Next Page →
6: IDEAL RAMJET CYCLE

6: IDEAL RAMJET CYCLE

This is essentially the Brayton cycle. Here we have a flowing fluid, so that it is not useful to draw a pressure-volume diagram. We will instead represent the engine processes on a Temperature - Entropy (T- s) diagram. This is the same as an Enthalpy-Entropy diagram (a Mollier Chart) if cp is constant.

 

Region

Process

Ideal

Actual

a to 1

No change: supersonic flow: no external compression or suction ahead of inlet.

 

1 to 2

Adiabatic compression, with no work done except volume change

Isentropic:

p, T increase; To, po constant.

s constant.

po drops due to shocks and friction.

s increases.

2 to 3

Heat addition

Constant pressure.

To, T increase;

po constant.

po drops.

3 to 4

Adiabatic expansion. No work extracted except that of volume change.

To, po constant. T, p drop.

po drops slightly.

s increases slightly.

 

T-s Diagram

 

 

Notes:

1. Lines of constant pressure slant upwards.

2. Lines of constant pressure also diverge.

3. Even in the ideal engine, entropy must increase when heat is added. We thus see that as the fluid is brought back to the ambient pressure pa, the temperature reached is higher than the ambient temperature. This is another way of expressing the Brayton cycle efficiency.

 

 


Next Page →