Dear Colleagues,

Stringent regulations concerning pollutant and greenhouse gas emissions, safety and cost concerns, and fuel load and stability issues all call for the need to improve combustion devices. This can only be achieved with an in-depth understanding of realistic flames, which, in most cases, are turbulent. A fundamental understanding of such flames, as well as advanced numerical simulations, require experimental data that should ideally be time-resolved and volumetric, due to the inherent unsteadiness and three-dimensionality of the flames.

With this Special Issue, we aim to present recent work on tomographic and volumetric imaging in flames. We advise that papers should not exceed 12 pages in length. Review papers are also welcome, but please contact us if you plan to prepare one.

Prof. Dr. Khadijeh Mohri
Prof. Dr.-Ing. Andreas M. Kempf
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Energies is an international peer-reviewed open access semimonthly journal published by MDPI.

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• Flame tomography
• Extracting quantitative information from flame reconstructions
• Phantom study strategies for tomography algorithm development
• Novel tomography algorithms
• High accuracy camera calibration methods
• Laser-based techniques
• Light field imaging
• Time-resolved data
• High resolution data
• Industrial applications
• Thermo-acoustics applications
• Methods for the analysis of volumetric data

Type: Article
Title: Image Reconstruction for Chemical Species Tomography Using Full Spectral Data
Authors: N. Polydorides ¹, M. Lengden ², C. Liu ¹, G. Humphries ², S.-A. Tsekenis ¹, D. Wilson ² and H. McCann ^1,*
Affiliations
¹   School of Engineering, Institute for Digital Communications, University of Edinburgh, Edinburgh, UK; [email protected] (N.P.); [email protected] (C.L.); [email protected] (S.-A.T.);
²   Department of Electrical and Electronic Engineering, University of Strathclyde, Glasgow, UK; [email protected] (M.L.); [email protected] (G.H.); [email protected] (D.W.);
* Correspondence: [email protected] 
Abstract: We consider the linear inverse problem of Chemical Species Tomography using spectral absorption data, such as those acquired in wavelength modulation spectroscopy. We introduce a novel approach that exploits the rich spectral information to overcome some of the limitations of CST. Under isobaric measurement conditions and known species temperature, our approach results in a linear well-posed estimation problem. The concentration images thus obtained have enhanced noise robustness, and spatial resolution significantly better than previous inversion methods for the same number of optical paths. We demonstrate the approach by CST of carbon dioxide in combustion exhaust, through both simulation and experimental measurement.
Keywords: Carbon dioxide concentration, chemical species tomography, linear attenuation, inverse problem, image reconstruction, Near-IR absorption, spectroscopy

Type: Article
Title: Two-dimensional tomographic simultaneous multi-species visualization in laminar and turbulent flames
Authors: Thomas Häber ^1,*, Rainer Suntz ¹ and Henning Bockhorn ²
Affiliations:
¹ Institute of Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT); [email protected], [email protected]
² Engler-Bunte-Institute, Chair of Combustion Technology, Karlsruhe Institute of Technology (KIT); [email protected]
* Correspondence: [email protected]
Abstract: In recent years, the tomographic visualization of laminar and turbulent flames has received much attention. In most cases, either the integral 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* (310nm), CH* (430nm) and soot (750nm) in laminar methane/air, as well as turbulent methane/air and ethylene/air diffusion flames. Taking advantage of the strong spatial (and temporal) coupling of OH* and CH* in laminar and moderate turbulent flames, multi-species tomography enables us to quantify the reconstruction quality independent of any phantom studies. This is especially import in turbulent flames, where it is difficult to separate measurement noise from turbulent fluctuations. We will show 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. We will also investigate 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*.
Keywords: optical emission tomography, combustion, chemiluminescence, Tikhonov regularization, algebraic reconstruction technique, laminar diffusion flame, turbulent diffusion flame, local equivalence ratio

Type: Article
Title: Two-Dimensional Tomographic Simultaneous Multi-Species Visualization in Laminar and Turbulent Flames
Authors: Thomas Häber ^1,*, Rainer Suntz ¹ and Henning Bockhorn ²
Affiliations:
¹ Institute of Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT); [email protected], [email protected]
² Engler-Bunte-Institute, Chair of Combustion Technology, Karlsruhe Institute of Technology (KIT); [email protected]
* Correspondence: [email protected]
Abstract: In recent years, the tomographic visualization of laminar and turbulent flames has received much attention. In most cases, either the integral 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* (310nm), CH* (430nm) and soot (750nm) in laminar methane/air, as well as turbulent methane/air and ethylene/air diffusion flames. Taking advantage of the strong spatial (and temporal) coupling of OH* and CH* in laminar and moderate turbulent flames, multi-species tomography enables us to quantify the reconstruction quality independent of any phantom studies. This is especially import in turbulent flames, where it is difficult to separate measurement noise from turbulent fluctuations. We will show 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. We will also investigate 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*.
Keywords: optical emission tomography, combustion, chemiluminescence, Tikhonov regularization, algebraic reconstruction technique, laminar diffusion flame, turbulent diffusion flame, local equivalence ratio

Title: Image Reconstruction for Chemical Species Tomography Using Full Spectral Data
Authors: N. Polydorides 1, M. Lengden 2, C. Liu 1, G. Humphries 2, S.-A. Tsekenis 1, D. Wilson 2 and H. McCann 1,*
Affiliation: 1 School of Engineering, Institute for Digital Communications, University of Edinburgh, Edinburgh, UK; [email protected] (N.P.); [email protected] (C.L.); [email protected] (S.-A.T.); 2 Department of Electrical and Electronic Engineering, University of Strathclyde, Glasgow, UK; [email protected] (M.L.); [email protected] (G.H.); [email protected] (D.W.); * Correspondence: [email protected]
Abstract: We consider the linear inverse problem of Chemical Species Tomography using spectral absorption data, such as those acquired in wavelength modulation spectroscopy. We introduce a novel approach that exploits the rich spectral information to overcome some of the limitations of CST. Under isobaric measurement conditions and known species temperature, our approach results in a linear well-posed estimation problem. The concentration images thus obtained have enhanced noise robustness, and spatial resolution significantly better than previous inversion methods for the same number of optical paths. We demonstrate the approach by CST of carbon dioxide in combustion exhaust, through both simulation and experimental measurement. Keywords: Carbon dioxide concentration, chemical species tomography, linear attenuation, inverse problem, image reconstruction, Near-IR absorption, spectroscopy

Title: Two-dimensional tomographic simultaneous multi-species visualization in laminar and turbulent flames
Authors: Thomas Häber 1,*, Rainer Suntz 1 and Henning Bockhorn 2
Affiliation: 1 Institute of Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT); [email protected], [email protected] 2 Engler-Bunte-Institute, Chair of Combustion Technology, Karlsruhe Institute of Technology (KIT); [email protected] * Correspondence: [email protected]
Abstract: In recent years, the tomographic visualization of laminar and turbulent flames has received much attention. In most cases, either the integral 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* (310nm), CH* (430nm) and soot (750nm) in laminar methane/air, as well as turbulent methane/air and ethylene/air diffusion flames. Taking advantage of the strong spatial (and temporal) coupling of OH* and CH* in laminar and moderate turbulent flames, multi-species tomography enables us to quantify the reconstruction quality independent of any phantom studies. This is especially import in turbulent flames, where it is difficult to separate measurement noise from turbulent fluctuations. We will show 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. We will also investigate 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*.