Thermal modelling and optimisation of a high temperature blackbody radiator
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Blackbody radiators, or graphite tube furnaces, are commonly used in the calibration of pyrometers for temperature range up to 3 000 °C. These radiators are usually constructed from graphite cylindrical shaped cavities insulated by graphite felt or similar materials. The calibration uncertainties associated with one of these radiators, a 48 kW Thermogage furnace, are 1 °C at 1 000 °C and a wavelength of 650 nm rising to 2 °C at 2 000 °C. These uncertainties are mainly due to deviations of the blackbody emissivity from 100%. The emissivity has been calculated to be 99.2% at a temperature of 1 000 °C and a wavelength of 650 nm, increasing to 99.9% in some cases. To improve this Thermogage furnace’s temperature calibration uncertainty to the level required, the emissivity must be increased to 99.9% over the full temperature range. This can be achieved by improving the temperature uniformity of its cavity inner walls. Therefore, the aim of this work is to achieve this emissivity increase by optimising the temperature uniformity of the blackbody furnace graphite tube. A quasi 2-D numerical model has been developed to predict the temperature profile of the Thermogage furnace’s tube. This has been used to optimise the temperature uniformity based on input parameters such as the thermophysical properties of ATJ graphite and WDF graphite felt. These thermophysical properties have been thoroughly investigated and implemented into the quasi 2-D numerical model. The numerical predictions generated have been validated by comparing them to the measured temperature profile and radial heat fluxes of the graphite tube. Once an agreement has been achieved between the measured and the modelled results, the quasi 2-D numerical model has been used to generate numerical predictions of the temperature profile based on design methodologies that include changing the cross sectional area and the length of the graphite tube as well as using different insulating gases. With a new tube design, a better temperature uniformity has been achieved and thus improvement in the cavity emissivity resulting into temperature uncertainty of better than 0.02 °C for operating temperatures from 1 000 to 1 600 °C and at a wavelength of 650 nm.
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