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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. 

Increasing demands for cloud-based computing and storage, the Internet of Things and machine learning-based applications have necessitated the use of more eficient cooling technologies. Direct-to-chip liquid cooling using cold plates has proven to be one of the most effective methods to dissipate the high heat luxes of modern high-power CPUs and graphics processing units (GPU). While the published literature has well-documented research on the thermal aspects of direct liquid cooling, a detailed account of reliability degradation is missing. The present investigation provides an in-depth experimental analysis of the accelerated degradation of copper cold plates used in high-power direct-to-chip liquid cooling in data centers. 

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. 

Abstract This paper proposes a computational fluid dynamics (CFD) simulation methodology for the multi-design variable optimization of heat sinks for natural convection single-phase immersion cooling of high power-density Data Center server electronics. Immersion cooling provides the capability to cool higher power-densities than air cooling. Due to this, retrofitting Data Center servers initially designed for air-cooling for immersion cooling is of interest. A common area of improvement is in optimizing the air-cooled component heat sinks for the fluid and thermal properties of liquid cooling dielectric fluids. Current heat sink optimization methodologies for immersion cooling demonstrated within the literature rely on a server-level optimization approach. This paper proposes a server-agnostic approach to immersion cooling heat sink optimization by developing a heat sink-level CFD to generate a dataset of optimized heat sinks for a range of variable input parameters: inlet fluid temperature, power dissipation, fin thickness, and number of fins. The objective function of optimization is minimizing heat sink thermal resistance. This research demonstrates an effective modeling and optimization approach for heat sinks. The optimized heat sink designs exhibit improved cooling performance and reduced pressure drop compared to traditional heat sink designs. This study also shows the importance of considering multiple design variables in the heat sink optimization process and extends immersion heat sink optimization beyond server-dependent solutions. The proposed approach can also be extended to other cooling techniques and applications, where optimizing the design variables of heat sinks can improve cooling performance and reduce energy consumption. 

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 In recent years there has been a phenomenal development in cloud computing, networking, virtualization, and storage, which has increased the demand for high performance data centers. The demand for higher CPU (Central Processing Unit) performance and increasing Thermal Design Power (TDP) trends in the industry needs advanced methods of cooling systems that offer high heat transfer capabilities. Maintaining the CPU temperature within the specified limitation with air-cooled servers becomes a challenge after a certain TDP threshold. Among the equipments used in data centers, energy consumption of a cooling system is significantly large and is typically estimated to be over 40% of the total energy consumed. Advancements in Dual In-line Memory Modules (DIMMs) and the CPU compatibility led to overall higher server power consumption. Recent trends show DIMMs consume up to or above 20W each and each CPU can support up to 12 DIMM channels. Therefore, in a data center where high-power dense compute systems are packed together, it demands efficient cooling for the overall server components. In single-phase immersion cooling technology, electronic components or servers are typically submerged in a thermally conductive dielectric fluid allowing it to dissipate heat from all the electronics. The broader focus of this research is to investigate the heat transfer and flow behavior in a 1U air cooled spread core configuration server with heat sinks compared to cold plates attached in series in an immersion environment. Cold plates have extremely low thermal resistance compared to standard air cooled heatsinks. Generally, immersion fluids are dielectric, and fluids used in cold plates are electrically conductive which exposes several problems. In this study, we focus only on understanding the thermal and flow behavior, but it is important to address the challenges associated with it. The coolant used for cold plate is 25% Propylene Glycol water mixture and the fluid used in the tank is a commercially available synthetic dielectric fluid EC-100. A Computational Fluid Dynamics (CFD) model is built in such a way that only the CPUs are cooled using cold plates and the auxiliary electronic components are cooled by the immersion fluid. A baseline CFD model using an air-cooled server with heat sinks is compared to the immersion cold server with cold plates attached to the CPU. The server model has a compact model for cold plate representing thermal resistance and pressure drop. Results of the study discuss the impact on CPU temperatures for various fluid inlet conditions and predict the cooling capability of the integrated cold plate in immersion environment. 

Abstract Data centers are critical to the functioning of modern society as they host digital infrastructure. However, data centers can consume significant amounts of energy, and a substantial amount of this energy goes to cooling systems. Efficient thermal management of information technology equipment is therefore essential and allows the user to obtain peak performance from a system and enables higher equipment reliability. Thermal management of data center electronics is becoming more challenging due to rising power densities at the chip level. Cooling technologies like single-phase immersion cooling allow overcoming many such challenges owing to their higher thermal mass, lower fluid pumping powers, and potential component reliability enhancements. It is known that immersion cooling deployments require extremely low coolant flow rates, and, in many cases, natural convection can also be used to sufficiently dissipate the heat from the hot server components. It, therefore, becomes difficult to ascertain whether the rate of heat transfer is being dominated by forced or natural convection. This may lead to ambiguity in choosing an optimal heat sink solution and a suitable system mechanical design due to unknown flow regimes, further leading to sub-optimal system performance. Mixed convection can be used to enhance heat transfer in immersion cooling systems. The present investigation quantifies the contribution of mixed convection using numerical methods in an immersion-cooled server. An open compute server with dual CPU sockets is modeled on Ansys Icepak with varying power loads of 115W, 160W and 200W. The chosen dielectric fluid for this single-phase immersion-cooled setup is EC-100. Steady-state Computational Fluid Dynamics (CFD) simulations are conducted for forced, natural, and mixed convection heat transfer in a thermally shadowed server configuration at varying inlet flow rates. A baseline heat sink and an optimized heat sink with an increased fin thickness and reduced fin count are utilized for performance comparison. The effect of varying Reynolds number and Richardson number on the heat transfer rate from the heat sink is discussed to assess the flow regime, stability of the flow around the submerged components which depends on the geometry, orientation, fluid properties, flow rate and direction of the flow. The dimensionless numbers’ influence on heat transfer rate from a conventional air-cooled heat sink in immersion versus an immersion-optimized heat sink is also compared. The impact of server orientation on heat transfer behavior for the immersion optimized heat sink is also studied on heat transfer behavior for the immersion optimized heat sink. 

Abstract Data centers have complex environments that undergo constant changes due to fluctuations in IT load, commissioning and decommissioning of IT equipment, heterogeneous rack architectures and varying environmental conditions. These dynamic factors often pose challenges in effectively provisioning cooling systems, resulting in higher energy consumption. To address this issue, it is crucial to consider data center thermal heterogeneity when allocating workloads and controlling cooling, as it can impact operational efficiency. Computational Fluid Dynamics (CFD) models are used to simulate data center heterogeneity and analyze the impact of two different cooling mechanisms on operational efficiency. This research focuses on comparing the cooling based on facility water for Rear Door Heat Exchanger (RDHx) and conventional Computer Room Air Conditioning (CRAH) systems in two different data center configurations. Efficiency is measured in terms of ΔT across facility water. Higher ΔT will result in efficient operation of chillers. The actual chiller efficiency is not calculated as it would depend on local ambient conditions in which the chiller is operated. The first data center model represents a typical enterpriselevel configuration where all servers and racks have homogeneous IT power. The second model represents a colocation facility where server/rack power configurations are randomly distributed. These models predict temperature variations at different locations based on IT workload and cooling parameters. Traditionally, CRAH configurations are selected based on total IT power consumption, rack power density, and required cooling capacity for the entire data center space. On the other hand, RDHx can be scaled based on individual rack power density, offering localized cooling advantages. Multiple workload distribution scenarios were simulated for both CRAH and RDHx-based data center models. The results showed that RDHx provides a uniform thermal profile across the data center, irrespective of server/rack power density or workload distribution. This characteristic reduces the risk of over- or under-provisioning racks when using RDHx. Operational efficiency is compared in terms of difference in supply and return temperature of facility water for CRAH and RDHx units based on spatial heat dissipation and workload distribution. RDHx demonstrated excellent cooling capabilities while maintaining a higher ΔT, resulting in reduced cooling energy consumption, operational carbon footprint (?), and water usage. 

Abstract The increasing demand for high-performance computing in applications such as the Internet of Things, Deep Learning, Big data for crypto-mining, virtual reality, healthcare research on genomic sequencing, cancer treatment, etc. have led to the growth of hyperscale data centers. To meet the cooling energy demands of HPC datacenters efficient cooling technologies must be adopted. Traditional air cooling, direct-to-chip liquid cooling, and immersion are some of those methods. Among all, Liquid cooling is superior compared to various air-cooling methods in terms of energy consumption. Direct on-chip cooling using cold plate technology is one such method used in removing heat from high-power electronic components such as CPUs and GPUs in a broader sense. Over the years Thermal Design Power (TDP) is rapidly increasing and will continue to increase in the coming years for not only CPUs and GPUs but also associated electronic components like DRAMs, Platform Control Hub (PCH), and other I/O chipsets on a typical server board. Therefore, unlike air hybrid cooling which uses liquid for cold plates and air as the secondary medium of cooling the associated electronics, we foresee using immersion-based fluids to cool the rest of the electronics in the server. The broader focus of this research is to study the effects of adopting immersion cooling, with integrated cold plates for high-performance systems. Although there are several other factors involved in the study, the focus of this paper will be the optimization of cold plate microchannels for immersion-based fluids in an immersion-cooled environment. Since immersion fluids are dielectric and the fluids used in cold plates are conductive, it exposes us to a major risk of leakage into the tank and short-circuiting the electronics. Therefore, we propose using the immersed fluid to pump into the cold plate. However, it leads to a suspicion of poor thermal performance and associated pumping power due to the difference in viscosity and other fluid properties. To address the thermal and flow performance, the objective is to optimize the cold plate microchannel fin parameters based on thermal and flow performance by evaluating thermal resistance and pressure drop across the cold plate. The detailed CFD model and optimization of the cold plate were done using Ansys Icepak and Ansys OptiSLang respectively. 

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.