Significance Laser-induced fluorescence (LIF) can be used to perform non-intrusive, in-situ, and temporally and spatially resolved measurements of combustion characteristics with strong species selectivity and good sensitivity. This paper reviews research progress in the development and application of multi-species planar LIF (PLIF), tracer PLIF and PLIF-based velocimetry in combustion diagnostics to measure instantaneous flame structure, fuel concentration, temperature, and velocity. This work also discusses the typical examples, characteristics, and challenges of conducting PLIF in fundamental combustion diagnostics and practical engine measurements. The technological trends about high-repetition PLIF, volumetric LIF (VLIF), and simultaneous multi-parameter measurements are also presented.

Progress The PLIF can visualize the two-dimensional distributions of multi-species generated during the combustion process and show the instantaneous flame structure. The formaldehyde (CH₂O) can be used as an indicator of the flame preheating zone, and OH radicals can be regarded as a flame marker of the product zone. Simultaneous PLIF measurements of the CH₂O and OH can obtain the heat release zone of a premixed turbulent flame shown in Fig.6(a). Fig.6(b) and Fig.6(c) show CH₂O and OH PLIF images that are acquired simultaneously as well as the distributions of the heat release zone that are obtained by the pixel-by-pixel product of OH and CH₂O. CH radicals in flames are usually employed to indicate the flame reaction zone. The CH radicals are difficult to be measured by PLIF due to their relatively low concentration in flames. The signal-to-noise ratio of the CH PLIF can be significantly improved by using a high-energy tunable Alexandrite laser at ~387 nm and the C-X excitation at ~314 nm. The distributed reaction zone can be identified in a high-speed jet flame by broadening CH distributions that can be observed by the CH-PLIF. The HCO and CH₃ radicals that indicate instantaneous flame structure can be measured by single-shot HCO PLIF and photofragmentation LIF, and the two-dimensional distributions of HCO and CH₃are shown in Fig. 6(c) and Fig. 6(d), respectively. The OH, CH, and CH₂O PLIF can be used to obtain instantaneous flame structures in a cavity-based scramjet combustor, which helps to better understand the flameholding modes and mechanism in a supersonic flow. Feature extraction of the turbulent flame front, the flame surface density, the progressive variable, and the ridge can be achieved from the PLIF images, which gives quantitative information of the flame structure and sheds light on interactions between combustion and turbulence.

The tracer PLIF can measure fuel concentration, equivalence ratio, and temperature during the mixing process by adding fluorescent tracers into small-scale burners or practical combustion systems. Characteristics of the frequently used tracer molecules are described, such as acetone, 3-Pentanone, toluene, and NO. Typical applications of the tracer PLIF in showing the fuel distribution, equivalence ratio, and temperature distribution during the mixing process of different engines are introduced. The two-line atomic fluorescence (TLAF) can be used to indicate the two-dimensional distribution of the flame temperature by seeding atomic elements into a flame. Temperature characteristics of typical atomic elements (e.g. gallium, indium, and thallium) are compared, and different seeding methods for the indium atom are introduced. Linear and nonlinear TLAF methods with the indium atom seeding can be used to show two-dimensional distributions of the flame temperature. Nonlinear TLAF is a promising technique for two-dimensional temperature measurements in a harsh environment with an acceptable signal-to-noise ratio. The challenges of conducting the tracer PLIF in quantitative measurements are presented. Accurate calibrations of the fluorescence intensity in different conditions of temperature and pressure play a key role in the quantitative measurements of the tracer PLIF and TLAF techniques.

The PLIF techniques can be used in molecular tagging velocimetry (MTV) to non-intrusively measure the velocity distribution of the flow field. In the MTV technique, a ‘write’ laser pulse is employed to generate flow tracer (e.g. NO, Kr and OH) with a relatively long-lifetime fluorescence through the process of photodissociation, excitation, or photochemical reaction, and then a ‘read’ laser pulse is used to tag the location displacement and the delay time of the tracers. The NO PLIF, Kr PLIF, and OH PLIF are usually adopted during the ‘read’ process of the MTV technique. The air photolysis and recombination tracking (APART)/vibrationally excited NO monitoring (VENOM), krypton tagging velocimetry (KTV), and hydroxyl tagging velocimetry (HTV) have been widely used in measuring the velocity distribution in a cold or reacting flow ranging from low to hypersonic velocity.

Conclusion and Prospect LIF is a non-intrusive and in-situ technique, which can be used to accurately measure instantaneous flame structure, fuel concentration, temperature, and velocity of flames and engine combustion. The repetition rate and measurement dimension of LIF techniques are required to be further improved. With the development of high-energy and high-repetition pulsed lasers, the high-speed PLIF technology (10-1000 kHz) can show the dynamic evolution of instantaneous flame structure during the process of flame ignition, flameout, and combustion oscillation. The VLIF technology can be employed to demonstrate the three-dimensional structure of the flame and realize four-dimensional (three-dimensional+t) measurements in combination with a high-speed laser. The PLIF can be synchronized with PIV, Rayleigh scattering, and other techniques to realize the simultaneous visualization of instantaneous flame structure, flow velocity, and flame temperature, which helps to further reveal the interaction mechanism of combustion and turbulence.

The applications of the PLIF techniques in practical engines need to solve many problems, such as complex optical path adjustment and optical window design, stray light suppression, and signal-to-noise ratio optimization. The combustion information obtained by the PLIF techniques is still limited. The PLIF techniques need to be combined with other measurement methods, theoretical models, and numerical simulations to better understand the characteristics and mechanisms of combustion.