Ramjet Flashcards
2) Why do we need to use the fuels with high energy content?
- The change in the stagnation enthalpy rate of the gas flow
between the exit and entrance of the engine is equal to the added chemical enthalpy rate of the injected fuel flow which is directly proportional to the exit velocity. - Therefore, the higher the energy of fuels (enthalpy) the more thrust can be generated.
- Hydrocarbon fuels have a very high energy content which allows for extended powered flight.
3) Please analyse how to improve the cycle efficiency? from the simplified equation used to value the cycle efficiency of a ramjet
To increase cycle efficiency if we assume the nozzle and burner are ideal (PA=PD and PB=PC)
1 - Cycle efficiency is proportional to the inlet pressure ratio PB/PA
2 - Cycle efficiency is proportional to the combustor temperature difference TC-TB
3 - As the inlet total pressure ratio PoA/PoB decreases cycle efficiency decreases
4 - The cycle is anchored by the inlet compression process, which is a characteristic of the Brayton cycle.
5) Which aspects influence the combustion efficiency and stability limits?
- Type of fuel
- Equivalence ratio which is the ratio of the actual fuel-air ratio to the stoichiometric fuel-air ratio
- Air stagnation pressure
- Air stagnation temperature
- Afterburners as they increase the temperature of the gas ahead of the nozzle increasing the exit velocity
6) Why does a ramjet have difficulty in operating when cruising above Mach 5?
This is due to the ram compression causing a very high temperature, these high temperatures can exceed material limits. It is also difficult to burn more fuel to produce more thrust at speeds greater than Mach 5.
7) Discuss the loses in a real ramjet
Stagnation pressure loses due to:
- Inlet shocks and viscous friction
- Burner heat addition and friction
- Nozzle shocks and viscous friction
Stagnation temperature loses due to:
- Heat conduction and radiation to walls
- Incomplete combustion
What is the significance of specific thrust in air-breathing engines?
Specific thrust indicates the efficiency of thrust generation relative to air mass flow, crucial for performance evaluation.
How does a ramjet operate, and what are its limitations?
Ramjets compress incoming air using the aircraft’s speed, effective at high velocities but incapable of producing thrust at zero airspeed.
What are the key differences between subsonic and supersonic ramjet inlets?
Subsonic ramjet inlets are simpler, relying on normal shock waves, while supersonic inlets involve oblique shock waves and complex geometries to handle higher air speeds effectively.
How does the thermodynamic cycle of an ideal ramjet work?
The ideal ramjet thermodynamic cycle involves adiabatic compression, constant pressure heat addition, isentropic expansion, and heat rejection at constant pressure.
How do supersonic inlets, like the Concorde’s, function?
They use movable ramps to manage airflow at different speeds, optimizing engine performance at Mach 2.
What is TSFC and its importance in jet engines?
SFC, or Thrust Specific Fuel Consumption, measures fuel efficiency in jet engines, indicating fuel used per unit of thrust. It’s key for assessing engine cost-effectiveness and efficiency.
