Research On Split Natural Cooling System Based On Micro Heat Pipe Arrays In Data Center

With the rapid development of computing science and technology of 5G,the data processing capacity of the server equipment in data center(DC)is growing rapidly with a large amount of heat and high temperature,which seriously affects its stable operation.The cooling equipment in DC is required to operate continuously for 8760h in the whole year and a certain amount of surplus is needed.The problem of energy consumption is increasingly serious.In 2017,the power consumption of the cooling system accounts for more than 40%of the total energy consumption.How to reduce the energy consumption of air conditioning(AC)system has become the focus of attention in this field.Due to the low ambient temperature in the transition season and winter,the outdoor natural cold energy can be fully utilized to reduce the operation time and energy consumption of AC and realize the green development of DC.According to the requirements of temperature,humidity and cleanliness specified in DC,indoor and outdoor air-water heat exchanger(AWHE)are designed,which compose of several core heat transfer components and is easy to disassemble and maintain.The closed water-cooled circulation system is used to connect the indoor and outdoor AWHE,and a split natural cooling system(SNCS)based on the micro heat pipe array(MHPA)is developed.The SNCS avoids the direct mixing of indoor and outdoor air and overcomes the restriction of outdoor air by factors such as cleanliness and humidity,etc.Compared with the conventional heat pipe loop heat transfer system,the matching and layout of the system are more flexible,the heat transfer performance and stability performance are better and it can save more space,meanwhile,compared with outdoor cooling tower and other devices,it solves the problems of freezing and large amount of water replenishment.The heat transfer,flow and energy saving characteristics are studied as follows:First of all,the performance research and optimal selection of MHPAs are carried out.The performance of MHPAs with different length of evaporation section under small evaporation section area and different working medium is studied when the heating temperature is 5-20oC,the cooling temperature is-15-10oC.MHPAs show the best performance of heat conduction and temperature distribution when the length of evaporation section is 120 mm and the filling medium is R141b under the condition of natural convection and forced convection.Through the summary of the research conclusions and the earlier stage of the research group on the performance of MHPA,this paper provides a reference for the selection of MHPA in different application fields and the selection of indoor and outdoor AWHE.Secondly,the experimental platform of indoor and outdoor AWHE is proposed,the heat transfer performance under different processes,flow characteristics,heat transfer factor j and friction factor f are studied.The results show that the heat loss rate of AWHE is lower,and the heat transfer performance of countercurrent is better than that of forward flow.The overall heat transfer performance is better than outdoor AWHE,because of the high working temperature and large evaporation area of indoor AWHE.The same serrated fins and structural forms are used for indoor and outdoor AWHE,the pressure drop are at the same level.Under the maximum temperature difference of inlet cold and hot fluid,the maximum heat transfer efficiency is 81.4%,the maximum heat transfer rate of outdoor AWHE is 7.5 k W,which is 18.6%lower than the indoor AWHE of 9.3 k W,and the maximum exergy efficiency of indoor AWHE is 38.5%,which is 5.7%higher than outdoor AWHE.The maximum pressure drop of air side and water side is 339.8 Pa and 8.86 k Pa,respectively.The comprehensive evaluation index j/f ^1/2 is 10.8%higher than that of the louver fin heat exchanger.The fitting curve of?-NTU and pressure drop with flow rate is obtained,which provides theoretical basis for the theoretical design and equipment selection.Thirdly,according to the different performance of indoor and outdoor AWHE,firstly,under the room-based heat dissipation condition,the combined form(N=1)is studied.The heat transfer resistance of indoor AWHE is 39.1%less than that of outdoor AWHE,and the system imbalance is obvious.Then,the optimization analysis of the SNCS with different combination forms(N=0.75,N=0.6)is carried out.The SNCS shows the best heat transfer and pressure drop performance when N=0.75.The maximum heat exchange of the system is 8.7 k W,and the maximum energy efficiency coefficient(EER)is 14.01.On this basis,according to the heat channel-based heat dissipation condition,the further optimization analysis of the SNCS shows that when N=0.75,the maximum heat transfer rate is 12.4 k W,and the maximum EER is 17.15,which is 42.5%and 22.4%higher than the room-based heat dissipation condition,respectively,and the fitting curves of the system EER under the two heat dissipation conditions are obtained,which provide reference for the practical application in different regions.Then,the serrated fin is taken as the research object,and the serrated fin unit model is established.The heat transfer factor j and friction factor f are obtained by numerical simulation,and verified with the experimental value and theoretical experience value.The verified j and f,porosity and physical parameters are given to the simplified porous media module on the air side of indoor and outdoor AWHE as known conditions,the simulation verification and performance optimization of AWHE are carried out,in order to obtain the compact and efficient AWHE.Finally,on the basis of the optimized structure of AWHE,the SNCS modules are applied to the data room.According to six typical cities in different climatic regions,the cost-effectiveness ratio(REC),annual power saving,investment payback period and energy efficiency index of DC before and after transformation under two heat dissipation conditions are compared and analyzed hourly throughout the year.Under the room-based heat dissipation condition,the small operation air flow rate at 1500m³/h and measures to increase the number of modules shall be adopted to achieve the lowest cost-effectiveness ratio,the minimum PUE of DC after transformation is 1.75,the overall PUE is 11.9%-16.5%lower than that before transformation.Under the heat channel-based heat dissipation condition,the larger operation air flow rate at 2500m³/h shall be adopted,and the minimum REC shall be obtained by minimizing the number of SNCS modules,the overall REC is within 0.15 RMB/k Wh,the investment payback period is within 1.7 years,and the minimum PUE is 1.57,the overall PUE is15.1%-25%lower than DC before transformation.Through the above research,the optimal operation conditions and the number of SNCS modules are obtained,and according to different environmental temperature conditions of different climatic regions,the optimal operation strategy of the SNCS after transformation and the most suitable for the application of the SNCS in different climatic regions are obtained,which provides theoretical basis and guidance for the modular application of the SNCS in DC.