Direct Numerical Simulations of Nucleate Pool Boiling

Nucleate pool boiling plays a crucial role in thermal management in various engineering applications including, aviation, spacecraft, electronic components, and nuclear reactors. While it is an efficient heat removal mechanism, the mechanistic theory of nucleate pool boiling remains incomplete, and its exact mechanisms are not yet fully understood. Nucleate pool boiling can be categorized into three types: i) saturated - where the bulk liquid temperature equals the saturation temperature of the fluid, ii) subcooled - where the bulk liquid temperature is below the saturation temperature, and iii) superheated - where the bulk liquid temperature exceeds the saturation temperature. This research aims to advance the understanding of nucleate pool boiling improving energy efficiency and reducing costs. The primary focus of this research is to investigate the effect of subcooling, an area where two opposing schools of thought exist: one asserting that subcooling significantly influences heat flux, and the other suggesting negligible impact. The underlying physics of subcooling in nucleate boiling remains barely understood, though it is hypothesized that subcooling could lead to a reverse trend in the boiling curve. To close this knowledge gap, we utilize Direct Numerical Simulations (DNS) to model subcooled nucleate pool boiling. Additionally, we studied one case of superheated pool boiling, and one case of saturated pool boiling. Furthermore, we examined the effect of gravity on subcooled and saturated pool boiling by conducting simulations in multiple gravity conditions in two-dimensions, and focusing on International Space Station (ISS) gravity in three-dimensional computations. All computations utilize an in-house DNS solver that resolves the Navier Stokes equations coupled with mass, energy, and momentum conservation equations for incompressible flow. The Ghost Fluid Method is implemented to account for sharp jumps in pressure, velocity, and temperature across the multiphase boundary, while the level set technique tracks the liquid-vapor interface. A dynamic contact angle model is implemented. Nucleation sites distribution is modeled using a Monte Carlo Halton sequence and provided as an input to the solver. The pressure Poisson equation is solved using the Ghost Fluid Method and Hypre library. Parallelization of the solver is achieved via MPI for distributed memory systems. Additionally, a computer vision algorithm is developed to track bubbles and extract properties such as equivalent bubble diameter, centroid coordinates, and area. The effect of subcooling is investigated by varying the saturation temperature, the effect of superheat by adjusting the Stefan number, and the effect of gravity by modifying the Froude number. The following findings are highlighted: When subcooling increases, the heat flux decreases until it reaches an inflection point. After that point, the heat flux increases again. The inflection point was registered at a subcooling temperature equal to 11 degrees Celsius under Earth gravity conditions, and a subcooling temperature equal to 8 degrees Celsius under microgravity conditions. This behavior was correlated with the bubble dynamics. Statistical analysis of the root mean square fluctuations of the temperature, velocity, and turbulent heat flux also reflected the existence of this inflection point. Furthermore, details of the coherent structures reveal the presence of arch vortices at near wall, while ring and mushroom-like vortices were observed farther away from the wall under Earth gravity conditions. Vortices in microgravity conditions were described mainly as arch vortices with few spiral vortices observed during negligible bubble departures. Moreover, superheat nucleate pool boiling was found to be governed by bubble breakup and hairpin vortices.

Author

• Youssoufi, Sara

Language

• English

Keyword

• Computational Fluid Dynamics
• Fluid Mechanics
• Heat Transfer
• Cooling in Multiphase Flows
• Data Science
• Nucleate Pool Boiling

Date created

• 2025

Type of Work

• Dissertation

License

• All rights reserved

Rights statement

• In Copyright

GW Unit

• Mechanical & Aerospace Engineering

Degree

• Ph.D.

Advisor

• Balaras, Elias

Committee Member(s)

• Bardet, Philippe M.
• Riaz, Amir
• Plesniak, Michael W.
• Rau, Matthew J.

Persistent URL

•  https://scholarspace.library.gwu.edu/etd/8g84mn115