Thermographic Phosphor Thermometry Research Articles

Characterization of manganese-activated magnesium fluorogermanate with regards to thermographic phosphor thermometry

Apart from the temperature, four parameters were investigated for their impact on the phosphorescence characteristics of Mg4FGeO6:Mn with regards to phosphor thermometry: the dopant concentration, the laser pulse energy, gas compositional and pressure effects as well as irreversible changes due to heat treatments. Five specially produced phosphors with different dopant concentrations as well as commercially available Mg4FGeO6:Mn were investigated in the form of coatings and pure powder. The phosphorescence was excited by the third harmonic of a pulsed Nd:YAG laser (355 nm). The lifetime decay as well as the emission spectra of the subsequently emitted phosphorescence were determined. Generally, the decay time decreased with increasing dopant concentration, with increasing laser pulse energy and for coatings also with increasing maximum temperature and duration of heat treatments, whereas the impact of the laser power was minimized by a modified evaluation routine of the decay time. Gas compositional and pressure effects did not have a significant influence on the decay time of Mg4FGeO6:Mn. Neither the variation of the dopant concentration nor the exposure to heat treatments influenced the shape of the emission spectra in any way.

Gas compositional and pressure effects on thermographic phosphor thermometry

In the present study, the influence of gas compositional and pressure conditions on thermographic phosphor thermometry was investigated. A heatable pressurized and optical accessible calibration chamber was built to measure the phosphorescence decay time at different temperatures as well as at different partial and absolute pressures. At room temperature, the absolute pressure could be increased to 30 bar. To vary the gas composition, nitrogen, oxygen, carbon dioxide, methane, helium as well as water vapour were used. Three different phosphors were investigated: Mg4FGeO6:Mn, La2O2S:Eu and Y2O3:Eu. Phosphorescence was excited by the third and the fourth harmonics of a pulsed Nd:YAG-laser (355 nm and 266 nm, respectively) and recorded temporally resolved by a photomultiplier. Mg4FGeO6:Mn as well as La2O2S:Eu were not influenced significantly by varying partial and absolute pressures. In contrast, Y2O3:Eu showed a strong sensitivity on the oxygen concentration of the surrounding gas phase as well as irreversible changes in the phosphorescence decay time after increasing the absolute pressure.

Two-colour phosphor thermometry for surface temperature measurement

Thermographic phosphor thermometry enables surface temperatures to be measured over a wide range extending from sub 0 °C to in excess of 1400 °C by utilising the temperature-dependent luminescent properties of lanthanide-doped ceramics. The technique is particularly advantageous where surface temperatures are high due to the difficulty in applying conventional techniques and is therefore particularly well suited to the study of surface temperatures and heat transfer in gas turbines. The paper describes a two-colour implementation of the technique based on the intensity ratio of emissions from two distinct lines in the spectrum. An imaging system based on a single CCD camera has been developed and is described as are the image processing routines. YAG:Dy and YSZ:Dy phosphors were calibrated over a temperature range from 300 to 900 K with a repeatability in the data of around ±0.6%. The response of the phosphors and the performance of the imaging system were demonstrated in a transient heating/cooling study of ceramic and metallic plates.

The application of laser measurement techniques to aerospace flows

Laser measurement techniques are becoming more commonly applied to many areas of thermofluids and heat transfer. An area of their application, which presents highly challenging requirements, is in the measurement of aerospace flows. In this case the experimentalist may encounter high mass flowrates, high-speed flows and in some cases high temperatures, and the measurements may also have to be taken with restricted optical access. Such types of flow have already been measured using established laser techniques such as particle image velocimetry, laser Doppler anemometry, laser-induced fluorescence and liquid crystal thermography. Other more recent techniques which have seen application include Doppler global velocimetry and molecular tagging velocimetry for velocity measurement and thermographic phosphor thermometry for surface temperature measurement. This paper reviews the most recent advances in laser techniques and their application to aerospace environments.

Oxygen quenching of phosphorescence from thermographic phosphors

Thermographic phosphor thermometry is a technique for surface temperaturemeasurement which may be employed in the hot sections of a gas turbineallowing temperature detection up to around 1400 °Cwith uncertainties better than for other remote standard techniques such aspyrometry. The phosphors have been regarded as pressure insensitive and indeedhave been used to provide reference temperature data for the correction ofpressure-sensitive paint data. The authors wish to employ the technique in gasturbine combustors where oxygen partial pressure varies widely due to itsconsumption in the combustion reaction. An experiment was therefore conductedto confirm the pressure/oxygen insensitivity of two high-temperaturephosphors. However, this revealed a response to oxygen partial pressure thatimplies an uncertainty in temperature measurements within the primaryzone of a combustor of typically 1%. There does not appear to have beenany previous report of such a response in the literature and this notetherefore serves as a caution to those employing the thermographic phosphorthermometry technique where oxygen partial pressure varies significantly.

Thermographic phosphor thermometry for film cooling studies in gas turbine combustors

The paper describes the application of thermographic phosphors for surface temperature measurements in gas turbine combustors and on surrounding components. Thermographic phosphors enable many of the problems associated with the application of conventional thermometry techniques in a combustion environment to be overcome. They consist of a ceramic host matrix with a lanthanide ion dopant. When illuminated with UV light, they exhibit luminescence which is temperature dependent by virtue of variations in the relative intensities of distinct emission lines or in the time constant of the exponential decay that occurs once excitation has ceased. The authors have calibrated the response of a range of phosphors and demonstrated that a useful dynamic response exists up to at least 1400±C. Furthermore, to demonstrate the utility of the technique, surface temperatures have been measured over a small region within a model gas turbine combustor. In this region the temperature distribution was measured with an estimated uncertainty of ±10 K, with the absolute temperature values confirmed by thermocouple measurements within the experimental uncertainties of this technique.