Anisotropic heat transfer in composites based on high-thermal conductive carbon fibers

The authors have developed a shell-and-tube type thermal energy storage unit using carbon-fiber brushes as a thermal conductivity promoter. In this paper, experiments on heat transfer in the brush/n-octadecane composites are discussed, leading to further improvement of the unit. Experimental apparatuses imitate shell-and-tube type heat exchangers. Temperatures in the brush/n-octadecane composite are measured. The carbon brush improves the transient thermal responses in the entire composite. The effect of the brush on the thermal responses is especially obtained in the region where the directions of the fibers agree with those of the heat flows from the tubes to the center of the brush. The thermal responses are improved with an increase in the diameter of the brush. However, when the brush diameter is too large, the branching of the fibers near the tubes create a region with a low density of the fibers. This low-density area prevents the further enhancement of heat transfer. Accordingly, there is an optimum diameter of the brush. In addition, the thermal conductivity in the composite will further improve when the low-density area of the fibers does not exist.