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DOE OSTI2016

A high-fidelity approach towards simulation of pool boiling

A novel numerical approach is developed to simulate the multiscale problem of pool-boiling phase change. The particular focus is to develop a simulation technique that is capable of predicting the heat transfer and hydrodynamic characteristics of nucleate boiling and the transition to critical heat flux on surfaces of arbitrary shape and roughness distribution addressing a critical need to design enhanced boiling heat transfer surfaces. The macro-scale of the phase change and bubble dynamics is addressed through employing off-the-shelf Computational Fluid Dynamics (CFD) methods for interface tracking and interphase mass and energy transfer. The micro-scale of the microlayer, which forms at early stage of bubble nucleation near the wall, is resolved through asymptotic approximation of the thin-film theory which provides a closed-form solution for the distribution of the micro-layer and its influence on the evaporation process. In addition, the sub-grid surface roughness is represented stochastically through probabilistic density functions and its role in bubble nucleation and growth is then represented based on the thermodynamics of nucleation process. This combination of deterministic CFD, local approximation, and stochastic representation allows the simulation of pool boiling on any surface with known roughness and enhancement characteristics. The numerical model is validated for dynamics and hydrothermal characteristics of a single nucleated bubble on a flat surface against available literature data. In addition, the prediction of pool-boiling heat transfer coefficient is verified against experimental measurements as well as reputable correlations for various roughness distributions and different surface orientations. Finally, the model is employed to demonstrate pool-boiling phenomenon on enhanced structures with reentrance cavities and to explore the effect of enhancement feature design on thermal and hydrodynamic characteristics of these surfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Towards Predictive Reacting Flow LES

Grid dependency and the interaction of numerical errors with those from physical models can negatively impact the predictive capabilities of the traditional large eddy simulation (LES) approach. This paper attempts to address these issues for the simulation of turbulent reacting flows, where the canonical problem of a temporally evolving reacting jet is used as a test case. The impact of implicit and explicit filtering of the governing system of equations, and the impact of removing grid dependency from physical models through application of a constant filter width, is assessed. Four different grid resolutions are employed in the study. Differences are experienced in mean velocity and temperature profiles with grid resolution refinement when using the traditional implicit filtering approach, demonstrating the grid dependency of the obtained solutions. These differences are substantially mitigated by fixing the filter width used in physical models to a constant value, as the grid resolution is refined. Explicit filtering utilizing discrete filters reduces these differences even further, but to a smaller degree. This latter improvement comes at a very substantial computational cost. Higher Reynolds number cases need to be studied, where the LES resolution is far from that of a DNS, in order for a more conclusive assessment of the benefit of the explicit filtering approach to be made. However, even if explicit filtering is found to have a large positive impact on the predictive capabilities of reacting flow LES, its computational cost is likely to prevent it from being routinely applied to large, complex geometries, at least for the case where discrete filters are used.

Cocks, Peter A.↗