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This study analyzed the ability of computational fluid dynamic software to accurately predict the flow coefficient of three bottom-load ball valves to develop a design which can accurately control flow rate.

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Thesis (MScEng (Mechanical and Mechatronic Engineering))--Stellenbosch University, 2008. / Aortic valve replacements are frequently performed during heart surgery. However, since this is quite a stressful procedure, many patients are turned down for medical reasons. Stented valves, designed and manufactured for percutaneous insertion, eliminate many of the risks involved in open-heart surgery, thus providing a solution to patients not deemed strong enough for open-chest aortic valve replacements. The aortic valve is a complex structure, and therefore numerical simulation is necessary to obtain flow and stress data to support the design of a prosthetic heart valve in the absence of viable physical measuring methods. To aid in the design of a prosthetic heart valve, various finite element valve models were created, and the fluid structure interaction (FSI) between the valves and the blood was simulated using commercial finite element software. The effect of the geometry of the leaflets on the haemodynamic behaviour over the cardiac cycle was investigated. It was found that leaflet dimensions should be chosen judiciously, because of their considerable effect on the stress distribution and performance of the valve. A simple leaflet geometry optimisation was done for a 20 mm and 26 mm valve, respectively, by means of existing geometry relationships found in the literature. Simulations were done to obtain the maximum leaflet attachment forces that can be used by a stent designer for fatigue loading, or to investigate the structural strength of the stent. These simulations were numerically validated. The effect of leaflet thickness and stiffness on resistance to opening, stress distribution and strain were investigated. Results showed that leaflet thickness has a greater effect on the performance of the valve than leaflet stiffness, and thereby validated the results of similar tests contained in the literature. After simulating over-, as well as under-dilation of a stented valve, it was found that problems associated with over-dilation can be minimised to a certain extent by increasing the coaptation1 region of the leaflets. A simple pulse duplicator was designed based on a four-element Windkessel model. The pulse duplicator was used to study the performance of the prototype valves by means of high-speed photography, the results of which were fed into one of the numerical finite element models and compared to real valve performance. Some of the prototype valves showed efficiencies of 88%.

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With a flat slide valve concept, when compared to conventional piston spool valves, reduced leakage and increased service life could be achieved. In order to achieve a reduction of leakage flows and guarantee the adjustability of the valves at the same time, a correct design of the pressure compensation is essential. The magnitude of force depends on both the operating point of the valve and the position of the slider. Due to the design of the flat slide valve, it is possible to use ceramic semi-finished products for the main stage, which consist of control plates and a slide plate. The geometries are simple enough to be inexpensively manufactured with sufficient accuracy, using a ceramic pre-product using laser cutting technology. This article introduces the concept of the flat slide valve for a proportional 4/3-way valve. The design of the valve, including the design of the metallic main stage is presented. This includes the design of the flow channels, which have to be suitable for ceramic materials. Furthermore the design of the actuator and hydraulic circuit for testing is presented. With the shown design, the function of a 4/3-way valve, known from piston spool valves, can be implemented with a linear behavior between slide plate deflection and opening flow cross section.

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