Open and Contained Cold Aisle Experimentally Validated CFD Model Implementing CRAC and Server Fan Curves for a Data Center Test Laboratory

Abstract

This paper presents the results of an experimentally validated Computational Fluid Dynamics (CFD) model for a data center with fully implemented fan curves on both the servers and the Computer Room Air Conditioner (CRAC). Open and contained cold aisle systems are considered experimentally and numerically. This work is divided into open (uncontained) cold aisle system calibration and validation, and fully contained cold aisle system calibration and leakage characterization. In the open system, the CRAC unit is calibrated using the manufacturer fan curve. Tiles flow measurements are used to calibrate the floor leakage. The fan curves of the load banks are generated experimentally. A full physics based model of the system is validated with two different CRAC fan speeds. The results showed a very good agreement with the tile flow measurements, with an approximate average error of 5%, indicating that the average model prediction of the tile flow is five percent lower that the measured values. In the fully contained cold aisle system, a detailed containment CFD model based on experimental measurements is developed. The model is validated by comparing the flow rate through the perforated floor tiles with the experimental measurements. The CFD results are in a good agreement with the experimental data. The average error is about 6.7%. Temperature measurements are used to calibrate other sources of containment and racks leaks including mounting rails and clearance between racks. The temperature measurements and the CFD results agree well with average error less than 2%. Detailed and equivalent modeling methods for the floor and containment system are investigated. It is found that the simple equivalent models are able to predict the flow rates however they did not succeed in providing detailed fluid flow information. While the detailed models succeeded in explaining the physical phenomena and predicting the flow rates with noticeable tradeoffs regarding the computational time. Important conclusions can be drawn from this study. In order to accurately model the containment system, both the CRAC and the load banks fan curves should be simulated in the numerical model. Unavoidable racks and containment leaks could cause inlet temperature increase even if the cold aisle is overprovisioned with cold air. It is also noted that heat conduction through the floor tiles causes a slight increase the inlet temperature of the cold aisles. Finally, it is noteworthy that using detailed modeling is necessary to understand the details of the thermal systems, however simpler and faster to compute equivalent models can be used in extended optimization studies that show relative rankings of different designs.

Implementing rack thermal capacity in a room level CFD model of a data center

“…This commercial software has been used extensively to simulate the flow and energy transport in data center modeling. Published simulations have shown relatively good agreement in comparative studies with experimental data and provided a reasonable interpretation to practical observations [5]. The server level model is developed and validated based on measurements conducted in [9].…”

“…In a recent survey [1], it is shown that more than 55% of the sample has PUE greater than 1.8. CFD has been used in data center thermal analysis as it demonstrated to produce reliable results when compared with experimental data [2][3][4][5]. The reliability of data centers depends on developing a fundamental understanding of the airflow pattern and the system response in emergency situations.…”

An experimental and theoretical investigation of the effects of supply air conditions on computational efficiency in data centers employing aisle containment

“…Experimental and numerical investigations have shown this approach to be effective in reducing electricity consumption and improving cooling effectiveness [8], [9], [15]- [18]. In DCs employing aisle containment, a positive pressure differential, ∆p CH , is usually maintained between the cold and hot aisles, in order to prevent recirculation [9] [17]. This tends to drive air through any gaps present in the containment system or server racks, hence bypass is not completely prevented by containment [9], [17], [18].…”

“…In DCs employing aisle containment, a positive pressure differential, ∆p CH , is usually maintained between the cold and hot aisles, in order to prevent recirculation [9] [17]. This tends to drive air through any gaps present in the containment system or server racks, hence bypass is not completely prevented by containment [9], [17], [18].…”

“…Numerical investigations have either neglected the potential for bypass within server racks (i.e. cold air passing through server racks whilst bypassing the server inlets) [8], [13], [15], [16], [19], or have incorporated this phenomenon without experimental calibration [17] [20]. One experimental study investigated the impact of aisle containment on cooling, but did not measure bypass within racks, effectively assuming that all air flow through racks passed through the servers [9].…”