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

11.3 Temperature Difference and Heat Transfer 323

food, such as freshly baked cookies, by blowing on it, you are using the principles of convec-

tion heat transfer. The cooling of computer chips by blowing air across them is another example

of cooling something by convection heat transfer. There are two broad areas of convection heat

transfer:forced convectionandfree (natural) convection. Forced convection refers to situations

where the flow of fluid is caused or forced by a fan or a pump. Free convection, on the other

hand, refers to situations where the flow of fluid occurs naturally due to density variation in the

fluid. Of course, the density variation is caused by the temperature distribution within the fluid.

When you leave a hot pie to cool on the kitchen counter, the heat transfer is by natural con-

vection. The heat loss from the exterior surfaces of the hot oven is also by natural convection.

To cite another example of natural convection, the large electrical transformers that sit out-

doors at power substations are cooled by natural convection (on a calm day) as well.

For both the forced and the free convection situations, the overall heat transfer rate between

the fluid and the surface is governed by Newton’s law of cooling, which is given by

(11.18)

wherehis the heat transfer coefficient in W/m

2

K (or Btu /hft

2

R),Ais the area of the

exposed surface in m

2

(ft

2

),Tsis the surface temperature in °C (°F), andTfrepresents the tem-

perature of moving fluid in °C (°F). The value of the heat transfer coefficient for a particular

situation is determined from experimental correlation; these values are available in many books

about heat transfer. At this stage of your education, you need not be concerned about how to

obtain the numerical values of heat transfer coefficients. However, it is important for you to

know that the value ofhis higher for forced convection than it is for free convection. Of course,

you already know this! When you are trying to cool down rapidly, do you sit in front of a fan

or do you sit in an area of the room where the air is still? Moreover, the heat transfer coefficient

his higher for liquids than it is for gases. Have you noticed that you can walk around comfort-

ably in a T-shirt when the outdoor air temperature is 70F, but if you went into a swimming

qhA 1 TsTf 2

CPU being cooled by a fan.

Source: Photo by Stephen Engler

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