Measurement of radial thermal conductivity of a cylinder using a time-varying heat flux method

Despite a variety of techniques available for thermal conductivity measurement, there is a lack of methods to directly measure the radial thermal conductivity of a cylinder. This presents a critical gap in heat transfer metrology, particularly in presence of anisotropic thermal conduction, such as in a Li-ion cell, where radial thermal conductivity plays a key role in determining performance and safety. This paper reports a technique for measurement of radial thermal conductivity of a cylinder by accounting for variable heat flux into the cylinder when heated on the outside. It is shown that heat flux into the cylinder from a thin heater wrapped around its surface varies significantly with time. This variation, which was neglected in past work, is accounted for by developing a variable heat flux model for experimental data analysis. For two different materials, measurements of radial thermal conductivity using this approach are shown to be in close agreement with standard thermal conductivity measurement using the transient plane source method. Radial thermal conductivity of a 26650 Li-ion cell is measured to be 0.39 W/mK. Besides contributing a new approach for thermal metrology in cylindrical systems, this work also improves the understanding of thermal phenomena in Li-ion cells.