Heat Transfer Performance Analysis Of The CO₂ Two-phase Thermosyphon Loop And Its Application In Data Center

Energy consumption of buildings like data centers is very huge and grows year by year.While there is a great energy saving potential in the air conditioning systems in data centers.At present,as an energy efficient air conditioning solution,the two-phase thermosyphon loop?TPTL?has already been put into application in data centers.However,due to the deficient knowledge of the internal operating machenism of the TPTL,there are still lots of problems during using it.Besides,the working fluid of the TPTL is mainly conventional refrigerants like Freon.From the perspective of environmental protection,CO₂ is adopted as working fluid in this study.The internal operating machenism of the TPTL is investigated by experiment.Combined with the engineering application,the performance of the TPTL and the feasibility of CO₂ as the working fluid are analyzed.The operating states of the CO₂ TPTL are investigated.Typical operating states are classified into several zones and the characteristics of each zone are illustrated.Besides,the operating rules of the periodic fluctuation state of the TPTL are summarized.According to the experimental results,there is ineffective circulation during the stable operating of the CO₂ TPTL.The TPTL realizes its maximum heat transfer ability when the fill ratio is around 100%.The maximum heat transfer ability of the TPTL increases dramatically with the increasing of the riser and downcomer diameter.Therotical model for stable operating simulation of the TPTL is built.According to the simulated results,the reasons why the fill ratio and the riser and downcomer diameter influence the performance of TPTL are explained.Based on the differences between the physical properties of CO₂ and R22,comparative study between CO₂ TPTL and R22 TPTL is conducted including the flow regime,the maximum heat transfer ability,the driving temperature difference and so on.The maximum heat transfer ability of the CO₂ TPTL is much higher than that of the R22 TPTL under same fill ratio and construction of the TPTL.The thermal resistance of the CO₂ TPTL is lower than that of the R22TPTL because of the higher boiling and condensing heat transfer coefficient of CO₂.Thus,under same heat transfer rate,the d riving temperature difference of CO₂ TPTL is lower.Double-evaporator TPTL is experimentally investigated considering that there are usually several evaporators connected in parallel in the TPTL system in real application.It was found that the self-regulating ability is very limited when the heating powers of the two evaporators are different.The mass flow rate of the high heating power evaporator is higher than that of the low heating power evaporator,but the mass flow rate is not distributed in proportion to the heating power.The problem of the present conventional central air supply system in data center is analyzed according to some site measurement result.An engineering application case showed that TPTL system is a feasible and energy efficient solution.In regarding to CO₂ TPTL,a side face air supply rack is proposed.Different air conditioning systems in data center are compared with each other.The present research gives further knowledge of the internal operating machenism of CO₂ TPTL.It's supposed to offer a theoretical basis and a design concept for the application of TPTL in data centers.