In this 13-part series of articles, The Combustion Institute recognizes the 2023 Distinguished Papers selected from among the scientific papers presented during the 39th International Symposium on Combustion. Congratulations to Hongchao Chu, Lukas Berger, Temistocle Grenga, Zhao Wu, and Heinz Pitsch for winning the DPA in the Turbulent Flames colloquium.

When asked about the motivation for their paper, “Effects of differential diffusion on hydrogen flame kernel development under engine conditions,” Hongchao Chu, representing his group, said that there is significant potential for reducing greenhouse gas and pollutant emissions by using hydrogen as a fuel in internal combustion engines, particularly in heavy-duty vehicles. Given the significantly different molecular properties of hydrogen compared to other fuels, e.g. its high molecular diffusivity, the behavior of hydrogen flame kernels differs significantly from that of conventional large hydrocarbon fuels. Understanding the early development of flame kernels in engine-relevant conditions is crucial for engine emissions and thermal efficiency. Thus, a knowledge gap for further investigation is presented.

It is known that lean hydrogen/air flames are prone to thermodiffusive instabilities due to differential diffusion, which refers to the high molecular diffusion of hydrogen compared to other species. Although it has been seen that this instability can cause a huge effect – namely it can increase burn rates in turbulent flames by a factor of 3-5 – the specific impact of differential diffusion on the early flame kernel growth was not clear. This is particularly true under realistic engine conditions characterized by elevated in-cylinder pressure and high unburned temperature. This research successfully demonstrated that under engine conditions, differential diffusion leads to strong thermodiffusive instabilities, significantly increasing the stretch factor and thereby facilitating early flame kernel growth.

This research has both immediate and long-term impacts. Crucial quantitative insights into the challenging effects that need to be addressed for modeling hydrogen flame kernel development in engine-relevant conditions are demonstrated by the detailed analysis in this study. It has significant implications for the design of hydrogen engines and will advance the understanding and optimization of the combustion process in such engines.

Chu, Berger, Grenga, Wu, and Pitsch have developed a unique high-fidelity DNS database that will facilitate subsequent model development and validation. This is particularly beneficial for model developers and engine designers. The numerical methods utilized in this study are also advantageous for other researchers interested in DNS.

This research took place at the Institute for Combustion Technology, RWTH Aachen University, Germany. The study began in the spring of 2021 and concluded in December 2021.

Over 1,500 papers were submitted to the 39th International Symposium on Combustion. All papers were categorized into one of 13 colloquia, and then distributed to Colloquium Coordinators and Co-Chairs. Each paper received at least three reviews from qualified individuals through the peer-review process. Less than 50 percent of the papers submitted were accepted for presentation.

Following the symposium, one paper presented in each colloquium is awarded the distinction of Distinguished Paper. Visit here to view the presentation. The 13 Distinguished Papers undergo committee review for consideration for the Silver Combustion Medal. A paper selected for this honor exemplifies quality, achievement, and significance to advance a field of combustion science, and will be awarded during the CI’s 40th International Symposium – Emphasizing Energy Transition in Milan, Italy.