Clement Kleinstreuer

Experimental and theoretical studies of nanofluid thermal conductivity enhancement: a review

Nanofluids, i.e., well-dispersed (metallic) nanoparticles at low- volume fractions in liquids, may enhance the mixtures thermal conductivity, knf, over the base-fluid values. Thus, they are potentially useful for advanced cooling of micro-systems. Focusing mainly on dilute suspensions of well-dispersed spherical nanoparticles in water or ethylene glycol, recent experimental observations, associated measurement techniques, and new theories as well as useful correlations have been reviewed.It is evident that key questions still linger concerning the best nanoparticle-and-liquid pairing and conditioning, reliable measurements of achievable knf values, and easy-to-use, physically sound computer models which fully describe the particle dynamics and heat transfer of nanofluids. At present, experimental data and measurement methods are lacking consistency. In fact, debates on whether the anomalous enhancement is real or not endure, as well as discussions on what are repeatable correlations between knf and temperature, nanoparticle size/shape, and aggregation state. Clearly, benchmark experiments are needed, using the same nanofluids subject to different measurement methods. Such outcomes would validate new, minimally intrusive techniques and verify the reproducibility of experimental results. Dynamic knf models, assuming non-interacting metallic nano-spheres, postulate an enhancement above the classical Maxwell theory and thereby provide potentially additional physical insight. Clearly, it will be necessary to consider not only one possible mechanism but combine several mechanisms and compare predictive results to new benchmark experimental data sets.

Flow structures and particle deposition patterns in double-bifurcation airway models. Part 1. Air flow fields

The understanding and quantitative assessment of air flow fields and local micronparticle wall concentrations in tracheobronchial airways are very important for estimating the health risks of inhaled particulate pollutants, developing algebraic transfer functions of global lung deposition models used in dose-response analyses, and/or determining proper drug-aerosol delivery to target sites in the lung. In this paper (Part 1) the theory, model geometries, and air flow results are provided. In a companion paper (Part 2, Comer et al. 2001), the history of particle deposition patterns and comparisons with measured data sets are reported. Decoupling of the naturally dilute particle suspension makes it feasible to present the results in two parts. Considering a Reynolds number range of 500 ≤ Re D ≤ 2000, it is assumed that the air flow is steady, incompressible and laminar and that the tubular double bifurcations, i.e. Weibels generations G3-G5, are three-dimensional, rigid, and smooth with rounded as well as sharp carinal ridges for symmetric planar, and just rounded carinas for 90° non-planar configurations. The employed finite-volume code CFX (AEA Technology) and its user-enhanced FORTRAN programs were validated with experimental velocity data points for a single bifurcation. The resulting air flow structures are analysed for relatively low (Re D = 500) and high (Re D = 2000) Reynolds numbers. Sequential pressure drops due to viscous effects were calculated and compared, extending a method proposed by Pedley et al. (1977). Such detailed results for bifurcating lung airways are most useful in the development of global algebraic lung models.

Computational mechanics of Nitinol stent grafts

A finite element analysis of tubular, diamond-shaped stent grafts under representative cyclic loading conditions for abdominal aortic aneurysm (AAA) repair is presented. Commercial software was employed to study the mechanical behavior and fatigue performance of different materials found in commercially available stent-graft systems. Specifically, the effects of crimping, deployment, and cyclic pressure loading on stent-graft fatigue life, radial force, and wall compliances were simulated and analyzed for two types of realistic but different Nitinol materials (NITI-1 and NITI-2) and grafts (expanded polytetrafluoroethylene-ePTFE and polyethylene therephthalate-PET). The results show that NITI-1 stent has a better crimping performance than NITI-2. Under representative cyclic pressure loading, both NITI-1 and NITI-2 sealing stents are located in the safe zone of the fatigue-life diagram; however, the fatigue resistance of an NITI-1 stent is better than that of an NITI-2 stent. It was found that the two types of sealing stents do not damage a healthy neck artery. In the aneurysm section, the NITI-1&ePTFE, NITI-1&PET, and NITI-2&PET combinations were free of fatigue fracture when subjected to conditions of radial stress between 50 and 150mmHg. In contrast, the safety factor for the NITI-2&ePFTE combination was only 0.67, which is not acceptable for proper AAA stent-graft design. In summary, a Nitinol stent with PET graft may greatly improve fatigue life, while its compliance is much lower than the NITI-ePTFE combination.

Transient airflow structures and particle transport in a sequentially branching lung airway model

Considering oscillatory laminar incompressible three-dimensional flow in triple planar and nonplanar bifurcations representing generations three to six of the human respiratory system, air flow fields and micron-particle transport have been simulated under normal breathing and high-frequency ventilation (HFV) conditions. A finite-volume code (CFX4.3 from AEA Technology, Pittsburgh, PA) and its user-enhanced FORTRAN programs were validated with experimental velocity data points for a single bifurcation. The airflow structures and micron-particle motion in the triple bifurcations were analyzed for a representative normal breathing cycle as well as HFV condition. While both the peak inspiratory and expiratory velocity profiles for the low Womersley case (α=0.93) agree well with those of instantaneously equivalent steady-state cases, some differences can be observed between flow acceleration and deceleration at off-peak periods or near flow reversal, especially during inspiratory flow. Similarly, the basic fe...

Targeted Drug-Aerosol Delivery in the Human Respiratory System

Inhalation of drug aerosols is a modern pathway to combat lung diseases. It is also becoming the preferred route for insulin delivery, pain management, cancer therapy, and nanotherapeutics. Popular delivery devices include nebulizers, metered-dose inhalers, and dry-powder inhalers. They are all nondirectional and hence have typically low particle deposition efficiencies in desired nasal or lung areas. Thus, for specific disease treatment with costly and/or aggressive medicine, it is necessary to provide targeted drug-aerosol delivery to predetermined sites in the human respiratory system. Experimental measurements and computer models of particle transport and deposition in nasal and lung airway models are presented. Furthermore, the underlying methodology and performance of pressurized metered dose inhalers as well as new smart inhaler systems are discussed. To maximize respiratory drug delivery to specific sites, an optimal combination of particle characteristics, inhalation waveform, particle release position, and drug-aerosol dosage has to be achieved.

Laminar-to-turbulent fluid-particle flows in a human airway model

As in many biomedical and industrial applications, gas–solid two-phase flow fields in a curved tube with local area constrictions may be laminar, transitional and/or turbulent depending upon the inlet flow rate and tube geometry. Assuming steady incompressible air flow and non-interacting spherical micron-particles, the laminar-to-turbulent suspension flow problem was solved for a human airway model using a commercial software with user-supplied pre- and post-processing programs. All flow regimes (500<Relocal<104) were captured with an low-Reynolds-number k–ω turbulence model. Considering different steady inspiratory flow rates (15⩽Q⩽60 l/min) and Stokes numbers, the three-dimensional simulation results show the following: (i) The onset of turbulence after the constriction in the larynx can be clearly observed when the inspiratory flow rate changes from low-level breathing (Qin=15 l/min) to high-level breathing (Qin=60 l/min). The flow reattachment length in the trachea becomes shorter, the axial velocity profile becomes more blunt, and the secondary flow decays faster with the occurrence of transition to turbulence. (ii) Particles follow the basic relationship between airflow and particle motion very well at the lower inspiratory flow rate (Qin=15 l/min); however, particle motion seems to be random and disperse, i.e., influenced by flow fluctuations in case of high inspiratory flow (Qin=60 l/min). (iii) Turbulence can enhance particle deposition in the trachea near the larynx to some extent, but it is more likely to affect the deposition of smaller particles (say, St<0.06) throughout the airway at relatively high flow rates (Qin=30 and 60 l/min) due to turbulent dispersion. However, the particle size and inhalation flow rate (i.e., Stokes number) are still the main factors influencing particle deposition when compared with turbulent dispersion alone. The methodology outlined can be readily applied to other two-phase flows undergoing changing flow regimes in complex tubular systems.

Cyclic micron-size particle inhalation and deposition in a triple bifurcation lung airway model

Abstract Laminar oscillatory inhalation flow as well as micron-particle transport and wall deposition in a representative triple bifurcation airway model have been simulated using a commercial finite-volume code with user-enhanced programs. The computer model has been validated with experimental particle deposition data and deposition patterns for double bifurcations. The transient air flow, particle transport, and wall deposition patterns were analyzed and summarized in terms of particle deposition efficiencies (DEs) and surface density maps. Particle deposition may increase under cyclic flow conditions, i.e., DE-values are typically larger for cyclic flow than for steady flow at the mean flow rate of a given inhalation pulse. The maximal relative difference between these two DE-values may be as high as 50% for 0.02⩽Stmean⩽0.12 during normal inspiration (Q in =30 l / min ) . However, matching inlet Reynolds and Stokes numbers are proposed, which generate under quasi-steady flow conditions very similar DE-values and deposition patterns as in equivalent pulsatile flow. The relative differences between DE-values for cyclic and matching steady cases are less than 5%. The quantitative results of this work are of interest to researchers either conducting health risk assessment studies for inhaled particulate pollutants or analyzing drug aerosol deposition at desired lung target sites.

Rheological effects on pulsatile hemodynamics in a stenosed tube

Abstract Transient laminar axisymmetric flow through a tube with a smooth local area reduction of 75% is considered. Using an experimentally validated control volume method, the influence of three rheology models (i.e., Newtonian, power law and Quemada) are investigated. Three Womersley numbers ( Wo=r 0 ω/ν 1/2 :=4.0, 7.5 and 12.5) are compared for a sinusoidal input pulse which varies between 0 and 400 with a mean Reynolds number of 200. The primary application is pulsatile flow in axisymmetric stenosed artery segments. Results show that for the highest Womersley number considered here, a second co-rotating vortex is formed distal to the primary vortex. Also, the shear-thinning rheological models have a secondary effect on the flow field that primarily appears in terms of subtle changes to the hemodynamic wall parameters (i.e., the time-averaged wall shear stress, spatial wall shear stress gradient, and oscillatory shear index). The non-Newtonian models affect the entrainment of fluid-like particles in the post-stenotic region measurably. The particle residence time (PRT), defined as the ratio of transient to steady residence times, is found to be less than or equal to unity for the majority of fluid elements for all rheologies and Womersley numbers considered. The highest fraction of PRT 10, is found to increase with increasing Wo and is decreased by the non-Newtonian formulations. This is due to a low-shear high-viscosity band that impedes the progress of fluid particles into the near wall region.

Relation between non-uniform hemodynamics and sites of altered permeability and lesion growth at the rabbit aorto-celiac junction

Using the rabbits aorto-celiac junction as a representative atherosclerotic model, the hemodynamics of a bifurcating blood vessel are numerically simulated and three hemodynamic parameters are compared. The wall shear stress (WSS), the oscillatory shear index (OSI), and the spatial wall shear stress gradient (WSSG) are considered in this study. Locally enhanced wall permeabilities and intimal macrophages are generally considered to be involved in atherogenesis, and here the primary concern is with the hemodynamic influence on these early stages of the disease process. In comparing the segmental averages of the indicator functions and previously published intimal white blood cell densities, only the WSSG shows a statistically significant correlation. All three indicators have selective strengths in determining sites of early lesion growth around the aorto-celiac flow divider. At the proximal end of the flow divider on the lateral side of the orifice, there are elevated values of the OSI as well as WSSG and low WSS values. Regions of elevated wall permeabilities compare with the regions of elevated WSSG along the lateral and distal portions of the flow divider. Largely dependent upon the present input pulse with reverse flow, the OSI indicates relatively high values throughout the flow domain, however, it is important when utilized in conjunction with low WSS regions. This study presents a rationale for further quantitative correlative studies in the rabbit model based on additional histological data sets.

Computational design of a bypass graft that minimizes wall shear stress gradients in the region of the distal anastomosis

PURPOSE Recent experimental and theoretic studies show that large wall shear stress gradients characterize disturbed flow patterns associated with the location of myointimal hyperplasia, atheroma, or both. Graft-to-artery anastomoses that minimize wall shear stress gradients may reduce the degree of myointimal development and the propensity for thrombosis. This study analyzes the distribution of distal anastomotic wall shear stress gradients for conventional geometries and for the optimized geometry assuming idealized merging of the graft with the artery. METHODS A validated computational fluid dynamics program was used to solve the transient three-dimensional partial differential equations and auxiliary equations that describe laminar incompressible blood flow. Time-averaged wall shear stresses and wall shear stress gradients were calculated for three distal graft-artery anastomoses: a standard end-to-side, a Taylor patch, and an optimized geometry. The latter was obtained iteratively by minimizing the local wall shear stress gradients and was analyzed under resting and exercise inflow waveforms. RESULTS Both the standard and Taylor patch anastomoses have relatively high wall shear stress gradients in the regions of the toe and heel. For all flow inputs studied nonuniform hemodynamics in the optimized graft design are largely eliminated, and the time-averaged wall shear stress gradients are greatly reduced throughout the anastomotic zone. At resting flow the Taylor patch produces slightly lower wall shear stress gradients in the anastomotic region than the standard end-to-side anastomosis. The optimized design reduces wall shear stress gradients to almost one half of that of the standard and Taylor patch geometries. At exercise flow wall shear stress gradients almost triple in the standard anastomosis and increase approximately 30% in the Taylor patch. In contrast, the geometrically optimized design is basically independent of the type of flow input waveform in terms of time-averaged wall shear stress gradients and disturbed flow patterns. CONCLUSION This study demonstrates that it is possible to design a terminal graft geometry for an end-to-side anastomosis that significantly reduces wall shear stress gradients. If the wall shear stress gradient is confirmed to be a major hemodynamic determinant of intimal hyperplasia and restenosis, these results may point to the design of optimal bypass graft geometries.