Investigation of blood cells migration in large stenosed artery

Atherosclerosis is one of the main diseases responsible for the high global mortality rate involving heart and blood vessel disorders. The build-up of fatty materials in the inner wall of the human artery prevents sufficient oxygen and nutrients reaching the organs of the body. Atherosclerosis is a chronic, long term condition, which develops and progresses over time; however, the disease does not present any symptoms until an advanced stage is reached, which results in potential permanent debility and sometimes sudden death. This thesis is concerned with the progression of atherosclerosis in an artery with mild stenosis that has resulted in a 30% reduction in its diameter. To this end, data on the low wall shear stress has been correlated with the atherosclerotic prone region. In a stenosed artery, this region corresponds to the separation zone that is formed distal to the lumen reduction. Atherosclerosis is a complex phenomenon, and not only involves wall shear stress, but also cellular interactions. Previous research has shown that even in the absence of wall biological effects, the blood cell distribution is strongly influenced by the hydrodynamics of the fluid. The mechanisms of blood cell distribution and the dynamic behaviour of the blood flow were investigated by developing a physical model of the stenosed artery, and by using particles to represent the presence of the blood cells. Particle Image Velocimetry system was employed and the size of particles were the 10μm and 20μm.The flow field was characterised and the particle distribution was measured. The characteristics of steady flow in the stenosed artery at Reynolds numbers of 250 and 320 revealed the importance of fluid inertia and the shear gradient distal to stenosis. Unequal distribution of the particles modelling the blood cells was observed, as more particles occupied the recirculation zones than the high shear region and central jet. The particle migration was found to depend on the particle size, particle concentration and fluid flow rates. The results suggested that the presence of similar effects in the real human arterial system may be significant to the progression of atherosclerotic plaques. At lower Reynolds number of 130, a particle depleted layer was observed at the wall region. In physiological flow the cell free layer will prevent the transport of oxygen and nitrogen oxide (NO) to the muscle tissues. A numerical method was used to simulate the flow characteristics measured in the experiment. The numerical results revealed the importance of the hydrodynamic mechanism of particle migration. Drag and lift forces were found to affect the residence time of particles in the recirculation region. The findings of this work have suggested that for a complex geometry like a large stenosed artery at physiological flow rates, hydrodynamic forces are important in cell migration in the flow separation zone. Even without biological forces, the cells migrate to the low wall shear stress region. For computational dynamics studies, this study has demonstrated the need for higher-order modelling at the cellular level in order to establish the particle migration mechanisms

Numerical modelling of blood cells distribution in flow through cerebral artery aneurysm

Recent aneurysm studies have focused on the correlation between different parameters and rupture risk; however, there have been conflicting findings. Computational fluid dynamics (CFD) allows for better visualization but idealized aneurysm models may neglect important variables such as aneurysm shape and blood flow conditions. In this paper, one case of an aneurysm was studied with CFD using a non-Newtonian Power Law Model to investigate the correlation between wall shear stress and blood cells distribution. Results show that velocity of blood flow decreased as it entered the aneurysm and the neck of the aneurysm experienced a greater magnitude of wall shear stress than the remainder of the cerebral artery. Besides, the blood cells generally begin at low velocities and increase after the first curve of the artery. Findings and further studies with larger cases of patients will improve treatment and prevention of aneurysm ruptures

SELECTED ASPECTS OF SELF-HEATING BEHAVIOUR OF FAT CONTAINING FOOD POWDERS

The self-heating of milk powder deposit in drying devices has been identified as having the potential to cause a serious thermal hazard. The consequences of self-heating can result in disruption to equipment and installation, lost production time, degradation of product quality and that of the prime importance, ie. the danger to life. The preventive and protective measures have been designed and implemented in most dairy plants. In order to aid such efforts, the process conditions which could lead fires have been further investigated. The thermal kinetics of the dairy powders have been studied previously by some researchers but a number of related issues still require more in-depth study. A series of mathematical models simulating the self-heating behaviour of reactive materials have been developed and modified over time but experimental validation is not substantial. This project presents a study evaluating the thermal ignition kinetics of whole milk powder. It is an extension of a previous work in which a novel measurement procedure and a numerical simulation model were proposed (Chong, 1997). In this work, several new aspects has been addressed and experimental validations made. In particular, a much larger sample size was used, so that the effect of heat accumulated inside the sample was more pronounced to make the measurements more accurate. Previously, two reaction regime for whole milk powder were identified in the temperature range of 125°C to 160°C. However, the relationship drawn was not 100% convincing. The number of points plotted at low temperature range were perhaps insufficient to illustrate a definitive trend. In the current work, a similar test was conducted at ambient temperatures ranging from ll6°C (a much lower ambient temperature) to 150°C. The experimental data from this work were plotted together with the previous results which prove the reaction regime discovered by Chong(l997). Hence, the existence of at least two reaction mechanisms during self-heating of whole milk powder have been validated . The experimental conditions employed in this were then simulated using a numerical simulation programme. The experimental results recorded were compared with the simulation data. Some modifications were made to the program, based on the recent available information. The model was able to predict the self-heating behaviour of the milk powder in a larger sample size. From the sensitivity analysis, a more reliable thermal conductivity of milk powders was identified. Hence, the model produced more precise results compared with the experimental data. Further simulations were made on the effect of initial sample temperature and initial water content on the self-heating behaviour. In addition, the measurement techniques and activation energy of various composition of milk powder have been reported previously but the underlying reactions have still been less explained. Fat oxidation of milk powder has been claimed to be a major reaction triggering ignition. For this reason, a set of experiments was carried out to study the effect of fat content on self-ignition propensity. It was found that a large amount of fat can inhibit thermal runaway reaction. However, the presence of small fat component could also enhance the exothermicity. Therefore, a critical fat content in which self ignition may be more easily initiated has been suggested

Investigation of blood cells migration in large stenosed artery

Atherosclerosis is one of the main diseases responsible for the high global mortality rate involving heart and blood vessel disorders. The build-up of fatty materials in the inner wall of the human artery prevents sufficient oxygen and nutrients reaching the organs of the body. Atherosclerosis is a chronic, long term condition, which develops and progresses over time; however, the disease does not present any symptoms until an advanced stage is reached, which results in potential permanent debility and sometimes sudden death. This thesis is concerned with the progression of atherosclerosis in an artery with mild stenosis that has resulted in a 30% reduction in its diameter. To this end, data on the low wall shear stress has been correlated with the atherosclerotic prone region. In a stenosed artery, this region corresponds to the separation zone that is formed distal to the lumen reduction. Atherosclerosis is a complex phenomenon, and not only involves wall shear stress, but also cellular interactions. Previous research has shown that even in the absence of wall biological effects, the blood cell distribution is strongly influenced by the hydrodynamics of the fluid. The mechanisms of blood cell distribution and the dynamic behaviour of the blood flow were investigated by developing a physical model of the stenosed artery, and by using particles to represent the presence of the blood cells. Particle Image Velocimetry system was employed and the size of particles were the 10μm and 20μm. The flow field was characterised and the particle distribution was measured. The characteristics of steady flow in the stenosed artery at Reynolds numbers of 250 and 320 revealed the importance of fluid inertia and the shear gradient distal to stenosis. Unequal distribution of the particles modelling the blood cells was observed, as more particles occupied the recirculation zones than the high shear region and central jet. The particle migration was found to depend on the particle size, particle concentration and fluid flow rates. The results suggested that the presence of similar effects in the real human arterial system may be significant to the progression of atherosclerotic plaques. At lower Reynolds number of 130, a particle depleted layer was observed at the wall region. In physiological flow the cell free layer will prevent the transport of oxygen and nitrogen oxide (NO) to the muscle tissues. A numerical method was used to simulate the flow characteristics measured in the experiment. The numerical results revealed the importance of the hydrodynamic mechanism of particle migration. Drag and lift forces were found to affect the residence time of particles in the recirculation region. The findings of this work have suggested that for a complex geometry like a large stenosed artery at physiological flow rates, hydrodynamic forces are important in cell migration in the flow separation zone. Even without biological forces, the cells migrate to the low wall shear stress region. For computational dynamics studies, this study has demonstrated the need for higher-order modelling at the cellular level in order to establish the particle migration mechanisms.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Wall Shear Stress Prediction Using Computational Simulation on Patient Specific Artery with Aneurysm

An aneurysm is formed when a blood vessel becomes dilated or distorted. It will cause the vessel to expand to a size greater than its original diameter. In this study, Wall Shear Stress (WSS) of cerebral artery with aneurysm was predicted using Computational Fluid Dynamics (CFD). WSS in the artery is one of the indicators for brain artery disease progression. Based on the results, the maximum value of blood velocity and WSS on patient specific artery with aneurysm are 3.23 m/s and 60.1 Pa, respectively. The location of high WSS is before and after the aneurysm bulge. The WSS is above the normal physiological value where the artery wall is exposed to high stress. Hence, the vessel at this location is anticipated to become weaker and could be further dilated

Wall Shear Stress Prediction Using Computational Simulation on Patient Specific Artery with Aneurysm

An aneurysm is formed when a blood vessel becomes dilated or distorted. It will cause the vessel to expand to a size greater than its original diameter. In this study, Wall Shear Stress (WSS) of cerebral artery with aneurysm was predicted using Computational Fluid Dynamics (CFD). WSS in the artery is one of the indicators for brain artery disease progression. Based on the results, the maximum value of blood velocity and WSS on patient specific artery with aneurysm are 3.23 m/s and 60.1 Pa, respectively. The location of high WSS is before and after the aneurysm bulge. The WSS is above the normal physiological value where the artery wall is exposed to high stress. Hence, the vessel at this location is anticipated to become weaker and could be further dilated