Numerical evaluation of wall temperature measurement method developed to estimate thermal stress at T-junction pipe

Abstract

Thermal fatigue cracking may initiate at a T-junction pipe where high and low temperature fluids flow in from different directions and mix. Thermal stress is caused by a temperature gradient in a structure and by its variation. In this study, an experimental method was developed to estimate stress distributions at a T-junction pipe. FEM analysis and experiments to measure wall temperatures at the inner surface of the pipe with thermocouples were used in this method. The numerical simulations were performed to decide the optimum measuring points in the axial and circumferential directions for thermocouples. The numerical simulation results also showed that the attenuation of temperature amplitude and phase delay from the inner surface to the thermocouple measurement points was not negligible for 0.5 mm diameter sheathed thermocouples. A transfer function was calculated to obtain wall temperatures at the inner surface from measured data. In addition, the numerical simulation results showed that the amplitude and the phase of temperature fluctuations differed depending on existence of voids around thermocouples. These results showed that thermocouples should be installed in pipes without voids to measure accurate temperature fluctuations. It was confirmed that the voids disappeared when thermocouples were brazed in a vacuum atmosphere. Such thermocouples are expected to provide reliable experimental data from which the proper thermal stress distributions can be estimated by this method