Theory of Heat Transfer — Winter 2023

List down the applications of extended surface with example in day-to- day life.

Explain the different modes of heat transfer in detail.

Derive equations of temperature distribution and heat dissipation for infinite fin.

What do you understand by insulation? Explain critical thickness for cylinder?

Use of aluminum material as a cooking utensils are not desirable. Evaluate

Asteel pipe 3 cm in diameter has its outer surface at 200°C, is placed in air at 30°Cwith heat transfer coefficient of 8.5 W/m2.K. It is proposed to add insulation (k = 0.07 W/m.K) on its outer surface to reduce the heat loss by 40%. Estimate the thickness of insulation required, if pipe temperature and heat transfer coefficient remain unchanged.

An exterior wall of a house consists of a 0.1 m layer of common brick having thermal conductivity 0.7 W/m.0C. It is followed by a 0.04 m layer of gypsum plaster with thermal conductivity of 0.48 W/m.0C. What thickness of loosely packed rock wool Insulation (k = 0.065 W/m.0C) should be added to reduce the heat loss or (gain) through the wall by 80%?

What is lumped system analysis? What are the assumptions and when is it applicable?

What is meant by thermal resistance? Explain the electrical analogy for solving heat problems

Define Prandtl number. What is the physical interpretation when its value is lesser or greater than one? Show with neat sketches.

Explain the significance of the following terms: (1) Nusselt Number. (2) Grashoff number (3) Reynolds number.

How does the fluid flow inside the duct differ from fluid flow over the bodies?

Derive the momentum equation for hydrodynamic boundary in differential form with neat sketch. Write equation for stretching factor? State its significance for solving momentum equation.1

Differentiate parallel flow and counter flow heat exchanger.

Write the advantages of the effectiveness-NTU method over the LMTD method.

With a neat sketch explain Film and dropwise condensation.

State & explain Lambert’s cosine law.

Explain the following terms in heat exchanger? (1) Effectiveness (2) NTU

Prove that the effectiveness of parallel flow heat exchanger is given by 1−exp [−NTU(1+C)] 𝜀 = 1+C

What do you mean by fouling factor? State the causes of fouling?

Making use of Plank’s law of distribution, establish the relation for the Wien’s displacement law.

Derive the Stefan-Boltzmann law from the Plank’s law of thermal radiation. What is the value of Stefan-Boltzmann constant?

What do you understand by absorptivity? How can it be improved for an opaque body?

Define a black body. Give examples of some surfaces which don’t appear black but have high value of absorptivity.

Define boiling? Draw boiling curve which shows all the boiling regimes and explain nucleate boiling regime in brief.