Heat Transfer Characteristics Of Pulsating Heat Pipe And Its Application

Pulsating heat pipe (PHP) has been regarded as a new and efficient heat transfer device that operates via the complex vapor-liquid two-phase pulsating flow and phase change heat transfer to achieve the heat transport. It possesses the advantages of simple structure, good heat transfer performance, strong environmental adaptability, especially the unique advantage of long-distance and pumpless heat transport in the anti-gravity condition. Therefore, the pulsating heat pipe has a promising application in microelectronics cooling and industrial waste heat utilization.Until now, the mechanism and inherent law of coupled evaporation-condensation phase change as well as the complex vapor-liquid two-phase flow inside the PHP are not completely known. Especially, the effect of channel size on the vapor-liquid two-phase pulsating flow behavior and heat transfer characteristics is still waiting to be explored. In addition, the startup and heat transfer performances of PHP in the anti-gravity condition remains to be further investigated. Particularly, there is still lack of study on the application of long-distance and pumpless heat transport by the PHP in anti-gravity condition. In this context, three experiments of gas-liquid flow and heat transfer in PHP are conducted, including the visualization of vapor-liquid two-phase pulsating flow, the heat transfer performance in anti-gravity environment and the energy storage of waste heat.In order to explore the mechanism of phase change heat transfer and dynamic behaviors of vapor-liquid two-phase flow in PHP, a visualization and thermal performance test system is assembled. The visualization experiment are conducted to investigate the effect of channel dimension on the flow pattern transition, operation modes and heat transfer performance of the PHP. The results indicate that the vapor-liquid two-phase flow patterns in PHP mainly include bubbly flow, plug flow and annular flow. As the channel dimension decreases, the formation of the vapor slugs at evaporation section due to many tiny bubbles aggregation is gradually transformed to the single bubble growth and expansion, and the breakup and deformation of the vapor plugs more likely occur in the condenser section. Four operation modes of vapor-liquid phases are observed during the operation of the PHP, including pulsation operation, transitory circulation, circulation with transition of direction and directional circulation. The pulsating characteristics of the wall temperature in each section are directly determined by the operation mode. The PHP often keeps slight pulsation state at low heating power while directional circulation state at high heating power. Besides, the PHP with moderate diameter has larger pulsation amplitude, higher pulse frequency and better heat transfer performance.To verify the unique advantage of the PHP to transport heat by gas-liquid phase change in anti-gravity environment, an experimental study on the heat transfer performance of the copper PHP is conducted. Based on this experiment apparatus, the heat transfer performance of PHP in the horizontal, gravitational and anti-gravity conditions are compared, and the effects of the filling ratio, number of bends and height on the heat performance of PHPs in the anti-gravity condition are analyzed. The results indicate that the PHP starts-up is typically via the gradual start-up in the anti-gravity condition and it is easier to start up as the bend number increases. The optimal filling ratio for the heat transfer performance of PHP is about 70%, and it is difficult to achieve normal operation if the filling ratio is less than 50%. With increasing the height of the PHP, the heat transfer performance and heat transfer limit is reduced. Under the same heat load condition, the overall heat transfer performance of the PHP in the horizontal condition is better than that in the anti-gravity condition but inferior to that in the gravitational condition. With the increase of heat load, differences in overall heat transfer performance among these three conditions gradually decreases, indicating that the influence of gravity effect is weakened.In order to verify the feasibility of application of PHP for the waste heat utilization in Antarctic expedition support platform, an experiment of PHP is conducted to achieve the energy storage of waste heat of flue gas. In this experiment, the air is heated by quartz glasses so as to imitate flue gas of diesel generator, and the PHP is utilized to transfer waste heat of fuel gas to the oil storage tank at the bottom of the tube of fuel gas. The results indicate that the PHP can efficiently transfer heat in the anti-gravity condition and achieve the heat storage of waste heat from flue gas. This means that the anti-freeze and heat preservation of oil storage tank in Antarctic expedition support platform can be achieved by using the PHP to utilize the waste heat of flue gas from diesel generators.