Investigating temporal variation in the apparent volume fraction measured by time-resolved laser-induced incandescence

In many time-resolved laser-induced incandescence (TiRe-LII) experiments, it is common practice to relate the intensity emitted by laser-heated nanoparticles to the detected LII signal through a factor (here called the intensity scaling factor, ISF) that includes the particle volume fraction and other parameters that may not be the focus of the analysis. While, in the absence of evaporation or sublimation, the ISF should theoretically remain constant with respect to time, recent multi-wavelength measurements show that, in reality, it may vary with both time and fluence. We consider four candidate effects that contribute to this behavior: particle annealing; polydispersity in the nanoparticle-size distribution; background luminosity due to emission from nanoparticles in the line-of-sight before and behind the probe volume; and the temporal resolution of the detector. We demonstrate these effects by simulating TiRe-LII data for in-flame soot at atmospheric pressure, using new simplified heat transfer and annealing models. Analysis of experimental signals collected from flame-generated soot at atmospheric pressure reveals trends in the ISF similar to those predicted by simulations. These temporal variations provide important insights that can help to diagnose problems in TiRe-LII experiments and improve TiRe-LII models.