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Assignment 2 Heat Transfer MECE3930U

All questions are compulsory. Submit the assignment on or before 15th Feb 2016

(Monday) 1:00 pm in the drop box # 15. (Note: if the date of submission falls on a holiday,

the new submission date automatically becomes the next working day.)

1.

The temperature of gas stream is to be measured by a thermocouple whose junction can be

approximated as a 1.2 mm diameter sphere. The properties of the junction are k=35 W/m.K,

=8500kg/m3, and cp=320 J/kg.K, and the heat transfer coefficient between the junction and

the gas is h = 90 W/m2.K. Determine how long it will take for the thermocouple to read 99

percent of the initial temperature difference. (25 Marks)

2.

An ordinary egg can be approximated as a 5.5 cm diameter sphere whose properties are

roughly k = 0.6 W/m.K and = 0.14X10-6m2/s. The egg is at initial temperature of 8oC and is

dropped into boiling water at 97oC. Taking the convection heat transfer to be h = 1400

W/m2.K, determine how long will it take for the center of the egg to reach 70oC. You may solve

this problem using one term approximation method. (25 Marks)

3.

A short brass cylinder (= 8530 kg/m3, cp= 0.389 kJ/kg.K, k=110W/m.K, and =3.39X10-5 m2/s)

of diameter 8 cm and height 15 cm is initially at a uniform temperature of 150oC. The cylinder

is now placed in atmospheric air at 20oC, where heat transfer takes place by convection with

a heat transfer coefficient of 40 W/m2K. Calculate: (25 Marks)

(a)

The center temperature of the cylinder

(b)

The center temperature of the top surface of the cylinder 15 minutes after the start of

the cooling.

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4.

Consider the base plate of a 800-W household iron having a thickness of L=0.6 cm, base area

of A = 160 cm2, and thermal conductivity of k = 20 W/m.oC. The inner surface of the base plate

is subjected to uniform heat flux generated by the resistance heaters inside. When steady

operating conditions are reached, the outer surface temperature of the plate is measured to

be 85oC. Disregarding any heat loss through the upper part of the iron and taking the nodal

spacing to be 0.2 cm, (25 Marks)(a)

Obtain the finite difference formulation for the nodes and

(b)

Determine the inner surface temperature of the plate by solving those equations.

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