William Dempster

On the experimental testing of fine Nitinol wires for medical devices

Nitinol, a nickel titanium alloy, is widely used as a biocompatible metal with applications in high strain medical devices. The alloy exhibits both superelasticity and thermal shape memory behaviour. Basic mechanical properties can be established and are provided by suppliers; however the true stress-strain response under repeated load is not fully understood. It is essential to know this behaviour in order to design devices where failure by fatigue may be possible. The present work develops an approach for characterising the time varying mechanical properties of fine Nitinol wire and investigates processing factors, asymmetric stress-strain behaviour, temperature dependency, strain rate dependency and the material response to thermal and repeated mechanical loading. Physically realistic and accurately determined mechanical properties are provided in a format suitable for use in finite element analysis for the design of medical devices. Guidance is also given as to the most appropriate experimental set up procedures for gripping and testing thin Nitinol wire.

Prediction of the Flow and Force Characteristics of Safety Relief Valves

The design of safety relief valves depends on knowledge of the expected force-lift and flow-lift characteristics at the desired operating conditions of the valve. During valve opening the flow conditions change from seal-leakage type flows to combinations of sub-sonic and supersonic flows It is these highly compressible flow conditions that control the force and flow lift characteristics. This paper reports the use of computational fluid dynamics techniques to investigate the valve characteristics for a conventional spring operated 1/4” safety relief valve designed for gases operating between 10 and 30 bar. The force and flow magnitudes are highly dependent on the lift and geometry of the valve and these characteristics are explained with the aid of the detailed information available from the CFD analysis. Experimental determination of the force and flow lift conditions has also been carried out and a comparison indicates good correspondence between the predictions and the experiment. However, attention requires to be paid to specific aspects of the geometry modeling including corner radii and edge chamfers to ensure satisfactory prediction.Copyright

A computational fluid dynamics model to evaluate the inlet stroke performance of a positive displacement reciprocating plunger pump

A computational fluid dynamics model of the middle chamber of a triplex positive displacement reciprocating pump is presented to assess the feasibility of a transient numerical method to investigate the performance of the pump throughout the 180° of crank rotation of the inlet stroke. The paper also investigates, by means of computational fluid dynamics, the pressure drop occurring in the pump chamber during the first part of the inlet stroke in order to gain a better understanding of the mechanisms leading to cavitation. The model includes the compressibility of the working fluid and the lift of the inlet valve as a function of the pressure field on the inlet valve surfaces. It also takes into account the valve spring preload in the overall balance of forces moving the valve. Simulation of the valve motion was achieved by providing the solver with two user-defined functions. The plunger lift–time history was defined by the crank diameter and connecting rod length. This paper will demonstrate the feasibility and reasonable accuracy of the method adopted by comparison with experimental data.

Modelling Of Metal-to-metal Seals In A Pressure Relief Valve Using Advanced FE-analysis

This study investigates the behaviour of the contact faces in the metal-to-metal seal of a typical pressure relief valve. The valve geometry is simplified to an axisymmetric problem. A cylindrical nozzle, which has a valve seat on top, contacts with a disk, which is preloaded by a compressed linear spring. All the components are made of the steel AISI type 316N(L) defined using the multilinear kinematic hardening material model based on monotonic and cyclic tests at 20◦C. Analysis considerations include the effects of the Fluid Pressure Penetration (FPP) across the valve seat which exists at two different scales. There is certain limited fluid leakage through the valve seat at operational pressures, which is caused by the fluid penetrating into surface asperities at the microscale. At the macroscale, non-linear FE analysis using the FPP technique available in ANSYS revealed that there is also a limited amount of fluid penetrating into gap. Accurate prediction of the fluid pressure profile over the valve seat is addressed in this study by considering the FPP interaction on both scales. The shape of this pressure profile introduces an additional component of the spring force, which needs to be considered to provide a reliable sealing. The analysis showed that the evolution of the profile, which is caused by the isotropic softening of the material, is significant during the cyclic operation of the valve.

Improved Prediction of Shell Side Heat Transfer in Horizontal Evaporative Shell and Tube Heat Exchangers

This paper presents an improved prediction method for the heat transfer and pressure drop in the shell side of a horizontal shell and tube evaporator. The results from an experimental test program are used in which a wide range of evaporating two-phase shell side flow data was collected from a TEMA E-shell evaporator. The data are compared with shell side heat transfer coefficient and pressure drop models for homogeneous and stratified flow. The comparison suggests a deterioration in the heat transfer data at low mass fluxes consistent with a transition from homogeneous to stratified flow. The pressure drop data suggest a stratified flow across the full test range. A new model is presented that suggests the transition in the heat transfer data may be due to the extent of tube wetting in the upper tube bundle. The new model, which also takes into account the orientation of the shell side baffles, provides a vast improvement on the predictions of a homogenous type model. The new model would enable designers of shell side evaporators/reboilers to avoid operating conditions where poor heat transfer could be expected, and it would also enable changes in process conditions to be assessed for their implications on likely heat transfer performance.

An advanced CFD model to study the effect of non-condensable gas on cavitation in positive displacement pumps

Abstract An advanced transient CFD model of a positive displacement reciprocating pump was created to study its behavior and performance in cavitating condition during the inlet stroke. The “full” cavitation model developed by Singhal et al. was utilized, and a sensitivity analysis test on two air mass fraction amounts (1.5 and 15 parts per million) was carried out to study the influence of the dissolved air content in water on the cavitation phenomenon. The model was equipped with user defined functions to introduce the liquid compressibility, which stabilizes the simulation, and to handle the two-way coupling between the pressure field and the inlet valve lift history. Estimation of the performance is also presented in both cases.

A CFD Study on the Mechanisms Which Cause Cavitation in Positive Displacement Reciprocating Pumps

A transient multiphase CFD model was set up to investigate the main causes which lead to cavitation in positive displacement (PD) reciprocating pumps. Many authors such as Karsten Opitz [1] agree on distinguishing two different types of cavitation affecting PD pumps: flow induced cavitation and cavitation due to expansion. The flow induced cavitation affects the zones of high fluid velocity and consequent low static pressure e.g. the valve-seat volume gap while the cavitation due to expansion can be detected in zones where the decompression effects are important e.g. in the vicinity of the plunger. This second factor is a distinctive feature of PD pumps since other devices such as centrifugal pumps are only affected by the flow induced type. Unlike what has been published in the technical literature to date, where analysis of positive displacement pumps are based exclusively on experimental or analytic methods, the work presented in this paper is based entirely on a CFD approach, it discusses the appearance and the dynamics of these two phenomena throughout an entire pumping cycle pointing out the potential of CFD techniques in studying the causes of cavitation and assessing the consequent loss of performance in positive displacement pumps.

The Use of Effectiveness Concepts to Calculate the Thermal Resistance of Parallel Plate Heat Sinks

With this study, a new and more adaptable approach to the thermal design of the large heat sinks used in power electronics is proposed. This method, supported by the results from an extensive experimental program, recognizes that (1) the heat sink fins and the airflow adjacent to them form a simple cross-flow heat exchanger, and (2) conventional NTU-effectiveness methods can be adapted for use in the thermal analysis of the heat sink. This adaptation requires the development and evaluation of an equivalent heat capacity to describe the energy conducted along the fin. This method was initially used to evaluate the convective heat transfer coefficients between the fin and the cooling air. In this geometry, the developing airflow conditions make the prediction of representative values difficult. The correlation found to describe the test results was then used in an inverted analysis to predict and compare the experimental values for the heat sinks thermal resistance. The method is finally used in a design example where the fin spacing is optimized for a particular test duty. It is concluded that this new approach will make the design of large heat sinks more robust and reliable.

A CFD study on two-phase frozen flow of air/water through a safety relief valve

Abstract The air-water two phase critical flows through a safety relief valve commonly used in the refrigeration industry is examined with particular emphasis on the prediction of the critical mass flowrates using CFD based approaches. The expansion of the gas through the valve and the associated acceleration is coupled to the liquid phase and results in changes to the velocity slip with the possibility of influencing the choking conditions and the magnitude of the critical mass flows. These conditions are poorly reported in the literature for safety valves. This paper presents a study where the ability of established two phase multi-dimensional modelling approaches to predict such conditions are investigated. Comparison with the simplified mixture model will show that this model tends to underestimate mass flowrates for medium to high liquid mass fraction. However, the two fluid model can adequately account for the thermal and mechanical non equilibrium for these complex flow conditions with the use of simplified droplet sizing rules.

The testing and evaluation of trial refrigerants: Part 1. System description

This work is a contribution to the discovery and evaluation of new refrigerants. It has been stimulated by the uncertainty created by the Kyoto Protocol over the long-term availability of the hydroflorocarbons (HFCs) and also by the need for more knowledge of HFC-oil pairings. An economical method for testing refrigerants in terms of coefficient of performance (COP) and evaporative behaviour in a lubricated system is described. Part 1 describes the equipment, its instrumentation and operating procedures. Part 2 discusses the system output for four test refrigerants: two single fluids R134a and R22 and two test non-azeotropic blends each having a glide of around 9 K. Study of R22 was confined to evaporative behaviour. The ranking of refrigerants by COP and boiling length is given and the generality of use of these rankings is discussed.