A Numerical Approach to Determining Flammability Limits of Hydrocarbon Process Fluids

Abstract

Intrinsic to the computer modeling of explosions and fires is the concept of flammability limits. Conventionally, the term “flammability limit” is defined loosely as the concentration limits beyond which flame propagation is no longer possible. More formally, a fundamental flammability limit is defined as the mixture concentration at which a steady, laminar, one- dimensional, planar, and adiabatic flame fails to propagate. Fundamental flammability limits are reached when the heat release from chemical reactions becomes comparable to the radiative heat loss from the flame. The difficulty in predicting these fundamental limits, a priori, for a given combustible mixture is the dependence of the flammability limit on chemical kinetics. In this study we present a computational methodology for the determination of a mixture’s fuel lean and fuel rich flammability limits. Numerical calculations were performed using a modified version of the CHEMKIN PREMIX flame code. This code has been modified to allow for the capturing of the singular behavior around the turning point and allowing, thus, the accurate determination of a mixture’s flammability limits. The present methodology has been extensively validated to determine the flammability limits of single component and binary fuel mixtures. These validations are presented. Real hydrocarbon process fluids are complex mixtures that consist of hundreds of species spanning a wide range of molecular weights and chemical classes. The surrogate fuel approach, whereby the kinetics of the complex mixture is modeled using a few individual components, is now applied to determine the flammability limits of real hydrocarbons.