An investigation of transient three-dimensional buoyancy-driven flow and heat transfer in a closed horizontal annulus
Abstract Buoyancy-driven flow and heat transfer in a horizontal annulus bounded by two impermeable boundaries in the axial direction are numerically investigated in this work. A transient three-dimensional analysis of the end effects on heat transfer and the corresponding flow field is presented. The effects of the axial impermeable boundaries are shown to be characterized by retardation of the flow field through the viscous shearing force in regions near the end walls. The results show that the temperature distribution remains unchanged in the core region provided that the annulus length to outer radius ratio is larger than a critical value. Flow and temperature fields are found to be symmetrical with respect to the mid-axial plane of the annulus. A number of interesting features resulting from the sudden heating of the inner cylinder are presented. Finally, a thorough presentation of formation of flow and temperature fields as a result of the sudden heating of the inner cylinder is given.
An investigation and comparative analysis of two- and three-dimensional turbulent natural convection in a horizontal annulus
In the present study, simu1atioIf of turbulent buoyancy-driven flow in an annulus bounded by concentric, horizontal cylinders and adiabatif end walls has been carried out. Time-averaged equations of turbulent fluid motion and heat transfer were solved using a wall function approach coupled with the standard k-8 model. Discretization of the governing equations was achieved using a finite element scheme based on the Galerkin method of weighted residuals. Using a two-dimensional analysis, results were obtained for Rayleigh numbers ranging from lo6 to lo9 and the effects of Prandtl number and radius ratio on the flow and heat transfer characteristics were thoroughly examined. A wide range of parameters (lo6 < Ra -C 109, 0.01 i Pr < 5000, 1.5 i R i 11) was considered in the present study. A comprehensive comparative analysis establishing a unified treatment of previous experimental and computational results is presented. A good agreement was found between the results from this investigation and previous works. Results obtained from a three-dimensional model are also presented to describe more realistic three- dimensional flow characteristics. It is shown that, if the annulus is sufficiently long, there exists a core region over a substantial length of the cavity, which can be approximated by a two-dimensional model.
Comparative study of in vitro flow characteristics between a human aortic valve and a designed aortic valve and six corresponding types of prosthetic heart valves.
A model of a pulsatile cardiovascular system is described including an electronic control, a hydraulic driving unit, a volume storage, a peripheral resistance and measuring equipment. Six different artificial heart valves, a designed aortic valve (Aachen pockett valve II) and a human aortic valve have been tested in the outflow position of the pump. Three of them are presently being used in clinical applications. Measurements of pressure and volume flow have been obtained for different frequencies n, pump volumes Vp and time ratios between pressure and suction phase. From the experimental results a comparative evaluation was deducted for different working conditions. In comparison to natural human aortic valve (NK) large volume flow is verified by the Aachen pocket valve II (AT), the Gott-Dagget value (GD), the Starr-Edwards valve (SE) and the Lillehei-Kaster valve (LK). The Gott-Daggett valve has the smallest back flow volume. The Smeloff-Cutter and in particular the Wada-Cutter valve have the largest regurgitation. This is to be expected since the Cutter valves are purposely so designed that they do not close tightly in order to avoid blood sedimentation. The human aortic valve, the Gott-Daggett valve and the Aachen pocket valve demonstrate the most favourable opening-closing characteristics. The human aortic valve achieves a negligible pressure loss, followed by the Björk-Shiley and the Lillehei-Kaster valve.