John Hosford Joshua Thornock

Free ConvectionGeneral Assumptions

Cooling without the pad will be considered free convection as opposed to an enclosure

Cooling without the pad does not include fans previously installed or air flow within the casing

Air and surface temperature assumed 27°C and 67°C

AIRFLOW

Internal Flow – Non-Circular TubesGeneral Assumptions

Cooling with the pad will be considered internal flow in a non-circular tube closely representing a rectangle

Flow begins at the center of the laptop and moves to the outside so the non-circular tube length will be half of the length of the laptop

Determine Convection Coefficient for Free Convection

Properties @ Tf=(Tinf+Ts)/2

HORIZONTAL PLATEFREE CONVECTION

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Determine Convection Coefficient for Forced Convection

Three terms had to be estimated Cross sectional area (Ac) Perimeter (P) Mass flow (mdot)

Solve Hydrolic diameter (Dh)

Solve Reynold’s number

=>FLOW IS TURBULENT

Laptop Surface4 inches

Cooling pad surface

Side View

Air

Determine Convection Coefficient for Forced Convection

For turbulent flow use Equation 8.61

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Comparing CoefficientsThe actual heat transfer will depend on the temperatures of the surroundings and the computer surface. Still these coefficients show a drastic difference between having and not having the cooling pad. This is where we need to remember our assumptions. We ignored the fact that the computer has fans and convection inside the case. These more difficult calculations may give a more accurate answer, however, the analysis performed may give us a good idea that the cooling pad will aid the heat transfer a significant amount—increasing the heat transfer by approximately 20 times!

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ConclusionIn addition to providing a cooler surface to

contact your lap, laptop cooling pads greatly aid the transfer of heat away from a laptop’s surface. Although only the convection coefficient was determined in this study, the amount of heat wicked away from the laptop surface is only a linear function of this coefficient for a given temperature difference:

Therefore, the increase in heat transfer is proportional to the increase in convection coefficient—in this case, 20x.

AppendixA Hewlett Packard laptop and a Belkin laptop

cooling pad were the basis for this model.Properties:

Air at laptop surface (Ts): 67°C ( 152.6°F)

Ambient air (T∞): 27°C (8o.6°F)L=.1524m (6 inches, for a 14.1” laptop)

Properties of ambient air (27°C)

Properties of air at surface temperature (67°C)