Fakulti Kejuruteraan Mekanikal & Pembuatan

Universiti Tun Hussein Onn Malaysia

BDA 30603: Heat Transfer – Individual Assignment 6

Topic: 2D Heat Conduction and Lumped System Page 01 of 01

3-130 Hot and cold water pipes 12 m long run parallel to each other in a thick concrete layer. The

diameter of both pipes are 6 cm, and the distance between the centerlines of the pipes is 40 cm. the

surface temperature of the hot and cold pipes are 60 °C and 15 °C respectively. Taking the thermal

conductivity of the concrete to be k = 0.75 W.m-1

.K-1

, determine the rate of heat transfer between the

pipes.

3-132 A row of 1 m long and 2.5 cm diameter used uranium fuel

rods that are still radioactive are buried in the ground parallel to

each other with a center to center distance of 20 cm at a depth of

4.5 m from the ground surface at a location where the thermal

conductivity of the soil is 1.1 W.m-1

.K-1

. if the surface temperature of

the rods and the ground are 175 °C and 15 °C respectively,

determine the rate of heat transfer from the fuel rods to the

atmosphere through the soil.

3-141 Hot water at an average temperature of 90 °C passes through a row of eight parallel pies that

are 4 m long and have an outer diameter of 3 cm, located vertically in the middle of a concrete wall (k =

0.75 W.m-1

.K-1

) that is 4 m high, 8 m long, and 15 cm thick. If the surface of the concrete walls are

exposed to a medium at 32 °C, with heat transfer coefficient of 12 W.m-2

.K-1

, determine the rate of heat

loss from the hot water and surface temperature of the wall.

4-5C Consider a hot baked potato on a plate. The temperature of the potato is observed to drop by 4

°C during the first minute. Will the temperature drop during the second minute be less than, equal to, or

more than 4 °C? Why?

4-6C Consider a potato being baked in an oven. The temperature of the potato is observed to rise by

5 °C during the first minute. Will the temperature rise during the second minute be less than, equal to,

or more than 5 °C? Why?

4-20 A person found dead at 5PM in a room whose temperature is 20 °C. the temperature of the

body is measured to be 25 °C when found, and the heat transfer coefficient is estimated to be at 8 W.m-

1.K

-1. Modeling the body as a 30 cm diameter, 1.7 m long cylinder and using the lumped system as a

rough approximation, estimate the time of death of that person. Take k = 0.617 W.m-1

.°C -1

, ρ = 996

kg.m-3

, and cp = 4178 J.kg -1

.°C-1

4-23 A brick of 203 x 102 x 57 mm in dimension is being burned in a kiln

to 1100 °C, and then allowed to cool in a room with ambient air

temperature of 30 °C and convection heat transfer coefficient of 5 W.m-

2.K

-1. If the brick has properties of ρ = 1920 kg.m

-3, and cp = 790 J.kg

-1 .°C

-1,

and k = 0.617 W.m-1

.°C -1

determine the time required to cool the brick to

a temperature difference of 5 °C from the ambient air temperature.