The initial stage of flame development has become important with regard to promoting more stable lean/dilute combustion in spark ignition engines. Thus, a means of performing simultaneous, instantaneous measurements of the unburned gas flow and temperature to clarify phenomena related to local extinction is required. Inexpensive two-color inorganic fluorescent particles (f-particles), which the authors previously proposed for velocimetry imaging of the gas flow around solid objects, could be employed for this purpose. Two-color f-particles containing rhodamine B (RhB) and rhodamine 6G (R6G) emit fluorescence derived from each dye. The ratio of the fluorescence intensity at 577 nm to that at 550 nm shows an almost linear relationship with temperature between 333 and 473 K. However, in our previous study, the color of the f particles was found to have changed following the constant volume vessel (CVV) experiments at 473 K. This effect occurred because at least one of the two dyes in the SiO2 particles underwent pyrolysis during the 1 h trial. In contrast, the tests performed in an engine or using a heat gun to 593 K ambient for less than 2 min had no effect on the appearance of the f-particles. In the present study, thermal stability of f-particles was investigated at static ambient temperature condition using convection oven by changing storing time, temperature and species of dyes. As a result, the color of appearance and the intensity of fluorescence of the f-particles containing RhB after cooling to room temperature was unchanged when the condition was set at 473 K storing at around 2, 10, 30 and 60 min, however, these were clearly changed at 523 K storing at 60 min. In contrast, these were not unchanged in the case of the f-particles containing R6G. Therefore, the f-particles could be used at least under 523 K within 30 min.
