Influence Of Blockage Research Articles

Assessment of RANS and LES in predicting the flow in a 61-pin wire-wrapped bundle with a porous blockage

One of the most promising nuclear reactor designs is the Liquid-Metal Fast Reactor (LMFR). Its fuel bundle rods are separated with wire spacers that are helically wrapped around each fuel pin. Since this bundle presents a tight rod pitch to diameter ratio, accumulation of debris, cladding deformation or detachment of the wires may generate the formation of porous blockages. Understanding the coolant behavior in LMFR fuel bundles under the influence of blockages is mandatory for a complete nuclear safety evaluation. Therefore, this paper presents Computational Fluid Dynamics (CFD) results using the RANS k-ε Realizable and the LES models to simulate the flow of a 61-pin wire-wrapped bundle with the presence of a porous blockage that obstructed six interior subchannels. The simulated Reynolds numbers were 5,000 (transition regime) and 14,000 (turbulent regime). Results of the first and second order velocity statistics were compared with the Particle Image Velocimetry (PIV) experimental data acquired in the Thermal-Hydraulics Research Laboratory facility of Texas A&M University. The simulations results were found in reasonable agreement with the PIV data. Additional analysis of the flow structures, pressure distribution and power spectral densities were also performed to give more insight of the flow characteristics in LMFR reactors under the presence of porous blockages.

Influence of Blockage on the Dynamic Characteristics of a Fluid Conveying Aluminium Tube

Influence of Blockage on the Dynamic Characteristics of a Fluid Conveying Aluminium Tube

Sub-Channel Analysis of a Hexagonal Sub-Assembly: Influence of Blockage and Axial Power Distribution on Critical Heat Flux

Abstract The study of thermal hydraulics of a hexagonal sub-assembly is essential to ensure the safe operation of liquid metal cooled fast reactors. Identifying the dryout location in fuel sub-assembly (FSA) is a precursor to the determination of safe Critical Heat Flux (CHF) margins. In this study, a sub-channel analysis code coupled with a film thickness model is employed to predict the CHF location in a hexagonal sub-assembly. A simple post-CHF heat transfer model is proposed and validated against the experimental data. The nature of flow resistance changes and operating conditions would significantly influence the occurrence of CHF. To this end, the effect of blockage (0.0 ≤ b ≤ 0.3) and axial power distribution (APD) on CHF is systematically investigated in a hexagonal sub-assembly. It was observed that the presence of blockage causes coolant flow maldistribution which results in an early occurrence of CHF for higher mass flux (G > 1500 kgm−2s−1) and lower inlet subcooling (ΔTsub ≤ 30 K) conditions for b = 0.3. Furthermore, a comparative study of uniform and sinusoidal heat flux distributions is performed. It was noticed that sinusoidal APD causes the early occurrence of CHF compared to uniform APD.

Influence and Mitigation of Pedestrian Blockage at mmWave Cellular Networks

The large spectral bandwidth at millimeter-wave (mmWave) frequencies provides a mean to achieve very high data rates in wireless communication systems. A unique characteristic of mmWave is that mmWave links are very sensitive to blockage and have large propagation path loss, which exhibits low line-of-sight (LoS) probability, unstable connectivity and unreliable communication. This paper studies the influence of the blockage in pedestrian scenario, explains in detail how blockage affects the mmWave propagation characteristics. In particular, we study the behavior of the blockage due to human mobility and how it affects the timescale for outage due to blockage using knife-edge diffraction model (KED). One of the existing solutions to overcome the influence of blockage, is to associate the user equipment (UE) with other available base stations (BSs) by handover (HO) if the serving BS is blocked. In this paper, for a pedestrian scenario, we propose two reinforcement learning (RL) based user association algorithms, which accounts for the past experience of the blockage on the position of the UE. One focuses on the reward to increase the sum LoS probability and is named as blockage-aware user association (BAUA). The other focuses on the reward to balance the trade-off between the throughput and the LoS probability and is named as modified BAUA. We compare the proposed algorithm with the conventional user algorithms such as the maximum throughput based algorithm and the maximum SINR based algorithm. Simulation results show that to increase the sum LoS probability BAUA would be suitable, and to increase the average throughput maximum throughput based method would be suitable.

XPERIMENTAL STUDY OF THE INFLUENCE OF BLOCKAGE ON CRITICAL VELOCITY AND BACKLAYERING LENGTH IN A LONGITUDINALLY VENTILATED TUNNEL

This study has been conducted to analyze the effect of vehicular blockage on critical velocity and backlayering length experimentally through a 3 m model tunnel (1:50 of a standard tunnel length). Three vehicle sizes in two or three arrays, occupying 3-15% of tunnel crosssection in six scenarios, were used. Based on this study, critical velocity decreases in the case that ventilation velocity reaches the fire source directly. However, critical velocity increases when the path of ventilation flows are blocked by blockages due to an increase in heat release rate through vehicular obstructions. In addition, the influence of blockages on backlayering length and its dimensionless analysis by the ratio of longitudinal ventilation velocity to critical velocity is also presented. It is found that the backlayering length follows an exponential relation with and without blockages. Moreover, the analysis of backlayering length with respect to the blockage section length shows that as backflow occurs along the ceiling and flows over blockage section, its heat transfer to surrounding increases leading to shorter backlayering length due to the enhanced inertial force and heat loss.

Influence of blockage and shape of a bluff body on the performance of vortex flowmeter with wall pressure measurement

Experimental and numerical investigations are carried out with various bluff body shapes to identify an appropriate shape which can be used for vortex flowmeter application. In both the cases vortex shedding frequency is inferred from the fluctuation of wall pressure. The numerical simulations are carried out with cylindrical and triangular bluff bodies to understand the vortex shedding phenomenon and to identify an appropriate turbulence model for this class of flows with wall pressure measurement. The simulations reveal that the k– ε RNG model predicts the Strouhal number closer to the experimental results than other models. The experimental investigations are carried out with several bluff body shapes, such as triangular, trapezoidal, conical, cylindrical and ring shapes, with water as the working medium. In this context, the effects of sampling rate, tap location and blockage effects are explored. The results suggest that the axisymmetric tapping is better than differential pressure tapping in terms of signal amplitude. The non-dimensional location of the static pressure tap is found to be 0.714 times diameter of pipe times blockage. The trapezoidal bluff body is found to be the best among all the bluff bodies investigated in terms of signal amplitude and constancy of Strouhal number. The vortex flowmeter performance is also measured under disturbed flow conditions created by using gate valve and bends. These results are significant because they provide an optimum bluff body shape and blockage, and also present the performance of vortex flow meter under disturbed flow conditions which is rather seldom reported in the literature.

Laminar Boundary Layer Development Around a Circular Cylinder: Fluid Flow and Heat-Mass Transfer Characteristics

This paper presents a comprehensive computational work on the hydrodynamic, thermal, and mass transfer characteristics of a circular cylinder, subjected to confined flow at the cylinder Reynolds number of Red=40. As the two-dimensional, steady and incompressible momentum and energy equations are solved using ANSYS-CFX (version 11.0), the moisture distributions are computed by a new alternating direction implicit method based software. The significant results, highlighting the influence of blockage (β=0.200–0.800) on the flow and heat transfer mechanism and clarifying the combined roles of β and moisture diffusivity (D=1×10−8–1×10−5 m2/s) on the mass transfer behavior, are obtained for practical applications. It is shown that the blockage augments the friction coefficients (Cf) and Nusselt numbers (Nu) on the complete cylinder surface, where the average Nu are evaluated as Nuave=3.66, 4.05, 4.97, and 6.51 for β=0.200, 0.333, 0.571, and 0.800. Moreover, the blockage shifts separation (θs) and maximum Cf locations (θCf−max) downstream to the positions of θs=54.10, 50.20, 41.98, and 37.30 deg and θCf−max=51.5, 53.4, 74.9, and 85.4 deg. The highest blockage of β=0.800 encourages the downstream backward velocity values, which as a consequence disturbs the boundary layer and weakens the fluid-solid contact. The center and average moisture contents differ significantly at the beginning of drying process, but in the last 5% of the drying period they vary only by 1.6%. Additionally, higher blockage augments mass transfer coefficients (hm) on the overall cylinder surface; however, the growing rate of back face mass transfer coefficients (hm−bf) is dominant to that of the front face values (hm−ff), with the interpreting ratios of h¯m−bf/h¯m=0.50 and 0.57 and h¯m−ff/h¯m=1.50 and 1.43 for β=0.200 and 0.800.

Hydrodynamic-thermal boundary layer development and mass transfer characteristics of a circular cylinder in confined flow

The effects of blockage on the hydrodynamic, thermal and mass transfer characteristics of a circular cylinder (CC) and their association with each other are investigated numerically, by considering the influence of blockage ( β = 0.333–0.800) on the flow and heat transfer mechanisms in conjunction with moisture diffusivity ( D = 1 × 10 −8–1 × 10 −5 m 2/s) to show how much mass transfer behavior and phenomena are affected. As some comprehensive ANSYS-CFX runs are performed in the hydrodynamic and thermal fields around the CC, the moisture distributions within the CC are evaluated by Alternating Direction Implicit method. It is determined that blockage causes thinner hydrodynamic and thermal boundary layers, rises the frictional and thermal activities, and shifts the separation locations ( θ s) downstream to θ s = 50.20°, 41.98° and 37.30° for β = 0.333, 0.571 and 0.800. In the complete blockage scenario set, stagnation point heat transfer values are evaluated to be above those of the back-face, signifying the superior heat transfer enhancing capability of the stagnation point momentum activity when compared with the impact of downstream vortex system. The influence of moisture diffusivity on the overall drying times is determined to advance with stronger blockage. As the back face mass transfer coefficients ( h m-bf ) rise with a high β, the contrary is valid for front face values ( h m-ff ), with the interpreting ratios of h ¯ m-bf / h ¯ m = 0.51 and 0.57 and h ¯ m-ff / h ¯ m = 1.49 and 1.43 for β = 0.333 and 0.800.

Influence of blockage and upstream disturbances on the performance of a vortex flowmeter with a trapezoidal bluff body

Experimental investigations are conducted on vortex flowmeter with the differential wall pressure measurement method. The bluff body employed is trapezoidal in shape and water is used as the working fluid. Three different blockages (0.14, 0.24 and 0.3) are considered in this study. The performance of the vortex flowmeter is studied both under fully developed condition and in the presence of flow disturbances. The flow disturbance is created using 45° swirl generator and gate valve placed at different upstream distances. The performance of the flowmeter is also evaluated in the presence of a Laws Vanes flow conditioner placed downstream of the swirl generator. The blockage ratio of 0.3 is found to be the best among all the blockages studied under both disturbed and undisturbed conditions.

Heat transfer from heated bodies in air-water mist flow. 4th report. An experiment on a horizontal cylinder with uniform wall temperature under the influence of blockage.

An experimental study has been made on convective heat and mass transfer from a horizontal heated cylinder in downward flow of air-water mist at a blockage ratio of 0.4. Measured two-phase local heat transfer coefficients agree fairly well with the authors' numerical solutions, obtained previously of the front surface of a cylinder, over the ranges of a mass flow ratio of 0-4.5 ×10-2, a temperature difference between cylinder and air of 10∼43 K, a gas Reynolds number of (7.9∼23)×103, a Rosin-Rammler size parameter of 105∼168μm, and a dispersion parameter of 3.4∼3.7. Heat transfer augmentation, two-phase to single-phase, of over 19 times was attained at the forward stagnation point. As for heat transfer of the rear part of the cylinder, an empirical formula is derived by taking into account the dimensionless governing variables, i.e., coolant-feed and evaporation parameters.

Effect of Blockage on Periodic Pressure Fluctuations and Shock in Cavitating Flow

The intensity of the cavitation shock, the geometry of the cavity and the associated pressure fluctuations in the wake are measured for six different circular cylindrical models. The influence of blockage on these parameters is discussed in detail. In general, with an increase in the blockage ratio, the shock intensity becomes stronger with an increase in the cavitation number. This is noticeable even for small models. With the onset of the shedding-cavity-vortex flow, the shock intensity jumps to a higher value, and the Strouhal number increases. A higher blockage at a given flow velocity or a higher velocity at a fixed blockage tends to accentuate this phenomenon. Shedding of the cavity ceases for λ≥7 (σ≤0.6), however the rear of the cavity shows oscillatory instability resulting in the higher Strouhal number.

An actuator-disc model for the prediction of abrupt stall in an axial compressor rotor

The representation of loss in a cascade by the appearance of blockage has been extended to deal with blade rows by the use of a source distribution to represent this blockage, and in the case of the actuator disc approximation, the presence of sources is confined to an axi-symmetric dìstribution over the actuator disc. It is found that if a typical dependence of loss (and consequently diffusion ratio) upon incidence for each section of an axial compressor rotor is represented in this manner, the influence of blockage on the axial velocity distribution may be found using the potential equation combined with the usual actuator disc approximation. Study of the behaviour of the controlling ordinary differential equation for the axial velocity ahead of the disc reveals that as the flow is reduced, the equation contains a singularity within the range of radius and a meaningful solution does not exist. This result is interpreted as the limit to continuous operation and reasonable agreement between this predicted limit and the appearance of abrupt stall (experimentally) is found.