Theory of Heat Transfer — Summer 2021

What is thermal contact resistance? Upon what parameters does this resistance depend?

Define extended surfaces with neat sketch?

Awall 30 cm thick has size 5 m × 3 m made of red bricks (k = 0.35 W/m.K). It is covered on both sides by the layers of plaster 2 cm thick (k = 0.6 W/m.K). The wall has a window of size 1 m × 2 m. The window door is made of glass, 12 mm thick having thermal conductivity 1.2 W/m.K. Estimate the rate of heat flow through the wall. Inner and outer surface temperatures are 10°Cand 40°C, respectively.

What do you mean by transient analysis? Define Biot Number?

Define critical thickness? Explain critical radius for cylinder?

Derive equations of temperature distribution and heat dissipation for Fin insulated at tip.

Using dimensional analysis, obtain a general form of equation for forced Convective heat transfer

Write temperature profile equation and heat transfer from fin for 1) Infinte long fin 2) Tip insulated fin 3) Tip non insulated fin

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

Estimate the heat transfer rate from a 100 Wincandescent bulb at 140°Cto an ambient at 24°C. Approximate the bulb as 60 cm diameter sphere. Calculate the percentage of power lost by natural convection. Use following correlation and air properties ; Nu = 0.60 (Gr.Pr)1/4 The properties of air at 82°Care: ν = 21.46 × 10–6 m2/s, K = 30.38 × 10–3 f W/m.K, Pr = 0.699.

Define Nusselt Number. Explain its significance in convection heat transfer.

What do you mean by Dimensional analysis? What are the assumptions used in it?

Explain the Reynolds-Colburn analogy for laminar flow over a flat plate.

Define heat exchanger? Classify heat exchanger in detail?

Explain Film-wise and drop-wise condensation with neat sketch?

Derive an expression for log mean temperature difference of parallel flow heat exchanger

Explain the following terms with reference to heat exchanger :

NTU (ii) Effectiveness

Explain correction factor for multi-pass arrangement heat exchanger? Also define fouling factor?

State & explain Kirchhoff’s identity. What are conditions under which it is applicable

State & explain Lambert’s cosine law.

What are the radiation space and surface resistances? How are they expressed? For what kind of surface, the radiation surface resistance is zero?

Define total emissive power (Eb ) and intensity of radiation (Ib ). Show that Eb = π×Ib

Explain the following :

Grey Body (ii) Spectral Intensity of Radiation (iii) Black body

Explain the following terms:

Radiation shield (ii) Radiation geometrical factor.

Derive an expression for rate of radiation exchange, when a radiation shield is inserted between two large parallel plates.