Session J34: Reacting Flows: Chemical Kinetics

Comparison of Chemical Mechanisms for Simulation of Hydrogen/Ammonia Combustion

Ammonia and hydrogen are two fuels with the potential to enable future carbon emission-free combustion systems. Due to complex chemical pathways, chemical mechanisms describing the combustion of ammonia/hydrogen mixtures are often inaccurate. While simulations of combustion systems have the potential to accelerate the transition to a carbon-free future, they rely on accurate chemical mechanisms thereby motivating the need to assess existing mechanisms for accuracy. In this work, several existing chemical mechanisms describing hydrogen/ammonia combustion are assessed. Both premixed and non-premixed flames are considered and evaluated by comparing simulation results against existing experimental data. Premixed flame accuracy is assessed based on flame speeds from one-dimensional freely propagating flames, while non-premixed flame accuracy is assessed based on extinction strains of counterflow flames. For both configurations, sensitivity and reaction path flux analyses are performed to better understand the behavior and differences of the various chemical mechanisms investigated. From the aggregate of these analyses, recommendations for the most accurate mechanisms are drawn.   

Projection-based model reduction for thermo-chemical non-equilibrium gas mixtures

State-specific thermochemical collisional models are crucial for accurately describing the physics of systems involving non-equilibrium plasmas. Although these (nonlinear) models provide detailed insights into kinetic processes when internal energy levels significantly deviate from the equilibrium Maxwell-Boltzmann distribution, they are computationally expensive and impractical for large-scale, multi-dimensional simulations. While computational cost can be reduced significantly by using low-order models, these are typically derived using empirical arguments and physics-based assumptions that ultimately lead to inaccurate predictions. We propose to address this issue by developing projection-based reduced-order models that leverage the form of full-order governing equations. In particular, we identify an oblique projection operator by balancing the state and gradient covariance matrices associated with the linearization of the full-order equations about thermochemical-equilibrium steady-state solutions. The use of the linearized equations allows for fast data generation and training, while also providing a good approximation of the low-order subspace the nonlinear dynamics evolve on. Once the projection operator is identified, we obtain a reduced-order model via Petrov-Galerkin projection of the original nonlinear system. The overall procedure is akin to the well-known balanced truncation for linear time-invariant systems, and to the recently-developed CoBRAS formulation proposed by Otto et al., SISC, 2023. Reduced-order models are developed and tested on two distinct thermochemical systems: i) a  rovibrational collisional model for the O₂-O system, and ii) a vibrational collisional model for the combined O₂-O and O₂-O₂ systems. Our approach demonstrates both high accuracy and significant computational speedup for these nonlinear chemical kinetic systems, with training times on the order of minutes on a single GPU. The model effectively captures differences in orders of magnitude within the quantum state distribution function, resolving both low and high lying energy states.   

Probing chemical reactions via X-ray emission spectroscopy using a colliding droplet mixer

The high-intensity, high brightness X-ray pulses generated by X-ray free electron laser (XFEL) facilities are instrumental in studying fundamental processes with atomistic resolution. They allow  to study the electronic structure during transient reaction processes in the liquid phase at room temperature, in the absence of radiation damage.

By colliding two droplets acting as reagent carriers in air, a chemical reaction can be triggered via mixing and followed by time-delayed probe with X-ray spectroscopy. Colliding droplet mixing addresses two distinct challenges of other, more commonly used continuous flow sample delivery methods: First, two sufficiently small droplets (<50 pL) mix homogeneously within a few hundred µs, making reaction studies on the sub-millisecond time scale accessible. Second, by synchronizing the sample droplets with the X-ray source frequency (120 Hz), scarce samples such as metalloproteins can be measured. Here, we have characterized the mixing time scales of the reaction via fluorescence quenching reactions, and the stability of the droplet collisions via bright-field imaging. Also, we were able to follow an oxidation reaction of an iron model compound via element-specific Fe X-ray emission spectroscopy. We followed the changes in oxidation state over time, while retaining low sample consumption. Overall, this discontinuous flow mixing principle enables studies on a wide range of biologically and chemically relevant samples that are currently inaccessible.   

Direct-Injection Constant-Volume Combustion for Ignition Delay Measurement

The temperature dependence of ignition delay provides insight into the combustion mechanisms of heavy liquid fuels. In particular, negative temperature coefficient (NTC) behavior is predicted by chemical-kinetic models of hydrocarbon combustion. Experimental data at a wide range of conditions is necessary to validate chemical-kinetic models and examine the effects of fuel additives. This investigation explores the use of a small-scale direct-injection constant volume combustion chamber (CVCC) to obtain combustion pressure traces and ensuing ignition delays. A single-hole injector ensured near-homogeneous mixing between fuel and air. Temperature sweeps from 625 K to 875 K were conducted for iso-octane at 5 bar, n-heptane at 5 and 10 bar, and a 20% mixture of n-butyl acetate in heptane at 5 and 10 bar. A global equivalence ratio of 1.0 was maintained during all tests. Results showed strong NTC behavior for octane, but little for heptane. The observed ignition delays were similar in magnitude to those predicted by models in simulation. The butyl acetate/heptane blend showed longer ignition delays compared to heptane alone, and some NTC behavior was observed at similar temperatures. Overall, these results suggest that the direct-injection CVCC is a valuable platform for combustion experiments and can be used to evaluate diesel additives.