The Research Of Heat Transfer Mechanism For Loop Heat Pipe Enhanced By Pressure Head Of Evaporation

The electron chip,as the typical representative of high heat flux device,was rapidly developed with the development of modern electronic information technology.And the dissipation issues of the chip were focused by more and more fields.Nowadays,Loop Heat Pipe(LHP),as a high efficiency device used phase change,is widely used in the dissipation of data center,super computer,and notebook.The heat transfer characteristics of LHP are that,the heat is transferred by using the phase change of the working fluid and the driving force is normally the capillary force.Therefore,LHP becomes the first selection for the dissipation of computer chips with the advantages of high heat transfer efficiency,good isothermal condition,and non-moving part.However,the limitation of transport distance from evaporator to condenser results to the design of thermal control devices and other issues.The capillary force produced in the wick is usually considered as the only driving force for LHP in the traditional theory.However,some researches have shown that the pressure head of evaporation cannot be ignored during the LHP operation.Based on this,the enhanced pressure heat of evaporation for LHP was proposed and the heat transfer mechanism was studied.In the novel LHP,the wick is separated by the heating surface,and therefore the steam chamber is formed.The location of the phase change interface was transferred from the wick to the steam chamber.And therefore,the heat transfer mechanism of the separately heat and mass transfer was formed.The relative experiment and theoretical research are conducted.In the experimental section,three loop heat pipe devices were established,which were the LHP cooling by nature convection,the visual evaporator under nature convection,and the LHP cooling by water.As the exploratory study,the results of LHP cooling by nature convection showed that,such kind of LHP can operate under fix or varied heat loads,and showed good self-adjusting ability.The system ranged from 0.3 ^oC/W to 0.2 ^oC/W when the heat load changed from 32W to 55W.Through the visual evaporator experiment,two operational models of such LHP were observed:(1)a liquid film was formed in the steam chamber,and the pool boiling can be found in the evaporator,defined as“pool boiling”model;(2)there was no liquid film in the steam chamber.The liquid dropping from the wick was evaporated immediately,defined as“dropping-evaporating”model.In order to improve the cooling efficiency and find the operational characteristic and the optimal working condition,the relatively experiments were conducted by using the LHP cooling by cooling water.The experiments about the influence of different charging ratios,heat sink temperatures,working fluids and vacuum degrees on the operational characteristics were comprehensive studied.In the condition of different charging ratios,the result showed the optimal charging ratio was 35%,in which the performance of LHP was best.The LHP cannot successfully operate when the charging ratio was 15%and 85%.What's more,the temperature fluctuation phenomenon can be observed when the charging ratio became higher.In the condition of the heat sink temperature,there also existed an optimal heat sink temperature,which was 18 ^oC for these specific structure parameters.In this condition,the operational temperature,the start-up time,the thermal resistance for the LHP,and the heat transfer coefficient of the evaporator were 65.1^oC,1350s,0.18^oC/W,and 3.1×10⁴ W/(m²·k),respectively.In addition,the LHP enhanced by pressure head of evaporation showed a low thermal resistance when compared to the LHP driven by capillary force in other literatures^[175-177].In the mathematical model section,the node analysis method was employed and the mathematical models of LHP enhanced by pressure head of evaporation and other two LHPs driven by capillary force were established.The validation was also conducted.The mathematical model for the LHP enhanced by pressure head of evaporation was also established and validated by experiment data.The result showed the mathematical model can well explain the operational characteristics of such kind of LHP.Compared to experimental data,the biggest deviation occurred during the start-up process,which was caused by the deviation of the flow rate in the start-up process.Two mathematical models of LHP driven by capillary force were also established and validated.Compared to the performance of LHP enhanced by pressure head of evaporation in this paper,the operational temperature,the start-up time,and the transport distance were selected as the performance indicators.Due to the driving force was bigger in the LHP enhanced by pressure head of evaporation,it showed the best performance.What's more,the three mathematical models were also applied into the running status of CPU in data center and the result showed the LHP enhanced by pressure head of evaporation had the lowest operational temperature and fastest dynamic response time for varied heat load.At last,the numerical simulation was conducted for LHP enhanced by pressure head of evaporation.The dropping process in the wick,the evaporation process in the steam chamber,and the condensation process in the condenser were simulated,which was also validated by the visual experiment of evaporator.