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Kidess A.,Technical University of Delft | Kidess A.,Burgers Center for Fluid Mechanics | Tong M.,University College Dublin | Duggan G.,University College Dublin | And 6 more authors.
International Journal of Heat and Mass Transfer | Year: 2015

Through an integrated macroscale/mesoscale computational model, we investigate the developing shape and grain morphology during the melting and solidification of a weld. In addition to macroscale surface tension driven fluid flow and heat transfer, we predict the solidification progression using a mesoscale model accounting for realistic solidification kinetics, rather than quasi-equilibrium thermodynamics. The tight coupling between the macroscale and the mesoscale distinguishes our results from previously published studies. The inclusion of Marangoni driven fluid flow and heat transfer, both during heating and cooling, was found to be crucial for accurately predicting both weld pool shape and grain morphology. However, if only the shape of the weld pool is of interest, a thermodynamic quasi-equilibrium solidification model, neglecting solidification kinetics, was found to suffice when including fluid flow and heat transfer. We demonstrate that the addition of a sufficient concentration of approximately 1 μm diameter TiN grain refining particles effectively triggers a favorable transition from columnar dendritic to equiaxed grains, as it allows for the latter to heterogeneously nucleate in the undercooled melt ahead of the columnar dendritic front. This transition from columnar to equiaxed growth is achievable for widely differing weld conditions, and its precise nature is relatively insensitive to the concentration of particles and to inaccurately known model parameters. © 2015 Elsevier Ltd. All rights reserved.

Kidess A.,Technical University of Delft | Kidess A.,Burgers Center for Fluid Mechanics | Kenjeres S.,Technical University of Delft | Kenjeres S.,Burgers Center for Fluid Mechanics | And 4 more authors.
International Journal of Thermal Sciences | Year: 2016

Experimental observations of high-energy surface melting processes, such as laser welding, have revealed unsteady, often violent, motion of the free surface of the melt pool. Surprisingly, no similar observations have been reported in numerical simulation studies of such flows. Moreover, the published simulation results fail to predict the post-solidification pool shape without adapting non-physical values for input parameters, suggesting the neglect of significant physics in the models employed. The experimentally observed violent flow surface instabilities, scaling analyses for the occurrence of turbulence in Marangoni driven flows, and the fact that in simulations transport coefficients generally have to be increased by an order of magnitude to match experimentally observed pool shapes, suggest the common assumption of laminar flow in the pool may not hold, and that the flow is actually turbulent. Here, we use direct numerical simulations (DNS) to investigate the role of turbulence in laser melting of a steel alloy with surface active elements. Our results reveal the presence of two competing vortices driven by thermocapillary forces towards a local surface tension maximum. The jet away from this location at the free surface, separating the two vortices, is found to be unstable and highly oscillatory, indeed leading to turbulence-like flow in the pool. The resulting additional heat transport, however, is insufficient to account for the observed differences in pool shapes between experiment and simulations. © 2016 Elsevier Masson SAS. All rights reserved.

Hoang D.A.,Burgers Center for Fluid Mechanics | Hoang D.A.,Technical University of Delft | Haringa C.,Burgers Center for Fluid Mechanics | Haringa C.,Technical University of Delft | And 8 more authors.
Chemical Engineering Journal | Year: 2014

This paper reports an analysis of the parallelized production of bubbles in a microreactor based on the repeated break-up of bubbles at T-junctions linked in series. We address the question how to design and operate such a multi-junction device for the even distribution of bubbles over the exit channels. We study the influence of the three primary sources leading to the uneven distribution of bubbles: (1) nonuniformity in the size of bubbles fed to the distributor, (2) lack of bubble break-up, and (3) asymmetric bubble breakup caused by asymmetries in flow due to fabrication tolerances. Based on our theoretical and experimental analysis, we formulate two guidelines to operate the multi-junction bubble distributor. The device should be operated such that: (i) the capillary number exceeds a critical value at all junctions, Ca>Cacrit, to ensure that all bubbles break, and (ii) the parameter (ls/w)·Ca1/3 is sufficiently large, with ls/w the distance between the bubbles normalized by the channel width. More quantitatively, (ls/w)·Ca1/3>2 for fabrication tolerances below 2%, which are typical for devices made by soft lithography. Furthermore, we address the question whether including a bypass channel around the T-junctions reduces flow asymmetries and corresponding nonuniformities in bubble size. While bubble nonuniformities in devices with and without bypass channels are comparable for fabrication tolerances of a few percent, we find that incorporating a bypass channels does have a beneficial effect for larger fabrication tolerances. The results presented in this paper facilitate the scale-out of bubble-based microreactors. © 2013 Elsevier B.V.

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