Interface, Phase Change and Molecular Transport in Sub, Trans and Supercritical Regimes for N-Alkane/Nitrogen Mixtures
Abstract
Understanding the behavior of liquid hydrocarbon propellants under high pressure and temperature conditions is a crucial step towards improving the performance of modern-day combustion engines (liquid rocket engines, diesel engines, gas turbines and so on) and designing the next generation ones. Under such harsh thermodynamic conditions (high P&T) propellent droplets may experience anywhere from sub-to-trans-to-supercritical regime. The focus of this research is to explore the dynamics of the vapor-liquid two phase system formed by a liquid hydrocarbon fuel (n-heptane or n-dodecane) and ambient (nitrogen) over a wide range of P&T leading up to the mixture critical point and beyond. Molecular dynamics (MD) has been used as the primary tool in this research along with other tools like: phase stability calculations based on Gibb’s work, Peng Robinson equation of state, density gradient theory and neural networks. This work can be broadly considered to be addressing the following questions: 1. What P&T conditions trigger the transition of a hydrocarbon fuel from sub to supercritical regime? 2. Knowing that classical macroscopic models’ breakdown at high P&T conditions, what should be the process to analyze the vapor-liquid two phase system under such conditions? The analysis journey map can be laid out in the following steps: First, based on the classical work of Gibbs, a phase stability analysis is done to understand the difference between Type-I and Type-III mixtures. Our system of interest of n-alkane/nitrogen falls under the category of Type-III. Building up on this, the mixture critical point of multicomponent systems is estimated. Second, MD is used to understand the transition from the classical two-phase evaporation regime to the diffusion controlled mixing regime. Knowing how to estimate mixture critical points, a fuel is said to transition from sub-to-supercritical regime when the local P&T conditions exceed the mixture critical point. This also coincides with the surface tension vanishing. The multicomponent effect of fuel and ambient is also studied on the transition behavior. Third, comprehending the importance of interfacial region in the problem at hand, MD is used to take a closer look at the vapor-liquid interface under equilibrium conditions (at various P&T). Additionally, Density Gradient Theory (DGT) is used to predict the binary system interface for qualitative comparison purposes. Properties like interface thickness, enrichment at interface, velocity distribution functions (VDFs) are studied as a function of equilibrium P&T conditions. Fourth, interfacial transport holds a key role in modeling of two-phase vapor-liquid systems. Only way to circumvent the complex transport dynamics in the non-equilibrium transition layer is to formulate suitable boundary conditions (KBC in this case). The main building blocks required for KBCs are: density distributions, velocity distribution functions and evaporation/condensation coefficients. The framework for VDFs having been already laid out in the previous step, in this step MD simulations are used to estimate molecular mass fluxes using two methods (fixed boundary and two boundary). Evaporation coefficients are reported, for non-equilibrium scenarios (net evaporation), as a function of temperature.
Degree
Ph.D.
Advisors
Qiao, Purdue University.
Subject Area
Artificial intelligence|Mathematics
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