The article gives the results of numerical and theoretical research on features of high-enthalpy supersonic flows in narrow cylindrical ducts. Mathematical model is based on non-stationary axisymmetric Navier-Stokes complete equation system. Calculations are made for ducts of different length at laminar and turbulent flow modes with various turbulence models. The article analyzes influence of ducts geometric parameters on flow characteristics. The author studies peculiarities of Mach number distributions along radius of the ducts at different cross-sections. The article shows that supersonic flow deceleration occurs in the ducts and intensity of the deceleration depends on considered conditions and can cause formation sub-sonic flow zone of considerable length.