VARDHAMAN COLLEGE OF ENGINEERING
(AUTONOMOUS)
B. Tech VI Semester Regular Examinations, May 2018
(Regulations: VCE-R15)
HEAT TRANSFER
(Mechanical Engineering)
Date: 17 May, 2018 FN
Time: 3 hours
Max Marks: 75
Answer ONE question from each Unit
All Questions Carry Equal Marks
Unit I
1.

List and explain the three types of boundary conditions.
8M

Calculate critical radius of insulation for asbestos Ck=0.17W/m °C surrounding a pipe and exposed to room air at 20°C with h=3W/m2K. Calculate the heat loss from 200°C, 50mm diameter pipe when covered with critical radius of insulation and without insulation.
7M
2.

Derive general 3-dimensional heat conduction equation is Cartesian co-ordinates.
9M

Explain briefly:
i. Thermal Conductivity
ii. Thermal Diffusivity
iii. Overall heat transfer co-efficient
6M
Unit II
3.

Write short notes on heisler chart.
4M

A brick wall of 10cm thick is initially at a uniform temperature of 230 oC .The wall is suddenly exposed to a convective environment of 30 oC with a surface heat transfer coefficient of 60W/m2-K Determine:
i. The centre temperature at 2hour after the exposure to the ambient
ii. Energy removed from the wall per m2 during 2 hours
iii. What would be the time taken for the surface of the wall to reach a temperature of 40 oC.Take mass density ρ=2300 Kg/m3,Thermal conductivity K=0.69W/m-K,Thermal diffusivity α=0.5*10-6m2/sec
11M
4.

With neat sketch explain different types of fins.
5M

In order to reduce thermal resistance from the surface of a plane wall 50cm*50cm, 100 pin fins of 1cm diameter and 10cm long are attached. If the fins are made of a material of K=300W/m-K and the value of surface conductance with convective boundary condition is 15W/m2K. Calculate the decrease in thermal resistance and increase in heat transfer rate from the wall, if the temperature difference between wall and surroundings is 170oC.
10M
Unit III
5.

Derive co-relation for forced convection in terms of dimensionless numbers using dimensional analysis.
8M

A tube 5m long is maintained at 100 by steam jacketing a fluid flows through tube at rate of 175kg/hr at 30 The diameter of tube is 2cm. Find average heat transfer co-efficient. Take ρ=850Kg/m3 Cp=2000J/Kg γ=5.1x10-6m2/s, K=0.12W/m
7M
6.

Explain the development of thermal boundary layer over a flat plate.
8M

Write the differences between natural and forced convection.
7M
Unit IV
7.

With a neat sketch, discuss the phenomenon of pool boiling.
5M

A Circular plate of 25cm diameter with both surfaces maintained at a uniform temperature of 100 oC is suspended in a horizontal position in atmospheric air at 20 oC. Determine the heat transfer from the plate.
10M
Cont…2
2
8.

Define LMTD and obtain an expression for LMTD of parallel flow heat exchanger.
8M

A counter flow heat exchanger of heat transfer area 12.5 m2 is to cool oil Cp=2000J/KgK with water of Cp=4170J/KgK the oil enters at 100 with the flow rate of 2Kg/s while water enters at 20 with a flow of 0.48Kg/sec the overall heat transfer co-efficient is 400W/m2K. Calculate exit temperature of water and heat transfer.
7M
Unit V
9.

Explain briefly:
i. The concept of black body
ii. Stefan Boltzmann law
iii. Lambert cosine law
9M

Two parallel plates at T1=900K and T2=500K have emissivity's ε1=0.6 and ε2=0.9 respectively. A radiation shield having an emissivity ε31=0.15 on one side and ε32=0.06 on other side is placed between the plates. Calculate the heat transfer rate by radiation per square meter with and without radiation shield.
6M
10.

For a blackbody enclosed in a hemispherical space show the emissive power of the black body is time the intensity of radiation.
9M

State and explain:
i. Kirchhoff law
ii. Planck's law