GAS DYNAMICS AND JET PROPULSION – KME064
Section-wise Important Questions & Ready Answers
SECTION A
(Attempt all – 2 marks each)
(a) Compressibility Effect
Compressibility effect refers to the change in density of a fluid when subjected to pressure and velocity variations. This effect becomes significant when the Mach number exceeds about 0.3, and it is a key consideration in high-speed gas flows.
(b) Nozzle and Diffuser
A nozzle is a device used to increase the velocity of a fluid by converting pressure energy into kinetic energy. A diffuser performs the opposite function by decreasing fluid velocity and increasing pressure.
(c) Stagnation Enthalpy
Stagnation enthalpy is the total enthalpy of a fluid when it is brought to rest isentropically. It is expressed as the sum of static enthalpy and kinetic energy per unit mass.
(d) Reason for Shock Wave Formation in a Nozzle
Shock waves develop in a nozzle when the back pressure is higher than the design exit pressure for supersonic flow. This causes a sudden and irreversible change in pressure, temperature, and velocity.
(e) Conditions for Fanno Flow
Fanno flow occurs in a constant area duct with friction, steady one-dimensional flow, adiabatic conditions, and no external work interaction.
(f) Over-Expanded and Under-Expanded Flow
Flow is over-expanded when exit pressure is lower than ambient pressure, causing compression waves. It is under-expanded when exit pressure is higher than ambient pressure, leading to expansion waves.
(g) Escape Velocity
Escape velocity is the minimum velocity required for an object to escape the gravitational pull of a celestial body without further propulsion.
(h) Essential Components of Turbojet Engine
A turbojet engine consists of an air intake, compressor, combustion chamber, turbine, and exhaust nozzle. These components work together to produce thrust.
(i) By-Pass Ratio in Turbofan Engine
By-pass ratio is the ratio of mass flow rate of air bypassing the engine core to the mass flow rate passing through the core. Higher by-pass ratios improve fuel efficiency and reduce noise.
(j) Monopropellant
A monopropellant is a single chemical that produces thrust through decomposition without requiring an oxidizer. Hydrogen peroxide is a common example.
SECTION B
(Attempt any three – 10 marks each)
1. Stagnation to Static Temperature Ratio and Mach Number
For isentropic flow, stagnation temperature is related to static temperature and Mach number through energy conservation. As Mach number increases, the difference between stagnation and static temperature becomes significant.
2. Mach Number Variation in a Convergent-Divergent Duct
In a nozzle, Mach number increases from subsonic to supersonic after the throat. In a diffuser, Mach number decreases. In a venturi, acceleration and deceleration occur symmetrically.
3. Maximum Entropy in Fanno Flow
For Fanno flow, entropy reaches a maximum when Mach number equals one. Any further increase or decrease in Mach number results in reduced entropy.
4. Working of Ramjet Engine
A ramjet engine operates without a compressor or turbine. High-speed air compression occurs due to the forward motion of the aircraft. Combustion increases energy, and thrust is produced through a nozzle.
5. Liquid vs Solid Propellants
Liquid propellants offer higher efficiency and controllability, while solid propellants provide simplicity and reliability. The choice depends on mission requirements.
SECTION C
Q3(a) Compressible vs Incompressible Pressure Coefficient
At sonic conditions, compressibility effects become prominent, resulting in a noticeable deviation between compressible and incompressible pressure coefficient values, approximately 27.5% for air.
Q3(b) Isentropic Flow Relations
For one-dimensional isentropic flow, pressure, temperature, and density ratios are functions of Mach number and specific heat ratio, forming the basis of compressible flow analysis.
Q4(a) Normal Shock Wave
Across a normal shock, Mach number drops from supersonic to subsonic, static pressure and temperature increase sharply, while stagnation pressure decreases due to irreversibility.
Q4(b) Jet Engine Performance (Numerical)
Thrust is calculated from mass flow rate and velocity difference. Thrust power and propulsive efficiency are determined using standard propulsion equations.
