Application of the 3ω Hot Wire Technique for Measurement of Fluid Thermophysical Properties

The popular 3ω method of measuring thermophysical properties of solids is adapted for the simultaneous measurement of thermal conductivity and heat capacity in both liquids and gases. This technique is experimentally simple and has a lower susceptibility to random experimental noise, bulk fluid motion, radiation losses, and non-linear effects than other transient hot wire measurement methods. The compactness of the 3ω hotwire allows it to be used with different fluids in a variety of circumstances with very little specialized experimental equipment. Both the experimental setup and theoretical model are detailed. Experimental 3ω measurements were made in a variety of common fluids (air, water, and mineral oil) using commercially drawn 10μm platinum and 5μm tungsten hot wires which serve as both heating element and resistance thermometer. Measurements taken over a range of frequencies are numerically reduced to provide both thermal conductivity and heat capacity information. Experimental measurements and the corresponding analytical model are presented in terms of impedance or thermal resistance; a more physically meaningful and intuitive basis of comparison. Fluid properties are determined by curve-fitting an analytical model to experimental data using a least-squares approach. This technique allows both thermal conductivity and heat capacity (or thermal diffusivity) to be uniquely determined from a single measurement sequence.