Ph.D. Thesis Proposal by

Advisor: Prof. Vigor Yang

MULTI-SCALE MODELING OF HEAT TRANSPORT IN COMBUSTION OF ENERGETIC NANOMATERIALS

10 am Wednesday November 16

Montgomery-Knight Building Room 317

ABSTRACT

Energetic nanomaterials are attractive to combustion and propulsion, and energy generation applications. They have high volumetric energy density, and offer precision control of thermal transport properties and chemical reactivity via tailorable nanoscale properties. Examples include metal-based nanoparticles, nanoporous sheets, nano-suspensions, etc. One common approach is to disperse energetic fuel nanoparticles in oxidizer base-fluids to form energetic nano-suspensions. Combustion of nano-suspensions may be diffusion or kinetics limited, depending on their characteristic timescales. Enhanced heat conduction rate has significant impacts on the combustion behaviors of these materials.  A detailed understanding of the heat transport mechanisms, therefore, demands special attention.

Heat conduction in nano-suspensions is a widely debated topic. Anomalous thermal conductivity enhancements observed in experiments are often credited to interfacial heat transport and dynamic phenomena. Solid-liquid interfacial heat conduction may be enhanced or repressed by tuning interfacial (Kapitza) conductance. Hydrophilic interfaces offer a strong bonding between particle and base-fluid. Strong bonding between solid and liquid atoms at the interface results in the surface adsorption of fluid molecules, and reduced Kapitza resistance. Interfacial layer (nanolayer) is also significant in enhancing heat conduction. Nanolayer acts as a high density, high phonon mean free path conduit for efficient heat conduction between particle and fluid. Contributions from Brownian motion is also debated. Although it was believed that the random walk of nanoparticles can enhance overall thermal conductivity, Brownian motion effects are usually neglected owing to the high timescales associated with it. Formation of long-chain structures by nanoparticle aggregation can cause heat percolation effects. Several experimental evidences support the role of nanoparticle aggregation towards thermal conductivity enhancement.

In this work, a bottom-up approach is followed to obtain a multiscale model of conductive heat transport in energetic nano-suspension. Molecular dynamics simulations are performed to understand heat transport at the nanoscale. Aluminum, aluminum oxide, and water are the materials used in simulations. In the first stage, equilibrium molecular dynamics (EMD) simulations are performed to investigate the effect of particle volume fraction, particle size, interfacial bonding strength, temperature, and pressure on the thermal conductivity of single and multi-particle nano-suspensions. In the next stage, non-equilibrium molecular dynamics (NEMD) simulations are employed to study interfaces: behavior of interfacial conductance, and the effect of various parameters on it. On the basis of the explored nanoscale physics, a theoretical model for the effective thermal conductivity of energetic nanomaterials is developed. In the final stage, this model is used in a flame propagation problem. Thereafter, the effect of tunable nanoscale properties on the thermal conductivity and macroscale combustion properties is also investigated.

Committee:

Dr. Vigor Yang,AE

Dr. G. P. Peterson, ME

Dr. Jerry Seitzman, AE

Dr. Asegun Henry, ME

Dr. Julian Rimoli, AE