Critical

Question 1

On a enthalpy-entropy diagram, how does the actual stagnation state differ from the isentropic stagnation state

Answer 1

The stagnation enthalpy represents the total energy of the fluid per unit mass, which is in a constant adiabatic situation.

The static enthalpy represents the total energy per unit mass of a fluid which doesn’t include the kinetic energy of the fluid. if the kinetic energy of the fluid is zero, the static enthalpy is equal to the stagnation enthalpy.

Question 2

Explain the principle of a converging diverging nozzle

Answer 2

In the converging, the velocity, temperature and pressure of sound decreases whilst the velocity and mach number of fluid increase in a flow direction. The density decrease slowly and then quickly as the velocity increases.

In the throat, the pressure and mach number reach critical values.

In the diverging, the velocity of the fluid increases as the pressure decreases, this is due to the decreasing density of there fluid.

Question 3

Describe why a ramjet travelling at supersonic velocities may use a diffuser prior to the combustion process?

Answer 3

Ramjets are a logical evolution of turbofan engines for supersonic flight. The compressor efficiency drops dramatically due to shockwave and boundary layer separation losses when the blades are near sonic.

The removal of the gas turbine blades simplifies the engine mechanically which removes mechanical losses in the engine. It also prevents swirling, as in purely axial flow, and there are no radial components. This increases the tangential velocity which increases the thrust generated by the engine.

Question 4

Explain how the efficiency of a gas turbine engine has increased. Focus on compressor and turbine

Answer 4

Compressor
-Reducing tip losses to due leakage caused by the stationary walls and moving rotors
-Minimisation of surface to volume ratio
-Study of 3D flow fields to minimise secondary flow losses
-Use of low drag, laminar flow blade cascades
-Use of centrifugal compressor in small jet engines.

Turbine
-Use of single crystal blade structures
-Implementation of multi-pass cooling
-Use of thermal barrier coatings
-Casting bleed air holes into the trailing edge of the blades
-Use of Blisks and Blinds

Question 5

Describe the evolution of the piston engine in the first half of the 20th century

Answer 5

By the 1950’s power to weight and power outputs had increased significantly due to the high speeds and performance of military engines, and the increased speed, reliability and safety of civil aviation engines.

Specific weight of the engines had decreased from 9kg.kw of the Wright flyer to 0.6kg/kw, and the power of operational engines had peaked at 4300bhp.