Effect of Gravity on Liquid Plug Transport Through an Airway Bifurcation Model

Many medical therapies require liquid plugs to be instilled into and delivered throughout the pulmonary airways. Improving these treatments requires a better understanding of how liquid distributes throughout these airways. In this study, gravitational and surface mechanisms determining the distribution of instilled liquids are examined experimentally using a bench-top model of a symmetrically bifurcating airway. A liquid plug was instilled into the parent tube and driven through the bifurcation by a syringe pump. The effect of gravity was adjusted by changing the roll angle (ϕ) and pitch angle (γ) of the bifurcation (ϕ=γ=0deg was isogravitational). ϕ determines the relative gravitational orientation of the two daughter tubes: when ϕ≠0deg, one daughter tube was lower (gravitationally favored) compared to the other. γ determines the component of gravity acting along the axial direction of the parent tube: when γ≠0deg, a nonzero component of gravity acts along the axial direction of the parent tube. A splitting ratio Rs, is defined as the ratio of the liquid volume in the upper daughter to the lower just after plug splitting. We measured the splitting ratio, Rs, as a function of: the parent-tube capillary number (Cap); the Bond number (Bo); ϕ; γ; and the presence of pre-existing plugs initially blocking either daughter tube. A critical capillary number (Cac) was found to exist below which no liquid entered the upper daughter (Rs=0), and above which Rs increased and leveled off with Cap. Cac increased while Rs decreased with increasing ϕ, γ, and Bo for blocked and unblocked cases at a given Cap>Cac. Compared to the nonblockage cases, Rs decreased (increased) at a given Cap while Cac increased (decreased) with an upper (lower) liquid blockage. More liquid entered the unblocked daughter with a blockage in one daughter tube, and this effect was larger with larger gravity effect. A simple theoretical model that predicts Rs and Cac is in qualitative agreement with the experiments over a wide range of parameters.

Four Years of Successful Operation at Mercer Generating Station Following Feedwater Heater Tube Repair: An Alternative to Replacement

In the late 1990’s, two of the high-pressure feedwater heaters at PSEG Power LLC’s Mercer Generating Station began to experience tube leaks after almost 40 years of operation. Prior to the repair, these leaks were occurring at least once per month. Based on eddy current and video probe inspections, the area of tube degradation appeared to be isolated to the desuperheater region of the bundle. PSEG Power LLC performed a life assessment of the feedwater heaters and, based on the good condition of the remainder of the heaters, determined that repair of the desuperheater region could improve the reliability and significantly extend the operating life of the heat exchangers. The heater condition, coupled with a repair vs. replace economic model, led to the decision to sleeve the feedwater heaters. During the April 1999 maintenance outage, Framatome ANP performed repair work on the two high pressure feedwater heaters. This repair consisted of installing an 11 foot-2 inch (3404 mm) sleeve in all outlet tube ends, spanning the desuperheating region. The sleeving method used consisted of a three step process. After the sleeve was installed, its lower end was roll expanded within the tubesheet. Next, the entire length of the sleeve in the desuperheater section was expanded into close contact with the parent tube. The final installation step was upper hydraulic expansions, above the desuperheating zone end plate, in nondefective portions of the tube. This installed sleeve became the pressure boundary in the desuperheater section of the feedwater heater. After more than 4 years of operation, no tube leaks have been noted and the need to replace the components has been eliminated for the foreseeable future.

First report of Sphenothallus Hall, 1847 in the middle Cambrian

Sphenothallus Hall, 1847 is a widespread Paleozoic marine taxon that has been interpreted most recently as a tubiculous annelid or other ‘worm’ or as a thecate hydrozoan or scyphozoan cnidarian (e.g., Mason and Yochelson, 1985; Feldmann et al., 1986; Van Iten et al., 1992, 1996; Neal and Hannibal, 2000; Zhu et al., 2000). Members of this genus are characterized by a very gently tapered, finely lamellar apatitic tube bearing a closed subconical holdfast and a pair of robust longitudinal thickenings situated at the ends of the tube's greatest diameter (Zhu et al., 2000). Rarely, the tube exhibits an internal transverse wall that extends adaperturally along the inner surface of the tube proper and may also exhibit a subcylindrical terminal protuberance (Van Iten et al., 1992, figs. 5, 6). The transverse wall of Sphenothallus is essentially identical in gross morphology and microstructure to the schott (apical wall) of conulariids, an extinct group of thecate cnidarians (Van Iten, 1991, 1992a, 1992b; Jerre, 1994; Van Iten et al., 1996; Hughes et al., 2000) that may have been closely related to Sphenothallus (Van Iten et al., 1992). Clarke (1913, pl. 26, figs. 16-18; see also Van Iten et al., 1992, fig. 3) illustrated a compound specimen of S. sica consisting of numerous “daughter” tubes arranged in a highly regular manner (in opposition and evenly spaced) along a single, relatively large “parent” tube whose test wall appears to be confluent with the base of the “daughter” tubes. This compound specimen is distinctly different from the more common associations of epibiontic, holdfast-bearing Sphenothallus arranged in a less orderly fashion on Sphenothallus tubes or other shells (see for example Feldmann et al., 1986, fig. 2), and probably is best interpreted as a clonal colony. Sphenothallus tubes, both solitary and branched, are most similar to thecae of solitary and colonial hydrozoan and scyphozoan polyps, differing from these relatively delicate structures mainly in being mineralized and in bearing a pair of longitudinal thickenings. Like conulariids, Sphenothallus occurs in rocks originally deposited in a broad spectrum of marine facies (open carbonate shelf to restricted shale basin), but is perhaps most conspicuous in restricted basin and shelf slope facies represented by dark shales and lime mudstones.

Experimental Investigation of Oscillatory Flow Through a Symmetrically Bifurcating Tube

To provide a quantitative description of the convection field of gas transport through the lung under both low and high-frequency ventilation conditions, volume-cycled, purely oscillatory flow has been investigated in a symmetrically bifurcating model bronchial bifurcation. Significant differences in the flow properties that developed as the Reynolds number varied from 750 to 950 and the dimensionless frequency varied from 3 to 12 are described. At low frequency, the axial velocity field was found to approximate closely that of a steady flow through a bifurcation. However, even at α = 3, secondary velocity fields were confined to within a few diameters of the bifurcation, with less than 10 percent of the magnitude of the axial velocity. At high frequency they were still slower and more limited. These secondary velocity observations are discussed in terms of a physical mechanism balancing inviscid centripetal acceleration with viscous retardation. As the dimensionless frequency increased but the flow amplitude decreased, the magnitude of the axial drift velocity field was found to decrease. In addition, a burst of high-frequency velocity fluctuations was detected in both the axial and secondary velocity measurements in the parent tube, in low-frequency flow, during the deceleration phase of expiration. The position and timing of this burst suggest that it derives from the free shear layer in the parent tube. Stability criteria for the flow were therefore evaluated.

Inspiratory and Expiratory Steady Flow Analysis in a Model Symmetrically Bifurcating Airway

Steady inspiratory and expiratory flow in a symmetrically bifurcating airway model was studied numerically using the finite element method (FIDAP). Flows of Reynolds number of 500 and 1000 during inspiration and a flow of Reynolds number of 500 during expiration were analyzed. Since the geometry of the bifurcation model used in this study is exactly the same as the model used in the experimental studies, the computed results were compared to the experimental findings. Results show that most of the important flow features that were observed in the experiment, such as the skewed velocity profiles in the daughter branches during inspiration and velocity peak in the parent tube during expiration, were captured in the numerical simulation. Quantitatively, the computed velocity profiles are in good agreement with the measured profiles. This comparison validates the computational simulations.

Steady Expiratory Flow in a Model Symmetric Bifurcation

A model symmetric bifurcation was employed to simulate steady expiratory flow in the upper part of the human central airways. A two color, two component laser Doppler anemometer was used to measure both the axial flow and the secondary flow at three different Reynolds numbers of 518, 1036, and 2089, corresponding to Dean numbers of 98, 196, and 395. The test section is a symmetric bifurcation of constant cross-sectional area with a branching angle of 70 degrees. The flow rate into the two daughter branches was about the same. Results show that in the junction plane, velocity profiles in the daughter branches are skewed towards the inner walls. In the parent tube, just downstream of the flow divider, the velocity profile is biconcave with a dip at the center but this is rapidly transformed into a velocity peak. In a plane transverse to the bifurcation plane, parabolic velocity distribution was conserved through the daughter branches. In the parent tube, the transverse profiles became flat downstream of the flow divider and developed a defect at the center further downsteam towards the end of the parent tube part of the bifurcation. The velocity defect was confined to a small region in the vicinity of the centerline. Helical motion typified by symmetric vortices was observed in the daughter branches. In the parent tube, a set of four vortices induced by the turning of the flow was observed.

Sinusoidal Variation of Wall Shear Stress in Daughter Tube Through 45 Deg Branch Model in Laminar Flow

The present paper describes the experimental implemented work on the flow situation through a branch model having a daughter tube bifurcated from a parent tube at 45 deg. Experiments have been conducted utilizing an electrochemical method. The results show that, even in steady flow, the wall shear stress along the proximal wall in the daughter tube varies significantly with position in the form of a damped sine wave. For pulsating flow at the nondimensional pulsating frequency of α = 6.6, the above mentioned tendency appears to be severe, and the distribution of its amplitude in pulsating flow is similar to that of wall shear stress in steady flow.

Electrochemical Surface Shear Rate Evaluation in Models of the Renal Artery

Electrochemical measurements of surface shear rate were done in two renal artery models with 45 and 90-deg take-off angles to determine the extent to which the parent tube flow rate, flow division and branch shape influence flow at sites for atherogenesis. The dependence of surface shear rate on each of these independent variables was nonlinear; mathematical relationships which could predict surface shear rate at other flow rates, flow divison ratios or vessel shapes did not emerge. The results of this work along with reports of others, which demonstrate great variability in location and extent of atherosclerotic lesions, support the conclusion that pathological studies and flow studies should be done simultaneously if the role of flow in atherogenesis is to be elucidated.

Arterial bifurcation flows – effects of flow rate and area ratio

Velocity profiles and surface shear rates, for three model symmetrical bifurcations made of glass from dimensions based on the arterial system, were investigated. The models studied had area ratios of 0.75, 1.02, and 1.29, with a common included angle of 75°. Area ratio and parent tube flow rate were the two independent variables evaluated. Measurements were made with a tracer particle technique using cinephotography. Velocity profiles had their highest values on the inside, and lowest values on the outside, of the branch. Flow symmetry existed in the plane perpendicular to the plane of the bifurcation. Surface shear rates remained well above the daughter-tube developed values, between two and six diameters downstream from the carina. Shear rates below the daughter-tube developed value were found on the outside wall between the carina and two daughter-tube diameters downstream. Vortex-like flow was absent in this region for the 0.75 area ratio branch and was found above 900 Reynolds number in the 1.29 area ratio branch. The disturbed flow described by others in this region may not contain vortex-like streamlines for the physiologically important 0.75 area ratio.