Modelling induction skull melting design modifications

Induction Skull Melting (ISM) is used for heating, melting, mixing and, possibly, evaporating reactive liquid metals at high temperatures when a minimum contact at solid walls is required. The numerical model presented here involves the complete time dependent process analysis based on the coupled electromagnetic, temperature and turbulent velocity fields during the melting and liquid shape changes. The simulation is validated against measurements of liquid metal height, temperature and heat losses in a commercial size ISM furnace. The often observed limiting temperature plateau for ever increasing electrical power input is explained by the turbulent convective heat losses. Various methods to increase the superheat within the liquid melt, the process energy efficiency and stability are proposed.     

Modelling the dynamics of the tilt-casting process and the effect of the mould design on the casting quality

All titanium alloys are highly reactive in the molten condition and so are usually melted in a water-cooled copper crucible to avoid contamination using processes such as Induction Skull Melting (ISM). These provide only limited superheat which, coupled with the surface turbulence inherent in most conventional mould filling processes, results in entrainment defects such as bubbles in the castings. To overcome these problems, a novel tilt-casting process has been developed in which the mould is attached directly to the ISM crucible holding the melt and the two are then rotated together to achieve a tranquil transfer of the metal into the mould. From the modelling point of view, this process involves complex three-phase flow, heat transfer and solidification. In this paper, the development of a numerical model of the tilt-casting process is presented featuring several novel algorithm developments introduced into a general CFD package (PHYSICA) to model the complex dynamic interaction of the liquid metal and melting atmosphere. These developments relate to the front tracking and heat transfer representations and to a casting-specific adaptation of the turbulence model to account for an advancing solid front. Calculations have been performed for a 0.4 m long turbine blade cast in a titanium aluminide alloy using different mould designs. It is shown that the feeder/basin configuration has a crucial influence on the casting quality. The computational results are validated against actual castings and are used to support an experimental programme. Although fluid flow and heat transfer are inseparable in a casting, the emphasis in this paper will be on the fluid dynamics of mould filling and its influence on cast quality rather than heat transfer and solidification which has been reported elsewhere.     

FREE SURFACE FLOW AND HEAT TRANSFER IN CAVITIES: THE SEA ALGORITHM

This study concerns the mathematical modeling of heat transfer and free surface motion under gravity, in cavities partially filled with a liquid. This two-phase flow problem is solved using a single-phase technique that assumes the air and liquid occupying the volume of the cavity can be treated as a single fluid with a sharp property discontinuity at the interface. A two-valued scalar advection equation is solved to mark the extent of each fluid. This idea is simple in concept, but requires careful application for two reasons: (1) The interface must remain sharp throughout the simulation; and (2) the equations of motion have to be expressed in a way that prevents the numerical “smothering” of the lighter fluid by the heavy one during the iteration process. To satisfy (1), the Van Leer TVD differencing scheme is adopted for the scalar advection equation, with appropriate flux corrections in the momentum and enthalpy equations. To satisfy (2), the continuity equation is expressed in volumetric form.

The technique is incorporated in the scalar equation algorithm (SEA), It is applied to two problems, the first being the collapse of a liquid column in a sealed cavity with wall heat transfer, and the second the filling and simultaneous cooling of a mold with liquid aluminum.     

Investigation into the performance of turbulence models for fluid flow and heat transfer phenomena in electronic applications

Heat is extracted away from an electronic package by convection, conduction, and/or radiation. The amount of heat extracted by forced convection using air is highly dependent on the characteristics of the airflow around the package which includes its velocity and direction. Turbulence in the air is also important and is required to be modeled accurately in thermal design codes that use computational fluid dynamics (CFD). During air cooling the flow can be classified as laminar, transitional, or turbulent. In electronics systems, the flow around the packages is usually in the transition region, which lies between laminar and turbulent flow. This requires a low-Reynolds number numerical model to fully capture the impact of turbulence on the fluid flow calculations. This paper provides comparisons between a number of turbulence models with experimental data. These models included the distance from the nearest wall and the local velocity (LVEL), Wolfshtein, Norris and Reynolds, k-/spl epsiv/, k-/spl omega/, shear-stress transport (SST), and k/spl epsiv//kl models. Results show that in terms of the fluid flow calculations most of the models capture the difficult wake recirculation region behind the package reasonably well, although for packages whose heights cause a high degree of recirculation behind the package the SST model appears to struggle. The paper also demonstrates the sensitivity of the models to changes in the mesh density; this study is aimed specifically at thermal design engineers as mesh independent simulations are rarely conducted in an industrial environment.     

A computational model for defect prediction in shape castings based on the interaction of free surface flow, heat transfer, and solidification phenomena

High-integrity castings require sophisticated design and manufacturing procedures to ensure they are essentially macrodefect free. Unfortunately, an important class of such defects—macroporosity, misruns, and pipe shrinkage—are all functions of the interactions of free surface flow, heat transfer, and solidication in complex geometries. Because these defects arise as an interaction of the preceding continuum phenomena, genuinely predictive models of these defects must represent these interactions explicitly. This work describes an attempt to model the formation of macrodefects explicitly as a function of the interacting continuum phenomena in arbitrarily complex three-dimensional geometries. The computational approach exploits a compatible set of finite volume procedures extended to unstructured meshes. The implementation of the model is described together with its testing and a measure of validation. The model demonstrates the potential to predict reliably shrinkage macroporosity, misruns, and pipe shrinkage directly as a result of interactions among free-surface fluid flow, heat transfer, and solidification.     

A domain decomposition algorithm for inverse welding problems

A mathematical model and a numerical scheme for the inverse determination of heat sources generated by means of a welding process is presented in this paper. The accuracy of the heat source retrieval is discussed. Received: 30 January 2001 / Accepted: 30 May 2001     

Synovialectomy and reconstruction of the retinaculum flexorum in median nerve decompression: technique and early results

We studied the efficiency of platelet collection by the Mobile Collection System (MCS) using two types of experimental protocols and evaluated the effect of storage at 22 degrees C on the platelet concentrates (PC). MCS is a new blood cell separator that combines discontinuous flow features with a new computerized operating system and can be used to harvest either full units of apheresis PC (SDP protocol) or half units of PC together with one to two units of plasma (PLP protocol). On the average, 1.98 x 10(11) +/- 0.46 x 10(11) (mean +/- SD) platelets were obtained by the PLP protocol and 3.01 x 10(11) +/- 0.70 x 10(11) and 4.2 x 10(11) +/- 1.12 x 10(11) by the early and later versions of the SDP protocols, respectively. The mean number of WBC per PC ranged from 3.3 to 4.7 x 10(8). During the storage period pH stayed above 7.0. On the average, the production of one molecule of lactate corresponded to the consumption of 0.538 molecules of glucose, indicating that less than 8% of glucose was consumed by the oxidative pathway. There were only small increases in LDH and B thromboglobulin concentrations. Furthermore, the ability of platelets to recover from osmotic shock and to aggregate following exposure to dual agonists declined only slightly during storage, indicating that both viability and function of platelets collected by the MCS were preserved during storage.     

Dynamic Model for Metal Cleanness Evaluation by Melting in a Cold Crucible

Melting of metallic samples in a cold crucible causes inclusions to concentrate on the surface owing to the action of the electromagnetic force in the skin layer. This process is dynamic, involving the melting stage, then quasi-stationary particle separation, and finally the solidification in the cold crucible. The proposed modeling technique is based on the pseudospectral solution method for coupled turbulent fluid flow, thermal and electromagnetic fields within the time varying fluid volume contained by the free surface, and partially the solid crucible wall. The model uses two methods for particle tracking: (1) a direct Lagrangian particle path computation and (2) a drifting concentration model. Lagrangian tracking is implemented for arbitrary unsteady flow. A specific numerical time integration scheme is implemented using implicit advancement that permits relatively large time-steps in the Lagrangian model. The drifting concentration model is based on a local equilibrium drift velocity assumption. Both methods are compared and demonstrated to give qualitatively similar results for stationary flow situations. The particular results presented are obtained for iron alloys. Small size particles of the order of 1 μm are shown to be less prone to separation by electromagnetic field action. In contrast, larger particles, 10 to 100 μm, are easily “trapped” by the electromagnetic field and stay on the sample surface at predetermined locations depending on their size and properties. The model allows optimization for melting power, geometry, and solidification rate. This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France.