Volumetric Laser-induced Fluorescence Research Articles

High-speed two-color scanning volumetric laser-induced fluorescence

Many problems in fluid mechanics require single-shot 3D measurements of fluid flows, but are limited by available techniques. Here, we design and build a novel flexible high-speed two-color scanning volumetric laser-induced fluorescence (H2C-SVLIF) technique. The technique is readily adaptable to a range of temporal and spatial resolutions, rendering it easily applicable to a wide spectrum of experiments. The core equipment consists of a single monochrome high-speed camera and a pair of ND: YAG lasers pulsing at different wavelengths. The use of a single camera for direct 3D imaging eliminates the need for complex volume reconstruction algorithms and easily allows for the correction of distortion defects. Motivated by the large data loads that result from high-speed imaging techniques, we develop a custom, open-source, software package, which allows for real time playback with correction of perspective defects while simultaneously overlaying arbitrary 3D data. The technique is capable of simultaneous measurement of 3D velocity fields and a secondary tracer in the flow. To showcase the flexibility and adaptability of our technique, we present a set of experiments: (1) the flow past a sphere, and (2) vortices embedded in laminar pipe flow. In the first experiment, two channel measurements are taken at a resolution of 512 × 512 × 512 with volume rates of 65.1 Hz. In the second experiment, a single-color SVLIF system is integrated on a moving stage, providing imaging at 1280 × 304 × 256 with volume rates of 34.8 Hz. Although this second experiment is only single channel, it uses identical software and much of the same hardware to demonstrate the extraction of multiple information channels from single channel volumetric images.

A state-of-the-art review on laser-induced fluorescence (LIF) method with application in temperature measurement

Laser-induced fluorescence (LIF) is a robust and vigorous method for non-intrusive measurement of temperature, pressure, concentration, and pH in fluids. The application of LIF for measurement is also extendable to solid and gaseous environments. In this method, the fluorescent molecules are transferred to a higher energy state by absorbing the energy of a laser beam and subsequently undergo spontaneous emission of light in the form of fluorescence during the transition to the ground. The emitted fluorescence is commonly captured using a setup consisting of a solvent, fluorescent material, detecting cameras, lenses, and filters, which enables the determination of the thermophysical parameters of the measuring environment using pertinent mathematical formulae. This method can be used for precise measurements in a wide range of engineering and medical applications. Despite the numerous studies on the utilization of LIF for measurement, there is not a comprehensive review paper with a focus on various types of LIF techniques and their applications in temperature measurement. This paper seeks to address this gap by fully discussing these approaches, demonstrating temperature measurements in different environments and circumstances, and outlining their benefits and drawbacks. Accordingly, one-dye one-color, two-dye two-color, one-dye two-color, one-dye three-color, two-dye three-color, volumetric LIF, and encapsulation techniques are fully discussed as the main LIF approaches and their applications for the temperature measurement in fluid flows, droplets, microfluidics, and gas environments.

Three-dimensional concentration field imaging in a swirling flame via endoscopic volumetric laser-induced fluorescence at 10-kHz-rate

Three-dimensional concentration field imaging in a swirling flame via endoscopic volumetric laser-induced fluorescence at 10-kHz-rate

High-speed volumetric imaging of formaldehyde in a lifted turbulent jet flame using an acousto-optic deflector

The development of high-speed volumetric laser-induced fluorescence measurements of formaldehyde ($$\hbox {CH}_2\hbox {O}$$-LIF) using a pulse-burst laser operated at a repetition rate of $${100} \hbox { kHz}$$ is presented. A novel laser scanning system employing an acousto-optic deflector (AOD) enables quasi-4D $$\hbox {CH}_2\hbox {O}$$-LIF imaging at a scan frequency of $${10}\hbox { kHz}$$. The diagnostic capability of time-resolved volumetric imaging is demonstrated in a partially premixed DME/air lifted turbulent jet flame near the flame base. Simultaneous imaging of laser beam profiles is performed to account for the laser pulse energy fluctuation and laser sheet inhomogeneity. With the accurate registration of laser sheet positions, the volumetric reconstruction of $$\hbox {CH}_2\hbox {O}$$-LIF signals is performed within a detection volume of $$17.3 \times 11.9 \times {2.3}\, \hbox { mm}^3$$ with an average out-of-plane spatial resolution of $${250}\,\upmu \hbox {m}$$. A surface detection algorithm with adaptive thresholding is used to determine the global maximum intensity gradient by calculating gradient percentiles. The flame topology characteristics are investigated by evaluating the 3D curvatures of $$\hbox {CH}_2\hbox {O}$$ surfaces. Curvatures calculated using 2D data systematically underestimate the full 3D curvature due to the lack of out-of-plane information. The inner surfaces near the turbulent fuel jet exhibit higher probabilities of large mean curvature than the outer surfaces. The saddle and cylindrical structures are dominant on both the inner and outer surfaces and the elliptic structures occur with lower probability. The results suggest that the damping of turbulent fluctuations by the temperature increase through the $$\hbox {CH}_2\hbox {O}$$ region reduces the curvature, but the local structure topology remains self-similar.

A quantitative evaluation method of 3D flame curvature from reconstructed flame structure

Three-dimensional (3D) imaging techniques, such as computed tomography of chemiluminescence and volumetric laser-induced fluorescence, are advanced combustion diagnostic tools that can provide rich insights on the spatial distribution of combustion intermediates. Besides the flame structure that is directly reconstructed, some deep and underlying information such as the geometric features are quite useful for a complete understanding of combustion mechanisms and, therefore, is desired to be further explored. However, to the best of our knowledge, due to the concave and convex variations on flame surface and the mathematical challenges, almost no attempts have been made to experimentally calculate the 3D geometric features of a turbulent swirl flame. In this work, we propose a point-extracted method that can effectively extract a point cloud from the reconstructed turbulent swirl flame. In addition, the two principal curvatures, mean curvature and the Gaussian curvature were evaluated with the so-called triangle-mesh-based method (TMB). One main contribution of this work is to perform a comparison between TMB and direct numerical simulation for curvature evaluation. The quantities obtained by this method are expected to be valuable for the validation of combustion modeling and for the investigation of combustion mechanisms. The 3D curvatures including principle curvatures, mean curvature and Gaussian curvature were experimentally evaluated in a turbulent swirl flame for the first time. The Gaussian curvatures can be related to three typical kinds of curved surfaces, i.e. elliptical, cylindrical and saddle surfaces. For the swirl flame used in this work, the cylindrical surface is regarded as a “base flame surface”, based on which some humps called flame wrinkles would form at some positions. At the tip of each hump, the flame will present an elliptical cup-like surface, where the Gaussian curvatures are relatively large. The transition region from cylindrical surface to elliptical surface mostly manifests a saddle surface.

Spray impingement wall film breakup by wave entrainment

Fuel spray impingement on engine wall and piston in the spark-ignition direct-injection (SIDI) setting has been considered a major concern in the aspect of engine emission and combustion efficiency. Excess wall film will result in deterioration of engine friction, incomplete combustion, and substantial cycle-to-cycle variations. These effects are more pronounced during engine cold-start process. Therefore, the formation of wall film on engine wall/piston and the dynamic process of the wall film interacting with impinging spray and spray-induced gas flow are of great significance for reducing wall film mass. However, the dynamic process of wall film was not investigated thoroughly in existing literatures. This work will present a high-speed, simultaneous measurement of a single-hole spray structure, as well as wall film geometry and thickness, via Mie scattering and volumetric laser-induced fluorescence, respectively. Quantitative film thickness measurement was achieved via fluorescence intensity signal calibration with a known, wedge-shape liquid film apparatus. Remarkable wall film droplet entrainment at the leading edge of the liquid film waves was revealed in the measurement, which has not been adequately depicted or analyzed in existing spray impingement studies. A considerable amount of liquid droplets detaches from the liquid film via liquid film fingering, during which process the quantity of liquid mass on the wall is decreased. Quantitative analysis of such phenomenon is performed and we estimated that a liquid mass equivalent to 30–40% of the residual liquid film mass is detached from the liquid film via wave entrainment. Furthermore, through the comparative study of the side view of the spray and the liquid film caused by spray impingement, it is shown that non-uniform spray structure is likely the cause of liquid film wavy motions. These observations suggest that wave entrainment should be considered by numerical models and experimental designs to accurately predict spray impingement phenomenon.

Two-color volumetric laser-induced fluorescence for 3D OH and temperature fields in turbulent reacting flows.

Single-shot, two-color, volumetric laser-induced fluorescence was demonstrated for three-dimensional (3D), tomographic imaging of the structural properties of the OH radical and temperature field in a turbulent hydrogen-air flame. Two narrowband laser sources were tuned to the Q1(5) and Q1(14) transitions of the (1,0) band in the A2Σ←X2Π system and illuminated a volumetric region of the flame. Images from eight unique perspectives collected simultaneously from each of the two transitions were used to reconstruct overlapping OH fields with different Boltzmann fractions and map the 3D temperature distribution with nanosecond precision. Key strategies for minimizing sources of error, such as detector sensitivity and spatial overlap of the two fields, are discussed.

Rapid tomographic reconstruction based on machine learning for time-resolved combustion diagnostics.

Optical tomography has attracted surged research efforts recently due to the progress in both the imaging concepts and the sensor and laser technologies. The high spatial and temporal resolutions achievable by these methods provide unprecedented opportunity for diagnosis of complicated turbulent combustion. However, due to the high data throughput and the inefficiency of the prevailing iterative methods, the tomographic reconstructions which are typically conducted off-line are computationally formidable. In this work, we propose an efficient inversion method based on a machine learning algorithm, which can extract useful information from the previous reconstructions and build efficient neural networks to serve as a surrogate model to rapidly predict the reconstructions. Extreme learning machine is cited here as an example for demonstrative purpose simply due to its ease of implementation, fast learning speed, and good generalization performance. Extensive numerical studies were performed, and the results show that the new method can dramatically reduce the computational time compared with the classical iterative methods. This technique is expected to be an alternative to existing methods when sufficient training data are available. Although this work is discussed under the context of tomographic absorption spectroscopy, we expect it to be useful also to other high speed tomographic modalities such as volumetric laser-induced fluorescence and tomographic laser-induced incandescence which have been demonstrated for combustion diagnostics.

Toward real-time volumetric tomography for combustion diagnostics via dimension reduction.

Volumetric tomography for combustion diagnostics is experiencing significant progress during the past few years due to its capability of imaging evolving turbulent flows. Such capability facilitates the understanding of the mechanisms behind complicated combustion phenomena such as lean blowout, acoustic oscillations, and formation of soot particles. However, these techniques are not flawless and suffer from high computational cost which prevents them from applications where real-time reconstructions and online monitoring are necessary. In this Letter, we propose a new reconstruction method that can effectively reduce the dimension of the inversion problem, which can then be solved with a minimum computational effort. This method and a classical iterative method were tested against each other using a proof-of-concept experiment in which endoscopic computed tomography of chemiluminescence (CTC) was implemented. The results show that the proposed method can dramatically reduce the computational time and, at the same time, maintain similar reconstruction accuracy, as opposed to the classical approach. Although this Letter was discussed under the context of CTC, it can be applied universally to other modalities of volumetric tomography such as volumetric laser-induced fluorescence.

On the quantification of spatial resolution for three-dimensional computed tomography of chemiluminescence: erratum.

This erratum clarifies an accidental omission of a citation.

Kilohertz VLIF (volumetric laser induced fluorescence) measurements in a seeded free gas-phase jet in the transitionally turbulent flow regime

This paper reports the demonstration of instantaneous three-dimension (3D) measurements in turbulent flows at repetition rates up to 10 kHz using VLIF (volumetric laser induced fluorescence). The measurements were performed based on the LIF signal of iodine (I2) vapor seeded in the flow. The LIF signals of I2 vapor were generated volumetrically by a thick laser slab and then simultaneously captured by a total of seven cameras from different perspectives, based on which a 3D tomographic reconstruction was performed to obtain the 3D distribution of I2 vapor concentration. Single-shot measurements obtained in a duration of hundreds of nanoseconds (limited by the pulse duration of the excitation laser) were demonstrated in a 50 × 50 × 50 mm3 at a repetition rate up to 10 kHz. These measurements demonstrated the feasibility and potential of VLIF for resolving the 4D spatiotemporal dynamics of turbulent flows. Based on the experimental results obtained, this work also studied the VLIF signal level and its effects on the reconstruction accuracy under different the measurement conditions, illustrating the capabilities and limitations of performing high speed VLIF measurements.

On the quantification of spatial resolution for three-dimensional computed tomography of chemiluminescence.

Three-dimensional computed tomography of chemiluminescence (CTC) for combustion diagnostics is attracting a surged research interest due to recent progress in sensor technologies and reduced costs of high-speed cameras. For example, it has been applied to recover the 3D distributions of intermediate chemical species such as CH* and OH*, heat release rate, and flame topology. Although these applications were demonstrated to be successful, there are still a few drawbacks of this technique that have not be cured. For example, to the best of the authors' knowledge, all the imaging models that have been developed so far ignore the imperfections of cameras such as lens distortion and skewness. However, this will unavoidably introduce errors into the weight matrix. In addition, spatial resolution of a CTC system is a critical performance parameter. However, it has only been studied qualitatively and no quantitative quantification method is reported so far. This work aims to solve these problems by improving the imaging model and developing a method based on edge spread function for the quantification of spatial resolution. Although this work is conducted under the context of CTC for combustion diagnostics, it also provides useful insights for other tomographic modalities such as volumetric laser-induced fluorescence and tomographic laser-induced incandescence.

KHz-rate four-dimensional fluorescence tomography using an ultraviolet-tunable narrowband burst-mode optical parametric oscillator

Recent work on four-dimensional (4D) tomographic imaging of reacting and non-reacting flows has utilized fluorescence, incandescence, or scattering from flow tracers, particulates, or aerosols, typically at the harmonics of an Nd:YAG laser. This paper presents high-speed 4D, volumetric laser-induced fluorescence measurements using an ultraviolet-tunable narrowband laser source to reach electronic transitions of chemical species of interest, such as the hydroxyl radical (OH), which may exist at parts-per-million (ppm) levels. A custom injection-seeded optical parametric oscillator pumped by a high-speed burst-mode laser was used for volumetric excitation of OH fluorescence, which was then captured by a pair of quadscopes to record eight unique views simultaneously at 10 kHz using only two high-speed intensified cameras. Successful tomographic imaging using high-energy, tunable narrowband radiation from a high-speed laser source lays the foundation for spatiotemporal, multidimensional analyses of a wide range of reacting and non-reacting flows of practical interest.

Analysis of 3D combustion measurements using CH-based tomographic VLIF (volumetric laser induced fluorescence)

Recent results have experimentally demonstrated the feasibility of obtaining 3D (three-dimensional) combustion measurements using tomographic VLIF (volumetric laser-induced fluorescence), specifically of the CH radical, representing the flame surface. To elucidate the fundamental capabilities and limitations of the VLIF technique, this work reports an analysis of its performance in terms of signal level, size of the field of view (FOV) in 3D, and accuracy. Compared to the established PLIF (planar LIF) technique that uses a thin laser sheet to excite the target species in a plane, the VLIF technique uses a thick laser slab to excite the target species in a volume. As a result, the VLIF technique involves more performance metrics compared to PLIF, and the relationship between these metrics is also different from that in the PLIF technique. Therefore, both experimental and computational studies were conducted to analyze the performance metrics of VLIF. First, experiments were conducted on well-controlled flames to examine the relationship among excitation energy, signal level, and FOV in 3D. Second, based on these experimental data, numerical simulations were performed to benchmark the VLIF technique under a range of conditions. These results illustrate the relationship among signal level, 3D FOV, and accuracy and are expected to be valuable for the optimal design of the VLIF technique.

Direct comparison of two-dimensional and three-dimensional laser-induced fluorescence measurements on highly turbulent flames.

This Letter reports the first direct comparison between two-dimensional (2D) and three-dimensional (3D) laser-induced fluorescence (LIF) applied to highly turbulent flames, with the goal of experimentally illustrating the capabilities and limitations of volumetric LIF (VLIF). To accomplish these goals, planar LIF (PLIF) and VLIF measurements were simultaneously performed on turbulent flames based on the CH radical. The PLIF measurements imaged a planar cross-section of the target flames across a 2D field-of-view (FOV) of 42 mm×42 mm. The VLIF measurements imaged the same region in the target flame with a 3D FOV of 42 mm×42 mm×5 mm, with 5 mm being the thickness of the measurement volume. The VLIF signals generated in this volume were captured by five intensified cameras from different perspectives, based on which a 3D tomographic reconstruction was performed to obtain the 3D reconstruction of the CH radical (as a marker of the flame front). The PLIF measurements were then compared to a cross-section of the VLIF measurement to demonstrate the feasibility and accuracy of instantaneous 3D imaging of flame topography and flame surface area in highly turbulent flames.

Single-shot 3D flame diagnostic based on volumetric laser induced fluorescence (VLIF)

We demonstrate single-shot three-dimensional (3D) flame measurements based on volumetric laser-induced fluorescence (VLIF). For this effort, the excitation laser beam was expanded into a “slab” with a height of 40mm and thickness of 10mm to excite the CH radicals in the target flames. The volumetric LIF signals were then simultaneously captured by five intensified cameras, and the images were fed into a tomographic algorithm as inputs to obtain the 3D distribution of CH. The results show that single-shot 3D measurements with sufficient accuracy can be obtained in a volume of 9.3×9.3×32.7mm3 with an excitation pulse energy of ∼10mJ. With a total of five intensified cameras, 3D measurements were obtained with a voxel size of 0.15mm over a total of 64×64×224≈106 voxels, which can be further improved by the use of additional cameras or advanced tomographic inversion techniques. Comparison with PLIF measurements suggested that the spatial resolution of the CH-based VLIF technique to characterize the flame surface was about 0.4mm. Measurements were first performed on stable laminar flames, for comparison and validation against PLIF, and then on turbulent jet flames, to illustrate the feasibility and accuracy of VLIF for instantaneous 3D flame measurements and motivate further investigation.

Single-shot volumetric laser induced fluorescence (VLIF) measurements in turbulent flows seeded with iodine.

This work reports the experimental demonstration of single-shot visualization of turbulent flows in all three spatial dimensions (3D) based on volumetric laser induced fluorescence (VLIF). The measurements were performed based on the LIF signal of iodine (I2) vapor seeded in the flow. In contrast to established planar LIF (PLIF) technique, the VLIF technique excited the seeded I2 vapor volumetrically by a thick laser slab. The volumetric LIF signals emitted were then simultaneously collected by a total of five cameras from five different orientations, based on which a 3D tomographic reconstruction was performed to obtain the 3D distribution of the I2 vapor in the target flow. Single-shot measurements (with a measurement duration of a few ns) were demonstrated in a 50 mm × 50 mm × 50 mm volume with a nominal spatial resolution of 0.42 mm and an actual resolution of ~0.71 mm in all three dimensions (corresponding to a total of 120 × 120 × 120 voxels).