ИСТИНА

Hybrid rocket engines are often considered as an intermediate class between the liquid rocket engines and solid rocket motors. For conventional solid fuels (i.e., such cured polymers as HTPB), gas-phase combustion is maintained by condensed phase pyrolysis and fuel vapor diffusion in the boundary layer. In this case, the convective heat transfer from the flame zone to the condensed phase fuel grain plays the governing role. The mass blowing off the gasifying surface promotes convective heat transfer blockage, reducing the solid fuel regression rate. Low-melting solid fuels overcome this intrinsic limitation of the conventional fuels, due to the droplet formation. This type of the solid fuels is characterized by the formation of a liquid layer on the solid fuel grain surface. While some part of the molten fuel is vaporized by the heat transfer from the reaction zone to the condensed phase (as in the 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 contribute to the convective heat transfer blockage. As a consequence, the overall regression rate of low-melting solid fuels is 3-4 times higher than that of the conventional formulations. In this paper, a computational model of low-melting solid fuel regression in the combustion chamber of hybrid rocket engine is developed. The numerical model is based on a system of RANS equations with a turbulence model for the gas phase. For the condensed phase of the solid fuel, we take into account its heating by 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 is consider as an incompressible, high-viscosity liquid. We take into account the dependence of melt viscosity on temperature in the form of Arrhenius’s law. On the interface between the 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 volume-of-fluid (VOF) approach. Simulation results are presented, demonstrating the variation of liquid-gas interface. It is shown that the molten layer becomes unstable under certain conditions, which results in the formation of waves on the surface of liquid layer with the amplitude growing in time. After reaching some critical value, the waves decay into the ensemble of droplets which enter the gas flow. The computational model predicts the droplet formation due to interaction of the waves with gas flow. Parametric studies of the regression rate are performed for a range of inlet gas velocities and temperatures; also, the effects of molten fuel viscosity on the regression rate are demonstrated. The results of numerical calculations are compared with well-known experimental data.