Overall emissivity of S-type thermocouple with spherical bead

The reading of a thermocouple is its bead temperature, which is normally not equal to the actual gas temperature because of the radiation loss and conduction loss of the bead. Since surface radiation contributes most of the heat loss of a thermocouple, which is proportional to the thermocouple emissivity, it is critical to obtain the thermocouple emissivity accurately. Due to the small size of thermocouples, it is difficult to measure the emissivity with traditional tools. The overall emissivity of a thermocouple is usually obtained by matching the measured and CFD simulated temperatures of standard flames. Of course, the overall emissivity is related to the emissivity of the wires and the bead. However, the relationship among them is unknown now, which is revealed here through CFD simulations. Under the same flow conditions, different wire emissivity and bead emissivity of an S-type thermocouple are set for the simulation, and the corresponding overall emissivity are searched to match the bead temperatures. So, the relationship among the overall emissivity, the wire emissivity, and the bead emissivity is revealed. The emissivity of the Pt wire has the most influence on the overall emissivity, and the bead emissivity has the least influence on the overall emissivity. A correlation is fitted to calculate the overall emissivity from the wire emissivity and the bead emissivity.