Creation of a transonic region by means of gas fuel burning in a supersonic flow with a Mach number M = 2 in a channel with a variable cross section is studied numerically. The averaged Navier - Stokes equations closed by the k-? model of turbulence are solved. The transonic region is created by using gas-dynamic pulses of a throttling jet, which initiates gaseous fuel combustion in the channel region with a constant cross section. The fuel (ethylene or kerosene) is supplied axially into the axisymmetric channel. It is shown that initiation of kerosene combustion requires using gas-dynamic pulses of higher power than those in the ethylene case. The evolution of the kerosene combustion process in channels of different lengths is studied. It is shown that the pulse action duration required for initiation increases with the length of the channel region with a constant cross section.