Victoria Timchenko

A computational study of transient plane front solidification of alloys in a Bridgman apparatus under microgravity conditions

A mathematical model of heat, momentum and solute transfer during directional solidification of binary alloys in a Bridgman furnace has been developed. A fixed grid single domain approach (enthalpy method) is used. The effects of coupling with the phase diagram (a concentration-dependent melting temperature) and of thermal and solutal convection on segregation of solute, shape and position of the solid/liquid interface are investigated. A vorticity–stream function formulation is used for calculation of the velocity fields. The results presented include calculations at 1 and 10 μg, both neglecting and including the dependence of melting temperature on concentration.

An evaluation of synthetic jets for heat transfer enhancement in air cooled micro‐channels

Purpose – The development of novel cooling techniques is needed in order to be able to substantially increase the performance of integrated electronic circuits whose operations are limited by the maximum allowable temperature. Air cooled micro‐channels etched in the silicon substrate have the potential to remove heat directly from the chip. For reasonable pressure drops, the flow in micro‐channels is inherently laminar, so that the heat transfer is not very large. A synthetic jet may be used to improve mixing, thereby considerably increasing heat transfer. This paper seeks to address this issue.Design/methodology/approach – CFD has been used to study the flow and thermal fields in forced convection in a two‐dimensional micro‐channel with an inbuilt synthetic jet actuator. The unsteady Navier‐Stokes and energy equations are solved. The effects of variation of the frequency of the jet at a fixed pressure difference between the ends of the channel and with a fixed jet Reynolds number, have been studied with ...

Large-Eddy Simulation of Turbulent Natural Convection in Vertical Parallel-Plate Channels

A numerical investigation examining natural convection in a vertical parallel-plate channel with the simultaneous presence of laminar, transitional, and turbulent regimes is conducted using large-eddy simulation. The compressible three-dimensional Favre-filtered mass, momentum, and energy conservation equations are closed using the Smagorinsky and the Vreman subgrid-scale models. A two-stage predictor-corrector numerical methodology for low-Mach-number compressible flows is adopted. Time-averaged wall temperature and field profiles are well captured by the Vreman model, while the Smagorinsky model underpredicts wall behavior considerably. It is demonstrated that the present code is capable of capturing the flow development which is unachievable by conventional Reynolds-averaged Navier-Stokes approaches.

A computational study of binary alloy solidification in the MEPHISTO experiment

Abstract A computational model is presented for the study of the solidification of a binary alloy. The enthalpy method has been modified and incorporated into both an in-house code SOLCON (Heat Transfer 98, 1998, p. 241) and the commercial CFD code CFX (CFX 4.2: Solver, 1997). The model has been used to simulate experiments on solidification of a bismuth–tin alloy which were performed during the 1997 flight of the MEPHISTO-4 experiment on the US Space Shuttle Columbia. The effects of thermal and solutal convection in the microgravity environment and of concentration-dependent melting temperature on the phase change processes are included. Comparisons of numerical solutions with actual microprobe results obtained from the MEPHISTO experiments are presented.

Study of three LES subgrid-scale turbulence models for predictions of heat and mass transfer in large-scale compartment fires

ABSTRACT Numerical assessment was performed to investigate the wall-adaptive features offered by two subgrid-scale (SGS) turbulence models: Wall-Adapting Local Eddy Viscosity (WALE) and Vreman against the Smagorinsky model. The gas temperature and velocity field predictions were enhanced using WALE over Smagorinsky, especially at the flaming and near-wall regions since WALE considers both strain and rotation rates of the turbulent structure and the turbulent viscosity approaches zero at the wall. Conversely, the simulation results by Vreman were under-predicted against the experimental data. The WALE model could notably enhance the simulation accuracy for large-scale compartment fires due to significant improvements of the flow diffusivity modeling.

LES and Multi-Step Chemical Reaction in Compartment Fires

Numerical studies on two large-scale compartment buoyant fires were performed utilizing a fully coupled Large Eddy Simulation (LES) and strained laminar flamelet combustion model, extended from single-step to multistep chemical reaction. Two Subgrid-Scaled (SGS) turbulent models, Smagorinsky and WALE, were examined for center and corner fires. Compared with the experimental and existing numerical data, the WALE model with a multistep chemical reaction gave the best prediction for the upper hot layer and doorway gas temperatures. Specie concentrations including oxygen, carbon dioxide, and carbon monoxide were also found to compare well against the experimental measurements when the WALE model with a multistep chemical reaction was adopted.

A Criterion for the Formation of Micro Synthetic Jets

A synthetic jet actuator is a zero net mass flow device, which under appropriate conditions generates a continuous jet always directed away from the orifice. Because of limited experimental and computational data on micro-sized jets, there is a need for a criterion to determine the onset of the sustained jet regime. A numerical study of axisymmetric micro synthetic jets for a frequency range from 250 to 50,000 Hz, orifice diameters range from 20 to 200 μm, and Reynolds numbers from 6.5 to 35 has been performed in order to identify a general jet formation criterion. The parametric study has allowed us to develop a new criterion for the onset of micro synthetic jets with Stokes numbers less than 7.Copyright

Application of dynamic global-coefficient subgrid-scale models to turbulent natural convection in an enclosed tall cavity

Large-eddy simulations examining natural convection in an enclosed cavity with the simultaneous presence of laminar, transitional, and turbulent flow regimes were conducted. The Rayleigh number based on height of the cavity is 4.6 × 1010. Different dynamic global-coefficient procedures to compute the Vreman [A. W. Vreman, “An eddy-viscosity subgrid-scale model for turbulent shear flow: Algebraic theory and applications,” Phys. Fluids 16, 3670 (2004)] model coefficient were implemented for the subgrid-scale tensors in both the momentum and energy equations. Based on comparison with experimental and existing numerical data, it is shown that the dynamic model derived from the “global equilibrium” hypothesis gives favorable results in the mean flow and turbulence quantities. Nevertheless, because of higher subgrid-scale dissipation, transition to a turbulent flow is postponed when the Vreman model coefficient is either uniform or determined dynamically using the Germano identity approach. This suggests that m...

Flow Structure and Heat Transfer Enhancement in Laminar Flow With Protrusion-Dimple Combinations in a Shallow Rectangular Channel

The effect of introducing combinations of spherical dimples and protrusions in a shallow rectangular channel on the flow and heat transfer in the laminar regime has been studied numerically. Four different cases were investigated. These consisted of: an isolated dimple, an isolated protrusion both placed on the centerline of one of the wide face of the channel, a combination of a dimple located on the centerline of the wide face of the channel and a protrusion located downstream but shifted to the side, and finally, a combination in which the protrusion and the dimple are reversed. The resultant, very complex flow structure and thermal fields in the channel are presented. The introduction of a single dimple results in a small enhancement of heat transfer and a very small reduction in pressure drop relative to those obtained in a smooth channel. However, a significant enhancement in heat transfer obtained from a single protrusion is associated with marginal increase in pressure drop. The addition of a protrusion downstream of the dimple leads to an increase of 30% in heat transfer augmentation above that which pertains for the isolated protrusion without any increase in the pressure drop. With the reversal of the positions of the protrusion and the dimple no effect on either the pressure drop or the heat transfer has been observed.Copyright

NUMERICAL STUDY OF G-JITTER DURING DIRECTIONAL SOLIDIFICATION

A study of directional solidification of a weak binary alloy (specifically, Bi-1% Sn) using a fixed grid single domain approach has been undertaken. The enthalpy method is used to solve for the temperature field over the computational domain, including both the solid and liquid phases. The vorticity-stream function formulation is used to describe thermosolutal convection in the liquid region. Results on the solute field and segregation are presented, showing the effects of the periodic disturbances for a range of amplitudes and frequencies (multicomponent) and for actual acceleration data obtained during a space flight.