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
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A Design Methodology for Controlling Local Airflow Delivery in Data Centers Using Air Dampers
Optimizing data centers for energy efficiency plays a key role in the successful operation of modern data centers. One important factor is the proper management of airflow. In most air-cooled data centers, the required airflow for cooling of IT equipment is supplied from a raised floor to server racks through perforated tiles. In recent years, different approaches have been implemented to increase the efficiency of air delivery through tiles such as the use of directional tiles, adding understructure scoops or using air dampers [1].
Because the IT load of each rack in the data center is constantly changing due to the processing demands of the IT hardware at a given time, simultaneous manual tuning of the airflow at the panel level is impossible or at least impractical. The amount of airflow delivered to the Cold Aisle Containment (CAC) can be adjusted using Variable Airflow Panels (Dampers) that can be controlled remotely. In this study, we design and optimize a fuzzy control system to control the open area ratio of air dampers in order to adjust the local airflow rate in the ES2 data center.
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- Award ID(s):
- 1738793
- NSF-PAR ID:
- 10095767
- Date Published:
- Journal Name:
- ITHERM
- ISSN:
- 1936-3958
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The dynamic nature of today’s data centers requires active monitoring and holistic management of all aspects of the facility, from the applications to the air conditioning. The most significant aspect of implementing a dynamic data center is the requirement to actively monitor and manage the infrastructure assets. It is vital to ensure information technology (IT) equipment has access to sufficient air (provisioned) at a proper temperature to assure their optimal and continues operation. Hot air recirculation, elevated fan speed, and hot spots are known consequences of an under-provisioned cold aisle. On the other hand, over-provisioning a cold aisle can lead to a significant loss in energy due to bypass of cooling air and leakages. Besides, the number of active servers in an aisle may be varied by load balancers due to short or long-term IT load changes. This demonstrates the need for an active airflow management scheme that is able to respond to airflow demand in different aisles of a data center. In this study, remotely controllable air dampers are implemented to regulate airflow delivery to a cold aisle containment (CAC) during workload changes in a data center. The energy saving opportunities are investigated and practical considerations are discussed.more » « less
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Most of the thermal management technologies concentrate on managing airflow to achieve the desired server inlet temperature (supply air operating set point) and not to manage/improve the amount of cool air (CFM) that each computer rack (i.e. IT servers) should receive in order to remove the produced heat. However, airflow is equally important for quantifying adequate cooling to IT equipment, but it is more challenging to obtain a uniform airflow distribution at the inlet of computer racks. Therefore, as a potential option for improving airflow distribution is to eliminate the sources of non-uniformities such as maldistribution of under-floor plenum pressure field caused by vortices. Numerous researchers focus on the adverse effects of under-floor blockages. This study focused to numerically investigate the positive impact of selectively placed obstructions (on-purpose air-directors); referred as partitions; Quantitative and qualitative analysis of underfloor plenum pressure field, perforated tiles airflow rate and racks inlet temperature with and without partitions using two Computational Fluid Dynamics (CFD) models, which were built using Future Facilities 6SigmaRoom CFD tool. First, a simple data center model was used to quantify the partitions benefits for two different systems; Hot Aisle Containment (HAC) compared to an open configuration. Second, the investigation was expanded using a physics-based experimentally validated CFD model of medium size data center (more complicated data center geometry) to compare different types of proposed partitions. Both models results showed that partition type I (partitions height of $\frac{2}{3}$ of plenum depth measured from the subfloor) eliminates the presence of vortices in the under-floor plenum and hence, more uniform pressure differential across the perforated tiles that drives more uniform airflow rates. In addition, the influence of proposed partitions on the rack inlet temperature was reported through a comparison between open versus hot aisle containment. The results showed that the partitions have a minor effect on the rack inlet temperature for the hot aisle containment system. However, the partitions significantly improve the tiles flowrate. On the other hand, for the open system, the presence of partitions has improved the tiles airflow rate, rack inlet temperature and hence eliminate the hot spots formation at computer rack inletmore » « less
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In raised floor data centers, tiles with high open area ratio or complex understructure are used to fulfill the demand of today’s high-density computing. Using more open tiles reduces pressure drop across the raised floor with the potential advantages of increased airflow and lower noise. However, it introduces the disadvantage of increased non-uniformity of airflow distribution. In addition, there are various tile designs available on the market with different opening shapes or understructures. Furthermore, a physical separation of cold and hot aisles (containment) has been introduced to minimize the mixing of cold and hot air. In this study, three types of floor tiles with different open area, opening geometry, and understructure are considered. Experimentally validated detail models of tiles were implemented in CFD simulations to address the impact of tile design on the cooling of IT equipment in both open and enclosed aisle configurations. Also, impacts of under-cabinet leakage on the IT equipment inlet temperature in the provisioned and under-provisioned scenarios are studied. Finally, a predictive equation for the critical under-provisioning point that can lead to a no-flow condition in IT equipment with weaker airflow systems is presented.more » « less
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