ИСТИНА

Hybrid rocket engines are often considered as an intermediate class between liquid rocket engines and solid rocket motors. For conventional solid fuels (i.e., cured polymers as HTPB), the combustion process is ruled by condensed phase pyrolysis and fuel vapor diffusion in the boundary layer. In this case the combustion process in hybrid rocket engines is driven by convective heat transfer from the flame zone to the condensed phase fuel grain. The mass blowing from the gasifying surface promotes convective heat transfer blockage, which results in reducing the solid fuel regression rate. Low-melting solid fuels overcome this intrinsic limitation of the conventional fuels, thanks to the droplet formation. This type of the solid fuels is characterized by the formation of a liquid layer on the surface of the solid fuel grain. While part of the melted fuel is vaporized by the heat transfer from the reaction zone to the condensed phase (as in conventional formulations), a fraction of it leaves the surface in the form of liquid droplets captured and entrained by the oxidizer stream. Being in the condensed phase, these droplets do not concur to the convective heat transfer blockage. As a consequence, the overall regression rate of low-melting solid fuels is 3-4 times the one of conventional formulations. In this paper we develop a computational model of low-melting solid fuel regression in the combustion chamber of hybrid rocket engine. The numerical model is based on a system of fully compressible RANS equations with k-ε turbulence model for gas phase. Turbulent combustion is described by the Eddy Dissipation Concept (EDC) model. For condensed phase of the solid fuel we take into account its heating under action of the heat flux from the gas flow, melting of the solid fuel and formation of the molten layer on the grain surface. The molten layer of solid fuel we consider as incompressible, high viscous liquid in boundary-layer approximation. On the interface of molten layer and gas flow we take into account the heat exchange and mechanical interaction due to pressure and tangential stress. Equations are solved on a Cartesian mesh, with the complex geometry liquid-gas interface described by the moving embedded boundary based on the level-set approach. The embedded boundary is propagating at the velocity determined by the motion of liquid layer and by the local regression rate proportional to heat flux incident onto the surface from the combustion zone. Variation of liquid-gas interface is demonstrated. We show that the molten layer loses its stability under certain condition. This instability results in formation of waves on the surface of liquid layer the magnitude of which increases in time and after reaching some critical value, the waves decay into the ensemble of droplets which enter the gas flow. The computational model of droplet formation due to interaction of the waves with gas flow is developed. The results of numerical calculations are compared qualitatively with well-known experimental data.