(AUTONOMOUS)
B. Tech VII Semester Supplementary Examinations, May 2018
(Regulations: VCE-R11/R11A)
HEAT TRANSFER
(Aeronautical Engineering)
Date: 21 May, 2018 AN Time: 3 hours Max Marks: 75
Answer ONE question from each Unit
All Questions Carry Equal Marks
UNIT I
1. State the laws governing three basic modes of heat transfer. 6M
Derive general heat conduction equation in Cartesian coordinates for an isotropic
material.
9M
2. Briefly explain boundary conditions of first, second and third kind. 6M
A brick wall 25cm thick is in contact with concrete of 5cm thick. k of brick is 0.69W/m K
and that of concrete is 0.93W/mK. If the temperature of exposed brick face is 300C and
that of concrete is 50C, determine the heat lost per hour through a wall of 10m x 5m.
Also find interface temperature.
9M
UNIT II
3. What purpose is served by attaching fins to a surface? Obtain the expression for
temperature distribution for very long fin. Why is the assumption of 1-D conduction an
approximation? Under what condition is it a good approximation?
9M
A thin silicon chip and an 8-mm thick aluminium substrate by a 0.02mm thick epoxy
joint. The chip and substrate are each 10mm on a side and their exposed surfaces are
cooled by air, which is at a temperature of 25oC and provides a heat transfer
coefficient of 100W/m2-K. If the chip dissipate 104 W/m2 under normal conditions, will
it operate below a maximum allowable temperature of 85oC?
6M
4. What is the physical interpretation of Fourier number? Is the lump capacitance
method of analysis likely applicable for cooling of a hot solid metal insulator? Give
reasons.
6M
A new process for treatment of a special material is to be evaluated. The material, a
sphere of radius ro=5mm is initially in equilibrium at 400oC in a furnace. It is suddenly
removed from the furnace and subjected to cooling in a air at 20oC for a period of time
until the center temperature reaches a critical value of 335oC. If
calculate
9M
UNIT III
5. Discuss the significance of the following:
i. Nusselt number
ii. Reynold's number
iii. Grashoff number
6M
Engine oil at 60oC flows over a 5m long flat plate on its upper surface whose surface
temperature is 20oC with a velocity of 2m/s. Determine the drag force and rate of heat
transfer per unit width of the entire plate.
9M
6. How is the thermal entry length defined for flow in a tube? In what region is the flow
in a tube fully developed? Show the velocity profile distribution along the length of the
pipe.
6M
Water at 15oC and μ=1.138X10-3N-s/m2 is flowing steadily in a 5cm
internal diameter tube made of stainless steel at a rate of 5.5lt/s. Determine the
pressure drop and the required pumping power input for flow through a 60m long
section of the tube? Take roughness for steel pipe as 0.0175.
9M
Cont…2
2
UNIT IV
7. Differentiate between the following:
i. Nuclear and Film boiling
ii. Film wise and Drop wise condensation
6M
A vertical square plate 30cm x 30cm is exposed to steam at atmospheric pressure. The
plate temperature is 980C. Calculate the heat transfer and mass of steam condensed
per hour.
9M
8. Obtain an expression for LMTD of a counter flow heat exchanger. State the
assumptions made.
6M
A heat exchanger is required to cool 55000kg/hr of alcohol from 660C to 400C using
40000kg/hr of water entering at 50C. Calculate:
i. Exit temperature of water
ii. Heat transfer
iii. Surface area required for parallel and counter flows
Take overall heat transfer coefficient=580W/m2K, CP(alcohol)=3760J/kgK and
CP(water)= 4180J/kgK.
9M
UNIT V
9. With reference to thermal radiation, explain the following:
i. Absorptivity
ii. Reflectivity
iii. Transmissivity
iv. Solid angle
v. Black body radiation intensity
10M
The temperature of a black surface 0.2 m2 in area is 5400 C. Calculate:
i. Total rate of energy emission
ii. Intensity of normal radiation
iii. Wavelength of maximum monochromatic emissive power
5M
10. State the following:
i. Planck's law
ii. Wein's displacement law
iii. Lambert's law
6M
Two large parallel plates of emissivities 0.8 and 0.6 are at 5000C and 1000 C. If a
radiation shield of emissivity 0.04 is inserted between the two plates, estimate the
percentage reduction in heat transfer.
9M