Abstract
In this study, the 3ω method was used to determine the thermal conductivity of nanofluids (ethylene glycol containing multi-walled carbon nanotubes (MWCNTs)) with temperature gradients. The thermal modeling of the traditional 3ω method was modified to measure the spatial variation of thermal conductivity within a droplet of nanofluid. A direct current (DC) heater was used to generate a temperature gradient inside a sample fluid. A DC heating power of 14 mW was used to provide a temperature gradient of 5000 K/m inside the sample fluid. The thermal conductivity was monitored at hot- and cold-side 3ω heaters with a spacing of 0.3 mm. Regarding the measurement results for the hot and cold 3ω heaters, when the temperature gradient was applied, the maximum thermal conductivity difference was determined to be 3% of the original value. By assuming that the thermo-diffusion of MWCNTs was entirely responsible for this difference, the Soret coefficient of the MWCNTs in the ethylene glycol was calculated to be −0.749 K−1.
Highlights
Particle transport in the presence of a temperature gradient is known as thermo-diffusion.Traditionally, thermo-diffusion is described as the net force exerted on a particle by the molecules of a fluid medium
To detect the concentration gradients of nanoparticles in fluids with temperature gradients, we propose measuring localized thermal conductivities inside nanofluid samples
This paper presented a method for measuring the thermo-diffusion of multi-walled carbon nanotubes (MWCNTs) in ethylene glycol (EG), based on thermal conductivity measurements in the presence of a temperature gradient
Summary
Particle transport in the presence of a temperature gradient is known as thermo-diffusion. Numerical analysis results suggested a significant heat transfer enhancement for nanofluids in channel flows, based on the thermal dispersion of nanoparticles [9]. To detect the concentration gradients of nanoparticles in fluids with temperature gradients, we propose measuring localized thermal conductivities inside nanofluid samples. The temperature variation in 3ω heaters can be controlled to within 1 K during measurement [20] This limits the effects of natural convection, which typically acts as the most significant source of experimental uncertainty in thermo-diffusion detection. Differences in MWCNT concentrations at the hot- and cold-side 3ω heaters were calculated based on the thermal conductivity measurements. The thermo-diffusion of MWCNTs was quantified based on a simplified nanoparticle diffusion equation in the steady state
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