A Correction Method For Gas Temperature Measurement With Thermocouple

Thermocouple is the most used tool in high temperature gas temperature measurement.However,thermocouple reading(thermocouple bead temperature)is always lower than the gas temperature due to heat transfer effects.It is necessary to correct the thermocouple measurement temperature to get the real gas temperature.The bead temperature of the thermocouple is determined by the energy balance of the bead.The thermocouple heat loss includes the convection and radiation between bead and high temperature gas,the radiation between bead and environment and the conduction between bead and thermocouple wire.These heat transfer modes together determine the temperature difference between bead temperature and gas temperature.By accurately calculating these heat transfer rate,the true gas temperature can be obtained from the measured temperature.The main contents of the paper include the experimental measurement,the 3D CFD simulation and 1D simplified heat transfer simulation.Five S type thermocouples with different sizes are used to measure the standard flames(the gas composition,temperature and velocity are known)to compare the gas temperature and the measured temperature.The three-dimensional CFD simulations are conducted to study the heat transfer process of the thermocouple bead and wire of the thermocouple in the standard flames.The influence of the heat transfers mode on the bead temperature is analyzed.A one-dimensional numerical simulation method is developed based on energy balance.The accuracy of the one-dimensional simulation program is verified with the CFD results and the measurement results.The one-dimensional simulation program can calculate the bead temperature based on the gas temperature or calculate the gas temperature based on the bead temperature.The accuracy of the convection heat transfer rate of the thermocouple bead and the thermocouple wire with the gas depends on the accuracy of the convection correlation.The traditional sphere and cylinder convection correlations cover a broad range of Reynolds number,so their accuracy for small Reynolds number might be not good.Based on the CFD results,better correlations are obtained for small Reynolds number.The new correlation will improve the one-dimensional simulation accuracy.The surface emissivity of thermocouple bead and wire reported in the literature are quite different,and they are related to the surface roughness,which is related to the manufacturing process.The thermocouple surface emissivity is obtained using the one-dimensional simulation result.According to the simulation data and experimental data,the influence of the conduction,gas radiation,and bead shape on bead temperature is quantitatively analyzed.Finally,the paper gives suggestions on how to accurately measure the temperature of hot combustion gas.