Study on the channel layout of pulsating heat pipe with orientation-independent performance under a localized heating condition

Abstract

In this study, a novel channel layout of pulsating heat pipe (PHP) is first proposed to ensure the orientation-independent performance under a localized heating condition. A dual-diameter channel, which ensures the PHP to have orientation-independent, is employed to a merged channel layout presented by Lim and Kim. The PHP with the channel layout is fabricated using MEMS techniques. Overall dimensions of the PHP are 30×47×1.45 mm3. HFE-7000 is used as the working fluid and the filling ratio is fixed at 45% by volume. In order to evaluate the thermal resistances of the PHP in the vertical orientation and the horizontal orientation, a series of the experiments is conducted in the vertical orientation and the horizontal orientation. Although the dual-diameter channel is employed to the merged channel layout, it is experimentally found that the PHP with the channel layout does not operate normally in the horizontal orientation. To figure out the causes of the problems, the flow behaviors inside the PHP are analyzed using high-speed photography. The results show that the operating mechanism of the dual-diameter channel does not implemented in the merged channel layout. At the top of the condenser section, because all of the merged channels are connected each other, the liquid slug moves from one large-diameter channel not only to a small-diameter channel but also to the other large-diameter channel, which is not flow behavior intended by the dual-diameter channel. In the evaporator section, because there is only one path from the small-diameter channels to the large-diameter channels, not all liquid slugs in the small-diameter channels penetrate into the evaporator section. This results in the decrease of the rate of the liquid supply to the evaporator section, thereby the driving force for the continuous oscillation reduces. To overcome these problems, a novel channel layout for each of the condenser and the evaporator sections is proposed. First, in the condenser section, one large-diameter channel is connected to only one small-diameter channel. Second, for the evaporator section, by penetrating the channel walls between the small-diameter channels and the large-diameter channels less deeply into the evaporator section than the other channel walls. To verify the effectiveness of the proposed channel layout, the PHP with the proposed channel layout is fabricated. The overall dimensions of PHP are identical to those of the PHP with the merged channel layout presented by Lim and Kim. HFE-7000 is used as the working fluid and the filling ratio is fixed at 45% by volume. It has been experimentally confirmed that the thermal performance of PHP with proposed channel layout is about 48% higher than that of PHP with the merged channel layout in the horizontal orientation. The difference of the thermal resistances of the PHP with the proposed channel layout in the vertical orientation and the horizontal orientation are about 10%, which indicates that the proposed channel layout ensures orientation-independent performance of the PHP. Through high-speed photography, the cause of the enhanced thermal performance is found due to the proposed channel layout. In the condenser section, the liquid slugs move from the large-diameter channels to only the small-diameter channels. Then, in the evaporator section, all of the liquid slugs in the small-diameter channel penetrate into the evaporator section due to the proposed channel layout for the evaporator section. These flow behaviors lead to the normal operation of the PHP in the horizontal orientation. If the PHP has the channel layout proposed either for the condenser section or the evaporator section, the PHP does not operate normally in the horizontal orientation. Therefore, PHP should have the channel layouts proposed for both condenser and the evaporator sections in order to have the orientation-independent performance under the localized heating condition.