In future energy converters using synthetic fuels from renewable sources, the near-wall region is of particular importance. Chemically reacting multi-phase flows, near-wall combustion processes and reaction processes in exhaust systems are characterized by a number of additional boundary conditions and properties that differ significantly from reactive flows far from the wall.
In research area A the underlying physicochemical mechanisms are investigated in simplified, generic environments using innovative measurement techniques. The focus is on both fossil and electrofuels as well as exhaust aftertreatment using SCR catalysts, for example.
We develop and validate sub-models and high-resolution numerical simulations using experimental insights and data from A. The focus is on individual processes for future electrofuels as well as higher near-wall pressures and temperatures. From this, overall models for the interaction of chemical reactions, turbulent flow, multiphase processes and wall heat transfer are being developed.
For example, the wall can directly influence the reaction process via catalytic effects. Already at a low chemical activity, catalytic processes can be highly relevant for effects such as pollutant formation or flame stability via radical adsorption on the wall. This becomes obvious by the fact that walls of different qualities can have decisive influence on the stability of combustion systems. The reaction-diffusion system of the gas phase becomes more complex due to the high gradients at the wall. Furthermore, a number of pollutants, especially unburned hydrocarbons, carbon monoxide and soot are formed largely in regions close to the wall.
The aftertreatment of pollutants in the exhaust train always happens on surfaces that often represent partition walls of honeycomb structures (monoliths). Those monoliths can be coated with a catalytic material on which the rate of the purifying chemical reaction is increased, or they are designed as filters to absorb soot particles, nitrogen oxides or ammonia. Walls in the exhaust train, such as exhaust pipes or mixers also play an important role during the provision of additives used in emission control, such as ammonia. Aqueous ammonia is used as precursor and can form undesirable films and solid deposits at theses walls. In all those cases chemically reacting turbulent flows interact with solid walls.
Combustion and exhaust systems are characterized by low temperatures in close proximity to the wall as well as by high temperature gradients. This leads to special conditions during heat transfer. The fluid-wall heat transfer processes are of central significance and have to be understood in the context of transient processes to analyze near-wall reaction processes.