4D spatiotemporal evolution of combustion intermediates in turbulent flames using burst-mode volumetric laser-induced fluorescence

Simultaneous two-plane flame front detection using PIV based on defocusing

This Letter presents a simultaneous two-plane flame front detection method. It is based on a standard single-camera single-plane particle image velocimetry (PIV) system in combination with an inexpensive and compact image splitting device. The image splitting optics places images from two depth-offset planes onto the two halves of a camera sensor. A shallow depth of field ensures only one plane is in focus on each half of the sensor. By using a high-pass filter and a novel two-step filter we have devised, the out-of-focus particle images are effectively removed, while the in-focus particle images remain, allowing the turbulent flame fronts on two planes to be detected simultaneously. Our approach could be combined with conventional polarization/wavelength discrimination methods to achieve simultaneous multi-plane flame front reconstruction with similarly high in-plane spatial resolution to single-plane measurement and is suitable for practical combustion devices with limited optical access.

Correction procedure for a tomographic optical setup employing imaging fiber bundles and intensified cameras

For reliable tomographic measurements the underlying 2D images from different viewing angles must be matched in terms of signal detection characteristics. Non-linearity effects introduced by intensified cameras and spatial intensity variations induced from inhomogeneous transmission of the optical setup can lead, if not corrected, to a biased tomographic reconstruction result. This paper presents a complete correction procedure consisting of a combination of a non-linearity and flatfield correction for a tomographic optical setup employing imaging fiber bundles and four intensified cameras. Influencing parameters on the camera non-linearity are investigated and discussed. The correction procedure is applied to 3D temperature measurements by two-color pyrometry and compared to results without correction. The present paper may serve as a guideline for an appropriate correction procedure for any type of measurement involving optical tomography and intensified cameras.

Advances in imaging of chemically reacting flows

Many important chemically reacting systems are inherently multi-dimensional with spatial and temporal variations in the thermochemical state, which can be strongly coupled to interactions with transport processes. Fundamental insights into these systems require multi-dimensional measurements of the thermochemical state as well as fluid dynamics quantities. Laser-based imaging diagnostics provide spatially and temporally resolved measurements that help address this need. The state of the art in imaging diagnostics is continually progressing with the goal of attaining simultaneous multi-parameter measurements that capture transient processes, particularly those that lead to stochastic events, such as localized extinction in turbulent combustion. Development efforts in imaging diagnostics benefit from advances in laser and detector technology. This article provides a perspective on the progression of increasing dimensionality of laser-based imaging diagnostics and highlights the evolution from single-point measurements to 1D and 2D multi-parameter imaging and 3D high-speed imaging. This evolution is demonstrated using highlights of laser-based imaging techniques in combustion science research as an exemplar of a complex multi-dimensional chemically reacting system with chemistry-transport coupling. Imaging diagnostics impact basic research in other chemically reacting systems as well, such as measurements of near-surface gases in heterogeneous catalysis. The expanding dimensionality of imaging diagnostics leads to larger and more complex datasets that require increasingly demanding approaches to data analysis and provide opportunities for increased collaboration between experimental and computational researchers in tackling these challenges.

Computed tomography of chemiluminescence for the measurements of flames confined within a cylindrical glass

Computed tomography of chemiluminescence (CTC) is one kind of volumetric tomography which can recover 3D flame structures and has found extensive applications for spatiotemporally resolved measurements of flames. However, the existing CTC techniques rely on the pinhole model and fail when the flames are confined within a cylindrical glass due to image distortion caused by the refraction on both the internal and external surfaces of the glass. In this work, a refined camera model was developed by combining the pinhole camera model with Snell's laws using a reverse ray-tracing method to incorporate the effects of refraction. A proof-of-concept demonstration of CTC based on the refined camera model was conducted on a swirl flame confined within a 20-mm-thick K9 glass. The results proved the superiority of such technique against the existing version in terms of reconstruction accuracy. This work is expected to be especially useful for the study of combustion phenomena such as combustion instability for which the flames are typically confined within cylindrical combustors.

Rayleigh-scattering-based two-dimensional temperature measurement at 100-kHz frequency in a reacting flow

Two-dimensional, Rayleigh-scattering-based temperature measurements utilizing a turbulent jet flame were performed in this study at 100-kHz frequency. This tenfold increase in measurement speed-compared to the 10-kHz frequency considered previously-facilitated identification and tracking of several highly dynamic flow features. Findings of this study demonstrate that flow-feature dynamics become uncorrelated qualitatively and quantitatively prior to an elapse of 100 μs between successive measurements, thereby necessitating the temperature-measurement frequency to exceed 10 kHz. At the proposed 100-kHz measurement frequency, resolution of the Taylor microscale and integral scales have been demonstrated in both space and time for this flow.

Comparison of femtosecond and nanosecond two-photon-absorption laser-induced fluorescence of krypton

Two-photon-absorption laser-induced fluorescence of Kr was explored using both nanosecond- and femtosecond-duration laser excitation sources. Fluorescence signals following two-photon excitation at two wavelengths (212.56 nm and 214.77 nm) were compared while varying laser pulse duration, energy, and excitation wavelength as well as pressure and Kr mole fraction in mixtures with nitrogen. Our findings show that stronger fluorescence was observed when the excitation wavelength was tuned to 212.56 nm, regardless of the excitation-pulse duration. Moreover, an approximate 100-fold signal enhancement from nanosecond excitation (∼3  mJ/pulse, 10 ns duration) was observed as compared to femtosecond excitation (∼6  μJ/pulse, 90 fs duration).

10 kHz simultaneous PIV/PLIF study of the diffusion flame response to periodic acoustic forcing

Response of a laminar diffusion dimethyl-ether flame forced by an acoustic field is provided. A forcing frequency of 100 Hz, which is chosen based on the typical thermo-acoustic instability frequency in a practical combustor, is applied to the flame at a Reynolds number of 250. The development of the forced vortical structures present in this flame has been investigated utilizing a burst mode laser with a repetition rate of 10 kHz. Flame/vortex interaction is visualized by planar laser-induced fluorescence (PLIF) of formaldehyde, which is used to identify the early-stage fuel decomposition in the flame. The flame structure is also correlated with the velocity field, which is obtained utilizing particle imaging velocimetry (PIV). The resulting phase-resolved and time-averaged velocity and vortex images indicate that the amplitude of excitation has pronounced effects on the flame via modifying the local heat release.

Experimental investigation on an acoustically forced flame with simultaneous high-speed LII and stereo PIV at 20 kHz

An ethylene-air diffusion flame was acoustically forced with a frequency of 100 Hz at four amplitudes ranging from 40% to 140%. The average bulk velocity of the fuel was 0.6 m/s. The soot distribution and velocity fields were measured by simultaneous two-dimensional laser-induced incandescence (LII) and stereo particle image velocimetry (PIV) at 20 kHz laser repetition rate. The LII signal was calibrated by pulse-to-pulse laser energy variation, and it was observed that the soot regions extended along the central axis of the flame and shrank radially under acoustic forcing compared with the steady flame. The volume fraction of soot in the acoustically forced flame decreased with increased acoustic driving. In addition, the PIV results revealed that the resident time was strongly associated with the formation of an oval-shaped soot region, which was induced by external acoustic forcing.

3D tomography reconstruction improved by integrating view registration

Tomographic measurements involve two steps: view registration (VR) to determine the orientation of the projections and the subsequent tomography reconstruction. Therefore, the practical error in both steps impacts the overall accuracy of the final tomographic measurements. Past work treated these two steps separately. This work shows that the overall tomography accuracy can be enhanced substantially if these two steps are considered holistically because there is an opportunity for each step to leverage the information in the other step to improve the overall accuracy if they are considered holistically. Based on this recognition, this work has developed a new method called the reconstruction integration view registration (RIVR) method to implement such a holistic scheme. The key of this implementation involved the use of the Metropolis criterion to adjust the initial orientation provided by the traditional VR process dynamically. Both controlled experiments and accompanying numerical analyses were conducted to validate the RIVR method. Two sets of controlled experiments were conducted and analyzed, including a static uniform dye solution and turbulent flows, where the RIVR technique was demonstrated to significantly reduce the overall reconstruction error (by ∼37% and ∼35%, respectively) compared to past methods that treated VR and tomography separately.

High-repetition-rate burst-mode-laser diagnostics of an unconfined lean premixed swirling flame under external acoustic excitation

Lean premixed swirling flames are important in practical combustors, but a commonly encountered problem of practical swirl combustors is thermo-acoustic instability, which may cause internal structure damage to combustors. In this research, a high-repetition-rate burst-mode laser is used for simultaneous particle image velocimetry and planar laser-induced fluorescence measurement in an unconfined acoustically excited swirl burner. The time-resolved flow field and transient flame response to the acoustic perturbation are visualized at 20 kHz, offering insight into the heat release rate oscillation. The premixed mixture flow rate and acoustic modulation are varied to study the effects of Reynolds number, Strouhal number, and acoustic modulation amplitude on the swirling flame. The results suggest that the Strouhal number has a notable effect on the periodic movements of the inner recirculation zone and swirling flame configuration.

Numerical and experimental validation of a single-camera 3D velocimetry based on endoscopic tomography

Tomographic velocimetry as a 3D technique has attracted substantial research interests in recent years due to the pressing need for investigations of complex turbulent flows, which are inherently inhomogeneous. However, tomographic velocimetry usually suffers from high experimental costs, especially due to the formidable expenses of multiple high-speed cameras and the excitation laser source. To overcome this limitation, a cost-effective technique called endoscopic tomographic velocimetry has been developed in this work. As a single-camera system, nine projections of the target 3D luminous field at consecutive time instants can be registered from different orientations with one camera and customized fiber bundles, while this is possible only with the same number of cameras in a classical tomographic velocimetry system. Extensive numerical simulations were conducted with three inversion algorithms and two velocity calculation methods. According to RMS error analysis, it was found that the algebraic reconstruction technique outperformed the other two inversion algorithms, and the 3D optical flow method exhibited a better performance than cross correlation in terms of both accuracy and noise immunity. Proof-of-concept experiments were also performed to validate our developed system. The results suggested that an average reconstruction error of the artificially generated 3D velocity field was less than 6%, indicating good performance of the velocimetry system. Although this technique was demonstrated by reconstructing continuous luminous fields, it can be easily extended to discrete ones, which are typically adopted in particle image velocimetry. This technique is promising not only for flow diagnostics but other research areas such as biomedical imaging.

Assessment of plenoptic imaging for reconstruction of 3D discrete and continuous luminous fields

Volumetric tomography has become an indispensable tool for flow diagnostics. However, it usually suffers from high experimental costs as multiple cameras are required in a typical tomographic system. Plenoptic imaging (PI) is a promising alternative which can simultaneously record spatial and angular information using only one single camera. Although PI has been pioneered by a few groups for 3D flow imaging, this particular application is still at its early stage of development and there are some aspects that need further investigation. In this work, we will systematically assess three representative tomographic algorithms for PI via numerical studies. In addition, we show here how 3D PI inversion can be interpreted from a tomographic perspective and how to conveniently perform the calibration with an existing well-established method which can take into account the effect of lens distortion. A proof-of-concept experiment was also conducted, and the conclusions drawn were consistent with those from numerical studies. Although this work was discussed under the context of flow/flame imaging, the general conclusions are also applicable to other application fields, such as biomedical imaging.

Hybrid diagnostic for optimizing domain size and resolution of 3D measurements

This Letter reports a hybrid three-dimensional (3D) visualization approach for turbulent flows at the kilohertz range. The approach, named scanning volumetric laser induced fluorescence (SVLIF), combines 3D tomography with scanning to significantly enhance spatial resolution of 3D measurements in a given domain (or equivalently, to enlarge the domain size under a given resolution) compared to past tomographic approaches. The SVLIF technique (1) divides a large measurement domain into smaller sub-domains, (2) performs 3D tomographic measurement in each sub-domain by scanning the excitation laser pulses across them consecutively, and (3) combines the measurements in all sub-domains to form a final measurement. This hybrid approach enables the conversion of temporal resolution into spatial resolution or domain size to optimize 3D measurements in a wider design space. In this work, the SVLIF was demonstrated and validated at a scanning rate of 1.86 kHz in a volume of 38.4  mm×26.5  mm×25.2  mm with 7.1 million voxels, representing a ∼5 times enhancement in the number of voxels or the domain size compared to past tomographic techniques.

Time-resolved measurements of a swirl flame at 4 kHz via computed tomography of chemiluminescence

Computed tomography of a chemiluminescence (CTC) system was implemented to provide time-resolved 3D measurements of an unconfined turbulent swirl flame. This system was designed in a cost-effective manner and employed three customized view registration assemblies to simultaneously capture eight projections of the target flame at a repetition rate of 4 kHz. Both time-resolved and time-averaged tomographic reconstructions were performed based on data acquired for a duration of 250 ms. Both qualitative and quantitative validations suggested the correctness of our implementation. The time-resolved instantaneous reconstructions successfully captured the evolution of the structural features of the swirl flame such as local extinctions and the helical mode. Based on the reconstructions, the centroids of chemiluminescence for all the layers were calculated. The trajectory of these centroids provided insights into the flow motion and suggested a rotating helical structure of the swirl flame. These results demonstrated the feasibility of resolving the dynamics of turbulent swirl flames with a kHz temporal resolution using the relatively inexpensive CTC system.

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.