hot wire method

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Computerized, Transient Hot-Wire Computerized, Transient Hot-Wire Thermal Conductivity (HWTC) Thermal Conductivity (HWTC)

Apparatus Apparatus For NanofluidsFor Nanofluids

The 6th WSEAS International Conference on HEAT and MASS TRANSFER The 6th WSEAS International Conference on HEAT and MASS TRANSFER ((WSEAS - HMT'09WSEAS - HMT'09))

Ningbo, China, January 10-12, 2009Ningbo, China, January 10-12, 2009

M. Kostic & Kalyan C. SimhamM. Kostic & Kalyan C. SimhamDepartment of Mechanical EngineeringDepartment of Mechanical EngineeringNORTHERN ILLINOIS UNIVERSITYNORTHERN ILLINOIS UNIVERSITY

www.kostic.niu.edu

Overview INTRODUCTION OBJECTIVE THEORY OF HOT-WIRE METHOD PRACTICAL APPLICATION OF HOT-WIRE

METHOD DESIGN OF HOT-WIRE CELL INSTRUMENTATION DATA ACQUISTION CALIBRATION UNCERTAINTY ANALYSIS RESULTS CONCULSIONS RECOMMENDATIONS

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INTRODUCTIONNanofluids are colloidal suspensions of

nanoparticles, nanofibers, nanocomposites in common fluids

They are found to have enhanced thermal properties, especially thermal conductivity

Thermal conductivity values of nanofluids may be substantially higher than related prediction by classical theories

No-well established data or prediction formula suitable to all nanofluids

Experimental thermal conductivity measurement of nanofluids is critical

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Table 1: Summary of landmark development in nanofluids

* (reprinted with permission; reference listed within this table are with respect to (Manna et al 2005))

*

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Nanofluid Preparation Methods

• One Step (Direct Evaporation and Condensation) Method

Fig1: Improved new-design for the one-step, direct evaporation-condensation nanofluid production

apparatus, (Kostic 2006)

• Two Step Method or Kool-aid Method

• Chemical Method

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Thermal Conductivity• Material Property• Determines ability to conduct heat• Important for thermal Management

Classification of Thermal Conductivity Measurement

Techniques for FluidsSteady State

Methods

Non-Steady State Methods

Horizontal Flat Plate MethodVertical Coaxial Cylinder MethodSteady State Hot-Wire Method

Line Source (Hot-Wire) MethodCylindrical Source MethodSpherical Source MethodPlane Source Method

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Transient Hot-Wire Method for Fluids• Fast and Accurate

Advantages:

• Minimize (or even avoid) Convection

• Minimum Conduction and Radiation losses

Classification of Hot-Wire Methods• Standard Cross Wire Method• Single Wire, Resistance

Method• Potential Lead Wire Method• Parallel Wire Method

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OBJECTIVE

Design

Device to Suspend Hot-Wire Reduce Nanofluid Sample

Size Minimize End Errors Uniform Tension on Hot-Wire Separate Wires for Power and

Voltage Monitor Temperature Mechanism to Calibrate

Hotwire Tension Flexibility for Cleaning and

Handling

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OBJECTIVE

Electrical CircuitFlexible ConnectionsInstrumenta

tion

Data Acquisition

Optimize to Reduce Noise

Develop ProgramCalibratio

nStandard Fluids

Uncertainty Analysis

Thermal Conductivity

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Principle of Hot-Wire Method• An infinitely long and thin, ideal continuous line

source dissipating heat into an infinite medium, with constant heat generation

General Fourier’s Equation

Boundary Conditions

Ideal case:Line source has an infinite thermal conductivity and zero heat capacity

rTr

rrtT

f

11

0t 0rand

0t rand f

r kq

rTr

2lim

0

0,lim

trTr

Where T is the final temperature, T0 is the initial temperature,r is the radial distance andt is the timeq is heat flux is thermal diffusivitykf is Thermal Conductivity

0TTT

f

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• The temperature change at a radial distance r, from the heat source is conforms to a simple formula by applying boundary conditions

• At any fixed radial distance, in two instances in time the equation, the temperature change can be represented as

trEi

kq

TtrTtrTff 44

,,2

0

.........

!22

4

!11

44ln

4),(

222

20

tr

tr

rt

kqTtrTT fff

f

1

212 ln

4 tt

kqTT

f

series expansion of the exponential integration

=0.5772 is the Euler’s constant Where,

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• A plot of temperature against the natural logarithm of time results in a straight line, the slope being propositional to kf

Tdtdq

k f

)ln(4

Practical application of hot-wire method• The ideal case of continuous line is approximated

with a finite wire embedded in a finite medium

Figure 2.1 Typical plot of temperature change against time for hot-wire experiment (Johns et al 1988)

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Nanofluids Thermal Conductivity Methods By

Other AuthorsAuthor, Year Nanofluid Thermal Conductivity

Measurement MethodWang et al (1999) Horizontal flat plate method

Lee et al (1999), Yu et al (2003) and Vadasz (2006)

Vertical, single wire, hot-wire method

Assael et al (2004) Two wires, hot-wire method

Manna et al (2005) Thermal comparator

Ma (2006) Horizontal, single wire, hot-wire method

Simham (2008) Vertical, single wire, hot-wire method

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Hot-wire Method for Nanofluid• Nanofluids are electrically conducting

fluids• Availability of nanofluids• Thermal expansion of wire • Cleaning of the cellHot-Wire Method for Electrically Conducting Fluids

Problems identified by Nagasaka and Nagashima (1981) • Possible current flow through the liquid, resulting in ambiguous measurement of heat generated in the wire,

• Polarization of the wire surface, • Distortion of small voltage signal due to

combination of electrical system with metallic cell through the liquid.

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ooo

f

CtBt

AtkqT ln1ln

4Where,

...2

ln24

ln 2

w

f

w

o

i

f

o

fo k

krr

kk

rA

i

i

f

fo

w

w

i

iw

fo

kkr

kkr

kB 22

21

w

o

w

w

i

i

i

w

if

o

wiiww

ifwo r

rkkkrr

kkkr

C ln112

24118

222

222 4

ln2

1

o

f

i

i

f

fo

w

w

i

iw

f rkk

rkk

rk

oo CtBt ln1 is due to the presence of the insulation layer on the wire

oA T shifts (i.e. offsets) the plot of

against ln (t), without changing the slope

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Insulation Coating Influence on Thermal Conductivity

Measurement

• The results of numerical simulation and experimental test show that, for most of the engineering applications, the relative measurement error of the thermal conductivity caused by the insulation coating are very small if the slopes of the temperature rise – logarithmic time diagram are calculated for large time values

• No correction to insulation coating is necessary even for the conditions that the insulation coating thickness is comparable to the wire radius, and that the thermal conductivity of the insulation coating is lower than that of the measured medium

Yu and Choi (2006)

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Reasons For Adapting Single Wire Method

• Simplicity of Operation• Low Cost• Easy Insulation Coating• Easy Construction• Design Optimized

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Design Parameters• Size of the wire (i.e., Wire radius)• Type of insulation coating• Length of the wire• Sample size (length and radius of the

cell) Selected Design Parameters• Wire Diameter 50.8 µm• Teflon Insulation coating thickness 25.4

µm• Measured length of wire (after fabrication)

is 0.1484 m• Diameter of bounding wall is 0.0144 m • Length of sample is 0.165 m

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Calibration Gauge(to guard spring rod and calibrate the spring tension)

Spring Rod with ThreadingLocking Nut

(calibrated weight for required spring tension) To the Data Acquisition System

Connectors and Calibration Guage Holder

D-Type Connector

T-Type Thermocouples

Hot-Wire Voltage Output Wires

Power Supply Connector

Cell Cap with Rectangular Cuts(for wire outlet)

Special Shape Sliding Fit Hole(avoids turning of spring)

Striped Stranded Copper Wire (to provide flexiblity and avoid backlash)

Tension Spring (spring constant 0.02 N/mm)

Constant Voltage Input Wires

Wire Holder

Hot-Wire Guiding Block(off-centered)

Sliding Tube (aligns the hot-wire)

Wire Protection Clip # 1

Mea

sure

men

t Sec

tion

149.

2 m

m

Soldered Joint # 1

Teflon Coated Platinum Hot-WireØ 0.0508 mm

Coating Thickness 0.0245 mm

Soldered Joint # 2



Cell Base Plate

Off-Centered Alignment Ring

Insulated Copper Wire Ø 0.254 mm

Teflon Sealing

Threaded Hole in Base Plate(Assembly and Cleaning)

Outer Shell(test-fluid reservoir)

Inner Semi-Circular Hot-Wire Holder

Thermocouple at the Bottom L45°

Threaded Nut

Inner Wire Guide

Fig 2: Cross-sectional front view of improved transient hot-wire thermal Conductivity Cell

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D-Type Connector









Wire Holder




Mea

sure

men

t Sec

tion

149.

2 m

m

Soldered Joint # 1



Soldered Joint # 2



Cell Base Plate



Teflon Sealing





Threaded Nut

Inner Wire GuideFig 2: Top half cross-sectional front view of transient hot-wire thermal conductivity cell

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D-Type Connector









Wire Holder




Mea

sure

men

t Sec

tion

149.

2 m

m

Soldered Joint # 1



Soldered Joint # 2



Cell Base Plate



Teflon Sealing





Threaded Nut

Inner Wire Guide

Fig 3: Bottom half cross-sectional front view of transient hot-wire thermal conductivity cell

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Base Plate

Threaded Nut


Protection Clip

Semi-Circular Hot-Wire Holder

(Off Centered)

Thermocouple at the middle

Ø14.371mm

Ø17.424mm

Fig 4: Cross sectional top view of the hot-wire cell at the middle

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Calibration GuageD-Type Connector (thermocouples and wire voltagemeasurement using data acquisition system)

Locking Nut(calibrated weight fpr required

spring tension)

Power Supply Connector Connectors and Calibration Guage Holder

Fixing Nut


Sliding Hole

Tension Spring

Cell Base Plate

Wire Holder Fixing NutWire Holder

T-Type Thermocouple

Hot-Wire Voltage Output WiresConstant Voltage Input

Threaded Nut (soldered to outer shell )

Fig 5: Isometric view of transient hot-wire thermal conductivity cell

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Thermocouple at the TopL15°

Thermocouple at the MiddleL75°

Thermocouple at the BottomL45°

Rectangular hole on the Inner Cell(for guiding the wires out)

Sliding tube

Locking Screw(avoids the axial movementof calibration guage)

Wire Guiding Hole(to guide the aligned wires out)


Tension Spring

Fig 6: Left-side view of transient hot-wire thermal conductivity cell without the outer cell, base plate and protection pins

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Calibrated Weight(for required spring tensionwithin elastic modulus of the platinum hot-wire)

Calibration Guage (guards the spring rod

and protects the platinum wires for sudden shocks)


Sliding Tube(causes free movement without friction)

Off-Centered Alignment Ring (Provides Rigidity to the other end of the wire)

Uniform Tensionon the Platinum Wire

Fig 7: Calibration position of the hot-wire cell

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Spring Assembly

Where,Weight of spring rod,W1 = 0.00708 NWeight of locking nut, W2 = 0.1762 NWeight of tension spring,W3 = 0.0115

NWeight of sliding tube,W4 = 0.00490 N

ΔZcalZcal

ΔZ0

(1) Spring Rod

(2) Locking Nut

Cell Cap

(3) Tension Spring

(4) Sliding Tube

Fwa

Spring Constant ζs

Initial Spring Force Fsi

waF = 0.1997 N

4321 WWWWFwa

s

sical

FWWWZ

321

calZ = 0.0056 m

www.kostic.niu.eduFig 8: Fabricated transient hot-wire thermal conductivity apparatus cell

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Instrumentation

Figure 5.1 Schematics of electrical circuit with data acquisition system

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Measurement Procedure• The wire is heated with electrical constant

power supply at step time• The wire simultaneously serves as the heating

element and as the temperature sensor

• The temperature increase of the wire is determined from its change in resistance

• Thermal conductivity is determined from the heating power and the slope of temperature change in logarithmic time

• The change in resistance of the wire due to heating is measured in time using a Wheatstone bridge circuit

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Signal AnalysisBridge Balance

3

0

2

1

RR

RR w

32

10 R

RR

Rw

Resistance of the hot wire

21

1

30

0

RRR

RRRRR

VVww

wwinout

The bridge voltage output

in

out

in

out

wt

VV

RRR

VV

RRRR

R

212

2113

The Resistance change of Hot-Wire

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00

121

wTCR

w

w

wtwt

TCR RR

RRR

T

The Temperature change of Hot-Wire

3RRRV

Vwt

wtinRw

The Voltage Drop Across the Hot-Wire

wtw

Rw

RLV

q2

Heat Flux per Unit Length at any Instant of Time

Tdtdqk f

)ln(

4


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Computerized Data Acquisition

• Data acquisition hardware and software are optimized to minimize signal noise and enhance gathering and processing of useful data

Types of Data Measured• Bridge voltage output

• Bridge voltage input • Hot-wire Voltage• Temperature of fluid

Programming in LabVIEW• A program has been written in LabVIEW application

software to automatically calculate thermal conductivity

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Data Acquisition Hardware• PCI – 6024E, Multifunctional DAQ Board

(E–series family, PCI, PCMCIA bus, 16 single-ended/ 8 differential channel analog inputs, 12 bit input resolution, 200 kS/s maximum sampling rate, ± 0.05 V to ± 10 V input range, 2 analog inputs, 12 bit output resolution, 10 kSamples/s output range, 8 digital I/O, two 24 bit counter timer, digital trigger)

• SCXI – 1000, 4 Slot Signal Conditioning Chassis (shielded enclosure for SCXI module, low – noise environment for signal conditioning, forced air cooling, timing circuit)

• SCXI – 1102, 32 Differential Channel Thermocouple Input Module (programmatic input range of ± 100 mV to ± 10 V per channel, overall gain of 1 – 100, hardware scanning of cold junction sensor, 2 Hz low pass filtering per channel, relay multiplexer, over voltage protection of ± 42 V, 333 kS/s maximum sampling rate, 0-50 ºC operation environment temperature)

• SCXI – 1303, 32 Channel Isothermal Terminal Block for Thermocouple modules (SCXI front end mountable terminal block for SCXI-1100 and SCXI-1102/B/C, cold junction compensation sensor, open-thermocouple detection circuitry, isothermal construction for minimizing errors due to thermal gradient, cold junction accuracy for 15-35 ºC is 0.5 ºC and for 0-15 ºC & 25-50 ºC is 0.85 ºC, repeatability is 0.35 ºC)

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Data Acquisition Hardware• SCXI – 1122, 16 Differential Channel Isolated Universal

Input Module (DC input coupling, nominal range ± 250 V to ± 5 mV with overall gain of 0.01 to 2000, over voltage protection at 250 Vrms, maximum working voltage in each input should remain with 480 Vrms of ground and 250 Vrms of any other channel, cold junction compensation, bridge compensation, isolated voltage and current excitation, low pass filter setting at 4 kHz or 4 Hz, shunt calibration, 16 relay multiplexer, 100 Samples/s (at 4 kHz filter) and 1 Sample/s (at 4 Hz filter), two 3.333 V excitation level sources)

• SCXI – 1322, Shielded Temperature Sensor Terminal Block (SCXI front end mountable terminal block for SCXI -1122, on board cold junction sensor)

• SCXI – 1349, Shielded Cable Assembly (adapter to connect SCXI systems to plug-in data acquisition devices, mounting bracket for secure connection to the SCXI chassis)

• SH68-68-EP, Noise Rejecting, Shielded Cable (Connects 68-pin E Series devices (not DAQ cards) to 68-pin accessories, individually shielded analog twisted pairs for reduced crosstalk with high-speed boards)

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Start

BridgeBalance

MeasureResistance

Calculate Initial Resistance R w0

Average Temperature of the Cell

Voltage Output V out

Measure Temperature at Top, Middle and Bottom

of Hot-Wire Cell

YESIf Vout >= 0.001 V

Measure Bridge Output Voltage V out

and time tPlot Vout Vs Ln(t)

Measure Bridge Input Voltage V in

Measure Temperature at Top, Middle and Bottom

of Hot-Wire Cell

Average Temperature of the Cell

CalculateWire Voltage V Rw

Plot Temperature Vs Ln(time)

Calculate Change in Temperature

Calculate Slope of Temperature and Ln(Time) for Specified Time Range

Calculate Heat Input

per unit length 'q'

Calculate Thermal Conductivity

Store All

Data

Time Ranges

NO

Initialize VariablesR1, R2, R3, TCR, Lw

Measurement Time

Sampling Rate

Calculate TotalChange in Resistance

End

Figure 5.3: LabVIEW Program Algorithm for Thermal Conductivity Measurement

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CalibrationTwo Standard Fluids Ethylene Glycol and Water

210 21 tTtTTTr

Reference Temperature

Resistances of the Wheatstone bridge circuit are measured as

1R = 2270.6 Ω

2R = 2161.1 Ω

3R = 7.715 Ω

0wR = 8.106 Ω

wL= 0.1484 m

wTCR RZ

,

wR = 8.22 Ω

Z = 0.02652 Ω/°C is the the slope of dRw vs TZ

Where,

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0

2

4

6

8

10

12

14

16

0.01 0.1 1 10 100

time, t [s]

Wire

Tem

pera

ture

Cha

nge,

ΔT

[°C

]

Ethylene GlycolDistilled WaterLog. (EG (2.0s - 6.0s))Log. (Water (2.0s-6.0s))

Figure 6.1: Wire temperature change against time (in logarithmic scale) for ethylene glycol and distilled water

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Heat Input per Unit Length in Time

5.54

5.545

5.55

5.555

5.56

5.565

5.57

0 5 10 15 20 25 30 35 40 45 50

Time, t [s]

Hea

t Inp

ut p

er U

nit L

engt

h, q

[W/m

]

WaterEthylene glycol

Figure 6.2: Heat input per unit length against time (for ethylene glycol and water)

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Calibration Data from (1 s-10 s)

5

6

7

8

9

10

11

12

13

14

1 10time, t [s]

Wire

Tem

pera

ture

Cha

nge,

ΔT

[°C

]

Ethylene GlycolDistilled Water

Valid time range for data reduction

Figure 6.3: Calibration data from time (1 s – 10 s), shows the selected time range for data reduction as 2s – 6 s, for ethylene glycol and water

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Repeatablity of Ethylene Glycol Thermal Conductivity Measurement

0.240

0.245

0.250

0.255

0.260

0.265

0 1 2 3 4 5 6 7 8 9 10

Measurement Set

Ethy

lene

Gly

col T

herm

al C

ondu

ctiv

ity, k f

eg [W

/m°C

]

Repeatability of EGLinear (Reference Value)Linear (Mean)

Figure 6.4: Results of repeatability measurement of thermal conductivity for

Ethylene glycol, shows the bias and precision error in measurement

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Repeatability of Water Thermal Conductivity Measurement

0.580

0.590

0.600

0.610

0.620

0.630

0.640

0.650

0 1 2 3 4 5 6 7 8 9 10

Measurement Set

Wat

er T

herm

al C

ondu

ctiv

ity, k

fw [W

/m°C

]

Repeatablity of WaterLinear (Reference)Linear (Mean)

Figure 6.5: Results of repeatability measurement of thermal conductivity for distilled water, shows the bias and precision error in measurement

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Calibration Results

FluidReferen

ce [W/m°C]

Measured

[W/m°C]

Bias Error

Precision

Error(95 %)

Uncertainty

Ethylene Glycol

(32.5 °C)0.254 0.253 - 0.395

% 2.03 % 2.06 %

Distilled water

(~ 26 °C)0.612 0.619 1.2 % 2.23 % 2.52 %

Table 6.1: Uncertainty in repeatability of measured thermal conductivity

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Uncertainty in Thermal Conductivity

Tdtdqk f

)ln(

4

RwTCR

wwTCRf Z

qRRdtdqRk 1

4)ln(

4 00

Rearranging in terms of the measured resistance change in the wire

22

0

22

0

RwTCRf Z

R

fR

w

f

TCR

fq

fk u

Zk

uRk

uk

uqk

u

Uncertainty

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Uncertainty in Heat Input per Unit Length

2222

qLw

Rwt

VRw

q PuLqu

Rqu

Vqu

wwtRw

qPis the precision error in the average heat input per unit length

%. 1.63quq

2

3

22

3

RRw

Rwt

RwV

in

RwV u

RV

uRV

uVV

uwtinRw

Uncertainty in Wire Voltage

%. 0.706 RwV VuRw

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Uncertainty in Total Resistance Change

222

3

2

2

2

1321

outinwt V

in

wtV

out

wtR

wtR

wtR

wtR u

VR

uVR

uRR

uRR

uRR

u

%. 0.813 wtR Ruwt

Uncertainty in Measured Bridge Voltage Input

% 0.535 inV Vuin

Uncertainty in Measured Bridge Voltage Output

%0.1outV Vuout

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220 mmcmmmmd uuu

Uncertainty in ResistancesUncertainty in Multimeter

2211 RmmdR Buu

Uncertainty in Resistance R1



%. 0.1 11RuR

2222 RmmdR Buu

2233 RmmdR Buu

%. 0.2522RuR

%. 0.516 33RuR

03210

2

3

2

2

2

1ww RR

wR

wR

wR Bu

RR

uRR

uRR

u


%. 1.6300wR Ru

w

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Uncertainty in Temperature Coefficient of Resistance 22

wTCR R

w

TCRZ

TCR uR

uZ

u %. 2.275 TCRTCRu

Hot-Wire Resistance Vs Temperature

Rw = 0.026521 T + 7.698728r2 = 0.999036

8.45

8.5

8.55

8.6

8.65

8.7

8.75

8.8

8.85

8.9

8.95

29 31 33 35 37 39 41 43 45

Temperature, T [°C]

Hot

-Wire

Res

ista

nce

Rw [Ω

]

Figure 6.7 Calibration of Temperature Coefficient of Resistance of Teflon Coated Platinum Hot-Wire

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Uncertainty in Length of Hot-Wire

222FSeVCdL LLuu

w %. 0.0661 wL Lu

w

Uncertainty in Slope of Total Resistance Change against Logarithmic Time

2

00

21

lnln

RRRR N

ii

N

iiR

RZyxZa

ttN

NSS

RR ZaZ Stu 1%95,200

0.2314% RZ ZuR

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quq

TCRTCRu

00 wR Ruw

RZ ZuR

fk kuf

Table 7.2: Percentage uncertainties

Uncertainty (%)

1.629

2.274

1.627

0.231

3.245

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Nanofluid thermal conductivity Measurement

• Copper, particle size 35 nm

• Ethylene glycol and Water

Base Fluid:

Nanoparticles:

Concentration:• 1 volumetric %

Physical Stabilization:• Ultrasonication

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Copper in Ethylene Glycol NanofluidMeasured Thermal Conductivity Ratio of

1 vol% of Copper in Ethylene Glycol Nanofluid

Mean= 1.1282

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14

1.16

0 1 2 3 4 5 6

Measurement Set

Ther

mal

Con

duct

ivity

ratio

kn feg/k

feg

1% vol Cu in EG

Linear (Mean)

Figure 7.1: Nanofluid thermal conductivity measurement of 1 vol % of copper in ethylene glycol

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Copper In Water Nanofluid

Measured Thermal Conductivity Ratio of 1 vol% of Copper in Water Nanofluid

Mean = 1.1595

1.0000

1.0500

1.1000

1.1500

1.2000

1.2500

1.3000

1.3500

0 1 2 3 4 5 6

Measurement Set

Ther

mal

con

duct

ivity

Rat

io kn fw

/kfw

1% vol Cu in Water

Linear (Mean)

Figure 7.2: Nanofluid thermal conductivity measurement of 1 vol % of copper in water

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Improvements in Design• Overall volume of the cell after fabrication is 35 ml• Four wire arrangement to measure voltage drop

independently from power wiring • Incorporated a spring to provide a uniform tension

and avoid any slackness due to expansion • Effective off-centering mechanical design provides

additional room for wiring and thermocouples • Three thermocouples to verify the uniformity of the

fluid temperature • Electrical connection junctions are arranged on the

cell for flexibility in connections and handling • Boundary induced errors are minimized

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Conclusion• Designed and Fabricated a Hot-wire cell

with improvements• Designed and Fabricated a Wheatstone

bridge for Hot-wire cell• Optimized Data Acquisition Hardware• Developed a LabVIEW Program for

Measuring Thermal Conductivity• Calibrated the Apparatus with Standard

Fluids

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Conclusion• Bias Error is within 1.5 %• Precision Error is within 2.5 %• Total Uncertainty within 3.5 % at 95 %

Probability • Enhancement in Thermal Conductivity

with Copper in Ethylene glycol is 13 %• Enhancement in Thermal Conductivity

with Copper in Water is 16 %

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RECOMMENDATIONS• The uncertainty analysis shows that the resistors are the

major contributors of error. This error can be reduced by using very high precision resistors with extremely small temperature coefficient of resistance.

• In the present study, temperature coefficient of resistance was determined through calibration over limited temperature range. Precise calibration under well controlled conditions with a larger temperature range would be beneficial.

• At present, the resistances are manually measured. This process can be automated in future.

• The data acquisition and LabVIEW® can be programmed to evaluate curvature of temperature versus logarithmic-time dependence (at initial heat-capacity and later convection non-linear regions), and automate evaluation if linear range relevant for thermal conductivity measurement.

• The hot-wire tension can be more accurately controlled using a micrometer in place of the fixed calibration gauge.

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Acknowledgements

The authors acknowledge support by National Science Foundation (Grant No. CBET-0741078).

The authors are also grateful for help in mechanical design and fabrication to Mr. Al Metzger, instrument maker and technician supervisor at NIU.

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