An Investigation on Thermocouple-Based Temperature Measurements in Sooting Flames

Temperature is an important parameter in flame chemical structure calculations. However, accurate temperature measurements are challenging to obtain. This manuscript reports on temperature measurements in sooting ethyl-benzene flames. Measurements were conducted with an array of four decreasing size thermocouples. However, to minimize disturbance of the flame by the insertion of the insulated multi-thermocouple array, the four thermocouples were also inserted bare one at a time, and results were contrasted. In both cases, radiative heat transfer was accounted for by using the well-known Nichols method [1]. This method was somewhat modified, as the extrapolation to zero thermocouple bead size was done using third-order polynomials. Furthermore, as soot accumulates on the thermocouple beads, upon their insertion in sooting flames, the obtained signals were scrutinized to determine the point in their time-history that is appropriate for this analysis. The zero time extrapolation technique, as suggested by McEnally et al. [2], was used to correct for soot accumulation at the tip of the thermocouples. Other methods were also explored, where theoretical expressions were used with corrections applied for the radiative heat losses from the thermocouples either prior or after the accumulation of soot on the thermocouple bead. Results showed that higher temperatures were recorded when single bare thermocouples were inserted into the flame as compared to inserting four thermocouples together. The difference was attributed to the disturbance of the flow-and temperature-fields in the flame, especially by the ceramic sheath that holds thermocouples together, and the thermal interaction of the thermocouples when they were inserted into the flame simultaneously. Results also showed that a combination of modified Nichols’ method with McEnallys’ corrections for sooting flames is a preferred technique as it nearly eliminates assumptions. Resulting temperature values are supported by theoretical calculations with judicious assumptions on important parameters.