LIF

Laser Induced Fluorescence (LIF)

Specific molecules can become electronically excited by absorbing laser light at specific wavelengths causing them to fluoresce. LIF uses these principles to take measurements in internal combustion engines. By adding a tracer to the fuel or intake (which is not effected by the combustion process) the molecules of the tracer fluoresce when exposed to laser light at certain wavelengths, this can be used to make flow and fuel dispersion measurements. LIF has a wide variety of applications such as visualisation and species concentration, in addition LIF may be used to take quantitative measurements of temperature, velocity and pressure.

Planar Laser Induced Fluorescence (PLIF)

By expanding the beam into a light sheet, 2-D spatial measurements can be carried out. The diagram below describes a PLIF experiment to measure gasoline distribution measurements in a spark ignition engine. Instead of directly viewing the gasoline a fuel dopant is used, selected according to certain criteria. Here the dopant Biacetyl is used.

Dopants are selected according to their fluorescent properties for visualisation purposes these are, good absorption of the laser wavelength, quantum yield, low quenching by oxygen, sufficient red-shift (Stokes-shift), similar boiling point and solubility in fuel. The large stokes shift is required to separate the fluorescent signal from the elastically scattered stray light. Dopant with a similar boiling point to fuel is to ensure that the dopant vaporises with the fuel and is a good tracer in both liquid and gas phases. Here the wavelength used for LIF excitation is 355nm (near ultra-violet).

The following pictures are typical flow visualisation images, showing swirl and tumble flow patterns taken using the PLIF method.

Ref: [9],[10],[E]

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