Timothy Conover

Flow visualization studies of a swirling flow in a cylinder

Abstract The three-dimensional flow dynamics of a cylindrical cyclone with tangential inlet and tangential exit with an aspect ratio of 2 and 4 was studied using flow visualization. Photographs of circular and longitudinal cross sections of the cylinder revealed stable three-dimensional flow structure in the cylinder. The primary flow was observed to follow a helical pattern that surrounded a secondary flow. The secondary flow was characterized by a single vortex that circulated around the cylinder axis and occupied a large fraction of the cylinder diameter. The secondary flow was seen to include two cells of rotation in skewed planes for an aspects ratio of 2 and four cells for an aspect ratio of 4. Qualitatively, no change in flow structure was seen for a Reynolds number of 15,000–60,000.

Mapping of the Lateral Flow Field in Typical Subchannels of a Support Grid With Vanes

Lateral flow fields in four subchannels of a model rod bundle fuel assembly are experimentally measured using particle image velocimetry. Vanes (split-vane pairs) are located on the downstream edge of the support grids in the rod bundle fuel assembly and generate swirling flow. Measurements are acquired at a nominal Reynolds number of 28,000 and for seven streamwise locations ranging from 1.4 to 17.0 hydraulic diameters downstream of the grid. The streamwise development of the lateral flow field is divided into two regions based on the lateral flow structure. In Region I, multiple vortices are present in the flow field and vortex interactions occur. Either a single circular vortex or a hairpin shaped flow structure is formed in Region II. Lateral kinetic energy, maximum lateral velocity, centroid of vorticity, radial profiles of azimuthal velocity, and angular momentum are employed as measures of the streamwise development of the lateral flow field. The particle image velocimetry measurements of the present study are compared with laser Doppler velocimetry measurements taken for the identical support grids and flow condition.

The Effect of Support Grid Design on Azimuthal Variation in Heat Transfer Coefficient for Rod Bundles

Support grids are an integral part of nuclear reactor fuel bundle design. Features, such as split-vane pairs. are located on the downstream edge of support grids to enhance head transfer and delay departure from nucleate boiling in the fuel bundle. The complex flow fields created by these features cause spatially varying hert transfer conditions on the surfaces of the rods. Azimuthal variations in heat transfer for three specific support grid designs, a standard gird, split-vane pair grid, and disc grid, are measured in the present study using a heated, thin film sensor. Normalized values of the azimuthal variations in Nusselt number are presented for the support grid designs at axial locations ranging from 2.2 to 36.7 D h . Two Reynolds numbers, Re = 28,000 and Re = 42,000 are tested

Mock circulatory system of the Fontan circulation to study respiration effects on venous flow behavior.

We describe an in vitro model of the Fontan circulation with respiration to study subdiaphragmatic venous flow behavior. The venous and arterial connections of a total cavopulmonary connection (TCPC) test section were coupled with a physical lumped parameter (LP) model of the circulation. Intrathoracic and subdiaphragmatic pressure changes associated with normal breathing were applied. This system was tuned for two patients (5 years, 0.67 m2; 10 years, 1.2 m2) to physiological values. System function was verified by comparison to the analytical model on which it was based and by consistency with published clinical measurements. Overall, subdiaphragmatic venous flow was influenced by respiration. Flow within the arteries and veins increased during inspiration but decreased during expiration, with retrograde flow in the inferior venous territories. System pressures and flows showed close agreement with the analytical LP model (p < 0.05). The ratio of the flow rates occurring during inspiration to expiration were within the clinical range of values reported elsewhere. The approach used to set up and control the model was effective and provided reasonable comparisons with clinical data.

Control of Respiration-Driven Retrograde Flow in the Subdiaphragmatic Venous Return of the Fontan Circulation

Respiration influences the subdiaphragmatic venous return in the total cavopulmonary connection (TCPC) of the Fontan circulation whereby both the inferior vena cava (IVC) and hepatic vein flows can experience retrograde motion. Controlling retrograde flows could improve patient outcomes. Using a patient-specific model within a Fontan mock circulatory system with respiration, we inserted a valve into the IVC to examine its effects on local hemodynamics while varying retrograde volumes by changing vascular impedances. A bovine valved conduit reduced IVC retrograde flow to within 3% of antegrade flow in all cases. The valve closed only under conditions supporting retrograde flow and its effects on local hemodynamics increased with larger retrograde volume. Liver and TCPC pressures improved only when the valve leaflets were closed whereas cycle-averaged pressures improved only slightly (<1 mm Hg). Increased pulmonary vascular resistance raised mean circulation pressures, but the valve functioned and cardiac output improved and stabilized. Power loss across the TCPC improved by 12%–15% (p < 0.05) with a valve. The effectiveness of valve therapy is dependent on patient vascular impedance.

In Vitro Study of Pulmonary Vascular Resistance in Fontan Circulation With Respiration Effects

The Fontan operation is the final stage of palliative surgery for children born with single ventricle heart defects. The most common configuration is called total cavopulmonary connection (TCPC), wherein the inferior vena cava and superior vena cava are anastomosed directly to the pulmonary arteries; therefore the pulmonary circulation is driven by venous pressure only. The Fontan procedure, although successful in the early postoperative period, with time can decrease in efficiency or even fail within several years after the operation. The reasons of different clinical outcomes for some of the Fontan patients are not clear enough, even though it is commonly accepted that certain factors such as low pulmonary vascular resistance and proper shape and size of the TCPC construction are crucial for the succesful long term outcomes. Accordingly, one of the major problems is the increase in pulmonary vascular resistance due to altered hemodynamics after the surgery, causing venous hypertension and respiratory-dependent pulmonary regurgitation [1]. The main pulmonary arteries may also see increased resistance due to congenital malformations, surgical scarring, or deliberate surgical banding. Thus, the consequence of the increased pulmonary vascular resistance at both proximal and distal locations with respect to the TCPC junction, and its effect on the systemic pressures and flow rates, is the main objective of this study.Copyright

Design of an Experimental Mock Circulatory System for the Fontan Circulation

Each year, a small fraction of children are born with univentricular hearts, causing the lethal blue baby syndrome. Several preliminary operations are required to buy time until the child’s blood vessels grow to sufficient size. Once the child reaches an age of 3–5 years, the blood vessels have grown enough for the Stage 3 Fontan operation, in which the superior and inferior vena cavae are coupled directly to the pulmonary arteries in a cruciform junction [2,3]. After this operation, the heart is only pumping blood to the systemic circulation. Only residual pressure in the systemic veins and intrathoracic pressure change with respiration drive the flow into the lungs [5]. This circulation decreases the load on the heart, allowing the patients to survive with normal blood oxygen levels. This circulation decreases the load on the heart, allowing the patients to survive into their 20s and 30s. An aim of this study is to develop an experimental model of the Fontan circulation that can be readily adapted to simulate patient specific anatomies so as to assist in potential surgical decisions. Of interest is the study of chronic venous hypertension, a result of the Fontan circulation having no heart “vacuum” at the end of the vena cavae; it is known to cause liver failure. We also intend to examine the hypothesis of Hsia et al. [6] that decreasing sub-diaphragmatic venous flow reversal will improve functional outcome of the Fontan.Copyright

The Effect of Support Grid Design on Azimuthal Variations in Heat Transfer Coefficient for Rod Bundles

Support grids are an integral part of nuclear reactor fuel bundle design. Features, such as vane pairs, are placed on the downstream edge of support grids to enhance heat transfer and delay departure from nucleate boiling. The complex flow fields created by these features cause spatially varying heat transfer conditions on the surfaces of the rods. Azimuthal variations in heat transfer for a standard grid, split-vane pair grid, and disc grid are measured in the present study using a heated, thin film sensor. Normalized values of the variations in Nusselt number are presented for the support grid designs at axial locations ranging from 2.2 to 36.7 Dh . Two Reynolds numbers, Re = 28,000 and Re = 42,000 are tested. Results identify distinctive azimuthal variations in Nusselt number for all three of the support grid designs tested. The split-vane pair grid exhibits the largest variations in azimuthal heat transfer while the disc grid has the most uniform heat transfer.Copyright

In Vitro Validation of a Multiscale Patient-Specific Norwood Palliation Model.

In Norwood physiology, shunt size and the occurrence of coarctation can affect hemodynamics significantly. The aim of the study was to validate an in vitro model of the Norwood circulation against clinical measurements for patients presenting differing aortic morphologies. The mock circulatory system used coupled a lumped parameter network of the neonatal Norwood circulation with modified Blalock–Taussig (mBT) shunt with a three-dimensional aorta model. Five postoperative aortic arch anatomies of differing morphologies were reconstructed from imaging data, and the system was tuned to patient-specific clinical values. Experimentally measured flow rates and pressures were compared with clinical measurements. Time-based experimental and clinical pressure and flow signals within the aorta and pulmonary circulation branches agreed closely (0.72 < R2 < 0.95) for the five patients, whereas mean values within the systemic and pulmonary branches showed no significant differences (95% confidence interval). We validated an experimental multiscale model of the Norwood circulation with mBT shunt by showing it capable of reproducing clinical pressure and flow rates at various positions of the circulation with very good fidelity across a range of patient physiologies and morphologies. The multiscale aspect of the model provides a means to study variables in isolation with their effects both locally and at the system level. The model serves as a tool to further the understanding of the complex physiology of single-ventricle circulation.

Experimentally Modeling Patient-Specific Fontan Circulations Including Respiration Effects Using a Mock Circulatory System

The Fontan circulation is the result of a series of operations performed on children born with univentricular circulations (1). These congenital heart defects are uniformly fatal if left alone. After birth, an operation is performed to assure that the child receives enough blood flow to the lungs, but not too much in order to avoid pulmonary vascular disease. Once the child reaches 2–4 years of age, the child’s blood vessels are sufficiently large for the Fontan operation. The Fontan operation connects the great systemic veins directly to the pulmonary arteries, bypassing the right ventricle entirely. One method of the Fontan procedure, which is known as the total cavopulmonary connection (TCPC), achieves venous return to the pulmonary circulation without a ventricular power source. The load on the heart is reduced, and these patients can lead a normal life into adulthood; although late complications continue to prevent normal lifespan. One unique feature of the Fontan circulation is reliance of the inferior vena cava (IVC) flow on respiration, and flow reversal in the IVC and hepatic vein during expiratory phase of breathing (2). Hsia et al. (3) suggest that reducing flow reversal in the hepatic vein will improve the outcome of the Fontan operation. The goal of this study is to model experimentally the Fontan circulation for a variety of different patients using an adjustable mock circulatory system, which for the first time includes the influence of respiration.Copyright