Jet Propulsion/Thermodynamic Cycles/Ramjet cycle

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Ramjet-schematic-kk-20050810.png

A ramjet uses the open Brayton cycle. In the above diagram a 2D supersonic intake is shown downstream of which is a divergent subsonic diffuser.

The temperature rise in the diffuser is related to the freestream Mach number M_0:

\frac{T_2}{T_0}= 1 + \frac {\gamma-1}{2} M_0^2

Maximum efficiency is reached if temperature rise in combustor is small.

\eta = \frac {[(\gamma-1)/2] M_0^2}{1+ [(\gamma-1)/2] M_0^2}

where \gamma = c_p/c_v \approx 1.4 is the ratio of specific heats of air.

Ramjets are inefficient at subsonic speeds and their efficiency improves at supersonic speeds.

At hypersonic speeds the compression and dissociation processes make full diffusion unattractive and supersonic combustion is being researched. Scramjet slow the air down to low supersonic speeds and then burn high flame velocity fuels such as hydrogen or methane to try to get net thrust.