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

OH planar laser-induced fluorescence imaging system using a kilohertz-rate 283 nm UV Ti:sapphire laser

A narrow linewidth Ti:sapphire laser is developed and characterized for the generation of an ultraviolet nanosecond laser pulses for the planar laser-induced fluorescence (PLIF) imaging of hydroxyl (OH). With a pump power of 11.4 W at 1 kHz, the Ti:sapphire laser produces 3.5 mJ at 849 nm with pulse duration of 17 ns and achieves a conversion efficiency of 28.2%. Accordingly, its third-harmonic generation outputs 0.56 mJ at 283 nm in BBO with type I phase match. An OH PLIF imaging system has been built; a 1 to 4 kHz fluorescent image of OH of a propane Bunsen burner has been captured based on this laser system.

Algorithm optimization of cross-interfaces computed tomography into full field

Tomographic approaches in confined space require advanced imaging algorithms that can properly consider the refractive distortion as the imaging rays pass through the optical wall. Our previous work established an algorithm (cross-interfaces computed tomography, CICT) and practically solved tomographic problems in confined space. However, critical restriction was found in CICT, which is that images simulated at small azimuth angles are contaminated with noticeable signal loss and become unusable. Based on this recognition, this work has developed an improved tomography approach, namely, full-field cross-interfaces computed tomography (FCICT), to extend the available view angles to all perspectives. The key to this approach involves the 3D domain discretization using voxel parallelepipeds instead of traditional voxel layers to establish the ray-tracing relationship between imaging planes and the measurement domain. The imaging process of FCICT is first validated by quantitatively comparing the grid imaging locations in measured and simulated projections of a calibration plate. By evenly distributing the view angles in the whole azimuth angle range, the FCICT reconstruction is then numerically validated by reconstructing a simulated double-cone flame phantom. The reconstruction presents a high correlation coefficient of ${\sim}{98}\%$ with the original phantom. Finally, the FCICT is employed to reconstruct an ethylene-air premixed flame. Comparisons show that re-projections generated by the FCICT reconstruction are in accordance with measured flame images, with the mean correlation coefficients of more than 95%.

4D temperature measurements using tomographic two-color pyrometry

This work presents a new approach for high-speed four-dimensional (3D + t) thermometry using only two high-speed cameras which are equipped with different band pass filters to capture thermal radiation signals at two narrow wavelength bands. With the help of a customized fiber bundle and a beam splitter, a total number of nine projections at each band were recorded, and the temperature distribution was evaluated by tomographic two-color pyrometry. In order to validate the effectiveness of this method, the 3D temperature distribution of a premixed steady flat flame was evaluated. The determined temperatures were compared to those of other studies, as well as to the results from inverse Abel transform and line-of-sight data. Further, the 3D temperature evolution of a weakly turbulent diffusion flame was observed at a repetition rate of 7.5 kHz. Such 4D temperature measurements are expected to be valuable in understanding turbulent combustion mechanisms especially of practical devices.

Spatial-frequency encoded imaging of multangular and multispectral images

Modern imaging techniques increasingly require signals to be collected from multiple viewpoints and spectral bands to realize multi-dimensional and multi-species detections. For this purpose, multiple cameras are commonly required. Each camera collects signals from one viewpoint or one spectral band, resulting in a considerable experimental cost. Based on frequency modulation, this work proposes an encoded-imaging technique that can record multangular and multispectral images in one acquisition. The signals recorded from different viewpoints and spectral bands are superimposed in the spatial domain, while being separate in the frequency domain. This allows us to extract individual images based on their respective frequency components. In this work, a proof-of-concept experiment was conducted. The high correlation coefficient between the superimposition of the extracted images and a normal superimposed image demonstrates the effectiveness of this technique. In addition, an improved mathematical formulation was proposed to describe the higher spatial-frequency components, which were considered merely to be residual lines in previous studies. The proposed encoded-imaging technique may have potential for multangular and multispectral imaging, which is especially useful for tomographic reconstructions.

Development and validation of a reconstruction approach for three-dimensional confined-space tomography problems

This work reports the development and validation of a new tomography approach, termed cross-interfaces computed tomography (CICT), to address confined-space tomography problems. Many practical tomography problems require imaging through optical walls, which may encounter light refractions that seriously influence the imaging process and deteriorate the three-dimensional (3D) reconstruction. Past efforts have primarily focused on developing open-space tomography algorithms, but these algorithms are not extendable to confined-space problems unless the imaging process from the 3D target and its line-of-sight two-dimensional (2D) images (defined as "projections") is properly adjusted. The CICT approach is therefore proposed in this work to establish an algorithm describing the mapping relationship between the optical signal field of the target and its projections. The CICT imaging algorithm is first validated by quantitatively comparing measured and simulated projections of a calibration plate through an optical cylinder. Then the CICT reconstruction is numerically and experimentally validated using a simulated flame phantom and a laminar cone flame, respectively. Compared to reconstructions formed by traditional open-space tomography, the CICT approach is demonstrated to be capable of resolving confined-space problems with significantly improved accuracy.

Data-driven three-dimensional super-resolution imaging of a turbulent jet flame using a generative adversarial network

Three-dimensional (3D) computed tomography (CT) is becoming a well-established tool for turbulent combustion diagnostics. However, the 3D CT technique suffers from contradictory demands of spatial resolution and domain size. This work therefore reports a data-driven 3D super-resolution approach to enhance the spatial resolution by two times along each spatial direction. The approach, named 3D super-resolution generative adversarial network (3D-SR-GAN), builds a generator and a discriminator network to learn the topographic information and infer high-resolution 3D turbulent flame structure with a given low-resolution counterpart. This work uses numerically simulated 3D turbulent jet flame structures as training data to update model parameters of the GAN network. Extensive performance evaluations are then conducted to show the superiority of the proposed 3D-SR-GAN network, compared with other direct interpolation methods. The results show that a convincing super-resolution (SR) operation with the overall error of ∼4% and the peak signal-to-noise ratio of 37 dB can be reached with an upscaling factor of 2, representing an eight times enhancement of the total voxel number. Moreover, the trained network can predict the SR structure of the jet flame with a different Reynolds number without retraining the network parameters.

Two-Dimensional Tomographic Simultaneous Multispecies Visualization—Part II: Reconstruction Accuracy

Recently we demonstrated the simultaneous detection of the chemiluminescence of the radicals OH* (310 nm) and CH* (430 nm), as well as the thermal radiation of soot in laminar and turbulent methane/air diffusion flames. As expected, a strong spatial and temporal coupling of OH* and CH* in laminar and moderate turbulent flames was observed. Taking advantage of this coupling, multispecies tomography enables us to quantify the reconstruction quality completely independent of any phantom studies by simply utilizing the reconstructed distribution of both species. This is especially important in turbulent flames, where it is difficult to separate measurement noise from turbulent fluctuations. It is shown that reconstruction methods based on Tikhonov regularization should be preferred over the widely used algebraic reconstruction technique (ART) and multiplicative algebraic reconstruction techniques (MART), especially for high-speed imaging or generally in the limit of low signal-to-noise ratio.

Two-Dimensional Tomographic Simultaneous Multi-Species Visualization—Part I: Experimental Methodology and Application to Laminar and Turbulent Flames

In recent years, the tomographic visualization of laminar and turbulent flames has received much attention due to the possibility of observing combustion processes on-line and with high temporal resolution. In most cases, either the spectrally non-resolved flame luminescence or the chemiluminescence of a single species is detected and used for the tomographic reconstruction. In this work, we present a novel 2D emission tomographic setup that allows for the simultaneous detection of multiple species (e.g., OH*, CH* and soot but not limited to these) using a single image intensified CCD camera. We demonstrate the simultaneous detection of OH* (310 nm), CH* (430 nm) and soot (750 nm) in laminar methane/air, as well as turbulent methane/air and ethylene/air diffusion flames. As expected, the reconstructed distributions of OH* and CH* in laminar and turbulent flames are highly correlated, which supports the feasibility of tomographic measurements on these kinds of flames and at timescales down to about 1 ms. In addition, the possibilities and limitations of the tomographic approach to distinguish between locally premixed, partially premixed and non-premixed conditions, based on evaluating the local intensity ratio of OH* and CH* is investigated. While the tomographic measurements allow a qualitative classification of the combustion conditions, a quantitative interpretation of instantaneous reconstructed intensities (single shot results) has a much greater uncertainty.

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.

Development of a modular, high-speed plenoptic-camera for 3D flow-measurement

This paper describes the development of a Modular Plenoptic Adaptor (MPA) for rapid and reversible conversion of high-speed cameras into plenoptic imaging systems, with the primary goal of enabling single-camera, time-resolved 3D flow-measurements. The MPA consists of a regular imaging lens, a microlens array, a tilt-adjustable microlens mount and an optical relay, which are collectively installed onto a high-speed camera through a standard lens mount. Each component within the system is swappable to optimize for specific imaging applications. In this study, multiple configurations of the MPA were tested and they demonstrated the ability to refocus and shift perspectives within high-speed scenes after capture. Additionally, the MPA demonstrated 3D reconstruction of captured scenes with <1% spatial error across a volume spanning approximately 50×30×50mm3. Finally, the MPA also demonstrated reconstruction of a 3D droplets-field with sufficient quality to support qualitatively accurate plenoptic particle image velocimetry (PPIV) calculations.

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.

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.

Temporally resolved two dimensional temperature field of acoustically excited swirling flames measured by mid-infrared direct absorption spectroscopy

The detailed understandings of temperature profiles and flow-flame interaction in unsteady premixed swirling flames are crucial for the development of low emission turbine engines. Here, a phase-locked tomographic reconstruction technique measuring the large absorption cross section of CO₂ at its mid-infrared fundamental band around 4.2 μm is used to acquire the flame temperature and in situ CO₂ volume fraction distribution in a turbulent premixed swirling flame under different levels of external acoustic forcing amplitude. The temporally resolved temperature field variation reveals large temperature fluctuation in unsteady premixed swirling flames produced near the nozzle exit due to vortex-driven mixing of surrounding cold gas. The temperature fluctuation quickly dissipates when moving downstream of the flame with the flow velocity of the burnt gas. The accurate high temporal resolution thermodynamic measurements of the phase-locked tomographic thermometry technique reported in this work can be generally applied to periodic reacting flows.

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.