Numerical investigations of thermodynamic states formed near a reacting, supercritical CH[formula omitted]/O[formula omitted] flame

Abstract

The identification of the correct thermodynamic states forming in supercritical mixing and combustion is an important area of research in engineering. When a multi-component mixture evolves at pressures that are higher than the critical pressures of each individual component, a real-gas (RG) equation of state (EoS) must be used in order to achieve superior accuracy in the predictions. However, this may not be enough when the complex non-linear mixing allows the formation of multi-phase states. The physics and the modeling of multi-phase states occurring at supercritical conditions is the subject of many research efforts and it has already been shown to play in important role when coupled with fluid-mechanics. In this situation, the RG EoS needs to be augmented with a multi-phase model that can predict the formation of pockets of liquid and vapor in a certain region, therefore the presence of “pseudo” phases, commonly discussed in supercritical thermodynamics no longer applies. Currently, the concept of vapor-liquid equilibrium (VLE) seems to be one viable approach to describe this physics. In this paper, we focus the attention on the thermodynamic states formed in the vicinity of a supercritical CH4/O2 flame. In particular, we investigate the occurrence of multi-phase conditions that require state-of-the-art modeling in RG thermodynamics. We provide emphasis about the numerical issues that may occur when inverse thermodynamic problems are solved in presence of VLE (as for example obtaining temperature and pressure out of internal energy, density and overall mixture composition) and how they can be avoided. The VLE model is compared with the corresponding single phase (no VLE) counterpart and the results are analyzed and explained. Finally, we provide an a-posteriori method to explain the role of each component in the formation of VLE states and compare it with zero-dimensional, conventional mixing models. The idea behind this work is to establish the effect of VLE in a relatively simple, canonical configuration, as well as describe a method that helps to understand how these states are formed and how they can be predicted before-hand.