The purpose of this paper is to conduct a numerical simulation of the stroke patient's cerebral arteries and investigate the flow parameters due to the presence of stenosis. The computational fluid dynamics (CFD) simulations are based on simplified and realistic cerebral artery models. The seven simplified models (benchmarks) include straight cylindrical vessels with idealized stenosis with variable d/D (0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1). The realistic model of the cerebral artery is based on magnetic resonance imaging (MRI) for patient-specific cerebral arteries. The simulation for the realistic model of the cerebral artery is performed at boundary conditions measured by ultrasonography of the input and the output flow profiles (velocity and pressure). The obtained CFD results of the benchmarks are validated with actual data from the literature. Furthermore, a previous vascular contraction is assumed to be exist and the effect of this contraction area ratio on the blood flow regime is discussed and highlighted. Furthermore, CFD results show that a certain vascular contraction area critically affects the blood flow which shows increasing the wall shear stress WSS at the stenosis site. An increase in the blood velocity and vortex appears after the contraction zone, this lead to vessel occlusion and strokes. Article highlights The pressure drop across the arterial contraction is reduced when the area ratio d/D is increased. In some cases, the vortex can prevent blood flow from crossing, this leads to vessel occlusion especially at low d/D The WSS near the contraction area is high. Increasing the WSS can cause embolism that leads to lead to vessel occlusion.
Steam power plants are the largest industrial users for water. New restrictions for using water in cooling systems have led to a search for alternative cooling methods. This paper presents an experimental study of using a vapor compression refrigeration system (VCRS) for cooling a steam power plant condenser. The refrigeration system uses commercially available and environmental friendly R-410A to cool an intermediate chilled water loop which is used as a coolant for the steam condenser. Working under lower condenser pressure with higher coolant flow rates reduces the power required for the refrigeration system and rises the coefficient of performance (COP) and condensation rate. Based on the present experimental data an adjustment to a known empirical correlation for the Nusselt number in a shell and tube steam condenser is presented. The results show that decreasing the inlet coolant temperature increases condensation rate, heat rejection, COP, overall heat transfer coefficient, and R-410A to condensate mass flow ratio. Moreover, the increase in the rate of condensation and COP is most pronounced at lower steam condenser operating pressure and higher water coolant mass flow rate. The results reveal that using a VCRS is capable of providing a steam condenser with a more constant and lower coolant temperature than traditional wet and dry cooling technologies.
A step-by-step technique has been implemented in the analytical study of heat transfer and pressure gradient characteristics of refrigerants R-410A, R-407C, R-22, and R-134a used as cooling media in the condenser of a steam power plant. Refrigerants are optimized to replace water/air as coolant in the condenser of a steam power plant. Refrigerants have much lower temperatures and much higher heat transfer rates than water or air. The thermal resistances that affect heat transfer characteristics and surface condenser performance are included. The effect of inlet refrigerant temperature and mass flow rate are reported for the four refrigerants. Calculations are performed at two inlet refrigerant temperatures À21 C and À30 C and mass flow rate ranging from 92.905 to 132.905 kg/s. The results revealed that the overall heat transfer coefficient, heat transfer rate, and condensation rate increased with refrigerant mass flow rate, with higher values at lower inlet refrigerant temperatures. For a given refrigerant mass flow rate and inlet temperature, the analytical study indicated that R-410A has higher values of overall heat transfer coefficient, heat transfer rate and condensation rate than R-407C, R-22, and R-314a, respectively. Moreover, it is found that R-410A, at À30 C and 132.905 kg/s, is superior in condensing all steam entering the condenser than the other refrigerants; this corresponds to higher exergy efficiency. The condenser pressure was observed to be slightly higher for R-410A than the other refrigerants.
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