measurements were made; these results are used to verify the
CFD model used for all scenarios. Floor Plans C through J are
based on actual data centers, which have been previously
Total Flo
ated ile Me
r low # of Per
40% leakage
modeled with CFD. Only the basic floor plans including
3 s) (cfm)
3 s) (cfm)
CRAC, perforated floor tile, and plenum-ducted IT equipment
locations are retained for the present study.
Two common perforated tile types are considered:
nominally 25%-open and nominally 56%-open areas. As
shown in Fig. 2, more-restrictive (e.g. 25% open) tiles typically
feature drilled holes while high-flow tiles (e.g. 56% open) may
more accurately be described as grates. Each scenario modeled
features either all 25%-open or all 56%-open perforated tiles;
no mixed-tile scenarios were considered. Based on
manufacturer's published data, the loss coefficients (as defined
in Eq. (1)) for the 25% and 56% perforated tiles are taken as
51.3 and 3.4 respectively. Note that some of the mean
perforated tile airflow rates listed in Table 2 are unrealistically
high for 25%-open tiles. Recall that the airflow uniformity
results hold equally well at lower airflow rates. The airflow
rates listed are the values used in the CFD models and are of
value primarily for reference.
Three leakage rates are considered: 0% (no leakage), 20%,
and 40%. These leakage rates are defined as a percentage of
total CRAC supply airflow minus any plenum-ducted IT
airflow. These leakage rates are probably conservative in light
of anecdotal evidence indicating that it is not uncommon for
50% or more of the airflow supplied by the CRAC's to exit the
plenum via non-perforated-tile openings.
Four plenum depths are considered which cover the range
of typical design values: 0.30 m (12 in), 0.46 m (18 in), 0.61 m
(24 in), and 0.91 m (36 in).
The combination of design parameters discussed above
results in 240 scenarios. Additionally, a supplemental floor-
plan study is carried out using Floor Plan A with a 0.61 m (24
in) plenum and 20% leakage. The effect of row length on
airflow uniformity is investigated; the room is made wider and
narrower by adding and removing perforated tiles while
keeping all other geometry and conditions fixed.
Copyright 2005 by ASME
models. It is noted that for floor plans including plenum-ducted
CFD MODELING CONSIDERATIONS
IT equipment (E, G, and H), airflow uniformity results hold at
different flow rates only when IT airflow is scaled up or down
Only the plenum airflow is modeled in this investigation
in proportion with the total CRAC and leakage airflow.
with a zero pressure boundary condition imposed above the
raised floor. In practice, the flow in the plenum is fairly
FLOVENT V5.1 by Flomerics [8] was used for all CFD
decoupled from the room flow for fairly restrictive perforated
modeling. A structured Cartesian grid and the k-ε turbulence
tiles like the 25%-open type. With less restrictive tiles, like the
model were used for all simulations. Total simulation time for
56%-open type, the plenum and room (above the raised floor)
the primary 240 scenarios was approximately 1 week on a 3.4
airflow are somewhat coupled. In this case, room airflow
GHz Pentium 4 computer.
dynamics can affect the perforated tile airflow distribution.
Since only the plenum is modeled here, the results are strictly
CALIBRATION AND EXPERIMENTAL VERIFICTION
applicable only to the case of uniform room pressure above the
OF CFD MODEL
Floor Plan A with 0.23 m (9 in) and 0.61 m (24 in) deep
As mentioned above, obstructions are not included in the
plenums was used to assure that results were grid-independent.
CFD models. While obstructions may significantly impact
The maximum length of any side of any grid cell was
airflow uniformity, the effects are typically fairly localized.
systematically reduced until predicted tile airflow results
Furthermore, given the fairly random nature of obstructions, it
stabilized. Ultimately, a grid size was selected with the
is difficult to quantify the effects of obstructions in any useful
following characteristics: a maximum cell size of 15 cm (6 in),
a minimum cell size of 2.5 cm (1 in), and a minimum of 8 cells
in the plenum-depth direction. These grid settings were then
Perforated tiles are modeled with the pressure-airflow
applied to all cases.
relationship given in Eq. (1) and the loss coefficients indicated
Experimental Verification of CFD Model
As discussed above, leakage airflow is driven by a pressure
In order to verify the modeling methodology used in this
difference and may vary widely across the data center floor.
paper, the experimental results reported in [6] are used.
One reasonable approach to modeling leakage would be to
Specifically, one of the six layouts studied in [6] is used here
represent the raised floor as a resistance with a particular loss
for this verification and is depicted in Table 1 as Floor Plan B
coefficient, which establishes the local leakage airflow based
with the results shown in Fig. 3. The case studied is an array of
on local pressure difference across the raised floor. While
4 x 15 perforated tiles situated between the 2 CRAC units. The
physically realistic, this model is not ideally suited for present
experiments were performed on a portion of a large raised-floor
purposes because, following this approach, the total amount of
data center located at IBM in Poughkeepsie, New York. This
leakage airflow is not known a priori; it is an output from the
area measured 6.06 (20 ft) x 20 m (66 ft) with a raised floor
simulation. Therefore, the approach taken here is to simply
height of 29.2 cm (11.5 in). The floor tiles were 610 mm (2 ft)
remove (via a mass sink located just under the raised floor) the
on a side with the position of the CRAC units as noted in the
leakage airflow uniformly across the floor plan. This is
plan view shown in the figure. The CRAC units were
convenient for defining leakage rates (e.g. 0%, 20%, and 40%)
positioned such that the momentum of the air exhausting from
and is deemed adequate in light of the real variations in leakage
the blowers within the CRAC units were directed toward each
paths and other assumptions in the models.
other, that is, the air was exhausting such that it collided in a
region between the two CRAC units. To accurately measure
CRAC's are modeled with fixed airflow rates. This is
the flow exhausting from the tiles and to account for all the
justifiable as the external (e.g., plenum and perforated tile)
flow exiting the CRAC units, this testing area of the floor was
resistance is much less than the internal CRAC resistance,
blocked off around the perimeter of the testing area from the
which may be 250 to 750 Pa (1 to 3 in H2O) or more. Blower
raised floor down to the concrete subfloor. This 29.2 cm (11.5
outlets are modeled explicitly with the exception of Floor Plan I
in) height (as measured from the subfloor to the bottom of the
for which the airflow is specified uniformly over the entire
tile) was carefully sealed with cardboard and duct tape. In
CRAC footprint. Turning vanes are not included in any cases
addition, any electrical or plumbing openings in this area were
as the details of which vary from vendor to vendor.
Furthermore, as turning vanes generally keep stronger jets in
tact farther from the CRAC, they lead to decreased airflow
The test results shown in Fig. 3 and Table 3 are with both
uniformity compared to simply allowing the airflow from the
CRAC units operating and neither CRAC unit having a turning
CRAC to diffuse more evenly over the subfloor.
vane. Each perforated tile is a nominally 25% open perforated
tile. Actual measurements of the tile showed an array of 0.64
IT equipment which draw air directly from the plenum
cm (0.25 in) diameter holes resulting in a 19.5 % opening, not
("plenum-ducted" in Table 1) are also modeled with fixed
25% as the manufacturer states. The impedance of the
airflow rates and the (outward) flow is specified as leaving the
perforated tiles were measured on a flow bench with the
plenum uniformly over the entire IT equipment footprint. Note
following results:
that most IT equipment does not draw air directly from the
plenum and therefore is not included in the plenum-only
DP[Pa] = 419 {Q[m3/s]}1.99 (7a)
Copyright 2005 by ASME
reverse direction impinges on the left wall (at x = 0), turns 180
deg., and exits from the tiles in the middle. Most of the fluid
exiting the unit B is discharged as a jet towards unit A. A small
DP[in H20] = 4.05x10-7 {Q[cfm]}1.99 (7b)
amount of fluid impinges on the east wall (at x = 20 m), turns
around, and is also exhausted through the tiles in the middle.
The longitudinal velocities (directed along the x axis) are larger
near unit B, causing large pressure variation in this region. The
peak in the airflow velocity distribution is located closer to unit
A and corresponds to the location where the two opposing
CFD SIMULATION RESULTS AND CONCLUSIONS
Airflow (cfm) -50
Example Showing Airflow Uniformity is Independent of
Table 4 summarizes the scenarios considered. For Floor
Plans A and H, corresponding scenarios were modeled with the
total airflow rate halved and doubled. CRAC airflow, leakage
airflow, and ducted-IT airflow were all halved or doubled
(though the percentage of leakage airflow is constant) although
only mean airflow per tile is shown in the Table 4.
Airflow (cfm) -50
Fig re 3 – CFD
Airflow uniformity is, indeed, largely independent of flow
rate. The small variations that do exist reflect the fact that all
the losses in the plenum do not scale identically with airflow
The air momentum is such that the air streams from both
CRAC units collide near the center of the perforated tile region.
Effect of Parameters Considered on Uniformity
The flow distribution shows a fairly symmetric distribution as
would be expected with the minimums occurring nearest the
Table 5 shows all results from the main matrix of 240
CRAC units and the maximum near the center of the perforated
scenarios considered. The top half of the table covers 25%-
tile region. Although some asymmetry is evident, it may be
open tile scenarios; the bottom half covers 56%-open tile
due to the differences in total airflow rates from the CRAC
scenarios. Results show considerable tile-to-tile airflow
units, from the asymmetric location of CRAC blower outlets, or
variations. In fact, all the floor plans exhibit some backflow
the asymmetry of non-perforated tiles adjacent to the CRAC
under at least one scenario with 56%-open tiles. Selected
units. (As many of these details were not included in the CFD
results are discussed in greater detail below.
model, the only sources of asymmetry in the CFD model are
the asymmetry of the room layout and asymmetric location of
CRAC blower outlets.) The flow from some of the perforated
tiles nearest the CRAC units showed some flow downward into
the raised floor plenum.
The predicted flow rates from the model are in good
agreement with the measured values. Although not shown, the
flow exiting CRAC unit A (see Table 1) splits into two streams:
one moving in the forward direction (toward the right) and the
other in the reverse direction. The fluid in the forward stream
exits from the tiles close to unit A. The stream flowing in the
Copyright 2005 by ASME
Figure 4 shows the effect of floor plan on airflow
uniformity with 20% leakage and a 0.61 m (24 in) plenum.
Other than Floor Plan B (which was primarily intended for
validating the CFD model, not as a practical layout) the
standard deviation from the mean is on the order of 10% for
25%-open tiles and 50% for 56%-open tiles. The probability
distributions for these values of standard deviation are also
shown in Fig. 4. A reasonable spread around the mean is
associated with the 25%-open tiles while the spread associated
with the 56%-open tiles is very broad. 68% of the 25%-open
tiles will be within ±10% of the mean airflow per tile whereas
68% of the 56%-open tiles are within only ±50% of the mean.
So, from a standard deviation perspective, most 25%-open tile
floor plans deliver reasonable uniformity while most 56%-open
tile floor plans do not. Even with 25%-open tiles and small
standard deviations, some floor plans, C and F in particular,
have considerable minimum and maximum variations with
some backflow occurring under most scenarios. From Table 5
and Fig. 4, we see that the floor plans achieving the best
uniformity are those in which the CRAC's supply air in the
direction parallel to the perforated-tile rows with room and
CRAC layouts, which are rectangular and symmetric. Floor
Plan G performs very well, however, the large fraction of
plenum-ducted IT equipment makes this floor plan fairly
atypical. The best typical layout is Floor Plan A, which is a
standard 7-tile pitch hot aisle/cold aisle layout [1].
Standard Deviation
Distribution Density
u e 4 – Airflow U
g and 0.61 m (24 in) P
Figure 5 shows the effect of plenum depth with 25%-open
tiles and 20% leakage. The five floor plans shown are fairly representative of the more typical (of real data center) floor plans considered. Uniformity improves only modestly with plenum depth except for Floor Plan C (in which the CRAC's supply air in the direction perpendicular to the perforated tile rows). Considering the results of all floor plans studied (Table 5), there is no great benefit to increasing the plenum depth beyond 0.61 m (24 in). As plenum obstructions were not included in the models, actual plenums should be designed with clear airflow space of 0.61 m (24 in) or more.
Copyright 2005 by ASME
with backflow first occurring at about 45 tile widths. Although
not shown in Fig. 7, the maximum tile airflow always occurs
near the center of the room with the minimum tile airflow
occurring at the perforated tiles located nearest the CRAC's.
There is no maximum distance across which air can be supplied; however, beyond 20-30 tile widths uniformity
becomes a limiting factor. As more and more tiles are added, the mean airflow per tile and the average plenum pressure drop.
Standard Deviation 10%
However, the structure of the supply airflow (jets) near the
CRAC's does not change substantially as perforated tiles are
added. Consequently, pressure disturbances near the CRAC become larger relative to tile resistance (which scale with the
airflow rate squared) as the row is lengthened. Although
somewhat counterintuitive, supplying additional airflow will
not help, as the uniformity is independent of airflow rate.
Finally, it is stressed that these comments are made with respect
to Floor Plan A of Table 1. If, instead, CRAC's were located
only on one side of the room (with a wall at the other) we could
expect uniformity to become poor beyond 10-15 tile widths.
Figure 6 shows the effect of leakage airflow with 25%-
open tiles and 0.61 m (24 in) plenums. The same five floor
plans as discussed above in the context of plenum depth are
again taken as representative. Uniformity degrades with
increasing airflow leakage; the net effect is similar in
magnitude to the effect of plenum depth over the respective
parameter ranges considered here. Recall that the greater the
tile resistance relative to airflow path resistance, the better the
uniformity. Airflow lost to leakage in the plenum effectively
increases the resistance (creates a greater pressure loss) along
the airflow path making the perforated tile resistance less dominant. While minimizing leakage airflow will improve
uniformity, from a cooling perspective the airflow is not
completely wasted as it goes to cooling the room and
equipment in the vicinity of the leakage openings.
ig e 7 – Airflow U
DESIGN RECOMMENDATIONS TO ACHIEVE
AIRFLOW UNIFORMITY
Based on the above findings, the following design
recommendations are made with respect to maximizing
perforated tile airflow uniformity:
• Use only the more-restrictive perforated tiles (e.g. 25%
open) for general deployment. High-flow perforated tiles
(e.g. 56% open) should be used only in special
circumstances. Examples include: the number of high-
flow tiles constitute only a small fraction of perforated tiles
present, an isolated plenum has been created and
uniformity verified through measurement, and hot IT-
equipment exhaust is ducted directly back to the CRAC's
(so that uniform air delivery is not critical).
The effect of row length on airflow uniformity was
• Create standard hot aisle/cold aisle layouts (like Table 1,
investigated using Floor Plan A with a 0.61 m (24 in) plenum
Floor Plan A) utilizing rectangular, symmetric layouts.
and 20% leakage. The room was made wider and narrower by
Locate CRAC's at the end of the hot aisle (far from
adding and removing perforated tiles while keeping all other
perforated tiles) and avoid rows longer than about 20
geometry and conditions fixed. Figure 7 shows the results.
perforated tiles or about 10 perforated tiles for layouts with
Uniformity degrades fairly quickly beyond 20-30 tile widths
CRAC's at only one end with a wall at the other.
Copyright 2005 by ASME
• Design plenums for clear airflow space of 0.61 m (24 in) or
• Minimize leakage airflow through non-perforated tile
openings in the raised floor.
• Keep chilled water pipes and cables away from the exhaust
regions of A/C units and only use turning vanes where the A/C units are used in an inline layout where each is used to
boost the static pressure of the next in line [6].
• Do not simply increase airflow rate without addressing the
other factors listed above, as this will not improve
REFERENCES
"Thermal Guidelines for Data Processing Environments", American Society of Heating,
Refrigerating, and Air-Conditioning Engineers (ASHRAE) 2004
[2] "Optimizing Data Centers for High-Density
Computing", Hewlett-Packard Technology Brief, http://h200005.www2.hp.com/bc/docs/support/Support
The Uptime Institute. "Alternating Cold and Hot Aisles Provides More Reliable Cooling for Server Farms". http://www.upsite.com/TUIpages/whitepapers/tuiaisles.h
[4] VanGilder, J. and Lee, T., 2003, "A Hybrid Flow
Network-CFD Method for Achieving Any Desired Flow
Partitioning Through Tiles of a Raised-Floor Data Center", InterPACK 2003, Maui, Hawaii.
[5] Kang, S., Schmidt, R., Kelkar, K., Patankar, S., "A
Methodology for the Design of Perforated Tiles in
Raised Floor Data Centers Using Computational Flow Analysis", IEEE-CPMT Journal, Vol. 24, No. 2, pp. 177-
[6] Schmidt, R. et al, 2001, "Measurements and Predictions
of the Flow Distribution Through Perforated Floor Tiles In a Raised-Floor Data Center", InterPACK 2001, Kauai,
[7] Holman, J.P., 1989, Experimental Methods for
Engineers, McGraw-Hill, Inc., New York., pp.49-50, 57.
FLOVENT V5.1 Software by Flomerics, Flomerics Ltd., 81 Bridge Road, Hampton Court, Surrey KT8 9HH, UK,
Copyright 2005 by ASME
Airflow Uniformity through Perforated Tiles in a Raised-Floor Data Center
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