Experimental Characterization of the Transient Response of Air/Water Crossflow Heat Exchangers for Data Center Cooling Systems

2015 ◽  
Author(s):  
Marcelo del Valle ◽  
Alfonso Ortega
Keyword(s):  
Heat Exchanger ◽  
Water Flow ◽  
Transient Response ◽  
Data Center ◽  
Heat Exchangers ◽  
The Other ◽  
Experimental Characterization ◽  
Cooling Systems ◽  
Hybrid Cooling

Data Center hybrid air/liquid cooling systems such as rear door heat exchangers, overhead and in row cooling systems enable localized, on-demand cooling, or “smart cooling.” At the heart of all hybrid cooling systems is an air to liquid cross flow heat exchanger that regulates the amount of cooling delivered by the system by modulating the liquid or air flows and/or temperatures. Due the central role that the heat exchanger plays in the system response, understanding the transient response of the heat exchanger is crucial for the precise control of hybrid cooling system. This paper reports on the transient experimental characterization of heat exchangers used in data centers applications. An experimental rig designed to introduce controlled transient perturbations in temperature and flow on the inlet air and liquid flow streams of a 12 in. × 12 in. heat exchanger test core is discussed. The conditioned air is delivered to the test core by a suction wind tunnel with upstream air heaters and a frequency variable axial blower to allow the control of air flow rate and bulk temperature. The conditioned water is delivered to the test core by a water delivery system consisting of two separate water circuits, one delivering cold water, and the other hot water. By switching from one circuit to the other or mixing water from both circuits, the rig is capable of generating step, ramp and frequency perturbations in water temperature at constant flow or step, ramp or frequency perturbations in water flow at constant temperature or combinations of temperature and water flow perturbations. Experimental data are presented for a 12×12 heat exchanger core with a single liquid pass under different transient perturbations.

Experimentally Verified Transient Models of Data Center Crossflow Heat Exchangers

2014 ◽  
Author(s):  
Tianyi Gao ◽  
Bahgat Sammakia ◽  
James Geer ◽  
Milnes David ◽  
Roger Schmidt
Keyword(s):  
Heat Exchanger ◽  
Data Center ◽  
Heat Exchangers ◽  
Distribution Systems ◽  
Data Centers ◽  
Computational Time ◽  
Cooling Systems ◽  
Rear Door ◽  
Compact Models ◽  
Liquid Heat

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.

Numerical and Experimental Characterization of the Transient Effectiveness of a Water to Air Heat Exchanger for Data Center Cooling Systems

2015 ◽  
Author(s):  
Tianyi Gao ◽  
Marcelo del Valle ◽  
Alfonso Ortega ◽  
Bahgat G. Sammakia
Keyword(s):  
Heat Exchanger ◽  
Mass Flow ◽  
Data Center ◽  
Heat Exchangers ◽  
Cooling System ◽  
Cross Flow ◽  
Inlet Temperature ◽  
Experimental Measurements ◽  
Cooling Systems ◽  
Flow Heat

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. × 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.

Comparative Thermal and Energy Analysis of a Hybrid Cooling Data Center With Rear Door Heat Exchangers

2013 ◽  
Author(s):  
Tianyi Gao ◽  
Emad Samadiani ◽  
Bahgat Sammakia ◽  
Roger Schmidt
Keyword(s):  
Energy Consumption ◽  
Data Center ◽  
Heat Exchangers ◽  
Data Centers ◽  
Cooling System ◽  
High Density ◽  
Cooling Systems ◽  
Density Data ◽  
Rear Door ◽  
Hybrid Cooling

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.

Perimeter Cooling Unit and Localized Row-Based Cooling Unit Transient Air Flow Effects Modeling and Characterization in Data Centers

2015 ◽  
Author(s):  
Tianyi Gao ◽  
James Geer ◽  
Bahgat G. Sammakia ◽  
Russell Tipton ◽  
Mark Seymour
Keyword(s):  
Thermal Management ◽  
Data Center ◽  
Data Centers ◽  
Cooling System ◽  
Air Flow ◽  
Cooling Systems ◽  
Dynamic Thermal Management ◽  
Hybrid Cooling ◽  
Cooling Unit ◽  
Cooling Air

Cooling power constitutes a large portion of the total electrical power consumption in data centers. Approximately 25%∼40% of the electricity used within a production data center is consumed by the cooling system. Improving the cooling energy efficiency has attracted a great deal of research attention. Many strategies have been proposed for cutting the data center energy costs. One of the effective strategies for increasing the cooling efficiency is using dynamic thermal management. Another effective strategy is placing cooling devices (heat exchangers) closer to the source of heat. This is the basic design principle of many hybrid cooling systems and liquid cooling systems for data centers. Dynamic thermal management of data centers is a huge challenge, due to the fact that data centers are operated under complex dynamic conditions, even during normal operating conditions. In addition, hybrid cooling systems for data centers introduce additional localized cooling devices, such as in row cooling units and overhead coolers, which significantly increase the complexity of dynamic thermal management. Therefore, it is of paramount importance to characterize the dynamic responses of data centers under variations from different cooling units, such as cooling air flow rate variations. In this study, a detailed computational analysis of an in row cooler based hybrid cooled data center is conducted using a commercially available computational fluid dynamics (CFD) code. A representative CFD model for a raised floor data center with cold aisle-hot aisle arrangement fashion is developed. The hybrid cooling system is designed using perimeter CRAH units and localized in row cooling units. The CRAH unit supplies centralized cooling air to the under floor plenum, and the cooling air enters the cold aisle through perforated tiles. The in row cooling unit is located on the raised floor between the server racks. It supplies the cooling air directly to the cold aisle, and intakes hot air from the back of the racks (hot aisle). Therefore, two different cooling air sources are supplied to the cold aisle, but the ways they are delivered to the cold aisle are different. Several modeling cases are designed to study the transient effects of variations in the flow rates of the two cooling air sources. The server power and the cooling air flow variation combination scenarios are also modeled and studied. The detailed impacts of each modeling case on the rack inlet air temperature and cold aisle air flow distribution are studied. The results presented in this work provide an understanding of the effects of air flow variations on the thermal performance of data centers. The results and corresponding analysis is used for improving the running efficiency of this type of raised floor hybrid data centers using CRAH and IRC units.

Minimum Irreversibility Criteria for Heat Exchanger Configurations

1999 ◽  
Vol 121 (4) ◽  
pp. 241-246 ◽  
Author(s):  
F. E. M. Saboya ◽  
C. E. S. M. da Costa
Keyword(s):  
Heat Capacity ◽  
Heat Exchanger ◽  
Heat Exchangers ◽  
Cross Flow ◽  
Second Law Of Thermodynamics ◽  
The Other ◽  
Maximum Heat ◽  
Transfer Rates ◽  
Mass Flow Rates ◽  
Flow Heat

From the second law of thermodynamics, the concepts of irreversibility, entropy generation, and availability are applied to counterflow, parallel-flow, and cross-flow heat exchangers. In the case of the Cross-flow configuration, there are four types of heat exchangers: I) both fluids unmixed, 2) both fluids mixed, 3) fluid of maximum heat capacity rate mixed and the other unmixed, 4) fluid of minimum heat capacity rate mixed and the other unmixed. In the analysis, the heat exchangers are assumed to have a negligible pressure drop irreversibility. The Counterflow heat exchanger is compared with the other five heat exchanger types and the comparison will indicate which one has the minimum irreversibility rate. In this comparison, only the exit temperatures and the heat transfer rates of the heat exchangers are different. The other conditions (inlet temperatures, mass flow rates, number of transfer units) and the working fluids are the same in the heat exchangers.

Experimental Characterization of Two-Phase Flow Through Valves Applied to Liquid-Assisted Gas-Lift

2020 ◽  
Vol 142 (6) ◽  
Author(s):  
Renato P. Coutinho ◽  
Paulo J. Waltrich ◽  
Wesley C. Williams ◽  
Parviz Mehdizadeh ◽  
Stuart Scott ◽  
...  
Keyword(s):  
Water Flow ◽  
Two Phase Flow ◽  
Numerical Study ◽  
Water Flow Rate ◽  
Phase Flow ◽  
Experimental Characterization ◽  
Two Phase ◽  
Flow Through ◽  
Gas Lift

Abstract Liquid-assisted gas-lift (LAGL) is a recently developed concept to unload wells using a gas–liquid fluid mixture. The success deployment of the LAGL technology is related to the behavior of two-phase flow through gas-lift valves. For this reason, this work presents an experimental and numerical study on two-phase flow through orifice gas-lift valves used in liquid-assisted gas-lift unloading. To the knowledge of the authors, there is no investigation in the literature on experimental characterization of two-phase flow through gas-lift valves. Experimental data are presented for methane-water flow through gas-lift valves with different orifice port sizes: 12.7 and 17.5 mm. The experiments were performed for pressures ranging from 1.00 to 9.00 MPa, gas flow rates from 0 to 4.71 m3/h, and water flow rate from 0 to 0.68 m3/min. The experimental results are compared to numerical models published in the literature for two-phase flow through restrictions and to commercial multiphase flow simulators. It is observed that some models developed for two-phase flow through restrictions could successfully characterize two-phase flow thorough gas-lift valves with errors lower than 10%. However, it is first necessary to experimentally determine the discharge coefficient (CD) for each gas-lift valve. The commercial flow simulators showed a similar performance as the models available in the literature.

Effect of Flow Maldistribution and Viscosity Variations on Transient Response of Plate Heat Exchangers: A Comparison With Uniform Flow and Constant Viscosity Model

Volume 1 ◽  
2004 ◽  
Author(s):  
H. Shokouhmand ◽  
N. Khareghani
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Transient Response ◽  
Heat Exchangers ◽  
Uniform Flow ◽  
Viscosity Model ◽  
Step Response ◽  
Plate Heat ◽  
Constant Viscosity ◽  
Flow Maldistribution

In this paper, transient response of plate heat exchangers under flow maldistribution and viscosity variations is discussed. This transient response is compared with the response achieved from uniform flow and constant viscosity through the exchanger. Flow maldistribution (unequal flow in channels) is calculated for U and Z types of plate heat exchangers. This flow maldistribution along with viscosity variations, during the growth of the temperature profile in each channel, affect the convective heat transfer coefficient in the transient period of heat transfer, and make it to be different from that of the other channels. These conditions make the transient response of a plate heat exchanger to have some deviations from the uniform flow and constant viscosity model response, which is discussed in this paper. The governing equations of heat transfer are solved using finite difference methods. Frequency response as well as step response of the heat exchanger is implemented as a time dependent initial conditions.

scholarly journals Experimental characterization of water flow through smooth rectangular microchannels

2005 ◽  
Vol 17 (9) ◽  
pp. 098105 ◽  
Author(s):  
R. Baviere ◽  
F. Ayela ◽  
S. Le Person ◽  
M. Favre-Marinet
Keyword(s):  
Water Flow ◽  
Experimental Characterization ◽  
Rectangular Microchannels ◽  
Flow Through
Sign in / Sign up

Export Citation Format

Share Document