In this study, heat spreading characteristics of a flat heat pipe-type heat spreader were determined in terms of a dimensionless effective thermal conductivity ratio. The effective thermal conductivity ratio was defined as a ratio of the effective thermal conductivities of the heat spreader under test and a reference material (e.g. Cu) having the same external dimensions as the heat spreader. Based on this definition, the measurement model showed that the effective thermal conductivity ratio consisted of only four temperature differences, which were precisely measurable using calibrated thermometers.
In this study, to prove effectiveness of the suggested method, we constructed a flat heat pipe-type heat spreader having dimensions of 102 mm × 102 mm × 5 mm (width × length × thickness), and investigated the effect of various influencing factors such as the heat load (heat flux), working fluid charge ratio, and internal vapor space structure using the suggested effective thermal conductivity ratio. The constructed heat spreader was injected with distilled water as the working fluid at different charge ratios from the minimum value to the maximum for stable operations. For the internal vapor space structure, heat spreaders having a reduced vapor space thickness with varying expansion angle of the flow guides were fabricated, and their heat spreading characteristics were compared.
As to effect of the working fluid charge ratio, the effective thermal conductivity ratio near the center increased with increasing charge ratio as the vapor flow space thickness was reduced with increasing charge ratio, resulting in increased vapor speed; the center region effective thermal conductivity ratio showed monotonic increase with increasing heat load due to the increased vapor mass flow rate. However, when an excess amount of the working fluid was injected, due to the accumulation of the liquid-phase working fluid near the condensing section, abrupt temperature changes were observed near outer edge of the heat spreader. In addition, due to the temperature changes along the coolant flow path, the liquid accumulation was observed to distribute asymmetrically. At 200 W, the maximum effective thermal conductivity ratio of 27.1±7.1 was measured for about 70 % charge ratio and at 2 cm radial distance from the center, which corresponded to a dimensional effective thermal conductivity of 10800 W m^(-1) K^(-1)±2800 W m^(-1) K^(-1) (at approximately 95 % level of confidence).
Regarding the effect of the internal vapor space structure, as the thickness of the vapor space was reduced, the effective thermal conductivity ratio near the center was improved due to the increased vapor speed. At 120 W, the heat spreader having 1 mm thick vapor flow space and 4° flow guide expansion angle showed an effective thermal conductivity ratio of 43.9±27.8, which corresponded to a dimensional effective thermal conductivity of 17600 W m^(-1) K^(-1)±11000 W m^(-1) K^(-1) (at approximately 95 % level of confidence). With increasing heat load, the effective thermal conductivity ratio decreased at 140 W and 160 W and then increased gradually beyond 160 W. With a flow guide expansion angle of 12°, the heat spreader showed lower effective thermal conductivity ratios at low heat loads but possessed higher values at higher heat loads than the heat spreader having 4° flow guide expansion angle. The effect of the internal vapor structure was found to be insignificant as the radial distance from the center increased.