H. Ezzat Khalifa

Experimental and computational study of perforated floor tile in data centers

Current CFD simulation studies of large data centers cannot model the detailed geometries of the perforated tiles due to grid size limitation. These studies often assume that the tile flow can be modeled as constant velocity based on a fully open tile. In this case, mass flux is enforced at the expense of under-predicting momentum flux; the error in momentum flux can be as high as a factor of four for a 25% open perforated tile. Since jet entrainment is a strong function of its initial momentum flux, this error can be significant with respect to predicting the mixing of the surrounding room air into the tile flow. Combined experimental and computational studies were carried out to quantify the importance of the detailed tile geometry, and it was found that proper prediction of the mixing process must account for the tile opening patterns. Suggestions of how to model the floor perforated tiles in data center CFD simulations are then presented.

Particle Levitation Due to a Uniformly Descending Flat Object

This article presents analytical and computational fluid-dynamics (CFD) solutions of the unsteady flow resulting from a horizontal circular disk moving downward at a constant velocity toward a horizontal floor seeded with spherical micro-particles, and the effect of this flow on particle detachment and levitation. The selected configuration is a simplification of numerous practical applications in which particle resuspension is important, for example a foot or an object impacting a dusty floor, or a squeeze film thrust bearing with particle contamination. The resulting radial and axial velocity field, coupled with a particle detachment model and the particle equations of motion were employed to compute particle trajectories in the gap. The CFD solutions were utilized to describe the high-speed radial wall jet and the vortices developing outside the disk and to explain their role in particle levitation and entrainment. It is shown that as the gap narrows the resulting radial velocity close to the disk perimeter is high enough to detach and levitate μ m-size particles, and that the vortices shed by the descending disk and its high-velocity radial wall jet create an upward convective motion that contributes to particle resuspension from the floor and entrainment in any far-field flow that might be present around the descending disk.

Development and experimental validation of a thermo-hydraulic model for data centers

Most data center design analyses focus on design point operation, which, in reality, is rarely realized due to the reliability needs and the constantly changing operating states of data centers. The objective of this work is to develop and experimentally validate a model that will allow data center operators and designers the ability to evaluate the energy impact of various data center loop configurations and operating strategies. The modeling methodology couples a thermodynamic model of the cooling equipment and a hydraulic pipe network for hydraulic characterization. Inherent in this model is the ability to capture off-design operating conditions of the data center cooling infrastructure caused by changes in ambient conditions and fluctuations in required IT load. The thermo-hydraulic model is validated based on an existing IT data center and chiller plant located in Poughkeepsie, New York. The model is used to study the data center under several climatic and operating conditions—namely, winter and summer. The resulting performance in terms of the ratio of the total facility power to the IT power usage effectiveness was found to be 1.57 and 1.73, respectively, for the two climatic and operating modes.

Optimization of Enclosed Aisle Data Centers Using Bypass Recirculation

used for exploring optimization possibilities in air-cooled data centers. The model is used to evaluate parametrically the total energy consumption of the data center cooling infrastructure for data centers that utilize aisle containment. The analysis highlights the importance of reducing the total power required for moving the air within the computer room air conditioners (CRACs), the plenum, and the servers, rather than focusing primarily or exclusively on reducing the refrigeration system’s power consumption. In addition, the benefits of introducing a bypass recirculation branch in enclosed aisle configurations are shown. The analysis shows a potential for as much as a 60% savings in cooling infrastructure energy consumption by utilizing an optimized enclosed aisle configuration with bypass recirculation, instead of a traditional enclosed aisle in which all the data center exhaust is forced to flow through the CRACs. Furthermore, computational fluid dynamics is used to evaluate practical arrangements for implementing bypass recirculation in raised floor data centers. A configuration where bypass tiles, with controllable low-lift fans, are placed close to the discharge of CRACs results in increased mixing and is shown to be a suitable method for providing nearly thermally uniform conditions to the inlet of the servers in an enclosed cold aisle. Other configurations of bypass implementation are also discussed and explored. [DOI: 10.1115/1.4005907]

Design of Simulated Server Racks for Data Center Research

Conducting experiments on real high-density computer servers can be an expensive and risky task due to the risks associated with unintended inlet temperatures that exceed the server’s red-line temperature limit. Presented herein is the development of the simulated chassis that mimic real computer servers. Briefly, twelve high-power simulated chassis were designed and built to accurately simulate the actual operating conditions of a real computer chassis in a data center. Each simulated chassis is designed to have approximately 300 Pa pressure drop at a flow rate of 600 cfm to represent a real IBM server chassis. Additionally, the simulated chassis are designed to match the thermal mass of a real server. Eight of the simulated chassis were designed to have constant speed fans and variable heating power while the remaining four chassis were designed to have variable speed fans and variable heating power. Further discussions about the design phase of the simulated chassis are the substantial part of this paper. Underlining the challenges and safety issues with high-power chassis, guidelines for designing and constructing a chassis that simulates the real environment of a typical data center are presented.Copyright

Transient Thermal Response of Servers Through Air Temperature Measurements

Transient CFD analysis of data centers requires appropriate representations of the transient thermal characteristics of servers. Thermal conductance and thermal capacitance are two determining characteristics for the response of servers under unsteady conditions. Previous studies proposed tests that require detailed temperature and thermal capacitance measurements for each of the server component, requiring access to individual components inside the server. In this paper, we propose a method for obtaining the transient thermal characteristics of a server from server inlet and outlet temperatures under transient operating conditions.Copyright

Experimental investigation of CRAH bypass for enclosed aisle data centers

Aisle containments in data centers help provide uniform server inlet air temperatures. This allows the cooling system to run at a higher evaporator temperature and more efficiently. On the other hand, CRAH units run at higher speeds to ascertain that racks receive sufficient air flow. Since CRAH fan power already constitutes an important component of data center power use, such increases in the fan power can overshadow the energy savings due to more efficient chiller operation. CRAH bypass configuration is proposed to achieve optimum operating condition for enclosed aisle data centers. This configuration utilizes fan-assisted perforated floor tiles to induce a fraction of tile flow from the room through bypass ports or leakage paths and help decreasing the amount of air flow passing through the large flow resistances of CRAH units. Experimental results show that there is an optimum operating condition for the specific data center test cell that is designed to represent an enclosed aisle data center utilizing the proposed CRAH bypass configuration. Here, the flow characteristics of major system components and experimental measurements have been used to calibrate a flow network model (FNM) for the design optimization and trade-off analysis of the proposed system. Calibrated FNM along with a thermodynamic model (TM) of the cooling infrastructure provides an estimate of the energy use at various fractions of CRAH bypass air and chilled water temperatures. This study introduces the design of the experimental setup for testing CRAH bypass configuration for enclosed aisles and for calibrating models to predict the cooling infrastructure energy saving potential of the proposed technique.

A hybrid lumped capacitance-CFD model for the simulation of data center transients

Transient thermal events in air-cooled data centers may lead to undesirable operating conditions such as the formation of hot spots and associated degradation of equipment reliability. These transients may be caused by cooling equipment failures, server load changes, or other time-dependent scenarios in data center operations. This paper introduces a fast-executing hybrid computational fluid dynamics (CFD)/Lumped-Capacitance model for predicting server inlet temperatures resulting from common transient events such as server shutdown, partial or total chilled water interruption, or partial or total failure of the computer room air handlers (CRAH). The model uses initial steady-state CFD or experimental data in combination with several lumped-capacitance models of the various thermal masses in the data center, including the servers, the room enclosure, the CRAHs and the underfloor plenum. The inclusion of these thermal capacitances and their associated thermal conductance was found to be an important contributor to the overall transient response of the data center air-space. The model predictions have been compared with experimental data obtained in a three-rack data-center test cell and found to agree well with the experimental measurements. Examples of the application of the model to more realistic data center configurations are also given.

Energy and environmental assessment of on-site power and cooling for data centers

High density data centers consume mega watts of electricity for both computing and cooling. Because of the fast paced growth of data center energy demand, effective energy conservation measures must be implemented. The use of on-site combined power generation and cooling systems for data centers that are located on campuses is discussed and compared to grid-powered data centers. The placement of the power plant on site not only reduces transmission losses, but also allows the utilization of the power plant waste heat to generate cooling for the data center and both heating and cooling for adjacent campus buildings. This is why on-site power generation for data centers is an appealing option. Further, with on-site power generation, it is possible to supply the data center with direct current (DC)power, thus, avoiding the cascade of waste in the multiple AC/DC conversions typical of conventional data centers. This article presents a detailed modeling effort to assess the efficiency and environmental and energy cost benefits of such on-site co-generation systems for campus-embedded data centers by utilizing a primary energy analysis.

CFD Analysis of Personal Ventilation with Volumetric Chemical Reactions

This article studies the concentration distribution in the microenvironment of a person and inhalation exposure in a typical office space when modeling first- and second-order reactions. First, wall adsorption of ozone and d-limonene and the resulting volumetric reaction are validated, and 2D computational concentration profiles are compared to the experimental results from Ito (2007b) with reasonable agreement. The validated model is then implemented in a 3D simulation of a typical office space with a seated thermal manikin, floor diffuser, desk, and optional personal ventilation (PV) system. Three cases were modeled: (1) no PV, (2) a PV system consisting of a single round jet, and (3) a novel low-mixing co-flow nozzle that directs fresh air toward the breathing zone (BZ). All concentration distributions showed significant differences compared to the traditional well-mixed assumption. The inhalation exposure was analyzed as an intake fraction (iF) and compared for the three cases. The co-flow nozzle was able to reduce the amount of reaction product inhaled by 5 times compared to a case without PV.