Measurement of Fluctuating Temperature in Turbulent Flame by Dual Thermocouple Method

Abstract

For the measurement of fluctuating temperature in turbulent flame, a technique compensating the first-order lag resulting from the thermal inertia of the thermocouple is usually applied. However, in the regimes characterized by large fluctuations in temperature and velocity which lead to significant variations in heat transfer and hence in its time-constant, the former method employing averaged time-constant (mean time-const. method) can then leads to signal distortion and departure from the true gas temperature. To eliminate this defect, new technique (dual ther-mocouple method) is developed. In this technique, temperature signals (T1, T2) from two fine wire thermocouples with different diameters (d1, d2) arranged with short distance are detected and the gas temperature (Tg) is obtained from an operation of following equation.Measurements made in a propane turbulent free jet flame by dual thermocouple method and mean time-const. method are compared and discussed. Time averaged mean temperatures are almost equal in both method, however, particularly in the zone in which vigorous combustion reaction occurs and marked spatial temperature gradient exists, significant differences are observed in RMS temperature fluctuations and probability density functions.