(SEM III) THEORY EXAMINATION 2022-23 THERMODYNAMICS

This question paper is designed for students studying Engineering Thermodynamics, Refrigeration, Heat Transfer, and Power Plant Engineering. It tests deeper conceptual understanding, numerical accuracy, and the ability to apply thermodynamic laws to real engineering situations. The paper includes topics ranging from basic thermodynamic properties to advanced refrigeration cycles, entropy analysis, steam tables, Rankine cycle, availability, and irreversibility.

The question paper is divided into three major sections: A, B, and C, ensuring a holistic evaluation of theoretical knowledge, descriptive writing skills, problem-solving skills, and application of thermodynamic principles.

SECTION A — Short Conceptual Questions (20 Marks)

Section A contains 10 short-answer questions (each of 2 marks). These questions evaluate the student’s ability to recall and explain basic thermodynamic definitions and principles.

The topics include:

1. Continuum Concept in Thermodynamics

Understanding matter as a continuous distribution without molecular gaps—an essential assumption in classical thermodynamics.

2. State, Path, and Cycle

Basic terms used to describe thermodynamic systems and processes.

3. COP (Coefficient of Performance)

Students must define COP and derive the relationship between:

COP of refrigerator

COP of heat pump

4. Clausius Theorem

Fundamental to the second law of thermodynamics and entropy.

5. Helmholtz & Gibbs Functions

The question tests understanding of thermodynamic potentials used for equilibrium analysis.

6. Coefficient of Volume Expansion

Important property of real substances undergoing heating.

7. First Law Efficiency vs Second Law Efficiency

Comparison of energy-based vs exergy-based efficiency.

8. Saturation, Subcooled Liquid, Superheated Vapour

Basic definitions from steam cycles and phase-change processes.

9. Unit of Refrigeration

Ton of refrigeration and its significance.

10. Types of Refrigerants

Chemical names, classification, and modern eco-friendly refrigerants (R-134a, R-410A, etc.).

This section tests core conceptual clarity, fundamental definitions, and thermodynamic understanding.

SECTION B — Descriptive + Numerical Questions (30 Marks)

Students must answer any three questions (each 10 marks).
These questions emphasize problem-solving, derivation, thermodynamic laws, and steam property analysis.

B(a) Isolated System + Temperature Concept + Numerical on Air Expansion

Difference between heat, temperature, and internal energy

Rigid insulated tank (adiabatic) expansion

Maximum work using thermodynamic relationships

This question tests first law analysis, ideal gas relations, and adiabatic process logic.

B(b) Derivation of Entropy Equation

Deriving the change in entropy for an ideal gas:

ΔS=Rln⁡(V2V1)+Cvln⁡(T2T1)\Delta S = R \ln\left(\frac{V_2}{V_1}\right) + C_v \ln\left(\frac{T_2}{T_1}\right)ΔS=Rln(V1​V2​​)+Cv​ln(T1​T2​​)

Tests mathematical formulation and understanding of entropy, a key thermodynamic property.

B(c) Joule-Thomson Coefficient + Inversion Curve

Students must explain:

Throttling process

Cooling/heating region

Inversion curve significance in liquefaction systems

Related to refrigeration & gas liquefaction.

B(d) Steam Property Numerical

Given steam initially at high pressure:

Constant volume expansion

Calculation of dryness fraction

Final internal energy

Entropy change

Work done

Uses steam tables, constant-volume process, and property diagrams.

B(e) Actual Vapour Compression Cycle

Students must sketch:

Actual vs ideal VCR cycle on P–h and T–s diagrams

Effect of evaporator pressure (superheat)

Effect of condenser pressure (subcooling)

This evaluates understanding of refrigeration performance and COP variations.

SECTION C — Applied Thermodynamics & Advanced Problems (20 Marks)

Section C consists of 4 advanced questions, each with two options.
Students must answer one part from each question (10 marks each).

Q3 — Thermodynamic Cycles or Equation of State Problems

(a) PMM-II & Quasi Static Expansion Numerical

Discusses impossibility of Perpetual Motion Machine of Second Kind

Gas undergoing quasi-static expansion

Internal energy equation given

Find maximum internal energy and net heat transfer

Tests knowledge of energy interactions, work, quasi-static laws, and U = f(P,V) relations.

(b) Three-Process Cycle Numerical**

Given:

Compression with PV = constant

Heat rejection

Work input/output interactions

Students must compute:

Q12Q_{12}Q12​

Q31Q_{31}Q31​

This question evaluates the application of the first law of thermodynamics over cycles.

Q4 — Heat Engine & Heat Pump / Entropy of Universe

(a) Combined Heat Engine + Heat Pump System

Understanding:

Energy flows

Efficiency & COP

Heat interactions in combined systems

(b) Entropy Change of Universe

Two cases:

Copper block dropped in lake

Two copper blocks exchanging heat

Tests deep understanding of second law, entropy, and irreversibility.

Q5 — Clausius-Clapeyron Relation / Availability & Irreversibility

(a) Clausius-Clapeyron

Fundamental equation used for phase-change diagrams, vapour pressure curves.

(b) Air Cooling in a Pressure Vessel

Students calculate:

Availability (exergy) in initial & final state

Irreversibility of a real process

This tests a student’s mastery of exergy analysis, a high-level thermodynamic concept.

Q6 — Rankine Cycle or Psychrometric Chart

(a) Moisture at Turbine Exit

Compute:

Maximum allowable boiler pressure

Rankine efficiency

Based on 15% moisture limit

Requires strong understanding of steam power plants.

(b) Psychrometric Processes

Explaining:

Heating & dehumidification

Cooling & humidification

Sensible heating

Sensible cooling

Uses psychrometric charts and HVAC principles.

Q7 — Refrigeration Cycles (Carnot / Bell-Coleman)

(a) Reversed Carnot Cycle

Discuss:

PV & TS diagrams

Limitations of ideal Carnot refrigeration cycle

(b) Air-Refrigeration (Bell-Coleman) Cycle Numerical**

Calculate:

COP

Refrigerating effect

Uses ideal gas relations and compressor/cooler processes.