Flow Path Resistance in Heterogeneous Porous Media Recast into a Graph-Theory Problem

This work aims to describe the spatial distribution of flow from characteristics of the underlying pore structure in heterogeneous porous media. Thousands of two-dimensional samples of polydispersed granular media are used to (1) obtain the velocity field via direct numerical simulations, and (2) conceptualize the pore network as a graph in each sample. Analysis of the flow field allows us to distinguish preferential from stagnant flow regions and to quantify how channelized the flow is. Then, the graph’s edges are weighted by geometric attributes of their corresponding pores to find the path of minimum resistance of each sample. Overlap between the preferential flow paths and the predicted minimum resistance path determines the accuracy in individual samples. An evolutionary algorithm is employed to determine the “fittest” weighting scheme (here, the channel’s arc length to pore throat ratio) that maximizes accuracy across the entire dataset while minimizing over-parameterization. Finally, the structural similarity of neighboring edges is analyzed to explain the spatial arrangement of preferential flow within the pore network. We find that connected edges within the preferential flow subnetwork are highly similar, while those within the stagnant flow subnetwork are dissimilar. The contrast in similarity between these regions increases with flow channelization, explaining the structural constraints to local flow. The proposed framework may be used for fast characterization of porous media heterogeneity relative to computationally expensive direct numerical simulations. Article Highlights A quantitative assessment of flow channeling is proposed that distinguishes pore-scale flow fields into preferential and stagnant flow regions. Geometry and topology of the pore network are used to predict the spatial distribution of fast flow paths from structural data alone. Local disorder of pore networks provides structural constraints for flow separation into preferential v stagnant regions and informs on their velocity contrast.

Evolution of Fracture Aperture in Quartz Sandstone under Hydrothermal Conditions: Mechanical and Chemical Effects

Fractures efficiently affect fluid flow in geological formations, and thereby determine mass and energy transport in reservoirs, which are not least exploited for economic resources. In this context, their response to mechanical and thermal changes, as well as fluid–rock interactions, is of paramount importance. In this study, a two-stage flow-through experiment was conducted on a pure quartz sandstone core of low matrix permeability, containing one single macroscopic tensile fracture. In the first short-term stage, the effects of mechanical and hydraulic aperture on pressure and temperature cycles were investigated. The purpose of the subsequent intermittent-flow long-term (140 days) stage was to constrain the evolution of the geometrical and hydraulic fracture properties resulting from pressure solution. Deionized water was used as the pore fluid, and permeability, as well as the effluent Si concentrations, were systematically measured. Overall, hydraulic aperture was shown to be significantly less affected by pressure, temperature and time, in comparison to mechanical aperture. During the long-term part of the experiment at 140 °C, the effluent Si concentrations likely reached a chemical equilibrium state within less than 8 days of stagnant flow, and exceeded the corresponding hydrostatic quartz solubility at this temperature. This implies that the pressure solution was active at the contacting fracture asperities, both at 140 °C and after cooling to 33 °C. The higher temperature yielded a higher dissolution rate and, consequently, a faster attainment of chemical equilibrium within the contact fluid. X-ray µCT observations evidenced a noticeable increase in fracture contact area ratio, which, in combination with theoretical considerations, implies a significant decrease in mechanical aperture. In contrast, the sample permeability, and thus the hydraulic fracture aperture, virtually did not vary. In conclusion, pressure solution-induced fracture aperture changes are affected by the degree of time-dependent variations in pore fluid composition. In contrast to the present case of a quasi-closed system with mostly stagnant flow, in an open system with continuous once-through fluid flow, the activity of the pressure solution may be amplified due to the persistent fluid-chemical nonequilibrium state, thus possibly enhancing aperture and fracture permeability changes.

Measurement of Two-Dimensional Void Fraction Distributions of Rising Bubbles in a Simulated Sub-Channel by Wire-Mesh Sensors at Conditions of Forced Convective and Stagnant Flows

Clarifying thermal-hydraulic characteristics in a nuclear reactor core is important in particular to enhance the thermo-fluid safety of nuclear reactors. Spacers installed in subchannels of fuel assemblies have the role of keeping the interval between adjacent fuel rods constantly. Similarly, in case of PWR the spacer has also the role as the turbulence promoter. When the transient event occurs, two-phase flow is generated by boiling of water due to heating of fuel rods. Therefore, it is important to confirm the two-phase flow behavior around the spacer. So, the effect of the spacer affecting the two-phase flow was investigated experimentally at forced convective flow condition. Furthermore, in order to improve the thermal safety of current light water reactors, it is necessary to clarify the two-phase flow behavior in the subchannels at the stagnant flow condition. So, the bubbly flow data around a simulated fuel rod were obtained experimentally at the stagnant flow condition. A wire-mesh sensor was used to obtain a detailed two-dimensional void fraction distribution around the simulated spacer and fuel rod. As a result of this research, the bubbly behavior around the simulated spacer and fuel rod was qualitatively revealed and also bubble dynamics in the sub-channels at the conditions of forced convective and stagnant flows were evaluated. The present experimental data are very useful for verifying the detailed three-dimensional two-phase flow analysis codes.

Abstract T P174: Intra-Arterial Signal on Arterial Spin Labeling Perfusion MRI Can Identify Stagnant Flow in Patients with Acute Middle Cerebral Artery Occlusion

Introduction: T2*-weighted imaging (T2*WI) can detect acute endovascular clots by the susceptibility vessel sign (SVS). Stagnant flow in front of middle cerebral artery (MCA) occlusion sites may contribute to intra-arterial high-intensity signal on arterial spin labeling (ASL) magnetic resonance imaging (MRI), making it another potential marker of MCA occlusion. We compared intra-arterial high-intensity signal and SVS in patients with symptomatic and asymptomatic MCA occlusion and patients without major vessel occlusion. Methods: We identified transient ischemic attack or ischemic stroke patients by (a) 3-T MRI, diffusion-weighted imaging, ASL, T2*WI, and magnetic resonance angiography (MRA) performed within 24 h after stroke onset and (b) the presence of MCA occlusion (n=34 patients) or the absence of major vessel occlusion (n=24 patients). We included asymptomatic patients with MCA occlusion (n=6). The presence or absence of intra-arterial high-intensity signal and SVS was recorded as was its coincidence with the presence of MCA occlusion on MRA. Results: In patients with acute ischemic stroke the sensitivity of intra-arterial high-intensity signal was significantly higher than of the SVS (88% vs 50%; p<0.05). The accuracy of intra-arterial high-intensity signal was also higher than of the SVS (93% vs 71%; p<0.05). Neither the intra-arterial high-intensity signal nor the SVS was observed in patients without major vessel occlusion. The presence or absence of intra-arterial high-intensity signal was highly consistent with the presence or absence of MCA occlusion on MRA (κ=0.74). Positivity for the intra-arterial high signal was higher in symptomatic than asymptomatic patients with MCA occlusion (88% vs 17%; p<0.05), suggesting that acute rather than chronic arterial occlusion contributes to the visibility of the intra-arterial high-intensity signal. Conclusions: The intra-arterial high-intensity signal on ASL could identify stagnant flow in front of occlusion sites due to acute arterial occlusion.

Intra-Arterial Tirofiban Infusion for Partial Recanalization with Stagnant Flow in Hyperacute Cerebral Ischemic Stroke

Early reocclusion is a major concern associated with poor clinical outcomes in patients with an ischemic cerebral stroke. This occurs most frequently in patients with partial initial recanalization. This study focuses on partial recanalization with stagnant antegrade flow after intravenous (IV) tPA or spontaneously, treated with the administration of intra-arterial (IA) tirofiban. Three patients with initial M1 occlusion on diagnostic studies had an occluded segment that was recanalized with stagnant flow after IV tPA or spontaneously. In all cases, IA tirofiban was administrated. We evaluated the distal blood flow and the degree of vascular narrowing in the pre and post-procedure angiography and at follow-up in addition to the clinical status. In all patients, severe vascular narrowing with stagnation of blood flow was detected in the initial M1. After infusion of IA tirofiban, improvement of the distal blood flow was achieved rapidly within 40 minutes in all patients. The severe vascular narrowing resolved rapidly in two patients without residual stenosis. In one patient, moderate vascular narrowing was still present. The median baseline National Institutes of Health Stroke Scale (NIHSS) scores were 18 and the median post-procedural NIHSS scores were 2 at two weeks. No intracerebral hemorrhage occurred in any of the patients. Treatment with IA tirofiban was safe and effective in patients with partial initial recanalization. It can be suggested that detection of any partial recanalization is time for administration of glycoprotein IIb-IIIa receptor inhibitor in hyperacute ischemic stroke.

Quality Defects and Analysis of the Microfluidic Chip Injection Molding

Compared with hot embossing, microfluidic chips injection molding is higher efficiency process and more suitable for mass production, but the quality control for injection molding is much more complex. Experiments indicate that the incomplete replication of the micro-channel and the sink mark for microfluidic chips are the chief defects to the molding. Simulation and theoretical analysis show that the stagnant flow of the melt in micro-channel and the shrinkage difference of the chips in different directions are the main reasons for molding defect. A set of new methods that how to control process parameter, design mold, and select polymer material is proposed to reduce or avoid the defects.

PIV Measurements of Flow in a Centrifugal Blood Pump: Steady Flow

Magnetically suspended left ventricular assist devices have only one moving part, the impeller. The impeller has absolutely no contact with any of the fixed parts, thus greatly reducing the regions of stagnant or high shear stress that surround a mechanical or fluid bearing. Measurements of the mean flow patterns as well as viscous and turbulent (Reynolds) stresses were made in a shaft-driven prototype of a magnetically suspended centrifugal blood pump at several constant flow rates (3–9L∕min) using particle image velocimetry (PIV). The chosen range of flow rates is representative of the range over which the pump may operate while implanted. Measurements on a three-dimensional measurement grid within several regions of the pump, including the inlet, blade passage, exit volute, and diffuser are reported. The measurements are used to identify regions of potential blood damage due to high shear stress and∕or stagnation of the blood, both of which have been associated with blood damage within artificial heart valves and diaphragm-type pumps. Levels of turbulence intensity and Reynolds stresses that are comparable to those in artificial heart valves are reported. At the design flow rate (6L∕min), the flow is generally well behaved (no recirculation or stagnant flow) and stress levels are below levels that would be expected to contribute to hemolysis or thrombosis. The flow at both high (9L∕min) and low (3L∕min) flow rates introduces anomalies into the flow, such as recirculation, stagnation, and high stress regions. Levels of viscous and Reynolds shear stresses everywhere within the pump are below reported threshold values for damage to red cells over the entire range of flow rates investigated; however, at both high and low flow rate conditions, the flow field may promote activation of the clotting cascade due to regions of elevated shear stress adjacent to separated or stagnant flow.