Study On Heat Transfer Characteristics Of A Pump-driven Two-phase Cooling Loop For Data Centers

In the 21 st century, the energy problem is one of the three basic problems of modern social development.In the energy consumption of building air conditioning, air conditioning energy consumption of data centers are rising rapidly in these years.During the transition season and winter, there is much cold source in outdoor for data centers free cooling.Considering the data center load is high, the advantage of phase-change and working fluid antifreezing problem in winter, a pump-driven two-phase loop(PTL) heat exchange unit is chosen.The working fluid is R22.A PTL experimental apparatus and a PTL heat exchange unit in data center free cooling were studied in this paper. One dimensional steady state mathematic model of PTL was developed.The PTL cycle characteristics and heat transfer mechanism were studied, the main work includes:According to the cycle theory and the state of working fluid in evaporator outlet and condenser inlet, four cycle model were superheat cycle, two-phase cycle, subcooling-two-phase loop and subcooling cycle. One dimensional steady state mathematic model of PTL was developed, based on heat transfer and pressure loss equation. The calculation flow chart of the model was studied.The positive cycle is better than the reverse cycle in the calculating speed and computational accuracy.The isometric is better than the isoenthalpy partitioning.The pressure loss of different parts in the PTL was studied using PTL model. The pressure loss from evaporator inlet to the reservoir inlet: there is a linear relationship between the evaporator inlet subcooling degree of R22 and the heat exchanger heat transfer.The greater of pressure loss, the greater of R22 temperture change in the evaporator. There is a subcooling heat transfer in the evaporator when the pressure loss is very high.The pressure loss from condenser outlet to the reservoir inlet: there is a linear relationship between the condenser outlet subcooling degree of R22 and the heat exchanger heat transfer. The greater of pressure loss, the greater of R22 temperture change in the condenser. There is a subcooling heat transfer in the evaporator and condenrser when the pressure loss is very high. With the increase of pressure loss in the evaporator and condenser, the heat transfer increased firstly and then decreased. With the increase of pressure loss from evaporator outlet to condenser inlet, the heat transfer decreased. Heat transfer attenuation rate of the former is greater than the latter.The temperature of R22 increased firstly and then decreased in the evaporator. There are a temperature different and heat transfer pinch point when the R22 is saturated liquid in the evaporator. The heat transfer distribution of evaporator mainly concentrates in the inlet and oulet. The temperature of R22 gradual decreased in the condenser. There are a temperature different and heat transfer turning point when the R22 is saturated liquid in the condensr.As the increase of heat exchange area, the heat transfer growth gradually changes slowly. The heat transfer distribution of evaporator mainly concentrates in the inlet and oulet. The greater of heat exchange area, the greater of heat exchange area of the middle section in the evaporator. According to the principle of temperature uniformity, a multi loop PTL was analyzed. The higher single loop temperature efficiency, the less number of loops.A PTL experimental apparatus was set up. The cycle characterization and heat transfer mechanism of PTL were studied in this paper.With the increase of mass flow rate, heat transfer increased firstly and then decreased, increased again at last. There are two main influence factors, one is the heat transfer coefficient and another is the pressure loss. Firstly, heat transfer coefficient rises faster, and pressure loss to change slowly.As the mass flow rate continues to rise, heat transfer coefficient rises slowly, pressure loss change is bigger. With the increase of mass flow rate, ther subcooling heat transfer is biger then the two-phase heat transfer, so heat rise again. Heat exchanger increased from 1 to 6 units, the heat transfer growth gradually changes slowly. There are a temperature different and heat transfer pinch point when the R22 is saturated liquid in the evaporator.A PTL heat exchanger unit for date center free cooling experimental apparatus was set up. The heat transfer characteristics, flow characteristics and indoor and outdoor temperature difference were studied in this paper.The COP of the unit was 3.75 when the indoor-outdoor temperature difference was 10°C, then increased to 9.37 when the temperature difference was 25°C.When the vapor quality of R22 at the evaporator outlet was between 0.3 and 0.6, the mass flow rate affected the heat transfer rate slightly.The relationship between the heat transfer rate and the temperature difference was linear.With the decrease of outdoor temperature, the control strategy of the unit is: adjustment the frequency of condenser fan firstly. When the unit COP is increased slowly, adjustment the frequency of evaporater fan, indoor fan minimum volume flow rate for maximum allowable temperature difference air supply in data center. According to the indoor return air temperature, adjustment the frequency of pump lastly.