Measurement of number densities in supersonic flows using a method based on laser-induced acetone fluorescence

A method that is based on laser-induced fluorescence (LIF) and that involves the use of acetone as a seed can be utilized for measurement of supersonic flows with high temporal and spatial resolutions. In the present study, the feasibility of using this method to measure the number density in supersonic flows at temperatures lower than the atmospheric temperature is investigated. An experiment in which low-temperature conditions are created by supersonic isentropic flows in a convergent–divergent nozzle is carried out. LIF is realized by exciting acetone with the fourth harmonic of a Nd:YAG laser, and the LIF signal is detected by a CCD camera with an image intensifier. In order to theoretically determine the LIF characteristics, a multistep decay model that can be used for temperatures of 300–900 K is extended so that it can be used in the low-temperature regime. The constant included in the model is redetermined by fitting the model to the present experimental data. The LIF calculated by using this extended model is found to agree very well with the experimental data. The model also indicates that the number density measured by the LIF method in the temperature range 170–290 K is insensitive to temperature.