This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Chapter 1 Basics of Heat Transfer
116 A 15 cm × 20 cm circuit board houses 120 closely spaced 0.12 W logic chips. The amount of heat
dissipated in 10 h and the heat flux on the surface of the circuit board are to be determined.
Assumptions 1 Heat transfer from the back surface of the board is negligible. 2 Heat transfer from the front
surface is uniform.
Analysis (a) The amount of heat this circuit board dissipates during a 10h period is &
Q = (120)(0.12 W) = 14.4 W Chips,
0.12 W &
Q = QΔt = (0.0144 kW)(10 h) = 0.144 kWh &
Q (b) The heat flux on the surface of the circuit board is As = (0.15 m )(0.2 m ) = 0.03 m 2 15 cm &
Q
14.4 W
&
=
= 480 W/m 2
qs =
2
As 0.03 m 20 cm 117 An aluminum ball is to be heated from 80°C to 200°C. The amount of heat that needs to be
transferred to the aluminum ball is to be determined.
Assumptions The properties of the aluminum ball are constant.
Properties The average density and specific heat of aluminum are
given to be ρ = 2,700 kg/m3 and C p = 0.90 kJ/kg.°C. Metal
ball Analysis The amount of energy added to the ball is simply the change in its
internal energy, and is determined from
Etransfer = ΔU = mC (T2 − T1) where E m = ρV = π
6 ρD3 = π
6 (2700 kg / m3 )(015 m)3 = 4.77 kg
. Substituting,
Etransfer = (4.77 kg)(0.90 kJ / kg. ° C)(200  80)° C = 515 kJ Therefore, 515 kJ of energy (heat or work such as electrical energy) needs to be
transferred to the aluminum ball to heat it to 200°C. 118 The body temperature of a man rises from 37°C to 39°C during strenuous exercise. The resulting
increase in the thermal energy content of the body is to be determined.
Assumptions The body temperature changes uniformly.
Properties The average specific heat of the human body is given to be 3.6
kJ/kg.°C.
Analysis The change in the sensible internal energy content of the body as a
result of the body temperature rising 2°C during strenuous exercise is ΔU = mCΔT = (70 kg)(3.6 kJ/kg.°C)(2°C) = 504 kJ 14 ...
View
Full
Document
 Fall '10
 Dr.DanielArenas
 Thermodynamics, Mass, Heat

Click to edit the document details