# Reactive Flows

Reactive flows are all types of fluid flows with chemical reactions occouring within the fluid phase, at the interphase between different fluid phases or at interphases to solids. Gasphase combustion phenomena are the most important application of reactive flow modelling, while homogeneous combustion (gas-liquid or gas-solid) is still a minor research field. Simulation of reactive flows is getting more and more important, since they offer a possibility to gain insight to processes having harsh conditions, e.g. high temperture and pressure combustion. Additionally, optimization of existing processes can be performed by modeling studies following design-of-experiment (DOE) approaches. In this way, the effort and costs for prototypes and field test can be reduced to a minimum.

However, modelling reactive systems is numerically expensive since the non-linear reaction kinetics leads to stiff-ODE systems in the species and energy conservation equations. Special solution algorithms, capable of handling these stiff systems of equations, are required to solve the equations. The best known stiff ODE solvers are the SIBS and RADAU5 algorithm.

Strategies to reduce the numerical effort to solve reactive flows include chemistry tabulation. Chemistry tabulation basically means that the solution of the given inital value problem (initial compostion, temperature, pressure) is stored. Subsequently, if similar starting values occur, the stored results are retrived and interpolated according to the differences in the starting values. These tabulation strategies differ depending on the employed combustion model. Some tabulation methods construct the tabulation on a predefined grid in the composition space, while others construt the tables "in-situ" during simulation run time.

A critical choice in simulating reactive flows is the combustion model. Different models, all having certain advantages and disadvantages, exist. Special emphasis is put on turbulent combustion and its strong and complex iteraction between turbulence and chemistry. Common turbulence-chemistry interaction models are based on three different principles: geometrical analysis, one-point statistcs and turbulent mixing. The flamelet assumption is usually associated with the geometrical analysis, where every flame elemet is assumed to behave like a laminar flame. In contrast to the flamelet model, one-point statistic models do not require assumptions on the flame structure. Turbulent mixing models assume reaction time scales smaller than mixing time scales. The Eddy Break-Up (EBU) model or Eddy Dissipation Concept (EDC) are the best know turbulent mixing combustion models.

Our reserach group is experinced in modelling different kinds of reactive flows. We are heavily involved in multiphase reacting flows, e.g. gas-solid flow considering gas and solid phase reactions.