Velocity measurements by flow tagging employing laser enhanced ionisation and laser induced fluorescence

A new flow tagging technique for measurements of gas velocities over a wide range of flow conditions is proposed. This method employs the laser-based spectroscopic techniques of laser-enhanced ionisation (LEI) and laser induced fluorescence (LIF). Initially the flow to be studied is seeded with a small amount of sodium. The flow is then tagged by utilising LEI to deplete a substantial fraction (80%) of the neutral sodium species in a well defined region upstream of the flow. Finally, single laser pulse planar LIF is used to create a two-dimensional (2D) inverse image of the depleted tagged region downstream of the flow at a specific time delay. The feasibility of this method was demonstrated in two different types of air-acetylene flame by measuring instantaneous 2D flow structures. Flame velocities were determined with an accuracy of better than 10%.