Measurement of the thermal conductivity of fluids using laser spot lock-in thermography

In this work, we propose a new method to retrieve the thermal conductivity of fluids, k_f, using laser spot lock-in infrared thermography. The measurement cell consists in two paralepidid blocks separated by a narrow gap of variable and known width, L. This gap can be filled with a gas or a liquid, producing a thermal contact resistance, R_th, which quantifies the thermal barrier to heat propagation. By focusing the laser spot close to the interface between the two solids, this thermal contact resistance can be obtained by fitting the temperature field recorded by the infrared camera to its analytical expression. Using the well-known relationship R_th = L/k_f, the thermal conductivity of the fluid is obtained from a simple linear relation. Measurements performed in test samples show a good agreement between determined thermal conductivities and literature reported values, demonstrating the validity of the method.