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www.kostic.niu.eduComputerized, Transient Hot-Wire Thermal
Conductivity (HWTC) Apparatus For NanofluidsThe 6th WSEAS
International Conference on HEAT and MASS TRANSFER (WSEAS - HMT'09)
Ningbo, China, January 10-12, 2009
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www.kostic.niu.eduOverviewINTRODUCTIONOBJECTIVETHEORY OF
HOT-WIRE METHODPRACTICAL APPLICATION OF HOT-WIRE METHODDESIGN OF
HOT-WIRE CELLINSTRUMENTATIONDATA ACQUISTIONCALIBRATIONUNCERTAINTY
ANALYSISRESULTSCONCULSIONSRECOMMENDATIONS
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www.kostic.niu.eduINTRODUCTIONNanofluids are colloidal
suspensions of nanoparticles, nanofibers, nanocomposites in common
fluidsThey are found to have enhanced thermal properties,
especially thermal conductivityThermal conductivity values of
nanofluids may be substantially higher than related prediction by
classical theories No-well established data or prediction formula
suitable to all nanofluidsExperimental thermal conductivity
measurement of nanofluids is critical
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www.kostic.niu.eduTable 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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www.kostic.niu.eduNanofluid Preparation MethodsOne Step (Direct
Evaporation and Condensation) MethodFig1: 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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www.kostic.niu.eduThermal ConductivityMaterial
PropertyDetermines ability to conduct heatImportant for thermal
ManagementClassification of Thermal Conductivity Measurement
Techniques for FluidsSteady State MethodsNon-Steady State
Methods
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www.kostic.niu.eduTransient Hot-Wire Method for FluidsFast and
AccurateAdvantages:Minimize (or even avoid) ConvectionMinimum
Conduction and Radiation lossesClassification of Hot-Wire
MethodsStandard Cross Wire MethodSingle Wire, Resistance
MethodPotential Lead Wire MethodParallel Wire Method
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www.kostic.niu.eduOBJECTIVEDesignDevice to Suspend
Hot-WireReduce Nanofluid Sample SizeMinimize End ErrorsUniform
Tension on Hot-WireSeparate Wires for Power and VoltageMonitor
TemperatureMechanism to Calibrate Hotwire TensionFlexibility for
Cleaning and Handling
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www.kostic.niu.eduOBJECTIVEElectrical CircuitFlexible
ConnectionsInstrumentationData AcquisitionOptimize to Reduce
NoiseDevelop ProgramCalibrationStandard FluidsUncertainty
AnalysisThermal Conductivity
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www.kostic.niu.eduPrinciple of Hot-Wire MethodAn infinitely long
and thin, ideal continuous line source dissipating heat into an
infinite medium, with constant heat generation General Fouriers
Equation Boundary Conditions Ideal case:Line source has an infinite
thermal conductivity and zero heat capacity
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www.kostic.niu.eduThe temperature change at a radial distance r,
from the heat source is conforms to a simple formula by applying
boundary conditionsAt any fixed radial distance, in two instances
in time the equation, the temperature change can be represented as
series expansion of the exponential integration
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www.kostic.niu.eduThermal ConductivityA plot of temperature
against the natural logarithm of time results in a straight line,
the slope being propositional to kfPractical application of
hot-wire methodThe ideal case of continuous line is approximated
with a finite wire embedded in a finite mediumFigure 2.1 Typical
plot of temperature change against time for hot-wire experiment
(Johns et al 1988)
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www.kostic.niu.eduNanofluids Thermal Conductivity Methods By
Other Authors
Author, YearNanofluid Thermal Conductivity Measurement
MethodWang et al (1999)Horizontal flat plate methodLee et al
(1999), Yu et al (2003) and Vadasz (2006)Vertical, single wire,
hot-wire methodAssael et al (2004)Two wires, hot-wire methodManna
et al (2005)Thermal comparatorMa (2006)Horizontal, single wire,
hot-wire methodSimham (2008)Vertical, single wire, hot-wire
method
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www.kostic.niu.eduHot-wire Method for NanofluidNanofluids are
electrically conducting fluidsAvailability of nanofluidsThermal
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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www.kostic.niu.eduWhere,
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www.kostic.niu.eduInsulation Coating Influence on Thermal
Conductivity MeasurementThe 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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www.kostic.niu.eduReasons For Adapting Single Wire
MethodSimplicity of OperationLow CostEasy Insulation CoatingEasy
ConstructionDesign Optimized
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www.kostic.niu.eduDesign ParametersSize of the wire (i.e., Wire
radius)Type of insulation coatingLength of the wireSample size
(length and radius of the cell)Selected Design ParametersWire
Diameter 50.8 mTeflon Insulation coating thickness 25.4 mMeasured
length of wire (after fabrication) is 0.1484 mDiameter of bounding
wall is 0.0144 m Length of sample is 0.165 m
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www.kostic.niu.eduFig 2: Cross-sectional front view of improved
transient hot-wire thermal Conductivity Cell
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www.kostic.niu.eduFig 2: Top half cross-sectional front view of
transient hot-wire thermal conductivity cell
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www.kostic.niu.eduFig 3: Bottom half cross-sectional front view
of transient hot-wire thermal conductivity cell
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www.kostic.niu.eduFig 4: Cross sectional top view of the
hot-wire cell at the middle
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www.kostic.niu.eduFig 5: Isometric view of transient hot-wire
thermal conductivity cell
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www.kostic.niu.eduFig 6: Left-side view of transient hot-wire
thermal conductivity cell without the outer cell, base plate and
protection pins
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www.kostic.niu.eduFig 7: Calibration position of the hot-wire
cell
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www.kostic.niu.eduSpring AssemblyWhere,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
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www.kostic.niu.eduFig 8: Fabricated transient hot-wire thermal
conductivity apparatus cell
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www.kostic.niu.eduInstrumentationFigure 5.1 Schematics of
electrical circuit with data acquisition system
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www.kostic.niu.eduMeasurement ProcedureThe wire is heated with
electrical constant power supply at step timeThe wire
simultaneously serves as the heating element and as the temperature
sensorThe temperature increase of the wire is determined from its
change in resistanceThermal 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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www.kostic.niu.eduSignal AnalysisBridge BalanceResistance of the
hot wire The bridge voltage output The Resistance change of
Hot-Wire
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www.kostic.niu.eduThe Temperature change of Hot-Wire The Voltage
Drop Across the Hot-Wire Heat Flux per Unit Length at any Instant
of TimeThermal Conductivity
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www.kostic.niu.eduComputerized Data AcquisitionData acquisition
hardware and software are optimized to minimize signal noise and
enhance gathering and processing of useful data Types of Data
MeasuredBridge voltage outputBridge voltage input Hot-wire
VoltageTemperature of fluid Programming in LabVIEWA program has
been written in LabVIEW application software to automatically
calculate thermal conductivity
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www.kostic.niu.eduData Acquisition HardwarePCI 6024E,
Multifunctional DAQ Board (Eseries 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 C 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 15-35C is 0.5C and for 0-15C & 25-50C is 0.85C,
repeatability is 0.35C)
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www.kostic.niu.eduData Acquisition HardwareSCXI 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 250Vrms, maximum working voltage
in each input should remain with 480Vrms of ground and 250Vrms 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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www.kostic.niu.eduFigure 5.3: LabVIEW Program Algorithm for
Thermal Conductivity Measurement
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www.kostic.niu.eduCalibrationTwo Standard Fluids Ethylene Glycol
and WaterReference TemperatureResistances of the Wheatstone bridge
circuit are measured as = 0.1484 m , Where,
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www.kostic.niu.eduFigure 6.1: Wire temperature change against
time (in logarithmic scale) for ethylene glycol and distilled
water
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www.kostic.niu.eduFigure 6.2: Heat input per unit length against
time (for ethylene glycol and water)
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www.kostic.niu.eduFigure 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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www.kostic.niu.eduFigure 6.4: Results of repeatability
measurement of thermal conductivity for Ethylene glycol, shows the
bias and precision error in measurement
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www.kostic.niu.eduFigure 6.5: Results of repeatability
measurement of thermal conductivity for distilled water, shows the
bias and precision error in measurement
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www.kostic.niu.eduCalibration ResultsTable 6.1: Uncertainty in
repeatability of measured thermal conductivity
FluidReference [W/mC]Measured [W/mC]Bias Error Precision
Error(95 %)UncertaintyEthylene Glycol (32.5 C)0.2540.253- 0.395
%2.03 %2.06 %Distilled water(~ 26 C)0.6120.6191.2 %2.23 %2.52 %
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www.kostic.niu.eduUncertainty in Thermal ConductivityRearranging
in terms of the measured resistance change in the wire
Uncertainty
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www.kostic.niu.eduUncertainty in Heat Input per Unit Lengthis
the precision error in the average heat input per unit length
Uncertainty in Wire Voltage
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www.kostic.niu.eduUncertainty in Total Resistance Change
Uncertainty in Measured Bridge Voltage InputUncertainty in Measured
Bridge Voltage Output
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www.kostic.niu.eduUncertainty in ResistancesUncertainty in
MultimeterUncertainty in Resistance R1Uncertainty in Resistance
R2Uncertainty in Resistance R3Uncertainty in Resistance R3
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www.kostic.niu.eduUncertainty in Temperature Coefficient of
Resistance Figure 6.7 Calibration of Temperature Coefficient of
Resistance of Teflon Coated Platinum Hot-Wire
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www.kostic.niu.eduUncertainty in Length of Hot-WireUncertainty
in Slope of Total Resistance Change against Logarithmic Time
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www.kostic.niu.eduTable 7.2: Percentage uncertainties
Uncertainty(%)1.6292.2741.6270.2313.245
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www.kostic.niu.eduNanofluid thermal conductivity
MeasurementCopper, particle size 35 nmEthylene glycol and WaterBase
Fluid:Nanoparticles:Concentration: 1 volumetric %Physical
Stabilization:Ultrasonication
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www.kostic.niu.eduCopper in Ethylene Glycol NanofluidFigure 7.1:
Nanofluid thermal conductivity measurement of 1 vol % of copper in
ethylene glycol
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www.kostic.niu.eduCopper In Water NanofluidFigure 7.2: Nanofluid
thermal conductivity measurement of 1 vol % of copper in water
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www.kostic.niu.eduImprovements in DesignOverall volume of the
cell after fabrication is 35 mlFour 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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www.kostic.niu.eduConclusionDesigned and Fabricated a Hot-wire
cell with improvementsDesigned and Fabricated a Wheatstone bridge
for Hot-wire cellOptimized Data Acquisition HardwareDeveloped a
LabVIEW Program for Measuring Thermal ConductivityCalibrated the
Apparatus with Standard Fluids
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www.kostic.niu.eduConclusionBias 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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www.kostic.niu.eduRECOMMENDATIONSThe 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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www.kostic.niu.eduAcknowledgementsThe 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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www.kostic.niu.eduThank You
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