Influences of Physical Properties and Coating on Frequency Response of Fine-Wire Thermocouple Probes

Theoretical analysis of the frequency response of a fine-wire thermocouple probe was performed. A strict solution of the response was derived and used to investigate influences of the difference in physical properties between two butt-welded thermocouple wires. The solution obtained enables us to understand complex behaviors of the frequency response, even if the physical properties of the two thermocouple wires are greatly different as seen in the copper-constantan (type-T) thermocouple. Additionally, the frequency response at an arbitrary point in the cross-sectional area of the thermocouple wire was investigated. The result shows that a common assumption that the cross-sectional temperature distribution is kept uniform becomes invalid in a high-frequency region, though the nonuniformity can usually have negligible effects on the thermocouple response. The present theoretical analysis provides useful information for revealing the response characteristics of a fine-wire thermocouple probe and for improving the reliability of the fluctuating temperature measurement in high-temperature turbulent flows.