Numerical Investigation of Data Center Raised-Floor Plenum

Abstract

Data center raised-floor plenum effectiveness is numerically investigated using a computational fluid dynamics (CFD) package to determine the most appropriate data center raised floor plenum height (size). The current study considers raised floor plenum height between 30.5 cm and 152.4 cm (12–60 inch) with the standard 15.2 cm (6-inch) increment while maintaining the supply airflow rate constant. Three factors are considered for optimum plenum size: the individual airflow rate from each perforated tile, the level of airflow rates uniformity between different perforated tiles and the top rack inlet air temperature at 2.13 m (7-ft) above the raised floor. The results show that raised floor plenum with height of 76.2 cm (30-inch) or higher had the most uniform airflow rates through the perforated tiles and most uniform temperature across the top of the racks. The findings also indicate that the uniformity of the static pressure inside the plenum increases with plenum height. The current results show that data center plenums with height range of 76.2–91.4 cm (30–36 inch) are found to be the best option based on the current thermo-fluid considerations however thermo-economics analysis should be addressed in future work.
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ASME 2015 International Mechanical Engineering Congress & Exposition
November 13-19, 2015, Houston, Texas, USA
IMECE2015-50884
NUMERICAL INVESTIGATION OF DATA CENTER RAISED-FLOOR PLENUM
Abdlmonem H. Beitelmal
Qatar Environment and Energy Research Institute
Qatar Foundation
Doha, Qatar
ABSTRACT
Data center raised-floor plenum effectiveness is
numerically investigated using a computational fluid
dynamics (CFD) package to determine the most
appropriate data center raised floor plenum height (size).
The current study considers raised floor plenum height
between 30.5 cm and 152.4 cm (12-60 inch) with the
standard 15.2 cm (6-inch) increment while maintaining
the supply airflow rate constant. Three factors are
considered for optimum plenum size: the individual
airflow rate from each perforated tile, the level of
airflow rates uniformity between different perforated
tiles and the top rack inlet air temperature at 2.13 m (7-
ft) above the raised floor. The results show that raised
floor plenum with height of 76.2 cm (30-inch) or higher
had the most uniform airflow rates through the
perforated tiles and most uniform temperature across the
top of the racks. The findings also indicate that the
uniformity of the static pressure inside the plenum
increases with plenum height. The current results show
that data center plenums with height range of 76.2-91.4
cm (30-36 inch) are found to be the best option based on
the current thermo-fluid considerations however thermo-
economics analysis should be addressed in future work.
INTRODUCTION
Energy consumption is a critical factor in the design
and operation of data centers. The data center is the
. Environmental Protection
Agency (EPA) reported that data centers consumes at
least 1.5% of the total United States of America
electricity consumption [1] and this value is expected to
increase over time. This increase is motivated in part by
the technological advancement and increased demand
for data centers with more capacity and high
performance compute systems in addition to compaction
or consolidation of hardware to make better use of
physical floor space. Blade servers are great examples of
such compaction, and represent one of the fastest
growing enterprise server markets. The thermal
requirement of Information Technology (IT) equipment
is usually specified by temperature and airflow rate. This
requirement is met by delivering the proper amount of
airflow at the pre-specified temperature to the inlet side
of the IT equipment. Some studies [2-5] have discussed
the lack of energy efficiency in state of the art data
centers and showed that for every one watt of power
used by the IT equipment, an additional 0.5-1.0 Watt of
power is required by the cooling resources to maintain
proper inlet conditions for the same equipment i.e. an
additional 50%-100% is used to operate the cooling
resources. The over provisioning of the cooling systems
design can add millions of dollars to the annual thermal
management cost for large data centers. The capital cost
required to deploy cooling systems at this scale is also
considerable. Hence efficient cooling of data centers is
the way to minimizing power consumption, reducing the
harmful emissions to the environment and providing a
cost savings prospective.
Typical high density data centers are designed based
on hot/cold aisles layout. The conditioned air is drawn at
specific temperature and humidity into the IT systems by
the integrated fans or blowers. Hot air rejected from the
IT equipment is exhausted into the hot aisles and
recycled through the room which is known as room
return or through a ceiling plenum and in this case is

2
called ceiling return, back to the computer room air
handler (CRAH) units. Ideally, the hot return air streams
should return to the CRAC units without mixing with the
cool air from the supply. However, most data centers
have an open air return design (no ceiling plenum)
which results in mixing the servers air exhaust with
some of the conditioned air streams and reducing the
energy efficiency of the data center. The complicated air
flow distribution in the open environment makes thermal
management of such data centers challenging. The lack
of proper thermal environment inside a data center is a
serious threat not only to system reliability and
performance, but also warranties and service agreements
of systems manufacturers.
The design treatment of the heat rejected by the
computational, networking and storage resources into
the data center is specified simply by matching the
installed cooling capacity to the total data center design
power rating. However, this approach does not guarantee
       
       
may be formed. Hence a thermal requirement specified
by a combination of a temperature range and a minimum
airflow rate is required for reliable data center operation.
This specific requirement can be met by delivering
sufficient amount of cooled air to the inlet of each rack
at a temperature within the acceptable temperature range
.
The most common data centers are designed based
on raised-floor architecture and hence the focus of this
paper.
Fig. 1 Typical raised-floor data center configuration
Figure 1 depicts a simple data center configuration
with one CRAH pressurizing under-floor plenum
enclosure. The floor plenum should be free from any
significant blockages that can disturb airflow and the
static pressure inside the plenum. Plenum blockages may
be caused by network cable trays, power cables and
unused legacy cables from past installations. Overhead
racks trays design maybe a good solution to potential
plenum blockages.
A conventional raised-floor data center is typically
equipped with perforated tiles with fixed percent open
area and CRAH units with constant flow rates.
Perforated tiles are critical elements of the raised-floor
data centers as they are the channels by which the cooled
airflow is delivered to the data center IT systems. The
CRAH units pressurize the under-floor plenum and
maintain a positive static pressure differential between
the plenum pressure and above the raised-floor pressure.
The pressure differential is the driving force for the
cooled airflow across the perforated tiles with the
volumetric airflow being proportional to the percent
open area. Most CRAH unit manufacturers attempt to
thermally manage the data center environment based on
the CRAH unit return air temperature although there has
being a move to use the supply air temperature instead.
In either case, the set point temperature of each CRAH
unit is selected manually and is based on a qualitative
assessment of the data center environmental conditions
which makes provisioning data center cooling capacity
even more challenging.
DATA CENTER THERMAL MANAGEMENT
CRAH units and perforate tiles are two knobs that
are available to manage and fine tune the thermal
environment inside a data center and eliminate hot spots.
The data center thermal management is achieved by
selecting the proper CR  individualized set
point temperatures and individualized airflow rate. Fine
tuning the localized airflow is achieved by selecting the
proper number, size and layout of the perforated tiles.
Selecting the number and size of the CRAH units are
typically performed quantitatively by matching them to
the total heat load generated inside the data center while
the data center layout of the CRAH units and the number
and layout of the perforated tiles are performed
qualitatively based on the IT equipment heat load
distribution. Caution should be taken since the IT system
utilization and consequently the heat load is
continuously fluctuating and as a result the cooling
capacity requirement is also fluctuating.
Off-the shelf data center perforated tiles are
available with a fixed percent open area ranging
Cold
Aisle
Server rack
CRAH
Unit
Perforated
Tile
Hot
Aisle
Plenum
Racks
Exhaust

3
typically between 25% and 56% although perforated
tiles with higher percent open area is also available in
the market. Facility engineers and operators are required
to physically move, relocate and replace perforated tiles
for local airflow adjustments and to match new cooling
requirements or prevent localized hot spots that may
result from insufficient cooled air circulation. The
relocation of perforated tiles is not a recommended long
term strategy since it introduces changes to the plenum
static pressure distribution, to the available airflow from
each individual perforated tile and may even disrupt the
overall data center thermal environment in addition to
being impractical and labor intensive. This issue
becomes particularly more pronounced when the data
center plenum height is less than 2-ft in height [6, 7].
Perforated tiles with automated dampers have been
introduced as a solution to the challenges that data
centers operators face in tuning the localized airflow to
match the continuous change in the heat load
distribution. These controllable perforated tiles tune the
airflow by adjusting the percent-open area between 0%
and 50% [6]. The adjustable dampers provide flexibility
to an existing standard perforated tile [7]. Monitoring
and controlling these channels will provide local thermal
management that could lead to energy savings [8, 9].
The increased fluctuation in the plenum static pressure
negatively affects the airflow uniformity through the
perforated tiles. This results in over provisioning of
cooling capacity to compensate for the airflow loss in
the affected areas and reduces the overall data center
energy efficiency. Published reports and studies are
available that highlight energy efficiency challenges and
opportunities in data centers [10-12]. New energy
efficiency metrics have being proposed to standardize
data center energy performance assessment based on
first principle and thermodynamic approach [13].
This paper presents the results of a numerical study
on the effect of a plenum height/size on the perforated
tile individual airflow rate and rack inlet temperature
distribution at 7-ft above the raised floor. This study
utilizes FloVent software, a commercially available
computational fluid dynamics (CFD) package. The
method takes into account the data center size, the
plenum dimensions, the CRAH set point temperature,
location and air flow rate, the perforated tiles number,
size and locations. The results are presented in terms of
the temperature, static pressure and volumetric flow rate
distribution. This paper provides insight into optimum
data center plenum sizing underscoring the power of the
numerical design approach. The importance of
predicting the performance of a given data center prior to
construction and facilitating changes to the layout of the
IT and cooling resources are evident.
NUMERICAL MODEL
The partial differential equations describing the
complex interactions between the dynamics of fluid flow
and heat transfer are highly coupled and non-linear
making them difficult to be solve analytically. Resolving
these equations numerically requires the use of various
approximation techniques such as finite volume, finite
element and finite difference. The partial differential
equations are initially discretized and transformed into
algebraic form to be solved iteratively until a pre-
specified convergence level is reached. The convergence
level is determined based on the variables residual
monitored relative to the problem scale at hand.
The governing equations describing the airflow are
continuity, momentum and energy equations.

      (1)
Equation (1) is the continuity equation also known
as the conservation of mass equation. The variables
   are the density, velocity and time, respectively.
This equation states that the mass entering a control
volume is equal to the mass leaving the control volume
plus the change of mass within the same control volume.

          (2)

Equation (2) is the conservation of Momentum. The
variables (  ) are pressure, gravity and the viscous
shear tensor, respectively. This equation is derived from
      the net
force is equal to mass times acceleration. The forces
exerted on the fluid volume in this case are pressure,
viscous and body forces. This equation is a vector
equation and therefore has a magnitude and a direction.

 - (3)

4
Equation (3) is the conservation of Energy. The
variable ( ) is the total internal energy and the thermal
conductivity, respectively. This equation is derived from
the first law of thermodynamics and states that the net
exchange of energy across the control surfaces is equal
to the change between the initial and final state of the
system.
The numerical model of the data center requires
information on the actual and maximum heat load of the
IT equipment and the maximum available air flow and
capacity of the CRAH units. Caution should be
exercised to make sure that each CRAH  capacity
in the numerical model does not exceed its actual
physical (rated) capacity at any given time. This can be
done by coding the CRAH unit maximum allowable
capacity into the numerical model and verifying that the
limit have not being exceeded by checking the final
results of the numerical model. The model, once
converged, solves for the temperature profile and airflow
distribution among other variables, and enables
optimization of the installed cooling capacity in the data
center.
STEADY STATE MODEL
The principal features of the mathematical model
presented in this paper are defined as a steady-state
three-dimensional model with negligible radiation effect.
The motion of the fluid layers within the space of the
data center is affected by fluctuating currents caused by
mixing, entrainment and circulation and therefore it is
best represented using the k-Epsilon (k- 
model that calculates the kinetic energy and its
dissipation rate.The k-ε model solves for two variables:
k; the turbulent kinetic energy, and ε; the rate of
dissipation of kinetic energy. The k-epsilon model has
good convergence rate and relatively low memory
requirements. The numerical model is built taking into
account all of the data center details that affect air flow
and temperature distribution including plenum size,
        
      
load. The data center has a total area of 29 m2 (312 ft2)
with one row of ten industry standard server racks and
one perforated tile strategically located for each server
rack to provide the necessary cooling capacity. The
server racks are modeled as enclosures with open ends
based on the standard industry racks dimensions. The
model defines the servers as heat sources with their
corresponding airflow rates and each perforated tile as a
planar resistance to reflect the percent open area of 47%.
The selection of the 47% open perforated tiles is based
on experience and what is typically available as an off-
the shelf product. The maximum load of each rack
selected for this study is 10kW with a maximum heat
load of 100 kW for the overall data center. One standard
CRAH unit supplies the necessary cooling capacity for
the overall data center and is modeled to supply the
cooled air at a given temperature. In this model, the
CRAH unit supply temperature is set at 20 oC.
RESULTS AND DISCUSSION
This study presents a numerical analysis to predict
the impact of the changes in the plenum size on the
plenum static pressure, the airflow through each
perforated tile and temperature distribution above the
raised-floor. The objective of the analysis is to show that
the availability of adequate cooling resources to match
the heat load is not sufficient by itself to prevent hot
spots that may lead to server failures if and when non-
uniformity of airflow distribution reach a critical level
[4]. The numerical model is built based on the
assumption that there is no obstruction to the airflow
inside the plenum and that the plenum walls are
adiabatic i.e. the temperature at the exit of the perforated
tile is equal to the air temperature supplied by the CRAH
unit.
Figure 2 shows a sample temperature distribution
plots for various plenum sizes. The temperature shown
on the plots is the spatial temperature distribution at
213.4 cm (7-ft) above the raised floor. This location is
selected because it is typically the worst case scenario
for the inlet temperatures. The plots show hot spots
    
sizes 61 cm (24-inch) and smaller. A hot spot is defined
as the inlet air temperature at or above 30 oC. The inlet
air temperature distribution becomes more uniform as
the plenum size increases and the uniformity in
temperature becomes more pronounced for plenum sizes
91.4 cm (36-inch) and larger.

5
30.5 cm (12”) Plenum 76.2 cm (30”) Plenum
45.7 cm (18”) Plenum 91.4 cm (36”) Plenum
61.0 cm (24”) Plenum 152.4 cm (60”) Plenum
Fig. 2: Sample temperature plots at 213.4 cm (7-ft) above the raised floor for various plenum sizes.
The volumetric airflow rate from the each perforated
tile is plotted and shown in Fig. 3. Although the current
results are based on the model at hand and on the given
assumptions, they still valuable in terms of showing the
trend of how the volumetric air flow rates distribution
among the perforated tiles vary as a result of the plenum
size changes. There exists a plenum size range where the
volumetric airflow is more uniform and that is when the
plenum size is between 76.2-91.4 cm (30-36 inch).
Figure 4 displays the inlet temperature taken at
213.4 cm (7-ft) above the raised floor and at the inlet of
the top server in the rack. The plot shows that the inlet
temperature to some of the servers for 30.5 cm (12-inch)
and 45.7 cm (18-inch) plenum sizes approaches the
maximum operating threshold temperature set by the
manufacturer. Each system has a built-in thermal
threshold that may vary from one type of system to
another but typically is set to 36 oC at which the system
graceful shutdown starts. Data centers owners and
operators work hard to avoid this issue and prevent
    g the critical
thermal threshold range and therefore it makes very
good sense to numerically evaluate the data center
environment numerically and before the actual systems
are in place. It is very difficult to mitigate this issue once
the data center is already in operation and the only
solution is to over provision the cooling resources and
manually redistribute/reselect the perforated tiles around
the affected systems. The current model results show
temperature slightly above 33 oC and approaching the
critical thermal threshold level.
Temp > 30oC
Servers’ Exhaust
Server Racks
CRAH Unit
Servers’ Inlet
T (oC)

6
Fig. 3: Individual perforated tiles volumetric flow rate
for various plenum sizes
Fig. 4: Inlet temperature at 7-ft (213.4 cm) above the
raised floor
The maximum room and inlet rack temperatures at 213.4
cm (7-ft) above the raised-floor as a function of the
plenum height are shown in Fig. 5. The plot shows both
temperatures to follow the same trend which indicates
the linear relationship between the inlet and the exhaust
temperatures. The plot also show that at plenum height
of 61 cm (24-inch) or smaller help the generation of hot
spots that may leads to over provisioning and reduces
the energy efficiency of the data center.
Fig. 5: Maximum room temperature and Inlet
temperature at 213.4 cm (7-ft) above the raised floor
versus plenum height
Tables 1 and 2 display sample numerical results
obtained from the CFD model. The results display the
        
inlet temperatures at 213.4 cm (7-ft) above the raised
floor as the raised-floor plenum size varied.
Perforated
Tile no.
Plenum Height
45.7 cm (18")
61.0 cm (24")
76.2 cm (30")
91.4 cm (36")
cm3/min
T , oC
cm3/min
T , oC
cm3/min
T , oC
cm3/min
T , oC
-5
16.0
33.8
22.5
28.0
31.9
22.9
40.8
21.3
-4
43.4
27.0
47.8
23.6
48.6
22.1
48.1
21.2
-3
59.2
20.7
54.3
21.7
52.1
21.5
49.7
21.0
-2
63.2
20.1
59.0
20.8
54.5
20.7
51.4
20.7
-1
59.0
20.0
57.2
20.4
53.6
20.8
50.7
20.6
1
59.0
20.0
57.2
20.4
53.6
20.8
50.7
20.6
2
63.2
20.1
59.0
20.8
54.5
20.7
51.4
20.7
3
59.2
20.8
54.3
21.7
52.1
21.5
49.7
21.0
4
43.4
27.0
47.8
23.6
48.6
22.1
48.1
21.2
5
16.0
33.8
22.5
28.0
31.9
22.9
40.9
21.3
Max. Room Temp.
44.6
38.8
35.6
34.8
Table 1: Airflow rate and temperature values for various plenum sizes at 213.4 cm (7-ft) above the raised floor
0
10
20
30
40
50
60
70
-5 -4 -3 -2 -1 1 2 3 4 5
Perforated tile flow rate, cm3/min
Perforated tile number
30.5 cm Plenum
45.7 cm Plenum
61.0 cm Plenum
76.2 cm Plenum
91.4 cm Plenum
106.7 cm Plenum
121.9 cm Plenum
137.2 cm Plenum
152.4 cm Plenum
15
20
25
30
35
40
-5 -4 -3 -2 -1 1 2 3 4 5
Temperature,oC
Perforated tile number
30.5 cm Plenum
45.7 cm Plenum
61.0 cm Plenum
76.2 cm Plenum
91.4 cm Plenum
106.7 cm Plenum
121.9 cm Plenum
137.2 cm Plenum
152.4 cm Plenum
0
5
10
15
20
25
30
35
40
45
50
020 40 60 80 100 120 140 160 180
Temperature, oC
Plenum height, cm
Maximum Room Temperature
Maximum Rack Inlet Temperature

7
Perforated
Tile no.
Plenum Height
106.7 cm (42")
121.9 cm (48")
137.2 cm (54")
152.4 cm (60")
cm3/min
T , oC
cm3/min
T , oC
cm3/min
T , oC
cm3/min
T , oC
-5
47.0
20.8
48.1
20.7
48.9
20.9
49.6
21.2
-4
47.8
20.7
47.6
20.6
47.7
20.7
47.9
20.7
-3
48.2
20.6
47.9
20.5
47.5
20.6
47.3
20.6
-2
48.9
20.4
48.3
20.4
47.9
20.5
47.6
20.6
-1
48.8
20.4
48.7
20.3
48.5
20.5
48.3
20.6
1
48.8
20.4
48.7
20.3
48.6
20.5
48.3
20.6
2
48.9
20.4
48.3
20.4
47.9
20.5
47.6
20.6
3
48.3
20.6
47.9
20.5
47.6
20.6
47.3
20.6
4
47.8
20.7
47.6
20.6
47.8
20.7
47.9
20.7
5
47.1
20.8
48.2
20.7
49.0
21.0
49.6
21.3
Max. Room Temp.
34.7
35.0
35.4
36.0
Table 2: Airflow rate and temperature values for various plenum sizes at 213.4 cm (7-ft) above the raised floor
It is clear from the tables that the bulk of the airflow
leaves the plenum through the middle perforated tiles for
plenum heights smaller than 76.4 cm (30-inch) and
tapers down towards both ends of the perforated tiles
row. The volumetric flow rate from each of these two
perforated tiles measured less than 50% of the average
air flow rate through any of the other individual
perforated tile. The variation between the perforated tiles
volumetric airflow rate ranges between 3-5% at the low-
end and 12-13% at the high-end of the total and
maximum volumetric flow rate of 481.4 cm3/min
(17,000 CFM) specified for the CRAH unit by the
manufacturer. The results also show the maximum
variation between any two perforated tiles is as high as
75% for 45.7 cm (18-inch) plenum size. This variation
between the low-end and the high-end volumetric flow
rates decreases to 20% for 91.4 cm (36-inch) plenum
size, to about 4% for the 42-inch (106.7 cm) plenum size
and to about 2.3% for the 121.9 cm (48-inch) plenum
size. Interestingly this variation goes slightly up to about
3% for the 137.2 cm (54-inch) plenum size and to close
to 5% for the 152.4 cm (60-inch) plenum size. The
temperature distribution at 213.4 cm (7-ft) above the
raised-floor plenum moves towards more uniformity for
plenum height of 76.2 cm (30-inch) and larger. The
results indicate that an optimum raised-floor plenum size
range exist for the constant airflow inlet condition
specified in this study.
SUMMARY AND CONCLUSION
A numerical model of a hypothetical density data
center is built to investigate the effect of plenum height
on the level of uniformity of the static pressure within
the plenum just below the perforated tiles. The static
pressure just below the perforated tiles directly affects
the perforated tiles airflow distribution and the inlet
airflow temperature to the server racks. The steady-state
model is created and the effect of the plenum height/size
on the airflow distribution and inlet server temperature is
analyzed. The steady state model shows that the rack
inlet temperature increases to an unacceptable level
when the plenum height is 61.0 cm (24-ft) or smaller.
This issue can be resolved by proper
redistribution/addition of the perforated tiles around the
affected servers. However this approach may affect other
areas negatively and disrupt the overall airflow
distribution and cooling capacity of the data center.
Numerical modeling and pre-analysis of the data center
environment is the best approach to predict and resolve
such issues before it happens. Furthermore, economic
assessment should be conducted when selecting the
plenum size for a new data center as larger plenums
require larger capital investment; the cost of the plenum
is not included in the current analysis. The following are
the specific key points summary of this study:
 The uniformity of the static pressure inside the
plenum increases with plenum height. This uniformity
becomes more pronounced as the plenum height
increases beyond the 61 cm (24 inch).

8
 The data center plenums with height range of 76.2-
91.4 cm (30-36 inch) are the best option based on the
current thermo-fluid considerations and for the specified
geometry, however thermo-economics analysis should
be addressed in future work.
 High density data centers architecture and design
approach must be augmented with detailed
computational thermo-fluids analyses. Providing
sufficient cooling resources based on the total compute
heat load in the data center is not sufficient by itself, as
localized high power density will lead to localized hot
spots.
 Modeling the thermo-fluids behavior in the data
center helps with uptime assessment, create policies and
procedures to avoid data center failure and enable
reliable data center operation.
 Modifying the number or layout of the perforated
tile, adding or removing plenum obstructions such as
wires and cables will affect the static pressure uniformity
inside the plenum and may disrupts the airstream paths
however this behavior will be more pronounced for
smaller plenums heights. It is highly recommended that
all power and network cables, wires used inside the data
center are connected through rack-top cable trays.
 One way of maintaining control over the static
pressure uniformity inside a plenum is to create dividers
(compartmentalization) and allow each CRAH unit to
feed air into a specific compartment within the plenum.
This approach provides a granular control of the static
pressure uniformity inside the plenum and allow for
tighter control of the volumetric airflow through the
perforated tiles. This method may take away the CRAH
units redundancy unless at least one side divider is
designed to be removable either automatically or
manually when the adjacent plenum section static
pressure goes below a certain threshold pressure value.
FUTURE WORK
Future work will include more detail evaluation and
analysis of an actual size data center. The current study
should be extended to investigate the effect of multiple
CRAH units and their zone of influence variation as the
plenum size changes. Direct versus indirect chilled air
delivery into the plenum effect on the static pressure and
consequently on the perforated tiles airflow rates should
be investigated.
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