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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Assessment of a Smoothed Particle Hydrodynamics Method for Thin Film on Textured Surfaces

A large number of previous studies suggest that the behavior of the near-wall liquid micro-layer plays a key role in boiling heat transfer and boiling crisis; see examples. The thin liquid layer interacts with heater surface and 'external' two-phase flow. The net effects of these interactions define the micro-hydrodynamics that leads to dryout. As a result, a good understanding of the thin-layer hydrodynamics is significant for the accurate prediction of CHF. Recently, with the advent of nanotechnology including micro-scale manufacturing, it becomes possible to perform high heat-flux boiling experiments with controlled surface conditions. Although many reports show rich observations about the effects of surface characteristics on CHF, physical mechanisms remain not fully understood. In order to have better understanding of the above phenomena, several computational models are developed, ranging from the lubrication theory to CFD methods. The former models are limited to one-dimensional formulation and unable to deal with the dry-out phenomena. It turns out that CFD methods have a greater potential for the mechanistic study of boiling. Towards the study objective, a particle-based CFD method called Smoothed Particle Hydrodynamics (SPH) is adopted. SPH is a Lagrangian particle method for solving systems of partial differential equations. Originally SPH is proposed for fluid dynamics in the context of astrophysical problems. Because mass and density are introduced during the particle approximation, SPH is also applied in fluid dynamic problems, where mass and density are important parameters to determine, without mass loss (a limitation of interface capturing schemes in mesh-based methods). Besides, geometrically complex and/or dynamic boundaries and interfaces can be handled without undue difficulty in SPH. Currently, most SPH simulations focus on large-scale scenarios (meter scale), like flooding and solitary wave. Also some works have been done with SPH in pore-scale. As for micro- or nano-scale simulation, people usually apply Dissipative Particle Dynamics (DPD) and Molecular Dynamics (MD) for particle-based simulation. Almost no work has been done on micro-scale with SPH, so assessments are needed to investigate the capability of SPH in application of micro-hydrodynamics. The Phenomena Identification and Ranking Table (PIRT) is applied to align application's technical requirements with associated physics and materials, models, simulation code capabilities and verification, validation and uncertainty quantification activities. The figures of merit are determined as: (1) Timing of micro-layer rewetting dry surface; (2) Effects of surface features. The corresponding key phenomena are determined as: (1) Hydrodynamics; (2) Interface Dynamics; (3) Energy; (4) Surface Effect. This study deals with isothermal scenario and no energy term is considered. Based on the current needs, a new SPH package, Micro-SPH, is compiled based on SPHysics with surface tension and wettability built in. Test cases are performed with Micro-SPH to assess the capability of SPH to each phenomenon identified. NEUTRINO with incompressible SPH solver is employed to investigate micro-fluid dynamics on a surface with uniformly distributed cylindrical pillars for application purposes. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2013

Yield asymmetry design of magnesium alloys by integrated computational materials engineering

Deformation asymmetry of magnesium alloys is an important factor on machine design in the automobile industry. Represented by the ratio of compressive yield stress (CYS) against tensile yield stress (TYS), deformation asymmetry is strongly related to texture and grain size. A polycrystalline viscoplasticity model, modified intermediate Φ-model, is used to predict the deformation behavior of magnesium alloys with different grain sizes. Validated with experimental results, integrated computational materials engineering is applied to find out the route in achieving desired asymmetry via thermomechanical processing. For example, CYS/TYS in rolled texture is smaller than 1 under different loading directions. In other textures, such as extruded texture, CYS/TYS is large along the normal direction. Starting from rolled texture, asymmetry will increase to close to 1 along the rolling direction after being compressed to a strain of 0.2. Our modified Φ-model also shows that grain refinement increases CYS/TYS. Along with texture control, grain refinement also can optimize the yield asymmetry. After the grain size decreases to a critical value, CYS/TYS reaches to 1 because CYS increases much faster than TYS. By tailoring the microstructure using texture control and grain refinement, it is achievable to optimize yield asymmetry in wrought magnesium alloys.

36 MATERIALS SCIENCE↗