Author ORCID Identifier

https://orcid.org/0000-0003-2244-5505

Date Available

10-10-2024

Year of Publication

2024

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Mechanical Engineering

Advisor

Savio Poovathingal

Co-Director of Graduate Studies

Alexandre Martin

Abstract

Multiphase flows occurring at high speeds are relevant to many aerospace applications, including reentry of space capsules, rocket and explosives combustion, and degradation of objects entering the atmosphere. The diffuse interface method, which continues to grow in popularity and robustness of available numerical methods, makes possible the capturing of complex liquid geometries in high speed reacting flows, as demonstrated in this work by a study of the breakup of molten aluminum droplets until their complete destruction. This work presents details of the development of a diffuse interface computational framework to capture complex, novel flow features that occur in multiphase flows when there are high temperature gradients between fluid phases, and the fluids change phases and chemical composition based on local thermodynamic conditions. This is in contrast to sharp interface frameworks, which typically use a finite rate of mass transfer at the interface. Differences and similarities between the two types of solvers are discussed. A quasi-conservative positivity preserving solver for the five equation model is used with thermal relaxation to the fully conservative four equation model to simultaneously model both dispersed and bubbly liquid flows. Details of the framework are presented and validated, including a multiphase method to capture viscous stresses, a method to account for thermal diffusion in the diffuse limit, phase transition, and chemical reactions. The method for thermal diffusion reduces computational cost such that hot, high conductivity multiphase flows at small length scales can be computed. The method for chemical reactions is presented such that two liquids can be present in the multiphase framework as well as relevant gases, although a total of three fluid species are used here in each case for compatibility with common diffuse interface frameworks. Fitting of fluid parameters for the equation of state is shown in detail. A series of axisymmetric interactions between heated molten aluminum droplets and shocks are presented for lower temperature, nonreacting droplets, vaporizing droplets, and further heated droplets which produce liquid aluminum oxide. Droplets of two sizes are simulated until breakup, showing a physically accurate stripping decomposition of larger droplets, but a flattening effect in the case of smaller droplets. Early shock impact physics are consistent with others found in the literature, where reflecting, high speed internal pressure waves cause pressures which are low enough to induce cavitation. Phase transition has a destructive impact on the outer portions of the droplet during breakup as well as at the aftbody, but largely does not decompose the forebody. The oxidation reaction of pure liquid aluminum into liquid alumina increases the temperature of the droplet and wake, but does not increase the droplet's time scale for full destruction. Wake physics are captured and analyzed late into the breakup stage. The total time required for breakup is accurate compared to experiments for the smaller droplets but underestimated for the larger droplets which is consistent with the fast decomposition of bubbly objects inherent to diffuse interface solvers.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2024.412

Funding Information

We would like to thank the University of Kentucky Center for Computational Sciences and Information Technology Services Research Computing for their support and use of the Lipscomb Compute Cluster and associated research computing resources. This material is based on work supported by National Aeronautics and Space Administration Kentucky under award number: 80NSSC20M0047.

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