Development of a Radial Composite Heat Sink for Use in Enclosed Engine and Thermal Systems

Open Access
- Author:
- Wolfgang, Christopher William
- Graduate Program:
- Mechanical Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 17, 2013
- Committee Members:
- Timothy Miller, Thesis Advisor/Co-Advisor
Robert Edward Masters, Thesis Advisor/Co-Advisor
Karl Martin Reichard, Thesis Advisor/Co-Advisor
Zoubeida Ounaies, Thesis Advisor/Co-Advisor - Keywords:
- Carbon Nanofiber
Heat Sink
Thermal Conductivity
Radial Composite
Heat Transfer
Thermal Dispersion - Abstract:
- Composite materials have seen increasing use as structural materials in industrial applications over the past few decades due to their excellent strength to weight ratio. Carbon nanofiber composites in particular have demonstrated excellent mechanical strength coupled with good thermal and electrical conductivity. High axial dependence of carbon nanofiber thermal conductivity, coupled with multiple manufacturing issues regarding composite production in unique geometries, has limited their utility in composite materials for thermal dispersion systems. The objective of this thesis is to develop a radial composite heat sink with a high thermal conductivity to weight ratio (compared to copper/aluminum). Research will be performed to study the thermal conductivity of carbon nanofiber composites as a function of fiber orientation and will be compared to typical heat sink materials. Results from the fiber orientation analysis showed that heat transfer in the transverse direction of the Hex-Tow® IM7 carbon nanofiber corresponded to a drop in thermal conductivity of 47% from the axial orientation. Small angle offsets of 5°, 10°, and 15° also showed rapid decrease in thermal conductivity from the fully axial orientation of 11.8%, 15.9%, and 21% respectively. A radial heat sink prototype was developed approximating a fully radial setup by limiting fiber orientation angle to 10° maximum oset. This prototype was produced as a 60° wedge with inner radius of 1.0 inch and 3.0 inch outer radius using four prepreg patterns to vary ply connection points and fiber orientation between successive layers using a unidirectional prepreg roll of HexTow® IM7 carbon nanofiber and the HexPly® 8552 anime cured epoxy resin. Testing of the prototype showed an overall thermal conductivity corresponding to approximately a 25° average fiber orientation. It is assumed that this low thermal conductivity is due to the inner surface being modified from the cylindrical geometry to a flat 1.0 inch long surface for contact with the heat source. This re-orientation of heat flux increased the overall fiber orientation from between 5 and 10° to between 20 and 30°. Results from this approximation match well with a near radial heat sink estimation, however further work is required before production of a full heat sink. Comparison of the thermal conductivity to density ratio for higher performance carbon nanofibers (Cytec's K-1100 fiber) showed an improvement over copper alloy 110 of 9.3x and over aluminum alloy 6061 of 6.6x, far surpassing the desired increase of a 2x increase over the k/rho ratio of the baseline metal samples.