Christopher Abram

Simultaneous temperature, mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles.

This paper presents an optical diagnostic technique based on seeded thermographic phosphor particles, which allows the simultaneous two-dimensional measurement of gas temperature, velocity and mixture fraction in turbulent flows. The particle Mie scattering signal is recorded to determine the velocity using a conventional PIV approach and the phosphorescence emission is detected to determine the tracer temperature using a two-color method. Theoretical models presented in this work show that the temperature of small tracer particles matches the gas temperature. In addition, by seeding phosphorescent particles to one stream and non-luminescent particles to the other stream, the mixture fraction can also be determined using the phosphorescence emission intensity after conditioning for temperature. The experimental technique is described in detail and a suitable phosphor is identified based on spectroscopic investigations. The joint diagnostics are demonstrated by simultaneously measuring temperature, velocity and mixture fraction in a turbulent jet heated up to 700 K. Correlated single shots are presented with a precision of 2 to 5% and an accuracy of 2%.

High-precision flow temperature imaging using ZnO thermographic phosphor tracer particles

Zinc oxide (ZnO) particles are characterised as a tracer for temperature measurements in turbulent flows, in the context of the thermographic particle image velocimetry technique. Flow measurements are used to compare the temperature precision of ZnO to that obtained using a well-characterised thermographic phosphor, BAM:Eu(2+), under the same conditions. For this two-colour, ratio-based technique the strongly temperature-dependent redshift of the luminescence emission of ZnO offers improved temperature sensitivity, and so at room temperature a threefold increase in the temperature precision is achieved. A dependence of the intensity ratio on the laser fluence is identified, and additional measurements with different laser pulse durations are used to independently show that there is also a dependence on the laser excitation irradiance, irrespective of fluence. A simple method to correct for these effects is demonstrated and sources of error are analysed in detail. Temperature images in a Re = 2000 jet of air heated to 363 K with a precision of 4 K (1.1%) are presented. The sensitivity of ZnO increases across the tested temperature range 300-500 K, so that at 500 K, using a seeding density of 2 x 10(11) particles/m(3), a precision of 3 K (0.6%) is feasible. This new phosphor extends the capabilities of this versatile technique toward the study of flows with small temperature variations.

Thermographic laser Doppler velocimetry using the phase-shifted luminescence of BAM:Eu 2+ phosphor particles for thermometry

Simultaneous point measurements of gas velocity and temperature were recently demonstrated using thermographic phosphors as tracer particles. There, continuous wave (CW) excitation was used and the spectral shift of the luminescence was detected with a two-colour intensity ratio method to determine the gas temperature. The conventional laser Doppler velocimetry (LDV) technique was employed for velocimetry. In this paper, an alternative approach to the gas temperature measurements is presented, which is instead based on the temperature-dependence of the luminescence lifetime. The phase-shift between the luminescence signal and time-modulated excitation light is evaluated for single BaMgAl10O17:Eu2+ phosphor particles as they cross the probe volume. Luminescence lifetimes evaluated in the time domain and frequency domain indicate that in these experiments, interferences from in-phase signals such as stray excitation laser light are negligible. The dependence of the phase-shift on flow temperature is characterised. In the temperature sensitive range above 700 K, precise gas temperature measurements can be obtained (8.6 K at 840 K) with this approach.

Thermographic Particle Image Velocimetry

Planar high-speed temperature and velocity measurements in gaseous flows can be performed simultaneously using micrometer-sized thermographic phosphor particles as tracers. Validation of this concept and applications in turbulent flows will be presented.