Heat pipe is a highly effective passive heat transfer device using phase change within small temperature difference. It is noted that heat pipe should be operated under heat transfer limit for practical heat pipe heat exchanger applications. The measurement in local and overall heat transfer coefficient is significant to anticipate the heat transfer limit. The wall temperatures and inner working fluid temperatures were measured to determine the heat transfer coefficient. The adiabatic part with transparent Pyrex glass was visualized to understand flow behaviors inside the thermosyphon. The dynamic behaviors of condensed working fluid were visualized for the specific tilted angle and power inputs at pseudo steady-state. At low heat input of 250W, the thin condensed liquid film is observed to be returned from condenser to evaporator. With increasing heat input of 500W, the nucleate boiling starts to occur in evaporator. More activated vapors turn to make wavy motion in free surface of the returned condensed liquid film which is thickened. In power input of 1,250W, the vigorous flow motion happens periodically and the interaction between vapor and liquid bursting reaches a maximum heat transfer which is led to the heat transfer limit in the thermosyphon. Over heat transfer limit (2,000 and 2,500W), the overall heat transfer is decreased when the degree of bursting motion between vapor and liquid is gradually reduced.

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