Improved positioning criterion and imaging scheme of deep UV planar laser-induced fluorescence technology

In summary, we have developed a physical model to describe the evolution of the nitric oxide-based planar laser-induced fluorescence technology (NO-PLIF) signal in a high-enthalpy wind tunnel and analyzed the factors that affect the contrast of fluorescent stripes obtained from two successive ultraviolet (UV) laser exposures. Based on this, we improved the flow velocity calculation algorithm with a centroiding positioning criterion, which is more accurate for flows with velocities of above 3 km/s and has better adaptability to background noises compared to the conventional peak value positioning criterion. Meanwhile, we have identified that a shortened frame interval is an effective means to improve the measurement accuracy of a hypervelocity flow, where a dual-frame high-speed UV imaging system is developed with a frame interval that is two orders of magnitude shorter than existing UV cameras. Consequently, transient N₂ flows with velocities ranging from 3.2 to 5.2 km/s were successfully measured. Further efforts will pay on extending the proposed positioning criterion to the cross-correlation algorithm for higher velocity measurement accuracy.