development of an experimental facility for …

1 Copyright © 2005 by ASME

Proceedings of ICMM2005 3rd International Conference on Microchannels and Minichannels

June 13-15, 2005, Toronto, Ontario, Canada

Paper No. ICMM2005-75070

DEVELOPMENT OF AN EXPERIMENTAL FACILITY FOR INVESTIGATING SINGLE-PHASE LIQUID FLOW IN MICROCHANNELS

Mark E. Steinke Thermal Analysis and Microfluidics Laboratory

Mechanical Engineering Department Kate Gleason College of Engineering

Rochester Institute of Technology, Rochester, NY USA

Satish G. Kandlikar Thermal Analysis and Microfluidics Laboratory

Mechanical Engineering Department Kate Gleason College of Engineering

Rochester Institute of Technology, Rochester, NY USA

J. H. Magerlein T.J. Watson Research Center

IBM Corporation Yorktown Heights, NY USA

Evan Colgan T.J. Watson Research Center

IBM Corporation Yorktown Heights, NY USA

Alan D. Raisanen IT Collaboratory,

Director of Technology Rochester Institute of Technology

Rochester, NY USA

ABSTRACT

An experimental facility is developed to investigate single-phase liquid heat transfer and pressure drop in a variety of microchannel geometries. The facility is capable of accurately measuring the fluid temperatures, heater surface temperatures, heat transfer rates, and the differential pressure in a test section.

A microchannel test section with a silicon substrate is used to demonstrate the capability of the experimental facility. A copper resistor is fabricated on the backside of the silicon to provide heat input. Several other small copper resistors are used with a four point measurement technique to acquire the heater temperature and calculate surface temperatures. A transparent Pyrex cover is bonded to the chip to form the microchannel flow passages. The details of the experimental facility are presented.

The experimental facility is intended to support the collection of fundamental data in microchannel flows. It has the capability of optical visualization using a traditional microscope to see dyes and particles. It also has the capability to perform micro-particle image velocimetry in the microchannels to detect the flow field occurring in the microchannel geometries. The experimental uncertainties have been carefully evaluated in selecting the equipment used in the experimental facility.

The thermohydraulic performance of microchannels will be studied as a function of channel geometry, heat flux and liquid flow rate. Some preliminary results for a test section, with a channel width of 100 micrometers, a depth of 200 micrometers, and a fin thickness of 40 micrometers are presented.

INTRODUCTION Cooling of high heat flux microprocessor chips using

single-phase liquid flow presents an attractive option as the heat fluxes exceed the present limit of air cooling (approximately 800 kW/m2). The majority of available literature and ongoing research programs are focusing on two-phase flow systems to perform the high heat flux removal. However, the single-phase cooling option offers considerable advantages over a two-phase flow system.

Kandlikar and Grande (2003, 2004) describe the fabrication techniques and possible trends in microchannels. They remark that the fluid flow in microchannels is further reaching than just efficient heat transfer. It could open up completely new fields not possible only a few years ago.

The upper limit of single-phase liquid-cooled systems is not clearly established. It is therefore necessary to conduct experiments in an effort to arrive at an acceptable cooling system performance and the ability to predict the performance. Establishing this boundary will also help to find ways to extend this limit with other advanced techniques, such as incorporating enhanced structures within microchannel flow passages.

The single-phase cooling system will have less system complexity, less variability resulting from flow boiling, and smaller pressure drops compared to a two-phase flow system. In addition, the heat rejection side of the system will also have less complexity because the need for a condensation process is absent.

Steinke and Kandlikar (2004a) identified single-phase heat transfer enhancement techniques for use in microchannels and minichannels. They speculate that this increase in heat transfer


performance from these techniques could place a single-phase liquid system in competition with a two-phase system. Thus, simplify the over all complexity and reliability. However, they point out that the added pressure drop resulting from the techniques should be carefully evaluated.

An experimental test facility that allows for the determination of important parameters influencing heat transfer and fluid flow in microchannels, as well as determining the associated experimental uncertainties, is desired. The facility should also have the ability to incorporate a variety of visualization techniques that are helpful in microfluidics.

LITERATURE REVIEW

There are over 150 papers that address the single-phase flow of liquids in microchannels. Bailey et al. (1995) provided a review of heat transfer and pressure drop in microgeometries. Their main focus was the pressure drop occurring in the microgeometry. Palm (2001) presented a review of single-phase and two-phase flow in microchannels. Some of the important parameters that govern the behavior were identified. Palm concluded that more work is needed to advance the understanding of fluid flow and heat transfer in microchannels. Sobhan and Garimella (2001) performed a comparative analysis of some of the existing works on microchannels. They cite the need for further study on all fronts to allow for the explanation of the discrepancies reported in literature. Recently, Morini (2004) performed a literature review and focused on the pressure drop and friction factor correlations available. They also point to the need for more experimental work to develop a greater fundamental understanding.

Table 1 shows selected works that have acquired data for heat transfer and pressure drop in microchannels. The most well known work is by Tuckerman and Pease (1981). This is often considered to be the pioneering work in microchannels. They used a pressure vessel to drive the flow through the microchannels. There are several parameters for each work reported in the table. The parameters include the hydraulic diameter, Reynolds number, mass flux, and heat flux. There are many methods for driving the flow through a microchannel.

The two most common methods used are a pressurized vessel and a pump. The pressure vessels are typically containers that contain the working fluid and can be pressurized using an inert gas, usually nitrogen. The other method is to use a pump. The style of pump used in the test loops varies. However, the most desirable feature for all of them is a steady flow rate. The most common pump used for steady flow is a gear pump. The gear pump is a positive displacement pump that delivers a constant volume of fluid at a steady rate, despite changes in upstream pressure. By contrast, a centrifugal pump’s flow rate depends upon the upstream pressure and therefore is less desirable.

A wide range of Reynolds numbers and heat fluxes are encountered in single-phase microchannel flow. A test facility that produces a wide range of flow rates would be desirable. In addition, an appropriate power supply for delivering high current would be required to generate large heat fluxes.

OBJECTIVES OF PRESENT WORK

The main objective of the present work is to develop an experimental facility capable of providing accurate data for single-phase liquid heat transfer and pressure drop in

microchannels. The experimental system should provide accurate measurements to generate microchannel heat transfer and pressure drop data. Additionally, the test section should provide the ability to perform microchannel flow visualization. The development of the test section for a variety of fluid flow, heat transfer, and geometry arrangements will be performed.

EXPERIMENTAL TEST FACILITY

The experimental test facility contains all of the supporting equipment that supplies a metered working fluid to the test section and the measurement of heat transfer and pressure drop. Figure 1 shows the design schematic for the experimental system.

General Description The flow loop can be operated in an open-system or a

closed-system mode. The type of experiment conducted determines which is better. The system represented in Fig. 1 is an open flow loop. The flow loop begins with a polycarbonate supply tank.

A positive displacement, micro-gear pump drives the flow through the loop. The gear pump is a Micropump brand pump that delivers 1 to 300 mL/min of flow at a maximum pressure of 4.8 bars. This style of pump is chosen to give a constant flow rate at the test section regardless of the pressure drop occurring in it. This is because the pressure drop can change significantly due to different microchannel geometries being tested. The micropump is chosen because of its small footprint, accurate flow rates, and ability to dispense small volumes.

Next, a membrane filter follows the pump. The filter is housed in a stainless steel casing that can maintain high pressures and temperatures. The membrane used in the filter housing a standard 47 mm diameter replaceable nylon filter. There are a variety of pore sizes readily available, ranging from 0.2 µm to 25 µm in diameter. Typically, a 1.0 µm pore size is used in the filter.

The next item in the flow loop is a flow meter bank that contains flow meters in a parallel arrangement with different flow rate ranges. Each flow meter has a different range to improve the accuracy of the flow measurement. The advertised accuracy of the rotameter is 2% of full scale. However, the measured accuracy is actually 1.25% for the bottom range and less than 0.5% for the mid an upper flow ranges. The flow meters are a rotameter type. For the flow range of the pump, the rotameter gives reasonable accuracy for a moderate cost. For even lower flow rates, a coriolis flow meter is utilized. The flow meters have an overall flow range of 0.01 mL/min to 4,000 mL/min. The individual flow meter ranges usually increase by an order of magnitude. In other words, one flow meter would have a 3 to 75 mL/min flow range and the next available flow meter would have a 30 to 300 mL/min flow range.

A miniature shell and tube heat exchanger and a recirculation bath are used to control the fluid temperature entering the test section. The tube diameters are 3 mm and the entire heat exchanger is constructed from stainless steel. The inlet heater is designed to heat the working fluid up to 90 °C. A typical load for the heat exchanger is 100 W.

The experimental system is instrumented to measure temperatures, pressures and flow rates. Integrated resistors on the test device are used to measure local temperatures.


Table 1: Selected Works for Single-Phase, Liquid Flow in Microchannel Passages. Author Year Fluid Shape Dh ( µµµµm ) Re G ( kg / m2s ) q” ( W / cm2 ) Method for Driving Flow

Tuckerman & Pease 1981 water rectangular 92 - 96 291 - 638 2615 - 5678 187 - 790 pressure Missaggia et al. 1989 water rectangular 160 2350 12463 100 pressure Riddle et al. 1991 water rectangular 86 - 96 96 - 982 950 - 9545 100 - 2500 pressure

Gui & Scaringe 1995 water trapezoid 338 - 388 834 - 9955 2110 - 21937 12 - 112 pressure Peng et al. 1995 methanol rectangular 311 - 646 1530 - 13455 174 - 1179 0.2 - 10 pump

Peng & Peterson 1996 water rectangular 133 - 200 136 - 794 579 - 4448 5 - 45 pump Vidmar & Barker 1998 water circular 131 2452 - 7194 16005 - 46954 506 - 2737 pump

Ravigururajan & Drost 1999 R124 rectangular 425 135 - 1279 3.2 - 30.6 0.8 - 13 pump Lee et al. 2002 water rectangular 85 119 - 989 1196 - 9944 35 pump Qu & Mudawar 2002 water rectangular 349 137 - 1670 335 - 4093 100 - 200 pump

Figure 1: Experimental Flow Circuit. Open system mode shown.


They are formed by depositing copper in a very thin line, more details given in a later section. The remaining system temperatures are read using thermocouples; do to the inexpensive yet accurate measurement capabilities. E type thermocouples are chosen for their accuracy and their temperature range.

Pressure is measured using a differential pressure transducer from Omega. The pressure transducer range, from 0.02 to 6.8 bars, is selected to give the highest level of accuracy.

Flow Visualization Studying the fluid flow in microchannels can be achieved

in party by visualizing the flow field. There are two main visualization techniques that can be employed with the experimental test facility, . The experimental facility is capable of performing both flow visualization techniques by utilizing an optically clear microchannel wall.

A standard optical microscope has been included in the experimental facility. Long distance objectives have been incorporated to increase the working distance. A high-speed CCD camera is attached to the microscope to capture images of the flow.

A micro-particle image velocimetry (µPIV) system is utilized with a visualization test section to investigate the velocity field in the microchannel. A high frame rate camera is utilized with this system to visualize higher Reynolds number flows. The velocity vectors in the microchannel can be determined using this technique. The flow distribution in the microchannels, the flow in the header region, and the entrance length in the microchannel are some of the aspects that can be studied with this experimental facility.

TEST SECTION DESIGN The design of the test section used to conduct the

experiments in the present work is discussed in this section. The test section is located in the test flow circuit and provides support for the test device. The test fixture is part of the test section and provides a platform to contain the electrical and fluidic connections.

Test Device The test device contains the microchannels, the heater and

the temperature sensors. The geometries of the microchannels that have been fabricated are presented in Table 2. Figure 2 shows the location of the geometry variables used in Table 2. The parameters are the microchannel width is a, fin thickness separating the microchannels is s, depth of the microchannel is b, the total flow length of the microchannel is L, and the number of microchannels is n.

There are twelve different microchannel configurations selected for the study. The channels are 10 mm in total length and are 8 mm wide. The number of channels varies to completely fill the 8 mm width. The resulting number of channels varies between 18 and 100. The test section is fabricated using two different layers bonded using an adhesive. The first layer is a silicon substrate that contains the microchannels, the heater, and the temperature sensors. The

second layer is made of Pyrex 7740 and contains the fluidic connections.

Figure 2: Geometric Parameters for the Microchannels

Table 2: Channel Geometries of the Test Device.

a s pitch n L Number µµµµm µµµµm µµµµm µµµµm

G1-001 40 40 80 100 10000 G1-002 40 100 140 57 10000 G1-003 70 40 110 73 10000 G1-004 70 100 170 47 10000 G1-005 100 40 140 57 10000 G1-006 100 100 200 40 10000 G1-007 200 40 240 33 10000 G1-008 200 100 300 27 10000 G1-009 200 200 400 20 10000 G1-010 250 40 290 28 10000 G1-011 250 100 350 23 10000 G1-012 250 200 450 18 10000

The microchannels are fabricated using deep reactive ion

etching (DRIE). A very high aspect ratio with straight sidewalls can be achieved using this method. Figure 3(a) shows an example of the microchannels etched into the silicon. There are also inlet and outlet headers formed in the silicon. The back side of the header is rounded to help direct the flow and minimize stagnation regions. Finally, the entrance to each channel is rounded to reduce the pressure drop due to the entrance. The radius is one half the fin thickness, s.

An insulating layer of silicon nitride is deposited on the backside of the device. Then, a layer of copper is deposited and patterned on the backside to form the heater and sense resistors. Finally, a nitride cap is deposited to help minimize corrosion of the copper. The copper is patterned into a long serpentine resistor for the heater. The total length is 82 mm with a resistance of 9.7 ohms. The current is controlled to allow for the introduction of a targeted heat flux of 200 W/cm2. The current supplied to the heater can be as large as 5 amps.

The six temperature sensor resistors are kept very small to increase the spatial resolution. A constant current is supplied to each sense resistor. The resistance of the sensor is a function of temperature allowing for the temperature to be determined.

b

a s


Each sense resistor requires four connections, two for supply current and two for potential reading. The sensor will return a line averaged temperature along the length of the resistor. Figure 3(b) shows the backside of the device with the heater, sensors, and the bonding pads.

Figure 3: Microchannels in Silicon Substrate. (a) microchannels etched into front side of silicon, (b) electrical layout of copper back side.

To discuss the location of features on the test device, let

the origin be located at the center of the device. There are six temperature sensors located on the backside of the device. The locations of the sensors are at -4.750 mm, -3.375 mm, -1.875 mm, 0.375 mm, 3.375 mm, and 4.750 mm. The first and last sensor is placed beneath the inlet and outlet plenums. The remaining four sensors are interlaced within the serpentine of the heater.

The silicon substrate contains the microchannels with three walls. A Pyrex cover piece is placed on top of the silicon substrate to form the complete microchannel test device. The cover piece has the same overall dimensions of the silicon substrate. The thickness of the cover plate is 750 µm. The cover piece is laser drilled with two holes to match with the location of the inlet and outlet headers. The diameter of the plenum hole is 1.5 mm.

Adhesive Bonding Adhesive bonding is the method chosen to attach the Pyrex

cover to the silicon substrate. This method provides wide latitude with the surface preparation as well as surface roughness. However, both parameters are still important in the success of the bond interface. The adhesive used is M-Bond 610. It is a two-component, solvent thinned, epoxy-phenolic adhesive. The operational temperature range is from -269 ºC to 370 ºC for a short period and the long term temperature range is -269 ºC to 260 ºC. The approximate composition is 60% Tetrahydrofuran, 30% Bisphenol F epoxy resin, 10% Methyl Ethyl Ketone.

Test Fixture The test fixture is the part of the test section that provides

the support structure for the microchannel test section. The fixture houses the electrical and the fluidic connections. The material used for the test fixture is black delrin. Delrin is an

acetal resin from DuPont. It is a lightweight plastic that is very stable and does not experience any significant creep.

The test fixture is comprised of the following major parts; the pogo probe block, the main block, the bottom retaining plate, the device retaining plate, and the mounting plate. The pogo probes are inserted into the pogo probe block. The pogo probes are a high current version that can handle up to 10 amps. Figure 4 shows the pogo probes inserted into their respective block. Their pattern there matches the contact pad pattern in Fig. 3(b). The pogo probe block is inserted into the main block. Figure 5 shows the test section assembly.

Figure 4: Pogo Probe Block with Probes Inserted.

Figure 5: Test Fixture Assembly.

Figure 6: Device Retaining Plate.

A critical part in the test fixture assembly is the device retaining plate shown in Figure 6. This part provides a clamping force on the microchannel test section and the fluidic connections. The piece is made of an optically clear polycarbonate material. The fluid enters the piece and moves into an inlet plenum. The fluid then enters the test device and leaves through the exit plenum. The device is sealed using two double sealing O-rings.

Two miniature thermocouples are fabricated and placed in the plenums. The thermocouple bead is 0.5 mm in order to

Sensor Resistor

Power Resistor

(a) (b)

Inlet

Outlet

Pressure

Thermocouple

Injection Port

Test Device

Pogo Probes Pogo Probe

Block


minimize the effect on the flow. They are also coated with an epoxy to seal them and prevent corrosion.

Pressure ports are located in each of the plenums to get the pressure drop across the test device. The plenums themselves are carefully designed to allow for sufficient volume to reduce the fluid flow velocity. This ensures that the measured pressure drop is indeed measuring the change in static pressure and has no dynamic pressure component included with the measurement. Experiments are conducted to ensure that true static pressure is measured.

EXPERIMENTAL UNCERTIANTY The experimental uncertainties can become quite large for

a microchannel heat exchanger because the magnitudes of some of the measurements are very small. In addition, the propagation of errors in the system can become troublesome.

The physical size of the system being measured presents a challenge. It is very difficult to fabricate a thermocouple small enough to have any significant size resolution and attach it to the chip at a desired location. A better approach is to use a resistance element that has a known temperature dependence.

The magnitudes of the measurements represent a problem as well. The heat transfer in microchannels is very efficient. Therefore, the changes in temperature or the temperature difference can be very small. The ∆T can be on the order of only a few degrees. A typical experimental uncertainty value for temperature is ± 0.5 °C on a two point calibration. An experimental uncertainty of ± 0.04 °C can be achieved by using the data collection procedure presented later.

Fortunately, several of the standards for experimental uncertainties still apply. The two best standards for determining experimental uncertainties are ASME PTC 19.1 (1998) and NIST Technical Note 1297 (1994). There are many similarities between these standards and others. The primary standard discussed here will be the ASME standard. In general, the total uncertainty is comprised of two parts; bias error and precision error given by:

22

22 ��

�

��

�+�

�

��

�=N

BU

σ (1)

where U is the total uncertainty, B is the bias error, σ is the standard deviation, N is the number of samples. The bias error is a measure of the systematic error and the precision error is a measure of the random errors in the system.

There are several rules to follow when propagating errors from measured variable to calculated values. In general, Eq. (2) gives the uncertainty of a calculated parameter.

�=

��

��

�

∂∂=

n

ii

ip u

pU

1

2

σσ (2)

where p is the calculated parameter. The uncertainty in any parameter is the sum of the uncertainties of the components used to calculate that parameter.

A measurement technique is utilized to reduce the experimental uncertainties. In a data burst DAQ (data acquisition) mode, several hundred samples of the same quantity, like temperature, are measured at a high frequency. The mean and standard deviation of the sample set is determined and used to report the parameter being measured.

The systematic errors are ones that result from biases in the experimental setup and can be minimized by using calibrations. These errors are typically repeatable and can be eliminated. The uncertainties for the major parameters are presented in Table 3 as well as the parameter mean values. Table 3. Uncertainties for Mean of Experimental Data.

Parameter Mean Uncertainty (%) Q ( mL/min ) 40 0.5 G ( kg/m2s ) 741 5.1 Re 212 6.1 q” ( W/cm2 ) 29 3.3 T ( °C ) 40 0.2 θ ( °C cm2 / W ) 0.4 6.0 ∆p ( kPa ) 2.9 3.0 f 0.4 6.5

Steinke and Kandlikar (2005a, 2005b) present a more

detailed discussion of the experimental uncertainties occurring in microchannel pressure drop and heat transfer. The uncertainty for calculated parameters is derived in terms of the measured parameters. This allows for the contribution from each measured parameter to be determined.

The following equation is recommended for use in microchannel fluid flow. The uncertainty in the fRe product is shown in Eq. (3), Steinke and Kandlikar (2005a).

2/1

222

222

222

Re

225

53

2

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=

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baU

baU

bU

aU

Q

U

LU

p

UUU

f

U

bab

aQL

p

f

µρµρ

(3)

where Q is the volumetric flow rate, ρ is the density, µ is the viscosity, ∆p is the pressure drop, a is the microchannel width, b is the microchannel depth, and L is the microchannel length.

It can be seen that the most dominant terms in the fRe uncertainty are the measurements of microchannel width and height. Therefore, even with the most careful pressure drop measurements, there is still going to be a large uncertainty due to the measurement errors of the microchannel dimensions and the flow rate. SYSTEM CALIBRATION AND EXPERIMENTAL PROCEDURE

The approach used for the system calibration and the experimental procedure are described in this section. The three main topics include the system calibration, the experimental procedure, and the preparation of the working fluid.

Experimental Calibration The experimental system requires calibration and some

specific data acquisition techniques to generate the most accurate data possible for a given system configuration. The


thermocouples, temperature sensors, pressure transducers, flow meters, and the pump all require calibration and will be discussed in detail.

The thermocouples are calibrated using a heated block and an ice bath. The ice bath gives a reference temperature of 0 °C. A calibrated hot block from Omega is used to give the other known temperature points. The range of operation for the thermocouple calibrator is 40 °C to 480 °C. The operating range for the experimental system is from 0 °C to 200 °C. The thermocouples are calibrated using twenty points chosen within the operational range of the system. A linear curve fit is assumed and applied to the collected points to determine the calibration equation. Finally, the calibration process is repeated using the equation that was determined. The temperature measurements are verified in this manner and excellent agreement is found using this method. All of the temperatures are measured to within ± 0.03 °C.

The temperature sensors on the test device are calibrated by applying a known temperature to the device. Recall that these sensors are actually copper resistors and the resistance is a function of temperature. The efficient heat transfer that occurs in the microchannels will be used to our advantage. Temperature controlled water is passed through the test device at a moderate flow rate. A sufficient amount of time is allowed to let the test device reach the fluid temperature, within ± 0.05 °C. Several external thermocouples are used to ensure that an accurate temperature and steady state temperature is achieved. Finally, the potential is measured for that applied temperature. A linear curve fit is assumed and applied to the data. Once again, the calibration is repeated with the calibration equation to verify the accuracy of the fit. The test devices are calibrated prior to experimentation and calibrated just after experimentation to ensure good linearity.

The pressure transducers are calibrated using known pressures and the measured response of the transducer. A pressure calibrator from Omega is used to apply a known value of pressure. The range of the pressure calibrator is -100 kPa to 200 kPa. The high side pressure port on the differential pressure transducer is exposed to the known values of pressure. Over twenty points are taken with in the range of the specific pressure transducer. A linear curve fit is assumed and used to generate the calibration equation.

The flow meters and pump are calibrated at the same time. The pump is set for a specific flow rate. The fluid is collected in a flask and timed. Then, the mass of the fluid is measured on a calibrated balance for that period of time. Several points are taken over the range of operation of the pump and for each of the flow meter ranges. A polynomial curve fit is applied to get the calibration equation for the pump and flow meters.

Experimental Procedure A data acquisition (DAQ) system monitors several

thermocouples, pressure transducers, and sensing resistors. In addition, the data acquisition system controls the pump used to drive flow in the system. The DAQ is based upon the signal conditioning SCXI system from National Instruments. This system conditions all incoming signals and gives the ability to have high channel counts.

The DAQ system used in the experimental system is capable of sampling at a rate of 100,000 samples per second. The present work is focused on steady state performance.

However, the fast sampling will be used to increase the accuracy. The procedure for data collection begins with the sampling rate. The channels are sampled at a rate of 1.0 kHz. The data collected is in the form of a waveform. The channel information contains both amplitude and frequency information. A thousand samples for each channel are collected. The data are analyzed to determine the mean and the standard deviation, σ, of the data set. Then, the mean and twice the standard deviation, 2σ, are recorded. The calibration for each measurement is applied offline of the data collection.

A test device is loaded in the test fixture. First, the top retaining plate is used to create the fluidic connections. Then, the electrical connections are made via the pogo probes. The flow loop is brought to the desired inlet temperature by using a test section bypass. Next, the flow enters the test device and the input power is applied. After all of the data has been collected at the specified heat flux, the input power is changed while the flow rate remains constant. Therefore, the data is collected by varying the heat flux for a fixed mass flow rate.

The system reaches steady state before the data collection begins. The measured values are considered to be at steady state if the values do not fluctuate more than the uncertainty values. Typically, the data collection begins 5 minutes after the system reaches steady state. The data is a time averaged mean of over 5,000 points.

Working Fluid Preparation The working fluid for the present work is distilled, de-

ionized and degassed water. The present work is focused on generating fundamental single-phase flow data for a variety of microchannel geometries. It is therefore important to eliminate all of the interfering variables. The amount of dissolved gas in the water will affect the heat transfer performance at very large ∆Ts. The non-condensables can outgas during the experiments and cause heat transfer enhancement.

The amount of dissolved gas in the water needs to be precisely controlled to eliminate the heat transfer changes resulting from out-gassing of dissolved gases. Steinke and Kandlikar (2004b) conducted an experimental investigation concerning the control of dissolved gases. They demonstrated that the effects due to the out-gassing of dissolved gases can be eliminated if the water is treated to reduce the dissolved oxygen content to 5.4 parts per million (ppm) at 25 °C. To be conservative, the dissolved oxygen level used in these experiments is maintained at 3.2 ppm. The water is checked randomly throughout the course of the experiments.

A brief outline of the degassing procedure is given. A pressure vessel is filled with de-ionized, distilled water. The vessel is heated to generate steam. A deadweight corresponding to 1 atm of pressure is applied. When the vessel reaches the desired pressure, the deadweight is removed. A vigorous boil results from the sudden change in pressure. As a result, the dissolved gases are released from the water.

EXPERIMENTAL RESULTS Some experimental results obtained from the Single-Phase

Experimental Facility will be presented to demonstrate the parameters that can be investigated with the system. The results shown are for microchannel with a width of 250 µm, a


depth of 200 µm, and a fin thickness of 200 µm. The heat transfer performance will be characterized in terms of the heat flux, temperature difference and unit thermal resistance of the microchannel test section. The pressure drop performance will be characterized in terms of the total pressure drop and apparent friction factor. Table 4 presents the range of parameters investigated for this microchannel geometry.

Table 4. Range of Experimental Data

Parameter Min Max Q ( mL/min ) 12 68 G ( kg/m2s ) 219 1262 Re 61 364 q” ( kW/m2 ) 34.9 548.9 T ( °C ) 24.8 54.6 θ ( °C m2 / W ) 4100 16900 ∆p ( kPa {psi} ) 0.7 {0.10} 4.8 {0.70} f 0.11 0.8

The following results do not reflect the maximum

performance for a silicon microchannel. The smaller pitches and microchannel widths will provide a much higher heat transfer performance. There are many papers that present better performance. For example, Colgan et al. (2005) investigated the performance of silicon microchannels with some enhancement features and dissipated more than 2,750 kW/m2. The test section used has the coarsest pitch and the widest microchannels. The intention is to demonstrate some of the parameters that can be determined from the experimental facility and the associated uncertainty.

Figure 7 shows the difference between the average temperature of the resistive sensor on the back side of the test section and the inlet water temperature versus heat flux. The data are represented by points and the uncertainty is shown as error bars. As expected, the temperature difference increases linearly with heat flux and decreases with increasing mass flux.

0

5

10

15

20

25

30

0.0 0.2 0.4 0.6 0.8

Heat Flux, q" MW/m2

∆∆ ∆∆T a

vg,(T

j-Tin

)

°° °°C

G = 219G = 342G = 513G = 726G = 977G = 1262

Figure 7: Temperature Difference vs. Heat Flux. G01-012; a = 250 µµµµm, b = 200 µµµµm; G = 219, 342, 513, 726, 977, 1262 kg/m2s; Re = 61, 91, 132, 187, 250, 324.

The slope of the line in Fig. 7 is the unit thermal resistance of the test section. Figure 8 shows the unit thermal resistance for the different mass fluxes. The unit thermal resistance is a common parameter in the electronics cooling industry. It is meant to give a direct comparison of performance for different cooling techniques and a lower value is desired. The unit thermal resistance can be calculated using Eq. (4).

( )"" q

TT

q

T ijj −=

∆=θ (4)

0

20

40

60

80

100

120

140

160

180

200

0.00 0.10 0.20 0.30 0.40 0.50 0.60

Heat Flux, q" MW/m2 θ

° θ

° θ

° θ

°C

m2

/ MW

G = 219G = 342G = 513G = 726G = 977G = 1262

Figure 8: Unit Thermal Resistance vs. Heat Flux. G01-012; a = 250 µµµµm, b = 200 µµµµm; G = 219, 342, 513, 726, 977, 1262 kg/m2s; Re = 61, 91, 132, 187, 250, 324.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400Reynolds Number, Re

Hea

t Flu

x, q

" M

W/m

2

G = 219G = 342G = 513G = 726G = 977G = 1262

Figure 9: Heat Flux vs. Reynolds Number. G01-012; a = 250 µµµµm, b = 200 µµµµm; G = 219, 342, 513, 726, 977, 1262 kg/m2s; Re = 61, 91, 132, 187, 250, 324.

For a mass flux of 1262 kg/m2s and a ∆Tavg of 23.5 °C, the

unit thermal resistance is 0.40 °C cm2/W and the thermal resistance would be 0.15 °C/W. A typical air-cooled heat sink


thermal resistance alone would be approximately 1 °C/W. This does not take into account the thermal resistances of the silicon, thermal interface material, contact resistances, and the heat spreader. Therefore, this technique is already an improvement over air cooled heat sinks. A typical two-phase system would have a thermal resistance would be approximately 0.10 °C/W. Once again, the present work does not attempt to get maximum performance from the microchannels. However, the thermal resistances of the single-phase and the two-phase system can be on the same order of magnitude.

The heat flux versus Reynolds number is shown in Figure 9. The junction temperatures were limited to 55 °C to minimize corrosion of the copper resistors. The heat flux was adjusted for each mass flux to obtain the data as long as all of the junction temperatures remained less than 50 °C. As a result, the number of data points collected for each mass flux is not the same. As expected, the higher mass fluxes can support higher heat fluxes. The largest heat flux dissipated is 54.9 W/cm2. An increase in the maximum heat flux is demonstrated with an increase in Reynolds number.

The pressure drop in the test section is measured in the inlet and outlet plenums. The entrance and exit losses are determined and subtracted from the measurement to determine only the pressure drop occurring in the microchannel. Figure 10 shows the resulting pressure drop in the microchannels. As expected, the pressure drop increases with the Reynolds number. The small variation seen in the Reynolds number is due to the water properties being calculated using the mean water temperature. The mean temperature will increase as the outlet temperature rises with an increase in heat flux.

Figure 10: Pressure Drop vs. Reynolds Number. G01-012; a = 250 µµµµm, b = 200 µµµµm; G = 219, 342, 513, 726, 977, 1262 kg/m2s; Re = 61, 91, 132, 187, 250, 324.

The apparent friction factor can now be calculated from the

pressure drop. The inlet and exit losses are removed from the measured pressure drop. The conventional expansion and constriction area correlations were used. The apparent friction factor is calculated using Eq. (5), Kakaç et al. (1987).

( )2

Re2

h

app

D

LVfp

µ=∆ (5)

where fapp is the apparent friction factor, Re is the Reynolds

number, µ is the viscosity, V is the mean velocity, L is the microchannel length, and Dh is the hydraulic diameter. The theoretical value for fRe can be used to normalize the friction factor data to give the non-dimensional fRe ratio, C*. The non-dimensional fRe ratio versus Reynolds number is shown in Figure 11. This is a more appropriate method to compare theoretical prediction of the apparent friction factor, Steinke and Kandlikar (2005a). A C* value of 1.0 would be a perfect match between experimental data and theoretical value.

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0 100 200 300 400Reynolds Number, Re

Cap

p*

Capp* = 1.0G = 219G = 342G = 513G = 726G = 977G = 1262

Figure 11: Capp

* vs. Reynolds Number. G01-012; a = 250 µµµµm, b = 200 µµµµm; G = 219, 342, 513, 726, 977, 1262 kg/m2s; Re = 61, 91, 132, 187, 250, 324.

There seems to be good general agreement with the friction

factor data. The Capp* ratio takes into account the developing

flows in the microchannels. The experimental uncertainties are shown as error bars in the figure. There are a few cases that still have a 5% difference that the theoretical value and is an encouraging result. A more detailed discussion on the friction factors and the procedure for correcting for the entrance and exit losses and the developing flow region is presented by Steinke and Kandlikar (2005a).

CONCLUDING REMARKS A single-phase experimental test facility has been designed

and fabricated. The system is capable of delivering a working fluid to a test section with a variety of inlet temperatures and flow rates. A wide range of mass fluxes and heat fluxes can be applied. The equipment has been carefully selected to give the maximum amount of accuracy in the flow ranges.

The experimental facility also has the ability to perform flow visualization using a traditional optical microscope and a micro-particle image velocity system.

The system can deliver fluid from a flow range of 1 mL/min to 4,000 mL/min. It can provide an input power to the test section of 1.5 kW.

The experimental uncertainties have been carefully controlled to produce experimental data with very low uncertainty. Steinke and Kandlikar (2005a, 2005b) present a


more detailed discussion of the experimental uncertainties occurring in microchannel pressure drop and heat transfer.

The experimental results shown represent some of the parameters that can be determined using the single-phase test facility. The facility can now be used to determine the performance of single-phase microchannels.

The unit thermal resistance shown in Fig. 7 seems to be dependent upon the Reynolds number for low mass fluxes and independent of the heat flux at the higher Reynolds numbers. The diabatic friction factors are in agreement with conventional laminar Poiseuille flow theory after accounting for the entrance and exit losses and developing region effects.

NOMENCLATURE a channel width, m b channel height, m B systematic error Dh hydraulic diameter, m f Fanning friction factor G mass flux, kg m-2 s-1 L channel length, m n number of channels N number of measurements in sample p pressure, Pa P power, W q heat transfer rate, W q” heat flux, W m-2 Q volumetric flow rate, L s-1

R thermal resistance, °C W-1 Re Reynolds number s fin thickness, m T temperature, °C

V mean velocity, m s-1 ui uncertainty of parameter i U uncertainty

Greek

∆ difference η fin efficiency µ viscosity, N s m-2

ρ density, kg m-3 σ standard deviation θ unit thermal resistance °C m2 W-1

Subscripts app apparent avg average f fluid h heater i inlet o outlet s surface AKNOWLEDGEMENTS

The fabrication of the test devices was performed at IBM T.J. Watson Research Center. The device bonding and experimental investigation was performed at Rochester Institute

of Technology in the Thermal Analysis and Microfluidics Laboratory. In addition, the support from the IT Collaboratory for device bonding is appreciated. REFERENCES

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