Circulating water is used as the working medium in water-based chillers. Salt ions, such as calcium in circulating water, may precipitate during long-term operation, resulting in the attenuation of heat transfer performance. The application of small-diameter tubes in heat exchangers may lead to more prominent fouling problems. This study developed an accelerated fouling method to evaluate water fouling risk. The most typical operating conditions for water-based chillers were selected as the experimental conditions. The test samples included small diameter (5 mm) smooth tubes, with 7 mm smooth tubes selected for the control experiment. The experimental conditions include a circulating water inlet temperature of 60 ℃, a flow rate of 1 m/s, a foulant mass concentration of 800 mg/L, and a test time of 0-400 h. The results revealed that the total fouling mass was 39.5% higher and required fouling time was 17.6% shorter when comparing the 5 mm and 7 mm smooth tubes; the small diameter tubes had a larger total fouling mass and higher fouling rate. The heat transfer coefficients of the 5 mm and 7 mm tubes after fouling decreased by 12.5% and 9.7%, respectively, and the pressure drops increased by 50.6% and 10.4%, respectively, demonstrating a more severe heat transfer performance deterioration of small diameter tubes after fouling. The microscopic observation results of the fouling layer morphology demonstrated that the fouling layer in 5 mm tubes is a form of compact lamellar scaling, which is more difficult to remove compared with the 7 mm tube; therefore, the fouling risk should be considered when promoting the application of small diameter tubes in chillers.

A CO₂ data center cooling and heating system integrated with dedicated mechanical subcooling and dual-temperature evaporation technology (DMS-DE) is proposed to realize green and efficient cooling for data centers and improve comprehensive energy efficiency. A thermodynamic and carbon emission performance system model was established and compared with the basic CO₂ system (Base) and a single evaporating-temperature CO₂ system with dedicated mechanical subcooling (DMS-SE). The results demonstrated that the DMS-DE system had the maximum coefficient of performance (COP), optimal subcooling degree, and discharge pressure. Therefore, adopting the DMS-DE can significantly increase the system COP and exergy efficiency. Compared with Base and DMS-SE, COP increased by 14.1% and 9.0%, and the exergy efficiency increased by 13.24% and 4.31%, respectively. The life cycle carbon emissions of the DMS-DE system were reduced by 16.1% and 9.3% compared with Base and DMS-SE, respectively. This study can provide a technical reference for the highly efficient and clean operation of combined heating and cooling utilization for data center scenarios.

A compressor outlet tube is a transmission component of sound power, and its sound power loss directly affects the performance of pulse tube cryocoolers. Flexible bellows can adjust the relative positions of compressors and cold fingers in applications compared with traditional rigid smooth tubes. This study analyzed the flow characteristics of two types of connected pipes by simulation, and the influence of different types of connected pipes on the performance of the entire machine was verified experimentally to determine the influence of flexible bellows on the cryocooler. The simulation results demonstrated that mixed flow appears at the ripple of the bellows, resulting in greater resistance loss, when compared with a rigid smooth pipe. Under the same inlet parameters, the outlet mass flow and pressure amplitude were lower, and the sound power loss was greater. The experimental results demonstrated that the input power required by the bellows was higher when the cooling capacity was the same. When the cooling temperature was 37.5 K and the cooling capacity was 0.5 Wthe input power of bellows and smooth tubes was 119 W and 112 W, respectively; when the cooling capacity was 3.0 W, the input power of bellows and smooth tubes was 279 W and 259 W, respectively.