Cold Aisle Research Articles

Investigation of the Mechanism of Temperature Rise in a Data Center With Cold Aisle Containment

This paper theoretically investigates the relationships among factors that affect the temperature rise of server racks and experimentally tests the influence of variable space contained arrangements on the thermal performance. To express the flow and heat transfer process of cold air in servers and analyze the critical factors affecting the temperature rise, a simplified mathematical model representing servers is developed using experimental results. An experiment is conducted within a modular data center in which cold air is supplied from a raised floor. The experiment employed a variable space of cold aisle containment and measured the resulting temperature rise, as well as pressure difference of racks and other parameters, in the simplified mathematical model. By comparing the experimental results and theoretical calculation, the theoretical model is proved to be reasonable and valid. The model predicts that the critical factors affecting the temperature rise of racks consist of static and dynamic pressure difference, total pressure of the fans, geometric structure, power consumption, resistance of doors, and opening area of servers. The result shows that the factor affected by the cold aisle contained system is the static pressure, while for the dynamic pressure difference, the contained architecture has a slight positive effect. Although the average temperature rise is quite decreased in the contained system, the static pressure distribution is nonuniform. A half-contained system which reduced contained space ratio to 50% is measured to cause a 22% increase of the static pressure difference, making a more uniform temperature distribution.

Impact of Server Thermal Design on the Cooling Efficiency: Chassis Design

There are various designs for segregating hot and cold air in data centers such as cold aisle containment (CAC), hot aisle containment (HAC), and chimney exhaust rack. These containment systems have different characteristics and impose various conditions on the information technology equipment (ITE). One common issue in HAC systems is a pressure build-up inside the HAC (known as backpressure). Backpressure also can be present in CAC systems in case of airflow imbalances. Hot air recirculation, limited cooling airflow rate in servers, and reversed flow through ITE with weaker fan systems (e.g., network switches) are some known consequences of backpressure. Currently, there is a lack of experimental data on the interdependency between overall performance of ITE and its internal design when backpressure is imposed on ITE. In this paper, three commercial 2-rack unit (RU) servers with different internal designs from various generations and performance levels are tested and analyzed under various environmental conditions. Smoke tests and thermal imaging are implemented to study the airflow patterns inside the tested equipment. In addition, the impact of hot air leakage into the servers through chassis perforations on the fan speed and the power consumption of the servers are studied. Furthermore, the cause of the discrepancy between measured inlet temperatures by the intelligent platform management interface (IPMI) and external sensors is investigated. It is found that arrangement of fans, segregation of space upstream and downstream of fans, leakage paths, the location of baseboard management controller (BMC) sensors, and the presence of backpressure can have a significant impact on ITE power and cooling efficiency.

Experimental investigation on thermal management for small container data center

The energy consumptions of data centers are increasing dramatically and cooling systems using for controlling the operating temperature usually consume more than half of these energy. Thus, effective strategies on thermal management in data centers may have benefit on energy savings. In order to improve the power usage effectiveness (PUE) of data centers, the effects of air supply flowrate by computer room air handler (CRAH), intake flowrate of rack cooling fans, air supply grilles and heat load distribution on the cooling performance of a typical small container data center having overhead air supply system are investigated in this paper. The rack cooling index (RCI) and supply heat index (SHI) are incorporated to evaluate the local and average cooling performance of data racks. It is found that the intake flowrate of rack cooling fans plays dominant role, which is suggested to be greater than the air supply flowrate in normal operation. Meanwhile, the volumetric flowrate distribution of grilles is studied. The additional front grille before entering the cold aisle may improve the RCI and SHI of front rack (rack A), in which severe hot-air recirculation may occur in typical operation. Conversely, additional rear grille at the exit of cold aisle has negative effect on cooling performance of the last rack (rack E). Based on the measurement of local temperature distribution, the non-uniform heat load distribution in racks is proposed. Result shows that the RCI can be improved to 64% if more power is placed at lower half part, which is eligible for energy saving. However, the non-uniform heat load distribution shows limited improvement on the rear rack (rack E).

Experimentally Validated Computational Fluid Dynamics Model for Data Center With Active Tiles

Abstract This paper presents an experimentally validated room-level computational fluid dynamics (CFD) model for raised-floor data center configurations employing active tiles. Active tiles are perforated floor tiles with integrated fans, which increase the local volume flow rate by redistributing the cold air supplied by the computer room air conditioning (CRAC) unit to the under-floor plenum. The numerical model of the data center room consists of one cold aisle with 12 racks arranged on both sides and three CRAC units sited around the periphery of the room. The commercial CFD software package futurefacilities6sigmadcx is used to develop the model for three configurations: (a) an aisle populated with ten (i.e., all) passive tiles; (b) a single active tile and nine passive tiles in the cold aisle; and (c) an aisle populated with all active tiles. The predictions from the CFD model are found to be in good agreement with the experimental data, with an average discrepancy between the measured and computed values for total flow rate and rack inlet temperature less than 4% and 1.7 °C, respectively. The validated models were then used to simulate steady-state and transient scenarios following cooling failure. This physics-based and experimentally validated room-level model can be used for temperature and flow distributions prediction and identifying optimal number and locations of active tiles for hot spot mitigation in data centers.

Model Predictive Control for Energy-Efficient Operations of Data Centers with Cold Aisle Containments

In this paper, we study a problem of controlling cooling facilities and computational equipments for energy-efficient operations of data centers. Although a plethora of approaches have been proposed in previous literatures, there is a lack of rigorous methodologies for effectively addressing the problem. To bridge this gap, we propose MPC frameworks for jointly and optimally tuning the Computer Room Air Conditioner (CRAC) supplying air temperature as well as the number of active servers for minimizing the overall energy consumption. We specifically find that, when a standard model of data centers with contained cold aisles and cooling facilities are used, the optimization problems arising from the MPC framework can be transformed to convex optimization problems that can be solved efficiently. We present several numerical simulations to illustrate the effectiveness of our theoretical results.

Parametric optimisation of raised-floor data centres to enhance the thermal performance

Advancement in IT technology and the increasing demand in the processing and storage requirements have led to an increase in hardware densities, causing a significant cooling challenge in data centre operation. Extensive energy consumption and operational costs in data centre management are the main concerns that have led many engineers and researchers to develop numerous methods and recommendations to improve the efficiency of the cooling system. Establishing the temperature and airflow pattern for a typical data centre with hot and cold aisles is essential for the efficient thermal management. In this study, thermal analysis of a raised-floor data centre, using computational fluid dynamics (CFD), is undertaken. Factors including room layout, CRAC location, percentage opening of perforated tile, air distribution system that contribute to efficient cooling in data centre design were studied. To further understand the capability of each design, five investigations were conducted of separate components of the configuration to observe their individual impacts on the overall data centre performance. Various design solutions to reduce the energy consumption and cost in the data centre are proposed based on the findings.

Thermal Performance Evaluation of a New Close-Coupled Cooling Solution including Cooling Failure Analysis

The objective of this work is to introduce and evaluate a new end-of-aisle cooling design which consists of three cooling configurations. The key objectives of close-coupled cooling are to enable controlled cooling of information technology (IT) equipment, flexible and modular design, and the containment of hot air exhaust from the cold air. The thermal performance of the proposed solution is evaluated using computational fluid dynamics modeling. A computational model of a small size data center room has been developed. The room is modeled to be a hot aisle containment setup, i.e., the hot air exhaust exiting for each row is contained and directed within a specific volume. The cold aisle is separated from the hot aisle by means of banks of heat exchangers (HXs) placed on either side of the containment aisle. Based on the placement of rack fans, the design is divided into three sub-designs—Case 1: passive HXs with rack fan walls; Case 2: active HXs (coupled with fans) with rack fan walls; Case 3: active HXs (coupled with fans) with no rack fans. The cooling performance is calculated based on the thermal and flow parameters obtained for all three configurations. The computational data obtained has shown that the Case 1 is used only for lower system resistance IT. However, Cases 2 and 3 can handle denser IT systems. Case 3 is the design that can consume lower fan energy and handle denser IT systems. The article also discusses the cooling behavior of each type of design under cooling failure conditions with Case 2 showing better cooling redundancy compared with other two cases.

Optimization of Enclosed Aisle Data Centers With Induced CRAH Bypass

Aisle containment systems reduce temperature nonuniformities in traditional air-cooled data centers and pave the way for higher cooling air temperature and more efficient operation of the cooling system and wider application of economizers. However, enclosing the aisle does not guarantee the lowest possible cooling infrastructure power. Enclosed air (EA) configurations require computer room air handler (CRAH) fans to provide the entire rack air flowthrough the perforated tiles, as well as the leakage flow that inherently exists in data centers. In this paper, we propose the installation of induction bypass (BP) fans beneath the cold aisle (CA) perforated tiles that draw air from the plenum, placing the latter under a slightly depressed pressure. In doing so, warm air from the hot aisle is drawn into the plenum, where it is mixed with a smaller amount of cold air from the CRAH. As a result, the CRAH fans operate at lower speeds and consume less power, but the air passing through the CRAH is cooled to a lower temperature so that the air supplied to the enclosed CA remains at or slightly below the server redline temperature (~27 °C). Detailed optimization based on hour-by-hour annual energy simulations for seven U.S. cities has shown that the proposed induced CRAH BP scheme can reduce combined chiller and air-moving power consumption substantially in EA data centers with or without an airside economizer.

Experimental Demonstration and Flow Network Model Verification of Induced CRAH Bypass for Cooling Optimization of Enclosed-Aisle Data Centers

Aisle containment separates hot and cold aisles in air-cooled data centers to prevent hot air recirculation into cold aisle and decrease temperature nonuniformity among the servers. Uniform server inlet temperatures allow the cooling system to operate more efficiently at a higher evaporator temperature. Lower cooling power consumption can be achieved by optimizing the combined power consumption of the chillers and the computer room air handlers (CRAHs) fans. CRAH bypass (BP) is proposed, in which additional fans at the tiles induce a fraction of tile flow from the room into the plenum through low-resistance ports or leakage paths, thus decreasing the airflow passing through the high pressure resistance of the CRAH heat exchangers and filters and associated fan power usage. A further advantage of the proposed induced CRAH BP is the elimination of leakage as a result of reversing the room-plenum pressure difference. However, in order to keep the enclosed-aisle temperature below acceptable limits (typically ≤27 °C), the chiller needs to operate at a lower, less efficient temperature. This paper presents the experimental verification of a flow network model (FNM) in a data center test cell demonstrating the proposed induced CRAH BP concept. Exercise of the verified FNM in conjunction with a thermodynamic model of the cooling infrastructure (TDM) confirms that there is an optimum BP fraction that minimizes the combined chiller and CRAH fan power consumption.

Airflow Management on the Efficiency Index of a Container Data Center Having Overhead Air Supply

Effect of airflow managements on the efficiency index of a small container data center having overhead air supply is reported in this study. Seventeen arrangements and configurations regarding the airflow and blockage arrangements are experimentally examined and compared. Test results indicate an appreciable hot air recirculation occurring for rack arrangement without any blockage, and the hot spot occurs at the second rack alongside the cold aisle. The hot spot had moved to the first rack when the blockage plate is installed on the rack top. Rack locations relative to air handler casts a negligible effect on the efficiency index, and it is comparatively more effective by sealing the trailing of the cold aisle. A smaller cold-aisle spacing helps to lower the temperature distribution, and an additional opening of the supplied vent will not help in removal of hot spot. Shutting off the grille in the center of cold aisle is also unable to fix the hot air recirculation and may even incur hot air reversal. The hot air reversal can be removed by adding additional blockage plate at the flow reversal section. Higher supplied air flow rate also improves the efficiency index considerably.

Studying the fan-assisted cooling using the Taguchi approach in open and closed data centers

This research examined the application of fan-assisted cooling within open and contained raised-floor data centers with the intention of assessing the effectiveness of the system’s cooling method. A computational fluid dynamics (CFD) approach that incorporated both airflow and thermal analysis was employed to examine the detailed effectiveness of fan-assisted air-cooling in a data center that exhibited the behavior of under-cabinet leakage. The CFD technique was also used to assess the impact that three distinct aspects had on the performance of the fan-assisted air-cooling system: cold aisle containment, straightened air flow and fan-to-tile distance. In addition, the mixed-level Taguchi statistical approach was applied to understand how variations in these parameters directly and interactively modified the heat transfer in the surroundings of the inlets of the server racks. The results of the research reveal that fan-assisted perforations, taking the combination of the key factors (the main and interact effects) into account, would represent a viable means through which the cooling systems employed in both open and closed aisle data centers can be enhanced. The well-constructed CFD-based Taguchi methodology might yield a good level of validity for use in substantial and timely optimization procedures to further improve the cooling effectiveness and efficiencies of data centers.

A novel approach to the data center hybrid cooling design with containment

ABSTRACTHybrid cooling solutions, which consist of the installation of heat exchangers near IT equipment, aim to eliminate hot spots in data centers containing high-density computer cabinets. In addition, cold aisle containment is often used to reduce hot air recirculation to improve energy efficiency. However, the viability and efficiency of each hybrid cooling strategy with or without containment depend on the IT load and equipment arrangement, and no formal procedure exists for selecting the most efficient strategy for a given application. Therefore, this study provides a computational approach for ranking the performance of different cooling strategies based on their capacity and cooling efficiency. The results of analyses indicate that applying containment is beneficial in (1) lowering the maximum temperature of the air entering the racks as airflow rates are increased, and (2) increasing the uniformity of rack inlet temperatures. However, applying containment also requires additional mechanical work by the computer room air handler (CRAH) fan, which may raise the data center power usage effectiveness (PUE). Application of the computational approach discussed here highlights the use of hybrid cooling to lower PUE by reducing the CRAH fan power.

Investigation and improvement of air distribution system’s airflow performance in data centers

To improve cooling and energy utilization efficiency, under-floor flexible baffles (UFBs) was applied to transform the under-floor airflow pattern in this paper. The application of UFBs impacted the airflow pattern within the Cold aisle containment (CAC). As a result, the exhaust air temperatures of racks were reduced and the exhaust air temperature distribution of the racks was better distributed. The results indicated that, although the use of CAC reduced the exhaust air temperature of racks and the maximum temperature of local hotspot, the air temperature distribution of the racks presented as a non-uniform distribution and the temperature of local hotspot was still high. However, not only could UFBs further reduce exhaust air temperature, but also it made the exhaust airflow more uniform. When the angle between the baffle and CRACs outlet air orientation was at 45, the maximum temperature of local hotspots dropped most, which was from 30.5℃ to 28.4℃ and achieved a 2.1℃ temperature drop. Under the circumstance, the exhaust air temperature distribution was more uniform compared to the data center with CAC but without UFBs.

Effect of raised floor height on different arrangement of under-floor air distribution performance in data center

In the more and more expanding and complicated data center, requirements on air conditioning system and the air distribution arrangement are getting higher and higher. One of the resolutions is improving the airflow uniformity and cooling the local over-hot area by raising the floor in the data center. With a typical mode of four rows of cabinets in the data center as a sample. This paper uses Computational fluid dynamic (CFD) to investigate the influence of the six raised floor heights (0.2m, 0.4m, 0.6m, 0.8m, 1m and 1.2m) on the airflow distribution of three types of under-floor air distribution (the open aisle, the cold aisle containment and the hot aisle containment). Optimization is done on performance indexes, including supply/return heat index (SHI/RHI), return cooling index (RCI) and return temperature index (RTI), with which the condition of subcooling / overheating of all cabinets and the characteristics of the airflow organization in the data center are reflected and the thermal environment in the data center is revealed. Normally, the cooling effect and the uniformity of the airflow can be improved by raising the floor height. However, this won’t work well when the floor height reaches a certain level. Therefore, based on the comparative analysis on the 18 different cases in this investigation, the recommended range of raised floor height in data centers of different construction can be obtained and verified in the experimental tests. The experimental results turn out to be in accordance with the simulation results, which proves the range recommended in this investigation is valuable.

Enclosed aisle effect on cooling efficiency in small scale data center

Taking actual computer room of small scale data center in project as an example, airflow distribution of computer room in different aisle types were simulated, and temperature and velocity field were analyzed. An experiment was also carried out for the newly used computer room. The study shows that cold and hot air mix at two sides and top of free open cold and hot aisles, which reduces the utilization of cold air. Cooling effect of free open, semi-enclosed and full enclosed cold aisle turns better; and the effect of semi-enclosed cold aisle is obvious, which is recommended to use. Computer room’s temperatures of full enclosed hot aisle are lower than that of full enclosed cold aisle. So the former has a slight advantage, but requires more aisle partitions and higher cost. When the heat quantity of computer room is small, the natural cooling capacity of outdoor air in winter can meet the cooling requirement. Test data also show that enclosed cold aisle can improve the cooling efficiency.

Optimization of Cooling System for Data Center Case Study: PAU ITB Data Center

Data center is a facility that consumes a lot of energy. From the total energy used by data center, IT equipments as its major component consume energy by 50%, and cooling system consumes energy by 37%. There are various strategies to improve the efficiency of the cooling system in order to save energy consumption. Because data center operates continuously, so that the potential of disturbance and shut down due to overheating should be avoided. Optimization is performed by using the Computational Fluid Dynamics (CFD) simulation. Assessment of PAU data center shows that the metric Rack Cooling Index (RCI) for data center cooling systemis 100% and the metric Power Usage Effectiveness (PUE) is 2.04. This value indicates that the energy consumption of data centercooling system is not efficient.Simulation results indicate the presence of mixing air between hot air and cold air and cause less efficient of the cooling process. Optimization is carried out by adding blanking panels, cold aisle containment, and changing the layout of the components and shelves. The result shows that the value of RCI achieved at 100% for RCIhigh and 99% for RCIlow. Meanwhile, the PUE value is reduced to 1.92. The energy used by cooling system is also reduced by 1.28kW.

Optimization of Airflow Organization for a Small-scale Date Center Based on the Cold Aisle Closure

In order to maintain a stable thermal environment of the data center, air-conditioning systems for the data center are in high energy consumption operation state yearly. Airflow organization in a data center impacts the cooling effectiveness of the air-conditioning systems significantly. This paper investigated a small-scale data center by combine of measurement and CFD simulation. To supply accurate boundary conditions, site measurement was conducted to validate the simulation results. In this paper, three optimization approaches of airflow organization were proposed and simulated by CFD software. The cooling performance of these approaches were compared by the temperature of 16 points on the same plane (Z=1.5m) in the room. The results indicated that cold aisle closure could eliminate by-pass airflow but also worsen thermal environment. Setting air return on both sides of hot aisle got most significant improvement in cooling performance. The temperature of hot points could be decreased by 0.5~5°C.

On cold-aisle containment of a container datacenter

In this study, influences of various blockage arrangements as well as cold-aisle containment on the overall efficiency of a container datacenter are reported. The test container datacenter contains ten racks and the cold air is supplied from drop ceiling. The experiments are performed with each rack equally delivering a power of 3kW. Eight different layouts are examined. Appreciable hot air recirculation prevails especially at the entrance of cold aisle. Adding blockage plates to the computer room air handler (CRAH) at the entrance of computer racks only improve marginally the associated rack cooling index (RCI). Layout with enclosing the end of cold aisle while let open the entrance of cold aisle also performs poorly. Full containment of the cold aisle with high jet air flowrate from the supplied grilles gives the worst performance. Layout with enclosing at the entrance of cold aisle but let open the end of cold aisle shows a much superior temperature distribution and a rather high RCI of 99%. Higher supplied flow may induce jet airflow pattern, causing the Coanda effect to result in fluctuating behaviors of RCI and SHI alongside the cold aisle (zigzag phenomenon). A lower supplied air flowrate without Coanda effect for a full containment design give the best overall performance.

파티션 시스템 적용을 통한 기존 데이터센터 서버실의 냉방 에너지 절감 성능평가

본 연구에서는 기존 데이터센터 서버실에서 공기분배시스템으로 파티션 시스템의 적용성을 평가하기 위해 기존 시스템 그리고 파티션의 설치 높이와 위치를 변수로 하는 파티션 시스템과 컨테인먼트 시스템에 대해 총 21가지 경우의 컴퓨터시뮬레이션을 실시하고 다음과 같은 결론을 얻었다. 공기온도 및 기류 분포 해석결과를 이용하여 평가해 본 공기분배시스템별 냉방 에너지 절감 성능은 컨테인먼트 시스템, 파티션 시스템, 기존 시스템 순서로 우수한 것을 확인할 수 있었다. 기존 시스템과 파티션 설치높이 0.1~0.7m까지의 파티션 시스템은 재순환 공기에 의해 냉복도에서 1.0~1.2m 높이를 기점으로 공기 온도가 급격히 상승하면서 중간 높이의 서버와 상부 서버의 인입구 온도차는 <TEX>$11{\sim}15^{\circ}C$</TEX>로 큰 차이를 보여, 재순환 공기가 상부 서버의 과열의 원인을 제공하는 것으로 나타났다. 파티션 시스템에서 냉복도 상부에 파티션을 설치할 경우에 파티션의 적정 높이는 서버 랙 상부에서 천장까지 높이의 90%(0.9m)이상, 열복도 상부에 파티션을 설치하는 경우는 파티션 높이가 80%(0.8m)이상은 되어야 재순환 공기를 충분히 차단하여 서버 인입구 공기온도가 RCI 권장온도 범위를 만족하는 냉방 에너지 절감효과를 볼 수 있는 것으로 나타났다. In this study, a computer simulation of the three types of air distribution systems, open type system, aisle partition system and aisle containment system, to evaluate the applicability of the aisle partition system on a data center server room. The variables of the simulation were the height and location of the partition fixed on the top server rack. The energy efficiency of the air distribution systems were confirmed to be excellent in the order of the aisle containment system, aisle partition system, and open type system. In the cold aisle partition system, the height of the partition that can be effective in saving cooling energy by obstructing sufficient air recirculation was found to be more than 0.9m. In the hot aisle partition system, the height of the partition was found to be more than 0.8m.

Experimental investigation on the thermal performance of water cooled multi-split heat pipe system (MSHPS) for space cooling in modular data centers

A water cooled multi-split heat pipe system (MSHPS) was proposed to cool the space in modular data centers. It combines high thermal performance heat pipe technology with cold aisle containment architecture technology to remove heat effectively and minimize the short-circuiting of the hot air within the IT server. Experimental investigations were conducted to identify the optimal refrigerant filling ratio and the effect of different inlet air temperatures and different inlet cooling water temperatures on the unit of MSHPS. As a result of experiments, the optimal refrigerant filling ratio was found in the range from 33% to 42% with cooling capacity range from 6100W to 6200W and the system thermal resistance of evaporation process ranged from 0.0028K/W to 0.0030K/W. The effect of inlet temperature of air and cooling water were analyzed in terms of the cooling capacity, refrigerant temperature and pressure distribution along the unit of MSHPS. These results are useful for the design and operational control of the MSHPS for modular data centers.