The phase transition from subcritical to supercritical conditions, referred to as transcritical behavior, significantly impacts the evaporation and fuel–air mixing in high-pressure liquid-fuel propulsion systems. Transcritical behavior is characterized as a transition from classical two-phase evaporation to a single-phase gas-like diffusion regime as surface tension and latent heat of vaporization reduce. However, the interfacial behavior represented by the surface tension coefficient and evaporation rate during this transition which are crucial inputs for Computational Fluid Dynamics (CFD) simulations of practical transcritical fuel spray is still missing. This study aims at developing new evaporation rate and surface tension models for transcritical n-dodecane droplets using molecular dynamics (MD) simulations irrespective of the droplet size. As MD simulations are primarily limited to the nanoscale, the new models can bridge the gap between MD and continuum simulations and enable the direct application of these findings to microscopic droplets. A new characteristic timescale, i.e., “undroplet time,” is defined which marks the transition from classical two-phase evaporation to single-phase gas-like diffusion behavior. The undroplet time indicates the onset of droplet core disintegration and penetration of nitrogen molecules into the droplet, which occurs after the vanishment of the surface tension. By normalizing the time with respect to the undroplet time, the rate of surface tension decay, evaporation rate, and the rate of droplet mass depletion become independent of the droplet size. Calculation of pairwise correlation coefficients for the entire MD results shows that both surface tension coefficient and evaporation rate are strongly correlated with the background temperature, while pressure and droplet size play a less significant role past the critical point. Therefore, new models for surface tension coefficient and evaporation rate spanning from sub- to supercritical conditions are developed as a function of background pressure and temperature, which can be used in continuum simulations. The identified phase change behavior based on the undroplet time shows a good agreement with the phase change regime maps obtained using microscale experiments and nanoscale MD predictions.
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Developing a Modern CFD Framework with Parallel Algorithms and Mesh Adaption
We discuss recent advances in a Computational Fluid Dynamics (CFD) framework that uses a combination of of Arbitrary Langrangian Eulerian (ALE) Dynamics and Adaptive Mesh Refinement (AMR). We describe updates and on-going work on the framework that allow for build portability on generic HPC (High Performance Computing) platforms. We also describe some of the more advanced algorithms that are available in the framework such as those which model surface tension effects in two and three dimensions. We introduce a new method for curvature and normal vector calculation in 2D, which we call the method of osculating circles. We benchmark this method and compare with other other volume of ŕuid (VOF) approaches to simulating surface tension effects. We predict how these algorithms will scale on these latest platforms such as the new Perlmutter system at NERSC which is a HPE Cray EX supercomputer with both GPU-accelerated and CPU-only nodes. We discuss the application of surface tension models to the interaction of a hydrogen droplet heated by an x-ray free electron laser with another hydrogen droplet.
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- Award ID(s):
- 2005259
- NSF-PAR ID:
- 10420551
- Date Published:
- Journal Name:
- Proceedings of the International Conference on Computational Fluid Dynamics
- ISSN:
- 2330-6580
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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