Thermocouple-based thermometry for laminar sooting flames: Implementation of a fast and simple methodology

This work reports the implementation and the validation of the extrapolation method for thermocouple temperature measurements corrected from the radiation losses in sooting flames. This simple method relies on the use of thermocouples having different size diameters and enables a fast and direct determination of the flame temperature by extrapolation to zero diameter. We propose here a detailed study of the possibilities and limitations offered by this method based on experimental measurements and comparison with well-established methods carried out in a laminar diffusion sooting flame. In details, a specific fast insertion setup using four different sized thermocouples has been implemented to record temperature values at different heights in the flame. From these data, we highlight that a linear calibration curve correlates the raw measured temperatures to the flame temperatures corrected of the radiation losses can be easily and rapidly obtained. The impact of soot deposition on the thermocouple on the temperature measurement is also discussed. To assess the reported thermocouple methodology, a direct comparison is made between the temperature profile determined along the vertical central axis of the flame by the extrapolation method with OH and NO LIF thermometry measurements as well as numerical simulation. Finally, we also report the comparison of experimental and simulated radial temperature profiles highlighting the adequate dynamic of the method for temperature profile determination in high temperature gradient conditions (500 K/mm). This work demonstrates that the extrapolation method is an efficient and fast method to determine accurate temperature profiles in flames, even in presence of soot particles up to a few hundred ppb, which can be useful for the development of fast and cheap sensors for either laboratory or larger-scale applications.