Effect Of Blockage Ratio Research Articles

Effects of Copper Foam Wall/Obstacles on the Deflagration-to-Detonation Transition

To investigate the process of deflagration-to-detonation transition (DDT) of the copper foam wall in the pulse detonation engine, detonation combustion experiments were carried out using ethylene and oxygen-enriched air (oxygen concentration: 40%) as fuel and oxidant. The effects of copper foam pore size, thickness, installation length, and structure on the flame acceleration were studied experimentally, as were the effects of blockage ratio and mixture equivalence ratio. The results show that among the four pore sizes selected, the 10 PPI case produces the fastest flame development on the porous flat wall. For conditions with successful detonation, the 10 PPI case reduces the DDT distance and time by 34.97 and 42.57%, respectively, compared with the 40 PPI case. Regarding the installation length of copper foam, it is found that copper foam with a large pore size (10 PPI) and a short installation length achieves stable detonation. The general structure of copper foam was also analyzed, including the porous flat wall, porous obstacle, and solid obstacle with the same installation length. These three structures effectively accelerate the initiation of detonation. Particularly, porous obstacles show more pronounced effects on flame acceleration than the commonly used solid obstacles. These results are instructive for optimizing the short-distance ignition ability of pulsed detonation engines.

Influences of longitudinal ventilation and two same of obstacles placed upstream and downstream of the fire source on gas temperature distribution in a tunnel

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.

CFD simulations of vapour cloud explosions using PDRFoam

Computational Fluid Dynamics (CFD) has gained a lot of popularity in the consequence assessments of gas explosions in the past couple of decades. This work reviews Porosity/ Distributed Resistance (PDR) approach-based CFD solver (PDRFoam) for gas explosions. The PDR approach is used to model the effect of small-scale obstacles and only solve for the large scale congestion. PDR-based modified Favre averaged equations for mass, momentum, enthalpy, and regress variable are solved using PDRFoam solver, which is developed as a new application in OpenFOAM. The evaluation of the solver is done against medium-scale standard benchmark experiments. Further, the effect of block-age ratio, different types of fuels, and different obstacle diameters is explored. Using PDRFoam simulations, the pressure–time series, the flame arrival times, and the flame speeds are predicted and the same are compared with experiments. Compared to standard body-fitted grid simulations, the PDR approach is computationally economical; The predicted results are in good agreement with experiments.

Effects of blockage ratio on the propagation characteristics of hydrogen-rich gas rotating detonation

The propagation characteristics of a hydrogen-rich gas/air rotating detonation wave (RDW) were investigated for different blockage ratios (BRs). Two rotating detonation chamber (RDC) widths were used in combination with different RDC exit widths to obtain different RDC BRs. The variations in the RDW propagation modes and wave velocities at different BRs and equivalence ratios (ERs) were studied and analyzed. The experimental results show that four types of RDW propagation modes can be obtained (single wave, single wave/counter-double waves hybrid mode, triple waves, and unstable triple waves) by changing the BRs and ERs. For BR > 0.64, the RDW exhibits a triple waves mode. The RDC width also affects the RDW propagation mode. The results show that at low BRs, the change in the RDW propagation mode owing to the injection pressure difference is the main influence mechanism. As the BR increases, the influence of the reflected shock wave from the exit of the RDC increases, which plays an important role in the generation of the triple waves mode. The stability of RDW propagation can be improved by appropriately increasing the blockage ratio in the RDC. The 26 mm RDC width at BR = 0 results in a maximum wave velocity of 1933.8 m/s. Moreover, the stability of the RDW propagation is poor at low and high BRs.

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

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

Study on the Effect of Blockage Ratio on Maximum Smoke Temperature Rise in the Underground Interconnected Tunnel

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.

Investigating the Effects of Blockage Ratio on the Performance of a Surface-piercing Propeller in Free Surface Water Tunnel Tests

This paper investigates the effect of the immersion ratio parameter on the hydrodynamic performance of three surface-piercing propellers with diameters of 0.125, 0.132 and 0.140m at different advancing speeds. Experimental tests have been carried out in the free surface water tunnel of the Babol Noshirvani University of Technology. The results showed that the maximum thrust coefficient of three propellers occurs in the velocity range of 3-3.5 m/s. This interval represents the transition area of the three propellers. Also, the effect of the blockage ratio on the hydrodynamic coefficients of three propellers relative to the advance coefficient has been studied. By increasing the immersion depth raises the propeller's wet surface and increases the thrust and torque hydrodynamic coefficients. However, growing the propeller's diameter to 0.140m causes the effect of the blockage ratio parameter by increasing the immersion and the propeller's torque experiences a decreasing trend. Therefore, maximum propeller efficiency value with diameter 0.140m in immersion ratio 0.60 and 0.70, incresing 38% and 44%, respectively; relative to other proepllers. Also, the curve of the efficiency gradient of three propellers in the optimum immersion ratio of 0.40 compared to the advancing coefficient shows that the maximum efficiency gradient occurs in the range of 0.7 to 0.9.

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

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

Flame Acceleration and DDT in a Channel with Continuous Triangular Obstacles: Effect of Blockage Ratio

ABSTRACT Experiments were conducted in a stoichiometric hydrogen-oxygen mixture at initial pressure of 1 atm to study the flame acceleration and deflagration-to-detonation transition (DDT) in a 20 mm high channel equipped with continuous triangular obstacles. Effect of blockage ratio (br) was explored by considering different obstacle heights of 0, 1 mm, 3 mm, 5 mm, and 7 mm, corresponding to br = 0, 0.1, 0.3, 0.5, and 0.7, respectively. High-speed schlieren photography and OH* chemiluminescence recording were used to visualize the processes. Results show that blockage ratio has a significant effect on the scales and velocities of the vortices generated in the gaps between neighboring obstacles and consequently on the subsequent flame-vortex interactions. Higher br leads to faster flame acceleration via stronger flame-vortex interactions. The intricate shock-flame interactions cause successive local explosions in the br = 0.3, 0.5, and 0.7 cases, creating conditions for DDT. An eventual choking regime occurs in the br = 0.7 case due to energy losses by continuous diffractions at the obstacle vertices. It was found that the onset of DDT is triggered when the critical height of the unobstructed passage is approximately greater than three detonation cell sizes. The mechanism of DDT changes as the br decreases. One important feature observed is that the br = 0.1 case has the shortest DDT distance. In addition, the final flame speed (~740 m/s, 1.4 times unburned sound speed) before DDT for br = 0.1 is significantly smaller than those (~1300 m/s, 2.4 times unburned sound speed) for br ≥ 0.3. The reason is that the low obstacles retain the characteristics of the obstacle, while mimicking the wall roughness, resulting in both shock compression of unburned gas and viscous heating in the boundary layer ahead of the flame front. The combined effects promote the occurrence of DDT for the case with br = 0.1.

Effect of blockage ratio on flow of a viscoelastic wormlike micellar solution past a cylinder in a microchannel.

We present experiments on the flow of a viscoelastic wormlike micellar solution around cylinders (radius R) confined in straight microchannels (width W). Thirteen flow geometries are tested where the blockage ratio is varied over a wide range 0.055 ≤ BR = 2R/W ≤ 0.63. Experiments are performed at negligible Reynolds number, and for Weissenberg numbers Wi = λU/R up to 1000, where U is the average flow speed and λ is the relaxation time of the fluid. Micro-particle image velocimetry is used to characterise the flow state at each BR and Wi. In all of the geometries, a first critical Weissenberg number marks a transition from symmetric flow to an asymmetric but time-steady flow state, while a second higher critical Weissenberg number marks the onset of time-dependent flows. However, we report a clear shift in behaviour over a narrow intermediate range of 0.33 ≲ BR ≲ 0.41. Channels with BR ≤ 0.33 fall in a 'low' BR regime, with instabilities that originate from the downstream stagnation point, while those with BR ≥ 0.44 fall in a 'high' BR regime, with instabilities developing at the upstream stagnation point. Behaviour within the newly-identified intermediate BR regime is complex due to the competing influence of the two stagnation points. We summarise all our results in a flow state diagram covering Wi-BR parameter space, clearly defining the different regimes of blockage ratio for the first time. Our results contribute to the understanding of the complexities of viscoelastic flow in this benchmark geometry.

Effect of blockage ratio on flow characteristics in obstructed open channels

Effect of blockage ratio on flow characteristics in obstructed open channels

Effects of an obstacle on the deflagration behavior of premixed liquefied petroleum gas-air mixtures in a closed duct

This work aims to investigate the influences of an obstacle on the deflagration behaviors of premixed liquefied petroleum gas (LPG)-air mixtures. Experiments were conducted in a closed tube, with a length/diameter ratio of 5.9, at ambient pressure and temperature. The effects of blockage ratios (BRs, 0–0.9) on the flame propagation features, the combustion time, the explosion pressure, the pressure rising rate, and the deflagration index of LPG-air mixture were observed and analyzed. The results indicated that the flow field structure of the unburned gas induced by the obstacle increased the degree of flame front wrinkling and sharply increased the flame propagation speed. During the subsequent propagation, various extremely irregular flame shapes emerged on the flame front, and the flame propagation speed continually accelerated as the distance from the ignition source increased. However, 1.5 m from the ignition source, the flame propagation speed began decreasing, after which the flame alternately accelerated and decelerated. Furthermore, the obstacle drastically changed the LPG explosion pressure dynamics: the maximum explosion pressure increased, the time to reach maximum explosion pressure decreased, and the deflagration index increased, namely, the explosion hazard level increased. When crossing the obstacle, under the action of the Rayleigh-Taylor instability regime, the sudden acceleration of the flame in the duct generated the pressure waves. These pressure waves interacted with the flame front and produced flame disturbances. Thereby the flame front was distorted and deformed, and the flame front area and the local mass and energy transmission were increased.

Thermal-hydraulic performance and design parameters in a curved-corrugated channel with L-shaped baffles and nanofluid

High heat generation is seen as a major issue in most mechanical and manufacturing industries, carrying with huge sub-problems. One of the possible cooling techniques is the combination of two or more passive methods, in particular for those parts with complex geometry. In this study, flow structure and heat transfer characteristics of the novel channel namely: curved-corrugated channel are numerically studied with using ZnO-water nanofluid and presence of L-shaped baffles. The influences of corrugations, baffles manner arrangement, and geometric parameters; corner angle (γ= 30°,45°,60°, and 90°) and blockage ratio (BR=0.25,0.3. 0.35, and 0.4), at different Reynolds number (8000–32000) and volume fraction of ZnO particles (0–4%) are evaluated using thermal-hydraulic performance method. The multi-phase mixture and the κ-ε model are used to simulate turbulent nanofluid flows inside the curved-corrugated channels at constant temperature condition (T = 355 K). The results reveal that the formation of vortex flow and increased turbulence due to effects of corrugations and baffles can improve the heat transfer enhancement. Inline arrangement of baffle is superior to the staggered arrangement in thermal-hydraulic performance (PEC), and at lowest angle 30° provide the best PEC. Regarding the friction ratio and compared to those of a plain channel, the effect of blockage ratio is considerable as it yields a multiplier impact of the corner angle. Reducing the blockage ratio and the corner angle, i.e. 0.25 and 30°, yields to the best PEC at 1.99. New correlations for Nusselt number and friction factor for baffled curved-corrugated channel with using nanofluid are also reported.

Blowout limit of premixed flame in a micro preheated combustor with a flame holder at different blockage ratios

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.

Effect of Blockage Ratio on Heat Transfer and Pressure Drop in Rotating Ribbed Channels at High Rotation Numbers

At high rotation numbers, the rotational effects on heat transfer and flow could be diverse among the channels with different blockage ratios. However, most studies are conducted under low rotation number (less than 0.25) and selected blockage ratio. This paper experimentally investigates the effect of rib blockage ratio (ranges from 0 to 0.3) on pressure loss and heat transfer in a rotating square channel under high rotation number (up to 0.81). The ribs staggered on leading and trailing walls were oriented 90° to the mainstream flow. The Reynolds number and the wall-to-fluid temperature ratio varied from 20 000 to 40 000 and 0.08 to 0.2, respectively. The results showed that a larger blockage ratio resulted in a better heat transfer but a higher pressure drop. The optimum blockage ratio was 0.1 for the best thermal performance. The rotational effects were weakened in the passage with a higher blockage ratio, where the critical rotation number could not be observed. Moreover, the heat transfer enhancement induced by rotation was more significant when the temperature ratio increased. Finally, the correlations were developed for the pressure drop and the convective heat transfer on the leading and trailing edges.

CFD simulations of hydrogen deflagration in slow and fast combustion regime

This paper presents CFD modelling of hydrogen-air deflagrations. The proposed mathematical models are validated with the experiments provided to SARNET2 (Severe Accident Research NETwork of Excellence 2) project [A. Bentaib, A. Bleyer, N. Chaumeix, B. Schramm, P. Kostka, M. Movahed, H.S. Kang and M. Povilaitis, Final results of the SARNET Hydrogen deflagration Benchmark Effect of turbulence on flame acceleration. (2012), pp. 1–15; A. Bentaib, A. Bleyer, N. Meynet, N. Chaumeix, B. Schramm, M. Höhne, P. Kostka, M. Movahed, S. Worapittayaporn, T. Brähler, H. Seok-Kang, M. Povilaitis, I. Kljenak and P. Sathiah, SARNET hydrogen deflagration benchmarks: Main outcomes and conclusions. Ann. Nucl. Energy 74 (2014), pp. 143–152. doi:10.1016/j.anucene.2014.07.012]. The goal of SARNET2 project was to investigate the effect of blockage ratio on flame propagation and pressure evolution during deflagration of a lean mixture of hydrogen and air. Three blockage ratios were tested: BR = 0, 0.33 and 0.63. The experiments were carried out in the ENACCEF facility. The proposed mathematical models test the effect of turbulence models and radiative heat losses on flame front position and absolute pressure in the testing facility. A good agreement between the calculated and experimental results for the three BRs is found for the SST k-ω model alone and in combination with the γ transition model when radiative heat losses are accounted for in the modelling. In addition, an unstable tulip flame is evidenced in our CFD simulations what corroborates previous reports on premixed combustion in a closed tube.

Thermo-Hydraulic Performance of Round Tubes Installed with Double V-Shaped Winglet Vortex Generators: Effect of Blockage Ratio

Thermo-Hydraulic Performance of Round Tubes Installed with Double V-Shaped Winglet Vortex Generators: Effect of Blockage Ratio

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.

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.

CFD study of blockage ratio and boundary proximity effects on the performance of a tidal turbine

The effects of blockage ratio and boundary proximity on tidal turbine performance are quantified in this study using computational fluid dynamics (CFD). Reynold averaged Navier–Stokes (RANS) equations are solved using the commercial CFD code ANSYS CFX. Steady-state analyses are performed using a rotating frame of reference technique, and a shear stress transport turbulence model is used. Results from the numerical model are validated with experimental data. RANS CFD simulations are performed over tip speed ratios (TSRs) from 1 to 9. The simulation-based performance curve approximately match the experimental performance curve, with errors ranging from 4 to 9%. Blockage ratios from 0.02 to 0.19 are evaluated for a constant depth of two rotor diameters to study the effect of blockage on turbine performance. Results show that shaft power increased by 13 and 47% for TSRs of 5.11 and 9.05, respectively, when the blockage ratio increased from 0.02 to 0.19. Additionally, for these TSRs and blockage ratios the turbine thrust increased by 7 and 10%. The presence of a boundary layer resulted in a calculated thrust increases between 1.6 and 2.3% and power between 2.7 and 3.5% for the entire range of evaluated blockage ratios.