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Planar Laser-Induced Fluorescence, or PLIF, is an optical measurement technique based upon fluorescence emitted from chemical species excited by planar laser light. Essentially a sheet of laser light is passed through a flow field, and the subsequent fluorescence relaxation event is captured on a digital camera.
The presence and concentration of a species may be determined from its characteristic fluorescence when excited by a laser source. The technique is commonly used to measure instant whole-field concentration or temperature maps in liquid flows.
PLIF has been shown to be of use for a range of velocity, concentration, temperature and pressure measurements in different environments. Current applications for PLIF can be found in process engineering (mixing in stirring vessels, heating and cooling systems), biomedical engineering (transport of drugs in biological flows such as in model veins) and fluid dynamics research (turbulent mixing and heat transfer modelling, indoor climate etc.)
The process
Typically, a well-established optical absorption transition of the species to be studied is chosen and matched to the laser excitation wavelength. A narrow band filter is selected to differentiate the fluorescence in the region of characteristic wavelengths. This is associated only with the species of interest and any background signal interference is thus suppressed.
In the process the ground state atom or molecule is pumped to an excited state by a laser light sheet whose wavelength is tuned to excite a particular transition. A fraction of the ground state molecules of the species in the flow absorb the incident light and are excited to a higher electron energy state.
The excited species will after only a few nanoseconds to microseconds, de-excite and emit light at a wavelength larger than the excitation wavelength. This fluorescence is captured on a 2D CCD imaging sensor and enabes the acquisition of spatial information on the concentration and activity of the species.
Applications:
Combustion dynamics
PLIF imaging is a powerful tool for understanding fluid dynamics and combustion physics. The technique is very well-suited to applications performed at high pressures and temperatures that are relevant to real engines and gas turbines.
Recent research has involved the development of 2D and 3D time-resolved planar imaging and simultaneous measurements of multiple scalars. The data is expected to elucidate previously undetected physical processes in turbulent combustion for which new models can be expanded.
Some good examples of these models are the time-sequenced imaging of combustion instabilities in internal combustion engines, ultra-lean gas turbines and new aircraft injection systems.
Experiments involving re-ignition at high altitude with laser and plasma igniters can provide significant insight into the immensely complex flows prevailing in these combustors. Typical types of species investigated within plasmas and in particular in combustion studies include OH, CH, NO, NH, CN, CO, and O2 radicals as well as excitations within atomic and ionic species.
PLIF measurements are able to characterise the transport properties within the flame or plume, the turbulence, temperature, pressure as well as the concentrations of species.
Andor’s iStar 334T fast gated intensified CCDs series is designed to offer the ultimate integrated detection solution for high resolution, ns-scale time-resolved imaging. A series of experiments in 2009 used two intensified CCD iStar DH734-25U-03 cameras (1024 × 1024 pixels) to detect the fluorescence from excited CH radicals in flames. The multi-plane system was found to be useful for the study of 3D turbulent flame structures.
In a similar way, multi-scalar diagnostics will provide significant contributions to our understanding of turbulent combustion and, in particular, the effects of turbulent mixing on flame chemistry.
PLIF in aqueous flows
PLIF is an highly useful analytical method for analysing mixing processes even in aqueous solution. In a recent experiment using the iStar 312 CCD time resolved camera and an Nd:YAG green laser, an experiment was designed to examine and record the mixing process in a water flow using a laser sensitive rhodamine dye.
The laminar water-jet flow was established in a semi-enclosed mixing chamber using a water cross-flow loop. A green laser light sheet was directed to the centre plane of the water jet to excite the rhodamine 6G dye in the fluid.
The iStar camera collected the bright orange fluorescence signal from the dye and processed the images to provide concentration maps and show Eddy currents in the flow. The application of PLIF towards the study of aqueous flow becomes very important in pressurised water reactors (PWR).
In PWR, boron dilution is an important safety issue for reactor cooling. PLIF can be used in the visualization measurement of concentration distribution of boron in down-comer channels. In these experiments an aqueous solution of a fluorescent dye is used to simulate the boron solution.
The visual mixing phenomena are recorded using the high-speed iStar 312T camera a rhodamine B(RhB), along with a 532-nm excitation laser and cylindrical optics designed to provide a laser sheet beam. Results showed the diffusion process of boron and the concentration distribution of boron, which were important for the study of inhomogeneous boron dilution in the design and research of nuclear reactor systems.
The future of PLIF
There has been a remarkable increase in PLIF diagnostic capability over the last two decades, and the field continues to develop at a fast pace. Its capabilities go far beyond the mere observation of two-dimensional signal intensities.
The development status of PLIF has now passed the threshold between qualitative and quantitative images. In the near future, the availability of new laser sources operating at high frequency rates, detectors, and data processing schemes will allow for the acquisition of more accurate data, and measurements that could be more specifically tailored to the needs of the modelling community.
In 2016 Andor introduced the sCMOS iStar series’ state-of-the-art gating interface, which provides true <2 ns optical gating performance on a range of high QE Gen 2 and Gen 3 intensifiers. The fully integrated, triple output DDG features an ultra-low insertion delay and excellent timing accuracy down to few 10’s of picoseconds, allowing for fast plasma imaging, combustion studies.
The future of PLIF lies in two areas. Firstly, the application of two and three laser signals to provide 2 and 3D renderings of the mixing processes occurring in flame plasmas or solutions. Secondly the use of PLIF as a hyphenated technique in conjunction with complementary techniques such as particle image velocimetry (PIV). These developments will allow for the simultaneous measurement of a fluid velocity field and concentration of species in it.
References
Tingjie Zhao et al., Application of Planar Laser-Induced Fluorescence to Measurement of Concentration Field in the Downcomer, PBNC 2016: Proceedings of: The 20th Pacific Basin Nuclear Conference pp 681-690
