Flow Blockage Ratio Research Articles

Multi-fidelity modelling of shark skin denticle flows: insights into drag generation mechanisms.

We investigate the flow over smooth (non-ribletted) shark skin denticles in an open-channel flow using direct numerical simulation (DNS) and two Reynolds averaged Navier-Stokes (RANS) closures. Large peaks in pressure and viscous drag are observed at the denticle crown edges, where they are exposed to high-speed fluid which penetrates between individual denticles, increasing shear and turbulence. Strong lift forces lead to a positive spanwise torque acting on individual denticles, potentially encouraging bristling if the denticles were not fixed. However, DNS predicts that denticles ultimately increase drag by 58% compared to a flat plate. Good predictions of drag distributions are obtained by RANS models, although an underestimation of turbulent kinetic energy production leads to an underprediction of drag. Nevertheless, RANS methods correctly predict trends in the drag data and the regions contributing most to viscous and pressure drag. Subsequently, RANS models are used to investigate the dependence of drag on the flow blockage ratio (boundary layer to roughness height ratio), finding that the drag increase due to denticles is halved when the blockage ratio δ/h is increased from 14 to 45. Our results provide an integrated understanding of the drag over non-ribletted denticles, enabling existing diverse drag data to be explained.

Study on the influence of flow blockage in severe accident scenario of CAP1400 reactor

Deformed fuel rods can cause a partial blockage of the flow area in a subchannel. Such flow blockage will influence the core coolant flow and further the core heat transfer during the reflooding phase and subsequent severe accidents. Nevertheless, most of the system analysis codes simulate the accident process based on the assumed flow blockage ratio, resulting in inconsistencies between simulated results and actual conditions. This paper aims to study the influence of flow blockage in severe accident scenario of the CAP1400 reactor. First, the flow blockage model of ISAA code is improved based on the FRTMB module. Then, the ISAA-FRTMB coupling system is adopted to model and calculate the QUENCH-LOCA-0 experiment. The correctness and validity of the flow blockage model are verified by comparing the peak cladding temperature. Finally, the DVI Line-SBLOCA accident is induced to analyze the influence of flow blockage on subsequent CAP1400 reactor core heat transfer and core degradation. From the results of the DVI Line-SBLOCA accident analysis, it can be concluded that the blockage ratio is in the range of 40%–60%, and the position of severe blockage is the same as that of cladding rupture. The blockage reduces the circulation area of the core coolant, which in turn impacts the heat exchange between the core and the coolant, leading to the early failure and collapse of some core assemblies and accelerating the core degradation process.

Thermohydraulic Performance Improvement in Heat Exchanger Square Duct Inserted with 45° Inclined Square Ring

Thermal performance development, heat transfer structure, and flow behavior in the heat exchanger square duct equipped with a 45° inclined square ring are investigated numerically. The effects of flow blockage ratios and spacing ratios for the inclined square ring on fluid flow and heat transfer are considered. The Reynolds number (Re = 100–2000, laminar regime) based on the hydraulic diameter of the square duct is selected for the present work. The numerical domain of the square duct inserted with the 45° inclined square ring is solved with the finite volume method. The SIMPLE algorithm is picked for the numerical investigation. The heat transfer characteristics and flow topologies in the square duct inserted with the inclined square ring are plotted in the numerical report. The heat transfer rate, pressure loss, and efficiency for the square duct placed with the inclined square ring are presented in forms of Nusselt number, friction factor, and thermal enhancement factor, respectively. As the numerical results, it is detected that the heat transfer rate of the heat exchanger square duct inserted with the inclined square ring is around 1.00–10.05 times over the smooth duct with no inclined square ring. Additionally, the maximum thermal enhancement factor for the heat exchanger square duct inserted with the inclined square ring is around 2.84.

Improvements of COBRA-TF on the effect of flow blockage during a LB LOCA with consideration of fuel relocation phenomenon

Coolability of the partially blocked core in a large break loss-of-coolant accident (LB LOCA) is one of the most important thermal-hydraulic concerns for nuclear reactor safety analysis. During blowdown phase and early stage of reflooding phase in the LB LOCA, the prevailing wall-to-vapor convective heat transfer plays an important role on the decay heat removal process. Experiments on single-phase convective heat transfer to vapor were conducted in 5×5 heater rod bundles containing 3×3 ballooned rods of 90% flow blockage ratio with consideration of fuel relocation phenomenon. The obtained experimental data were used to assess the single-phase heat transfer enhancement models of the COBRA-TF code. The assessment results showed underprediction of local heat transfer downstream of the flow blockage. Therefore, a new correlation has been proposed to improve the prediction capability of the conventional models by more accurately describing the flow blockage effect. The new correlation predicted the local heat transfer satisfactorily within a 20% discrepancy of the experimental data for various kinds of flow blockage configurations.

Numerical heat transfer study in a round tube with 60° V-shaped rings

Turbulent flow and heat transfer behaviors in a 3-dimensional isothermal tube mounted repeatedly with 60° V-shaped rings (V-rings) are numerically investigated. The computation based on the finite volume method was conducted for the airflow rate in terms of Reynolds number (Re) in the range of 4000 to 20,000. The ring parameters include three flow blockage ratios, (BR=e/D=0.1, 0.15 and 0.2) and a single ring pitch ratio, PR=1. The computed result shows that friction factor and Nusselt number (Nu) increase with the increment in BR and the maximum thermal enhancement factor (η) of about 1.73 with Nu/Nu0 = 4.5 is found at BR=0.1 and Re=4000. For the need of maximum heat transfer enhancement (Nu/Nu0), the case of BR=0.2 V-ring is selected and modified since this case provides the highest heat transfer. The modification is made by cutting off some area of both ends of the BR=0.2 V-ring and the effect of cutting off (called end-cut ratio, ER=c/e) on thermal performance is also examined. The study reveals that for the end-cut V-ring, the friction factor is considerably decreased while the heat transfer rate reduces moderately with increasing ER. The end-cut V-ring yields the maximum η of 1.64 at ER=0.9, Re= 4000 with Nu/Nu0 = 5.4. Thus, the Nu/Nu0 of the end-cut one is about 20% higher than that of the BR=0.1 V-ring.

Numerical Study on Turbulent Forced Convection and Heat Transfer Characteristic in a Circular Tube with V-Orifice

Performance assessments on heat transfer, pressure loss, and thermal enhancement factor in the circular tube heat exchanger inserted with the V-orifices are investigated numerically. The influences of the blockage ratio, gap spacing ratio, and orifice arrangement are reported for turbulent regime, Re=3000–10,000. The finite volume method and SIMPLE algorithm are selected to solve the present problem. The mechanisms on flow and heat transfer characteristics are described. The periodic concepts on flow and heat transfer are also studied. The numerical results show that the gap spacing ratio is main reason for the changes of the flow and heat transfer topologies. The gap distance helps to adjust the optimum point of the thermal performance, especially at high flow blockage ratio. In addition, the optimum thermal performance of the present system is around 2.25 at the lowest Reynolds number, Re=3000.

Numerical investigation on turbulent forced convection and heat transfer characteristic in a square channel with discrete combined V-baffle and V-orifice

Turbulent forced convection, heat transfer and performance improvement in a square channel with discrete combined baffles (DCB), which combined V-baffle and V-orifice, are investigated numerically. The influences of the flow blockage ratios (BR=0.05, 0.10 and 0.15) and V-tip directions (V-tip pointing downstream called “V-Downstream” and V-tip pointing upstream called “V-Upstream”) are examined with a single pitch spacing ratio, PR=1, and attack angle, α=30o, for the Reynolds number, Re=5000–20,000. The computation results are reported in terms of flow visualizations, heat transfer characteristics, performance assessments. The results are compared with the smooth channel and the previous works. As the results, the DCB enhances the heat transfer rate and thermal efficiency due to the disturbance of the thermal boundary layer. The improvement of the heat transfer rate is around 2.8–6 times higher than the smooth channel depended on BR, V-tip directions and Re. In addition, the computational result reveals that the optimum thermal enhancement factor, TEF, is around 1.72 at BR=0.1, Re=3000 and V-Upstream.

Variations of bed elevations due to turbulence around submerged cylinder in sand beds

This paper presents the spatio-temporal variations in bed elevations and the near-bed turbulence statistics over the deformed bed generated around the submerged cylindrical piers embedded vertically on loose sediment bed at a constant flow discharge. Experiments were carried out in a laboratory flume for three blockage ratios in the range of 0.04–0.06 using three different sizes of submerged cylinders individually placed vertically at the centerline of the flume. Clear-water experimental conditions were maintained over the smooth sediment bed surface with a constant flow discharge (\(Q = 0.015\,{\rm m}^3/{\rm sec}\)), thereby giving three different cylinder Reynolds numbers \(Re_{D_c} = \frac{U_mD_c}{\nu }\) (=10200, 12750, 15300) away from the cylinder locations, where \(U_m\) is the maximum mean velocity, \(D_c\) is the cylinder diameter and \(\nu\) is the kinematic viscosity of fluid. Instantaneous sand bed elevations around the cylinders were recorded using a SeaTek 5MHz ultrasonic ranging system of net 24 transducers to estimate bed form migration, and the near-bed velocity data at transducer locations over the stable deformed bed around the pier-like structures were collected using down-looking three-dimensional (3D) Micro-acoustic Doppler velocimeter to estimate the bottom Reynolds shear stresses and the contributions of bursting events to the dominant shear stress component. The flow perturbation generated due to relatively lower flow blockage ratio favored to achieve the stable bed condition more rapidly than the others, and larger upstream scour-depth and deformed areas were noticed for greater flow blockage ratio due to larger cylinder diameter. For larger blockage ratio in the upstream of scour-hole near the bed, occurrences of probabilities of both boundary-ward interactions (Q1 and Q3) were the dominant; whereas in the downstream of the scoured region, occurrences of probabilities of second and third quadrant events (Q2 and Q4) were dominant. On the other hand, for the lower blockage ratio, quadrant (Q2) was dominant over Q4 in the downstream of scour-hole, and in the upstream of scour-hole, quadrant Q4 was the dominant.

Numerical Investigation on Turbulent Forced Convection in Heating Channel Inserted with Discrete V-Shaped Baffles

The numerical examinations on flow visualization, heat transfer characteristic and performance improvement in a square duct inserted with discrete V-shaped baffles (DVB) are reported. The DVB is designed to reduce the pressure loss when compared with the V-shaped baffle. The influences of the flow blockage ratios (b/H, BR = 0.05-0.20), V-tip directions (V-Downstream and V-Upstream) are investigated for Reynolds number, Re = 3000-20,000, with a single pitch ratio and flow attack angle of 1 and 30°, respectively. As the results, the DVB can generate the vortex flow and impinging flow through the test section that assists to develop the thermal efficiency. The augmentations on the heat transfer coefficient and pressure drop are around 2.5-5.75 and 4-22.5 times above the smooth duct, respectively. The best thermal efficiency in forms of thermal enhancement factor, TEF, around 2.32 is detected at Re = 3000, BR = 0.10 and V-Upstream.

Heat transfer from a short plate behind an obstacle in tandem

Heat transfer from a heated plate placed behind an obstacle in cross-flow was investigated in this experimental study. In the experiment three types of obstacles were investigated circular; rectangular and triangular cross-sectioned, in order to test the effects of geometrical shape of the obstacle on heat transfer from the test plate for various orientation angles of the plate ( β = 0°, +45°, 90° and −45°). The experiments have been conducted for three different obstacle sizes of each geometry. During the study the effects of obstacle size, and separation between the obstacle and test plate for different orientation angles were examined for range of Reynolds number 2700 < Re l < 13,000, dimensionless separation number P ranging from 1.81 to 13.82 and flow blockage ratio varied of ζ = 9.33%, ζ = 18.66%, and ζ = 28% for D = l , 2 l and 3 l respectively. Based on the measurements, average Nusselt number was determined using lump heat capacitance method. From the results, it was found that the shape and size of the obstacle and plate orientation angle significantly affect the heat transfer from a plate in the wake flow region.

Numerical investigation of laminar heat transfer in a square channel with 45° inclined baffles

Abstract A numerical investigation of laminar periodic flow and heat transfer in a three-dimensional isothermal-wall square channel fitted with 45° inclined baffles on one channel wall is carried out in the present work. The finite volume method is introduced and the SIMPLE algorithm has been implemented for all computations. The fluid flow and heat transfer characteristics are presented for Reynolds numbers ranging from 100 to 1200. The 45° baffle mounted only on the lower channel wall has a height of b and an axial pitch length (L) equal to channel height (H). Effects of flow blockage ratios, BR = b/H = 0.1–0.5, on heat transfer and pressure loss in the square channel are examined and also compared with the typical case of the transverse baffle (or 90° baffle). It is found that apart from the rise of Reynolds number, the increase in the blockage ratio with the attack angle (α) of 45° results in considerable increases in the Nusselt number and friction factor values. The use of the 45° baffle can help to generate a streamwise main vortex flow throughout the channel leading to fast and chaotic mixing of flow between the core and the wall regions. In addition, the computational results reveal that the significant increase in heat transfer rate is due to impingement jets induced by a longitudinal vortex pair (P-vortex) of flow, appearing on the upper, lower and baffle trailing end side walls. The appearance of vortex-induced impingement flows created by the baffles leads to the maximum thermal enhancement factor of about 2.2 at BR = 0.4 and Re = 1200. The enhancement factor of the 45° baffle investigated is found to be higher than that of the 90° baffle for all Reynolds numbers and baffle heights.

Three-Dimensional Analysis of Turbulent Heat Transfer and Flow through Mixing Vane in A Subchannel of Nuclear Reactor

The present work evaluates the effects of mixing vane shape on the flow structure and heat transfer downstream of mixing vane in a subchannel of fuel assembly, by obtaining velocity and pressure fields, turbulent intensity, flow-mixing factors, heat transfer coefficient, and friction factor using three-dimensional RANS analysis. Four different shapes of mixing vane designed by the authors, are tested to evaluate the performances in enhancing the heat transfer, in comparison with commercialized split vane. Standard k-ε model are used as a turbulence closure model, and, periodic and symmetry conditions are set as boundary conditions. The flow blockage ratio is kept constant, but the twist angle of mixing vane is changed. The results with three turbulence models are compared with experimental data.

원자로 부수로내 혼합날개를 지나는 삼차원 열유동 해석

The present work evaluates the effects of mixing vane shape on the flow structure and heat transfer downstream of mixing vane in a subchannel of fuel assembly、 by obtaining velocity and pressure fields, turbulent intensity、flow-mixing factors, heat transfer coefficient and friction factor using three-dimensional RANS analysis. Four different shapes of mixing vane, wliich were designed by the authors、were tested to evaluate the performances in enhancing the heat transfer. Standard k-ε model is used as a turbulence closure model, and. periodic and symmetry conditions are set as boundary conditions. The flow blockage ratio is kept constant、 but the twist angle of mixing vane is changed. The results with three turbulence models were compared with experimental data.

Thermal Analysis and PLIF Imaging of Reacting Flow behind a Disc Stabilizer with a Central Fuel Jet

In this experimental work we studied the effects of fuel-to-air velocity ratio and air flow blockage ratio on the reacting flow pattern, thermal structure, and flame features in a bluff-body flame stabilizer. Spatial and temporal distributions of temperature and streamlines were measured by very fine thermocouples and LDV, respectively, and PLIF-OH image was also analyzed. The results show that all flame zones were controlled by the recirculating flow upstream, which was formed by interaction between the concentric jets. Spatial and temporal data of temperature and velocity fluctuations and images of hydroxyl radical concentration in the flow reveal a closed relationship between combustion and transient behavior of fluid dynamics. The large-scale motion of the recirculating flow played a determining role in the mixing, reaction, and stabilization in the entire flame area. Moreover, the velocity ratio and blockage ratio are both important factors to induce the position change of the high temperature reaction zone. The position of the reaction zone will move upstream from the outer shear layer of the air-driven vortex while the velocity ratio increases, and finally transfer to the inside area of the recirculation zone. The phenomenon becomes more apparent as the blockage ratio gets larger.