An Approach to Thermocouple Measurements That Reduces Uncertainties in High-Temperature Environments

Obtaining accurate temperature measurements with thermocouples in flame environments is challenging due to the effects of radiative heat losses, as these losses are difficult to quantify. Efforts to minimize radiative losses by, for example, suction pyrometry often result in a significant sacrifice in spatial resolution. In this work, a new experimental methodology is presented that both minimizes the temperature correction and allows the remaining correction to be accurately quantified. The approach is based on increasing and controlling the convective heat transfer to the thermocouple junction, which is accomplished by spinning the thermocouple at high speed. The rotation yields a large and known convective velocity over the thermocouple. Heat transfer can then be modeled for the thermocouple, and a functional relationship between temperature and rotational speed can be found. Fitting this model to the data allows for an accurate temperature correction. To test the feasibility of the rotating thermocouple technique for temperature measurement in high-temperature gases, experiments were conducted over a range of rotational speeds in a controlled flame where the temperature was known. The measured thermocouple temperatures as a function of rotational speed closely match the theoretical temperatures, yielding a straightforward approach to highly accurate gas temperature measurement. The results also demonstrate limited perturbation to the flow field, even at high rotational speeds. Finally, a method of deconvolution is described that significantly enhances the spatial resolution of the technique, approaching that of a stationary thermocouple.