Analysis of local conditions on graphite growth and shape during solidification of ductile cast iron

3D X-ray tomography recordings have been used to study graphite growth during solidification of ductile cast iron. Using data from such recordings, it is shown how local growth conditions influence growth rate and morphology of nodules during solidification. Experiments show that it is common for nodules to gradually change shape during solidification so that sphericity decreases. It is also found that different shaped nodules can evolve in direct contact with liquid iron and also after they are encapsulated in austenite. It is observed that a significant proportion of originally complete spherical nodules become less spherical via formation of protrusions on the surface; these new surfaces are observed to grow relatively faster. It is shown that encapsulation of the graphite nodule by austenite may be incomplete and that at the end of solidification, partial encapsulation and the effect of the number of nearest graphite nodules play a crucial role in determining the final graphite morphology

A computational fluid dynamics study of laminar forced convection improvement of a non-Newtonian hybrid nanofluid within an annular pipe in porous media

Porous inserts and nanofluids are among the conventional methods for the amelioration of heat transfer in industrial systems. The heat transfer rate could also be improved by utilizing porous substances with a higher thermal conductivity in these systems. This research work presents a two-dimensional (2D) numerical examination of the laminar forced convection of an Al2O3-CuO-carboxy methyl cellulose (CMC) non-Newtonian hybrid nanofluid within an annular pipe in a porous medium. The porous medium was inserted within two inner or outer wall cases. For hybrid nanofluid flow modeling in porous media, a Darcy–Brinkman–Forchheimer formulation was employed. Additionally, a power-law technique was utilized as a fluid viscosity model for the considered non-Newtonian fluid. The governing equations were discretized according to the finite volume method (FVM) using the computational fluid dynamics (CFD) software package ANSYS-FLUENT. The cylinder walls’ thermal boundary conditions were exposed to a constant heat flux. For various Darcy numbers, the impacts of different volume fractions of the hybrid nanofluid (0% to 5%), the total Nusselt number, the pressure drop, and the performance number (PN) were evaluated. The outcomes indicate that the heat transfer coefficient increases considerably with a decrease in the Darcy number (0.1 to 0.0001), as well as with an increase in the porous thickness ratio. Moreover, it was found that the nanoparticles’ increased volume fraction would ameliorate the heat transfer and, more considerably, the PN factor. Furthermore, according to the outcomes in both cases I and II for a constant porous thickness ratio and Darcy number (rp=1,Da=0.0001) and a high volume fraction (φ=5%), the maximum total Nusselt number reached 1274.44. Moreover, applying a volume fraction of 5% with Da=0.1 and rp=1 reached the highest value of the PN index equal to 7.61, which is augmented as roughly 88% compared to the case of a zero volume fraction

In situ synchrotron investigation of degenerate graphite nodule evolution in ductile cast iron

Ductile cast irons (DCIs) are of increasing importance in the renewable energy and transportation sectors. The distribution and morphology of the graphite nodules, in particular the formation of degenerate features during solidification, dictate the mechanical performance of DCIs. In situ high-speed synchrotron X-ray tomography was used to capture the evolution of graphite nodules during solidification of DCI, including degenerate features and the effect of the carbon concentration field. The degeneration of nodules is observed to increase with re-melting cycles, which is attributed to Mg-loss. The dendritic primary austenite and carbon concentration gradients in the surrounding liquid phase were found to control nodule morphology by locally restricting and promoting growth. A coupled diffusion-mechanical model was developed, confirming the experimentally informed hypothesis that protrusions form through liquation cracking of the austenite shell and subsequent localised growth. These results provide valuable insights into the solidification kinetics of cast irons, supporting the design of advanced alloys

Unraveling compacted graphite evolution during solidification of cast iron using in-situ synchrotron X-ray tomography

In spite of many years of research, the physical phenomena leading to the evolution of compacted graphite (CG) during solidification is still not fully understood. In particular, it is unknown how highly branched CG aggregates form and evolve in the semi-solid, and how local microstructural variations at micrometer length scale affect this growth process. We present here the first time-resolved synchrotron tomography combined with a bespoke high-temperature environmental cell that allows direct observation of the evolution of CG and relates this dynamic process to the local surrounding microstructures in a cast iron sample during repeated melting and solidification. Distinct processes are identified for the formation of CG involving the nucleation, growth, development of branches and interconnection of graphite particles, ultimately evolving into highly branched graphite aggregates with large sizes and low sphericities. CG is found to nucleate with a spheroidal or a plate-like shape, developing branches induced by high carbon concentration, e.g. thin melt channels. Additionally, CG grows much faster than spheroidal graphite during subsequent cooling in solid state. The direct visualization of the dynamic solidification process provides unprecedented new insights into formation mechanisms of CG and correlating factors such as local microstructural variations, and guides the development of CG iron solidification models

Mathematical modelling of coupled heat and mass transport into an electronic enclosure

In contrast to high fidelity CFD codes which require higher computational effort/time, the well-known Resistor-Capacitor (RC) approach requires much lower calculation time, but also with a lower resolution of the geometrical arrangement. Therefore, for enclosures without too complex geometry in their interior, it is more efficient to use the RC method for thermal management and design of electronic compartments. Thus, the objective of this paper is to build an in-house code based on the RC approach for simulating coupled heat and mass transport into a (closed) electronic enclosure. The developed code has the capability of combining lumped components and a 1D description. Heat and mass transport is based on a FVM discretization of the heat conduction equation and Fick's second law. Simulation results are compared with corresponding experimental findings and good agreement is found. Second simulation was performed to study the response of temperature and moisture inside an enclosure exposed to the B2 STANAG climatic cyclic conditions

Drying of a tape-cast layer: Numerical modelling of the evaporation process in a graded/layered material

Evaporation of water from a ceramic layer is a key phenomenon in the drying process for the manufacturing of water-based tape cast ceramics. In this paper we present a coupled free-flow-porous-media model on the Representative Elementary Volume (REV) scale for coupling non-isothermal multi-phase compositional porous-media flow — for the ceramic layer — and single-phase compositional laminar free flow — for the air above it. The preliminary results show the typical expected evaporation behaviour from a porous medium initially saturated with water, and water–vapour transport to the free-flow region in accordance with the available results from the literature. We elaborate on and discuss the characteristic drying-rate curve for a single layer ceramic, and compare it with that of a graded/layered ceramic. We, moreover, show the influence of the mean diameter of particles of the porous medium (dp) — which directly affects the intrinsic permeability (K) based on the well-known Ergun’s equation — of each single ceramic layer on the drying behaviour of a graded/layered ceramic

Modeling of nanosecond pulsed laser processing of polymers in air and water

Laser ablation of polymers in water is known to generate distinct surface characteristics as compared to that in air. In order to understand the role of ambient media during laser ablation of polymers, this paper aims to develop a physics-based model of the process considering the effect of ambient media. Therefore, in the present work, models are developed for laser ablation of polymers in air and water considering all the relevant physical phenomena such as laser-polymer interaction, plasma generation, plasma expansion and plasma shielding. The current work focuses on near-infrared laser radiation (lambda-1064 nm) of nanosecond pulse duration. The laser-polymer interaction at such wavelengths is purely photo-thermal in nature and the laser-plasma interaction is assumed to occur mainly by inverse-bremsstrahlung photon absorption. The computational model is based on the finite volume method using the Crank-Nicholson scheme. The model predicts that underwater laser ablation results in subsurface heating effect in the polymer and confinement of the laser generated plasma, which makes it different from laser ablation in air. Plasma expansion velocities are much lower in water than in air. This results in an enhanced plasma shielding effect in the case of water. The predicted results of ablation depth versus fluence from the model are in qualitative agreement with those observed in experiments

Drying of a tape-cast layer: Numerical investigation of influencing parameters

In this study, the evaporation of water from a ceramic-water mixture is investigated numerically with the purpose of understanding the drying process of the thin sheets produced by the tape casting process. In the scope of this work, a Representative Elementary Volume (REV) scale model concept for coupling non-isothermal multi-phase compositional porous-media flow and single-phase compositional laminar free-flow developed by Jabbari et al. (2016), is used for a thorough analysis of the influential parameters. Specifically, we investigate the influence of ventilation speed magnitude, vmax, the equivalent diameter of particles of the porous medium, dp, the porosity of the porous medium, ϕ, the initial temperature in the free-flow region, Tff, and the initial temperature in the porous-medium region, Tpm, on the characteristic drying curves of a thin ceramic layer. We, moreover, conduct a statistical analysis based on numerical experiments in combination with a fractional factorial design of the aforementioned parameters. The analysis accounts for the effects of parameters on the characteristic drying curves of a thin ceramic layer. The effects of varying each of the parameters as well as their mutual interaction are shown with particular attention to the maximal drying rate as well as the final time for the drying process

Revisiting models for spheroidal graphite growth

Recent experiments resolved nucleation and growth of graphite during solidification of ductile cast iron in 3D and time using synchrotron X-ray tomography [1]. We use the experimental observations to analyse the relation between graphite growth rate and the state of the particle neighbourhood to pinpoint possible links between growth rate of individual graphite spheres and the overall solidification state. With this insight we revisit existing models for growth of spheroidal graphite and discuss possible modifications in order to describe the critical final stage of solidification correctly