# Does the flow in an afterburner have to be subsonic?

This question is prompted by the problem on page 350 of the fluid mechanics book by Landau and Lifshitz. The problem states, "A small amount of heat is supplied over a short segment of a tube in a perfect gas in steady flow in the tube. Determine the change in the gas velocity when it passes through the segment." This seems to me to be applicable to an afterburner. The solution is obtained by using conservation of mass flux and momentum flux and then increasing the energy flux by the heat added. The change in velocity turns out to be given by, $$$$dv=\frac{(\gamma-1)q}{\rho(c^{2}-v^{2})}$$$$ where $$\gamma$$ is the ratio of specific heats, $$q$$ is heat added per unit cross-section per unit time, $$\rho$$ is density, $$v$$ is velocity and $$c$$ is the local speed of sound. This formula says that if the flow is supersonic $$v>c$$ then $$dv<0$$ and the gas velocity slows down. The addition of heat only increases the gas velocity if the flow is subsonic. So, my question is, "Does this result imply that the flow in an afterburner is subsonic?"