MEHRAN UNIVERSITY OF ENGINEERING AND TECHNOLOGY,

JAMSHORO

THERMODYNAMICS II ASSIGNMENT

NAME RAHEEL KHAN

ROLL NO 23-22ME073
SUBJECT THERMODYNAMICS II
DEPARTMENT MECHANICAL
DATE 23/04/25

SUBMITTED TO: ENGR. MOHSIN MEMON

QNO1. Describe how a nozzle converts thermal energy into kinetic
energy.

ANS: A nozzle is a device that accelerates fluid flow by converting

thermal energy (enthalpy) into kinetic energy. This process is governed
by the principles of thermodynamics and fluid mechanics.

Process:

High-Pressure Entry: The fluid enters the nozzle at high pressure and
temperature, possessing significant thermal energy.

Expansion and Acceleration: As the fluid passes through the nozzle's

converging section, its pressure and temperature decrease while its
velocity increases, converting thermal energy into kinetic energy.

Conservation of Energy: According to the steady-flow energy

equation:
h1+V1^2/2=h2+V2^2/2
Applications:

Jet Engines: Accelerate exhaust gases to produce thrust.

Steam Turbines: Convert steam's thermal energy into mechanical work.

Rocket Engines: Propel spacecraft by expelling high-velocity exhaust

gases.

QNO 2. Explain the concept of choked flow and its significance in

nozzle design.

Choked Flow:
Choked flow occurs when the velocity of a compressible fluid reaches
the speed of sound (Mach 1) at the nozzle's throat (narrowest section).
Beyond this point, further decreases in downstream pressure do not
increase the mass flow rate.

Significance in Nozzle Design:

Maximum Mass Flow Rate: Choked flow sets the upper limit for mass
flow through the nozzle, crucial for designing systems like jet engines
and rockets.

Stable Operation: Ensures predictable and stable operation of

propulsion systems.

Design Optimization: Helps in determining the optimal throat area for

desired performance.

Mathematical Representation:

For isentropic flow of an ideal gas:

Pexit/Pentry ≤(2/ γ+1)^ γ /γ−1

where γ\gamma γ is the specific heat ratio

QNO 3. Compare the efficiency of a nozzle used in a jet engine

versus one used in a water sprinkler system.

Jet Engine Nozzle:

Purpose: Convert high-pressure, high-temperature gases into high-

velocity jets to produce thrust.
Efficiency: High thermodynamic efficiency due to effective conversion
of thermal energy into kinetic energy.

Design Complexity: Precisely engineered to handle compressible flows

and withstand extreme conditions.

Water Sprinkler Nozzle:

Purpose: Disperse water over an area for irrigation or cooling.

Efficiency: Lower thermodynamic efficiency; designed for uniform

distribution rather than energy conversion.

Design Simplicity: Handles incompressible flow with less stringent

design requirements.

Comparison:

Jet engine nozzles are significantly more efficient in terms of energy

conversion, as they are designed for propulsion by maximizing the
transformation of thermal energy into kinetic energy. In contrast, water
sprinkler nozzles prioritize coverage area over energy efficiency.

QNO 4. Explain the difference between converging and converging-

diverging nozzles in terms of flow and efficiency.

Converging Nozzle:

Structure: Decreasing cross-sectional area.

Flow Behavior: Accelerates subsonic flow; maximum velocity at the

throat is Mach 1.
Application: Suitable for subsonic applications like wind tunnels.

Converging-Diverging Nozzle (De Laval Nozzle):

Structure: Converging section followed by a diverging section.

Flow Behavior: Accelerates flow to supersonic speeds; subsonic flow

accelerates to Mach 1 at the throat and continues to accelerate in the
diverging section.

Application: Essential in supersonic applications like rocket engines.

Efficiency:

Converging-diverging nozzles are more efficient for applications

requiring supersonic flow, as they effectively manage shock waves and
minimize energy losses.

QNO 5. Compare the flow behavior in a nozzle and a diffuser. In

what type of applications is each used?

Nozzle:

Function: Converts pressure energy into kinetic energy; accelerates

fluid flow.

Flow Behavior: Decreasing pressure and increasing velocity.

Applications: Jet engines, rockets, spray systems.

Diffuser:

Function: Converts kinetic energy into pressure energy; decelerates

fluid flow.
Flow Behavior: Increasing pressure and decreasing velocity.

Applications: Jet engine intakes, HVAC systems, wind tunnels.

Comparison:

While nozzles are used to accelerate fluids for propulsion or spraying,

diffusers are employed to slow down fluids, increasing pressure for
processes like combustion or measurement.

QNO 6. Discuss the use of nozzles in industrial applications such as

spray systems, turbines, and injectors.

Spray Systems:

Purpose: Atomize liquids into fine droplets for applications like

painting, cooling, or irrigation.

Design: Tailored to produce specific spray patterns and droplet sizes.

Turbines:

Purpose: Direct high-velocity fluids onto turbine blades to generate

mechanical work.

Design: Engineered to optimize flow angle and velocity for maximum

efficiency.

Injectors:

Purpose: Deliver precise amounts of fuel into combustion chambers.

Design: Ensure fine atomization for complete and efficient combustion.

Importance:

Nozzles in these applications are critical for controlling flow

characteristics, ensuring efficiency, and achieving desired outcomes in
various industrial processes.

QNO 7. Discuss the recent technological advancements in gas

turbine blade materials and cooling techniques. How do these
improve performance?

Material Advancements:

Single-Crystal Super alloys: Enhance high-temperature strength and

creep resistance.

Thermal Barrier Coatings (TBCs): Protect blades from extreme

temperatures, extending lifespan.

Ceramic Matrix Composites (CMCs): Offer high-temperature

capability with reduced weight.

Cooling Techniques:

Internal Cooling Channels: Circulate cooler air through blades to

dissipate heat.

Film Cooling: Create a protective layer of cool air over blade surfaces.

Transpiration Cooling: Allow air to permeate through porous materials

for uniform cooling.

Performance Improvements:
These advancements enable turbines to operate at higher temperatures,
improving thermal efficiency, reducing fuel consumption, and lowering
emissions.

QNO 8. Discuss how increasing the pressure ratio affects the

thermal efficiency of the Brayton cycle. Is there an optimal pressure
ratio?

ANS: The Brayton cycle is a thermodynamic cycle that describes the

workings of a constant-pressure heat engine, such as a gas turbine. The
cycle consists of four processes: isentropic compression, constant-
pressure heat addition, isentropic expansion, and constant-pressure heat
rejection. One of the most critical parameters affecting the performance
of the Brayton cycle is the pressure ratio, which is the ratio of the
compressor outlet pressure to the compressor inlet pressure.

Effect of Increasing Pressure Ratio on Thermal Efficiency:

According to thermodynamic analysis, increasing the pressure ratio

generally improves the thermal efficiency of the Brayton cycle. This can
be shown using the efficiency formula for an ideal Brayton cycle:

As increases, increases due to a greater temperature rise across the

combustion chamber and a relatively lower temperature drop during
exhaust.

Why Efficiency Increases:

Higher Compression Efficiency: More energy is available for

conversion into useful work due to better utilization of heat input.
Higher Turbine Work Output: The greater expansion work from the
turbine provides more net power.

Is There an Optimal Pressure Ratio?

Yes, there is an optimal pressure ratio beyond which further increases

can reduce overall efficiency. This happens due to:

Increased Compressor Work: As pressure ratio increases, the work

required by the compressor also increases significantly.

Material and Mechanical Limitations: High pressures and

temperatures demand advanced materials and cooling technologies,
increasing complexity and cost.

QNO 9. Explain the concept of Lean Premixed Combustion (LPC)

and its role in NOx reduction.

Lean Premixed Combustion (LPC) is an advanced combustion technique

where fuel is mixed with air in a lean proportion (excess air) before
ignition. This ensures that the fuel-air mixture burns at a lower flame
temperature compared to traditional rich or stoichiometric combustion.

Role in NOx Reduction:

Lower Flame Temperature: Reduces thermal NOx formation, which is

exponentially dependent on temperature.

Uniform Temperature Field: Minimizes hotspots that cause NOx

spikes.
Improved Mixing: Enhances combustion efficiency and minimizes
incomplete combustion.

Applications:

LPC is widely used in gas turbines, aero engines, and industrial furnaces
for cleaner emissions and adherence to environmental standards

10. Draw a schematic diagram of a turbojet engine and label each

component. Explain the thermodynamic process occurring in each part.

Processes:

Inlet: Isentropic compression.

Compressor: Increases pressure and temperature (isentropic).

Combustion Chamber: Constant pressure heat addition.

Turbine: Expands hot gases to extract work (drives compressor).

Nozzle: Converts remaining thermal energy into kinetic energy (thrust).

QNO 11. List the types of rotary compressors and state one
industrial application for each.

Screw Compressor: Uses intermeshing screws; used in industrial

refrigeration.

Vane Compressor: Sliding vanes in a rotor; used in dental tools.

Scroll Compressor: Orbiting scrolls compress air; used in HVAC

systems.

QNO 12. Apply the steady-flow energy equation to a compressor

system and discuss the assumptions involved.

General Equation:

Q˙−W˙=m˙(h2−h1+2V22−V12+g(z2−z1))

For Compressors:

Adiabatic Assumption:

Negligible changes in KE and PE: and terms are small.

Simplified Equation:

Assumptions:

Steady flow process

Adiabatic compression (no heat exchange)

Kinetic and potential energy changes are negligible

This equation relates work done to enthalpy change during compression.

QNO 13. Define an air compressor. What are the major
classifications of air compressors based on construction and
working principle?

An air compressor is a mechanical device that compresses air by

reducing its volume, thereby increasing its pressure for industrial and
commercial use.

Classifications:

A. Based on Construction:

Portable

Stationary

B. Based on Working Principle:

Positive Displacement Compressors:

Trap a fixed volume of air and reduce its volume.

Types: Reciprocating, rotary screw, rotary vane.

Dynamic Compressors:

Increase air velocity using impellers and convert to pressure.

Types: Centrifugal, axial

QNO 14. Differentiate between positive displacement compressors

and dynamic compressors with examples.

Compressors are mechanical devices used to increase the pressure of a

gas or air by reducing its volume. Based on their working principle,
compressors are broadly classified into *positive displacement
compressors* and *dynamic compressors*. Both types serve the same
purpose but operate through entirely different mechanisms.

Positive Displacement Compressors:

Positive displacement compressors function by trapping a fixed volume

of air and then compressing it mechanically by reducing its volume. As
the air is compressed, its pressure increases, and then it is discharged
into a storage tank or pipeline.

This type of compressor delivers pulsating or intermittent flow, and is

best suited for applications that require high pressure at relatively low
flow rates. They are simple in design, reliable, and often used in small-
to medium-scale operations.

Examples of Positive Displacement Compressors:

1. Reciprocating Compressors: Use pistons and cylinders to compress

air. Commonly found in automobile workshops and small industries.

2. Rotary Screw Compressors: Use two intermeshing screws to trap

and compress air. Widely used in industrial applications requiring
continuous compressed air supply.

Dynamic Compressors:

Dynamic compressors work on a different principle. Instead of trapping

air, they use a high-speed rotating impeller or blade to impart velocity
(kinetic energy) to the air, which is then converted into pressure energy
 through a diffuser.

Examples of Dynamic Compressors

1. Centrifugal Compressors: Use a rotating impeller to throw air

radially outward, increasing its pressure. Used in chemical plants and
refrigeration systems.

2. Axial Compressors: Use a series of rotating and stationary blades to

gradually increase air pressure as it moves along the axis. These are
commonly found in jet engines and gas turbines.

QNO 15. Explain the difference between reciprocating, rotary, axial,

and centrifugal air compressors

Reciprocating Compressor:

Working: Uses a piston moving back and forth to compress air in

cylinder

Best for: Small to medium applications.

Pros: High compression ratio, simple design.

Cons: High maintenance, noisy, limited flow rate.

Rotary Compressor:

Working: Uses rotating rotors to compress air.

Best for: Continuous, high-flow applications.

Pros: Low noise, continuous air supply.

Cons: Lower efficiency at high pressures, costly.

Axial Compressor:

Working: Air flows through multiple rotating and stationary blades.

Best for: High-speed applications (e.g., jet engines).

Pros: Efficient at high flow rates.

Cons: Complex design, expensive.

Centrifugal Compressor:

Working: Uses a rotating impeller to increase air velocity, then converts

Best for: Large-scale, high-flow applications.

Pros: High flow rate, low maintenance.

Cons: Less efficient at low flow rates.