power point file

Robust Design of Air Cooled Server Cabinets

Nathan Rolander, Jeff Rambo, Yogendra Joshi, Farrokh MistreeASME InterPACK Conference19 July 2005

Systems Realization Laboratory

Microelectronics & Emerging Technologies Thermal Laboratory

Support for this work provided by the members of

CEETHERM

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Background: What is a data center?

10,000-500,000 sq. ft. facilities filled with cabinets which house data processing equipment, servers, switches, etc.

Tens to hundreds of MW power consumption for computing equipment and associated cooling hardware

Trend towards very high power density servers (30 kW/cabinet) requiring stringent thermal management

* Image: B. Tschudi, Lawrence Berkeley Laboratories1

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Introduction & Motivation

Up to 40% of data center operating costs can be cooling related*

Cooling challenges are compounded by a lifecycle mismatch: New computer equipment introduced ~ 2 years Center infrastructure overhauled ~ 25 years

* Source: W. Tschudi, Lawrence Berkeley Laboratories

How do we efficiently integrate high powered equipment into an existing cabinet infrastructure

while maximizing operational stability?

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Cabinet Design Challenges

Flow complexity The turbulent CFD models required to analyze the

air flow distribution in cabinets are impractical to use iterative optimization algorithms

Operational stability Variations in data center operating conditions,

coupled with model inaccuracies mean computed “optimal” solutions do not translate to efficient or feasible physical solutions

Multiple design objectives Objectives of efficient thermal management, cooling

cost minimization, & operational stability are conflicting goals3

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Approach Overview

Integration of three constructs to tackle cabinet design challenges:

Flow complexity

Operationalstability

Multipleobjectives

Challenge

Thermally efficient & robust server cabinet design

approach

Integration

POD* based turbulent modeling

Robust designprinciples

The compromise DSP**

Construct

* Proper Orthogonal Decomposition ** Decision Support Problem4

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Modal expansion of basis functions, :

Fit optimal linear subspace through a series of system observations, .

Maximize the projection of the basis functions onto the observations:

Introduction to the POD

1)()(),(

iii xtatxu

}1|||||),(max{ 2 u

)'(')'()',( xdxxxxC

φ

u

5

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Maximize the projection of the basis functions, onto the observations:


1)()(),(

iii xtatxu

}1|||||),(max{ 2 u

)'(')'()',( xdxxxxC

< , > denotes ensemble averaging

( , ) denotes L2 inner product

Constrained variational calculus

problem

φ

u

5

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1)()(),(

iii xtatxu

}1|||||),(max{ 2 u

)'(')'()',( xdxxxxC



Assemble observations

covariance matrix

u

φ

5

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1)()(),(

iii xtatxu

}1|||||),(max{ 2 u

)'(')'()',( xdxxxxC



Take cross correlation tensor

of covariance matrix

u

φ

5

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1)()(),(

iii xtatxu

}1|||||),(max{ 2 u

)'(')'()',( xdxxxxC



Take eigen-decomposition of the

cross-correlation tensor

u

φ

5

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POD Based Turbulent Flow Modeling

Vector-valued eigenvectors form empirical basis of m-dimensional subspace, called POD modes

Superposition of modes used to reconstruct any solution within the range of observations ~10% error*

Flux matching procedure applied at boundaries >> areas of known flow conditions, resulting in the minimization problem:

Values of found using method of least squares Resulting model has ~O(105) reduction in DoF*

1

min{|| ( ) ||} ,p

i ii

G a F p m

G is the flux goalF(.) is contribution to boundary flux from the POD modesa is the POD mode weight coefficient

ai

* see: Rambo HT2005-72143 paper for complete analysis6

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Robust Design Principles

Determine superior solutions through minimizing the effects of variation, without eliminating their causes. Type I – minimizing variations in performance

caused by variations noise factors (uncontrollable parameters)

Type II – minimizing variations in performance caused by variation in control factors (design variables)

A common implementation of Type I robust design is Taguchi Parameter Design

7

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Robust Design Application

Goals:Y

X

Objective Function

DesignVariable

Deviationat Optimal Solution

OptimalSolution

( )y f x

Optimization minimizes the objective function:

Deviationat Robust Solution

RobustSolution

Solution insensitivity is obtained by minimizing curvature:

2

2 2

1

n

y ii i

f xx

n is the no. control variables

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Robust Design Application

Constraints:

ConstraintBoundary

Robust Solution Bounds

RobustSolution

FeasibleDesignSpace

OptimalSolution

X2

X1DesignVariable

DesignVariable

Variability bounds in control parameters must be accounted for to to avoid infeasible solutions:

() 0j jEg gx , j = 1,…,p

1

nj

j ii i

gg x

x, j = 1,…,p

E (.) is the mean function

p is the no. constraints

n is the no. control variables

Optimal Solution Bounds

2ΔX2

9

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The Compromise DSP Mathematics

Hybrid of Mathematical Programming and Goal Programming optimization routines:

min ( )f x. . ( ) 0is t g x

( ) 0ih x

Objective function

Inequality constraints

Equality constraints

Mathematical Programming

( ) i i iA d d G x

G is a goal

Goal Programming

Goal function

is under/over achievement of goal

,i id d

1

( )m

i i ii

f Z W d d

x

Minimization of Archimedean Deviation Function

W is the goal weight vector

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Problem Geometry

Enclosed Cabinet containing 10 servers

Cooling air supplied from under floor plenum

Cabinet Profile Server Profile

Server 2

Server 3

Server 4

Server 5

Server 6

Server 7

Server 8

Server 9

Server 10

H

W

Vin ,

Section a

Section b

Section c

Cold Supply Air

Hot Exhaust Air

x

z

Lc

Server 1

Section c

Section b

Section a

W

H

Vin

Hot/Cold airflow

Isoflux Blocks Qa,b,c

Ls

Fan Model

Hs

x

z Fan

Ls

Hs

Qa,b,c

Dimensions:

H = 2 m

W = 0.9 m

Ls = 0.6 m

Hs = 2U ~ 0.09m

11

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Cabinet Modeling

9 Observations of Vin = 0:0.25:2 m/s for POD

k-ε turbulence model for RANS implemented in commercial CFD software

Finite difference energy equation solver used for thermal solution, using POD computed flow field

1 iteration ~ 12 secVin = 0.95 m/s

x x

y

12

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Server 2

Server 3

Server 4

Server 5

Server 6

Server 7

Server 8

Server 9

Server 10

H

W

Vin ,

Section a

Section b

Section c

Cold Supply Air

Hot Exhaust Air

x

z

Lc

Server 1

Section c

Section b

Section a

W

H

Vin

Hot/Cold airflow

Design Variables & Objectives

Response:Chip Temperatures (oC)

Control Variables:Inlet air velocity, Vin [0, 1] m/sSection a chip power, Qa [0, 200] WSection b chip power, Qb [0, 200] WSection c chip power, Qc [0, 200] W

Server Cabinet Model

13

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Goals:Minimize Inlet air velocityMinimize Chip TemperaturesMinimize Chip Temperature Variation

Constraints:Meet Target Cabinet Power Qtotal

All Chip Temperatures < 85oC



Objective:Minimize ΔTmax & cooling energy for a given cabinet heat generationrate Qtotal

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Goals:Minimize Inlet air velocityMinimize Chip TemperaturesMinimize Chip Temperature Variation



iterate

Constraints:Meet Target Cabinet Power Qtotal

All Chip Temperatures < 85oC

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Results

Baseline vs. Maximum efficient power dissipation

Server 2

Server 3

Server 4

Server 5

Server 6

Server 7

Server 8

Server 9

Server 10

Vin ,

Section a

Section b

Section c

x

z

Lc

Server 1

Section c

Section b

Section a

W

H

Vin

Hot/Coldairflow

Server 2

Server 3

Server 4

Server 5

Server 6

Server 7

Server 8

Server 9

Server 10

Vin ,

Section a

Section b

Section c

x

z

Lc

Server 1

Section c

Section b

Section a

W

H

VinVin

Hot/Coldairflow

14

Without server power re-distribution, increasing flow of cooling air alone is ineffective

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Results

Inlet air velocity vs. Total cabinet power level

Cooling air is re-distributed to different cabinet sections depending upon supply rate >> server cooling efficiency

Server 2

Server 3

Server 4

Server 5

Server 6

Server 7

Server 8

Server 9

Server 10

Vin ,

Section a

Section b

Section c

x

z

Lc

Server 1

Section c

Section b

Section a

W

H

Vin

Hot/Coldairflow

Server 2

Server 3

Server 4

Server 5

Server 6

Server 7

Server 8

Server 9

Server 10

Vin ,

Section a

Section b

Section c

x

z

Lc

Server 1

Section c

Section b

Section a

W

H

VinVin

Hot/Coldairflow

15

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Results

Maximum chip temperature and bounds

Maximum chip temperature constraint met as variation in response changes with varying power & flow rates16

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Conclusions

How do we efficiently integrate high powered equipment into an existing cabinet infrastructure

while maximizing operational stability?

17

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Conclusions

For the typical enclosed cabinet modeled, over 50% more power than baseline can be reliably dissipated through efficient configuration

Robust solutions account for variability in internal & external operating conditions, as well as a degree of modeling assumptions &inaccuracies

Server cabinet configuration design can be accomplished without center level re-design

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Questions?

Thank you for attending!

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Final Validation

Comparison of results obtained using robust design and compact model to FLUENT

Total Cabinet Power (W)

Mean Chip Temp. Difference ( oC)

1600 3 2100 9 2400 3

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Robust vs. Optimal Configuration

Pareto Frontier

0.8 0.85 0.9 0.95 1124

125

126

127

128

129

Inlet Air Velocity (m/s)

Chi

p H

eat

Flu

x (W

)

0.8 0.85 0.9 0.95 187

88

89

90

91


Chi

p H

eat

Flu

x (W

)

0.8 0.85 0.9 0.95 140

45

50

55

60


Chi

p H

eat

Flu

x (W

)

0.8 0.85 0.9 0.95 161.8

61.9

62

62.1

62.2

62.3


Mea

n C

hip

Tem

pera

ture

(

o C)

(y) (a)

(b) (c)

Feasiblespace

Feasiblespace

Feasiblespace

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Effects of Robust Solution

Optimal >> Robust :Temperature Variation

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1180

200

220

240

260

280

300

320

Optimal >> Robust Weighting Value

Sum

of

Tem

pera

ture

Var

iabi

lity

Optimal vs. Robust Server Temperature Variability

Qtotal = 2000 W

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System Model

Control Factors, xInlet air velocity, Vin [0, 1] m/s

Section a chip power, Qa [0, 200] WSection b chip power, Qb [0, 200] WSection c chip power, Qc [0, 200] W

Noise Factors, ZInlet air temperature, Tin = 25 oC

Res

pons

e, y

Inle

t Air

Vel

ocity

(m/s

)C

hip

Tem

pera

ture

s (o C

)To

tal c

abin

et p

ower

(W)

Sig

nal F

acto

rs, M

Inle

t air

velo

city

(min

imiz

e)C

hip

Tem

pera

ture

s (m

inim

ize)

Cab

inet

Pow

er (n

omin

aliz

e)

Server Cabinet System

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Design Objective Specification

System Design Objectives >> Goals Minimize flow rate of cooling air supplied to cabinet Minimize server chip temperatures Minimize sensitivity of configuration to changes in

cabinet operating conditions System Design Specifications >> Constraints

All server chips must operate at under 85oC Total cabinet power must meet target value

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