MAP | CALMAIL |
Combustion Modeling Lab at UC Berkeley

Current Research

Analysis of the errors associated with the transport properties of excited species in plasma-assisted combustion


Daniel Pineda, Tiernan Casey, Xian Shi (CML)


Introduction

Applying high voltage electric fields to flames and combustible mixtures generates rotationally-, vibrationally-, and electronically-excited species via electron-impact excitation reactions. Of particular interest to the plasma-assisted combustion community are the excited singlet oxygen species, O2(a1) and O2(b1), because they are highly reactive and O2(a1) in particular has been shown to enhance flame speeds and reduce ignition delay times by several researchers. Currently, the transport properties of these electronically excited species are unknown, and difficult to measure experimentally. As such, researchers have been using the transport properties of the ground state counterparts. As researchers conduct more complex numerical simulations in one and more dimensions, the transport properties of the species will need to be determined in order to generate more accurate chemical kinetic mechanisms with confidence. We attempt to derive the transport properties of these species using computational chemistry, specifically, electronic structure methods. Most importantly, we are working to determine the effects of changing the properties on various flame simulations. More broadly, we are investigating the application of uncertainty quantification (UQ) techniques on the transport data of several key combustion species, considering that there is significant variation in their transport properties determined from different experiments throughout the 20th century.


Analysis of the errors associated with molecular transport parameters in combustion modeling and their effects on one-dimensional flame simulations

Abstract: Recent efforts in quantifying the uncertainty of chemical kinetic mechanisms have raised important questions in the combustion community regarding acceptable agreement between models and experiments. Often, the uncertainty in transport data is either not considered or not quantified when validating kinetic mechanisms. Although different methods have been implemented in which molecular parameters are used to calculate the coefficients of diffusion, viscosity, and thermal conductivity subsequently used in chemical kinetic models, the molecular parameters themselves are subject to experimental uncertainty and thus their effects on combustion modeling should be quantified. For this initial framework, we examine decades of experiments in the literature and use Markov Chain Monte Carlo methods to estimate uncertainties in collision diameters and well depths of molecules frequently encountered in combustion simulations. We then propagate these uncertainties through simulations of laminar methane-air flames, establishing modeling uncertainties on flame speed due to transport.


Effects of updated transport properties of singlet oxygen species on steady laminar flame simulations

Abstract: A current problem is distinguishing the relative contributions of chemical kinetic and transport effects to the enhancement of plasma-assisted combustion. As numerical investigations in plasma-assisted combustion move to simulations in one or more dimensions, the transport properties of excited-state species thought to contribute to the combustion enhancement will need to be determined to accurately model these systems and discern their chemical kinetic contribution to the enhancement. We provide the first estimates of the intermolecular interactions energies of singlet oxyegn (O2(a1) and O2(b1)) using ab initio quantum chemistry methods, and find that the transport properties derived from these interaction energies are substantially different from those of the corresponding ground state. One-dimensional simualtions of both premixed and non-premixed steady low-pressure methane flames seeded with 2.5% O2(a1) in the oxidizer were performed. Results indicate that updating the transport properties of the singlet oxygen species has a very small effect on flame speed and structure, but that this effect is largest in premixed flame simualtions due to the reactions of O2(a1) with H atoms.


Transport properties of singlet oxygen species and their effects on one-dimensional combustion simulations

Abstract: One-dimensional simulations of premixed H2-O2 flames seeded with 5% O2(a1) in the oxidizer were performed. Results indicate that updating the transport properties of the singlet oxygen species has a very small effect on flame speed and structure, but that this effect is largest during flame extinction in transient simulations.