Study of CFD-Based Raised-Floor Data Center Cooling With Parametric CRAC Turbofan Blower Airflow Patterns

Commonly encountered thermal management challenges of today’s rapidly changing power density, raised-floor hot/cold aisle data centers include typically uncontrollable tile flow non-uniformity along the above-floor cold aisle. For example, the operational cooling provision intensity near the Computer Room Airflow Conditioner (CRAC) unit can be far less than that on the other side (far away from the CRAC unit). This undesired trend leads to an unbalanced aisle-level air cooling and subsequent inefficient power consumption. In this study, the CRAC turbofan blower flow boundary conditions were thoroughly investigated. Computational Fluid Dynamics (CFD) based simulations were employed to describe and evaluate the differently configured CRAC turbofan blower flow conditions (i.e., normal, angled, and sheared CRAC flow patterns) as well as their impacts upon the air cooling performance. This work indicates that the considered turbofan blower boundary condition, together with their underlying transportation mechanism within the plenum, might contribute an essential influence to the flow structure adjacent to the tile perforations. In particular, it was found that the sheared CRAC turbofan blower airflow pattern is capable of giving rise to favorable tile flow straightening manners. This finding further promotes an improvement of the consequently obtained aisle-level air cooling effectiveness and efficiencies, contributing to more advanced data center thermal management in the future.

Assessment of Impacts of Climate Change on Tile Discharge and Nitrogen Yield Using the DRAINMOD Model

The detrimental impacts of agricultural subsurface tile flows and their associated pollutants on water quality is a major environmental issue in the Great Lakes region and many other places globally. A strong understanding of water quality indicators along with the contribution of tile-drained agriculture to water contamination is necessary to assess and reduce a significant source of non-point source pollution. In this study, DRAINMOD, a field-scale hydrology and water quality model, was applied to assess the impact of future climatic change on depth to water table, tile flow and associated nitrate loss from an 8.66 ha agricultural field near Londesborough, in Southwestern Ontario, Canada. The closest available climate data from a weather station approximately 10 km from the field site was used by the Ontario Ministry of Natural Resources and Forestry (MNRF) to generate future predictions of daily precipitation and maximum and minimum air temperatures required to create the weather files for DRAINMOD. Of the 28 models applied by MNRF, three models (CGCM3T47-Run5, GFDLCM2.0, and MIROC3.2hires) were selected based on the frequency of the models recommended for use in Ontario with SRA1B emission scenario. Results suggested that simulated tile flows and evapotranspiration (ET) in the 2071–2100 period are expected to increase by 7% and 14% compared to 1960–1990 period. Results also suggest that under future climates, significant increases in nitrate losses (about 50%) will occur along with the elevated tile flows. This work suggests that climate change will have a significant effect on field hydrology and water quality in tile-drained agricultural regions.

Data Center Thermal Efficiency Improvement by Cooling Flow Vectoring Using Synthetic Jets

Electronic cooling represents a major portion of a Data Centers energy consumption, thus efficient thermal management dramatically impacts energy savings. This work proposes reducing the energy consumption associated with server air-cooling by vectoring (tilting) the main upward tile flow using adjacent synthetic jets. The particular fluid dynamics generated by synthetic jets allows controlling the angle at which the tile flow emanates, directing the cooling air toward areas with higher cooling demand. Three-dimensional simulations were performed using k–ε standard turbulence model with the commercial software Ansys Fluent. In order to quantify and localize the inefficiencies of the system, we estimated the Exergy Destruction distribution in the cold aisle and servers. In previous studies, this technique proved successful in finding optimum operation conditions in Data Center cooling. As opposed to a base case without flow control, the adjacent synthetic jets directed the incoming fluid to areas with higher cooling demand, thus saving energy by avoiding over-provisioning air into servers operating under normal demand. The decrease in the overall Exergy Destruction demonstrated that vectoring improves the system’s global energy efficiency.

Comparison of performance of tile drainage routines in SWAT 2009 and 2012 in an extensively tile-drained watershed in the Midwest

Subsurface tile drainage systems are widely used in agricultural watersheds in the Midwestern US and enable the Midwest area to become highly productive agricultural lands, but can also create environmental problems, for example nitrate-N contamination associated with drainage waters. The Soil and Water Assessment Tool (SWAT) has been used to model watersheds with tile drainage. SWAT2012 revisions 615 and 645 provide new tile drainage routines. However, few studies have used these revisions to study tile drainage impacts at both field and watershed scales. Moreover, SWAT2012 revision 645 improved the soil moisture based curve number calculation method, which has not been fully tested. This study used long-term (1991–2003) field site and river station data from the Little Vermilion River (LVR) watershed to evaluate performance of tile drainage routines in SWAT2009 revision 528 (the old routine) and SWAT2012 revisions 615 and 645 (the new routine). Both the old and new routines provided reasonable but unsatisfactory (NSE < 0.5) uncalibrated flow and nitrate loss results for a mildly sloped watershed with low runoff. The calibrated monthly tile flow, surface flow, nitrate-N in tile and surface flow, sediment and annual corn and soybean yield results from SWAT with the old and new tile drainage routines were compared with observed values. Generally, the new routine provided acceptable simulated tile flow (NSE = 0.48–0.65) and nitrate in tile flow (NSE = 0.48–0.68) for field sites with random pattern tile and constant tile spacing, while the old routine simulated tile flow and nitrate in tile flow results for the field site with constant tile spacing were unacceptable (NSE = 0.00–0.32 and −0.29–0.06, respectively). The new modified curve number calculation method in revision 645 (NSE = 0.50–0.81) better simulated surface runoff than revision 615 (NSE = −0.11–0.49). The calibration provided reasonable parameter sets for the old and new routines in the LVR watershed, and the validation results showed that the new routine has the potential to accurately simulate hydrologic processes in mildly sloped watersheds.

Comparison of performance of tile drainage routines in SWAT 2009 and 2012 in an extensively tile-drained watershed in the Midwest

Subsurface tile drainage systems are widely used in agricultural watersheds in the Midwestern U.S. Tile drainage systems enable the Midwest area to become highly productive agricultural lands, but can also create environmental problems, for example nitrate-N contamination associated with drainage waters. The Soil and Water Assessment Tool (SWAT) has been used to model watersheds with tile drainage. SWAT2012 revisions 615 and 645 provide new tile drainage routines. However, few studies have used these revisions to study tile drainage impacts at both field and watershed scales. Moreover, SWAT2012 revision 645 improved the soil moisture based curve number calculation method, which has not been fully tested. This study used long-term (1991–2003) field site and river station data from the Little Vermilion River (LVR) watershed to evaluate performance of tile drainage routines in SWAT2009 revision 528 (the old routine) and SWAT2012 revisions 615 and 645 (the new routine). Both routines provided reasonable but unsatisfactory uncalibrated flow and nitrate loss results. Calibrated monthly tile flow, surface flow, nitrate-N in tile and surface flow, sediment and annual corn and soybean yield results from SWAT with the old and new tile drainage routines were compared with observed values. Generally, the new routine provided acceptable simulated tile flow (NSE = 0.50–0.68) and nitrate in tile flow (NSE = 0.50–0.77) for both field sites with random pattern tile and constant tile spacing, while the old routine simulated tile flow and nitrate in tile flow results for the field site with constant tile spacing were unacceptable (NSE = −0.77– −0.20 and −0.99–0.21 respectively). The new modified curve number calculation method in revision 645 (NSE = 0.56–0.82) better simulated surface runoff than revision 615 (NSE = −5.95 ~ 0.5). Calibration provided reasonable parameter sets for the old and new routines in LVR watershed, and the validation results showed that the new routine has the potential to accurately simulate hydrologic processes in mildly-sloped watersheds.

Quantifying the Effects of Future Climate Conditions on Runoff, Sediment, and Chemical Losses at Different Watershed Sizes

Quantifying the effects of climate change on watershed hydrology and agricultural chemical losses is imperative when developing appropriate management practices for agricultural watersheds. Agricultural management practices are often assessed at the watershed scale; therefore, understanding the influence of climate change at different watershed sizes can provide insight into the effectiveness of watershed management strategies. In this study, the Soil and Water Assessment Tool (SWAT) and downscaled weather data generated using the MarkSim weather file generator were used to evaluate the potential impact of climate change in the hydrologically modified Cedar Creek (CCW), F34, AXL, and ALG watersheds located in northeastern Indiana. This study evaluated changes in surface flow, tile flow, sediment, and agricultural chemical losses based on an ensemble mean of the 17 general circulation models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We found no clear evidence that watershed size had an impact on the simulation of climate change effects on discharge or nutrient losses. Results of this study indicated that predicted surface flow decreased significantly toward the end of this century (ranging from 9% in CCW to 22% in ALG), while predicted subsurface tile flow increased significantly (ranging from 20% in CCW to 26% in AXL). The percentage increases in predicted sediment loss for the CCW, AXL, and ALG watersheds were significant at a = 0.05, although the magnitudes of overall sediment losses were low, especially in the smaller monitored watersheds (F34, AXL, and ALG) in which several best management practices are implemented. Differences in predicted atrazine, soluble N, total N, and total P losses between the baseline period (1961-1990) and the end of this century were not significant for any of the watersheds, while increased predicted soluble P losses were only significant for the larger CCW and F34 watersheds. Keywords: Climate change, MarkSim, Surface flow, SWAT, Tile flow.

Creating a Calibrated CFD Model of a Midsize Data Center

Calibrating a CFD model against measured data is the first step to successfully utilizing this technology for change-management and the optimization of an existing data center. To date, there has been very little published on this calibration process; more focus has been placed on the use of CFD at the design stage and the development of modeling techniques and solvers. Further, few studies which feature comprehensive comparisons of CFD-predicted and measured data have been published for real data centers, and many that have, demonstrated only modest agreement at best. This study provides another such comparison — for a 7,400 ft2 (687 m2), 138-rack, raised-floor facility. The goals of the study are to benchmark the level of agreement that can be practically obtained and also to investigate the level of modeling detail required. Additionally, specific practical advice covering both CFD modeling and experimental measurements is provided. A plenum-only CFD model is compared to measured tile airflow rates and a room-model, which uses measured tile flow rates as boundary conditions, is compared to temperatures measured at each rack inlet. The level of agreement is among the best published to date and demonstrates that a CFD model can be adequately calibrated against measured data and is of value for ongoing data center operation.