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As web-based AI applications are growing rapidly, server rooms face escalating computational demands, prompting enterprises to either upgrade their facilities or outsource to co-located sites. This growth strains conventional heating ventilation and air-conditioning (HVAC) systems, which struggle to handle the substantial thermal load, often resulting in hotspots. Liquid-to-air (L2A) coolant distribution units (CDUs) emerge as a solution, efficiently cooling servers by circulating liquid coolant through cooling loops (CLs) mounted on each server board. In this study, the performance of a 24-kW L2A CDU is evaluated across various scenarios, emphasizing cooling effect and stability. Experimental tests involve a rack with three thermal test vehicles (TTVs), monitoring both liquid coolant and air sides for analysis. Tests are conducted in a limited air-conditioned environment, resembling upgraded server rooms with conventional AC systems. The study also assesses the impact of high-power density cooling units on the server room environment, measuring noise, air velocity, and ambient temperature against ASHRAE standards for human comfort. Recommendations for optimal practices and potential system improvements are included in the research, addressing the growing need for efficient cooling solutions amidst escalating computational demands. 

The ability of traditional room-conditioning systems to accommodate expanding information technology loads is limited in contemporary data centers (DCs), where the storage, storing, and processing of data have grown quickly as a result of evolving technological trends and rising demand for online services, which has led to an increase in the amount of waste heat generated by IT equipment. Through the implementation of hybrid air and liquid cooling technologies, targeted, on-demand cooling is made possible by employing a variety of techniques, which include but are not limited to in-row, overhead, and rear door heat exchanger (HX) cooling systems. One of the most common liquid cooling techniques will be examined in this study based on different conditions for high-power density racks (+50 kW). This paper investigates the cooling performance of a liquid-to-air in-row coolant distribution unit (CDU) in test racks containing seven thermal test vehicles (TTVs) under various conditions, focusing on cooling capacity and HX effectiveness under different supply air temperatures (SAT). This test rig has the necessary instruments to monitor and analyze the experiments on both the liquid coolant and the air sides. Moreover, another experiment is conducted to assess the performance of the CDU that runs under different control fan schemes, as well as how the change of the control type will affect the supply fluid temperature and the TTV case temperatures at 10%, 50%, and 100% of the total power. Finally, suggestions for the best control fan scheme to use for these systems and units are provided at the conclusion of the study. 

The rapid growth in data center workloads and the increasing complexity of modern applications have led to significant contradictions between computational performance and thermal management. Traditional air-cooling systems, while widely adopted, are reaching their limits in handling the rising thermal footprints and higher rack power densities of next-generation servers, often resulting in thermal throttling and decreased efficiency, emphasizing the need for more efficient cooling solutions. Direct-to-chip liquid cooling with cold plates has emerged as a promising solution, providing efficient heat dissipation for high-performance servers. However, challenges remain, such as ensuring system stability under varying thermal loads and optimizing integration with existing infrastructure. This comprehensive study digs into the area of data center liquid cooling, providing a novel, comprehensive experimental investigation of the critical steps and tests necessary for commissioning coolant distribution units (CDUs) in direct-to-chip liquid-cooled data centers. It carefully investigates the hydraulic, thermal, and energy aspects, establishing the groundwork for Liquid-to-Air (L2A) CDU data centers. A CDU’s performance was evaluated under different conditions. First, the CDU’s maximum cooling capacity was evaluated and found to be as high as 89.9 kW at an approach temperature difference (ATD) of 18.3 ◦C with a 0.83 heat exchanger effectiveness. Then, to assess the cooling performance and stability of the CDU, a low-power test and a transient thermohydraulic test were conducted. The results showed instability in the supply fluid temperature (SFT) caused by the oscillation in fan speed at low thermal loads. Despite this, heat removal rates remained constant across varying supply air temperatures (SATs), and a partial power usage effectiveness (PPUE) of 1.042 was achieved at 100 % heat load (86 kW) under different SATs. This research sets a foundation for improving L2A CDU performance and offers practical insights for overcoming current cooling limitations in data centers. 

Abstract Demand is growing for the dense and high-performing IT computing capacity to support artificial intelligence, deep learning, machine learning, autonomous cars, the Internet of Things, etc. This led to an unprecedented growth in transistor density for high-end CPUs and GPUs, creating thermal design power (TDP) of even more than 700 watts for some of the NVIDIA existing GPUs. Cooling these high TDP chips with air cooling comes with a cost of the higher form factor of servers and noise produced by server fans close to the permissible limit. Direct-to-chip cold plate-based liquid cooling is highly efficient and becoming more reliable as the advancement in technology is taking place. Several components are used in the liquid-cooled data centers for the deployment of cold plate-based direct-to-chip liquid cooling like cooling loops, rack manifolds, CDUs, row manifolds, quick disconnects, flow control valves, etc. Row manifolds used in liquid cooling are used to distribute secondary coolant to the rack manifolds. Characterizing these row manifolds to understand the pressure drops and flow distribution for better data center design and energy efficiency is important. In this paper, the methodology is developed to characterize the row manifolds. Water-based coolant Propylene glycol 25% was used as the coolant for the experiments and experiments were conducted at 21 °C coolant supply temperature. Two, six-port row manifolds' P-Q curves were generated, and the value of supply pressure and the flowrate were measured at each port. The results obtained from the experiments were validated by a technique called flow network modeling (FNM). FNM technique uses the overall flow and thermal characteristics to represent the behavior of individual components. 

The escalating information technology (IT) loads in modern data centers (DCs) present formidable challenges for traditional room-conditioning systems. The heat dissipated from IT equipment has witnessed a significant surge due to the rapid development of data processing, retrieval, and storage, driven by changing technology trends and the growing demand for online services. This evolving landscape poses a substantial burden on air-cooling systems, pushing them to their limits, especially with the prevailing trend of rising power densities in microprocessors and the emergence of hot spots. Amidst these challenges, singlephase cold plate cooling is gaining traction as IT power densities experience a dramatic climb. However, the widespread adoption of this cooling method faces impediments such as the limited availability of chilled water supplies, constrained air distribution pathways, and the absence of elevated floors in many older DCs. In response to these limitations, liquid-to-air (L2A) cooling distribution units (CDUs) have emerged as an alternative method. By incorporating hybrid air and liquid cooling technologies, the industry aims to achieve precise, ondemand cooling through the utilization of various techniques. In the realm of hybrid cooling systems that integrate both air and liquid cooling technologies, a partial failure of the Computer Room Air Handlers (CRAH) introduces unique challenges. Such a failure has the potential to disrupt the delicate balance between air and liquid cooling components, leading to uneven heat dissipation. Moreover, the interdependence of liquid and air cooling in hybrid systems means that even a partial failure can trigger a domino effect, reducing the overall cooling efficiency of the system. This comprehensive study delves into the implications of partial failure in the CRAH unit within the highpower density racks of a hybrid-cooled DC. The investigation explores how this partial failure impacts various critical parameters, including cooling capacity (CC), supply air temperature (SAT), air flow rate, supply fluid temperature (SFT), and thermal testing vehicle (TTV) heater case temperatures. For the purposes of this study, two L2A in-row CDUs were utilized, with a combined total heat load of 129 kW supplied to three racks. The experimental setup is meticulously equipped with the necessary instruments for monitoring and assessing tests on both the liquid coolant and air sides. By addressing these issues, the research contributes valuable insights to the ongoing efforts to optimize data center cooling solutions in the face of evolving IT demands and technological advancements. 

Data center cooling systems have undergone a major transformation in the persistent pursuit of better performance and lower energy use. Liquid cooling systems, particularly direct-to-chip systems, have emerged as a promising solution to address the increasing heat dissipation challenges. One critical component of such systems is the filtration mechanism, responsible for safeguarding the integrity and efficiency of the cooling process. These factors are pivotal in ensuring the reliable and sustainable operation of liquid cooling systems in high-demand applications, where electronic components continually push the boundaries of heat generation. This study undertakes a thorough examination of filters of different mesh size used in direct-to-chip liquid cooling systems. The research is multifaceted, encompassing the evaluation of filter performance, pressure drop characteristics, and long-term durability. The methodology employed in this research combines testing with a coolant distribution unit and rack setup to provide a holistic perspective on filter functionality. Findings from this study shed light on the key parameters that influence filter performance. Ultimately, the results of this research promise to contribute significantly to the advancement of direct-to-chip liquid cooling systems, facilitating the continued evolution of electronics in diverse fields, such as high-performance computing, data centers, and emerging technologies. With a focus on enhancing system reliability, efficiency, and sustainability, this study seeks to provide a valuable resource for engineers and researchers in the pursuit of effective cooling solutions for cutting-edge electronic applications. 

Owing to the dramatic increase in IT power density and energy consumption, the data center (DC) sector has started adopting thermally- and energy-efficient liquid cooling methods. This study examines a single-phase direct-to-chip liquid cooling approach for three high-heat-density racks, utilizing two liquid-to-air (L2A) cooled coolant distribution units (CDUs) and a combined total heat load of 128 kW. An experimental setup was developed to test different types of CDUs, cooling loops, and thermal testing vehicles (TTVs) for different operating conditions. IR images and the collected data were used to investigate the effect of air recirculation between cold and hot aisle containments on the CDU’s performance and stability of supply air temperature (SAT). Three different types of cooling loops (X, Y, and Z) were characterized thermally and hydraulically. Results show that Type Y has the lowest cold plate thermal resistance and pressure drop, among others. In a later test that included a single rack at a heat load of 53 kW and a single CDU, the heat capture ratio for fluid was found to be 94%. Experiments show that using blanking panels on the back of the racks limits hot air recirculation and maintains a steady SAT in the cold aisle. Finally, the CDU performance was evaluated at a high heat load for the three racks at 128 kW, and the average cooling capacity of the units is 58.6 kW, and the effectiveness values for CDU 1 and CDU 2 are 0.83 and 0.82, respectively. 

As the online frameworks and services are growing rapidly with the evolution of web-based Artificial Intelligence (AI) applications, server rooms are upgrading in computational capacity and size to keep up with these demands. Enterprise companies with their limited capacity server rooms struggle to keep up with these increasing computational demands. Hence, some of them end up outsourcing their servers to co-located facilities (Co-Lo) and the others choose to upgrade their existing server rooms. Correspondingly, the thermal load associated with such upgrades is typically tremendous. Approximately around 40% of the power consumed by datacentres is dissipated as heat. Conventional HVAC systems fail to satisfy the requirements of such server capacities. Not only do they struggle to fulfil the cooling load, but their maldistribution of cool air into the server room forms a major cause for hotspots formation. To tackle this issue, Liquid-to-Air (L2A) Coolant Distribution Units (CDUs) are being used as a liquid-based cooling solution for rack-level cooling. This type of CDUs provide efficient cooling for servers through liquid coolant that is distributed into cooling loops mounted on top of each server board. The generated heat is curried away using this liquid coolant back to the CDU, which then dissipates it into the surrounding air using dedicated pumps, fans, and heat exchanger, hence the name Liquid-to-Air. In the present work, one of the most popular liquid cooling strategies is explored based on various scenarios. the performance of a 24-kW liquid to Air (L2A) CDU is judged based on cooling effect, stability, and reliability. The study is curried out experimentally, in which a test rack with three thermal test vehicles (TTVs) are used to investigate various operation scenarios. Both liquid coolant and air sides of this experimental setup are equipped with the required instrumentations to monitor and analyse the tests. All test cases were taken in a room with limited air conditioning to resemble the environment of upgraded server rooms with conventional AC systems. Moreover, the impact of using such high-power density cooling unit on the server room environment with restricted HVAC system is also brought to light. Environmental and human comfort parameters such as noise, air velocity, and ambient temperature are measured under various operation conditions and benchmarked against their ranges for human comfort as listed in ASHREE standards. At the end of this research, recommendations for best practice are provided along with areas of enhancement for the selected system. 

Effective cooling is crucial for high-power liquid-cooled servers to ensure optimal performance and reliability ofcomponents. Thermal characterization is necessary to ensure that the cooling system functions as intended, is energy efficient, and minimizes downtime. In this study, a proposed methodology for thermal characterization of a high-powerliquid-cooled server/TTV [server and TTVs (thermal test vehicle) are used interchangeably] is presented. The server layout includes multiple thermal test vehicle setups equipped with direct-to-chip cold plates, with two or more connected in series to form a TTV cooling loop. These cooling loops are connected in parallel to the supply and return plenums of the cooling loop manifold, which includes a chassis-level flow distribution manifold. To obtain accurate measurements, two identical server/TTV prototypes are instrumented with sensors for coolant flow rate and temperature measurements for every TTV cooling loop. Four ultrasonic flow sensors are installed in the flow verification server/TTV to measure the coolant flow rate to each TTV cooling loop. In the thermal verification server, thermistors are installed at the outlet of each GPU heater of TTV cooling loop to log temperature measurements. The amount of heat captured by the coolant in each TTV cooling loop is subsequently estimated based on the flow rates determined from the flow verification server.This methodology enables precise characterization of the thermal performance of high-power liquid-cooled servers,ensuring optimal functionality, energy efficiency, and minimized downtime. 

Abstract Direct Liquid Cooling (DLC) has emerged as a promising technology for thermal management of high-performance computing servers, enabling efficient heat dissipation and reliable operation. Thermal performance is governed by several factors, including the coolant physical properties and flow parameters such as coolant inlet temperature and flow rate. The design and development of the coolant distribution manifold to the Information Technology Equipment (ITE) can significantly impact the overall performance of the computing system. This paper aims to investigate the hydraulic characterization and design validation of a rack-level coolant distribution manifold or rack manifold. To achieve this goal, a custom-built high power-density liquid-cooled ITE rack was assembled, and various cooling loops were plugged into the rack manifold to validate its thermal performance. The rack manifold is responsible for distributing the coolant to each of these cooling loops, which is pumped by a CDU (Coolant Distribution Unit). In this study, pressure drop characteristics of the rack manifold were obtained for flow rates that effectively dissipate the heat loads from the ITE. The pressure drop is a critical parameter in the design of the coolant distribution manifold since it influences the flow rate and ultimately the thermal performance of the system. By measuring the pressure drop at various flow rates, the researchers can accurately determine the optimum flow rate for efficient heat dissipation. Furthermore, 1D flow network and CFD models of the rack-level coolant loop, including the rack manifold, were developed, and validated against experimental test data. The validated models provide a useful tool for the design of facility-level modeling of a liquid-cooled data center. The CFD models enable the researchers to simulate the fluid flow and heat transfer within the cooling system accurately. These models can help to design the coolant distribution manifold at facility level. The results of this study demonstrate the importance of the design and development of the coolant distribution manifold in the thermal performance of a liquid-cooled data center. The study also highlights the usefulness of 1D flow network and CFD models for designing and validating liquid-cooled data center cooling systems. In conclusion, the hydraulic characterization and design validation of a rack-level coolant distribution manifold is critical in achieving efficient thermal management of high-performance computing servers. This study presents a comprehensive approach for hydraulic characterization of the coolant distribution manifold, which can significantly impact the overall thermal performance and reliability of the system. The validated models also provide a useful tool for the design of facility-level modeling of a liquid-cooled data center.