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Bahgat Sammakia - One of the best experts on this subject based on the ideXlab platform.

  • Transient Thermal Performance of Rear Door Heat Exchanger in Local Contained Environment During Water Side Failure
    ASME 2017 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems, 2017
    Co-Authors: Kourosh Nemati, Husam A. Alissa, Mohammad I. Tradat, Bahgat Sammakia
    Abstract:

    The constant increase in data center computational and processing requirements has led to increases in the IT equipment power demand and cooling challenges of high-density (HD) data centers. As a solution to this, the hybrid and liquid systems are widely used as part of HD data centers thermal management solutions. This study presents an experimental based investigation and analysis of the transient thermal performance of a stand-alone server cabinet. The total heat load of the cabinet is controllable remotely and a Rear Door heat exchanger is attached with controllable water flow rate. The cooling performances of two different failure scenarios are investigated. One is in the water chiller and another is in the water pump for the Rear Door Heat eXchanger (RDHX). In addition, the study reports the impact of each scenario on the IT equipment thermal response and on the cabinet outlet temperature using a mobile temperature and velocity mesh (MTVM) experimental tool. Furthermore, this study also addresses and characterizes the heat exchanger cooling performance during both scenarios.

  • Numerical and experimental characterization of the transient effectiveness of a water to air heat exchanger for data center cooling systems
    ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems InterPACK 2015 colloc, 2015
    Co-Authors: Tianyi Gao, Marcelo Del Valle, Bahgat Sammakia, Alfonso Ortega
    Abstract:

    Copyright ? 2015 by ASME.The cross flow heat exchanger is at the heart of most cooling systems for data centers. Air/Water or air/refrigerant heat exchangers are the principal component in Central Room Air Conditioning (CRAC) units that condition data room air that is delivered through an underfloor plenum. Liquid/air heat exchangers are also increasingly deployed in close-coupled cooling systems such as Rear Door heat exchangers, in-row coolers, and overhead coolers. In all cases, the performance of liquid/air heat exchangers in both steady state and transient scenarios are of principal concern. Transient scenarios occur either by the accidental failure of the cooling system or by intentional dynamic control of the cooling system. In either scenario, transient boundary conditions involve time-dependent air or liquid inlet temperatures and mass flow rates that may be coupled in any number of potential combinations. Understanding and characterizing the performance of the heat exchanger in these transient scenarios is of paramount importance for designing better thermal solutions and improving the operational efficiency of existing cooling systems. In this paper, the transient performance of water to air cross flow heat exchangers is studied using numerical modeling and experimental measurements. Experimental measurements in 12 in. x 12 in. heat exchanger cores were performed, in which the liquid (water) mass flow rate or inlet temperature are varied in time following controlled functional forms (step jump, ramp). The experimental data were used to validate a transient numerical model developed with traditional assumptions of space averaging of heat transfer coefficients, and volume averaging of thermal capacitances. The complete numerical model was combined with the transient effectiveness methodology in which the traditional heat exchanger effectiveness approach is extended into a transient domain, and is then used to model the heat exchanger transient response. Different transient scenarios were parametrically studied to develop an understanding of the impact of critical variables such as, the fluid inlet temperature variation and the fluid mass flow rate variation, and a more comprehensive understanding of the characteristics of the transient effectiveness. Agreement between the novel transient effectiveness modeling approach and the experimental measurements enable use of the models as verified predictive design tools. Several studies are designed based on the practical problems related to data center thermal environments and the results are analyzed.

  • Analysis of transient and hysteresis behavior of cross-flow heat exchangers under variable fluid mass flow rate for data center cooling applications
    Applied Thermal Engineering, 2015
    Co-Authors: Tianyi Gao, Bruce Murray, Bahgat Sammakia
    Abstract:

    Effective thermal management of data centers is an important aspect of reducing the energy required for the reliable operation of data processing and communications equipment. Liquid and hybrid (air/liquid) cooling approaches are becoming more widely used in today's large and complex data center facilities. Examples of these approaches include Rear Door heat exchangers, in-row and overhead coolers and direct liquid cooled servers. Heat exchangers are primary components of liquid and hybrid cooling systems, and the effectiveness of a heat exchanger strongly influences the thermal performance of a cooling system. Characterizing and modeling the dynamic behavior of heat exchangers is important for the design of cooling systems, especially for control strategies to improve energy efficiency. In this study, a dynamic thermal model is solved numerically in order to predict the transient response of an unmixed-unmixed crossflow heat exchanger, of the type that is widely used in data center cooling equipment. The transient response to step and ramp changes in the mass flow rate of both the hot and cold fluid is investigated. Five model parameters are varied over specific ranges to characterize the transient performance. The parameter range investigated is based on available heat exchanger data. The thermal response to the magnitude, time period and initial and final conditions of the transient input functions is studied in detail. Also, the hysteresis associated with the fluid mass flow rate variation is investigated. The modeling results and performance data are used to analyze specific dynamic performance of heat exchangers used in practical data center cooling applications.

  • Experimentally verified transient models of data center Crossflow heat exchangers
    ASME International Mechanical Engineering Congress and Exposition Proceedings (IMECE), 2014
    Co-Authors: Tianyi Gao, Justin Geer, M. David, Bahgat Sammakia, Rainer Schmidt
    Abstract:

    Copyright ? 2014 by ASME.Heat exchangers are key components that are commonly used in data center cooling systems. Rear Door heat exchangers, in-row coolers, overhead coolers and fully contained cabinets are some examples of liquid and hybrid cooling systems used in data centers. A liquid to liquid heat exchanger is one of the main components of the Coolant Distribution Unit (CDU), which supplies chilled water to the heat exchangers mentioned above. Computer Room Air Conditioner (CRAC) units also consist of liquid to air cross flow heat exchangers. Optimizing the energy use and the reliability of IT equipment in data centers requires Computational Fluid Dynamics (CFD) tools that can accurately model data centers for both the steady state and dynamic operations. Typically, data centers operate in dynamic conditions due to workload allocations that change both spatially and temporally. Additional dynamic situations may also arise due to failures in the thermal management and electrical distribution systems. In the computational simulation, individual component models, such as transient heat exchanger models, are therefore needed. It is also important to develop simple, yet accurate, compact models for components, such as heat exchangers, to reduce the computational time without decreasing simulation accuracy. In this study, a method for modeling compact transient heat exchangers using CFD code is presented. The method describes an approach for installing thermal dynamic heat exchanger models in CFD codes. The transient effectiveness concept and model are used in the development of the methodology. Heat exchanger CFD compact models are developed and tested by comparing them with full thermal dynamic models, and also with experimental measurements. The transient responses of the CFD model are presented for step and ramp change in flow rates of the hot and cold fluids, as well as step, ramp, and exponential variation in the inlet temperature. Finally, some practical dynamic scenarios involving IBM buffer liquid to liquid heat exchanger, Rear Door heat exchanger, and CRAC unit, are parametrically modeled to test the developed methodology. It is shown that the compact heat exchanger model can be used to successfully predict dynamic scenarios in typical data centers.

  • Transient effectiveness characteristics of cross flow heat exchangers in data center cooling systems
    Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference, 2014
    Co-Authors: Tianyi Gao, Justin Geer, Bahgat Sammakia, Alfonso Ortega, Rainer Schmidt
    Abstract:

    ? 2014 IEEE.Heat exchangers are important facilities used in data center cooling systems. The effectiveness of the heat exchangers strongly influences the thermal performance of the cooling systems. Rear Door heat exchangers, in-row, overhead coolers and fully contained cabinets are some examples of executing liquid and hybrid cooling systems used in data centers. A liquid to liquid heat exchanger is an important component of the Coolant Distribution Unit (CDU), which supplies chilled water to the heat exchangers mentioned above. Computer Room Air Conditioner (CRAC) units are also typically liquid to air cross flow heat exchangers. It is important to characterize the dynamic behavior of these heat exchangers for the thermal management of data centers, in order to improve the design of control strategies and energy efficiency. In this paper, the transient response of a two dimensional unmixed-unmixed cross flow heat exchanger is investigated. The transient effectiveness concept is used to build the mathematical models which will be used to solve and analyze the transient problems of cross flow heat exchangers. Numerical solutions for solving the mathematical model and capturing the transient effectiveness characteristics are developed. The solution accuracy is tested by comparison with several published analytical and analog solutions. An experimental study of the effectiveness of an IBM Rear Door heat exchanger is used to develop the heat exchanger models and to verify the effectiveness of the numerical solution at the same time. Several transient cases are investigated numerically for step, ramp and exponential variations in the inlet temperature of the minimum capacity rate fluid. The transient effectiveness characteristics are tested in parametric studies. The results obtained show that transient effectiveness is a useful method in analyzing and predicting the dynamic performance of heat exchangers.

Tianyi Gao - One of the best experts on this subject based on the ideXlab platform.

  • Innovative server rack design with bottom located cooling unit
    Proceedings of the 15th InterSociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems ITherm 2016, 2016
    Co-Authors: Tianyi Gao, Eric Kumar, Manasa Sahini, Wendy Lu, Charles Ingalz, Ali Heydari, Xiaogang Sun
    Abstract:

    ? 2016 IEEE.One important motivation of data center mechanical system RandD is to improve the energy efficiency and reliability. Many new cooling solutions have been successfully used in production data centers, such as hybrid/liquid cooling systems and free cooling systems, and a better Power Usage Effectiveness (PUE) has been achieved when compared with traditional air cooling data centers. Liquid cooling can be assorted in different categories such as server liquid cooling, rack liquid cooling and pod liquid cooling. In terms of rack liquid cooling, there are several mature technologies such as a Rear Door heat exchanger, an in row cooler, an overhead heat exchanger, a water cooled cabinet and so on. The hybrid cooling solution can be understood as a rack liquid cooling solution operated in a hybrid environment with CRAH/CRAC units in either a raised or a non-raised floor data center. This paper proposes and investigates a new rack liquid cooling design which the cooling unit is located at the bottom of a customized server rack. The bottom cooling unit consists of an air duct and a heat exchanger. The rack is front Door and back Door contained, and air is moved by a fan wall installed on the back of the rack recirculating within the cabinet, passing through the cooling unit and cooling the IT. First of all, a description of the customized rack and the concept of the novel rack cooling solution is provided. Then, a thermal feasibility analysis of this proposed rack cooling solution is conducted using a combination of analytical and computational modeling. Several modeling cases are designed to characterize the sensitivities of some major design and operating parameters. The results and corresponding analyses will be used to guide the prototype development. The height of the rack cooling unit is one of the key design parameters: with a minimal height required by the cooling coil, the loss of node space on the rack can be reduced. Therefore the design and selection of the heat exchanger is of paramount importance. On one hand, the design should provide adequate cooling capacity and sufficient heat transfer area; on the other hand, the height should be minimized. The effects of the heat exchanger design on the cooling performance and air side pressure drop are modeled and analyzed quantitatively in this work. In addition, another two important design parameters namely the front Door and back Door containment sizes are parametrically modeled. Furthermore, the operating conditions including the chilled water supply temperature, water flow rate, fan operating duty circle are investigated and results are reported. An expected mechanical PUE of this novel rack design is proposed.

  • Analysis of transient and hysteresis behavior of cross-flow heat exchangers under variable fluid mass flow rate for data center cooling applications
    Applied Thermal Engineering, 2015
    Co-Authors: Tianyi Gao, Bruce Murray, Bahgat Sammakia
    Abstract:

    Effective thermal management of data centers is an important aspect of reducing the energy required for the reliable operation of data processing and communications equipment. Liquid and hybrid (air/liquid) cooling approaches are becoming more widely used in today's large and complex data center facilities. Examples of these approaches include Rear Door heat exchangers, in-row and overhead coolers and direct liquid cooled servers. Heat exchangers are primary components of liquid and hybrid cooling systems, and the effectiveness of a heat exchanger strongly influences the thermal performance of a cooling system. Characterizing and modeling the dynamic behavior of heat exchangers is important for the design of cooling systems, especially for control strategies to improve energy efficiency. In this study, a dynamic thermal model is solved numerically in order to predict the transient response of an unmixed-unmixed crossflow heat exchanger, of the type that is widely used in data center cooling equipment. The transient response to step and ramp changes in the mass flow rate of both the hot and cold fluid is investigated. Five model parameters are varied over specific ranges to characterize the transient performance. The parameter range investigated is based on available heat exchanger data. The thermal response to the magnitude, time period and initial and final conditions of the transient input functions is studied in detail. Also, the hysteresis associated with the fluid mass flow rate variation is investigated. The modeling results and performance data are used to analyze specific dynamic performance of heat exchangers used in practical data center cooling applications.

  • Numerical and experimental characterization of the transient effectiveness of a water to air heat exchanger for data center cooling systems
    ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems InterPACK 2015 colloc, 2015
    Co-Authors: Tianyi Gao, Marcelo Del Valle, Bahgat Sammakia, Alfonso Ortega
    Abstract:

    Copyright ? 2015 by ASME.The cross flow heat exchanger is at the heart of most cooling systems for data centers. Air/Water or air/refrigerant heat exchangers are the principal component in Central Room Air Conditioning (CRAC) units that condition data room air that is delivered through an underfloor plenum. Liquid/air heat exchangers are also increasingly deployed in close-coupled cooling systems such as Rear Door heat exchangers, in-row coolers, and overhead coolers. In all cases, the performance of liquid/air heat exchangers in both steady state and transient scenarios are of principal concern. Transient scenarios occur either by the accidental failure of the cooling system or by intentional dynamic control of the cooling system. In either scenario, transient boundary conditions involve time-dependent air or liquid inlet temperatures and mass flow rates that may be coupled in any number of potential combinations. Understanding and characterizing the performance of the heat exchanger in these transient scenarios is of paramount importance for designing better thermal solutions and improving the operational efficiency of existing cooling systems. In this paper, the transient performance of water to air cross flow heat exchangers is studied using numerical modeling and experimental measurements. Experimental measurements in 12 in. x 12 in. heat exchanger cores were performed, in which the liquid (water) mass flow rate or inlet temperature are varied in time following controlled functional forms (step jump, ramp). The experimental data were used to validate a transient numerical model developed with traditional assumptions of space averaging of heat transfer coefficients, and volume averaging of thermal capacitances. The complete numerical model was combined with the transient effectiveness methodology in which the traditional heat exchanger effectiveness approach is extended into a transient domain, and is then used to model the heat exchanger transient response. Different transient scenarios were parametrically studied to develop an understanding of the impact of critical variables such as, the fluid inlet temperature variation and the fluid mass flow rate variation, and a more comprehensive understanding of the characteristics of the transient effectiveness. Agreement between the novel transient effectiveness modeling approach and the experimental measurements enable use of the models as verified predictive design tools. Several studies are designed based on the practical problems related to data center thermal environments and the results are analyzed.

  • Experimentally verified transient models of data center Crossflow heat exchangers
    ASME International Mechanical Engineering Congress and Exposition Proceedings (IMECE), 2014
    Co-Authors: Tianyi Gao, Justin Geer, M. David, Bahgat Sammakia, Rainer Schmidt
    Abstract:

    Copyright ? 2014 by ASME.Heat exchangers are key components that are commonly used in data center cooling systems. Rear Door heat exchangers, in-row coolers, overhead coolers and fully contained cabinets are some examples of liquid and hybrid cooling systems used in data centers. A liquid to liquid heat exchanger is one of the main components of the Coolant Distribution Unit (CDU), which supplies chilled water to the heat exchangers mentioned above. Computer Room Air Conditioner (CRAC) units also consist of liquid to air cross flow heat exchangers. Optimizing the energy use and the reliability of IT equipment in data centers requires Computational Fluid Dynamics (CFD) tools that can accurately model data centers for both the steady state and dynamic operations. Typically, data centers operate in dynamic conditions due to workload allocations that change both spatially and temporally. Additional dynamic situations may also arise due to failures in the thermal management and electrical distribution systems. In the computational simulation, individual component models, such as transient heat exchanger models, are therefore needed. It is also important to develop simple, yet accurate, compact models for components, such as heat exchangers, to reduce the computational time without decreasing simulation accuracy. In this study, a method for modeling compact transient heat exchangers using CFD code is presented. The method describes an approach for installing thermal dynamic heat exchanger models in CFD codes. The transient effectiveness concept and model are used in the development of the methodology. Heat exchanger CFD compact models are developed and tested by comparing them with full thermal dynamic models, and also with experimental measurements. The transient responses of the CFD model are presented for step and ramp change in flow rates of the hot and cold fluids, as well as step, ramp, and exponential variation in the inlet temperature. Finally, some practical dynamic scenarios involving IBM buffer liquid to liquid heat exchanger, Rear Door heat exchanger, and CRAC unit, are parametrically modeled to test the developed methodology. It is shown that the compact heat exchanger model can be used to successfully predict dynamic scenarios in typical data centers.

  • Transient effectiveness characteristics of cross flow heat exchangers in data center cooling systems
    Thermomechanical Phenomena in Electronic Systems -Proceedings of the Intersociety Conference, 2014
    Co-Authors: Tianyi Gao, Justin Geer, Bahgat Sammakia, Alfonso Ortega, Rainer Schmidt
    Abstract:

    ? 2014 IEEE.Heat exchangers are important facilities used in data center cooling systems. The effectiveness of the heat exchangers strongly influences the thermal performance of the cooling systems. Rear Door heat exchangers, in-row, overhead coolers and fully contained cabinets are some examples of executing liquid and hybrid cooling systems used in data centers. A liquid to liquid heat exchanger is an important component of the Coolant Distribution Unit (CDU), which supplies chilled water to the heat exchangers mentioned above. Computer Room Air Conditioner (CRAC) units are also typically liquid to air cross flow heat exchangers. It is important to characterize the dynamic behavior of these heat exchangers for the thermal management of data centers, in order to improve the design of control strategies and energy efficiency. In this paper, the transient response of a two dimensional unmixed-unmixed cross flow heat exchanger is investigated. The transient effectiveness concept is used to build the mathematical models which will be used to solve and analyze the transient problems of cross flow heat exchangers. Numerical solutions for solving the mathematical model and capturing the transient effectiveness characteristics are developed. The solution accuracy is tested by comparison with several published analytical and analog solutions. An experimental study of the effectiveness of an IBM Rear Door heat exchanger is used to develop the heat exchanger models and to verify the effectiveness of the numerical solution at the same time. Several transient cases are investigated numerically for step, ramp and exponential variations in the inlet temperature of the minimum capacity rate fluid. The transient effectiveness characteristics are tested in parametric studies. The results obtained show that transient effectiveness is a useful method in analyzing and predicting the dynamic performance of heat exchangers.

Alfonso Ortega - One of the best experts on this subject based on the ideXlab platform.

  • VHTX: A code for simulation of steady state and dynamic response of single or multiple networked cross flow heat exchangers in data center thermal management systems
    2017 33rd Thermal Measurement Modeling & Management Symposium (SEMI-THERM), 2020
    Co-Authors: Alfonso Ortega, Marcelo Del Valle, Carol Caceres
    Abstract:

    Crossflow heat exchangers are key components of both centralized (e.g. CRACs) and decentralized (Rear Door, in-row) cooling equipment utilized in data center thermal management systems. Modeling of their behavior in steady state is well documented but transient or dynamic operation, whether by intent or as a result of a system failure, has not been well documented. In “smart cooling” scenarios, cooling should be modulated with heating (i.e. IT) load which can vary with time and space as IT load varies within a rack and within the data center room. Cooling is optimally utilized when the cooling load follows or even anticipates the heating (IT) load and as such the heat exchanger operates in a dynamically controlled mode. In data center operation, there is also interest in understanding their behavior in case of system malfunctions such as pump or chiller failures which results in transient operation. The MATLAB™ simulation code VHTX was developed in order to simulate the performance of crossflow heat exchangers in both steady and dynamic operation. It is a standalone code for simulation of heat exchanger networks and core code elements are also being embedded into or coupled with other simulation environments such as MATLAB SIMULINK™ for control investigations, VTAS for data center system thermodynamic and energy analysis, and CFD codes for room simulations. This paper describes the basic formulation of the VHTX solver and its validation against research quality data on heat exchanger cores. It is shown that the code can accurately predict the coolant flow distribution within the heat exchanger core and its dynamic response to temporal events such as modulation of the coolant flow rate or temperature to match the air side thermal load. A case study simulating a typical Rear Door heat exchanger is presented as an example of the use of the code in a data center simulation.

  • Quasi-steady modeling of data center heat exchanger under dynamic conditions
    2017 16th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2017
    Co-Authors: Marcelo Del Valle, Alfonso Ortega, Kourosh Nemati, Carol Caceres, Bahgat Sammakia
    Abstract:

    The transient modeling of cross flow heat exchangers requires the use of complicated analytical solutions or complex numerical schemes to solve the set of coupled partial differential equations representing the air, coolant and wall temperatures. Since most of the dynamic mass flow or temperature perturbations encountered in data center environment are slow compared with the timescale of the heat exchanger, using a simpler quasi-steady model instead of a complex transient solution represents a good alternative. The present work shows the use of a quasi-steady heat exchanger model to recreate the transient response of a 12×12 in heat exchanger core under ramp and sinusoidal perturbations. A criteria to identify the regimes where the quasi-steady model is valid is developed. Finally the model is used to model a Rear Door heat exchanger and the results compared with experimental results.

  • Numerical and experimental characterization of the transient effectiveness of a water to air heat exchanger for data center cooling systems
    ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems InterPACK 2015 colloc, 2015
    Co-Authors: Tianyi Gao, Marcelo Del Valle, Bahgat Sammakia, Alfonso Ortega
    Abstract:

    Copyright ? 2015 by ASME.The cross flow heat exchanger is at the heart of most cooling systems for data centers. Air/Water or air/refrigerant heat exchangers are the principal component in Central Room Air Conditioning (CRAC) units that condition data room air that is delivered through an underfloor plenum. Liquid/air heat exchangers are also increasingly deployed in close-coupled cooling systems such as Rear Door heat exchangers, in-row coolers, and overhead coolers. In all cases, the performance of liquid/air heat exchangers in both steady state and transient scenarios are of principal concern. Transient scenarios occur either by the accidental failure of the cooling system or by intentional dynamic control of the cooling system. In either scenario, transient boundary conditions involve time-dependent air or liquid inlet temperatures and mass flow rates that may be coupled in any number of potential combinations. Understanding and characterizing the performance of the heat exchanger in these transient scenarios is of paramount importance for designing better thermal solutions and improving the operational efficiency of existing cooling systems. In this paper, the transient performance of water to air cross flow heat exchangers is studied using numerical modeling and experimental measurements. Experimental measurements in 12 in. x 12 in. heat exchanger cores were performed, in which the liquid (water) mass flow rate or inlet temperature are varied in time following controlled functional forms (step jump, ramp). The experimental data were used to validate a transient numerical model developed with traditional assumptions of space averaging of heat transfer coefficients, and volume averaging of thermal capacitances. The complete numerical model was combined with the transient effectiveness methodology in which the traditional heat exchanger effectiveness approach is extended into a transient domain, and is then used to model the heat exchanger transient response. Different transient scenarios were parametrically studied to develop an understanding of the impact of critical variables such as, the fluid inlet temperature variation and the fluid mass flow rate variation, and a more comprehensive understanding of the characteristics of the transient effectiveness. Agreement between the novel transient effectiveness modeling approach and the experimental measurements enable use of the models as verified predictive design tools. Several studies are designed based on the practical problems related to data center thermal environments and the results are analyzed.

  • Thermodynamic Analysis of Hybrid Liquid-Air-Based Data Center Cooling Strategies
    Volume 8B: Heat Transfer and Thermal Engineering, 2014
    Co-Authors: A. Bhalerao, Alfonso Ortega, Aaron P. Wemhoff
    Abstract:

    Energy savings in data center cooling systems equate to cost savings for the data center operator. Recently, investigators have considered alternative hybrid liquid-air technologies for data center thermal management such as in-row coolers, Rear Door heat exchangers, and overhead coolers. This study employs the in-house data center modeling tool Villanova Thermodynamic Analysis of Systems (VTAS) software package to ascertain the influence of hybrid liquid-air components on overall data center exergy destruction. The results show that the exergy destruction decreases for a hybrid liquid-air system using only an in-row cooler. As the in-row cooler removes a greater fraction of heat from the hot aisle, the total exergy destruction in the data center increases by a small amount. This analysis is extended for various configurations containing an in-row cooler, an overhead cooler, a Rear Door heat exchanger, and a CRAH unit to show that using hybrid liquid-air technologies without a CRAH unit is the most thermodynamically favorable.Copyright © 2014 by ASME

  • An exergy-based analysis of the effects of Rear Door heat exchange systems on data center energy efficiency
    Fourteenth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2014
    Co-Authors: Aaron P. Wemhoff, Alfonso Ortega
    Abstract:

    A holistic system-level analysis of the energy and mass transfer in a data center cooling system is used to determine the exergy destruction by each component in the system. The analysis, performed using our in-house analysis code, allows for identification of system inefficiencies and comparison of the energy efficiency of different data center cooling strategies. In this paper, we describe a systematic analysis of the exergy destruction in a traditional air-cooled strategy and a hybrid liquid-air system containing Rear Door heat exchangers (RDHXs). The results show that the exergy destruction by RDHXs increases with the amount of rack heat removal. However, the removal of rack heat concurrently decreases the heat removal and exergy destruction by computer room air handling (CRAH) units. The resultant overall exergy destruction is increased when both RDHX and CRAH units are in operation, but this gain in exergy may be attributed to low heat exchanger effectiveness values. The analysis also shows that when all heat is removed by the RDHX, the exergy destruction is lower than when all heat is removed by the CRAH, suggesting that the data center energy efficiency can be increased through the use of localized hybrid liquid-air cooling schemes as compared to centralized air cooled strategies.

Juan De Dios Ortúzar - One of the best experts on this subject based on the ideXlab platform.

  • What is behind fare evasion in urban bus systems? An econometric approach
    Transportation Research Part A: Policy and Practice, 2016
    Co-Authors: Pablo Guarda, Patricia Galilea, Laurel Paget-seekins, Juan De Dios Ortúzar
    Abstract:

    Fare evasion is a problem in many public transport systems around the world and policies to reduce it are generally aimed at improving control and increasing fines. We use an econometric approach to attempt explaining the high levels of evasion in Santiago, Chile, and guide public policy formulation to reduce this problem. In particular, a negative binomial count regression model allowed us to find that fare evasion rates on buses increase as: (i) more people board (or alight) at a given bus Door, (ii) more passengers board by a Rear Door, (iii) buses have higher occupancy levels (and more Doors) and (iv) passengers experience longer headways. By controlling these variables (ceteris paribus), results indicate that evasion is greater during the afternoon and evening, but it is not clear that it is higher during peak hours. Regarding socioeconomic variables, we found that fare evasion at bus stops located in higher income areas (municipalities) is significantly lower than in more deprived areas. Finally, based on our results we identified five main methods to address evasion as alternatives to more dedicated fine enforcement or increased inspection; (i) increasing the bus fleet, (ii) improving the bus headway regularity, (iii) implementing off-board payment stations, (iv) changing the payment system on board and (v) changing the bus design (number of Doors or capacity). Our model provides a powerful tool to predict the reduction of fare evasion due to the implementation of some of these five operational strategies, and can be applied to other bus public transport systems.

  • What is behind fare evasion? The case of Transantiago
    Transportation Research Board 94th Annual Meeting, 2015
    Co-Authors: Pablo Guarda, Patricia Galilea, Laurel Paget-seekins, Juan De Dios Ortúzar
    Abstract:

    Fare evasion is a problem in many public transport systems around the world and policies to reduce it are generally aimed at improving control and increasing fines. We use an econometric approach to attempt explaining the high levels of evasion in Santiago, Chile, and guide public policy formulation to reduce this problem. In particular, a negative binomial count regression model allowed us to find that fare evasion rates on buses increase as: (i) more people board (or alight) at a given bus Door, (ii) more passengers board by a Rear Door, (iii) buses have higher occupancy levels (and more Doors) and (iv) passenger experience longer headways. By controlling these variables (ceteris paribus), results indicate that evasion is greater during the afternoon and evening, but it is not clear that it is higher during peak hours. Regarding socioeconomic variables, we found that fare evasion at bus stops located in higher income areas (municipalities) is significantly lower than in more deprived areas. Finally, based on our results we identified five main methods to address evasion as alternatives to more dedicated fine enforcement or increased inspection; (i) increasing the bus fleet, (ii) improving the bus headway regularity, iii) implementing off-board payment stations, iv) changing the payment system on board and v) changing the bus design (number of Doors or capacity). Our model provides a powerful tool to predict the reduction of fare evasion due to the implementation of some of these five operational strategies, and can be applied to other bus public transport systems.

Roger R Schmidt - One of the best experts on this subject based on the ideXlab platform.

  • Comparative Thermal and Energy Analysis of a Hybrid Cooling Data Center With Rear Door Heat Exchangers
    Volume 2: Thermal Management; Data Centers and Energy Efficient Electronic Systems, 2013
    Co-Authors: Emad Samadiani, Bahgat Sammakia, Roger R Schmidt
    Abstract:

    Data centers consume a considerable amount of energy which is estimated to be about 2 percent of the total electrical energy consumed in the US, and their power consumption continues to increase every year. It is also estimated that roughly 30–40 percent of the total energy used in a data center is due to the thermal management systems. So, there is a strong need for better cooling methods which could improve the cooling capacity and also reduce energy consumption for high density data centers. In this regard, liquid cooling systems have been utilized to deal with demanding cooling and energy efficiency requirements in high density data centers. In this paper, a hybrid cooling system in data centers is investigated. In addition to traditional raised floor, cold aisle-hot aisle configuration, a liquid-air hybrid cooling system consisting of Rear Door heat exchangers attached to the back of racks is considered. The room is analyzed numerically using two CFD based simulation approaches for modeling Rear Door heat exchangers that are introduced in this study. The presented model is used in the second section of the paper to compare the hybrid cooling system with traditional air cooling systems. Several case studies are taken into account including the power increases in the racks and CRAC unit failure scenarios. A comparison is made between the hybrid cooling room and a purely air cooled room based on the rack inlet temperatures. Also in this study, total energy consumption by the cooling equipment in both air-cooled and hybrid data centers are modeled and compared with each other for different scenarios. The results show that under some circumstances the hybrid cooling could be an alternative to meet the ASHRAE recommended inlet air temperatures, while at the same time it reduces the cooling energy consumption in high density data centers.

  • Dynamic Analysis of Hybrid Cooling Data Centers Subjects to the Failure of CRAC Units
    Volume 2: Thermal Management; Data Centers and Energy Efficient Electronic Systems, 2013
    Co-Authors: Emad Samadiani, Roger R Schmidt, Bahgat Sammakia
    Abstract:

    Thermal management of high power data centers poses challenges due to the high operational cost which is made worse due to the many inefficiencies that arise in them. Additional challenges arise due to the dynamic behaviors that occur during normal operation, and also during emergencies such as power outages or failure of some or all of the cooling equipment. Water and hybrid air plus water cooled data centers are an alternate cooling solution combining liquid cooling systems, such as Rear Door heat exchangers located within the racks themselves, in addition to the traditional raised floor cold aisle air cooling system. Such a solution may be used when some of the equipment in a data center is upgraded to higher end and higher power equipment which may not be manageable with the existing air cooling system. For a data center with a hybrid cooling system, the cold air supply and the cold water supply should increase in case of an emergency, such as a CRAC (Computer Room Air Conditioner) units’ failure. In this paper, a detailed computational study is conducted to investigate the dynamic response of the impact of a CRAC failure on both water side and air side in a representative hybrid cooling room. The room studied is an air cooled data center using the common cold aisle approach, with Rear Door heat exchangers installed on all of the racks. CRAC failure is investigated in a hybrid cooling room. The variation and fluctuation in an average rack inlet temperature, and inlet temperatures at different detail locations are presented in plots, showing the dynamic performance of a hybrid cooling data center subjected to the different CRAC failure scenarios. Different response time studies are also presented in this paper.Copyright © 2013 by ASME

  • Experimental Characterization of Server Rack Energy Use at Elevated Ambient Temperatures
    ASME 2011 Pacific Rim Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Systems MEMS and NEMS: Volume 2, 2011
    Co-Authors: Madhusudan K Iyengar, Roger R Schmidt, Vinod Kamath, Bejoy Kochuparambil
    Abstract:

    It is now common for data center managers to question the impact on server energy usage of two recent impact factors: (1) the rise in the data center inlet air temperature to servers per 2008 ASHRAE guidelines, and (2) the fan speed increase from the use of rack level heat exchangers such as Rear Door Heat Exchangers. To help acquire a deeper understanding of the relevant issues, a system floor thermal test was built on the IBM New York data center benchmark floor which consisted of a standard 19″ rack filled with 39 3.0 GHz 1U servers that dissipated between 10–17 kW depending on extent of server utilization. Fan speed, chip temperature metrics, and server power data was collected using product debug codes and server level programs. A simulated air heat load was installed right in front of this server rack to allow the manipulation of air inlet temperature into the servers from 20 °C to 32 °C. Two different rack level configurations were considered for the experiments: (i) a perforated front Door and no Door at the Rear, and (ii) a perforated front Door and a Rear Door Heat Exchanger at the Rear. An exerciser program was used to vary the CPU utilization from Idle to 70% which represented a typical data center work load. Data was collected for 19 servers of the 39 servers (remaining were in use by Benchmark Lab) for the two rack configurations, for 4 inlet server air temperatures, and for two chip exerciser settings, i.e. 16 experiments. For the 70% exerciser setting (typical operation) and the base line rack configuration without rack level heat exchangers, the rise in server power for an increase in inlet air temperature was 5.2% for the 20 °C to 27 °C change and was 17% for the 20 °C to 31 °C change. For the 70% exerciser setting (typical), the increase in server power from the use of rack level heat exchangers (Rear Door Heat Exchanger) was less than 1.3% for all the conditions. Given the broad range of fan speed algorithms and cooling hardware in server products on the market and their change over each generation, significant further study will be required to characterize each category of systems for these conditions. However, the present study provides a template for quantifying server energy usage in a context that data center managers can understand and use.Copyright © 2011 by ASME

  • Rear Door Heat Exchanger Cooling Performance in Telecommunication Data Centers
    Volume 4: Electronics and Photonics, 2010
    Co-Authors: Venkata Naga Poornima Mynampati, Saket Karajgikar, Ibraheem Sheerah, Dereje Agonafer, Shlomo Novotny, Roger R Schmidt
    Abstract:

    The increase in the data center server heat density waves a scope for developing improved cooling technologies without raising the power consumption. It is commonly observed that 40% of the total data center energy is consumed by its cooling equipments. For higher server density cabinets, typical air cooling techniques leads to a substantial increase in the power consumption. Rear Door heat exchanger, an open looped cooling technique is one of the solutions for such scenarios. In this paper, emphasis is laid on the analytical determination of the optimum heat load after calculating the effectiveness of heat exchanger at given operating conditions of the data center and heat exchanger. Later, thermal analysis is performed and the working of heat exchanger is compared for different data center heat loads. Based on the results, a ‘rule of thumb’ is verified that Rear Door heat exchanger could be 100% efficient in cooling the cabinets of heat loads up to 27kW. Thus, for rack heat loads less than 27KW, CRAC units can be non-operational resulting in energy savings. Furthermore, effect of RDHx in different configuration is studied and compared.Copyright © 2010 by ASME

  • server rack Rear Door heat exchanger and the new ashrae recommended environmental guidelines
    ASME 2009 InterPACK Conference collocated with the ASME 2009 Summer Heat Transfer Conference and the ASME 2009 3rd International Conference on Energy , 2009
    Co-Authors: Roger R Schmidt, Madhusudan K Iyengar
    Abstract:

    The patented [1] Rear Door heat exchanger mounted to the Rear of IT equipment racks was announced in April, 2005 by IBM and has shown improvements in data center energy efficiency and reducing hot spots. It also allows data center operators to more easily implement some of the features of the newly approved ASHRAE data center recommended data center environmental guidelines [2]. This paper will describe several case studies of implementing the Rear Door heat exchanger in various data center layouts. The implementation of the water cooled Rear Door in these data centers will show the effects of various failure modes and how the new ASHRAE environmental temperature guidelines are still being met with the failure modes examined.Copyright © 2009 by ASME

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