Probe and optical diagnostics for scalar measurements in premixed flames

Procedures which allow the measurement of scalars in premixed recirculating flames using digitally-compensated thermocouples or, alternatively, laser-Rayleigh scattering, are described together with the error sources associated with the use of these techniques. The procedure for the numeric compensation of the thermocouple signals included experimental data on the characteristics of the effect of velocity and temperature on the time constant of the thermocouple and was optimised to guarantee the temporal resolution of the system. The Rayleigh system included its calibration in a propane/air flame and was optimised to avoid contamination of Mie scattering from small particles. The effect of the photomultiplier shot noise on the Rayleigh signal is quantified and used to compensate the results. The two techniques are shown to give rise to results in close agreement and to be suitable to characterise the turbulence nature of strongly recirculating premixed flames.