Theory of Heat Transfer — Winter 2022

Explain Fourier’s law of heat conduction.

Define Fin. Also classify the fins in detail.

Air stream at 240C is flowing at 0.4 m/s across a 100 Wbulb at 1300C. If the bulb is approximated by a 65 mm diameter sphere, calculate: (1) The heat transfer rate (2) The percentage of power lost due to convection. Using the relation: Nu=0.37 Re0.6 The thermo-physical properties at mean film temperature are as follows: ᶹ k= 0.03 W/m-K,Pr= 0.707, = 2.08 x 10-5 m2/s

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

“It is true that insulation is provided to reduce heat transfer rate but due to insulation heat transfer rate is not reduced always” Justify the statement analytically.

Awall of furnace is made up of inside layer of silica brick 120 mm thick covered with a layer of magnesite brick 240 mm thick. The temperature at the inside surface of silica brick wall and outside surface of magnesite brick wall are 7250C and 1100C respectively. The contact thermal resistance between the two walls at the interface is 0.00350C/Wper unit wall area. If thermal conductivities of silica and magnesite bricks are 1.7 Wm-Kand 5.8 W/m-K , Calculate (1) The rate of heat loss per unit area of walls and (2) The temperature drop at the interface.

Avery long 25 mm diameter copper (k= 380 W/m-K) rod extends from a surface at 1200C. The temperature of surrounding air is 250C and the heat transfer coefficient over the rod is 10 W/m2 K. Calculate: (1) Heat lost from the rod (2) How long the rod should be in order to be considered infinite?1

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

Explain Free and forced convection heat transfer with suitable example.

Explain the various parameters used in the free convection. Using Dimensional Analysis, obtain an expression for Nusselt Number in terms of Gr& Pr.

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

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

Avertical plate 0.4 m high and 0.41 m wide at 50°Cis exposed to steam at 100°C. Calculate the following (a) Film thickness at bottom of the plate (b) Maximum velocity at the bottom of the plate (c) Total heat transfer rate and heat flux. Assume at mean temperature of 75°C. ρ = 976 kg/m3, k = 0.668 W/m-K, μ=405 x10-6 kg/ms, h = 2258 kJ/kg fg

Define heat exchanger? Classify heat exchanger in detail?

Explain Film and dropwise condensation with heat sketch?

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

Explain NTU and Effectiveness of heat exchanger?

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

Derive an expression for logarithmic mean temperature difference (LMTD) in the case of counter flow heat exchanger.

State & explainLambert’s cosine law.

Define following: 1) Total Emissivity 2) Emissivity 3) Radiosity 4) Absorptivity

Two parallel rectangular surfaces 1m x 2m are opposite to each other at a distance of 4 m. The surfaces are black and at 100°Cand 200 °C. Calculate the heat exchange by radiation between the two surfaces?2

Explain the following :

Solid angle

Spectral Intensity of Radiation

Black body

Define Radiation shield and Radiation geometrical factor.