Background-oriented Schlieren Tomography Research Articles

Three-dimensional diagnosis of lean premixed turbulent swirl flames using tomographic background oriented Schlieren

The necessity of minimizing NOx emissions drives the pursuit of ultra-lean premixed combustion in aeroengines and gas turbines, characterized by susceptibility to combustion instabilities. To tackle this issue, swirling flow design is widely incorporated into lean premixed combustor design, enhancing flame stability, and shortening flame length. This study utilizes the tomographic background-oriented Schlieren (TBOS) to reconstruct the spatial distribution of the refractive index gradient of lean premixed turbulent swirl flames with an aeroengine combustor configuration. A parametric study of the TBOS reconstruction quality is conducted, and the results reveal that view sparseness primarily degrades the reconstruction quality compared to the specific iterative algorithm used. The classic visual hull approach is explored to address this challenge, highlighting the significance of visual hull size. Furthermore, to improve the reconstruction quality, a posterior support constraint method is proposed, involving the removal of voxels of nearly constant refractive index within the central volume surrounded by flames. Results demonstrate that implementing this posterior support constraint further improves the reconstruction quality of lean premixed turbulent swirl flames. Finally, the robustness of this posterior support constraint method is validated by introducing high-level noise to the light deflection data, showcasing the potential of combining the dedicated designed visual hull and proposed posterior support constraint in addressing the view sparseness challenge for TBOS measurements.

Background-oriented Schlieren tomography using gated recurrent unit.

Current Background-oriented schlieren tomography (BOST) methods rely primarily on iterative algorithms for reconstruction. Before reconstruction, a weight projection matrix was generated by performing 3D ray tracing using the projection relationship between the cameras, depending on the camera calibration parameters and large weight projection matrix which introduce artifacts and greatly reduce computational efficiency in the reconstruction. Considering that CT reconstruction uses spatial projection sequences from multiple directions, this study draws inspiration from the Recurrent Neural network(RNN) and utilizes spatial correlation between adjacent projection data to propose a background-oriented schlieren reconstruction method based on a gated recurrent unit (GRU) neural network. First, the model architecture is designed and implemented. Subsequently, numerical simulations were conducted using a methane combustion model to evaluate the proposed method, which achieved an average mean relative error (MRE) of 0.23%. Finally, reconstruction experiments were performed on the actual flow-field data above a candle flame, with a reprojection correlation coefficient of 89% and an average reconstruction time of only 1.04 s per frame. The results demonstrate that the proposed method outperforms traditional iterative reconstruction methods in terms of reconstruction speed and accuracy. This provides a feasible solution for the real-time reconstruction of three-dimensional instantaneous flow fields.

Reconstruction Method of 3D Turbulent Flames by Background-Oriented Schlieren Tomography and Analysis of Time Asynchrony

Background-oriented Schlieren tomography (BOST) is widely used for 3D reconstruction of turbulent flames. Two major concerns are associated with 3D reconstruction. One is the time asynchrony within the data acquisition of the high-speed camera. The other is that the ray tracing process requires significant computational consumption. This study proposes a ray tracing optimization method based on the k-d tree. The study results show that the average search nodes for each ray are only 0.018% of 3D flame with 3.07 million grid nodes. In addition, a parameter estimation method of the unknown azimuth power spectrum function is proposed. First, a typical Sandia turbulent jet diffusion flame dataset was built and validated accordingly, with experiments. The algorithm’s applicability to the 3D reconstruction of temperature and density fields is discussed on this basis. The root-mean-square error (RMSE) of the cross-section density for 3D reconstruction is below 0.1 kg/m3. In addition, the RMSE of the cross-section temperature is below 270 K. Finally, an uncertainty analysis of the flame reconstruction based on a physical model is performed by optimizing the ray tracing method. For the time asynchronous variance of 1 ms, the density uncertainty of the 3D reconstruction is below 1.6 × 10−2 kg/m3, and the temperature uncertainty is below 70 K. The method can provide an essential basis for the design of BOST systems and the 3D reconstruction of turbulent flames.

Analyzing 3D fields of refractive index, emission and temperature in spray-flame nanoparticle synthesis via tomographic imaging using multi-simultaneous measurements (TIMes)

Turbulent spray flames issuing from the SpraySyn 2.0 burner were investigated in the present work by means of tomographic imaging using multi-simultaneous measurements (TIMes). This small-scale standard burner is designed to study flame-spray synthesis of tailored nanoparticle systems. The applied measurement technique combined background-oriented schlieren tomography, computed tomography of light emission and two-color tomographic salt emission thermometry to reveal single-shot and time-averaged 3D fields of refractive index (linked to temperature, pressure and species concentration), CH* chemiluminescence (flame front indicator), SrOH emission from synthesized trace amount of Sr(NO3)2 dissolved in the liquid precursor solution (marker of the hot spray-flame products), and temperature of the hot spray-flame products, respectively. The salt emission thermometry technique was extended to tomographic measurements for the first time. Its implementation was based on the voxel-wise intensity ratio of reconstructed thermally-excited SrOH signals at two different regions in the visible spectrum that was converted to temperature using a single calibration curve based on multi-line OH planar laser-induced fluorescence measurements. The cost-effective setup for TIMes was composed of 43 machine-vision cameras equipped with bandpass spectrum filters, an innovative continuous semi-circular background pattern and LED light sources. The combined reconstructed fields provided information on the 3D flame structure in detail, showing the interaction between the flame front and spray stream, and the hot temperature regions. Spray-flame features were quantitatively analyzed, namely temperature, height, width, 3D tilt angle, and spreading half-angle of the flame plume. The temperature measurements proved to be robust against variations in the fuel/oxidizer equivalence ratio for the four investigated operating conditions. The spray flame issuing from the SpraySyn 2.0 burner at the standard operating condition was more stable than that in the previous burner generation, although more asymmetric near the nozzle exit and tilted, which might still cause heterogeneity of the synthesized nanoparticle system.

Coupled 3D evolutionary reconstruction technique for multi-simultaneous measurements

An integrated approach for the three-dimensional tomographic reconstruction of multiple scalar fields in premixed gaseous non-sooting reactive flow applications is presented. Measurement of different physical quantities are combined to enable an in-depth analysis of the combustion process in premixed turbulent flames. This is achieved by utilizing a flexible and versatile evolutionary reconstruction technique, where measured chemiluminescence signals and deflections of a background-oriented schlieren tomography (BOST) setup are processed in a coupled manner for the first time. The 3D tomographic inverse problem is stated as a coupled system and is holistically solved for the chemiluminescence intensity field and the refractive index field (from BOST data) in the flow domain. The chemiluminescence field is interpreted as a pseudo flame surface density function of a progress variable field that allows the analysis of flame front curvatures. The proposed method is tested for its consistency and sensitivities on synthetically generated measurement data of different complexities. Finally, the technique is applied to experimental measurement data of turbulent stratified flames with different levels of swirl.

Three-dimensional density measurements of a heated jet using laser-speckle tomographic background-oriented schlieren

Tomographic background-oriented schlieren (TBOS) is a promising optical technique for 3D density field measurement in turbulent flows. Path-integrated information on refractive index gradients is captured by cameras looking through the flow at textured background patterns in the form of apparent displacements of the patterns. A tomographic reconstruction of the three-dimensional refractive index gradients is performed, from which the instantaneous three-dimensional density field is obtained. Most TBOS measurements to date have been limited by large temporal blurring and/or large defocus blurring. Our 15-camera experimental set-up uses a pulsed laser for both illumination and creation of a laser-speckle background pattern. The high-power laser ensures adequate exposures in nanoseconds, so temporal integration is minimised in each measurement. The beam is spread to illuminate a surface that is observed by fifteen cameras that are evenly spaced circumferentially around the flow. Laser speckle patterns are observed by the cameras due to the diffuse reflection and interference of coherent light from roughness on the surface. We develop a methodology for selecting the optimal focal length, focus distance, and lens aperture by considering the compromise between displacement sensitivity, defocus blurring and speckle size. Low temporal- and spatial-blurring 3D temperature field measurements with excellent resolution of turbulent flow features are obtained and good agreement with thermocouple. Classical behaviours of the potential core instability such as vortex rollup and radial ejections are identified. The behaviour of the potential core is examined further, which reveals that the potential core undergoes considerable deformation and fragmentation during the transition to turbulence.

Tomographic imaging using multi-simultaneous measurements (TIMes) of emission and refractive index 3D fields in turbulent flames

The present work combines background-oriented schlieren tomography (BOST) with computed tomography of chemiluminescence (CTC) using the TIMes technique to provide simultaneous three-dimensional fields of refractive index (i.e., density) and CH* chemiluminescence (i.e., flame luminescence) on a single-shot basis. The feasibility of the proposed non-intrusive technique is assessed based on simulations and experiments. Its accuracy with respect to the number of cameras is evaluated using phantoms of a realistic simulated turbulent flame. The technique is experimentally demonstrated to investigate turbulent flames issuing through a double-annular bluff-body burner (Cambridge-Sandia stratified swirl burner) under a variety of operating conditions. The simultaneous measurements are performed using a cost-effective setup consisting of 29 cameras split between BOST and CTC. The reconstructed instantaneous and averaged results reveal the three-dimensional fields of the refractive index (related to the temperature, pressure and species concentration) and CH* chemiluminescence (indicator of the flame front), showing their structure interaction.

Direct background-oriented schlieren tomography using radial basis functions.

Background-oriented schlieren tomography (BOST) is effective for flow field measurement; however, different from general computed tomography (CT), the BOST utilizes the deflection of rays passing through an inhomogeneous field for measurement. It is sensitive to the refractive index gradient. Therefore, an additional integration step is typically employed to obtain the refractive index. In this article, a calculation method of projection matrix is proposed based on the radial basis function (RBF). The 3D distribution of the refractive index can be reconstructed directly. This method was first verified by numerical simulation. Then, the 3D instantaneous refractive index field above a candle flame was measured. The reprojection error was calculated by ray tracing. The results illustrate the accuracy and stability of the proposed method. This research provides a new and complete solution for the 3D instantaneous flow field (refractive index, density, or temperature) measurement.

Automated detection of hot-gas path defects by Support Vector Machine based analysis of exhaust density fields

Defects in the hot-gas path of aero engines have been shown to leave typical signatures in the density distribution of the exhaust jet. These signatures are superposed when several defects are present. For improved maintenance and monitoring applications, it is important to not only detect that there are defects present but to also identify the individual classes of defects. This diagnostic approach benefits both, the analysis of prototype or acceptance test and the preparation of Maintenance, Repair, and Overhaul. Recent advances in the analysis of tomographic Background-Oriented Schlieren (BOS) data have enabled the technique to be automated such that typical defects in the hot-gas path of gas turbines can be detected and distinguished automatically. This automation is achieved by using Support Vector Machine (SVM) algorithms. Choosing suitable identification parameters is critical and can enable SVM algorithms to distinguish between different defect types. The results show that the SVM can be trained such that almost no defects are missed and that false attributions of defect classes can be minimized.

Assessment of three-dimensional density measurements from tomographic background-oriented schlieren (BOS)

Tomographic background-oriented schlieren (BOS) is used to measure three-dimensional instantaneous density fields in turbulent flows. Density gradients are related to the refractive index through the Gladstone-Dale relation. Based on path-integrated apparent displacements in a background pattern from path variations in light rays travelling through the flow, a tomographic reconstruction of the three-dimensional refractive index gradients can be obtained. A Poisson equation of the reconstructed gradients is solved to obtain the refractive index field. We examine four sources of measurement error: (i) defocus blurring; (ii) spatial averaging in the finite difference scheme for solving the Poisson equation; (iii) limited-view tomographic reconstruction; and (iv) displacement field noise. Synthetic BOS displacements are generated from the instantaneous density fields of a heated jet at a Reynolds number of 10 000 and jet exit centreline-to-ambient density ratio of 0.83 computed via DNS. Synthetic background displacements are produced by raytracing through the refractive index field obtained using the Gladstone-Dale relation. The virtual BOS setup consists of 15 cameras placed circumferentially around the jet axis, based on a proposed experimental setup. We show that defocus blurring has the greatest impact on the measurement accuracy, using the RMS and peak errors between the measured and true DNS fields as metrics; its impact is greatest in the transitional region and decreases steadily downstream. Sources (ii) and (iv) have marginal impact (for noise 15% of mean displacements). Three reconstruction methods are tested: (i) filtered back-projection (FBP); (ii) an algebraic reconstruction technique (ART); and (iii) sequential FBP-ART. ART and FBP-ART are similarly accurate, producing under half the errors of FBP, which suffers from significant reconstruction artefacts. The reconstruction error is modest compared to the impact of defocus blurring.

Volumetric imaging of flame refractive index, density, and temperature using background-oriented Schlieren tomography

Volumetric imaging represents one of the major development trends of flow diagnostics due to both the advancement in hardware and the requirement for more information to further understand complicated turbulent and/or reactive flows. Background-oriented Schlieren tomography (BOST) has become increasingly popular due to its experimental simplicity. It has been demonstrated to be capable of simultaneously recovering the distributions of refractive index, density, and temperature of flows. However, its capability in thermometry has only been demonstrated under the axisymmetric assumption, which greatly limits its applicability. In this work, we dedicated to developing a cost-effective BOST system for the simultaneous retrieval of refractive index, density, and temperature distributions for the asymmetric flame. A few representative tomographic inversion algorithms were assessed as well. Both numerical and experimental demonstrations were conducted and the results show that our implemented BOST can successfully reconstruct the three-dimensional temperature distribution with a satisfactory accuracy.

Fast and robust volumetric refractive index measurement by unified background-oriented schlieren tomography

We propose a novel approach to background-oriented schlieren (BOS) tomography (BOST) that unifies the deflection sensing and reconstruction algorithms. BOS is a 2D flow visualization technique that renders light deflections due to refraction in the fluid. Simultaneous BOS measurements from unique views can be reconstructed by tomography to estimate the fluid’s 3D refractive index field. The cameras are focused through the fluid on textured background patterns. Deflections between an undistorted reference image and distorted image are typically determined by gradient-based optical flow (OF), which is a complex inverse problem and potential source of error in BOST. This paper presents an alternative approach to BOST that unifies the OF equations and deflection model. Our new operator simultaneously calculates the image distortions seen by each camera for a discrete refractive index distribution. Unified BOST (UBOST) thus reconstructs observed image distortions instead of inferred deflections, which are influenced by user-selected OF parameters. The UBOST operator has one third as many equations as the classical BOST operator. We show that our formulation reduces the effects of model error and the computational cost of reconstruction. These advantages are demonstrated with a numerical experiment using phantoms of varied complexity. Best practice UBOST reconstructions were more accurate than classical reconstructions of the exact deflections for each phantom. Moreover, UBOST estimates converged substantially faster, resulting in a $$\ge $$ 62.5% speedup with our solver.

Time-resolved three-dimensional imaging of flame refractive index via endoscopic background-oriented Schlieren tomography using one single camera

Due to the recent progresses in sensor and laser technologies, volumetric tomography has become a popular diagnostic for combustion studies. Background-oriented Schlieren tomography (BOST) is one promising modality of volumetric tomography due to its ease of implementation and versatility in imaging multiple scalars. It has been demonstrated for both instantaneous and time-resolved measurements of flames with multiple cameras. This work reports a time-resolved BOST system with only one single high-speed camera by employing a customized fiber bundle which is flexible, compact and portable. A proof-of-concept experiment was conducted by imaging the three-dimensional refractive index field of a premixed turbulent flame.

Instantaneous 3D flame imaging by background-oriented schlieren tomography

We apply background-oriented schlieren (BOS) imaging with computed tomography to reconstruct the instantaneous refractive index field of a turbulent flame in 3D. In BOS tomography, a network of cameras are focused through a variable index medium (such as a flame) onto a background of patterned images. BOS data consist of pixel-wise “deflections” between a reference and distorted image, caused by variations in the refractive index along the path between the camera and background. Multiple simultaneous BOS images, each from a unique perspective, are combined with a tomography algorithm to reconstruct the refractive index distribution (or optical density) in the probe volume. This quantity identifies the edges of the wrinkled turbulent flame surface. This is the first application of BOS imaging to flame tomography, setting the stage for low-cost 3D flame thermometry. Tomography is carried out within the Bayesian framework, using Tikhonov and total variation (TV) priors. The TV prior is more compatible with the abrupt spatial variation in the refractive index field caused by the flame front. We demonstrate the suitability of TV regularization using a proof-of-concept simulation of BOS tomography on an LES phantom. The technique was then used to reconstruct the instantaneous 3D refractive index field of an unsteady natural gas/air flame from a Bunsen burner using a 23-camera setup. Our results show how BOS tomography can capture and visualize 3D features of a flame and provide benchmark data for simulations.

Measurement of the fluctuating temperature field in a heated swirling jet with BOS tomography

This work investigates the potential of background-oriented schlieren tomography (3D-BOS) for the temperature field reconstruction in a non-isothermal swirling jet undergoing vortex breakdown. The evaluation includes a quantitative comparison of the mean and phase-averaged temperature field with thermocouple and fast-response resistance thermometer as well as a qualitative comparison between the temperature field and the flow field obtained from particle image velocimetry (PIV). Compared to other temperature-measuring techniques, 3D-BOS enables non-invasive capturing of the entire three-dimensional temperature field. In contrast to previous 3D-BOS applications, the present investigation makes use of the special character of the flow, which provides a global instability that leads to a rotational symmetry of the jet. Additionally, the rotational motion of the jet is used to obtain a tomographic reconstruction from a single camera. The quality of 3D-BOS results with respect to the physical setup as well as the numerical procedure is analyzed and discussed. Furthermore, a new approach for the treatment of thin occluding objects in the field of view is presented.

Automatic Detection of Defects in a Swirl Burner Array Through an Exhaust Jet Pattern Analysis

A significant challenge in improving the regeneration process of jet engines is the reduction of engine down-time during inspection. As such, early defect detection without engine disassembly will speed up the regeneration process. Defects in the engines hot-gas path (HGP) influence the density distribution of the flow and lead to irregularities in the density distribution of the exhaust jet which can be detected with the optical background-oriented Schlieren (BOS) method in a tomographic setup. The present paper proposes a combination of tomographic BOS measurements and supervised learning algorithms to develop a methodology for an automatic defect detection system. In the first step, the methodology is tested by analyzing the exhaust jet of a swirl burner array with a nonuniform fuel-supply of single burners with tomographic BOS measurements. The measurements are used to implement a support vector machine (SVM) pattern recognition algorithm. It is shown that the reconstruction quality of tomographic BOS measurements is high enough to be combined with pattern recognition algorithms. The results strengthen the hypothesis that it is possible to automatically detect defects in jet engines with tomographic BOS measurements and pattern recognition algorithms.

Combining ART and FBP for improved fidelity of tomographic BOS

Engine component defects along the hot-gas path (HGP) of jet engines influence the density distribution of the flow, and thus result in characteristic patterns in the exhaust jet. These characteristic patterns can be reconstructed with the optical background-oriented schlieren (BOS) method in a tomographic set-up, which in turn allows the identification of defects inside the engine through an exhaust jet analysis.The quality of the tomographic reconstruction strongly influences how easily defects can be detected inside the jet engine. In particular, the presence of high gradients in the reconstruction area has a strong impact on the reconstruction quality. An algebraic reconstruction algorithm (ART) is implemented and compared to a filtered-back projection (FBP) algorithm in terms of the capability of performing high-gradient tomographic BOS reconstructions. A combination of both algorithms is presented which significantly improves the reconstruction quality of high-gradient tomographic BOS in terms of artifact reduction. The combination of both algorithms is applied to both synthetic and real measurement data in this paper, in order to show possible applications and the achievable improvement of high-gradient tomographic BOS reconstructions.