Distortion in Thermal Field Around Inserted Thermocouples in Experimental Interfacial Studies, Part 4: End Effect

Abstract

The main objective of this investigation is to develop a model for predicting the systematic temperature measurement error due to the thermal disturbance in the region surrounding the thermocouple hot junction. A parametric finite element analysis has been conducted to model the general case of a three-dimensional thermocouple installation inserted in a blind hole. The variables considered in this study are the level of the heat flux in the measurement zone, as well as the thermal characteristics of the thermocouple wires, the filler material (cement), and the solid material in which the installation is placed. Analysis of the results showed that the pattern of the disturbed temperature field around the thermocouple sensing element is critically dependent on the ratio between the thermal conductivities of the filler material and the solid material. The results also showed that a reduction in the temperature gradient in the undisturbed field results in a considerable increase in the partial heat flow into the thermocouple wires, and consequently a significant systematic temperature measurement error. The effect of the eccentric positioning of the thermocouple on the uncertainty limits of the measurement error was found to be quite significant. A generalized model is presented to estimate the measurement error for any combination of the thermocouple installation attributes. Experimental verification of some aspects of this analysis has been carried out using a well-controlled experiment in which the thermocouple hole is scale-modelled. Comparison of the test results with the finite element predictions confirmed the accuracy and validity of the numerical modelling and results.