ASHRAE Journal - August 2021 - 46

TECHNICAL FEATURE
of thermal performance existed for the same (similar)
geometry in the data center.
Numerical simulations are preferred to assess the
influence of air distribution system configurations.
An on-site experimental investigation was conducted
to study the impact of the supply air temperature of
CRAHs on the thermal performance of the air containment
strategies. Both the simulation and experimental
results show that the use of HAC and CAC can effectively
improve the thermal environment in the data center.
Numerical Analysis
The detailed layout of this data center is shown in
Figure 2a. The IT room had 10 rows, and 10 rack servers
were placed in each row. The room-based cooling
method that uses six CRAH units to serve one white
space was applied. Figure 2b shows the power density of
the two types of rack servers placed in the IT room. Forty
A-type rack servers (16 kW/rack) and 60 B-type rack
servers (8.8 kW/rack) were installed. As for the direction
of the airflow, cold air was introduced through the
front vent, and hot air was discharged through the rear
vent. The leakage rate inside the rack servers was set
to 5% under the assumption that blanking panels were
installed in the empty slots of the cabinets. CFD software
was used for numerical investigation. It has a data
center-specific library with over 4,000 objects, which
are built and verified by the manufacturers. The heat
and mass transfer from the outside of the domain were
not considered.
Unstructured Cartesian grids were used to conduct
numerical analysis, and the total number of grids for the
baseline model was about 2.5 million. It was assumed
that the data center was not affected by external weather
and that there was no external cooling load, including
solar heat gain. It is important in all CFD simulations
to perform grid-independent study. This IT room was
modeled with existing racks and coolers by using a software
library in which the grid-independency testing has
been already verified.
Table 1 shows the boundary conditions for the numerical
analysis. In this numerical simulation, the provisioning
level of the air-conditioning resources is for a
given distribution of heat load as static provisioning.
The size of the applied cabinet also varied depending
on the power density of the rack server. It was assumed
that the temperature difference (ΔT) between the air
46
ASHRAE JOURNAL ashrae.org A U G UST 2021
FIGURE 2 Numerical model. (a) A test IT room, rack servers (A1 to J10) and
CRAH units, (b) rack server model and (c) CRAH unit model.
A.
Type-A: High-Density Rack
Server (16 kW/Rack) 40 Ea.
A
10
9
8
7
6
5
4
3
2
1
CRAH 1
CRAH 2
Power Per Cabinet (kW)
B.
Type-B
CRAH 3
CRAH 4
8.8 10.6 12.4 14.2 16
Type-A
C.
RA
RA
CRAH 5
CRAH 6
34 160
B
C
D
E
Type-B: Medium-Density Rack
Server (8.8 kW/Rack) 60 Ea.
F
G
H
I
J
SA
1200
600
TABLE 1 Boundary conditions of numerical analysis.
ROOM SIZE (M2)
410
IT ROOM
RAISED FLOOR
HEIGHT (MM)
TOTAL POWER
OF A ZONE (KW)
RACK
NUMBER OF
RACKS (EA)
IT AIRFLOW
RATE (M3/MIN)
NUMBER OF
CRAH UNITS
CRAH
TYPE OF
COOLING
AIR VOLUME
(M3/MIN)
MESH GRID SIZE
OF THE HAC
MODELING
CFD SOLVER
500
1,168
100
53 (A-Type)
30 (B-Type)
6
Chilled Water
System
770
2,447,462
k-Epsilon
Turbulence
Model
600
SA
ROOM HEIGHT
(MM)
CONTAINMENT
POROSITY (%)
TILE POROSITY
(%)
RACK IT LIMIT
(KW/RACK)
RACK POROSITY
(%)
TYPE OF CRAH
UNIT
COOLING
CAPACITY (KW)
SUPPLY AIR
TEMPERATURE
(°C)
MESH GRID SIZE
OF THE CAC
SOFTWARE
SA
EC Fan
4800
5 (With Cable
Cutouts)
25
16 (A-Type)
8.8 (B-Type)
5 (With Blanking
Panels)
Downflow With
EC Fan
230
15
2,003,457
Commercial
Software
inlet and outlet of the IT server was constant (15°C
[27°F]). Accordingly, the air volumes required by each
rack server were 53 m3/min (1,871 cfm) (A-type) and
30 m3/min (1,059 cfm) (B-type). The CRAH units sized
for the overall heat load are located in the IT room. The
exhaust hot air from the racks is pulled into the CRAHs,
12 810
2400
2000
Cold Aisle
Hot Aisle
Cold Aisle
Cold Aisle
Hot Aisle
Hot Aisle
Hot Aisle
Hot Aisle
Cold Aisle
Cold Aisle
Cold Aisle
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ASHRAE Journal - August 2021

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