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Thermal Conductivity of Liquids and Gases

Thermal conductivity data is of prime importance in designing heat exchangers. Heat transfer coefficients in these components are usually computed using correlations which require thermal conductivity data. Because of the importance of two-phase heat transfer processes in many processes, thermal conductivity of the saturated liquid and vapor are of greatest importance. It is difficult, however, to measure the thermal conductivity at saturation and thus single-phase measurements will be extrapolated to saturation conditions.

The Experimental Properties of Fluids Group has a unique capability to measure the thermal conductivity and thermal diffusivity of fluids with two versitile transient hot-wire apparatus.

These hot-wire apparatus can accurately measure these transport properties in the liquid, vapor and supercritical fluid phases of pure fluids and mixtures at temperatures from 30 K to 750 K with pressures to 70 MPa. The hot-wire cells use 4 micron diameter tungsten, 12.7 micron diameter platinum, or 25 micron diameter anodized tantalum (electrically insulated) hot wires depending on the fluid of interest. Electrically conducting fluids require the use of insulated hot wires.

Schematic of the Hot-wire Apparatus -- click to enlarge

Image by Marilyn Yetzbacher

In the transient hot-wire technique, small diameter wires are immersed in the fluid and used simultaneously as electrical resistance heaters and as resistance thermometers to measure the resulting temperature rise due to the resistance heating. The hot-wire cells are designed to approximate a simple 1-dimensional transient line-source of heat in an infinite medium as closely as possible to minimize corrections for the actual geometry. Two hot wires of differing length are operated in a differential mode to eliminate axial conduction effects due to the large diameter leads attached to the ends of each hot wire. Based on the transient line-source model, the thermal conductivity can be found from the slope of the measured linear temperature rise as a function of elapsed time with a typical uncertainty of less than 1%. The thermal diffusivity can be found from the intercept of this same linear temperature rise curve with a typical uncertainty of less than 10%. Independent measurements of the thermal diffusivity of fluids can also be made by dynamic light scattering (DLS) measurements at low angles with a typical uncertainty of 2%.

The two largest corrections to the ideal line-source model account for the finite dimensions of the wire and the finite dimensions of the fluid medium surrounding the wire. The correction for finite dimensions of the wire is largest at short elapsed times in low-density gas measurements. This correction can be reduced through the use of extremely small (4 micron diameter) hot wires. The correction for finite dimensions of the fluid medium is largest at long elapsed times in low-density (high thermal diffusivity) gas measurements. For low-density gas measurements, the hot-wire cells can also be operated in the steady-state mode to measure thermal conductivity with an uncertainty of about 2%.

The results of the transient hot-wire measurements are available for a many fluids in a series of NBS/NIST reports. Copies of these publications can be obtained by contacting Dr. Richard A. Perkins or in a machine readable format (ASCII, pdf, postscript) below.