Simultaneous measurements of velocity and concentration of gas flow using femtosecond laser-induced chemiluminescence

The mixing process in a gas flow plays a crucial role in chemical reactions, and simultaneous measurements of both velocity and mixture fraction are desired to fathom the process. Here, we report a scheme for simultaneously measuring both velocity and concentration by femtosecond laser-induced chemiluminescence. The femtosecond laser would induce chemical reactions that generate CN radicals in the B state. The transition of CN (X-B) would emit fluorescence with both strong intensity and long duration, and the decay of the fluorescence versus time showed prominent benefits for the simultaneous measurement. This measurement was accomplished by an ICCD camera worked in the on-chip multi-exposure mode, i.e., the camera had two exposures in succession to capture two luminescent lines on one image. The first line was used to measure the methane concentration and hence, the mixture fraction through a calibration procedure. The second line was the first line displaced by the flow in a known time interval and we demonstrate an algorithm to derive the one dimensional-two components velocity fields from the line shapes of the luminescent lines. The detection limit of the concentration is estimated to be 152 ppm and the minimum measurable velocity is estimated to be 5 m/s.