Heat Transfer Ch En 3453 / Me En 3650
Homework 7 – Due Friday Oct. 28 at 5 PM – Scan as pdf and submit through
Canvas
Useful Link:
http://higheredbcs.wiley.com/legacy/college/bergman/0470501979/anssel/answers_to_selected_exerc
ises_fhmt_7e.pdf
Problem 1 (7.3a in the textbook)
Consider steady, parallel flow of atmospheric air over a flat plate. The air has a temperature of 300 K
and free stream velocity of 25 m/s. Evaluate the boundary layer thickness at distances of 1, 10, and 100
mm from the leading edge. If a second plate were installed parallel to and at a distance of 3 mm from
the first plate, what is the distance from the leading edge at which boundary layer merger would occur?
Problem 2 (7.26 in the textbook)
Consider a rectangular fin that is used to cool a motorcycle engine. The fin is 0.15 m long and at a
temperature of 250 °C, while the motorcycle is moving at 80 km/h in air at 27 °C. The air is in parallel
flow over both surfaces of the fin, and turbulent flow conditions may be assumed throughout.
a. What is the rate of heat removal per unit width of the fin?
b. Generate a plot of the heat removal rate per unit width of the fin for motorcycle speeds ranging
from 10 to 100 km/h.
Problem 3 (7.17 in the textbook)
Air at a pressure of 1 atm and a temperature of 50 °C is in parallel flow over the top surface of a flat
plate that is heated to a uniform temperature of 100 °C. The plate has a length of 0.20 m (in the flow
direction) and a width of 0.10 m. The Reynolds number based on the plate length is 40,000. What is the
rate of heat transfer from the plate to the air? If the free stream velocity of the air is doubled and the
pressure is increased to 10 atm, what is the rate of heat transfer?
Problem 4 (7.92 in the textbook)
A tube bank uses an aligned arrangement of 10-mm-diameter tubes with ST = SL = 20 mm. There are 10
rows of tubes with 50 tubes in each row. Consider an application for which cold water flows through the
tubes, maintaining the outer surface temperature at 27°C, while flue gases at 427°C and a velocity of 5
m/s are in cross flow over the tubes. The properties of the flue gas may be approximated as those of
atmospheric air at 427°C. What is the total rate of heat transfer per unit length of the tubes in the bank?
Problem 5 (7.108 in the textbook)
The use of rock pile thermal energy storage systems has been considered for solar energy and industrial
process heat applications. A particular system involves a cylindrical container, 2m long by 1m in
diameter, in which nearly spherical rocks of 0.03-m diameter are packed. The bed has a void space of
0.42, and the density and specific heat of the rock are ρ=2300 kg/m3 and Cp = 879 J/kg*K, respectively.
Consider conditions for which atmospheric air is supplied to the rock pile at a steady flow rate of 1 kg/s
and a temperature of 90 °C. The air flows in the axial direction through the container. If the rock is at a
temperature of 25 °C, what is the total rate of heat transfer from the air to the rock pile?