Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

324 Chapter 11 Temperature and Temperature-Related Parameters

pool whose water temperature was 70F, you would feel cold? That is because the liquid water

has a higher heat transfer coefficient than does air, and therefore, according to Newton’s law of

cooling, Equation (11.18), water removes more heat from your body. The typical range of heat

transfer coefficient values is given in Table 11.4.

In the field of heat transfer, it is also common to define a resistance term for the convection

process, similar to theR-value in conduction. The thermal convection resistance is defined as:

(11.19)

Again,Rhas the units of C / W (K / W) or Fh / Btu (Rh / Btu). Equation (11.19) is com-

monly expressed per unit area of solid surface exposure and is calledfilm resistanceorfilm

coefficient.

(11.20)

whereRhas the units of

or

It is important to realize once again that neitherRnorRis dimensionless, and they provide a

measure of resistance to heat flow; the higher the values ofR, the more resistance to heat flow

to or from the surrounding fluid.

Example 11.9 Determine the heat transfer rate by convection from an electronic chip whose surface temper-

ature is 35C and has an exposed surface area of 9 cm

2

. The temperature of the surrounding
air is 20C. The heat transfer coefficient for this situation ish40 W/m
2
K.

TsTf35°C20°C15°C15 K

A 9 1 cm

2

2a

1 m

2

10,000 cm

2 b0.0009 m

2

ft

(^2) #
°F
Btu
h
°
ft
(^2) #
°R
Btu
h
¢
m
(^2) #
°C
W
a
m
(^2) #
K
W
b
R
1
h
R¿
1
hA
TABLE 11.4 Typical Values of Heat Transfer Coefficients
Heat Transfer Coefficient, Heat Transfer Coefficient,
Convection Type h(W/m

2

K) h(Btu /hft

2

R)

Free Convection

Gases 2 to 25 0.35 to 4.4

Liquids 50 to 1000 8.8 to 175

Forced Convection

Gases 25 to 250 4.4 to 44

Liquids 100 to 20,000 17.6 to 3500

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