Khalil Khanafer

Buoyancy-driven heat transfer enhancement in a two-dimensional enclosure utilizing nanofluids

Abstract Heat transfer enhancement in a two-dimensional enclosure utilizing nanofluids is investigated for various pertinent parameters. A model is developed to analyze heat transfer performance of nanofluids inside an enclosure taking into account the solid particle dispersion. The transport equations are solved numerically using the finite-volume approach along with the alternating direct implicit procedure. Comparisons with previously published work on the basis of special cases are performed and found to be in excellent agreement. The effect of suspended ultrafine metallic nanoparticles on the fluid flow and heat transfer processes within the enclosure is analyzed and effective thermal conductivity enhancement maps are developed for various controlling parameters. In addition, an analysis of variants based on the thermophysical properties of nanofluid is developed and presented. It is shown that the variances within different models have substantial effects on the results. Finally, a heat transfer correlation of the average Nusselt number for various Grashof numbers and volume fractions is presented.

Mixed convection flow in a lid-driven enclosure filled with a fluid-saturated porous medium

Abstract Volume averaged equations governing unsteady, laminar, mixed convection flow in an enclosure filled with a Darcian fluid-saturated uniform porous medium in the presence of internal heat generation is formulated. The two vertical walls of the enclosure are insulated while the horizontal walls are kept at constant temperatures with the top surface is moving at a constant speed. The developed equations are nondimensionalized and then solved numerically subject the appropriate initial and boundary conditions by the finite-volume approach along with the Alternating Direct Implicit (ADI) procedure. Comparisons with previously published work are performed and found to be in excellent agreement. A parametric study is conducted and a set of graphical results is presented and discussed to elucidate interesting features of the solution.

The effect of radiation on free convection flow of fluid with variable viscosity from a porous vertical plate

Abstract The effect of thermal radiation on the natural convection flow along a uniformly heated vertical porous plate with variable viscosity and uniform suction velocity was investigated numerically. The fluid considered in this study is of an optically dense viscous incompressible fluid of temperature-dependent viscosity. The laminar boundary layer equations governing the flow are shown to be nonsimilar. The governing equations are analyzed using a variety of methods: (i) a series solution for small values of ξ (a scaled streamwise coordinate depending on the transpiration); (ii) an asymptotic solution for large values of ξ; and (iii) a full numerical solution using the Keller box method. The solutions are expressed in terms of the local shear stress and the local heat transfer rate. The working fluid is taken to have Prandtl number Pr=1, and the effects of varying the viscosity variation parameter, γ, the radiation parameter, Rd, and the surface temperature parameter, θw, are discussed.

DOUBLE-DIFFUSIVE MIXED CONVECTION IN A LID-DRIVEN ENCLOSURE FILLED WITH A FLUID- SATURATED POROUS MEDIUM

This article presents a numerical study of mixed-convection heat and mass transport in a lid-driven square enclosure filled with a non-Darcian fluid-saturated porous medium. The two vertical walls of the enclosure are insulated, while the horizontal walls are kept at constant but different temperatures and concentrations with the top surface moving at a constant speed. The transport equations are solved numerically using the finite-volume approach along with the alternating direct implicit (ADI) procedure. Comparisons with previously published work are performed and found to be in excellent agreement. The results of the present investigation indicate that the buoyancy ratio, Darcy number, Lewis number, and Richardson number have profound effects on the double-diffusive phenomenon.

Effects of strain rate, mixing ratio, and stress-strain definition on the mechanical behavior of the polydimethylsiloxane (PDMS) material as related to its biological applications

Tensile tests on Polydimethylsiloxane (PDMS) materials were conducted to illustrate the effects of mixing ratio, definition of the stress-strain curve, and the strain rate on the elastic modulus and stress-strain curve. PDMS specimens were prepared according to the ASTM standards for elastic materials. Our results indicate that the physiological elastic modulus depends strongly on the definition of the stress-strain curve, mixing ratio, and the strain rate. For various mixing ratios and strain rates, true stress-strain definition results in higher stress and elastic modulus compared with engineering stress-strain and true stress-engineering strain definitions. The elastic modulus increases as the mixing ratio increases up-to 9:1 ratio after which the elastic modulus begins to decrease even as the mixing ratio continues to increase. The results presented in this study will be helpful to assist the design of in vitro experiments to mimic blood flow in arteries and to understand the complex interaction between blood flow and the walls of arteries using PDMS elastomer.

Tear size and location impacts false lumen pressure in an ex vivo model of chronic type B aortic dissection.

BACKGROUND Follow-up mortality is high in patients with type B aortic dissection (TB-AD) approaching one in four patients at 3 years. A predictor of increased mortality is partial thrombosis of the false lumen which may occlude distal tears. The hemodynamic consequences of differing tear size, location, and patency within the false lumen is largely unknown. We examined the impact of intimal tear size, tear number, and location on false lumen pressure. METHODS In an ex-vivo model of chronic type B aortic dissection connected to a pulsatile pump, simultaneous pressures were measured within the true and false lumen. Experiments were performed in different dissection models with tear sizes of 6.4 mm and 3.2 mm in the following configurations; model A: proximal and distal tear simulating the most common hemodynamic state in patients with TB-AD; model B: proximal tear only simulating patients with partial thrombosis and occlusion of distal tear; and model C: distal tear only simulating patients sealed proximally via a stent graft with persistent distal communication. To compare false lumen diastolic pressure between models, a false lumen pressure index (FPI%) was calculated for all simulations as FPI% = (false lumen diastolic pressure/true lumen diastolic pressure) x 100. RESULTS In model A, the systolic pressure was slightly lower in the false lumen compared with the true lumen while the diastolic pressure (DP) was slightly higher in the false lumen (DP 66.45 +/- 0.16 mm Hg vs 66.20 +/- 0.12 mm Hg, P < .001, FPI% = 100.4%). In the absence of a distal tear (model B), diastolic pressure was elevated within the false lumen compared with the true lumen (58.95 +/- 0.10 vs 54.66 +/- 0.17, P < .001, FPI% = 107.9%). The absence of a proximal tear in the presence of a distal tear (model C) diastolic pressure was also elevated within the false lumen versus the true lumen (58.72 +/- 0.24 vs 56.15 +/- 0.16, P < .001, FPI% 104.6%). The difference in diastolic pressure was greatest with a smaller tear (3.2 mm) in model B. In model B, DBP increased by 13.9% (P < .001, R(2) 0.69) per 10 beat per minute increase in heart rate (P < .001) independent of systolic pressure. CONCLUSIONS In this model of chronic type B aortic dissection, diastolic false lumen pressure was the highest in the setting of smaller proximal tear size and the lack of a distal tear. These determinants of inflow and outflow may impact false lumen expansion and rupture during the follow-up period.

Numerical study of laminar natural convection in tilted enclosure with transverse magnetic field

This paper numerically investigates the effect of the transverse magnetic field on flow field patterns and heat transfer processes in a tilted square cavity. The horizontal walls of the enclosure are assumed to be insulated while the vertical walls are kept isothermal. The power law control volume approach is developed to solve the conservation equations at Prandtl number of 0.71. Validation tests with existing data demonstrate the ability of the present scheme to produce accurate results. The effects of Grashof number, enclosure inclination angle, and Hartmann number are also investigated. The study covers the range of the Hartmann number from 0 to 100, the enclosure inclination angle from 0° to ‐90° with Grashof number of 104 and 106. The effect of the magnetic field is found to suppress the convection currents and heat transfer inside the cavity. This effect is significant for low inclination angles and high Grashof numbers. Additionally, it is noted that there is no variation of average Nusselt number with respect to inclination angle for high Hartmann number.

EFFECT OF HEATED WALL POSITION ON MIXED CONVECTION IN A CHANNEL WITH AN OPEN CAVITY

Mixed convection in an open cavity with a heated wall bounded by a horizontally insulated plate is studied numerically. Three basic heating modes are considered: (a) the heated wall is on the inflow side (assisting flow); (b) the heated wall is on the outflow side (opposing flow); and (c) the heated wall is the horizontal surface of the cavity (heating from below). Mixed convection fluid flow and heat transfer within the cavity is governed by the buoyancy parameter, Richardson number (Ri), and Reynolds number (Re). The results are reported in terms of streamlines, isotherms, wall temperature, and the velocity profiles in the cavity for Ri=0.1 and 100, Re=100 and 1000, and the ratio between the channel and cavity heights (H/D) is in the range 0.1-1.5. The present results show that the maximum temperature values decrease as the Reynolds and the Richardson numbers increase. The effect of the H/D ratio is found to play a significant role on streamline and isotherm patterns for differentheating configurations. The present investigation shows that the opposing forced flow configuration has the highest thermal performance in terms of both maximum temperature and average Nusselt number.

In Vitro Characterisation of Physiological and Maximum Elastic Modulus of Ascending Thoracic Aortic Aneurysms Using Uniaxial Tensile Testing

OBJECTIVE Ascending thoracic aortic aneurysms (ATAA) are a life-threatening condition due to the risk of rupture or dissection. This risk is increased in the presence of a bicuspid aortic valve (BAV). The purpose of this study was to provide data on the elastic modulus of aortic wall of ATAA using uniaxial tensile testing in two different areas of the stress-strain relationship: physiological and maximum range of stresses. The influence of tissue location, tissue orientation and valve type on these parameters was investigated. MATERIALS AND METHODS Tissues freshly excised from ATAA with bicuspid or tricuspid aortic valve were obtained from greater and lesser curvature (GC and LC) and the specimens were tested uniaxially in circumferential (CIRC) and longitudinal (LONG) orientation. Maximum elastic modulus (MEM) was given by the maximum slope of the stress-strain curve before failure. Physiological modulus (PM) was derived from the Laplace law and from ranges of pressure of 80-120 mmHg. Means of each group of specimen were compared using Students t-test to assess the influence of location, orientation and valve type on each mechanical parameter. RESULTS PM was found to be significantly lower than the MEM (p < 0.001). The MEM and PM were significantly higher (p < 0.01) in the CIRC (n = 66) than in the LONG orientation (n = 42). The MEM was higher in the circumferential orientation in the BAV group (p < 0.001 in GC and p < 0.05 in LC). MEM and PM in GC specimens were higher in the longitudinal orientation than the LC specimens (p < 0.05). CONCLUSION This study demonstrates the anisotropy of the aortic wall in ATAA and provides data on the mechanical behaviour in the physiological range of pressure.

Fluid-structure interaction analysis of turbulent pulsatile flow within a layered aortic wall as related to aortic dissection.

Turbulent pulsatile flow and wall mechanics were studied numerically in an axisymmetric three-layered wall model of a descending aorta. The transport equations were solved using the finite element formulation based on the Galerkin method of weighted residuals. A fully-coupled fluid-structure interaction (FSI) analysis was utilized in this investigation. We calculated Von Mises wall stress, streamlines and fluid pressure contours. The findings of this study show that peak wall stress and maximum shear stress are highest in the media layer. The difference in the elastic properties of contiguous layers of the wall of the aorta probably determines the occurrence of dissection in the media layer. Moreover, the presence of aortic intramural hematoma is found to have a significant effect on the peak wall stress acting on the inner layer.