Ramjet/scramjet Flashcards
Describe why a ramjet travelling at supersonic velocities may use a diffuser prior to the combustion process.
Ramjet engine without compressors has seemed as a logical evolution of the gas turbine appropriate to supersonic flight speeds, it is because the compressor efficiency drops dramatically due to shock wave and boundary-layer separation losses when the blade tip speeds are near sonic.
The removal of the gas turbine rotors simplifies the engine mechanical configuration, which eliminates mechanical loss that appears in in gas turbine engines. It also prevents swirling, as in purely axial flow, there are no radial components of airstream velocity, the tangential velocity would be larger, which directly contributes to larger thrust generation.
Either graphically or by description, indicate how the following quantities vary through all points from the diffuser to the entry to the combustion chamber, and subsequently the exit of the ramjet nozzle; pressure, temperature, velocity, stagnation pressure, stagnation temperature and density.
- The inlet or diffuser slows the air velocity relative to the engine from the flight velocity V0 to a smaller value V3. This decrease in velocity increases both the static pressure P3 and static temperature T3.
- However, the stagnation pressure and temperature remain unchanged for an ideal ramjet engine as the process is assumed isentropic.
Sketch and explain how the geometry of ramjets and scramjets may be utilised to control combustion Mach number.
https://app.box.com/s/zrhyh546jdezkzpn8ydytnyht5tunn6t/file/476130785521
Q1 C
Explain the difference between a ramjet and a scramjet
The combustion process in an ordinary ramjet takes place at low subsonic velocities; the combustion process of scramjet takes place at supersonic velocities
Explain why the ramjet is more appropriate for high speed flight than a traditional gas turbine engine.
The ramjet depends on the inlet to decelerate air to raise pressure in the combustion zone. The higher the velocity of the incoming air, the higher the pressure rise. Hence the ramjet works best at supersonic velocities. At subsonic velocities the ramjet is inefficient.
Explain, using a temperature-entropy diagram, why this upper limit leads to the requirement for a scramjet at high flight speeds
Slide 352 of Mega PDF
What sets an upper limit on the temperature of combustion in the ramjet?
At high speeds,high stagnation pressure and temperatures are experienced by an engine. To combust at subsonic velocities, the reduction in gas speed leads to high static pressures and temperatures which might exceed the material properties of the engine or lead to energy loss in dissociation reactions.
By allowing combustion at supersonic speeds, this reduces the static temperatures and pressures on the engine.
Describe how the geometry of a scramjet differs from that of a ramjet. Why are these
changes necessary?
The geometry of a scramjet differs from that of a ramjet in several ways due to the differences in their operating speeds. Ramjets operate at supersonic speeds, while scramjets operate at hypersonic speeds. In a ramjet, combustion occurs at low subsonic velocities, and the deceleration of high-velocity airstreams to subsonic velocity can result in large pressure losses and temperature rise, limiting the flight speed of the ramjet. On the other hand, in a scramjet, combustion occurs in the high supersonic and hypersonic flight regime, involving very high stagnation temperatures, pressures, and area contraction ratios, as well as complex variable geometry intake diffusers that adapt to flight across a wide speed range. Additionally, the subsonic combustor in a scramjet operates at high stagnation temperatures and low speeds, leading to gas undergoing energy-absorbing dissociation reactions. These changes in geometry are necessary to ensure efficient combustion and performance at hypersonic speeds in a scramjet engine.
State three reasons why the application of ideal analysis to ramjet behaviour may lead to
unrealistic results.
Real gas effects – Assumptions made in our ideal gas laws are not
necessarily valid at the working temperatures of ramjets
Fuel mass flow addition – This will significantly affect mass flow in the combustor and the nozzle.
Combustor stagnation pressure loss – related to dissipative losses caused by friction, separation and shock waves in the
combustor
Describe, for high speed aircraft, how a limiting cycle temperature leads to the requirement for supersonic combustion as flight speeds increase.
At high speeds, stagnation pressure and temperatures are experienced by an engine. To combust at subsonic velocities, the reduction in gas speed leads to high static pressures and temperatures which might exceed the material properties of the engine or lead to energy loss in dissociation reactions.
By allowing combustion at supersonic speeds, this reduces the static temperatures and pressures on the engine.
Based on these calculations, describe and explain the operational envelope of an ideal ramjet.
Base on the calculation, we can see that with a higher specific thrust, less fuel will be used. As fuel air ratio is constant in this calculation, the maximum flight Mach number will appear at where Thrust specific fuel consumption is infinitely close to zero, which sets a speed range of an ideal ramjet.
However, since the geometry is not included in parametric cycle analysis,
the plots of specific thrust and thrust specific fuel consumption, against Mach number are not portraying the behaviour of a specific engine.
