Hydrogen Laminar Flames

Hydrogen holds great promise as a chemical energy carrier for transporting and storing renewable energy. For the use of hydrogen, thermochemical energy conversion generating electricity, mechanical power, or high temperature heat has many advantages, such as robustness, versatility, and flexibility. However, hydrogen is very different from commonly used hydrocarbon fuels. Because of its special chemical and molecular-transport properties, it has very peculiar combustion behavior, which is characterized by especially high flame speeds roughly a factor of ten larger compared with methane, and by the occurrence of intrinsic flame instabilities for lean premixed combustion, which can substantially alter flame structure, surface, and dynamics. The focus of this chapter is on these two aspects in the context of laminar flames. First, the reasons for the particularly high unstretched laminar flame speeds are explained followed by a discussion on the effects of flame stretch. The main part of the chapter deals with the intrinsic flame instabilities including both hydrodynamic and thermodiffusive instabilities and their interplay, which are discussed in terms of theory, experimental evidence, numerical simulations, and modeling.