The numerical investigations of the rheological behaviors of nanofluids and blood flow.

摘要

The present research consisted of four different rheological simulation using computation fluid dynamics (CFD). In the first study, numerical simulations in the microchannel were performed. Microflow has become a popular field of interest due to the advent of micro-electro-mechanical systems (MEMS). The LBM is applied to simulate the 3 dimensional microchannel flows. Two different approaches are incorporated to investigate their impacts to the entire flow field. First, the thermal boundary with nanofluids applied the no-slip condition was simulated. Second, the thermal investigation of nanofluids with the slip conditions was investigated under the isothermal condition. In the simulation, nanofluid was treated as a single-phase fluid with a non-Newtonian viscosity, a function of shear rate and temperature. Because of the increased thermal conductivity of nanofluid, it was found that the performances were greatly improved for the microchannel when nanofluids were used as the coolants. In addition to heat transfer enhancement, the nanofluid did not produce extra pressure drop because of nanoparticle volume fraction.;In the second study, the study of the stent was performed. The analysis of a flow pattern in cerebral aneurysms and the effect of strut shapes are presented in this paper. The efficiency of stent is related to several parameters, including porosity and stent strut shapes. The goal of this paper is to identify numerically how the stent strut shape and porosity affect the hemodynamics properties of the flow inside an aneurysm. We use the lattice Boltzmann method (LBM) of a non-Newtonian blood flow. An extrapolation method for the wall and stent boundary is used to resolve the characteristics of a highly complex flow. To ease the code development and facilitate the incorporation of new physics, a new scientific programming strategy based on object-oriented concepts is developed. The reduced velocity, smaller average vorticity magnitude, smaller average shear rate and increased viscosity are observed when the proposed stent shapes and porosities are used. The rectangular stent is observed to be optimal and to decrease the magnitude of the velocity by 89.25% in 2D model and 53.92% in 3D model in the aneurysm sac. Our results show the role of the porosity and stent strut shape and help us to understand the characteristics of stent strut design.;In the third, hemodynamic stresses are involved with the development and progression of vascular diseases. This study investigates the influence of mechanical factors on the hemodynamics of the curved coronary artery in an attempt to identify critical factors of non-Newtonian models. Multiphase non-Newtonian fluid simulations of pulsatile flow were performed and compared to the standard Newtonian fluid models. Different inlet hematocrit levels were used with the simulations to analyze the relationship that hematocrit levels have with RBC viscosity, shear stress, velocity, and secondary flow. Our results demonstrated that high hematocrit levels induce secondary flow on the inside curvature of the vessel. In addition, red blood cell (RBC) viscosity and wall shear stress vary as a function of hematocrit level. Low wall shear stress was found to be associated with areas of high hematocrit. These results describe how RBCs interact with the curvature of artery walls. It is concluded that while all models have a good approximation in a blood behavior, the multiphase non-Newtonian viscosity model is optimal to demonstrate effects of changes in hematocrit. They provide a better stimulation of realistic blood flow analysis.;In the last study, intimal hyperplasia at arterial bypass graft is a major factor responsible for graft failure. Several techniques are used to explain intimal hyperplasia formation at the end-to-side anastomosis junction. Abnormal hemodynamics contributing to the development of disease at the junction is the one of most common theories. This study describes a means of modifying the area of bypass graft at the junction part. This procedure, called the laterally diffused bypass graft (LDBG), is able to alter the hemodynamics in the end-to-side anastomosis. The LDBG model, due to an expansion of the outer curvature in the graft, reduces the velocity on the artery bed, side and top junction walls. The recirculation with velocity vectors on the host artery is significantly altered near the heel region on the host artery. Wall shear stress (WSS) is decreased by up to 34% on the floor of artery centerline at the peak systole and by 61.9% on the top junction of artery during the systole deceleration. Corresponding time-averaged wall shear stresses (TAWSS) are found to decrease by 40.5%. Secondary flow is observed to be decreased significantly at the distal junction.