(SEM VI) THEORY EXAMINATION 2021-22 GAS DYNAMICS AND JET PROPULSION

GAS DYNAMICS AND JET PROPULSION (KME064)

Section-wise Detailed Answers – B.Tech Semester VI

SECTION A

(Attempt all questions – brief but descriptive explanations)

Q1(a) Define rocket propulsion

Rocket propulsion is a method of producing thrust by ejecting high-velocity gases generated from the combustion of propellants. According to Newton’s third law of motion, the backward expulsion of gases produces an equal and opposite forward force on the rocket. Unlike air-breathing engines, rocket engines carry both fuel and oxidizer, allowing them to operate in space where no atmospheric oxygen is available.

Q1(b) What is meant by gas dynamics?

Gas dynamics is the branch of fluid mechanics that deals with the study of motion of gases, especially when compressibility effects become significant. It mainly focuses on high-speed gas flows where changes in pressure, temperature, density, and velocity are important, such as in nozzles, diffusers, shock waves, and jet propulsion systems.

Q1(c) Classify the rocket engines

Rocket engines are broadly classified based on the type of propellant used. They may be solid propellant rocket engines, liquid propellant rocket engines, or hybrid rocket engines. Classification may also be based on feed systems, such as pressure-fed and pump-fed engines.

Q1(d) What is Fanno flow?

Fanno flow is a type of one-dimensional compressible flow that occurs in a constant-area duct where friction is present but no heat transfer takes place. Due to friction, entropy increases and the flow tends toward a choked condition where the Mach number becomes unity.

Q1(e) Define propulsive efficiency

Propulsive efficiency is defined as the ratio of useful power output (thrust power) to the rate of kinetic energy supplied to the jet. It indicates how effectively the engine converts the energy of exhaust gases into useful thrust for propulsion of the vehicle.

Q1(f) What type of compressor is used in turbojet engines?

Turbojet engines primarily use axial-flow compressors. These compressors are preferred because they allow large mass flow rates with high efficiency and smaller frontal area, making them suitable for high-speed aircraft propulsion.

Q1(g) Explain Mach cone

A Mach cone is a conical region formed when an object travels through a compressible medium at a speed greater than the speed of sound. Pressure disturbances propagate at the speed of sound, and when the object exceeds this speed, the disturbances form a cone whose angle depends on the Mach number.

Q1(h) Distinguish between Mach wave and normal shock

A Mach wave is an infinitesimally weak pressure wave that occurs when an object moves at exactly sonic speed. A normal shock, on the other hand, is a strong shock wave occurring perpendicular to the flow direction, causing sudden changes in pressure, temperature, density, and velocity.

Q1(i) What is a nozzle?

A nozzle is a duct of varying cross-section used to control the direction and magnitude of fluid velocity. In jet propulsion and rockets, nozzles convert thermal energy of gases into kinetic energy to produce thrust.

Q1(j) What are the types of rocket engines?

Rocket engines may be classified as solid rocket engines, liquid rocket engines, and hybrid rocket engines based on propellant type. Each type has specific advantages in terms of performance, storage, control, and application.

SECTION B

(Attempt any three – detailed explanations)

Q2(a) Differentiate between Rayleigh flow and Fanno flow

Rayleigh flow describes compressible flow in a constant-area duct where heat transfer occurs but friction is neglected. In this flow, addition or removal of heat significantly affects Mach number and stagnation properties. In contrast, Fanno flow occurs in a constant-area duct with friction but without heat transfer. Friction causes entropy increase and drives the flow toward sonic condition. Both flows are idealized models used to analyze real gas flow situations.

Q2(b) Explain the types of rocket engines with neat diagram

Rocket engines are classified based on propellant state and feeding method. Solid rocket engines use a solid mixture of fuel and oxidizer, offering simplicity and reliability but limited control. Liquid rocket engines store fuel and oxidizer separately and mix them in the combustion chamber, allowing better thrust control and higher efficiency. Hybrid engines combine solid fuel with liquid oxidizer, offering safer operation and moderate control. Diagrams typically show combustion chamber, nozzle, propellant tanks, and feed system.

Q2(c) Write a short note on space flights

Space flight refers to the movement of spacecraft beyond Earth’s atmosphere. It requires overcoming Earth’s gravitational force using powerful rocket propulsion. Space flights may be orbital, sub-orbital, or interplanetary. Rockets provide thrust during launch, after which spacecraft may rely on momentum and small thrusters for maneuvering. Space flights are used for satellite deployment, space exploration, scientific research, and communication.

Q2(d) Explain rocket turbo-pump feed system, ignition system, and combustion system

The turbo-pump feed system supplies fuel and oxidizer to the combustion chamber at high pressure using turbines driven by exhaust gases. The ignition system initiates combustion, often using electrical sparks or hypergolic propellants. The combustion system ensures efficient mixing and burning of propellants to produce high-temperature gases that expand through the nozzle to generate thrust.

Q2(e) Numerical on rocket nozzle performance (explanatory)

In a rocket nozzle problem involving throat area, chamber pressure, specific impulse, and propellant flow rate, performance parameters such as thrust coefficient, characteristic velocity, and specific propellant consumption are determined using standard rocket equations. These parameters indicate nozzle efficiency, propellant utilization, and overall engine performance.

SECTION C

(Attempt any one – long descriptive answers)

Q3(a) Difference between turbofan engine and turboprop engine

A turbofan engine produces thrust using a combination of jet exhaust and a large fan that accelerates bypass air, making it suitable for high-speed commercial aircraft with good fuel efficiency and reduced noise. A turboprop engine uses a gas turbine to drive a propeller, producing thrust mainly from the propeller, making it ideal for low-speed, short-range flights. Turbofans are efficient at higher speeds, while turboprops are more efficient at lower speeds and shorter runways.

Q3(b) Ramjet engine performance analysis (conceptual explanation)

A ramjet engine operates without a compressor, relying on high forward speed to compress incoming air. At Mach 1.4 and given altitude conditions, diffuser efficiency, combustion heat addition, and nozzle expansion determine overall performance. Parameters such as flight speed, air flow rate, pressure ratios, and fuel-air ratio are evaluated using compressible flow relations assuming ideal gas behavior.

Q4(a) Properties of solid propellants desired for rocket propulsion

Solid propellants should have high calorific value, stable combustion characteristics, high density, good mechanical strength, and long storage life. They should burn uniformly and safely without cracks or instability. These properties ensure reliable thrust and safe operation of solid rocket motors.

Q4(b) Construction and working of turbofan engine

A turbofan engine consists of a fan, compressor, combustion chamber, turbine, and nozzle. Incoming air is split into core air and bypass air. Core air undergoes compression, combustion, and expansion, while bypass air produces additional thrust. This configuration improves efficiency and reduces noise compared to turbojet engines.

Q5(a) Construction and working of ramjet engine

A ramjet engine consists of an inlet diffuser, combustion chamber, and nozzle. As the aircraft moves at high speed, air is compressed in the diffuser, fuel is injected and burned, and hot gases expand through the nozzle to produce thrust. Ramjets operate efficiently only at high supersonic speeds.
 

Q5(b) Choked flow through a nozzle
 

Choked flow occurs when the Mach number at the nozzle throat reaches unity. Beyond this condition, increasing downstream pressure difference does not increase mass flow rate. Choking is an important concept in rocket and jet engine design to control flow rate.
 

Q6(a) Turbo-prop engine with sketch explanation
 

A turboprop engine uses a gas turbine to drive a propeller through a reduction gearbox. Most thrust is generated by the propeller, while the exhaust provides minor thrust. Turboprops are efficient for short-haul flights and low-speed operations.
 

Q6(b) Turbojet engine performance analysis (descriptive)
 

In a turbojet engine, air intake, compression, combustion, and expansion produce a high-velocity jet. Given flight speed, air flow rate, fuel-air ratio, and calorific value, thrust, specific thrust, thrust power, and efficiencies are calculated using jet propulsion equations.
 

Q7(a) Rocket performance at different altitudes
 

Rocket thrust and specific impulse depend on ambient pressure. At higher altitudes where ambient pressure is low, thrust increases due to greater pressure difference at nozzle exit. Calculations involve effective jet velocity, actual jet velocity, and propellant flow rate to evaluate performance changes.
 

Q7(b) Explain turbojet engine in detail
 

A turbojet engine consists of an air intake, compressor, combustion chamber, turbine, and nozzle. Air is compressed, mixed with fuel, burned, and expanded through the nozzle to generate thrust. Turbojets are simple in construction and suitable for high-speed aircraft, though they are less fuel-efficient and noisier compared to turbofans.