<p>The simulation principles of slag formation process in the combustion chamber of power plants using high–energy materials such as solid and pasty propellants have been established. This process is determined by the processes of propellant burning, evolution of the multiphase flow inside a combustion chamber and the behavior of slag residues on structural elements. A sequence of using the characteristics of these processes to describe slag formation is shown. A new model for multiphase flow evolution is presented. This model is a key tool for describing the slag formation process. The features of this model are a detailed description of big-fraction solid particles transformation and considering the interaction of condensed and gaseous phases of the flow. The model is based on the combined Euler-Lagrange approach. To validate the model, an experimental study was conducted using a model engine based on aluminized paste propellant. The result of study brings a conclusion about the adequacy of simulation. The directions of further research are determined to solve the problem of simulating a process with account for varying composition parameters and conditions within the combustion chamber.</p>