Study On Flow Characteristics And Heat Transfer Mechanism Of Pulsating Heat Pipe At Liquid Nitrogen Temperature Range

The development of cutting-edge technologies such as cryogenic semiconductors and space exploration has created a demand for efficient heat transfer techniques in the liquid nitrogen temperature range.Pulsating heat pipes(PHPs),known for their high heat transfer performance,light-weight,flexible structure,and zero power consumption,are considered potential solutions.The operational mechanism of PHPs involves strong coupling between heat transfer and fluid flow.However,there is currently a lack of research on the flow characteristics of PHPs in the liquid nitrogen temperature range.The coupling characteristics between two-phase flow and heat transfer performance have not been clearly revealed,and the operational characteristics under different gravity conditions have not been systematically studied,thus limiting their application.To address the shortcomings in current research on PHPs in the liquid nitrogen temperature range,heat transfer and flow visualization experiments are conducted in this study,as well as numerical simulations.The main work and conclusions are as follows:(1)An experimental platform for heat transfer and flow visualization of PHPs in the liquid nitrogen temperature range is designed and constructed.For the first time,full-channel visualization experiments of cryogenic PHPs are achieved,and a method is proposed to evaluate flow data from three aspects: displacement and velocity of specific liquid slugs,average velocity of all liquid-vapor interfaces,and overall flow intensity.To comprehensively reveal the flow characteristics,flow visualization experiments are conducted over a wide range of filling ratios.The results showed that the typical flow patterns in the nitrogen PHP include bubbly flow,slug flow,churn flow,and annular flow.The flow pattern distribution and overall fluid motion are different from those in room temperature PHPs.Annular flow dominates under medium and low filling ratios.The proportion of slug flow increases with increasing filling ratio and decreases with increasing heat load.Moreover,the proportion of slug flow does not exceed 50% when the filling ratio is below 54%.The physical properties of nitrogen lead to unstable liquid-vapor interfaces and frequent flow pattern transitions inside the PHP.Short liquid slugs cannot exist for a long time,and the liquid tends to gather in the form of long liquid slugs.It is difficult to maintain an overall oscillation motion inside the PHP,and unidirectional circulation is easy to form.The fluid motion during stable operation is characterized by unidirectional circulation,with liquid slug velocities not exceeding 0.4 m/s.Relay long liquid slugs constitute the basic units of a stable fluid flow.The boiling of these long liquid slugs directly provided the continuous operation power for the PHP.(2)Synchronized heat transfer and visualization experiments are conducted.Two start-up modes are observed: bubble nucleation and interface evaporation.Only the former can activate all the channels,but it has a higher degree of superheat,leading to a hysteresis in heat transfer performance.The difference in heat transfer performance under different filling ratios is caused by the varying proportions of sensible heat and latent heat transfer,and the latent heat transfer efficiency is higher than sensible heat transfer in nitrogen PHPs.Experiment results under different inclination angles show that gravity has a significant influence on the operation of the nitrogen PHP.As the inclination angle decreases,the heat transfer performance gradually deteriorates.The stable boiling of the relay long liquid slugs can mitigate the influence of gravity.Once a unidirectional circulation is established,the heat transfer performance significantly improves and remains stable.Based on the experiment results,a three-dimensional numerical model of the nitrogen PHP is established using the Volume of Fluid(VOF)method and validated by experiment data.The simulation results show that in the thermally driven two-phase flow of the nitrogen PHP,the average pressure difference provided by each turn is similar,but the peak values always appear at different timing.This pressure difference drives the PHP to operate continuously and stably.Approximately 3/4 of the thermal resistance of the PHP exists in the fluid.The results on the coupling characteristics of heat transfer and fluid flow indicate that the improvement of heat transfer performance can be achieved by promoting the formation of unidirectional circulation flow.(3)Synchronized heat transfer and visualization experiments are conducted under different number of turns and inclination angles.In a PHP with a high number of turns,obvious oscillation motion occurs under medium heat flux.However,the oscillation motion is unstable and greatly influenced by heat load and inclination angle,resulting in variations in heat transfer performance.Unidirectional circulation flow is still the most efficient and stable motion.Within the experiment range,a higher number of turns can lead to a higher thermal conductivity.Increasing the number of turns helps mitigate the influence of gravity on the operational performance.However,gravity independence is still not achieved even with 20 turns.The low surface tension of nitrogen results in a higher impact of gravity on the operation of nitrogen PHPs compared to room temperature PHPs.Only the PHP with20 turns can operate in a horizontal orientation.The visualization results reveals that the horizontal operation is related to the oscillation motion under high number of turns conditions.The increased degree of freedom with higher number of turns makes it more prone to resonance,leading to significant movement of the liquid slug and thus supply liquid to the evaporator.The phase change in the evaporator further results in pressure difference which drives the fluid flow.It may be possible to achieve gravity independence in cryogenic PHPs by increasing the number of turns.