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Computerized , Transient HotWire Thermal Conductivity ( HWTC ) Apparatus for Nanofluids

A new and improved HWTC apparatus for thermal conductivity measurements of common fluids and nanofluids has been recently developed, designed and fabricated. A platinum (Pt) wire of 50.8 μm diameter with a Teflon insulation coating of 25.4 μm thickness was used as the hot-wire heater and temperature sensor for the present application. The new apparatus employs innovative solutions for easy calibration of uniform Pt-wire tension and thus minimizing the strain influence on temperature measurement (i.e., minimizing the well-known and unwanted “strain-gage effect” on Pt-wire electrical resistivity); measurement of Pt-wire voltage drop independently from power wiring (four wires); and an effective off-centered mechanical design to minimize the test fluid sample size (about 35 mL), but at the same time providing additional space for wiring, including three inside thermocouples for fluid temperature uniformity verification. Data acquisition hardware and LabVIEW application software are optimized to minimize signal noise and enhance acquisition and processing of useful data. The bias measurement error, based on calibration with distilled water and ethylene glycol, has been found to be within 1.5 %, and precision, i.e., repeatability error within 2.5 %. The total uncertainty in measured thermal conductivity (TC) has been estimated to be within 3.5 % at 95 % confidence probability. Two nanofluids of 1 % volumetric concentration of 35 nm size copper nanoparticles in ethylene glycol and in water, were prepared and their thermal conductivity measured using the apparatus. The mean enhancement in thermal conductivity of copper in ethylene glycol and copper in water nanofluids, were measured as 13 % and 16 %, respectively. The developed apparatus has proved to be efficient and with potential for further improvement.