DESIGN PROBLEM #2
50 Points
ME 259 - Heat Transfer
Spring Semester 2005
Due Wednesday, March 23
A new cooling solution is needed for the Octium TMX-900 processor when it is overclocked to 3.73 GHz. Under these conditions, the processor dissipates 115 W and the junction temperature must be less than 90°C for reliable operation. The processor footprint is 37.5 mm x 37.5 mm and the available height above the processor is 100 mm. The junction-to-case resistance is 0.05°C/W. You are responsible for a) finding an off-the-shelf CPU cooler and interface material that meets these design criteria when the ambient air temperature inside the computer enclosure is 38°C, and b) determining how many Kooltronic KB650 fans (60 Hz) are needed to maintain 38°C inside the enclosure when the outside air temperature is 30°C and the total heat load of the computer (without fans) is 400W. Note: the system pressure drop is estimated to be 0.25” H2O.
Some Design Guidelines:
The following assumptions are valid for your design computations:
1. Steady-state conditions.
2. One-dimensional conduction through any heat sinks and interface material (except for spreading resistance, see below).
3. Constant properties.
4. Negligible heat transfer from the sides of the processor, through the leads, or into the PCB. The cooler will be fastened to the processor package or PCB to provide the needed interface pressure. You do not need to design the fastening components.
Heat sinks that are larger than the chip “footprint” (length ´ width) are often used to attain the needed surface area. In such cases, the conduction through the base of the heat sink is two-dimensional; thus, the sink-to-ambient thermal resistance that is based upon one-dimensional conduction is not sufficient. Fortunately, the two-dimensional heat “spreading” effect can be accounted for by an additive thermal resistance given by the following spreading resistance formula:
where
Ab = surface area of heat sink base (bottom)
Ac = surface area of chip (top)
k = thermal conductivity of sink material (assume 2024 if Al alloy is not known)
R’sa = sink-to-ambient thermal resistance of chosen heat sink w/o spreading effect
tb = thickness of heat sink base
Report the important design information on a Design Summary page, followed by the supporting calculations. The calculations should be presented clearly so that your design procedure is easy to follow. Points will be subtracted for an unclear, unorganized analysis. Equation-solving software may be useful for parametric studies.
DESIGN SUMMARY
Chassis Fans
Vendor & model Kooltronic KB650
Number ______
Total fan power ______W
CPU Cooler
Vendor & model* ______
Base thickness ______mm
Length ______mm or Diameter ______mm
Width ______mm
Height ______mm
Rsa ______°C/W
Interface Material
Vendor & model* ______
Thermal impedance ______°C-cm2/W
Rcs ______°C/W
* include copy of product data sheet that shows all needed specifications