To solve the aerodynamic heating problems caused by high-speed flight, the active thermal protection method of transpiration cooling based on ceramic matrix composites has become a research hotspot. Three-dimensional needle-punched (3DN) fiber-reinforced ceramic-matrix composites (CMCs) based on the combination of Cf/SiC were used as porous media in this study. Firstly, in experiments of the flow-resistance and heat transfer characteristics, permeability, inertial constant, and volumetric convective heat transfer coefficient of 3DN Cf/SiC with a porosity of 41.0% were measured. Besides, these characteristics were also compared with those of sintered porous metal with equivalent porosity and pore size. The heat exchange capacity of CMC materials is comparable to that of sintered metals, while their flow resistance is relatively higher. Numerical simulations of transpiration cooling in a blunt-shaped head were conducted using a porous medium model. Considering the significant differences in flow characteristics, thermal conductivity, and density between CMCs and sintered metals, the control variable method was employed to investigate the effects of these properties on transpiration cooling performance. Under the nonuniform boundary condition, the excessively high hot-end temperature at the head region compromised the efficiency of transpiration cooling. The results indicate that the high flow resistance of CMCs is beneficial for mitigating the uneven distribution of coolant flow caused by non-uniform pressure. CMCs exhibit anisotropic thermal conductivity, where the high thermal conductivity in the parallel direction facilitates the transfer of high heat flux from the head to the wedge surface, thereby reducing the maximum temperature. Additionally, the low-density nature of the material allows for increased wall thickness, which provides a larger heat transfer space to regulate the heat absorption of the cooling medium under inhomogeneous heat flux boundaries, resulting in improved cooling performance. It is demonstrated that 3DN Cf/SiC ceramic matrix composites possess significant potential for thermal protection system applications.

With the rapid development of information technology and the wide application of intelligent arithmetic,the power density of a single cabinet continues to increase.Such high-power-density configurations cause severe cooling challenges while significantly increasing computing power.This study tests a two-phase microchannel self-driven cabinet-level air-cooling system for data centers dominated by air-cooling technology.The test results show that the system can achieve a heat-transfer capacity of 40 kW.The system is subsequently applied to a high-density cabinet demonstration project in Taiyuan.The measured data show that when the power of the cabinet reaches 30 kW under full-load conditions,this system can effectively satisfy its heat-dissipation requirements.In addition,this study constructs a heat-transfer model based on experimental data and further explores the heat-transfer capability of two-phase self-driven cabinet-level air-cooled terminals under different external conditions,with the aim of exploring the limits of air-cooling systems.The results show that the air-cooled terminal can realize a heat dissipation of 65.2 kW if the return air temperature of the cabinet-level terminal is maintained at 35℃,the temperature of the chilled water source is as low as 12℃,and the wind speed on the terminal reaches 5 m/s.Based on experimental validation and theoretical analysis,this study offers new possibilities and technical support for stock air-cooled data centers to further improve their arithmetic power.

To meet the demanding thermal management needs of modern data centers and energy storage systems, the gravity-driven two-phase loop (GDTPL) emerges as a highly efficient and reliable passive cooling solution by virtue of its pump-free, high-efficiency phase-change heat transfer characteristics. However, its application in the compact, multi-elevation, and non-uniform heat source layouts common to both fields has remained experimentally unverified. This study filled this gap through the design, construction, and systematic experimental investigation of a novel Multi-Elevation Heat Source GDTPL (MEH-GDTPL) system to systematically assess its performance under various thermal loads. The experimental results showed that the system achieves reliable and smooth startup under both uniform and non-uniform load conditions, and autonomously establishes steady twophase circulation. During steady-state operation, it demonstrated exceptional thermal uniformity: under uniform loads from 6400 to 8800 W, the maximum inter-level temperature difference was as low as 0.96 degrees C, and the per-row evaporation-side thermal resistance remained within 4.45-5.35 K/kW with small inter-row variations. Furthermore, its performance was validated in complex non-uniform scenarios, including alternating high-low power patterns and centrally concentrated heat loads. Under these conditions, the inter-evaporator temperature difference was effectively limited to within 1.62 degrees C, while the corresponding per-row thermal resistances remained below 6.0 K/kW. These results confirmed the system's structural adaptability and thermal consistency. This study provided the first definitive experimental validation of the MEH-GDTPL's feasibility and robust performance, offering a crucial high-performance passive cooling strategy for next-generation energy storage systems and high-density data centers.

Ammonia's high heat sink makes it suitable for highly efficient and compact thermal management systems in high thrust weight ratio aero-engines. This study experimentally investigated the flow boiling heat transfer of ammonia in a rectangular minichannel with a hydraulic diameter of 1 mm. Tests covered pressures from 1.22 to 2.31 MPa, mass fluxes of 152.62-394.19 kg/(m2 & sdot;s), and outlet vapor qualities of 0.027-0.98. Flow visualization identified typical flow regimes and their heat transfer mechanisms. It is indicated that heat transfer in the low vapor quality region is dominated by nucleate boiling (bubbly/slug flow). In the medium-to-high vapor quality region, heat transfer peaks due to conduction and evaporation within the liquid film of the annular flow regime. Heat transfer deteriorates in the dispersed flow regime due to the liquid film's dryout. Increasing pressure promotes the generation and detachment of smaller bubbles by reducing the vapor-liquid density ratio and surface tension, while also facilitating the earlier onset and enhanced stability of the annular flow regime. The higher two-phase flow velocities at elevated pressures lead to heat transfer enhanced across the entire vapor quality range. Based on the experimental data, a piecewise composite heat transfer correlation for saturated flow boiling incorporating scaling effects was developed. The proposed correlation predicts the experimental data with a mean absolute deviation of 12.07%, and the majority of the predictions fall within a +30% error band. This study provides a theoretical basis and a design tool for minichannel heat exchangers using ammonia as the working fluid.

The heat pipe/vapor compression integrated system has significant application potential in the field of energyefficient cooling for data centers. To address the different optimal refrigerant charge amount between the heat pipe and vapor compression loop in current systems, a novel integrated refrigeration system utilizing a new parallel-flow three-fluid heat exchanger was proposed and investigated. The optimal filling ratio for the heat pipe loop was determined as 48.8%. Under constant evaporator air flowrates, the superheat of the evaporator and the supercooling of the condenser decreased with the increase of outdoor temperature. In compressor-off mode when the outdoor temperatures ranged from -5 degrees C to 15 degrees C, the cooling capacity ranged from 2.66-10.4 kW with an energy efficiency ratio (EER) of 4.47-25.54. With compressor-on mode when the outdoor temperatures ranged from 15 degrees C to 35 degrees C, the cooling capacity reached 6.66-13.6 kW with an EER of 2.60-5.49. The annual energy consumption of the integrated system was simulated in different climate zones in China and compared with a traditional air conditioner and an integrated refrigeration system. The annual energy-saving rate was 3.9%35.1% relative to the traditional air conditioner, and up to 16.23% improvement over the integrated refrigeration system. This demonstrates successful integration of vapor compression and heat pipe technologies, with promising application potential in energy-efficient data center cooling.

Driven by surging AI computational demands, data center power densities are pushing conventional air cooling to its thermal limits, while liquid cooling remains constrained by high costs and retrofit challenges. Addressing the research gap in data center gaseous thermal management, this study pioneers the evaluation of helium as a high-performance, non-intrusive replacement coolant within existing architectures. Through finned-tube heat exchanger experiments and model calibration, helium's cooling capacity and energy efficiency were quantitatively compared with those of air. Results show that helium achieves an overall heat transfer conductance (KA) 1.45 times that of air. Under identical fan power, helium increases single-rack heat dissipation by 39%, reaching up to 36.7 kW. System-level analysis across five climates consistently demonstrates superior efficiency; in Beijing, cooling system energy consumption is reduced by 17.4% compared to air cooling. By validating helium's thermophysical advantages, this work establishes a viable, energy-efficient upgrade path that seamlessly bridges the gap between traditional air cooling and complex liquid systems.

Ceramic matrix composites (CMCs), distinguished by their high-temperature resistance, low density, and high specific strength, are extensively employed in the hot-end components of aero-engines. The pores inside CMCs can serve as seepage channels for transpiration cooling. This study obtained the permeability and inertial coefficient of two-dimensional plain-woven (2DPW) CMCs by experiments and then established a parametric modeling method and pore-scale simulation methodology specifically for 2DPW, and the internal flow mechanism was analyzed. Firstly, an experimental investigation was conducted to explore the flow characteristics, and the permeability and inertial coefficients among different porous medium were compared. Second, the internal geometric structure of the material was captured using a micro-computed tomography scanning method, and a simplified parametric modeling method was developed. Third, a representative volume element was constructed for pore-scale internal flow simulations. Numerically predicted pressure drops versus flow rate characteristics were validated against experimental data. Finally, an analysis was conducted on the internal flow and heat transfer characteristics, and the flow resistance as well as the volumetric convective heat transfer coefficient were acquired. The results indicated that the complex pore structure induced by inter-layer misalignment gives rise to high fluid tortuosity, resulting in significant variations in the magnitude and direction of the flow velocity, which contribute to the high inertial resistance. For the coupled flow and heat transfer process, the numerical simulation results show that the staggered 2D plain woven structure forces the fluid to pass through the inter-layer pores when flowing through the woven meshes of different layers, which generates substantial flow resistance while enhancing the heat transfer performance. The high-velocity flow within the narrow inter-layer pores provides the basis for high heat transfer, and the dual heat sources are derived from both the high-temperature solid matrix and the high-temperature swirling flow in the inter-bundle pores. This study provided guidance for the establishment of 2DPW geometric models, the analysis of flow and heat transfer characteristics, and the gradient porosity design in transpiration cooling.

[Objective]With the rapid advancement of artificial intelligence and high-performance computing,data centers are increasingly challenged to achieve effective thermal management and high energy efficiency.Traditional air-cooling methods struggle to handle rising power density,while liquid cooling offers superior heat transfer capabilities but often lacks deployment flexibility.To fully leverage the advantages of air and liquid cooling systems,this study proposes two server-level phase-change air-assisted liquid cooling(AALC)systems to enhance cooling performance,maintain modular deployment,and reduce energy consumption in high-density data centers.[Methods]Two technical configurations were investigated:an air-cooling retrofitting scheme(System 1)and a hybrid solution incorporating a heat pipe composite air conditioner(System 2).Both systems utilized a pump-free,gas-liquid phase-change heat transfer loop to enable passive fluid circulation and simplify the structural design.System 1 was designed to combine the structural simplicity and deployment flexibility of air-cooling systems with the high heat transfer efficiency of liquid cooling.It enabled seamless upgradation of high-density cabinets within limited space in existing air-cooled data centers without requiring large-scale infrastructure modifications.An experimental platform simulating a 50 kW cabinet heat load was developed using five electric heating modules.The thermal performance of System 1 was evaluated by monitoring the average temperature and surface uniformity across multiple evaporators.To further improve the energy-saving capability and year-round deployment potential across different climatic zones of China,System 2 was developed based on System 1 by incorporating a heat pipe composite air conditioner on the cold source side.This configuration retained the core advantages of passive two-phase cooling while enhancing the effectiveness of natural cooling.For System 2,annual power usage effectiveness(PUE)was simulated using hourly meteorological data from five representative Chinese cities.The total power consumption of the cooling system-including terminal fan power,outdoor fan power,and compressor power-was modeled using empirical polynomial-based expressions.In addition,economic metrics,such as annual energy savings and payback period,were evaluated.[Results]System 1 exhibited stable operation under a 50 kW cabinet heat load.The average temperature of the two-phase loop reached 39.85 ℃,and the maximum surface temperature difference among five vertical evaporators was only 1.9 ℃,demonstrating excellent thermal uniformity and heat dissipation capacity.The simulation results of System 2 showed up to 30％energy savings over conventional computer room air handler(CRAH)systems because of the enhanced use of natural cooling.In cities such as Beijing and Xi'an,the annual PUE was reduced to as low as 1.16.The hybrid operation considerably lowered fan and compressor energy consumption.Economic analysis indicated a payback period of approximately 1.89 years,confirming the financial viability of System 2.[Conclusions]The proposed AALC systems provide a practical and scalable solution for thermal management in high-density data centers.System 1 verifies the feasibility of passive phase-change cooling for flexible cabinet-level retrofitting.To further enhance the energy-saving potential,System 2 is designed by integrating a heat pipe composite air conditioner on the cold source side.The results demonstrate that the two systems offer substantial technical and economic advantages and provide a promising direction for the sustainable evolution of data center cooling technologies.

Transpiration cooling problem in carbon fiber reinforced silicon carbide matrix composites (C/SiC) was studied based on the context of cooling of jet engine hot-end components. Pore distributions of C/SiC with different fiber preforms were compared, and the C/SiC with three-dimensional needled preform (3DN C/SiC) was selected. The pore structure of 3DN C/SiC was analyzed and studied by X-ray computed tomography scanning (CT), and its porosity was obtained based on the Archimedes' principle. Finally, based on the Darcy-Forchheimer model, the pressure drop-flow rate curve was obtained and the permeability of 3DN C/SiC was calculated. By considering the influence of inertia and viscous forces, the characteristics and mechanism of the porous media flow inside 3DN C/SiC were analyzed. The results showed that 3DN C/SiC could achieve a permeability of 3.37 x 10-12 m2 under a porosity of 47.61 %, which was close to that of commonly used metal porous media. 3DN C/SiC also demonstrated good flow characteristics as a porous medium. Considering its other advantages, such as high temperature resistance, light weight, and high specific strength, 3DN C/SiC has excellent potential and prospects in jet-engine thermal protection systems.

A prototype of a multistage cooling system with a cooling capacity of 20 kW is assembled and studied. The traditional single-stage system is split into two sub-systems according to the temperature level, which reduces the temperature difference and irreversible heat transfer loss, and further improves the system's energy efficiency. The system mainly features three modes including free-air cooling, refrigeration-based cooling and waste heat recovery, which can achieve efficient cooling while capable of recovering the waste heat to the surrounding heat users. The efficiency of the proposed cooling and heat recovery system was assessed by measuring its energy reuse effectiveness (ERE). The energy savings and environmental benefits of the system were analyzed across five cities in China, each representing different climates. Results indicate that the system achieved an ERE of 0.91 in Beijing, 38.9% lower than the national average of the city. Additionally, an economic analysis showed a payback period of less than two years for the proposed system across all climates considered, which is 30 % shorter than that of a single-stage cooling system.

The rapid expansion of data centers has significantly increased energy consumption,with cooling systems accounting for about 40% of total use.Utilizing natural ambient cooling sources provides a simple and effective approach to enhancing energy efficiency.Radiative cooling(RC),though an emerging solution that can considerably reduce energy use,faces challenges in data centers due to the complex,multi-level nature of cooling systems,requiring careful adaptation across different scales,which hinders its widespread adoption in data centers.In this study,we designed radiative coolers for data center cooling systems to enhance efficiency,and then proposed an RC system integrating these structures and analyzed its energy-saving performance.The cooling properties of a real radiative cooling film applied to the cooler surface were experimentally tested,and the data were used for the simulation analysis of the proposed coolers.Five different radiative cooler structures were designed and optimized,and we conducted a comprehensive multi-level performance analysis of the optimized structures,including operational parameters such as flow rate and temperature,as well as the impact of location,climate,and regional adaptability.Subsequently,a novel hybrid cooling system incorporating radiative coolers for data centers was proposed.Comparative studies across different climate zones in China demonstrated that this hybrid system delivers substantial energy savings compared to traditional vapor-compression systems.Results showed that in Beijing,Urumqi,and Guangzhou,the annual temperature difference between the inlet and outlet of the radiative cooler ranges from 2.40℃ to 3.28℃,making it feasible for radiative cooling throughout the year in most parts of China.The annual Power Usage Effectiveness(PUE) in Beijing using the novel RC system is1.19,with an increase in Energy Efficiency Ratio(EER) of 60.74%.This study may contribute to the development of green,energy-efficient cooling technologies for future data centers.

Loop thermosiphon has significant advantages in solving the problem of high energy consumption in data center cooling scenarios. The evaporator is the most important part of the loop thermosiphon because it acts as a heat sink that directly absorbs heat from the IT equipment. Building an appropriate physical model of the evaporator is important for guiding the design process of a loop thermosiphon. Research on the two-phase fluid flow characteristics is necessary for determining the flow mechanism and obtaining an appropriate model. However, current studies mainly focused on the entire loop instead of the evaporator and were short of research on the void fraction modelling method. Therefore, this study focuses on the two-phase flow characteristics in the evaporator of a loop thermosiphon using R134a as coolant, especially the coolant slip ratio, pressure difference proportion, and relationship between coolant mass flow rate and total pressure difference in the vertical tube. First, we studied the mechanism of the vapor and liquid distribution in different operation modes of the loop thermosiphon. Second, we proposed a new experimental method to measure the total pressure difference in a vertical tube and obtained the relationship between the pressure difference and heat transfer capacity at different filling ratios. Finally, we selected the L-M friction model to calculate the friction pressure drop and compared the applicability of different void fraction models. The modified Smith model was adopted as the best void fraction model, and the flow characteristics, including slip ratio and pressure distribution, were studied. These results are important for designing loop thermosiphons.

With the development of big data and cloud computing, data centers have grown rapidly worldwide. A data center is a special building with high energy consumption. The energy consumption of traditional computer room air conditioning is assumed to be 45 % of the total energy consumption of the data center. Currently, the use of a loop thermosiphon as a cooling system is one of the most effective methods. Although loop thermosiphons have already been widely used, the boiling mechanism inside the evaporator needs to be studied further, particularly for pump-driven loop thermosiphons. In this study, we investigated the heat transfer and flow characteristics of a pump-driven loop thermosiphon. The inside tube diameter is 8.05 mm, the heat flux changes from 6317 to 28085 W/m2, and the mass flux changes from 437 to 1311 kg/m2s. First, the differences and similarities between pump- and gravity-driven modes are studied. Second, the matching relationship between the heat flux and mass flow rate is studied, and the mechanism of the flood limit for the pump-driven mode is illustrated. Finally, we compare different boiling correlations and propose a new correlation based on our experimental results.

Cooling systems within data centers are known for their substantial energy consumption. Predicting their energy usage typically involves two primary methodologies: constructing models grounded in physical principles and developing data-driven models. While physical models may lack broad applicability, artificial neural network (ANN) models often sacrifice interpretability. Striking a balance between accuracy and interpretability is a significant challenge in model development. In this study, we propose a novel approach that combines physical principles with ANN methodologies. By leveraging the strengths of both approaches, this combined model aims to enhance prediction accuracy while preserving interpretability and applicability. Experimental data specific to cooling systems were utilized to compare the predictive performance of the physical models, ANN models, and the combined model. Results demonstrate that the mean relative error (MRE) for the physical model was 7.95%, for the ANN model was 13.44%, and for the combined model was 6.54%. Additionally, the root mean square error (RMSE) values for the three models were 352.6, 258.3, and 181.9, respectively.

Zeotropic refrigerants possess the characteristic of temperature glide during phase-change process, making them suitable for temperature matching between cold and hot sources and reducing total temperature differences. In this manuscript, the applicability of two commonly used refrigerants, zeotropic refrigerant R407C and pure refrigerant R134a in data center cooling systems are investigated. Firstly, the heat transfer and flow characteristics of R407C and R134a are studied to obtain boiling heat transfer empirical correlations. Secondly, a data center cooling model is established, and modeling calculations are conducted based on experimental results. Finally, the influence of refrigerants on the energy saving effect are calculated based on the model. Experimental results demonstrate that R134a exhibits a 48.3% higher boiling heat transfer coefficient compared to R407C at heat flux 16500 W/m2. However, when considering a data center cooling system, using R407C as working fluid shows the advantage of temperature matching as heat transfer area increases. Based on modeling results, at large heat transfer area, the annual cooling load factor using R407C at low mass flow rate is 0.5% lower than that of using R134a at low mass flow rate, and 22.5% lower than that of using R134a at large mass flow rate for typical climate city Harbin.

In order to promote the green, energy-efficient and sustainable development of data centers, this study introduces a hybrid cooling system that incorporates the full-day radiative cooling technology into data centers and couples it within a heat pipe integrated cooling system, which efficiently operates alongside mechanical refrigeration and natural cold sources.The radiative cooling performance and regional adaptability in 12 major cities in China were analyzed using Energy Plus and mathematical modeling.Moreover, a 40-day test was conducted in Hebei to validate the practical operation of the radiative cooling system.The results demonstrated that during the summer, the average maximum net radiative cooling power can reach 40-50 W/m².The study also reveals the potential of combining the radiative cooling system with traditional air conditioning, leading to an average annual energy savings rate of 28% for data centers.The research provides valuable guidance for the design and implementation of radiative cooling systems in data centers.