Influence of volumetric multi-kHz laser irradiation on soot particle measurements in laminar diffusion flames

Abstract Volumetric multi-kHz laser irradiation can affect the measurements on soot particles. The severity of the influence causing local alteration of the particle and flame properties are investigated on a laminar diffusion flame. The aim is to identify a practical fluence threshold for which no or only minor influence on soot particles due to volumetric multiple laser irradiation occurs. To this end, the three quantities of interest (QoI) of particle temperature, soot volume fraction and primary particle size are measured and evaluated via 3D-two-color-pyrometry (3D-2CP) and 3D time-resolved laser-induced incandescence (3D-TiRe-LII). As laser source, a high repetition-rate burst-mode Nd:YAG pulse laser is employed. The progression of the three QoI is investigated for different fluences (up to ~ 160 mJ/cm 2 ) and repetition rates (up to 5 kHz) for selected individual pulses within a pulse train. For the determination of practical fluence limits, a maximum change of 25% in soot volume fraction is applied as a threshold. The fluence limit is highly dependent on laser repetition rate and flow velocity of the object of interest. For irradiation with eight laser pulses with 5 kHz, a fluence limit of ~ 60 mJ/cm 2 is identified.