Suhas V. Patankar

Use of Computational Fluid Dynamics for Calculating Flow Rates Through Perforated Tiles in Raised-Floor Data Centers

This paper describes a computational fluid dynamics model for calculating airflow rates through perforated tiles in raised-floor data centers. The model is based on the assumption that the pressure in the space above the raised floor is uniform, which allows the calculation to be limited to the space below the raised floor. It uses a finite-volume method, the k-∊ turbulence model, and a multigrid method. The model is applied to a real-life data center. The calculated results for velocity and pressure distributions are discussed. The flow rates through the perforated tiles are shown to be in good agreement with the measured values.

Techniques for Controlling Airflow Distribution in Raised-Floor Data Centers

In raised-floor data centers, the airflow rates through the perforated tiles must meet the cooling requirements of the computer servers placed next to the tiles. The data centers house a wide range of equipment, and the heat load pattern on the floor can be quite arbitrary and changes as the data center evolves. To achieve optimum utilization of the floor space and the flexibility for rearrangement and retrofitting, the designers and managers of data centers must be able to modify the airflow rates through the perforated tiles. The airflow rates through the perforated tiles are governed primarily by the pressure distribution under the raised floor. Thus, the key to modifying the flow rates is to influence the flow field in the plenum. This paper discusses a number of techniques that can be used for controlling airflow distribution. These techniques involve changing the plenum height and open area of perforated tiles, and installing thin (solid and perforated) partitions in the plenum. A number of case studies, using a mathematical model, are presented to demonstrate the effectiveness of these techniques.

Distributed Leakage Flow in Raised-Floor Data Centers

In raised-floor data centers, distributed leakage flow—the airflow through seams between panels on the raised floor— reduces the amount of cooling air available at the inlets of the computer equipment. This airflow must be known to determine the total cooling air requirement in a data center. The amount of distributed leakage flow depends on the area of the seams and the plenum pressure, which, in turn, depends on the amount of airflow into the plenum and the total open area (combined area of perforated tiles, cutouts, and seams between panels) on the raised floor. The goal of this study is to outline a procedure to measure leakage flow, to provide data on the amount of the distributed leakage flow, and to show the quantitative relationship between the leakage flow and the leakage area. It also uses a computational model to calculate the distributed leakage flow, the flow through perforated tiles, and the plenum pressure. The results obtained from the model are verified using the measurements. Such a model can be used for design and maintenance of data centers. The measurements show that the leakage flow in a typical data center is between 5-15% of the available cooling air. The measured quantities were used to estimate the area of the seams; for this data center, it was found to be 0.35% of the floor area. The computational model represents the actual physical scenarios very well. The discrepancy between the calculated and measured values of leakage flow, flow through perforated tiles, and plenum pressure is less than 10%.

Analysis of Airflow Distribution Across a Front-to-Rear Server Rack

The most common server racks in data centers are front-to-rear racks, which draw in the cooling air from the front side and discharge it from the backside. In a raised-floor data center the cooling air to these racks is provided by perforated tiles that are placed in front of them. In a high-density data center, these tiles discharge a considerable amount of airflow, which leads to a high-velocity vertical jet in front of the rack. Such a high-velocity jet may bypass the servers located at the bottom of the rack leading to their airflow starvation and potential failure. In this paper the effect of the high-velocity jet on the airflow taken by servers at various heights in the rack is studied. A computer model based on the Computational Fluid Dynamics (CFD) technique is used to predict the airflow distribution through servers stacked in the rack. Two cases are considered. In one case, the rack is placed in the middle of a row of racks in a prefect hot aisle-cold aisle arrangement. The boundary conditions around such a rack is symmetrical. In the other case, the rack is placed in a room with asymmetrical boundary conditions. The characteristics of the servers in the rack are taken from typical 1U and 2U servers manufactured by IBM. It is shown that in general the high-velocity jet has a mild effect on the airflow taken by the servers, and the airflow reduction is limited to servers at the bottom of the rack. Racks in a symmetrical configuration are more susceptible to the airflow starvation. In the most critical conditions, an airflow reduction of 15% is calculated for the server located at the bottom of the rack. Using the result obtained from the computational analysis, a simple model is developed to predict the reduction of the cooling air under the most critical situation for the server placed at the bottom of the rack.Copyright

A flow network analysis of a liquid cooling system that incorporates microchannel heat sinks

The objective of this study is to show the applicability of Flow Network Modeling (FNM) in analyzing liquid cooling systems that incorporate microchannel heat sinks. The study is divided in two parts. In the first part, an analytical model of a microchannel heat sink is proposed and its validity is established by comparison with a detailed CFD analysis. In the second part of the study, a liquid cooled system that accommodates three microchannel heat sinks is analyzed using Flow Network Modeling (FNM). In the FNM method, the pressure drop and heat transfer coefficient of the components in the system are calculated using analytical or empirical correlations. The goal of the analysis is to balance the liquid flow passing through each heat sink such that the chips case temperature remains below recommended values. This study illustrates the use of the FNM technique for analyzing liquid cooled systems and microchannel heat sinks.

PERFORMANCE ANALYSIS OF A DOUBLE-DECK CONVEYOR DRYER : A COMPUTATIONAL APPROACH

ABSTRACT This paper illustrates the use of numerical simulation models for evaluating the performance of a moving bed dryer. A finite-volume method is employed in developing a steady state, two-dimensional numerical model for a double-deck conveyor dryer. Using this numerical model, variations in the product moisture content and temperature along the length and across the height of the product beds are predicted. Similarly, the resulting variations in the temperature and relative humidity of the drying air are predicted in the entire two-dimensional domain of a dryer. Effect of air-to-product mass flow ratio and product residence lime on the average moisture content of the outgoing product are also evaluated for three different drying air temperatures.

Computational Modeling of Electroslag Remelting (ESR) Process Used for the Production of High-Performance Alloys

This paper presents a comprehensive computational model for the prediction of the transient Electroslag Remelting (ESR) process for cylindrical ingots based on axisymmetric two-dimensional analysis. The model analyzes the behavior of the slag and growing ingot during the entire ESR process involving a hot-slag start with an initial transient, near-steady melting, hot-topping and subsequent solidification of the slag and ingot after melting ends. The results of model application for an illustrative ESR process for Alloy 718 and its validation using results from an industrial trial are presented. They demonstrate the comprehensive capabilities of the model in predicting the behavior of the ingot and slag during the entire process and properties of the final ingot produced. Such analysis can benefit the optimization of existing process schedules and design of new processes for different alloys and different ingot sizes.

APPLICATION OF THE PARTIAL ELIMINATION ALGORITHM FOR SOLVING THE COUPLED ENERGY EQUATIONS IN POROUS MEDIA

This article discusses the solution of coupled energy equations in local thermal nonequilibrium models for porous media. The decoupled solution approach, in which the interaction between the solid and the fluid temperature fields is treated in an explicit manner, converges very slowly when the interface heat transfer coefficient and/or the specific surface area of the porous medium are large (large Biot number). An attractive alternative to the decoupled approach is the partial elimination algorithm, proposed by D. B. Spalding. In this algorithm, the discretization equations are rearranged so that the resulting equations are more implicit and take directly into account the coupling between the two phases. The convergence rates of these two solution procedures are studied with reference to convective heat transfer in a two-dimensional channel filled with a porous medium. The partial elimination algorithm converges much more quickly than the decoupled procedure, with the number of iterations required for convergence becoming constant for large Biot numbers.

Prediction of thermal history of preforms produced by the clean metal spray forming process

Abstract The clean metal spray forming (CMSF) process provides premium quality material for high performance aircraft engine components. In this process, ‘clean’ metal (metal free of oxides and other inclusions) is atomized via a scanning open atomizer and the resultant spray is collected as a solid billet preform. Under a program jointly funded by industry and the US Government, process models have been developed for the different stages of CMSF. The present paper focuses on models for the spray and deposition processes. The axisymmetric spray model has been benchmarked with experimental measurements of particle size distribution and mass distribution. It provides the input for the deposition model. A unique approach is used to make the deposition model both accurate and efficient. The models are discussed in detail and typical results presented.

Computational method for generalized analysis of pumped two-phase cooling systems and its application to a system used in data-center environments

Two-phase pumped-loop systems are being actively considered for cooling of high heat load electronics. In the present study, a computational method based on a two-level approach is developed for generalized system-level analysis of two-phase pumped-loop cooling systems containing multiple branches under steady-state conditions. Detailed one-dimensional analysis of components with distributed two-phase flow is performed to determine their flow and thermal characteristics. System-level analysis utilizes these compact representations for analyzing the component interaction in a generalized manner to predict the system performance. Component models have been developed for the microchannel-heat-sink, finned-tube evaporator and condenser, reservoir, and positive displacement pump. In order to illustrate the utility of the computational method in the design of practical two-phase cooling systems, it has been applied for the analysis of a pumped-loop system being explored for the cooling of hot-air exhausts from server racks in data centers. The system consists of multiple finned-tube evaporators in parallel branches, a water-cooled condenser, a reservoir, and a pump. Results of the analysis show the occurrence of flow maldistribution among the evaporators due to absorption of varying heat loads.