Application of computational fluid dynamics for the analysis of the furnace effect in the determination of the temperature of high temperature fixed points

Abstract

High-temperature fixed-points (HTFPs) have been intensely studied in the last two decades. Yet despite this there are still sources of poorly characterized uncertainty one of which is colloquially known as the "furnace effect". This has been attributed to different factors, examples of which include: the lack of uniformity of the furnace temperature, the thermal inertia of the furnace, the microstructure of the fixed point material, the heat dissipation through the walls of the furnace and cell, the effective emissivity of the black body cavity and the characteristics of the radiation thermometer. However, it has been shown that, all of these are too small to explain the magnitude of the temperature drop obtained and more recent research has indicated that this uncertainty is related to the specific configuration of the furnace and cell considered, so purely a geometric phenomenon. In that work it was shown that the furnace effect was caused by the interaction between the thermal radiation from the hot furnace tube and the inside of the blackbody cavity, specifically due to the reflection of the furnace thermal radiation from the HTFP cavity sidewall. To demonstrate this several modifications were made to a specially designed Cu fixed-point blackbody, these were; a means of reducing the diameter of the blackbody aperture, incorporation of radiation shield disks and making circumferential grooves on the inner surface of the HTFP blackbody. These modifications reduced the furnace effect for the copper fixed point by around 14 mK. The aim of this paper is to apply Computational Fluid Dynamics (CFD) to the analysis of the furnace effect to better understand its mechanism and to assess how much each mitigation strategy contributes to reducing the furnace effect. The results obtained using ANSYS©, have confirmed that the introduction of appropriate improvements in the black body cavity design reduces the magnitude of the furnace effect and that the most influential factor is the reduction of the cell aperture diameter, which, incidentally, confirms that this phenomenon is mainly caused by the reflected thermal radiation from the cavity sidewall. Here it was not possible to determine realistic values for the furnace effect, mainly due to the complexity of considering the variation of the specular component of the reflectance as a function of the angle of incidence of the irradiance, nevertheless qualitative results are in line with the observations.