Marcelo del Valle

Investigation of exergy destruction in CFD modeling for a legacy air-cooled data center

To keep up with the growing energy demand, legacy aircooled data centers have begun to implement simple but effective energy efficiency strategies or best practices. Improving “air management”, optimizing the delivery of cool air and the collection of waste heat, are among these efficiency strategies. One of the most powerful tools for identifying energy wasteful practices is the application of the second law of thermodynamics to estimate the exergy destruction in a system. Exergy is defined as the “available energy” in a system, hence wasteful practices are said to “destroy exergy.” Systematic analysis of data centers using this approach will identify the major contributors to wasteful energy practices including premature mixing of hot and cold air streams. In this study, a numerical model was developed for estimation of the data center airspace exergy destruction following the three-dimensional CFD simulation of room airspace. A second exergy destruction model available in the literature was implemented as well. Both models were tested on one simplified case with available analytical solution. They were then utilized to estimate the airspace exergy destruction in an existing research data center. Both approaches proved to be adequate, although the one proposed in this work presented significant sensitivity to numerical inaccuracies compared to the second model, even leading to zones of artificial negative exergy destruction.

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

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.Copyright

Transient modeling and validation of chilled water based cross flow heat exchangers for local on-demand cooling in data centers

Hybrid air/liquid cooling systems used in data centers enable localized, on-demand cooling, or “smart cooling” using various approaches such as rear door heat exchangers, overhead cooling systems and in row cooling systems. These systems offer the potential to achieve higher energy efficiency by providing local cooling only when it is needed, thereby reducing the overprovisioning that is endemic to traditional systems. At the heart of all hybrid cooling systems is an air to liquid cross flow heat exchanger which regulates the amount of cooling that the system provides by modulating the liquid or air flows or temperatures. Understanding the transient response of the heat exchanger is crucial for the precise control of the system. In this paper a 12 in. × 12 in water to air heat exchanger, with similar characteristics to the heat exchanger commonly found in data centers, is modeled using three partial differential equations solved by the use of a finite difference approach. The model is validated against experimental data obtained from an experimental rig designed to introduce controlled transient perturbations in temperature and flow on the inlet air and liquid flows to the heat exchanger. Experimental data were obtained for step change, ramp change, and sinusoidal variation in the inlet water temperature and mass flow rate. Steady state heat transfer coefficients are used in the air and liquid side of the heat exchanger. The heat transfer coefficient inside the tubes is calculated by the use of the Gnielinski correlation. A steady state technique is used to extract the air side heat transfer coefficient. With these parameters, it was found that the dynamic heat exchanger model agrees remarkably well with the transient experimental data. The modeling equations also provide insight into the characteristic response times of the heat exchanger in terms of the major independent non-dimensional parameters describing its design and operating conditions.

Numerical and compact models to predict the transient behavior of cross-flow heat exchangers in data center applications

Hybrid air/liquid cooling systems used in data centers enable localized, on-demand cooling, or “smart cooling” using various approaches such as rear door heat exchangers, overhead cooling systems and in row cooling systems. These systems offer the potential to achieve higher energy efficiency by providing local cooling only when it is needed, thereby reducing the overprovisioning that is endemic to traditional systems. At the heart of all hybrid cooling systems is an air to liquid cross flow heat exchanger which regulates the amount of cooling that the system provides by modulating the liquid or air flows or temperatures. Understanding the transient response of the heat exchanger is crucial for the precise control of the system. The aim of this work is the development of a rear door heat exchanger compact model using Artificial Neural Networks (ANN). The transient behavior of the heat exchanger is studied using a Finite Difference (FD) model. Different temperatures perturbations are introduced in the heat exchanger model to study its transient response. The finite different results are then used to train an ANN compact model. Both models are compared in terms of accuracy and computational resources.

SIMULATION OF TWO-PHASE FLOW AND HEAT TRANSFER IN MINI- AND MICRO-CHANNELS FOR CONCENTRATING PHOTOVOLTAICS COOLING

Advances in concentrating photovoltaics technology have generated a need for more effective thermal management techniques. Research in photovoltaics has shown that there is a more than 50% decrease in PV cell efficiency when operating temperatures approach 60°C. It is estimated that a waste heat load in excess of 500 W/cm2 will need to be dissipated at a solar concentration of 10,000 suns. Mini- and micro-scale heat exchangers provide the means for large heat transfer coefficients with single phase flow due to the inverse proportionality of Nusselt number with respect to the hydraulic diameter. For very high heat flux situations, single phase forced convection in micro-channels may not be sufficient and hence convective flow boiling in small scale heat exchangers has gained wider scrutiny due to the much higher achievable heat transfer coefficients due to latent heat of vaporization and convective boiling. The purpose of this investigation is to explore a practical and accurate modeling approach for simulating multiphase flow and heat transfer in mini- and micro-channel heat exchangers. The work is specifically aimed at providing a modeling tool to assist in the design of a mini/micro-scale stacked heat exchanger to operate in the boiling regime. The flow side energy and momentum equations have been implemented using a one-dimensional homogeneous approach, with local heat transfer coefficients and friction factors supplied by literature correlations. The channel flow solver has been implemented in MATLAB™ and embedded within the COMSOL™ FEM solver which is used to model the solid side conduction problem. The COMSOL environment allows for parameterization of design variables leading to a fully customizable model of a two-phase heat exchanger.© 2011 ASME

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

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.Copyright

Meteorological assessment and implementation of an air-side free-cooling system for data centers in Chile

Data center energy consumption in Latin America has increased considerably during last years. According to Datacenter dynamics, energy requirements during 2016 were expected to be around 3.85 GW. In Chile, the data center industry grew 14% between 2009 and 2010, whereas energy consumption increased 21.4% between 2012 and 2013. For this reason, many data centers in the country have started to evaluate efficient alternatives to reduce energy consumption such as the use of air containment techniques, air-side and water-side cooling systems. To date, existing free-cooling maps do not provide information about available hours during the year for implementing either air-side or water-side cooling systems in data centers in South America. This paper presents a thermo-dynamic analysis aimed to evaluate the potential use of air-side free-cooling systems in the Chilean data center industry. First, temperature and Relative Humidity (RH) variations, during three years, were obtained at 29 different stations throughout the entire country. The objective was to identify regions in Chile that meet data center thermal requirements proposed by the ASHRAE. Fiber-optic availability was also considered during the analysis. The thermodynamic model considered a white room with a thermal load of 20 kW, for which an air treatment unit was incorporated with the objective of providing cold air at 18° and 60% RH. An air treatment system was calculated at three different locations in Chile. These locations were selected since they offer high availability of fiber-optic connections (Chacalluta, Arica y Parinacota Region), strategic position for companies (Quinta Normal, Metropolitan Region), and low temperatures through the year (Carlos Ibanez, Aysen Region). Preliminary results have demonstrated that Chile is a relatively humid country. For this reason, cooling air must be dehumidified most of the time. The results also showed that even when low temperatures can be found in Carlos Ibanez, both Chacalluta and Quinta Normal offer excellent possibilities for the data centers industry. These two last locations offer more fiber-optic connections and temperature variations that lay within the range established by the ASHRAE.

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

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.

The Effectiveness of Data Center Overhead Cooling in Steady and Transient Scenarios: Comparison of Downward Flow to a Cold Aisle Versus Upward Flow From a Hot Aisle

The most common approach to air cooling of data centers involves the pressurization of the plenum beneath the raised floor and delivery of air flow to racks via perforated floor tiles. This cooling approach is thermodynamically inefficient due in large part to the pressure losses through the tiles. Furthermore, it is difficult to control flow at the aisle and rack level since the flow source is centralized rather than distributed. Distributed cooling systems are more closely coupled to the heat generating racks. In overhead cooling systems, one can distribute flow to distinct aisles by placing the air mover and water cooled heat exchanger directly above an aisle. Two arrangements are possible: (i.) placing the air mover and heat exchanger above the cold aisle and forcing downward flow of cooled air into the cold aisle (Overhead Downward Flow (ODF)), or (ii.) placing the air mover and heat exchanger above the hot aisle and forcing heated air upwards from the hot aisle through the water cooled heat exchanger (Overhead Upward Flow (OUF)). This study focuses on the steady and transient behavior of overhead cooling systems in both ODF and OUF configurations and compares their cooling effectiveness and energy efficiency. The flow and heat transfer inside the servers and heat exchangers are modeled using physics based approaches that result in differential equation based mathematical descriptions. These models are programmed in the MATLAB™ language and embedded within a CFD computational environment (using the commercial code FLUENT™) that computes the steady or instantaneous airflow distribution. The complete computational model is able to simulate the complete flow and thermal field in the airside, the instantaneous temperatures within and pressure drops through the servers, and the instantaneous temperatures within and pressure drops through the overhead cooling system. Instantaneous overall energy consumption (1st Law) and exergy destruction (2nd Law) were used to quantify overall energy efficiency and to identify inefficiencies within the two systems. The server cooling effectiveness, based on an effectiveness-NTU model for the servers, was used to assess the cooling effectiveness of the two overhead cooling approaches.Copyright

Simulation of Two-Phase Flow and Heat Transfer for Practical Design of Mini- and Micro-Channel Heat Exchangers

Mini- and micro-scale heat exchangers provide the means for large heat transfer coefficients with single phase flow due to the inverse proportionality of Nusselt number with respect to the hydraulic diameter. For very high heat flux situations, single phase forced convection in micro-channels may not be sufficient and hence convective flow boiling in small scale heat exchangers has gained wider scrutiny due to the much higher achievable heat transfer coefficients due to latent heat of vaporization and convective boiling. The purpose of this investigation is to explore a practical and accurate modeling approach for simulating multiphase flow and heat transfer in stacked mini- and micro-channel heat exchangers. The work is specifically aimed at providing the framework for the optimization of such devices. The model algorithm is described in detail and the effects of channel hydraulic diameter ranging from 150–300 μm and number of stacked layers on the thermal and hydrodynamic performance of the heat sinks are explored. The results from the two parameter study are used to suggest a design path for creating an optimal two-phase stacked microchannel heat exchanger.Copyright