Theory of Heat Transfer — Winter 2020

Explain the following terms:

Thermal diffusivity

Thermal Conductivity

Thermal contact resistance

What do you understand by fin effectiveness and fin efficiency?

Aflat plate, 1 m wide and 1.5 m long is to be maintained at 900C in air with a free stream temperature of 100C . Determine the velocity with which air must flow over flat plate along 1.5 m side so that the rate of energy dissipation from the plate is 3.75 KW. Take the following thermo- physical properties 500C. ρ= 1.09 kg/m3, k= 0.028 W/m0C, Pr= 0.7, µ = 2.03 x 10-5 kg/m-s , C = 1.007 KJ/Kg0C. p

What are Fourier and Biot numbers? Write their significance.

What is the “critical radius” of insulation? Derive an expression for the same for cylinders.

The composite wall of a furnace is made up with 120 mm of fire clay ( k= 0.25 ( 1 + 0.0009 t) W/m0C and 600 mm of red brick (k=0.8 W/m0C). The inside surface temperature is 12500C and the outside air temperature is 400C. Determine : (1) The temperature at the layer interface, and (2) The heat loss for 1 m2 of furnace wall.

Differentiate between velocity and thermal boundary layer.

Explain lumped heat capacity method of heat transfer and state its assumptions.

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

Distinguish between natural and forced convection heat transfer.

Show using momentum equation that in the case of incompressible boundary layer flow with negligible pressure gradient, ∂3u/∂y3= 0 at y = 0.

Discuss the concept of thermal boundary layer in case of flow over the plates. How it differ from velocity boundary?1

Define heat exchanger and classify in detail.

Write a note on Nucleate boiling?

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

Write assumptions used when we drive expression for LMTD for various types of heat exchangers.

What are the fouling factors? Explain their effect in Heat Exchanger design.

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

State and Prove Kirchoff’s law of radiation?

Differentiate between dropwise and filmwise condensation.

Derive a general relation for the radiation shape factor in case of radiation between two surfaces. Explain Wein’s displacement law of radiation.

Enumerate the factors on which the rates of emission of radiation by a body depend.

Define following: 1) Gray Body 2) Lambert’s Law of radiation 3) Transmissivity 4) Total Emissivity

Define Radiation Intensity? Prove that the intensity of radiation is given by I = E /  b b