Local Flow Turbulence Research Articles

Geometric optimization of pin fins for enhanced cooling in a microchannel heat sink

Microchannel heat sinks (MCHS) are widely used for active cooling of electronics. Considerable enhancements in thermal performance are achievable by finning the MCHS to extend heat transfer area and create local flow turbulence. Yet, the literature lacks multi-objective geometric optimizations of the fin design. This study optimizes the geometry of tapered squared cross-section pin-fins in a square base MCHS, where the fins’ dimensions are manipulated in terms of their area, clearance, tapering width, and tapering height ratios (AR, CR, TWR, and THR, respectively). The optimization is based on an experimentally validated computational model. The performance is analyzed for 256 finned MCHS configurations, compared with a reference unfinned MCHS. The results show a high dependency of thermal performance on AR and THR, where the hydraulic performance is more affected by AR. The executed genetic optimization procedure demonstrated non-dominated designs with thermal resistances ranging between 0.00078 and 0.0033 K/W and pressure losses ranging between 0.229 and 2.64 kPa. By considering these two performance metrics to be equally important, a balanced design with AR, CR, TWR, and THR of 0.284, 5.52 × 10−5, 0.6, and 1.0 was identified, which brings down the thermal resistance and pressure losses to only 0.000983 K/W and 0.7133 kPa, respectively.

Flow Structure and Transition to Local Turbulence Downstream of an Asymmetric Narrowing that Imitates Arterial Stenosis

The results of experimental studies and numerical simulation of the flow structure in the separation region downstream of an asymmetric narrowing of smooth canal that simulates 70% one-sided stenosis of the artery are presented. The Reynolds number was equal to 1800. The instantaneous flow velocity vector fields were measured using the SIV technique. The numerical solution was obtained by the large eddy simulation (LES) method. Setting the disturbances in numerical simulation close to the experimental conditions made it possible to obtain a satisfactory agreement between the calculated and experimental velocity fields and the components of the Reynolds stress tensor. The data on formation of the local flow turbulence region behind the constriction and subsequent downstream flow relaminarization are obtained. It is shown that a pair of secondary eddies localized within the region of flow separation is formed near the throat of the constriction.

Flow Structure and Transition to Local Turbulence Downstream of an Asymmetric Narrowing that Imitates Arterial Stenosis

The results of experimental studies and numerical simulation of the flow structure in the separation region downstream of an asymmetric narrowing of smooth canal that simulates 70% onesided stenosis of the artery are presented. The Reynolds number was equal to 1800. The instantaneous flow velocity vector fields were measured using the SIV technique. The numerical solution was obtained by the large eddy simulation (LES) method. Setting the disturbances in numerical simulation close to the experimental conditions made it possible to obtain a satisfactory agreement between the calculated and experimental velocity fields and the components of the Reynolds stress tensor. The data on formation of the local flow turbulence region behind the constriction and subsequent downstream flow relaminarization are obtained. It is shown that a pair of secondary eddies localized within the region of flow separation is formed near the throat of the constriction.

Assessment of turbulence and cavitation models in prediction of vortex induced cavitating flow in fuel injector nozzles

Vortex-induced cavitation develops in high pressure fuel injector equipment in areas where despite the very high surrounding pressures (>100 MPa), the rotational motion of the fluid causes large pressure drops in the core of a well-organized longitudinal vortex structure leading to cavitation with normally string type correspondingly also called string cavitation ; typically, the size of the string cavitation is small compared to the overall dimensions while they develop in a transient mode linked with the local flow turbulence. To address these complex interaction processes, some turbulence models have been tested in a diesel injector nozzle for which quantitative experimental data are available and are utilized to assess the predictive capability of relevant models; the flow has been considered compressible, turbulent and isothermal. The closure models considered include the RNG k−ε, SST k−ω, RSM, WALE LES as well as a developed very-large eddy simulation model (VLES). The employed mass-transfer cavitation models are the Schnerr and Sauer (SS), Zwart-Gerber-Belamri (ZGB), HEM and a modified ZGB model with a self-adaptive adjustment of condensation coefficient. The results indicate that the RNG k−ε model and SST k−ω model fail to predict the string cavitation induced by vortex flow in the nozzle due to the drawback of the isotropic vortex-viscosity hypothesis. The RSM model could capture an acceptable string cavitation morphology. The numerical results utilizing the VLES model and LES model are in considerably good agreement with the experimental data, while the VLES model can provide the predictions of cavitating flow as precise as that using the LES model but on a much coarser mesh. The distributions of tangential velocity are similar to the velocity characteristics of Rankin vortex or Taylor vortex, which can help understand the coherent vortex structures in string cavitating flow. It is also shown that the RSM turbulence model with modified ZGB cavitation model improves the accuracy of prediction in the vortex-induced cavitation compared to the other three cavitation models.

Improvement of plant reliability based on combination of prediction and inspection of wall thinning due to FAC

Abstract Improvement of plant reliability based on reliability-centered-maintenance (RCM) is going to be undertaken in nuclear power plants (NPPs). RCM is supported by three types of maintenances: risk-based maintenance (RBM); time-based maintenance (TBM); and condition-based maintenance (CBM). RBM is supported by suitable combinations of prediction, inspection and maintenance of elemental defects in stages of their propagations. Especially the combination of prediction and inspection is one of the key issues to promote RBM. As an example of RBM, the fusion of the prediction and inspection related to wall thinning due to flow-accelerated corrosion (FAC) has been introduced. Early prediction of FAC occurrence and its propagation should be confirmed throughout the entire plant systems which should be accomplished by inspections at the target locations followed by timely application of suitable countermeasures such as water chemistry improvements. At the same time, transparency and traceability as well as accuracy are strongly required for the prediction. A one dimensional FAC code based on analysis of materials and water interaction was applied for evaluation of FAC risk as well as FAC occurrence and propagation. The details of FAC mechanism have been reported along with the expected accuracy of prediction to prepare for ensuring their transparency and traceability. Based on prediction results, primary points for inspections will be determined. Unfortunately, it is still difficult to evaluate some key parameters in the plants, e.g., local flow turbulence. From the inspections, accumulated data will be applied to confirm the accuracy of the code, to tune some uncertainties of the key data for prediction, and then, to increase their accuracy. The synergetic effects of prediction and inspection on application of effective and suitable countermeasures are expected. In the paper, the procedures for the combination of prediction and inspection are introduced.

Impact of branching angle on neointimal coverage of drug-eluting stents implanted in bifurcation lesions

To investigate the impact of branching angle (BA) on neointimal coverage of drug-eluting stents (DESs) in bifurcation lesions. Previous experimental studies indicated that BA influences the local flow turbulence and wall shear stress, which are associated with neointimal coverage of DESs. Fifty-five bifurcation lesions in 47 patients were evaluated by serial optical coherence tomography (OCT) before DES implantation and at follow-up. Neointimal coverage was assessed in cross-sectional OCT images containing the side branch; regions including the side branch ostium (SO) and vessel wall (VW) were assessed separately. BA was measured using angiography (Angio-BA) and longitudinal OCT imaging (OCT-BA). In the SO region, a significant negative correlation was found between the uncovered strut percentage and Angio-BA or OCT-BA (r=-0.41, P=0.0024; r=-0.33, P=0.0167, respectively) and a significant positive correlation was found between Angio-BA and average neointimal thickness (r=0.31, P=0.025), whereas no correlation was observed between OCT-BA and average neointimal thickness (r=0.20, P=0.158). In the VW region, no correlation was found between Angio-BA or OCT-BA and the uncovered strut percentage or average neointimal thickness. BA influence the neointimal coverage over DES struts in the SO at coronary bifurcation lesions, but not in those attached to the VW.

Abstract 12311: Side Branch Angle Influences the Neointimal Coverage of Drug-Eluting Stent in Coronary Bifurcation Lesions: An Optical Coherence Tomography Study

Background: Stent implantation for coronary bifurcation is intrinsically accompanied with inappropriate strut apposition around side branch (SB), which has potential risk for future development of stent thrombosis. Previous experimental studies indicated that side branch angle (SBA) influences the local flow turbulence and wall shear stress. The purpose of this study was to investigate the relation between SBA and the neointimal coverage of drug-eluting stent (DES) which were implanted in coronary bifurcation lesions. Methods: Forty-seven bifurcation lesions treated with DES were evaluated with FD-OCT in 39 patients (mean age; 65.7±13.9 y.o) at follow-up angiography. Each lesions were divided into the bifurcation lesions without kissing balloon technique (KBT) after stent implantation (non-KBT group; 28 lesions) and those with KBT (KBT group; 19 lesions). Neointimal coverage was assessed based on cross-sectional OCT images containing SB at 0.4mm interval, and separately evaluated in terms of jailing struts over the SB ostium (SO) and non-jailing struts attaching to the vessel wall (VW). BA was measured on the longitudinal reconstruction images of FD-OCT before stent implantation. Incidence of uncovered struts and neointimal thickness were measured. Results: In non-KBT group, the percentage of uncovered struts was significantly higher in lesions with SBA≦60°compared with those with SBA>60°at both SO and VW region (56.9±39.8% vs. 25.6±26.8%, p<0.05; 10.6±7.6% vs. 3.3±4.4%, p<0.01, respectively) and significant negative correlation was found between SBA and the percentage uncovered struts at both SO and VW regions (r=–0.55, p<0.01; r=–0.47, p<0.05, respectively). In KBT group, the percentage of uncovered struts was not significant difference between lesions with SBA≦60°and those with SBA>60°at both SO and VW region (36.7±35.3% vs. 36.6±34.8%; 10.9±10.4% vs. 8.6±12.7%, respectively) and no correlation was found between SBA and the percentage of uncovered struts at both SO and VW region. Conclusions: The neointimal coverage of DES was affected by in bifurcation lesion without KBT, but is not affected in bifurcation lesion with KBT. KBT has potential benefit for neointimal coverage of DES at coronary bifurcation lesion with narrow SBA.

Numerical simulation of a radial diffuser turbulent airflow

In the present work are presented results from numerical simulations performed with the ANSYS-CFX ® code. We have studied a radial diffuser flow case, which is the main academic problem used to study the flow behavior on flat plate valves. The radial flow inside the diffuser has important behavior such as the turbulence decay downstream and recirculation regions inside the valve flow channel due to boundary layer detachment. These flow structures are present in compressor reed valve configurations, influencing to a greater extent the compressor efficiency. The main target of the present paper was finding the simulation set-up (computational domain, boundary conditions and turbulence model) that better fits with experimental data published by Tabatabai and Pollard. The local flow turbulence and velocity profiles were investigated using four different turbulence models, two different boundary conditions set-up, two different computational domains and three different flow conditions ( Re in – Reynolds number at the diffuser inlet). We used the Reynolds stress (BSL); the k– ε; the RNG k– ε; and the shear stress transport (SST) k– ω turbulence models. The performed analysis and comparison of the computational results with experimental data show that the choice of the turbulence model, as well as the choice of the other computational conditions, plays an important role in the results physical quality and accuracy.

Particle drag in a dilute turbulent two-phase suspension flow

The drag between phases plays an important role in the study of a turbulent two-phase suspension flow and its physical understanding will greatly promote progress in theoretical treatments of a whole range of important industrial and technical problems involving such a flow. The conventional practice of using the results of measurements based on a single particle in a laminar stream for the case of a turbulent flow of a dilute suspension is questioned. An analysis of the results of local measurements of upward turbulent flows of a solid particle-air two-phase suspension leads to the determination of the realistic particle drag coefficient over a wide range of flow conditions. It is established that the particle drag can be described by the simple Stokes law, based on an apparent turbulent viscosity of the fluid for the particles in the suspension flow. A correlation is provided for this apparent turbulent viscosity in terms of the particle size and concentration in the suspension, the local flow turbulence Reynolds number and the particle-to-fluid density ratio.