| With the rapid development of 5G technology,the heat dissipation of electronic equipment has become a key factor maintaining its stable operation.Pulsating heat pipe is a kind of heat dissipation device which realizes heat transfer through gas-liquid twophase flow and transformation.It has a good application prospect.Strengthening its heat transfer performance can provide solid technical support for the cooling of electronic devices.Based on this,this paper studies the effects of surfactant and ultrasonic on the performance of pulsating heat pipe by combining experiments and molecular dynamics simulation methods.Specific work is as follows:(1)The comprehensive performance test system of pulsating heat pipe was designed and built.The influence of surfactant sodium dodecyl sulfate(SDS)solution on the comprehensive performance of pulsating heat pipe was analyzed.It was found that the low concentration of surfactant solution was beneficial to the start-up characteristics of pulsating heat pipe,as well as the thermal performance was improved.Combined with theoretical analysis,it was revealed that the addition of surfactant reduced the surface wettability and Marangoni effect.Specifically,the lowest thermal resistance is 1.349 when the concentration is 0.001 % and the heating power is 30 W.(2)The composite mechanism of ultrasonic and surfactant was experimentally studied.When the surfactant concentrations were 0.01% and 0.1%,the heat transfer enhancement effect of ultrasound was the best.This can be attributed cavitation effect that the liquid working fluid in evaporation section was triggered by ultrasonic.Coupled with the role of surfactant,cavitation bubbles quickly rise from the condensation section and accelerate the oscillation of the liquid slug in the tube,which could greatly improve the heat transfer efficiency.In addition,the experiments were done to test dual-frequency ultrasonic exerting on evaporation and adiabatic sections and ultrasonic adding on evaporation section,respectively.The results show that the application of dual-frequency ultrasonic deteriorates the heat transfer performance,which provides a reference for the reasonable arrangement of ultrasonic in the application of pulsating heat pipe.(3)Based on the performance change of pulsating heat pipe with ultrasonic wave,the near-wall model at the micro level was established by LAMMPS to simulate the microscopic evolution of cavitation.The results show that the cavitation process could be divided into two stages: cavitation initial stage and rapid growth stage.Based on the cavitation chain reaction mechanism,the liquid cavitation process near the wall was analyzed.The results show that the cavitation effect leads the density fluctuation inside the wall,which consequently influences the boundary layer and brings obvious disturbance.(4)Molecular dynamics software was used to simulate the influences of wettability and initial system temperature on near-wall cavitation.The results show that the hydrophilic wall intensifies the density fluctuation and is more prone to bring cavitation effect,leading to 43 ps ahead of time to enter the rapid growth stage.While the hydrophobic wall is more likely to form gas layer,which weakens the cavitation effect.For the initial system temperature,the simulation results show that the molecular has more intense movement as the temperature increasing.Thus,the cavitation enters the rapid growth stage much earlier.Specifically,at 340 K,50 ps ahead of time,the maximum void volume fraction increased by 21.84 %. |
