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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?

enter image description here

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  • $\begingroup$ 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, an the area of the combustion chamber increases with the direction of the flow (area increase increases velocity for supersonic flow). $\endgroup$
    – Frosty
    Commented Jul 21, 2020 at 17:55
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    $\begingroup$ You should post that as an answer, rather than a comment, so that it can help future visitors to the site. Welcome to Aviation.SE! $\endgroup$
    – Steve V.
    Commented Jul 22, 2020 at 3:04

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$\begingroup$

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).

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  • $\begingroup$ i got the impression that mach number decreases also because speed of sound increases. $\endgroup$
    – Abdullah is not an Amalekite
    Commented Jul 22, 2020 at 7:51
  • $\begingroup$ That should also decrease mach number, I think both effects would be taking place then. $\endgroup$
    – Frosty
    Commented Jul 22, 2020 at 15:37
  • $\begingroup$ i would expect a pressure increase for that. i'm used to thinking of combustors as constant pressure, constant velocity, with the increase in volume from combustion countered by increasing cross sectional area down the combustor. $\endgroup$
    – Abdullah is not an Amalekite
    Commented Jul 22, 2020 at 15:50
  • $\begingroup$ Make sure you give yourself a check mark for the answer, so others know this has a solution. $\endgroup$
    – FreeMan
    Commented Jul 22, 2020 at 17:12
  • $\begingroup$ Abdullah, that was an interesting point. I asked my professor about this. The pressure in a traditional combustor stays constant because there is a decrease in density as temperature increases. Since mass flow rate must stay constant, there in turn must be an increase in velocity to account for the decrease in density. From what I understand, the combustor velocity increases to keep pressure constant, not the cross sectional area of the combustor. $\endgroup$
    – Frosty
    Commented Jul 22, 2020 at 19:21

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