Revisiting kinetic boundary conditions at the surface of fuel droplet hydrocarbons: An atomistic computational fluid dynamics simulation

Abstract

The role of boundary conditions at the interface for both Boltzmann equation and the set of Navier-Stokes equations have been suggested to be important for studying of multiphase flows such as evaporation/condensation process which doesn’t always obey the equilibrium conditions. Here we present aspects of transition-state theory (TST) alongside with kinetic gas theory (KGT) relevant to the study of quasi-equilibrium interfacial phenomena and the equilibrium gas phase processes, respectively. A two-state mathematical model for long-chain hydrocarbons which have multi-structural specifications is introduced to clarify how kinetics and thermodynamics affect evaporation/condensation process at the surface of fuel droplet, liquid and gas phases and then show how experimental observations for a number of n-alkane may be reproduced using a hybrid framework TST and KGT with physically reasonable parameters controlling the interface, gas and liquid phases. The importance of internal activation dynamics at the surface of n-alkane droplets is established during the evaporation/condensation process.

Molecular Conformational Manifolds between Gas-Liquid Interface and Multiphasic

“…One question raised was whether or not adding the interfacial layer using SAFT has an effect on the evaporation/condensation coefficient of n-dodecane. The answer is no, since SAFT, being a thermodynamics-based approach, cannot model properly transient processes such as internal activation dynamics effects [3].…”

“…where ω i is the magnitude of the transition-state imaginary frequency (see References [3,4] for more detail). While CKGT describes the collision effects among surrounding gases, vapor molecules, clusters, and droplets, assuming those are in the thermodynamic equilibrium states, QTST presents some kinetic quasi-equilibrium phenomena at an atomic level in the vicinity of fuel droplets.…”

“…We estimated this distribution function in the evaporation process based on the molecular theory of solvation for systems consisting of long-chain molecules, and concluded that "internal molecular dynamic" effects are important in this function rather than intermolecular interactions [3,4]. In order to understand the importance of transient internal activation dynamics during the evaporation process on hydrocarbon molecules, we calculated the evaporation rate, k i,jk , using a QTST-based…”

“…This coefficient can explain how the molecular heat and mass transfer take place in the vicinity of the liquid-vapor interface. However, a novel hybrid method of the quantum transition state theory and classical kinetic gas theory (QTST-CKGT) could make this connection in a new way without requiring the evaporation/condensation coefficient [3,4]. In Reference [1], it was assumed that the evaporation of n-dodecane is an enthalpic process with no entropy change in the temperature range of 298.15-648.15 K, as well as no energy barrier over the interfacial layers.…”

“…In Reference [1], it was assumed that the evaporation of n-dodecane is an enthalpic process with no entropy change in the temperature range of 298.15-648.15 K, as well as no energy barrier over the interfacial layers. However, recently the basis of the entropy-enthalpy competition mechanism [2] and the importance of confinement effects alongside the molecular conformational changes at the interface [3,4] were established during the evaporation/condensation process.…”