Peculiarities of Pyrometry in Particulate-Generating Flames

The pyrometry technique is a powerful diagnostic tool widely used in combustion to retrieve information on concentration and temperature of emitting particles. Thanks to the use of minimal optical access, fast and cheap qualitative and quantitative measurements can be carried out with high temporal and spatial resolution. It is widely applied to characterize carbon particles produced in hydrocarbon flames as well as nanoxide in synthesis flame. The technique is essentially based on the detection of light emission from particles in flame, assumed to be in equilibrium with the gas and at a uniform temperature in the probe volume. In this work, a synthesis flame is considered and the evidence of non-isothermality is proved, that means the coexistence of hot and cold particles in the probe volume. This coexistence strongly affects pyrometry measurements, both in terms of the effective temperature of the system under analysis and in terms of the concentration of the emitting particles. The non-isothermality is evidenced by comparing light emission from titania nanoparticles produced in a flame spray with that obtained irradiating the flame with a pulsed laser. In order to interpret the experimental results, a thought experiment is performed to model the possible occurrence in the probe volume of different percentages of particles with varying temperature. The results here presented are also discussed considering the implications of the pyrometry measurements to measure carbon particles in hydrocarbon flames.