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CFD for Data CentersSiddharth Premkumar Cad Technology Center

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With IT manufacturers improving the performance of datacom equipment year after year, the need for accurate designof data centers have become crucial to ensure safe and efficient design of data centers !ased on the "S#$"%publication for case studies on igh 'ensity 'ata Centers& this class aims to look at some of the most commonventilation strategies employed in actual e(isting data centers to understand ho) to model and set up these datacenters for C*' analysis and validate the results )ith actual published data This class )ill also look at an e(ample tocompare remodels of data centers based on the "S#$"% publication and )ould thereby help designers, engineersand pro+ect managers gain more insight into the design and operation of data centers

Learning Objectives "t the end of this class, you )ill be able to

-nderstand the different ventilation strategies being employed in data centers

Compare results for a data center model )ith published data

-nderstand modeling techniques for raised access floor data centers

Compare different designs

About the SpeakerSiddharth Premkumar .Sid/ is an "pplications %ngineer at C"' Technology Center, Inc and provides training andconsulting to clients and helps them to get the most out of their soft)are products Sid has done his 0asters1 in0echanical %ngineering from Syracuse -niversity and has e(perience )ith C*' and *%" programs

siddharthp2cadtechnologycenter com
mailto:[email protected]:[email protected]


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CFD for Data Centers

Ventilation strategies being employed in data centers

The need for CF Analyses on ata Centers

The follo)ing are some of the reasons to consider doing C*' analysis on a data center modelIT manufacturers increasing performance at the cost of increasing heat dissipationC*' analysis can reduce cost of retrofitsC*' can allo) e(ploring design alternativesC*' analysis can help locate hot spots

3et us no) e(plore some of the common ventilation strategies employed in data centers as sho)n in the table belo)

Ventilation Strategy Description$aised access floor supply "ir enters through perforated tiles located on a raised floor

and the racks are typically arranged in a hot aisle4coldaisle layout

$aised access floor supply )ith "#-1s on the floor belo) In this strategy, the air handling units ."#-1s/ are locatedon the floor belo) and so it allo)s all the mechanicalequipment to be grouped together

$aised access floor supply4ceiling return This strategy reduces mi(ing of the )arm and cold air andthe higher return air temperature encourages efficientoperation of the C$"C1s There could also be a ductedreturn used in this case )hich )ould further reduce thechances of mi(ing

$aised access floor supply )ith heat e(changers This strategy can be employed to reduce the overalltemperature of the space by removing some of the heatfrom the racks

5on raised access floor )ith or )ithout ceiling return This ventilation technique has the C$"C1s and serverracks located on the same level

The figures belo) sho) images of these ventilation strategies as described in the "S#$"% book on #igh 'ensity 'ataCenters6Case Studies and !est Practices, 7889

$aised access floor supply $aised access floor supply )ith "#-1s belo)

7


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Compare results for a data center model !ith published data

"on#raised access $oor data center

In this case study, )e )ill look at the case study for the Cedars6Sinai 0edical Center data center as published in thebook #igh 'ensity 'ata Centers6Case Studies and !est Practices, 7889 and look at the )ay it )as modeled, meshedand analy;ed

Material Assignment

The image belo) sho)s a snapshot of the case study "s indicated in this image, the non6raised access floor datacenter has an area of appro(imately


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$eferencing the image sho)n belo), as published in the case study, )e )ere able to create the model in $evit

*mage Courtesy of: ASH A!" Hig# Density Data Centers $ Case Stu%ies an% &est 'ractices" (00)

The figure belo) sho)s the model in $evit

"lso sho)n above is an add6in that can be used to launch the $evit model directly into "utodesk Simulation C*'?nce this is done, the model can be assigned materials in Simulation C*' The follo)ing assumptions )ere made forthe materials used

@


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Component MaterialWalls ConcreteCeiling Concrete*loor ConcreteServer racks "luminumStorage equipment "luminumC$"C1s "luminum*ire suppression equipment "luminumP'- "luminum-PS "luminum!"T "luminumConsole and net)orking equipment "luminum

Sho)n in the figure above is the tab that can be used to assign materials "lso on the left of the image, notice thataluminum and concrete is basically the material that is assumed for the various components of the data center The airvolume is automatically generated as long as )e have an air6tight volume in our $evit model The properties for the airvolume can be set to constant or variable depending on ho) the model needs to be used In our case, )e haveassumed that this is an interior room and constant properties for the air volume )ould be used

Boundary Conditions (BCs)


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?nce the material assignment is done, )e apply !C1s based on thedata provided for the Cedars6 Sinai 0edical Center data centerThe sources of heat generation )ere assigned a volume !C )hereasthe sources of air flo) )ere assigned a surface !C "lso, volumeslike the C$"C1s that did not contribute to the heat transfer )eresuppressed from the analysis to reduce the number of elementsgenerated during meshing and also so that the airflo) !C can beapplied correctly We )ill use the labels as sho)n in the figure belo) toidentify the various equipment in the room

The images belo) sho) the heat dissipation and flo) conditions as measured as part of the case study

A

%quipment Type Color C$"C=C$"C7

C$"C:C$"C>C$"C@C$"C>>> @ W eachP'-4-PS =::A W each

Surface !+it &oun%ary Con%itionC$"C = 99<


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Meshing

The ne(t step is the meshing process In this case study, )e performed the automatic meshing process )ith the defaultsettings The image belo) sho)s the meshed model

"s can be seen in the image above, the mesh is fairly coarse and you can see the nodes as the cyan colored points

Solve

"fter the meshing is done, )e can go into the solution module and define the physics )e need to includein our analysis We are going to include flo) and heat transfer in the model and also include the effects of radiation We perform the analysis over :88 iterations and the results are as sho)n belo)

B


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Results

"s part of the case study, temperature measurements )ere taken at the rack locations as sho)n in theimage belo)


"lso, the image belo) sho)s the actual measurements of temperatures at the rack locations referencedin the image above


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With this information, )e can use planes in Simulation C*' to plot temperatures in front of the rack and)e can compare the temperatures bet)een our model and the actual model as sho)n belo)

,nit num er Actual temperature Simulation CFDtemperature

Difference

=


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The average difference bet)een the C*' results and the actual measured results is (.(/ F

=7


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%nderstand modeling techni&ues for raised access $oor datacenters

'aised Access Floor

*or the purpose of this case study, )e )ill look at the data center located at The Deorgia Institute ofTechnology This model )as created in $evit )ith a total area of =


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The image belo) gives an idea of ho) to model this data center The =7 racks sho)n in the center portionof the model are the server racks the remaining > surrounding units represent the storage, managementand net)orking equipments


=>


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The figure belo) sho)s the model as it )as created in $evit

The model mainly includes C$"C units and server racks These units )ere modeled as sho)n in thefigure belo)

=@


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Material Assignment

The image belo) sho)s the materials that )ere assigned to the various parts of the model at a glance

The perforated floor )as modeled as a resistance material )ith a 8 7@ free area ratio since the case study

required the floors to have 7@F open tiles "n air volume )as used inside the C$"C unit for the boundaryconditions to be applied *or the server racks, an internal fan4blo)er material )as used )ith a flo) of=@:8 C*0 based on )hat )as measured for the case study "lso a slip factor of 8 : )as assumed )hichthe recommended value for a(ial fans


=


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Boundary Conditions

!ased on the planned cooling requirements for this data center, the follo)ing boundary conditions )ereapplied to the follo)ing pieces of equipment


"s sho)n in the figure above .on the left/, a volumetric flo) rate going into the C$"C units )as applied tosimulate the return flo) of air and a ;ero gauge pressure )as applied to the third C$"C unit to again

simulate the return flo) of air

The image on the right sho)s a volumetric flo) rate and temperature boundary condition that )as appliedto the C$"C at its e(it to simulate the air flo)ing into the room The values for the flo) rate andtemperatures )ere based on )hat )as measured for the case study as indicated belo)

=A


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The image above also sho)s that there are no boundary conditions applied on one of the C$"C units.C$"C :9/ and this is in accordance )ith the table sho)n above

Meshing

The image above sho)s the results of the automatic mesh si;ing process 5otice ho) the programautomatically determines areas )here the mesh needs to be refined and areas )here it can use a coarser mesh "nother point to note is that the air volume inside the C$"C units .the volume to )hich the

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boundary conditions have been applied/ has been suppressed in order to omit them from the analysis Ifthis )ere not done, the program does not recogni;e the inlets and outlets for the air flo)

Solution

?nce the model has been assigned materials, boundary conditions and meshed, )e are in a position torun the simulation Since )e are interested in the flo) and heat transfer behavior, )e have both modesturned on "lso )e are interested in studying the effects of radiation So )ith these conditions applied, )erun the simulation With the default values set for automatic convergence assessment, )e obtainconvergence in A:A iterations as sho)n in the convergence plot belo)

=B


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Compare i(erent esigns

In the previous e(ample, the data center )as modeled to have a total surface area of =


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The images belo) sho) the remodeled design for the data center


"lso highlighted in the image above, are some of the benefits that )ere predicted for the ne) design Weshall later see ho) )e can validate these predictions

The modeling technique including material assignment, boundary conditions and meshing is similar to theprevious e(ample The model is as sho)n belo)

7=


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"s can be seen from the figure above, the remodeled design .design 7/ had a total surface area of =888sq ft In the remodeled design the =7 racks are arranged in the middle portion )ith the storage,

net)orking and management racks located along the corners

77


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Results

The images belo) sho) the velocity profile in the under6floor for the t)o designs

In this case, the ma(imum velocity in the under6floor is about A


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With the remodeled design, the ma(imum velocity in the under6floor reaches about 77A7 C*0 +ust aspredicted by the case studyG *urther, summary planes can be created to compare the t)o designs side6by6side and the static pressure in the under6floor could be compared The static pressure for design =

)as 8 7B psf as against = @: psf for design 7, again in agreement )ith the prediction for the data centerremodel "nother consequence of the ne) design )as that the temperature in the under6floor )as higher.


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We can also cut a plane above the raised floor and use a summary image to compare the temperaturedistribution among the t)o designs

The image above sho)s that the ma(imum temperature in 'esign = is about =:A * *or the same plane in'esign 7, )e can see in the image belo) that the ma(imum temperature hovers in the late A8 * range

7@


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Thus using C*' analysis, )e )ere in a position to compare the t)o design alternatives for this datacenter model and conclude that 'esign 7 is the better alternative due to

#igher static pressures in the under6floor region

#igher velocity magnitudes in the under6floor region

3o)er temperatures above the floor

5egligible recirculation of )arm air above the perforated floor

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