Heat Transfer Important Questions for R20 JNTU Students
Heat Transfer is one of the most essential and scoring subjects for B.Tech Mechanical Engineering students under the R20 JNTU syllabus. It plays a vital role in understanding how heat energy moves within materials and systems like boilers, condensers, engines, and radiators.
This post covers chapter-wise Heat Transfer important questions, numerical focus areas, key formulas, and exam preparation tips. Every question and topic has been designed for R20 JNTU students aiming for top marks.
Unit 1: Introduction to Heat Transfer
Key Theory Questions
- What is Heat Transfer? Differentiate between Thermodynamics and Heat Transfer.
- Explain the three modes of heat transfer — conduction, convection, and radiation.
- State and explain Fourier’s Law of Heat Conduction.
- Define thermal conductivity (k) and list materials with high and low thermal conductivity.
- Write the general heat conduction equation in Cartesian coordinates.
- Explain the terms steady-state and transient conduction.
- What is contact thermal resistance? Why is it important in design?
- Define thermal diffusivity and its physical significance.
- Derive an expression for heat flow through a composite wall.
- Explain lumped system analysis and its validity criteria.
- What is the difference between Biot Number and Fourier Number?
- Explain one-dimensional, two-dimensional, and three-dimensional conduction with examples.
- What is the concept of thermal circuit?
- Define critical radius of insulation for a cylinder.
- Discuss real-world applications of conduction in engineering.
Numerical Focus Areas
- Basic formula: q = k × A × (ΔT / L).
- Composite wall problems using thermal resistance network.
- Transient heat conduction using lumped capacitance method.
- Critical radius calculation for cylinders and spheres.
- Estimation of heat loss through walls, pipes, and slabs.
Unit 2: Convection Heat Transfer
Key Theory Questions
Define convection and distinguish between natural and forced convection.
Explain Newton’s Law of Cooling: q = h × A × (T_s − T_∞).
Define film coefficient of heat transfer (h).
Explain hydrodynamic and thermal boundary layers.
Define Reynolds number (Re), Prandtl number (Pr), and Nusselt number (Nu).
Explain their significance in forced convection.
Derive Reynolds Analogy between momentum and heat transfer.
Discuss heat transfer in laminar flow over a flat plate.
Compare internal and external flow convection.
What are empirical correlations used in convection?
Explain film condensation vs dropwise condensation briefly.
Define Grashof number (Gr) and explain its importance in natural convection.
Discuss factors affecting convective heat transfer coefficient.
What is heat transfer enhancement and list some techniques.
Explain real-world applications of convection heat transfer in mechanical systems.
Numerical Focus Areas
Determine h using Nusselt correlation: Nu = hL/k.
Laminar flow over flat plate – solve using Nu = 0.332 × Re^0.5 × Pr^(1/3).
Natural convection correlations (e.g., Nu = 0.59 × (Gr × Pr)^0.25).
Use q = h × A × ΔT to calculate heat loss/gain.
Flow inside pipes using Dittus-Boelter equation: Nu = 0.023 × Re^0.8 × Pr^0.3.
Unit 3: Radiation Heat Transfer
Key Theory Questions
- Define thermal radiation and its characteristics.
- State Stefan-Boltzmann Law: E = σT⁴, where σ = 5.67×10⁻⁸ W/m²K⁴.
- Explain emissivity, absorptivity, and reflectivity.
- Define black body, gray body, and real body.
- Explain Kirchhoff’s Law of radiation.
- What is a shape factor (view factor)? Derive its expression for two surfaces.
- Define solid angle and its relation to radiation intensity.
- What is Planck’s Law?
- Derive the radiation heat exchange between two infinite parallel plates.
- Explain radiation shields and their purpose.
- Discuss the network analogy for radiation systems.
- Explain radiation errors in temperature measurement.
- Define solar radiation and its components.
- Explain gray-body heat exchange with emissivity factors.
- List applications of radiation heat transfer.
Numerical Focus Areas
- Use q = σA(T₁⁴ − T₂⁴) for radiation exchange.
- Calculate view factor (F₁₂) using A₁F₁₂ = A₂F₂₁.
- Problems on gray body radiation and emissivity.
- Multi-surface systems using radiation network analogy.
- Use of radiation shields to minimize heat loss.
Unit 4: Heat Exchangers
Key Theory Questions
- Define a heat exchanger and list different types.
- Differentiate between parallel-flow, counter-flow, and cross-flow heat exchangers.
- Explain LMTD (Log Mean Temperature Difference) and derive its formula.
- What are effectiveness (ε) and Number of Transfer Units (NTU)?
- Derive ε-NTU relations for parallel and counterflow exchangers.
- Define overall heat transfer coefficient (U).
- Explain fouling factor and its impact on performance.
- Write energy balance equations for both fluids.
- Discuss heat exchanger design steps.
- What are compact heat exchangers and where are they used?
- Define capacity ratio (Cmin/Cmax) and its influence on efficiency.
- What are regenerative heat exchangers?
- Discuss shell and tube vs plate heat exchangers.
- Explain thermal effectiveness.
- Explain heat exchanger failure causes and preventive measures.
Numerical Focus Areas
- LMTD formula: (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂).
- Effectiveness: ε = Q / (Cmin × (T_hot,in − T_cold,in)).
- NTU = UA / Cmin and ε = 1 − exp[-NTU(1 − Cmin/Cmax)].
- Determine U value using combined resistances:
1/U = 1/h₁ + Rf₁ + x/k + Rf₂ + 1/h₂. - Performance comparison of flow types using efficiency curves.
Unit 5: Boiling and Condensation
Key Theory Questions
- Define boiling and condensation heat transfer.
- Explain the pool boiling curve and its regions.
- Define critical heat flux (CHF) and Leidenfrost point.
- State Nusselt’s theory of film condensation.
- Differentiate between film condensation and dropwise condensation.
- What are the factors affecting boiling heat transfer?
- Explain surface orientation effects on condensation.
- Discuss heat transfer enhancement in boiling.
- Explain the condensation process in power plant condensers.
- What is boiling crisis and how is it prevented?
- What are nucleate, transition, and film boiling regions?
- Discuss real applications of boiling and condensation.
- Explain the effect of pressure on boiling heat transfer.
- What is flooding in condensation systems?
- Compare pool and flow boiling.
Numerical Focus Areas
- Condensation heat transfer coefficient: h = 0.943[(k³ρ²g(L − ρ)v hfg)/(μLΔT)]¹/⁴.
- Boiling heat flux: q = h × (T_s − T_sat).
- Critical heat flux: qmax = 0.131ρv hfg[g(ρL − ρv)/σ]¹/⁴.
- Film thickness and rate of condensation calculations.
Unit 6: Heat Transfer by Fins
Key Theory Questions
- What are fins? Explain their purpose.
- Derive the temperature distribution along a fin.
- Define fin efficiency (η) and fin effectiveness (ε).
- What assumptions are made in fin analysis?
- Explain rectangular, triangular, and pin fins.
- Derive heat transfer rate: Q = √(hPkA)(θb tanh mL).
- Discuss infinite fin and insulated tip cases.
- Define optimum fin length.
- Explain fin material selection criteria.
- Applications of fins in automobiles, heat exchangers, and transformers.
- What is fin parameter (m) and its physical significance?
- Compare short and long fins performance.
- Explain temperature distribution curve in fins.
- Discuss methods to increase fin performance.
- Define fin array efficiency.
Numerical Focus Areas
- m = √(hP/kA) for rectangular fins.
- Fin efficiency: η = tanh(mL)/(mL).
- Fin effectiveness: ε = (Qfin / Qno-fin).
- Determine temperature at the fin tip for insulated/uninsulated conditions.
- Compare different fin profiles for given base temperature.
Practice Numerical Problems
Key Theory Questions
- Determine heat transfer rate through a wall of 0.3 m thickness, area 10 m², ΔT=50 °C, k=0.8 W/m·K.
- Air flows over a flat plate (0.5 m × 0.5 m) at 3 m/s, find heat transfer coefficient if Nu=40 and k=0.026 W/m·K.
- Calculate radiation heat loss from a black plate (1 m²) at 700 K exposed to surroundings at 300 K.
- For a counter-flow heat exchanger, hot water enters at 100 °C and leaves at 60 °C; cold oil enters at 30 °C and leaves at 70 °C. Find LMTD.
- A fin (L=0.1 m, k=200 W/m·K, h=25 W/m²·K) has base temperature 100 °C and surrounding 25 °C — find fin efficiency.
- Calculate Biot number for a steel ball (r=20 mm, k=45 W/m·K, h=120 W/m²·K).
- A condenser handles steam at 0.1 bar condensing on a vertical plate 0.5 m high. Find the average heat transfer coefficient.
- Determine critical radius for insulation around a wire (k=0.3 W/m·K, h=10 W/m²·K).
- A wall is made of brick (k=0.72), plaster (k=0.22), and insulation (k=0.04). Find total thermal resistance.
- Calculate effectiveness of a parallel-flow exchanger if NTU=2 and Cr=1.
Tips to Prepare for Heat Transfer
Key Theory Questions
- Revise formulas daily — make flashcards for all units.
- Practice at least five numericals per topic.
- Memorize all dimensionless numbers (Re, Pr, Nu, Gr, Bi, Fo).
- Focus on derivations like Fourier’s Law, LMTD, and fin equations.
- Always draw and label diagrams neatly in exams.
- Understand unit conversions (W, kJ, °C, K, m²).
- Study previous JNTU R20 question papers to find repeating problems.
- Revise using standard textbooks like “Incropera – Fundamentals of Heat and Mass Transfer”.
Conclusion
Mastering Heat Transfer helps every mechanical engineering student understand real-life systems like IC engines, heat exchangers, boilers, and condensers. For R20 JNTU students, focusing on concepts + numericals + derivations ensures top results.
Keep revising the Heat Transfer important questions, practice key numericals, and connect theory with real-world applications. With consistent preparation, Heat Transfer becomes one of the easiest and highest-scoring subjects in your semester.
