Perforated Tiles Research Articles

Thermal Management and Energy Consumption in Air, Liquid, and Free Cooling Systems for Data Centers: A Review

The thermal management and reduction of energy consumption in cooling systems have become major trends with the continued growth of high heat dissipation data centers and the challenging energy situation. However, the existing studies have been limited to studying the influences of individual factors on energy saving and thermal management and have not been systematically summarized. Thus, this paper reviews the key factors in achieving thermal management and reducing energy consumption in each cooling system, the corresponding research, and optimization methods. To achieve these goals, in this paper, literature surveys on data center cooling systems are investigated. For data center air cooling, thermal management is mainly related to the uniform distribution of hot and cold air. Adjusting the porosity of perforated tiles can reduce energy consumption. For liquid cooling and free cooling systems, climate conditions, cooling system structural design, coolant type, and flow rate are key factors in achieving thermal management and reducing energy consumption. This paper provides the power usage effectiveness (PUE) values of the cooling systems in some cases. A summary of the key factors can provide directions for research on thermal management and energy reduction, and a summary of previous research can provide a basis for future optimization.

Experimental analysis of airflow uniformity and energy consumption in data centers

Temperature and airflow uniformity are experimentally investigated in this research study along with energy consumption analysis on the laboratory container size small data center. A raised floor plenum (RFP) data center system of size 6 (L) × 3.1 (W) × 2.3 (H) m with one Computer Room Air Handler (CRAH), two rows and each row contains five racks, five 8U simulator cabinets in each rack was constructed to test partial and closed cold aisle containment (CAC). Seven different cases were investigated with variables of perforated tiles (32%, 50 % & 70%), CRAH fan speed (60 Hz, 42 Hz), chiller return water temperature (CRWT) (15 °C & 17 °C) and heating load (full 50 kW, 75% load 37.5 kW). These cases were further analyzed using Rack Cooling Index (RCI) and Supply Heat Index (SHI) along with power consumption scenarios in terms of Coefficient of Performance (COP) and Power Usage Effectiveness (PUE). The uniqueness of this study is experimental investigations of system COP and PUE analysis for a wide range of important variables for partial and closed CAC. It was found that 50% porous tile provided the best airflow and temperature uniformity for the case of partial containment with full and 25% reduced heat load along with full and 30% reduced CRAH speed respectively. Slight recirculation was observed during the reduced CRAH fan speed case at the corner edge of data racks. However, due to additional supply airflow usage, these recirculation’s were within the acceptable SHI. Reduction of CRWT reduces the total Heating Ventilation and Air Conditioning (HVAC) power consumption and improves the overall system COP.

Simplified CFD Model for Perforated Tile with Distorted Outflow

Most of the perforated tile flow CFD models in the literature so far assume the air velocity coming out of the tile openings (pores) is uniform. However, in typical applications, such as data centers and indoor environments, perforated tile or diffuser outflow can be highly non-uniform due to many reasons (e.g., spatial variation of plenum pressure or varying local tile geometrical patterns). For an ideal (uniform) tile flow velocity that has the same flow rate as the non-uniform tile flow velocity, the tile flow momentum of the latter will always be greater because momentum scales with velocity squared. To illustrate the effect of tile flow velocity distortion, two generic CFD cases (one with uniform velocity and the other with non-uniform velocity) with multiple openings model are presented here. Their CFD results are compared to the momentum source model results and validated against previously published data of an isolated tile flow measurement. The momentum source model is one of the simplest/most practical CFD models that uses a body-force value for correcting the momentum deficit between perforated and fully open areas. Initially, the momentum source model results show good agreement with results of the uniform velocity case only. Thus, due to velocity distortion (non-uniform velocity) in the tile flow, the CFD results presented in this paper show a potential reason to modify the body-force value in the momentum source model with an adjustment factor (C). Several values of C factor are numerically investigated for the present distorted tile flow CFD case, and the best match is found to be at C, equaling approximately 1.6.

Experimental and numerical study on the thermal environment of a data center

Data centers are energy-intensive facilities, and a suitable thermal environment is vital for the safe and reliable operation of information technology equipment and energy saving. In this case study, field experiments and thermal environment evaluation were conducted in an existing data center. However, some problems were reported, including the relative humidity of the rack inlet air exceeding 60%, hot-cold air mixing, non-uniform temperature distribution, local overheating, and a low average temperature difference between the return and supply air of computing room air conditioning units (CRACs). To overcome these dilemmas, three cost-efficient and convenient methods for improving the thermal environment were proposed, including installing blanking panels, closing unnecessary perforated tiles, and increasing the set return air temperature of CRACs. The temperature field and flow field were simulated using the software 6SigmaRoom to predict the effects of the optimization methods. Finally, after the optimization scheme was implemented, field experiments were performed again to examine the application effects of optimization measures. The results demonstrate that the temperature distribution was more uniform, and the thermal environment was considerably improved. Meanwhile, the average temperature difference between the return and supply air of CRACs increased from 6.7 °C to 8.8 °C, resulting in a decrease in the supply air volume of CRACs of about 10903.37 m3/h, and saving the energy consumption of the fans around 96 kWh/day.

Optimization on jet‐induced ventilation to enhance the uniformity of airflow distribution in data center

AbstractImproving the utilization efficiency of cold airflow in data center (DC) has already attracted widespread concern. A broad consensus has been reached that cold/hot‐aisle containment technologies can reduce the mixture of cold and hot air to weaken the overheating phenomenon of the rack in a certain extent. However, local hot spots still occur in the front racks due to lower tile flow rate at the entrance of the cold aisle, which means the thermal environment of front racks can be further improved to achieve the more uniform airflow distribution horizontally and vertically in DC. In this paper, an innovative method of airflow optimization applying jet fans in the cold aisles is proposed to make up the lower tile flow rate, adjust the flow path of cold air from the perforated tiles to racks, and balance temperature heterogeneity. In addition, inductive velocity, nozzle height, horizontal position, and attachment distance of jet fans are optimized to explore the optimal parameters. Results show that the incorporation of jet‐induced ventilation can effectively improve the cooling performance and overall thermal environment in DC, while the amount of IT equipment that exceeded the ASHRAE recommended supply air temperature (SAT) is reduced by about 38%. The jet fan with optimal parameters has a broad application space in the raised floor DC.

An experimental and numerical investigation of novel solution for energy management enhancement in data centers using underfloor plenum porous obstructions

This investigation focuses on improving the airflow distribution in data centers. In many data centers vortices form in the plenum during operations. These vortices cause spatial and temporal non-uniformities and may give rise to hot regions in the data center which in turn impacts performance and reliability of the IT equipment. The current study identifies a novel approach using porous partitions in the plenum and demonstrates a significant generalized approach that is easily adoptable in existing and future data centers. For improving the overall data center energy efficiency and the cooling system effectiveness by eliminating a critical source of inefficiency. The results of quantitative and qualitative analyses of the underfloor plenum pressure field, perforated tiles airflow rate, and air temperature at the rack intake side with and without partitions are presented. Different data center configurations are studied using physics-based experimentally validated Computational Fluid Dynamics (CFD) model. The CFD model results showed that the partitions eliminated the presence of vortices in the underfloor plenum and thus enabled a more uniform pressure distribution and tile airflow delivery. Regarding rack inlet temperature, the results showed that the partitions significantly improved the air temperature at the rack inlet. Finally, a geometrical parametric study is performed. An ideal design is and demonstrated to improve the Supply Heat Index (SHI) by about 10%, while the amount of IT equipment that exceeded the ASHRAE recommended supply air temperature (SAT) was reduced by about 40%, and the floor leakage was cut in half.

Experimental and Computational Investigations of the Thermal Environment in a Small Operational Data Center for Potential Energy Efficiency Improvements

Abstract The share of equipment and power use in smaller data centers (DCs) is comparable with that of more massive counterparts. However, they grabbed less attention in the literature despite being less energy-efficient. This study highlights the challenges of setting up a computational fluid dynamics (CFD) model of a 180-m2 small-size high-performance computing (HPC) DC and the validation procedure leading to a reasonably accurate model for the investigation of the thermal environment and potential energy efficiency improvements. Leaky floors, uneven placement of computing equipment and perforated tiles preventing separation of hot and cold air, low-temperature operation, and excessive cooling capacity and fan power were identified sources of energy inefficiency in the DC. Computational fluid dynamics model predictions were gradually improved by using experimental measurements for various boundary conditions (BCs) and detailed geometrical representation of large leakage openings. Eventually, the model led to predictions with an error of less than 1 °C at the rack inlet and less than 5 °C at the rack outlet. The ultimate objective was to use the validated CFD model to test various energy efficiency measures in the form of operational or design changes in line with the best practices. Impact of leakage between the raised floor and the room, reduced airflow rate, cold-aisle and hot-aisle separation, workload consolidation, and higher temperature operation were among the phenomena tested by using the validated CFD model. The estimated power usage effectiveness (PUE) value reduced from 1.95 to 1.40 with the proposed energy efficiency measures.

CFD Investigation of Airflow Management in a Small Container Data Center

In this paper, the cooling performance of a small container data center is numerically investigated. It is indicated that adjusting the plenum depth and placing baffles in the plenum can improve the airflow uniformity efficiently. On the other hand, the perforated tiles with low porosity of 25% also can significantly improve the air flowrate distribution by 88.9% with the severe pumping power penalty of 2.79 times. Hence, the layout of perforated tiles with nonuniform porosities is optimized with the fixed average porosity of 50%, which can obtain almost absolutely uniform flowrate distribution in the cold aisle. In addition, the cold aisle containment (CAC) strategy is implemented, resulting in the improved rack cooling index (RCI) and supply heat index (SHI) to approach to 100% and 0%, respectively. However, the intake flowrates still suffer from nonuniformity due to the bypass effect, thus a novel criterion named flowrate uniformity index (FUI) is proposed. Besides, the drawer-type layouts with the shifting distance ( $\delta$ ) varying from 0 to 200 mm are investigated, and the case with $\delta = 150$ mm shows the best improvement in terms of the standard deviation of FUI ( $\sigma _{\mathrm {FUI}}$ ). Moreover, the $\sigma _{\mathrm {FUI}}$ of the case with a three-layer drawer-type layout can be further improved by 45.8%.

Numerical Investigation for the Arrangement of Perforated Tile under a Non-uniform Heat Source Condition in a Raised-floor Data Center

The research object of this paper was a typical raised-floor data center where the heat load of each rack was non-uniform. The non-uniform heat source caused that the temperature field in the room was also non-uniform and part of the rack outlet temperature was relatively high. In view of this problem, a numerical simulation was conducted to investigate the effect of the arrangement of perforated tile on the air distribution and the temperature field. The results indicate that a better arrangement of perforated tile could optimize the air distribution and make the temperature field in the room more uniform. The maximum rack outlet temperature was reduced by about 2.1°C under the proper condition of the tiles opening ratio, which means that the energy efficiency of the cooling system was accordingly improved. The adjustment of the tiles opening ratio could be a solution to the non-uniform heat source problem in a raised-floor data center.

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.

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.

Comprehensive study on the effects of plenum depths on air flow and thermal managements in data centers

A comprehensive numerical study of air flow and thermal management of data centers is conducted for different plenum depths, tiles perforation ratios and rack power densities. Temperature/air flow profiles and data centers thermal management are quantified using the performance measuring parameters SHI (Supply Heat Index), RHI (Return Heat Index) and RTI (Return Temperature Index). A CFD (Computational Fluid Dynamics) approach for air flow and temperature distribution simulation in the data centers is used. The results showed that increasing the plenum depth: (i) increases the uniformity of air flow rate in the data center, (ii) increases the possibility of recirculation of hot air at the racks tops and bypass of cold air at the sides of the terminal racks of the racks row, and (iii) changes the values of the measurable performance parameters to be outside the recommended ranges. The results recommend that a perforated tiles opening ratio of 25–30% and a plenum depth of 60 cm can be considered as the more suitable values for the data centers.

Experimental Failure Analysis of a Rear Door Heat Exchanger With Localized Containment

The accelerated growth of heat load in high-density data centers presents challenges to the design of effective cooling solutions. Both energy efficiency and the information technology (IT) equipment reliability are key requirements. Localized hybrid air-water cooling systems such as Rear Door Heat eXchangers (RDHXs) are an effective means to achieve these requirements. In this paper, the transient aerodynamic and thermal performance of a commercial RDHX was investigated experimentally. The RDHX was attached to an isolated server cabinet with a controllable heat load. A localized containment system was used to direct the airflow to the equipment in the cabinet that emanates from a single perforated tile within the enclosure. The water flow rate and supply water temperature to the RDHX was controlled, and a grid of 36 air velocity/temperature sensors was employed to monitor the airside of the cooling system. The cooling performance of the RDHX in an air blower failure scenario was investigated. The failure scenario was designed to diminish different parts of the overall system. The impact on the IT equipment and the cabinet outlet temperatures was assessed. Also, there was a significant reduction in the airflow to the IT equipment. The impact of the reduced airflow on different parts of the cabinet as well as the cooling performance of the heat exchanger was characterized during both failure and recovery.

Effect of CRAC units layout on thermal management of data center

Comprehensive numerical studies of thermal management of data centers were presented by several investigators for different geometric and operating conditions of data centers. In the present work, a technical note regarding the effect of the computer room air conditioning (CRAC) units layout arrangements is presented. Two arrangements of CRAC units layouts are investigated; namely locating CRACs units in line with the racks row and locating the CRACs units perpendicular to the rack row. Temperature distributions, air flow characteristics particularly air recirculation and bypass and thermal management in data centers are evaluated in terms of the measureable overall performance parameters: supply/return heat indices (SHI/RHI) and return temperature indices (RTI). The results showed that locating CRAC units perpendicular to the racks row has the following effects: (i) enhances the uniformity of the air flow from the perforated tiles along the rack row, (ii) reduces the hot air recirculation at the ends racks of the row and the cold air bypass at the middle rack of the row and (iii) enhances the data center performance parameters RTI, SHI and RHI.

On the characteristics of airflow through the perforated tiles for raised-floor data centers

Perforated tiles are widely used in raised-floor data centers. The cooling air across the perforated tiles must be distributed properly to adequately cool the equipment. This paper aims at investigating the dependence of the pressure loss coefficient across perforated tiles with respect to the geometrical factors and flow parameters. Numerical simulations of flow distribution of perforated tiles were performed for different pore-type and flow parameters. The pressure loss coefficient upon the Reynolds number, the tile area, the porosity, the diameter of the hole, the thickness of the hole and the arrangement is studied. The fitting equation was provided to calculate the pressure loss coefficient. And experimental tests were taken to verify the validity of the model, comparison with the other empirical equations is provided. Based on the comparison, the fitting equation is more applicable to study the pressure loss coefficient for perforated tiles in data center.

Experimental parametric study of servers cooling management in data centers buildings

A parametric study of air flow and cooling management of data centers servers is experimentally conducted for different design conditions. A physical scale model of data center accommodating one rack of four servers was designed and constructed for testing purposes. Front and rear rack and server’s temperatures distributions and supply/return heat indices (SHI/RHI) are used to evaluate data center thermal performance. Experiments were conducted to parametrically study the effects of perforated tiles opening ratio, servers power load variation and rack power density. The results showed that (1) perforated tile of 25% opening ratio provides the best results among the other opening ratios, (2) optimum benefit of cold air in servers cooling is obtained at uniformly power loading of servers (3) increasing power density decrease air re-circulation but increase air bypass and servers temperature. The present results are compared with previous experimental and CFD results and fair agreement was found.

Exergy Analysis of Data Centres-Effect of the Rack Location and Airflow Direction on the Thermal Performance

In this paper, a numerical analysis of flow and temperature distribution of a small raised-floor data centre is conducted in order to evaluate the thermal performance of the data centre. The flow patterns and temperature profiles established leads to a detailed exergy analysis of the data centre, which results in better understanding of irreversibilities in room airspace. The impact of the rack location in the data centre room and the airflow direction through perforated tiles on the thermal performance of the data centre is investigated using first-law and second-law of thermodynamics.

Modified Body Force Model for Air Flow Through Perforated Floor Tiles in Data Centers

Generally, porous jump (PJ) model is used for rapid air flow simulations (without resolving the tile pore structure) through perforated floor tiles in data centers. The PJ model only specifies a step pressure loss across the tile surface, without any influence on the flow field. However, in reality, the downstream flow field is affected because of the momentum rise of air due to acceleration through the pores, and interaction of jets emerging from the pores. The momentum rise could be captured by either directly resolving the tile pore structure (geometrical resolution (GR) model) or simulated by specifying a momentum source above the tile surface (modified body force (MBF) model). Note that specification of momentum source obviates the need of resolving the tile pore geometry and, hence, requires considerably low computational effort. In previous investigations, the momentum source was imposed in a region above the tile surface whose width and length were same as the tile dimensions with a preselected height. This model showed improved prediction with the experimental data, as well as with the model resolving the tile pore geometry. In the present investigation, we present an analysis for obtaining the momentum source region dimensions and other associated input variables so that the MBF model can be applied for general cases. The results from this MBF model were compared with the GR model and good agreement was obtained.

Numerical investigation and parametric study for thermal and energy management enhancements in data centers' buildings

A CFD numerical investigation of air flow and temperature distribution in data centers is presented for different operating and geometric conditions. Numerical experiments were conducted to parametrically study the effects of perforated tiles opening ratio, data center power density and locations of Computer Room Air Conditioning (CRAC) on the data center performance. Temperature distribution especially hot spot points, air flow characteristics particularly air recirculation and bypass and thermal management in data centers are evaluated in terms of the measureable overall performance parameters: supply/return heat indices (SHI/RHI), return temperature indices (RTI) and return cooling indices (RCI). Techniques of using cold aisle containments for enhancing the cooling effectiveness of data centers are investigated and evaluated. The results showed that (i) increasing the perforation ratio of perforated tails decreases the recirculation and bypass of air flow around the rack, (ii) increasing data centers' power density and supplied air flow rates increases the air bypass and air re-circulation, (iii) changing the spacing between the CRAC units and cold aisle slightly changes of air recirculation and bypass around the racks, and (iv) using appropriate cold aisle containment improves the performance of cooling of data centers, especially at high power density.

Airflow Management in a Contained Cold Aisle Using Active Fan Tiles for Energy Efficient Data-Center Operation

Generally, passive perforated tiles are used in a data center and the supplied airflow rate is underprovisioned; thus, the balance of the server air requirement is met by the hot air in the room, resulting in higher server inlet temperatures. Full provisioning of the supplied airflow rate and containing the cold aisle is expected to minimize the hot air leakage in the cold aisle, resulting in uniform and lower server inlet temperatures. Thus, the supply air temperature can be raised, resulting in energy savings at the chiller plant. Supplying extra air can be achieved using active perforated tiles, having multiple fans installed on them. In this paper, the underprovisioned case using passive tiles and the fully provisioned case using active tiles are investigated for both open and contained aisle conditions. Thermal field measurements suggest lower and uniform server inlet temperatures for the case with contained aisle as compared to open aisle and for the fully provisioned case using active tiles as compared to the underprovisioned case using passive tiles. System-level energy calculations suggests that containing the cold aisle results in lower (improved) power usage effectiveness (PUE); however, use of active tiles does not seem to improve the PUE.