MEMS 1043 Senior Design Project, Spring 2012

Final Report

Mechanism Enabling Retrofit for Cooling of Data Center Components

Matthew Kaminski, Naji Alibeji, Gregory Meyer, Chenell YorkDepartment of Mechanical Engineering and Materials Science

University of Pittsburgh, Pittsburgh, PA 15261, USA

1. ABSTRACT

A data center is a facility used to house computer system server boards and supporting infrastructure. Increasing computing capabilities and demand are resulting in greater room power densities and increases in the need for cooling of the electronic components within the server boards. Current cooling techniques which use forced air convection with the aid of fans are loud, inefficient, and expensive. This design focuses on a mechanism that enables a retrofit for cooling of data center components. The main features of this design are cooling blocks, which consists of a microchannel heat sinks and supporting housing. The microchannels consist of an arrangement of channels and fins whose purpose is to increase the area available for heat transfer from the component to the water. A separate cooling block is placed on each heat generating component within the server board. As water is pumped through the cooling blocks, it flows through the microchannels where it removes the heat generated from the component. The water travels through a series of tubes where it enters each successive cooling block. Ultimately, the water flows through a radiator where the water is cooled and sent back to the pump. Thermocouples were used during testing of the design to obtain temperature readings of the water and the heat generating components. These measurements were used to compare the actual thermal resistance of each cooling block to the calculated theoretical values. The optimal design will yield the minimum thermal resistance for each heat sink. This design demonstrates the feasibility of retrofitting a data center with a liquid-cooled thermal management solution. Additional tests are needed to better quantify the design’s effectiveness in removing the required amount of heat from the heat generating components.

NOMENCLATURE

Ac = microchannel wetted areaAcx = microchannel cross-sectional areaAs = base area of heat sinkCp = specific heatDh = hydraulic diameterf = Darcy friction factor constanth = convection heat transfer coefficientHch = microchannel depthk = thermal conductivityL = length of heat sinkm = mass flow rateN = number of microchannelsNu = Nusselt numberP = pressurePm = microchannel perimeterP0 = pump curve y-interceptQ = thermal design powerQ0 = pump curve x-interceptR = total thermal resistanceRe = Reynolds numbert = substrate thicknessTch = temperature at base of channelTfl_avg = average fluid temperatureTfl_in = fluid temperature at inlet of channelTfl_out = fluid temperature at outlet of channelTj = junction temperatureu = mean flow velocityV = volumetric flow rateW = width of heat sinkwch = microchannel widthwf = fin thickness

Greek symbols

μ = dynamic viscosityρ = density of fluidΔP = pressure dropΔT = temperature difference

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. 2. INTRODUCTION

DATA CENTER BACKGROUND

Data centers allow the storage of large amounts of equipment connected to communication systems, computers and electronics [1]. They house the highly specialized equipment responsible for the support of the computers, networks, data storage and security of a company’s business intelligence and business process [2]. The basic layout of a data center is shown in Figure 1.

Figure 1: Typical layout of a data center

The most fundamental data center has a computer network, contains backup power supplies, air conditioning, and security procedures [1]. Data centers are absolutely critical for companies as they act as the brain for most companies.

Data centers consume a tremendous amount of energy. According to a 2007 Scientific American Report, the overall total energy usage of data centers was 61 billion kilowatt-hours [3]. This corresponds to roughly 1.5% of the country’s entire electricity consumption [3]. Of the 61 billion kilowatt-hours consumed, as much as 40% of a data center’s energy bill comes from cooling equipment [4]. A breakdown of the power distribution for data centers can be seen in Figure 2.

Figure 2: Data center power distribution

As shown in Figure 2, data center cooling is a significant part of a data center’s total energy consumption. Failure to adequately cool data center components could result in irreversible damage to the components.

This design focused on cooling three components of the Foxconn G41MXE motherboard shown in Figure 3.

Figure 3: Foxconn G41MXE Series Motherboard with Core 2 Duo Processor (1), North Bridge Chipset (2), and South Bridge Chipset (3).

MOTIVATION FOR CURRENT WORK

The two main methods for cooling data center equipment are air cooling and liquid cooling. As seen in Figure 4, air cooling generally involves mounting a heat sink and a fan to each heat generating component

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of a server board [5]. Heat generated from the component is transferred to a heat sink by conduction. The fan blows air through the heat sink and removes the heat through convection [6].

Figure 4: Heat source/heat sink relationship for air cooling of a CPU [5]

Liquid cooling may be more efficient at removing the heat generated from server board components than air due to its higher thermal conductivity and specific heat capacity. When compared to traditional air-cooled methods, liquid cooling devices have the capability of reducing the power consumption of in-room cooling devices by as much as 90% [6]. For a liquid cooled system, liquid is pumped through a heat sink where it absorbs the heat from the component. The liquid exits the heat sink where it is then cooled by means of a radiator [5]. An image of liquid cooled computer component is shown in Figure 5.

Figure 5: Liquid cooled component

Air-cooled methods can create a very costly, complex, and inefficient infrastructure [6]. On the contrary, by allowing data centers to operate with higher-density racks, rack-level liquid cooling can reduce the data center IT footprint by as much as 80% [6].

Another advantage of liquid cooling is that it is much quieter than air cooling. As computer systems are constantly upgraded, this creates the need for additional fans throughout the data center facility, eliminating the need for high speed fans throughout a data center. This design focuses on retrofitting data center server trays with a liquid cooled system rather than completely rebuilding the server trays. Retrofitting a cooling device is a much cheaper alternative than completely replacing data center server trays.

MICROCHANNELS

A microchannel heat sink functions in the same manner as conventional centimeter scale channel designs with the exception that microchannels are on the scale of hundreds of micrometers. Microchannel heat sinks consist of an array of channels and fins which are used to increase the area available for heat transfer from the component to the fluid. Figure 6

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Final Report

depicts the layout of a microchannel heat sink.

FIGURE 6: Microchannel heat sink layout

The junction temperature is the temperature at the interface between the base of the heat sink and the top of the heat generating component. It is desired to keep this junction temperature as low as possible in order to ensure that the component remains below its maximum operating temperature. The microchannel heat sink is attached to the component of the server tray via a thermal paste. Heat is first transferred by conduction up from the component through the base of the heat sink and up through the fins. The fluid that flows through the microchannels picks up this heat and convects the heat away from the component. Microchannels exhibit dramatically better heat transfer performance than centimeter scale designs due to a large increase in the convection heat transfer coefficient. However, the better heat transfer performance must be balanced with the fact that microchannels also increase the pressure drop experienced by the fluid as it travels through the channels. Competition between these two parameters will yield optimal channel dimensions. This is explained in more detail in the thermal analysis section.

3. DESIGN

THERMAL ANALYSIS

The goal of the thermal analysis was to determine the number of channels, dimensions of the channels, and the required flow rate through the channels that would yield the lowest total thermal resistance for each heat sink. The total thermal resistance is comprised of

caloric resistance, convective resistance, and conduction resistance [7]. By minimizing the thermal resistance, the maximum amount of heat will be transferred from the component to the fluid. The analysis was performed in a MATLAB file in order to perform the iterative calculations. The final product of the analysis was a graphical user interface (GUI) which allows the user selects how many components need to be cooled, the dimensions of the components, and the maximum operating temperature and thermal design power (TDP) of the component. The MATLAB GUI is shown in Figure 7. Refer to Appendix B for the MATLAB code used to generate the GUI.

Figure 7: GUI used to determine optimal number of channels, dimensions of the channels, and the required flow rate through the channels for each heat sink

The maximum operating temperature and TDP can be gathered for each component from the manufacturer’s data sheets. The main requirement in the thermal analysis was to ensure that the junction temperature for each component remained below its maximum temperature while operating at TDP.

As discussed earlier, it was determined that the most cost effective way to cool each component would be to run the tubing in series from one component to the next. Therefore, the fluid that exits the first microchannel heat sink would be the same fluid that enters the second microchannel heat sink. This reduces the fluid’s ability to convect heat away since the fluid’s temperature will increase each time it flows through a heat sink. In addition, the pressure drops will

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t

WL

T j T chW f

H ch

W ch

H

MEMS 1043 Senior Design Project, Spring 2012

Final Report

accumulate each time the fluid passes through each additional heat sink.

In order to simplify the analysis and to ensure the microchannels could be machined, certain dimensions were fixed throughout the analysis. The dimensions of the base of each heat sink were the same dimensions used for the base of the heat generating component. Thus, the user will specify these values. After consulting the machine shop at the Swanson School of Engineering, it was determined that a fin aspect ratio of four would be the maximum value that would permit machining of the microchannels. In addition, there were also constraints on the minimum width of the channels since there are only certain mill bit sizes available in the machine shop. As discussed earlier, water is superior to air for this particular heat removal application. The higher density and specific heat of water allow for more effective heat removal than air; therefore, room temperature water was chosen as the working fluid for this design. Another property specified prior to beginning the analysis was the heat sink material. It was concluded that copper would yield the most desirable properties due to its high thermal conductivity and ease of machinability. Table 1 summarizes the dimensions that were fixed at the beginning of the analysis. Table 2 lists material properties of water and copper.

Table 1: Summary of dimensions that were fixed prior to beginning thermal analysis

Base thickness of

heat sink

Channel depth Fin width

Minimum Channel

width1 mm 0.8 mm 0.2 mm 0.5 mm

Table 2: Summary of the properties of the fluid and material used for the heat sinks [7]

Specific Heat Density Dynamic

viscosityThermal

conductivity

Water4183

JkgK

998.3 kgm3

1.002*

10−3 kgm∗s

0.6 W

m∗K

Copper N/A N/A N/A 400 W

m∗K

For the purposes of this design, a Foxconn G41MXE series motherboard was used as the basis for the design of the three heat sinks. Table 3 lists the components in the motherboard that require cooling. The PMP-300 Compact Pump from Koolance was chosen as the pump for this system because it provides an adequate head with low power consumption.

Table 3: Motherboard chip specifications [8, 9, 10]

Pentium Core 2

Duo CPU

North Bridge

Chipset: Intel G41

South Bridge

Chipset: Intel ICH7

Length 30 mm 11.5 mm 25.5 mmWidth 30 mm 10 mm 25.5 mmThermal Design Power

65 W 25 W 3.3 W

Maximum Operating Temperature

72 °C 102 °C 99 °C

The goal of this design is to achieve the lowest total thermal resistance for each heat sink while also minimizing the pressure drop through the heat sinks. The optimal design of each heat sink would ensure that the junction temperature of each component is minimized. By varying the number of channels used in each heat sink, a minimum junction temperature for each component can be found. However, there is competition between achieving the minimum junction temperature and achieving a minimum pressure drop through the channels. Equation 1 is used to calculate the junction temperature of a component.

T j=Q

Nh Ac+ Qt

k A s+ Q

m Cp+T fl¿

In order to optimize heat transfer from the component to the fluid, and thus minimize the junction temperature, several factors were considered. A tall, thin fin increases the number of channels and fins that comprise the heat sink, and thus there is more area

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(1)

MEMS 1043 Senior Design Project, Spring 2012

Final Report

available for heat transfer. As the number of channels increases, the junction temperature decreases as seen in Equation 1. This, however, must be balanced with the fact that a taller fin increases the hydraulic diameter of the channel which ultimately decreases the convection heat transfer coefficient. A decrease in the convection heat transfer coefficient will increase the junction temperature as seen in Equation 1. Therefore, it was desired for each channel to have the smallest hydraulic diameter possible because this would increase the convection heat transfer coefficient and lower the junction temperature. As discussed earlier, the ratio of the channel height to channel thickness must be below four due to machinability constraints. The challenge presented with using microchannel heat sinks is the large pressure drop that accumulates as the fluid flows through the channels. The pressure drop increases dramatically as the hydraulic diameter of the channel decreases. In addition, increasing the mass flow rate through the channels to achieve a lower junction temperature also increases the pressure drop. Therefore, the increase in heat transfer performance must be balanced with the increase in pressure drop. A larger pressure drop requires a larger pump which is not desired due to an increase in the cost of a pump and electricity usage.

Equation 2 below is the system curve for the pressure drop experienced by the water as it flows through three heat sinks. Varying the number of channels in each heat sink will yield a number of different system curves.

ΔP=32V μH ch

(L1

W ch1Dh1

2 +L2

W ch2Dh2

2 +L3

W ch3Dh3

2 )

The equation that describes the PMP-300 pump curve [11] is given below as Equation 3.

ΔP=P0−P0 VQ0

The intersections of the system curves and the pump curve yield a range volumetric flow rates that will ensure that the water from the pump can overcome the pressure losses as it travels through the heat sinks. These volumetric flow rates are easily converted into

mass flow rates, m, which can then be substituted into Equation 1 to yield a range of values for T j . The optimal number of channels, channel dimensions, and flow rate are determined at the point where the junction temperature is minimized for each heat sink. Equation 4 was used to calculate the total thermal resistance for each heat sink.

R=T j−T flavg

TDP

Table 4 lists the calculated total thermal resistance for each heat sink.

Table 4: Total thermal resistance for heat sinks

Heat Sink 1 Heat Sink 3 Heat Sink 3Total

Thermal Resistance

0.1905 ℃W

0.8974 ℃W

1.5603 ℃W

Please refer to Appendix A for a complete derivation of the above equations and for a more detailed analysis.

DESIGN REQUIREMENTS

The main objective of this project was to design a liquid cooled heat sink that can be used to retrofit outdated data center air cooled systems. Since this product is intended to be a retrofit, it must be capable of being installed in any server board configuration. Server trays come in many different variations, each with their own internal dimensions, server board layouts, and electrical components that require cooling. In order to effectively cool the electrical components, heats sinks apply a static load on the components with a specific amount of pressure in order to ensure effective thermal surface contact. This is typically done by directly mounting the heat sinks to the server board using bolts and standoffs. Other methods include retention springs that also use fixtures that bolt onto the server board to add support. The server boards have holes in the electrical board to accommodate for these options, but the bolt holes patterns and locations vary with different server trays models.

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(2)

(3)

(4)

MEMS 1043 Senior Design Project, Spring 2012

Final Report

Current liquid cooled retrofit heat sink options, such as the products offered by Koolance, use adjustable brackets and springs to mount the heat sinks to the server tray board as seen in Figure 8 [12]. However, these options are only compatible with a select few server board trays. One of the main concentrations of this project was to design a mounting mechanism that is capable of accommodating any server board tray. Therefore, the mounting mechanism must not depend on the bolt hole patterns in the server boards. Thus it was decided that using the weight of the server tray lid would be the best option to hold the cooling block into place.

Figure 8: Mounting cooling block from Koolance with either adapter bracket or retention springs [12]

Upon choosing the final design for the mounting mechanism, many designs were considered. Design one consisted of installing a network of tracks under the lid. These tracks would allow the mounting of the cooling block in any location in the server board. However, this design was eliminated because of the limited space inside of the board and the numerous obstacles that would restrict the path of the tracks. The second design involved drilling a grid of holes into the lid and directly mounting the cooling block to the lid. This design was also dismissed because it is preferred to not have to modify the existing lids. The consideration of using magnets to hold the cooling blocks in place was also eliminated because it is advised to not have magnets around the electrical components.

Every time a data center is retrofitted with this product, the design of the cooling block will need to adjust

depending on the size and specifications of the electrical components requiring cooling. If a standard solid model is used, the parts will have to be redesigned for every electrical component. To avoid this constant need for manually reconfiguring the parts, iParts and iAssemblies were used. iParts and iAssemblies use parametric tables which depend on key variables or dimensions to easily reconfigure the parts. Using iParts and iAssemblies make the inner workings of the models more complicated but in return they eliminate the need for manually reconfiguring the model. For further information about iParts and iAssemblies refer to Appendix C.

The final design consisted of a cooling block, spring, locator bracket, and adhesive. The cooling block sits directly onto the electrical component and is comprised of a microchannel base, headers, and a lid. Its purpose is to dissipate the heat from the heat generating component and keep the water contained within. Figure 9 shows a solid model of the final design. Please refer to Appendix E for the technical drawings of each part.

Figure 9: Solid model of final design

The copper microchannel base will contain the required number of channels indicated by the GUI. In addition, the microchannel base will also have eight 2-56 UNF threaded holes for the lid and headers to bolt to.

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Dowel Pin

Lid

Elbow Fitting

Locator Bracket

Microchannel Base

Headers

Spring

MEMS 1043 Senior Design Project, Spring 2012

Final Report

The lid sits directly onto the sides of the microchannel base and is bolted down. A recessed hole is located on top of the lid to help hold the spring in place. On the sides of the lid, tabs protrude out directly with a hole in the center of them. These tabs/holes are used to mate with the locator bracket and prevent the cooling block from shifting side to side. The locator bracket is attached to the lid using adhesive, and the pins mate with the cooling block and hold it in place as mentioned before. At first we planned to have a mechanism snap into place with the lid to hold it in place. After consideration we decided that a simple pin would suffice. An added benefit of using a simple pin is that the lid can be removed along with the locator bracket without having to undo the cooling block configuration.

The headers are designed to bolt onto both sides of the microchannel base. Each header has a 10-28 UNF threaded hole for the fittings on the top face of the header. This hole intersects with a channel on the side. This channel is just big enough to expose the microchannels to the water flow. Through holes on the header allow the header to be bolted to the microchannel base.

The spring sits in the recessed hole of the lid and expands past the locator bracket. When the lid is closed, the spring will be forced to compress and apply pressure to the cooling block. The stiffness of the spring was chosen based on the static load required by electrical component and the amount of compression the spring will experience.

The fitting chosen to connect the tubes to the header is an elbow to barb fitting. It was initially planned to use luer fittings which are typically used in medical applications. Figure 10 shows an assortment of different luer fittings available. They are small, reliable, and provide an easy quick release connection feature. However, with the limited amount of space in the server tray, it was difficult to find a combination of luer fittings that would effectively distribute the flow and provide the quick release connection. Therefore, it was decided to use a single elbow thread to barb fitting.

Figure 10: Luer fittings assortment

To assemble the cooling block, 2-56 UNC bolts were used to hold the microchannel base, headers, and lid together. The dowel pins were press fitted into the locator bracket. The cooling block was sealed using silicon, and Teflon tape was used to maintain a seal around the elbow fittings.

CONSTRUCTION

The materials used to construct the parts were purchased from McMaster-Carr. The microchannel base was made from a 6”x 6” slab of high ultra-conductive copper. The material chosen to make the headers, lids, and locator brackets was acrylic. Acrylic is cheap, easy to machine, and serves as a good thermal insulator. After some design reviews and iterations to the design, the drawings were finalized. A CNC machine was selected as the best method of machining the parts. After converting the 15 parts into a format compatible with the CNC machine, the code used to machine the parts was written by the machinist.

The first step was to cut the copper and acrylic stock into smaller work pieces and square them off. The parts where then machined using the CNC machine and the generated code. The microchannels in the base were cut using a special ordered 0.026” mill bit. After completing the parts, they were cleaned and the threaded holes were tapped. Figures 11, 12, and 13 shows the three completed microchannels, headers, and lids respectively.

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MEMS 1043 Senior Design Project, Spring 2012

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Figure 11: Complete microchannel base

Figure 12: Completed headers

Figure 13: Completed lids

The dowel pins were purchased from McMaster-Carr and grinded down to the correct length using a belt sander. The dowel pins for the locator bracket were press fitted using a precision vice. Each cooling block assembly included a microchannel base, lid, two headers, elbow fittings, locator bracket plate, two dowel pins, adhesive, silicon, four 2-56 UNC x 0.125” bolts, and four 2-56 UNC x 0.5” bolts. Figure 14 shows the assortment of parts for each cooling block assembly. For the smaller cooling blocks, the bolts from the lid and header intersected at some points, so the bolts from the header were shortened using a belt sander to an appropriate length.

Figure 14: Components included in one assembly

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An important feature of the cooling block is that it must be completely sealed to eliminate any leaks. This was challenging to achieve since there were many mating faces and small critical openings that had to be unobstructed. The first attempt included using scotch tape; the tape was layered onto the mating faces. The adhesive helped keep the tape in place while the openings for the channels and holes were cut using a surgical scalpel.

To ensure the integrity of the seal, water was pumped through the assembled cooling block for a few minutes. The scotch tape gasket was unsuccessful. The next attempt included a 1 mm gasket film from McMaster-Carr. However, it proved difficult to cut the rubbery gasket into a usable shape. The final attempt included the use of silicon. This was the last option considered because it was feared that the silicon would ooze into the header channel when compressed. This would have obstructed the flow and hindered the performance of the cooling block. To prevent the silicon from oozing, the mating faces were only wetted with the silicon leaving no excess to ooze into the channels. To further strengthen the seal, silicon was smeared on each seam. After testing this option, the seal proved to be reliable.

4. EXPERIMENT AND TESTING

As previously mentioned, the optimal design of each cooling block will minimize the thermal resistance between the heat generating component and the environment. Achieving this minimum thermal resistance will optimize the heat transfer performance of each cooling block. Due to excessive leakage in the North Bridge cooling block design, this cooling block was not tested. A bill of materials is shown below in Table 5.

Table 5: Bill of Materials

Part Description VendorUnit Price

($)QTY

Copper Slat McMaster 62.94 1

Acrylic Slat McMaster 7.04 1

.026" End Mill McMaster 23.61 3

Compression Spring

McMaster 7.36 1 pack

(5)

Elbow Fitting Value Plastics

Sample

2 packs (5)

Dowel Pin McMaster 4.77 2 packs

(5)

Flexible PVC Tubing McMaster 0.25 1ft

Double-Sided Tape McMaster 15.44 1

Adapter fitting, Tube to male threaded pipe

McMaster 5.26 1 pack

(10)

Threading Adapter, NPT 1 } over {8¿

M to G 1 } over {4¿ F

Koolance 2.17 5

TOTAL:

$195.51

EXPERIMENTAL SET-UP

Figure 15 is a schematic of the experimental setup used to test the CPU cooling block and the South Bridge cooling block.

Figure 15: Schematic of testing set-up

A thermofoil heater was used in conjunction with a power supply to simulate the heat generated from the data center components. Wires were soldered to the leads in the thermofoil heater, shown in Figure 16.

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MEMS 1043 Senior Design Project, Spring 2012

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Figure 16: Thermofoil heater with connected wires

Banana clips were used to connect these wires to the power source. In order to ensure the heat was evenly dissipated, a copper spreader was machined and then placed on top of the thermofoil heater. Prior to mounting the cooling block to the spreader, a groove was milled into the center of the spreader to allow a thermocouple to be placed at the junction of the spreader and the cooling block. This thermocouple was used to measure the junction temperature. A complete setup of the cooling block assembly and heating element is shown in Figure 17. Refer to Appendix D for complete cooling block assembly instructions.

Figure 17: Complete set-up of cooling block and heating component

TESTING

Experimental testing began by applying a known power input to each cooling block. This power input was used to simulate the heat that would be generated from a server board component. The power input to each cooling block was increased in increments of two watts, ranging from three to fifteen watts. The pump was then turned on and water from the reservoir tank was pumped through the system. The pump supplied the water with the necessary head to overcome the pressure losses experienced in the system. The flow from the pump was measured by the volumetric flow sensor as seen below in Figure 18.

Figure 18: Pump (left) with flow sensor (center) and reservoir tank (right)

The system was allowed to reach steady state before any true measurements were taken. Steady state was defined as the point of operation where every temperature in the system remained at a constant value. Once the system was operating at steady state, the true temperature measurements were recorded. Thermocouples were used at different points along system to gather temperature readings. The first thermocouple was placed after the flow sensor to record the temperature of the water before it entered the cooling block. Figure 19 shows the thermocouple used to measure the inlet temperature of the water.

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Figure 19: Thermcouple used to measure inlet water temperature

After the inlet temperature of the fluid was measured, the fluid traveled into the cooling block, where a second thermocouple was used to measure the junction temperature. The thermocouple used to measure the junction temperature is located between the two wires in Figure 17. A third thermocouple, Figure 20, was used to collect temperature data at the outlet of the cooling block.

Figure 20: Thermcouple used to measure outlet water temperature

After the water exits the cooling block, it travels into a radiator which then cools the water and recycles it to the reservoir tank where it can be used again to

continue the process. The reservoir tank and radiator are shown in Figure 21.

Figure 21: Reservoir (front) and radiator (back)

Temperature and flow measurements were collected for the CPU and South Bridge cooling block using a data acquisition unit and MATLAB code. However, as previously mentioned, data was not collected for the North Bridge cooling block due to excessive leakage. The goal was to determine the junction temperature as well as the inlet and outlet water temperatures for each cooling block at each power level. These values were used to calculate the convection heat transfer coefficient and the total thermal resistance of each cooling block.

5. RESULTS AND DISCUSSION

Based on the channel dimensions, material properties, and temperature measurements for each cooling block, the convection heat transfer coefficient was calculated for each cooling block. Figure 22 is a plot of the convection heat transfer coefficient at each power level for the CPU cooling block and South Bridge cooling block.

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2 4 6 8 10 12 14 16 180

50

100

150

200

250

Convection Coefficient vs Power Input

CPU Cool-ing Block

South Bridge Cooling Block

Heater Power Input (W)

Conv

ectio

n Co

effici

ent (

W/m

2)

Figure 22: Convection Coefficient vs. Power

Theoretically, the convection heat transfer coefficient should have remained constant at each power level since it is only a function of the channel dimensions. It was anticipated that the convection heat transfer

coefficient would be around 2917 Wm2 and 1455

Wm2 for

CPU cooling block and South Bridge cooling block respectively. As seen in Figure 22, not only was the convection heat transfer coefficient an order of magnitude less than the predicted value for each cooling block, but the convection heat transfer coefficient is larger for the South Bridge cooling block than the CPU cooling block. It is possible that the flow in the either cooling block may not have been fully hydrodynamically or thermally developed. If this were the case, then not all of the channels in the CPU cooling block may have had water running through them. This would result in a much lower convection heat transfer coefficient than expected.

The convection resistance accounts for nearly 90% of the overall thermal resistance. A plot of the total thermal resistance versus heat input was created for the CPU cooling block and the South Bridge cooling block as seen in Figures 23 and 24.

2 4 6 8 10 12 14 16 180

1

2

3

4

5

6

7CPU Cooling Block

Conduction

Convection

Caloric

Total

Heater Power Input (W)

Ther

mal

Res

ista

nce

(°C/

W)

Figure 23: Thermal Resistance vs. Power for CPU Cooling Block

2 4 6 8 10 12 14 16 180

2

4

6

8

10

12

14

16

18

South Bridge Cooling Block

Conduction

Convection

Caloric

Total

Heater Power Input (W)

Ther

mal

Res

ista

nce

(°C/

W)

Figure 24: Thermal Resistance vs. Power for South Bridge Cooling Block

As a result of the low convection heat transfer coefficients, the thermal resistance values obtained for both cooling blocks were much higher than the calculated values in Table 4. Theoretically, the total thermal resistance should be independent of the heat input. However, since the convection heat transfer coefficient varied for each cooling block, this resulted in a fluctuating total thermal resistance.

Uncertainty bars are represented by vertical bars on each plot. The flow rate was assumed to be constant in the GUI program; however, during the testing, it was

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MEMS 1043 Senior Design Project, Spring 2012

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found that the flow rate was steadily rising with the power input, which caused some discrepancies in the reported results.

As seen in Figures 22, 23, and 24, there exists an inverse relationship between the power input and the uncertainty. As the power into each cooling block was increased, the error was decreased. This can be accounted for by the relationship between the temperature difference and the uncertainty. As the value for ΔT grew, the uncertainty for the thermocouples lessened.

Due to the inability to seal the North Bridge cooling block, no testing was done and therefore no data was collected.

6. CONCLUSIONS

The current method of air cooling data center server tray components is noisy and inefficient. The goal of this project was to design a mechanism that enables a retrofit for cooling of data center components. The design was successful in that it provided a solution for retrofitting current air cooled server boards with a universal water cooled mechanism. However, preliminary testing of the CPU cooling block and South Bridge cooling block yielded insufficient results. Although two of the three cooling blocks proved to be functional with water as the working fluid, the total thermal resistance for each cooling block was much higher than expected.

7. RECOMMENDATIONS FOR FUTURE WORK

Recommendations for future work include redesigning the cooling block to limit the amount of mating faces in the assembly. This can be accomplished by machining the headers and the microchannels as one part with a lid designed to fit snugly against the microchannel base. Less hardware would result in less machining time, and the lower amount of mating faces in the assembly will reduce the risk of leakage.

A second recommendation would be to improve the mounting mechanism to something more stable and consistent. The current mounting mechanism of

double-sided tape could be improved by using something more stable and more user-friendly for the installer.

The GUI could also be improved to accommodate for more than three cooling components. A link could also be made between the GUI results and the solid model. Therefore, the GUI would not only calculate the necessary heat sink dimensions, but it would also transfer those dimensions into iParts and iAssemblies to create an accurate solid model.

Finally, additional testing of the three cooling blocks is needed to better quantify their effectiveness in removing the required amount of heat.

8. GLOBAL IMPACT OF THE WORK

The heat generated from data centers is a critical concern both economically and environmentally. If components within a data center overheat, this can create an expensive problem as many of these components are no longer usable. As a result, cooling is a very important and necessary feature of data centers. Data center electricity consumption will continue to grow with the increasing demand for computing power. To put things into perspective, in 2003 there were 5.6 million servers used in US data centers, and in 2007 there were 11.8 million [13]. A graphical representation of how power and cooling costs are increasing as the volume of business data continues to increase is shown in Figure 25 [14].

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Figure 25: Trend in various costs involved in data centers with time

Servers and their accompanying cooling components now consume more power than all of the televisions in the US [13]. One large 50,000 square foot data center consumes around 5 MW of power which is equivalent to the power it would take to power 5,000 homes [13].

Currently, data centers servers use the equivalent of one full year’s output from five 1,000 MW power plants [13]. This is equivalent to powering five million houses [13]. If energy efficiency could improve just 20 percent, that would be a power saving equivalent to the output from an entire 1,000 megawatt power plant, which would be enough to power one million homes [13]. With the growing demand for computing power, these numbers will most definitely grow.

Water cooling is environmentally friendly because the water is recycled after each pass through the system. This would have environmental benefits in decreasing pollution and decreasing the diversion of water from sensitive ecosystems [14]. All in all, using water as a coolant in data centers can provide numerous environmental and economic benefits to society. With the growing need for computing power in the future, the need for more energy efficient designs and cooling mechanisms for data centers has never been greater.

REFERENCES

[1] Web Hosting Top. N.p., 21 May 2010. Web. 15 Apr. 2012. http://webhostingtop.org/blog/269-what-is-a-data-center.

[2] Graybar. N.p., 2012. Web. 15 Apr. 2012. <http://www.graybar.com/applications/data-centers/what-are-data-centers>.

[3] Greenemeier, Larry. Scientific American. N.p., 19 Sept. 2007. Web. 15 Apr. 2012<http://www.scientificamerican.com/article.cfm?id=for-data-centers-informat>.

[4] 42u Data Center Cooling. N.p., n.d. Web. 24 Jan. 2012. <http://www.42u.com/42u-rack-cooling.htm>.

[5] Koolance: Superior Liquid Cooling Solutions. N.p., n.d. Web. 15 Apr. 2012. <http://www.koolance.com/cooling101_introduction>.

[6] Data Center Knowledge. Industry Perspectives, July 2010. Web. 15 Apr. 2012. http://www.datacenterknowledge.com/archives/2010/07/02/the-advantages-of-liquid-cooling/.

[7] Incropera, Frank P. Fundamentals of Heat and Mass Transfer / Frank P. Incropera [et Al.]. Hoboken, NJ: John Wiley, 2007. Print.

[8] Intel® Core™2 Extreme Quad-Core Processor QX6000Δ Sequence and Intel® Core™2 Quad Processor Q6000Δ Sequence Datasheet. Document 315592-005. Rev. 5. August 2007. Intel. 4 April 2012. http://download.intel.com/design/processor/datashts/31559205.pdf.

[9] Intel® G45, G41, Q45, Q35 and Q965 Chipsets for Embedded Applications Datasheet. Thermal Design Guide. Document 415360. Revision 1.5. February 2009. Intel. 4 April 2012. http://download.intel.com/embedded/chipsets/designgd/415360.pdf.

[10] Intel® I/O Controller Hub 7 (ICH7) Datasheet. Thermal Design Guidelines. Document 307015-001. Initial Release. April 2005. Intel. 4 April 2012. http://www.intel.com/content/www/us/en/io/intel-io-controller-hub-7-guide.html.

[11] "Pump, PMP-300 [no Nozzles] - Water Cooling Systems, Pc Liquid Cooling Kit, Cpu, Video Card, Hard Drive." Koolance.com. Web. 02 Apr. 2012. <http://www.koolance.com/water-cooling/product_info.php?product_id=950>.

[12]Cooling Block, CHC-120-VO6. Koolance.com. Web. 16 Apr. 2012. http://www.koolance.com/water-cooling/product_info.php?product_id=393.

[13] Pingdom Blog. Pingdom AB, 25 July 2008. Web. 24 Jan. 2012. <http://royal.pingdom.com/2008/07/25/us-data-

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centers-consuming-as-much-power-as-5-million-houses/>.

[14] NEC: Powered by Innovation. N.p., 1994-2012. Web. 15 Apr. 2012. <http://www.nec.com/en/global/environment/featured/eco_center/index.html>.

ACKNOWLEDGEMENTS

Dr. Mark Kimber, Ph.D., Department of Mechanical Engineering

Dr. Anne Robertson, Ph.D., Department of Mechanical Engineering

Andrew Holmes, Swanson School of Engineering Machine Shop

Ricardo Rivera-Lopez, Graduate Student, Department of Mechanical Engineering

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