SIMULTANEOUS VELOCITY AND SCALAR MEASUREMENTS IN PREMIXED RECIRCULATING FLAMES

The use of a laser-Doppler velocimeter has been extended to the analys is of turbulent heat transfer in a strongly sheared disc-stabilised pr opane-air flame through its combination with either laser Rayleigh sca ttering or digitally-compensated fine-wire thermocouples. The laser ve locimeter was based on a conventional forward scattering system from t he green light of a 5W Argon-Ion laser, while the Rayleigh signals use d the blue line of the same laser. The procedure for the numeric compe nsation of the thermocouple signals included analysis of the effect of velocity and temperature on the time constant of the thermocouple and was optimised to allow combined velocity-temperature samples acquired by a purpose-built digital interference with a frequency up to 2000 H z, without deterioration of the thermocouple by particle accretion. Th e maximum effective data rate for the combined Rayleigh/LDV system is shown to be around 130 Hz, which corresponds to a data rate of valid D oppler signals around 400 Hz and statistics based on more than 15 000 measurements is made possible. The results obtained with the two syste ms agree qualitatively, although the use of thermocouples attenuates t he measured velocity-temperature correlations. The results are used to assess the extent to which turbulent mixing in flames is altered by t he accompanying heat release and quantify the processes of non-gradien t diffusion in a strongly recirculating premixed flame.