why does the flow need to decelerate (by a thermal throat) in a scramjet?

Question

I have a question about the thermal throat in a scramjet. From my textbook:

"The scramjet, on the other hand, needs an area increase as combustion occurs. For this reason, on the scramjet the mechanical throat is substituted by a thermal throat that decelerates the flow through
tailored heat release."
why does the flow need to decelerate in a scramjet? 
if flow in a scramjet is supersonic, wouldn't heat release
accelerate the flow? (from general 1d form of equations du/u=1/(M^2-1)*(dA/A+dF/P-dQ/CpT))
Where is the thermal throat in this flow located?

Frosty · Answer

I found an answer, for those who are curious. The thermal throat described in a scramjet is not an ideal case. Essentially, when the flow is supersonic, heat addition done in the combustion chamber can slow the flow down to subsonic speeds (based off the 1-d heat velocity, Rayleigh Flow). When this happens the flow is considered thermally choked, and is a bad case for this engine. To prevent this thermal choke from happening, the area of the combustion chamber increases with the direction of the flow (area increase increases velocity for supersonic flow).

Answered by Frosty on November 7, 2021

why does the flow need to decelerate (by a thermal throat) in a scramjet?

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