Transport phenomena in a temperaturegradient studied by NEMD. A chemical reaction and a phase transition

Abstract

In this thesis we used non-equilibrium molecular dynamics (NEMD) to study non-equilibrium behaviors of two irreversible systems, both exposed to large temperature gradients. As modeling systems, we have chosen a simple chemical reaction, 2F↔F2, and a liquid-vapor interface of a Lenard-Jones spline fluid. The primary goal of this thesis is to investigate the nature of coupled transfer of heat and mass, and to obtain insight into the underlying molecular mechanisms, dynamic structure and properties of the non-equilibrium systems.

Heat and mass transports are central in mechanical as well as in chemical engineering. In order to predict transport properties of such systems, we need to confirm that there is a sound basis for the relevant transport equations. For the purpose, NEMD simulations have been used to study both equilibrium and dynamical behavior of the systems. To model the chemical reaction, Stillinger and Weber’s two- and three-body potentials were used. In addition to the two-body potential, the three-body potential is needed in order to sufficiently represent the main features of the reaction. Suitable NEMD techniques with the efficient reaction model were developed to study the fluorine reaction, in both stationary equilibrium and non-equilibrium states. Large temperature gradients were imposed through the boundaries in the NEMD box. With the NEMD simulations, the usefulness and validity of the theory of non-equilibrium thermodynamics (NET) have been investigated. The validity of the assumption of ’local equilibrium’ was tested for the chemical reaction in various temperature gradients. Furthermore, the quantitative definitions for the local ’chemical’ equilibrium were presented using the results from NEMD. The dynamic properties of the system are governed by the system’s entropy production. We gave the expression for the entropy production from NET to define the fluxes and forces in the system. Proper transport equations were presented for determination of the transport coefficients of the reacting system. Origins of transport properties, i.e. thermal diffusion coefficients (or Soret coefficient), heats of transfer, and Onsager coefficients, were discussed in a microscopic level. A dissipative or dynamic structure of the chemical reaction was displayed.

In addition we studied a phase transition, i.e. coupled heat and mass transfer across a liquid-vapor interface of a one-component system. The NEMD simulations with a Lenard-Johns spline potential were performed in different thermodynamic environments, e.g. with temperature gradients or/and concentration gradients. In the first place of this work, we proofed the validity of the assumption of local equilibrium at the surface where heat and mass transfer simultaneously. We then developed new algorithms to independently determine all four interfacial transfer coefficients for the surface. In the framework of NET, two sets of force- flux equations were defined by using the measurable heat flux on the vapor side as well as on the liquid side. The aim of this work is to find the interfacial coupling (Onsager) coefficients along the liquid-vapor coexistence curve and to add a proof of the Onsager’s reciprocal relations (ORR). To the best of our knowledge, this is the first time to test the validity of Onsager relations for a surface using NEMD.