Heat Transfer Characteristics Of Non-symmetry Micro-channel Flat Plate Pulsating Heat Pipe Under Variable Conditions

Chip power consumption is increasing,excessive temperature will degrade their performance and even affect their service life.The plate pulsating heat pipe has the advantages of simple structure and strong heat transfer ability,and it has excellent application prospect in the field of thermal design.In order to explore the heat transfer performance of pulsating heat pipe in the temperature range of 15 ℃～100 ℃ in evaporation section,15 ℃ or 5 ℃ of cooling water,and solve the problem of efficient and stable operation under horizontal or minus incline angles,a non-symmetry micro channel pulsating heat pipe was designed and a test platform was constructed.This paper comprehensively studied the heat transform performance of the ethanol-based 5-bend NCPHP at minus incline angles(-10°,-5°),horizontal(0°),positive incline angles(30°,60°,90°)and three sets of filling rates(30%,50%,70%)under increasing heating power,and the influence of different cooling water temperatures on the heat transfer performance is explored.Using VOF and LEE model,the numerical simulation at 30% and 50% liquid filling rate was carried out based on fluent under 90° incline angle.The main research contents and conclusions are as follows:1)A heat transfer performance experiment platform was built,and a separable liquid filling device was designed for small liquid filling capacity.The main body of the pulsating heat pipe adopted modular design for easy disassembly and replacement,and the vacuum diffusion welding process was used to make the plate pulsating heat pipe with good air tightness,which could ensure the reliability of the results.2)Under the action of non-symmetry channel,NCPHP can operate stably at 50%liquid filling rate and-5° incline angle.When the heating power is 40 W,the temperature in the evaporation section appears unstable fluctuation,while when the heating power is 50 W,NCPHP can operate stably.When the incline angle of NCPHP is 60° and the filling rate is 50%,the average thermal resistance at 60 W heating power is 0.44 K /W,which is the minimum value observed in this study.3)Under the condition of 30% low liquid filling rate,the dry-out phenomenon was observed in the evaporation section.After the dry-out phenomenon occurred,the NCPHP thermal resistance would increase if the heating power was continued to increase.NCPHP performance at 30% liquid filling rate is highly correlated with incline angle.When the incline angle reaches 60°,thermal resistance is no longer sensitive to the change of incline angle.At 70% liquid filling rate,the influence of incline angle decreases,and the thermal resistance decreases obviously only at a high heating power.4)Lower cooling water temperature can slow down the dry-out phenomenon at30% liquid filling rate.Before the dry-out phenomenon occurs,the temperature difference in evaporation section at different cooling water temperature decreases with the increase of heating power;after the dry-out phenomenon occurs,the temperature difference keeps increasing.The influence of cooling water temperature on temperature difference and average thermal resistance decreases under 50% filling rate.5)The working medium presents a flow mode of wide channel rising and narrow channel falling.At the start-up stage,the working medium experienced the processes of bubble formation,bubble growth and gas plug movement.Local dry-out of the elbow in the evaporation section was observed at low liquid filling rate,and the disappearance of the liquid bridge and obvious contact angle hysteresis in wide channels was observed.The resulting capillary pressure difference was an important factor in the flow resistance of working medium in NCPHP.6)Under the heating power of 30 W～60 W,the temperature in the evaporation section and the flow velocity of the working medium both have the first and second peaks.Under the heating power of 70 W,the second peaks are no longer obvious.The flow velocity of the working medium and the heat transfer in the condensing section increase with the increase of the heating power,while the thermal resistance decreases with the increase of the heating power.