Resolving the Shortcomings in Modern NH3 Kinetics Models using Detailed Species Time Histories and Direct Rate Measurements

Because of its zero-carbon content and established production methods, ammonia (NH3) has drawn much interest as a fuel for power generation and propulsion. Many research studies worldwide have been conducted over the past few years in an attempt to understand and predict its combustion chemistry. However, there are wide discrepancies among current models that are used to predict the combustion behavior of ammonia, and none can match the entire existing data set, so progress has been incremental and sometimes inconsistent. This project will resolve much of the current discrepancies and increase the reliability and predictive capability of ammonia chemistry models. The PI and his team will utilize state-of-the-art laser diagnostics and lab facilities to measure the model parameters that are currently missing. Research into hydrogen-based fuel sources such as ammonia will ultimately help to reduce the production of greenhouse gases globally. This multidisciplinary research project will allow graduate students from both mechanical engineering and physical chemistry backgrounds to interact on a daily basis. Ongoing, complementary projects in the PI’s lab and the Turbomachinery Laboratory’s undergraduate research program will broaden the number of participants while giving undergraduate students exposure to research using lasers for combustion chemistry. Although many results have been generated in recent years on ammonia chemical kinetics, most of the data have been for characterization of global reactivity, namely ignition delay times and laminar flame speeds. However, improved insight into the chemical kinetics of NH3 can be made by focusing on the measurement of detailed species time histories in a shock tube. Such measurements can be tailored for the validation of oxidation mechanisms and the direct measurement of rate coefficients. This project will measure species time histories for mechanism diagnosis and for making direct measurements of the rate constants of key reactions in the NH3 oxidation mechanism. By using a shock tube to produce the high-temperature conditions (1000 – 2500 K), laser absorption measurements of NH2, NH3, H2O, and N2O will be performed, and the reaction rates of at least 2 important reactions will be measured with high accuracy. The successful completion of the project will advance the fundamental understanding and prediction of ammonia oxidation. By focusing on the detailed information that concentration time histories of important species can provide, many of the discrepancies that currently exist among NH3 chemical kinetics models can be resolved. Direct measurements of the rate coefficients of individual, critical reactions at combustion temperatures will further improve the accuracy of detailed kinetics mechanisms for ammonia combustion. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.