Thermal And Flow Characteristics Analysis Of The Microchannel Separated Heat Pipe In Data Center

With the rapid development of information technology,the industrial scale and energy consumption of data center are increasing.The air conditioning system of data center,which accounts for a large proportion of its total energy consumption,has also become the focus of energy conservation research.Due to the characteristics of energy-saving,high efficiency and easy utilization of natural cooling source,microchannel separated heat pipe has been widely used in data center,and its structure and operation parameters have also been optimized.However,because of the compact structure and opaque wall material,the research and optimization on the refrigerant side of microchannel heat pipe still has space to improve.Therefore,this paper takes the flow and heat transfer characteristics of microchannel heat pipe as the research objective,uses experiment and simulation methods to solve the difficulties such as flow visualization and contact angle analysis.The influence of operation conditions,structural parameters,filling ratio,wall wettability and other parameters are mainly investigated,which has an important significance for performance optimization,energy conservation and operation safety of data center.First of all,to study the heat transfer characteristics of microchannel heat pipe and provide the basis for simulation,an experimental platform of microchannel separated heat pipe was built.Aiming at the air-cooled heat pipe air conditioning and water-cooled back-plate heat pipe,two heat pipe structure widely used in data centers,the heat transfer,superheat,inlet and outlet temperature difference,pressure distribution and other thermal characteristics were discussed under different conditions such as filling ratio,air volume,indoor and outdoor temperature difference.The results show that the filling ratio is one of the main factors affecting the thermal performance of the system.As the filling ratio increased to the optimal value,the heat transfer of the system increased gradually,and the parameters such as the air temperature,the temperature difference between the inlet and the outlet,and the superheat degree at the outlet decreased gradually.However,as the liquid filling rate further increased,the heat transfer performance of the system tended to deteriorate due to the influence of excessive refrigerant on the two-phase heat transfer area.In addition,increasing the temperature difference,air volume and other parameters could also increase the maximum cooling capacity,and partly affected the optimal range of filling ratio.Secondly,a simplified numerical model of microchannel heat pipe was established based on the models of louvered fins,phase transformation,contact angle and VOF.The reliability and accuracy of the numerical model were verified by experiment results.The simulation results show that the two-phase flow pattern in the evaporation section along the y-axis changed from pure liquid to bubble flow to slug flow.The flow pattern and the volume of the two-phase zone significantly affected the superheat degree of the wall and the thermal performance on the refrigerant side.In the condensation section,film condensation was the main form,and the refrigerant flows downward along the wall in the form of liquid film.The thickness of liquid film increased gradually with time,but also increased gradually along the direction of gravity,and the formation of condensate droplets were also observed in condensation section,which makes the outlet flow rate and volume fraction fluctuate periodically.Thirdly,on the basis of experiment and simulation,the thermal and flow characteristics of microchannel heat pipe under different wall contact angle were investigated.The impacts of contact angle on heat transfer,flow pattern,volume fraction distribution,wall and refrigerant temperature were analyzed,and the mechanism of contact angle change on refrigerant flow is revealed.It was found that with the increase of wall contact angle from 0° to 180°,vapor refrigerants tended to be distributed on two sides of the wall,leading to the regular slug flow to irregular vapor plug in the tube.At the same time,the wall temperature increased,and the average temperature difference between refrigerants near the wall and inside the heat pipe increased by 0.86?,and the heat transfer coefficient on the refrigerant side decreased by 68.9%,indicating that the wall hydrophilicity was conducive to the increase of critical heat flux.On the other hand,with the increase of contact angle,the nucleation time of bubbles in the tube was advanced,and the initial lowest point of nucleation was reduced by 55%.It is proved that the wall hydrophobicity is helpful to the nucleation of bubbles.Finally,in order to comprehensively use the advantages of hydrophobicity and hydrophilicity,the internal flow and thermal characteristics of hydrophilic-hydrophobic mixed microchannels were studied.It is found that in the microchannel,the evaporation degree of refrigerant increased nearly 3-5 times,and its overall heat transfer performance was significantly higher than that of hydrophilic and hydrophobic microchannels.Bubbles mainly nucleated on hydrophobic spots,colliding and aggregating with other bubbles in the rising process,which makes the vapor fraction in the tube periodic fluctuated by time-space.In addition,the hydrophobic area ratio and flow rate also significantly affected the mean value,amplitude and frequency of vapor fraction.