DETERMINATION OF THE TIME CONSTANT OF FINE-WIRE THERMOCOUPLES FOR COMPENSATED TEMPERATURE-MEASUREMENTS IN PREMIXED TURBULENT FLAMES

A new method for the determination of thermocouple time constants is d escribed, which reproduces the large temperature differences between t he thermocouple junction and ambient fluid that occur in premixed turb ulent flame applications. Time constants measured while the thermocoup le is both heating and cooling, as well as the dependency of the time constant on velocity and temperature, are reported. Velocities are var ied from 0.6 to 5.5 ms(-1), and junction temperatures range from 700K to 1500K while heating and from 400K to 1100K while cooling. Temperatu re differences between the thermocouple junction and ambient fluid up to 800K are achieved. Radiation lasses from the junction are accounted for. Predicted time constants based on commonly used correlations for heat transfer from a cylinder in cross-flow are compared to the data, and it is suggested that free molecular flow effects are partially re sponsible for the systematic deviation of the data from the theoretica l predictions.