Effect of H2-Air Premixing Characteristics on NOx Emissions in Microinjector Systems

Abstract With a push toward decarbonization, H2 has emerged as a popular carbon-free energy carrier with a strong focus on designing and modifying combustion systems to support its burning. With its high flame speed and flashback tendencies, jet flames have been explored as a dominant flame holding choice for H2 flames leading to the use of micromixer type injectors for dry, low-NOx next generation systems. The objective of this paper is to bridge fundamental research in micromixers with practical design. In this study, mixing and emission characteristics from H2 based micromixer systems have been assessed to build a low order modeling prediction tool. These systems comprise of six different fuel injection configurations, injecting H2 into air crossflow, followed by a mixing section of constant length. Three-dimensional steady RANS simulations were conducted to evaluate the relative mixing performance of these designs. The composition profile at various axial cuts along the mixing section is then supplied to an array of 1D premixed flat flame models to obtain a distribution of NOx emissions at a fixed residence time. Results of our modeling show that with increasing mixing length from L to 2L across all injectors, the coefficient of variation in equivalence ratio (φ) decreases. We observe that though the area weighted φavg is 0.5 across injectors, the range for φ at a given axial location varies based on the design as well as mixing length — represented in terms of an unmixedness level βφ. The relative emissions performance of injectors under consideration varies as a function of mixing length, further emphasizing its importance in micromixer configurations. Through this study, we have identified potential design parameters vital in the development of micromixers for H2 combustion.