The Experts below are selected from a list of 4911 Experts worldwide ranked by ideXlab platform

Weiwei Cai - One of the best experts on this subject based on the ideXlab platform.

  • Limited-projection volumetric tomography for time-resolved turbulent Combustion Diagnostics via deep learning
    Aerospace Science and Technology, 2020
    Co-Authors: Jianqing Huang, Qian Wang, Hecong Liu, Weiwei Cai
    Abstract:

    Abstract Time-resolved volumetric tomography (VT) has been applied extensively for turbulent flow/Combustion Diagnostics, due to its great capacity in reconstructing three dimensional scalar/vector fields. However, it usually suffers from high computational costs of conventional iterative methods in processing thousands of tomographic frames, and also the requirement of multiple high-speed camera/intensifiers to ensure sufficient spatial sampling, resulting in high experimental costs. In this work, we aim to take the full advantage of the recent progress in deep learning algorithms and develop an inversion method which not only reduces the processing time of a single frame down to the milliseconds level but also the number of projections required without sacrificing the imaging quality. Two distinct frameworks of convolutional neural network were designed and tested for VT reconstructions of turbulent flames for the first time. The results from proof-of-concept experiments implementing computed tomography of chemiluminescence (CTC) confirmed the feasibility of our method. Our data-driven approach can expedite the reconstruction process by a factor of ∼ 105 compared with conventional iterative methods (e.g., algebraic reconstruction technique). This work is expected to be valuable for all tomographic modalities which are seeking expedited reconstruction and reduced costs.

  • Toward real-time volumetric tomography for Combustion Diagnostics via dimension reduction.
    Optics letters, 2018
    Co-Authors: Hecong Liu, Jiaqi Zhang, Weiwei Cai
    Abstract:

    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.

  • Rapid tomographic reconstruction based on machine learning for time-resolved Combustion Diagnostics
    The Review of scientific instruments, 2018
    Co-Authors: Weiwei Cai, Yingzheng Liu
    Abstract:

    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.

  • Volumetric reconstruction for Combustion Diagnostics via transfer learning and semi-supervised learning with limited labels
    Aerospace Science and Technology, 1
    Co-Authors: Weiwei Cai, Jianqing Huang, Andong Deng, Qian Wang
    Abstract:

    Abstract Volumetric tomography (VT) is a powerful tool for Combustion Diagnostics due to its capacity in resolving flame structures in three-dimensional (3D). Recently, convolutional neural network (CNN) has been applied to solve the inversion problems of VT, which features an overwhelming advantage over classical iterative methods in terms of computational efficiency. However, a large number of labels have to be prepared for the supervised learning of CNN using iterative methods, compromising its efficiency advantage. Moreover, previous studies were limited to a single dataset and the generalization performance of CNN has not yet been tested. In this work, both transfer learning and semi-supervised learning were employed to construct the CNN networks with limited labels. The comparative studies between them and supervised learning confirmed that a significant improvement in reconstruction accuracy can be achieved even with limited labels. The correlation coefficient between the reconstruction and ground truth is larger than 0.98 for three commonly encountered application scenarios. The training strategies developed in this work are expected to be valuable for all VT modalities as applied to flow/Combustion Diagnostics.

Frank Beyrau - One of the best experts on this subject based on the ideXlab platform.

  • Laser Thermometry Techniques for Combustion Diagnostics
    Renewable Energy and the Environment Optics and Photonics Congress, 2012
    Co-Authors: Frank Beyrau
    Abstract:

    Several laser techniques with different characteristics for remote gas phase thermometry in the harsh environment of practical Combustion devices - such as internal Combustion engines and gas-turbine combustors - are presented, and their virtues and limitations discussed.

  • An external Raman laser for Combustion Diagnostics
    Combustion and Flame, 2011
    Co-Authors: Johannes Kerl, Thomas Sponfeldner, Frank Beyrau
    Abstract:

    Abstract We investigate the performance of a Ba(NO 3 ) 2 crystal based Raman laser as a compact and inexpensive alternative to dye lasers for certain applications in Combustion Diagnostics. Use of this Raman laser for double-pulse applications like particle image velocimetry (PIV) and for spectroscopic techniques such as laser induced fluorescence (LIF) is demonstrated. Very high conversion efficiencies can be achieved and the crystal shows no saturation effects when pumped in rapid succession, which is important for high repetition rate applications.

  • application of a beam homogenizer to planar laser Diagnostics
    Optics Express, 2006
    Co-Authors: Sebastian Pfadler, Monika Loffler, Frank Beyrau, Alfred Leipertz
    Abstract:

    The first application of a microlens array beam homogenizer to planar laser measurement techniques in Combustion Diagnostics is demonstrated. The beam homogenizing properties of two microlens arrays in combination with a Fourier lens for widespread applications are presented. An uniform line profile with very little temporal fluctuations of the spatial intensity distribution was generated resulting in a significant reduction of measurement noise and enabling an easier and faster signal processing.

Yu V. Romanovskii - One of the best experts on this subject based on the ideXlab platform.

  • Tunable diode laser spectroscopy as a technique for Combustion Diagnostics
    Spectrochimica Acta - Part B Atomic Spectroscopy, 2015
    Co-Authors: M. A. Bolshov, Yu A. Kuritsyn, Yu V. Romanovskii
    Abstract:

    Tunable diode laser absorption spectroscopy (TDLAS) has become a proven method of rapid gas Diagnostics. In the present review an overview of the state of the art of TDL-based sensors and their applications for measurements of temperature, pressure, and species concentrations of gas components in harsh environments is given. In particular, the contemporary tunable diode laser systems, various methods of absorption detection (direct absorption measurements, wavelength modulation based phase sensitive detection), and relevant algorithms for data processing that improve accuracy and accelerate the Diagnostics cycle are discussed in detail. The paper demonstrates how the recent developments of these methods and algorithms made it possible to extend the functionality of TDLAS in the tomographic imaging of Combustion processes. Some prominent examples of applications of TDL-based sensors in a wide range of practical Combustion aggregates, including scramjet engines and facilities, internal Combustion engines, pulse detonation combustors, and coal gasifiers, are given in the final part of the review.

Marcus Aldén - One of the best experts on this subject based on the ideXlab platform.

  • CW Laser radar for Combustion Diagnostics
    EPJ Web of Conferences, 2018
    Co-Authors: Elin Malmqvist, Marcus Aldén, Mikkel Brydegaard, Joakim Bood
    Abstract:

    A CW-laser radar system developed for Combustion Diagnostics is described. The system is based on triangulation to attain range information. A portable system has been constructed and here we show some result from measurements in various flames, for example Rayleigh scattering thermometry and monitoring of particle distributions with high temporal and spatial resolution. The concept can equally well be based on pulsed lasers, allowing suppression of background emission through gated detection.

  • CW-laser radar for Combustion Diagnostics
    Imaging and Applied Optics 2016, 2016
    Co-Authors: Elin Malmqvist, Marcus Aldén, Mikkel Brydegaard, Joakim Bood
    Abstract:

    A CW-laser radar system developed for Combustion Diagnostics is described. It is based on triangulation to attain range information. Some initial results from measurements in sooting flames are shown and some future perspectives are discussed.

  • Characterization of ammonia two-photon laser-induced fluorescence for gas-phase Diagnostics
    Applied Physics B, 2014
    Co-Authors: Christian Brackmann, Odd Hole, Bo Zhou, Zhongshan S. Li, Marcus Aldén
    Abstract:

    Two-photon laser-induced fluorescence (LIF) of ammonia (NH_3) with excitation of the C ′- X transition at 304.8 nm and fluorescence detection in the 565 nm C ′- A band has been investigated, targeting Combustion Diagnostics. The impact of laser irradiance, temperature, and pressure has been studied, and simulation of NH_3-spectra, fitted to experimental data, facilitated interpretation of the results. The LIF-signal showed quadratic dependence on laser irradiance up to 2 GW/cm^2. Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm^2, i.e., above irradiances attainable for LIF imaging. Maximum LIF-signal was obtained for excitation at the 304.8 nm bandhead; however, lower temperature sensitivity over the range 400–700 K can be obtained probing lines around 304.9 nm. A decrease in fluorescence signal was observed with pressure up to 5 bar absolute and attributed to collisional quenching. A detection limit of 800 ppm, at signal-to-noise ratio 1.5, was identified for single-shot LIF imaging over an area of centimeter scale, whereas for single-point measurements, the technique shows potential for sub-ppm detection. Moreover, high-quality NH_3-imaging has been achieved in laminar and turbulent premixed flames. Altogether, two-photon fluorescence provides a useful tool for imaging NH_3-detection in Combustion Diagnostics.

  • laser induced plasma in methane and dimethyl ether for flame ignition and Combustion Diagnostics
    Applied Physics B, 2011
    Co-Authors: Johannes W Troger, Johannes Kiefer, Zhongshan Li, Marcus Aldén
    Abstract:

    In this paper we report the investigation of the laser-induced breakdown and ignition behaviour of methane/air and dimethyl ether (DME)/air mixtures. Moreover, the optical emission from the induced plasma is utilized for determining the mixture composition quantitatively by means of laser-induced breakdown spectroscopy (LIBS). To the best of the authors’ knowledge, LIBS and laser ignition of DME have not been reported in literature before. The technique under investigation is finally employed for Combustion Diagnostics in laminar as well as turbulent flames. In the laminar premixed and non-premixed flames the LIBS spectra allow spatially resolved measurements of the equivalence ratio and enable studying the mixing of gases provided through the burner with the surrounding room air. In addition, the breakdown threshold of the applied laser pulse energy yields an estimate for the local temperature. In the turbulent cases single-shot LIBS spectra are recorded at fixed position allowing the derivation of local statistical fluctuations of the equivalence ratio in partially premixed jet flames. The results show that laser-induced breakdowns have a strong potential for flame Diagnostics and, under suitable conditions, for the ignition of combustible mixtures.

  • development of a picosecond lidar system for large scale Combustion Diagnostics
    Applied Optics, 2009
    Co-Authors: Billy Kaldvee, Joakim Bood, Andreas Ehn, Marcus Aldén
    Abstract:

    In the present work, a picosecond lidar system aiming at single-ended Combustion Diagnostics in full-scale Combustion devices with limited optical access, such as power plants, is described. The highest overall range resolution of the system was found to be <0.5 cm. A demonstration has been made in a nonsooty and sooty Bunsen burner flame. A well-characterized ethylene flame on a McKenna burner was evaluated for different equivalence ratios using Rayleigh thermometry. The results indicate both that picosecond lidar might be applicable for single-shot Rayleigh thermometry, even two-dimensional, and that there is a possibility to qualitatively map soot occurrence. Furthermore, differential absorption lidar has been investigated in acetone vapor jets for fuel visualization purposes.

Quang-viet Nguyen - One of the best experts on this subject based on the ideXlab platform.