Time-resolved Frequency Comb Research Articles

Validation of Computationally Efficient Simulations of Douglas Fir Pyrolysis and Combustion Using Time-Resolved Frequency Comb Laser Measurements

Computational simulations have the potential to provide low-cost, low-risk insights into wildland fire structure and dynamics. Simulation accuracy is limited, however, by the difficulty of modeling physical processes that span a wide range of spatial scales. These processes include heat transfer via radiation and turbulent advection, as well as both solid- and gas-phase chemistry. In the present study, we perform large eddy simulation (LES) with adaptive mesh refinement to model the multi-phase pyrolysis and combustion of dry Douglas fir, where temperature-based lookup tables corresponding to a multi-step pyrolysis mechanism are used to represent the composition of gas-phase pyrolysis products. Gas-phase and surface temperatures, mass loss, and water vapor mole fraction from the LES are shown to compare favorably with experimental measurements of a radiatively heated Douglas fir fuel sample undergoing pyrolysis and combustion beneath a cone calorimeter. Using frequency comb laser diagnostics, optical and infrared cameras, and a load cell, the experiments provide simultaneous in situ, time-resolved measurements of chemical composition, temperature, and mass loss. The present study thus combines cutting edge computational and experimental techniques with multi-step chemical pyrolysis modeling to provide a validated computational tool for the prediction of solid fuel pyrolysis and combustion relevant to wildland fires.

Direct measurements of DOCO isomers in the kinetics of OD + CO.

Quantitative and mechanistically detailed kinetics of the reaction of hydroxyl radical (OH) with carbon monoxide (CO) have been a longstanding goal of contemporary chemical kinetics. This fundamental prototype reaction plays an important role in atmospheric and combustion chemistry, motivating studies for accurate determination of the reaction rate coefficient and its pressure and temperature dependence at thermal reaction conditions. This intricate dependence can be traced directly to details of the underlying dynamics (formation, isomerization, and dissociation) involving the reactive intermediates cis- and trans-HOCO, which can only be observed transiently. Using time-resolved frequency comb spectroscopy, comprehensive mechanistic elucidation of the kinetics of the isotopic analog deuteroxyl radical (OD) with CO has been realized. By monitoring the concentrations of reactants, intermediates, and products in real time, the branching and isomerization kinetics and absolute yields of all species in the OD + CO reaction are quantified as a function of pressure and collision partner.

OD + CO → D + CO2 branching kinetics probed with time-resolved frequency comb spectroscopy

Time-resolved direct frequency comb spectroscopy was used to study the kinetics of the OD+CO→D+CO2 reaction, which is important for atmospheric and combustion chemistry. Complementing our recent work on quantifying the formation rate of the trans-DOCO radical, we report measurements of the kinetics of the chemically activated product channel, D+CO2, at room temperature. Simultaneous measurements of the time-dependence of OD and CO2 concentrations directly yield the products’ formation rate and its dependence on pressure and bath gas. Together with the trans-DOCO formation rate, these new measurements provide absolute yields of branching channels for both products of OD+CO in the low-pressure limit.

Mid-Infrared Time-Resolved Frequency Comb Spectroscopy of Transient Free Radicals.

We demonstrate time-resolved frequency comb spectroscopy (TRFCS), a new broadband absorption spectroscopy technique for the study of trace free radicals on the microsecond timescale. We apply TRFCS to study the time-resolved, mid-infrared absorption of the deuterated hydroxyformyl radical trans-DOCO, an important short-lived intermediate along the OD + CO reaction path. Directly after photolysis of the chemical precursor acrylic acid-d1, we measure absolute trans-DOCO product concentrations with a sensitivity of 5 × 10(10) cm(-3) and observe its subsequent loss with a time resolution of 25 μs. The multiplexed nature of TRFCS allows us to detect simultaneously the time-dependent concentration of several other photoproducts and thus unravel primary and secondary chemical reaction pathways.