow blocking. [ABSTRACT FROM AUTHOR]

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The paper aimed to study the flow structure at the wake of the bluff body with altered blockage ratio (BR) keeping the fixed aspect ratio (AR). The study was conducted by a finite volume technique using commercial software Ansys-Fluent. The CFD analysis of the bluff body is mainly subjected to the lower subcritical Reynolds number range (5000 to 15,000) along with blockage ratio as an important factor. The flow parameters such as drag coefficient, pressure, and kinetic energy variations are analyzed here for the Reynolds number (Re) and BR. It was observed that at fixed Re, the drag coefficient (CD) increases with an increase in the BR while decrease with increasing Re for a fixed value of BR.

This research was intended to investigate the effect of the geometry and blockage ratio of the flame connector on the combustion characteristics of wood pellets on a wood pellet stove. Combustion characteristics observed in this research included flame visualization, flame temperature, specific fuel consumption, and corrected-specific fuel consumption. Two types of geometry were used as a flame connector, i.e. disk and cone; for each geometry, the blockage ratio was varied at 0,6, 0,7, and 0,8. The blockage ratio was varied by change the number of the circular hole on the flame connector. Combustion in wood pellet stove without flame connector was then used as a comparison. The flame visualization test was applied to find out the color and dimensions of the flame, which was then used to determine the temperature on this flame. The water boiling test method version 4.2.3 was used to determine specific fuel consumption and corrected specific fuel consumption. The combustion chamber of the wood pellet stove has a dimension of 15 cm diameter and 20 cm height. Wood pellet supply used intermittent - continuous feeding system with a mass flow rate of 16 gram/minute. Flame visualization shows that stove with cone flame connector has a larger flame dimension and flame dimension decreases with the increasing of blockage ratio. The highest flame temperature occurred near the outlet of the combustion chamber, the same conditions were observed in all variations experiment. The application of flame connector involves greater specific fuel consumption due to longer boiling time. However, a correction factor by considering the amount of evaporated water results from corrected specific fuel consumption of wood pellet stove with cone flame connector which is smaller compared to the other type of wood pellet stove.

The present work describes the design process of a 3D-printed prototype of a three-blade horizontal-axis hydrokinetic water turbine (HAHWT). The employed blade profile is an EPPLER818, which was previously studied through the Q-Blade software according to the velocity range presumed (v < 1 m/s) in the experiments. The prototype performance was studied in a recirculating water channel at the Polytechnic Engineering School of Mieres (Oviedo University), with a gate of variable height at the channel end, which allows for performing different hydrodynamic scenarios upon varying the considered flow rate. The results show that the extracted power increases due to the equally increased blockage ratio, which represents the ratio between the turbine area and the channel area. However, an excessive increase in the blockage ratio corresponds to a power reduction effect due to the reduction in the effective area and the generation of a two-phase air-water condition.

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.

In this study, the coefficient of performance (COP) and relative coefficient of performance (COPR) of an air-based standing wave thermoacoustic refrigerator (ASWTAR) was investigated. All relevant components of the thermoacoustic refrigerator were fully available in the system. Heat pipes with a total heating power of 30W were installed into the cold-side and hot-side heat exchangers. The circular stacks with plugging ratios of 0.67, 0.71 and 0.74 and the spiral stacks with plugging ratio of 0.74 were tested and compared. The results show that the circular stack is the optimum stack geometry for ASWTAR. Similarly, a circular stack with a blockage ratio of 0.71 has the best performance, producing the lowest cold side temperature (Tc) and the highest temperature difference (ΔTm). Under no cooling load and 2W cooling load, Tc, ΔTm, COP and COPR were 22.4 °C, 30.5 °C, 0.93 and 9.57%, respectively. Finally, it is clear that due to the use of more suitable materials, the COP and COPR of a circular stack are greater than those of a spiral stack.

The aerodynamic resistance induced by a high-speed metro train entering and operating in a tunnel with a speed of 120 km/h is simulated using a three-dimensional, compressible turbulence model. An overset mesh method is adopted to solve the moving boundary problem, and the flow field around the train is simulated with a k-omega SST turbulence model to obtain accurate stress values at the walls. The selected model is verified with experimental and numerical data from the literature. Then, numerical simulations are performed to analyse the formation mechanisms of the pressure and friction drags. The aerodynamic drag basically stabilizes after the expansion wave passes the train head. The results reveal that the variations in friction resistance are related to the direction of the Mach wave, and this trend differs from that observed for pressure resistance. Mach wave influences the velocity where it passes, and further influences the friction drag. Result shows that friction drag of the train increases when encounter with compression wave propagated from the front or expansion wave propagated from the back, and decreases otherwise. The effects of the blockage ratio on the maximum and average pressure and friction resistance values of each train section are evaluated based on fitting functions. The predicted aerodynamic drag varies with the blockage ratio and for each train section, and the results are summarized and compared with predictions and experimental data from the literature. The variations in tunnel resistance and the overall trend are in good agreement with the previous results. Therefore, the findings presented in this study may provide a reference for the design of high-speed subway tunnels. [ABSTRACT FROM AUTHOR]

The present experimental work deals with the effect of partial blockage in the exit of the mixing channel on thermo-hydraulic performance of multi-pass jet plate solar air heater. This investigation is performed at IIT(ISM) Dhanbad campus (India) to obtain the results in terms of outlet temperature, collector efficiency, Temperature rise parameter (TRP), thermo - hydraulic performance parameter (THPP) and Nusselt number for double–pass cross and non–cross flow inline holes Jet Plate Solar Air Heater (JPSAH). Results for different ranges of operating and geometrical parameters such as, mass flow rate = 0.10–0.14 kg/s, Reynolds number, Re = 4741–10632, daily average solar flux incident on glass cover, I tavg. = 611–1113 W / m 2 and blockage ratio (H/Z 2) = 0, 0.33, 0.66 are presented. This work also compares the present results with simple double – pass solar air heater. It is found that the maximum percentage increments in THPP and Nu are 37 % and 79 % respectively at Re = 10247 and 9500 for BR = 0 compared to non-cross flow and simple double pass solar air heater. Based on the experimental data, correlations for the Nusselt number, friction factor and THPP have been developed. [ABSTRACT FROM AUTHOR]

In case of tunnel fire, vehicles can be blocked both upstream and downstream of the fire source. This paper presents a numerical study on influences of longitudinal ventilation and two same of obstacles placed upstream and downstream of the fire source on gas temperature distribution in a tunnel. The heat release rate, ventilation velocity and blockage ratio are varied. Results show that for a given ventilation velocity, as the blockage ratio increases, the inclination of fire plume gradually changes from the downstream to the upstream and the downstream smoke spread distance below the ceiling decreases. The Coanda effect (which refers to the adhesion of fluid on a convex surface) and the velocity vector field are introduced to explain the attachment of fire plume to the upstream obstacle. As the blockage ratio increases, the gas temperatures in the upstream of the fire source increase, while the gas temperatures in the downstream of the fire source decrease. The maximum temperature tends to decrease with increasing ventilation velocity. The effect of blockage ratio on maximum temperature is obvious for small ventilation velocities, while it becomes insignificant for large velocities. Then the maximum temperature model concerning the block effect and ventilation velocity are proposed. [ABSTRACT FROM AUTHOR]

In order to study the effect of thermal barrier coating deposition inside the film holes of turbine guide vanes on film cooling performance, film effectiveness on the suction side is measured by infrared thermal imaging technology. Film effectiveness is obtained at blockage ratios of 0, 0.2, 0.5, and 0.8, and blowing ratios of 0.7, 1.05, and 1.4. Film effectiveness decreases and the spanwise inhomogeneity becomes evident with an increase of the blockage ratio. When blowing ratios increase from 0.7 to 1.4, the surface averaged film effectiveness decreases by 55–60% at a large blockage ratio of 0.8, 21–27% at the middle blockage ratio of 0.5 and by no more than 11% at a small blockage ratio of 0.2. The rounded corners formed by blockage enhance the adhesion of the film at the small blockage ratio of 0.2, thereby improving the film cooling performance near the hole exit. There is a maximum increase of 0.2 in film effectiveness within four hole diameters.

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. [ABSTRACT FROM AUTHOR]

• The effect of rheological behavior of power-law fluids is evaluated numerically in confined and non-confined cases. • The Hopf bifurcation is defined for different blockage ratios and for different power-law fluids. • The effect of the power-law index on the promotion of mixing capacity in the flow is evaluated in 2D and 3D cases. • All numerical cases are solved using a recently proposed stabilized finite element method. • Numerical results can be used to understand more complex fluids under confinement and blockage conditions. In this work, the rheological behavior of power-law fluids on the flow dynamics is studied numerically. Studied cases include confined and non-confined problems that involve a single and two cylinders in tandem for 2-D and 3-D. For 2-D cases, the critical Reynolds numbers that define the Hopf bifurcation and its relation between the blockage ratio and the nature of the fluid are analyzed, including a complete CFD analysis. Results show that the blockage ratio, the addition of a second cylinder, and the rheological properties directly influence the flow's behavior. In this respect, the critical Reynolds number decreases by decreasing the power-law index and increasing the blockage ratio using shear-thinning fluids, and by increasing the power-law index in unconfined cases of shear-thickening fluids. For the 3-D case, different fluids are compared for a fixed Reynolds number and blockage ratio, proving that pseudoplastic fluids increase mixing capacity. [ABSTRACT FROM AUTHOR]

A micro preheated combustor with a flame holder was developed to take full advantage of both heat and flow recirculation. In the present study, the effect of blockage ratio (ξ) on the blowout limit of methane/air premixed flame is investigated. The blockage ratio increases by increasing the width of the flame holder in a combustion chamber with a constant height, while the inflow channel width correspondingly decreases. It is interesting to find that the blowout limit first increases and then decreases with increased ξ. Then, the underlying mechanisms are analyzed in terms of the flow field, stretch effect, heat transfer effect, and preferential transport. The analysis indicates that the direct effect of flow field on the present non-monotonic blowout limit is marginal. A larger stretch effect and heat-loss to the flame holder can induce the decrease in blowout limit from ξ = 0.6 to ξ = 0.7. The change rules of the heat recirculation and preferential transport effects are consistent with that of the blowout limit. The heat recirculation effect on the fresh mixture and preferential transport effect on the flame root are the best at ξ = 0.6, which contributes to the largest blowout limit at ξ = 0.6. Thus, a larger width of flame holder does not always guarantee a bigger blow-off limit. The present work not only provides guideline for reasonably choosing the size of the flame holder in the similar micro combustors but also helps us to gain insights into the combustion characteristics under the aforementioned competitive effects. • Blowout limit is studied at the synergistic effect of heat and flow recirculation. • Blowout limit first increases and then decreases with increased blockage ratio. • Heat recirculation and preferential transport effect determine the blowout limit. • A moderate width of flame holder is more suitable for the present combustor. • This study provides theoretical basis to optimize the combustor with bluff body [ABSTRACT FROM AUTHOR]

In this study, we numerically investigated the main factors affecting the flame blowout limit of bluff-body micro-combustors with a blockage ratio (δ) varying from 0.3 to 0.8; the combustion performance of these combustors is investigated as well. Measurements from the literature (Fan et al., 2012, Int J Hydrogen Energy, 37, 19,190–19,197) were used to validate the present numerical models. The predicted results are in good agreement with the experimental data, with a relative error of less than 10%. The results show that the blowout limit increases with δ in a non-monotonic manner. The establishment of a low-velocity recirculation behind the bluff body and recirculation of heat and key radicals help increase the flame blowout limit, whereas the stretching of reaction zones has an unfavorable effect. In contrast, heat loss contributes negligibly to the difference between the blowout limits in the micro-combustors for different values of δ. An extra pressure loss or initial power input is required to improve the blowout limits. An available size of the in-built bluff body should be carefully selected to maximize its efficiency and to considerably improve the blowout limit; however, this should be realized while ensuring a low cost of pressure loss when designing a micro-combustor. In general, δ = 0.5 is relatively optimal and recommended for the straight combustion channel discussed herein. Image 1 • Performance of a bluff-body micro-combustors with various blockage ratio (δ) was numerically investigated. • Blowout limit of the micro-combustor varies non-monotonically when increasing δ. • Blowout limit of the micro-combustor depends on the competition between the favorable and unfavorable factors. • Efficiency of the bluff-body to improve blowout limit should be carefully examined. • δ = 0.5 is relatively optimal and recommended for the straight combustion channel. [ABSTRACT FROM AUTHOR]