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Tomographic spectrometer for the temporally-resolved 2D reconstruction of gas phase parameters within a generic SCR test rig
Abstract
We present a tomographic spectrometer for the measurement of gas phase species parameter distributions within the cross section of a generic exhaust gas test rig built for the investigation of the physico-chemical processes during exhaust gas aftertreatment with selective catalytic reduction (SCR). A urea-water solution (UWS) is injected into the hot gas flow to supply ammonia (NH3) through thermolysis and hydrolysis processes as the reducing agent of nitrous oxides (NOx) within the SCR catalyst. To achieve a full NOx conversion, the pre-catalyst distribution of the major exhaust gas species is of great importance. The injection leads to several desirable and undesired highly complex processes including spray breakup and film formation on the opposing wall. To further optimize the pre-catalyst distribution while maximizing the efficiency of the overall process, the knowledge of mole fraction and temperature distributions are crucial.
The developed biaxial tomographic spectrometer utilizes a combined measurement method - laser absorption tomography by employing tunable diode laser absorption spectroscopy (TDLAS) data with the linear hyperspectral absorption tomography (LHAT) approach to achieve simultaneous multiparameter reconstructions. An image frame rate of 49 Hz was achieved following a phase averaging, and a tomographic signal-to-noise ratio limit was applied. Virtual experiments and measurements in homogeneous samples were performed to ensure the spectromter’s ability for qualitative and quantitative reconstructions. Measurements during water injection cycles into the hot gas flow have shown two main features within the distribution that can be linked to the presence of a vapor boundary layer at the channel bottom and an evaporated spray-induced cone feature. Temperature measurements of a thermocouple downstream of the measurement plane agree well with the reconstructed temperatures.
The spectrometer is intended to further improve the understanding of the pre-catalyst flow in SCR systems and the results can be taken as reference for corresponding numerical calculations.
... [2] used a transparent manifold system simulating a real exhaust pipe and employed laser diagnostics and a high-speed camera to determine the optimal injector/mixer distance for urea-water injection and investigate the distribution of droplets. Likewise, investigations on SCR sprays in test rigs with optical access have also been conducted [13,[15][16][17][18][19][20] along with some investigations conducted directly using engine dynos [21][22][23]. However, these test rigs either fell short in the complete investigation of spray development (limited optical access) or did not provide stable cross flow conditions (engine exhaust flow). ...
... However, these test rigs either fell short in the complete investigation of spray development (limited optical access) or did not provide stable cross flow conditions (engine exhaust flow). Moreover, a comprehensive understanding of SCR processes cannot be gained by the synopsis of the studies since the environments under which the experiments were performed, such as geometries and operating conditions differed vastly [15]. An efficient way to conduct a full investigation on spray development is through the usage of a wind tunnel which can provide a stable heated crossflow and a large optically accessible test section to enable the use of measurement equipment like Phase Doppler Anemometry (PDA), backlight imaging etc. ...
A new subsonic heated open-loop wind tunnel has been designed and manufactured specifically with a 0.25 × 0.25 m cross-section and 1.59 m long optically accessible quartz glass cross-section. The purpose of this wind tunnel was to investigate Selective Catalytic Reduction (SCR) sprays using Phase Doppler Anemometry (PDA). It is capable of reaching a flowrate equating to 50 m/s in the test section when operating at standard room conditions. A 145 kW heater is mounted downstream the blower to achieve air temperatures of up to 400 °C with maximum air mass flow of 1200 kg/h. The diffuser, flow conditioning and contraction cone sections of the wind tunnel have been designed to minimize velocity and temperature gradients within the test section while minimizing flow disturbances. A diffuser with a cone angle of 5o and an area ratio of 3.95 and a contraction cone with a contraction ratio of 4.05 was selected. The test section has been designed to give complete visible access through top and both sides, while the bottom is made of stainless steel. The two sides are made of quartz glass to provide the best signal transmission quality for usage with any optical equipment e.g. PDA. The top of the test section has sections made of Robax glass with detachable windows for easy mounting and dismounting of any equipment subject to be tested in the flow section. Experimental studies using PDA and seeded flow were conducted and verified against Computational Fluid Dynamics (CFD) and analytical solutions to verify the flow parameters inside the test section. The streamwise turbulence intensity in the test section was found to be less than 2% in the core with similar values for the cross-stream turbulence intensities. The complete design and construction of each section of the wind tunnel has been presented and discussed in this paper.
Together with the good flow quality, a heated crossflow of up to 400 °C with an air velocity of up to 10 m/s can be delivered upstream the flow section. It can be used to simulate various flow conditions e.g. flow from an engine exhaust, but could also allow to record many transient behaviors related with heated crossflows such as droplet wall film stripping, deposits formations in SCRs etc. Moreover, studies involving chemically composed flow can also be conducted since it is an open-loop wind tunnel and the gas flow is constantly being renewed.
... This requirement is particularly important in BOS and LAS due to the nonlinear relationship between the QoI and measured signal(s). One solution is to perform instantaneous or time-resolved tomography using simultaneous measurements from multiple cameras or laser beams to reconstruct the QoI in an interrogation plane or volume, as has been done in BOS (Ihrke et al 2007, Atcheson et al 2008, Nicolas et al 2017, Amjad et al 2020, chemiluminescence imaging (Floyd and Kempf 2011, Worth and Dawson 2012, Mohri et al 2017, and LAS (Wright et al 2010, Lengden et al 2020, van der Kley et al 2021. This approach requires considerable optical access, which is not always possible and increases the cost and complexity of an experiment. ...
Axisymmetric tomography is used to extract quantitative information from line-of-sight measurements of gas flow and combustion fields. For instance, background oriented schlieren (BOS) measurements are typically inverted by tomographic reconstruction to estimate the density field of a high-speed or high-temperature flow. Conventional reconstruction algorithms are based on the inverse Abel transform, which assumes that rays are parallel throughout the target object. However, camera rays are not parallel, and this discrepancy can result in significant errors in many practical imaging scenarios. We present a generalization of the Abel transform for use in tomographic reconstruction of light-ray deflections through an axisymmetric target. The new transform models the exact path of camera rays instead of assuming parallel paths, thereby improving the accuracy of estimates. We demonstrate our approach with a simulated BOS scenario in which we reconstruct noisy synthetic deflection data across a range of camera positions. Results are compared to state-of-the-art Abel-based algorithms. Reconstructions computed using the new transform are consistently more stable and accurate than conventional reconstructions.
... LAS is an optical diagnostic technique known for its quantitative, species-specific measurement capability, which, when combined with tomographic reconstruction methods [9,10,11], can be used to characterize reacting flowfields in the harsh combustion environments [12,13,14,15]. Simultaneous multi-species measurements, in particular, provide key insights into the physiochemical mechanisms governing combustion performance and further elucidate complex combustion physics [16,17,18]. In recent years, such sensors have been developed, demonstrated, and deployed across various propulsion applications, including gas turbine combustors [19], rotating detonation engines (RDEs) [20,21,22], scramjets [23,24,25], liquidpropellant rocket combustors [26], and hybrid rockets [12,27], among others. ...
In this work, a method for experimentally determining a local characteristic velocity, c∗, is presented for purposes of analyzing rocket combustion progress based on in-situ laser absorption spectroscopy (LAS) measurements of temperature and gas composition. Measuring c∗ from spatially-varying thermochemical properties provides an alternative to the classical c∗ evaluation, which uses global chamber pressure and mass flow rate measurements. Accordingly, the novel method provides more detailed insight to the underlying mechanisms (multi-phase thermochemistry, diffusive mixing, turbulence, etc.) governing combustion performance in the spatial domain. The method is demonstrated on an RP-2/O2 liquid-propellant rocket engine (LRE) and a PMMA/O2 hybrid rocket combustion experiment. Localized c∗ results for the LRE are obtained via in-chamber measurements of temperature, CO, and CO2 and are presented over a range of pressures (P= 28–83 bar) and mixture ratios (MR= 2.5–5). For the hybrid rocket combustion experiment, one-dimensional tomographic reconstruction techniques are used to spatially-resolve the flow-field thermochemistry and obtain spatially-resolved measurements of temperature, CO, CO2, and H2O. These measurements are compiled to obtain spatially-resolved c∗ images of the combustion zone. The c∗ results from both experiments are compared to the theoretical c∗ expected from chemical equilibrium, providing for a method to assess combustion performance or progress locally within the combustion zone.
... Full Bayesian methods are now also being used in combination with laser diagnostics, for example to infer surface chemistry parameters from LAS measurements (Adowski and Bauman 2018), to estimate characteristics of aqueous films using Fourier transform infrared spectrometry (Pan et al. 2017), to predict the heat of vaporization of iron based on measurements from laser induced incandescence (Sipkens et al. 2018), and to estimate absorbance spectra from LAS-transmitted intensity (Emmert et al. 2019). Bayesian methods have also been extensively used in the development of chemical species tomography for a range of applications Grauer et al. 2016;Schmidt et al. 2020;Emmert et al. 2020;van der Kley et al. 2020). Specific methods of Bayesian inference include hierarchical and variational techniques (Fox and Roberts 2012), and Stroud et al. (2018) and sources therein provide additional details on parameter estimation using maximum-likelihood techniques, as well as full Bayesian approaches such as ensemble Kalman filtering. ...
Given spatially sparse or lower dimensional experimental measurements, approximate Bayesian computation (ABC) and numerical simulations can be used to estimate unknown characteristics of complex multi-physics engineering systems. Here, we describe the ABC approach and use it to estimate the speed of high-temperature gases exiting an industrially relevant catalytic burner, as well as to estimate the completeness of combustion within the burner. Using vertical profiles of absorption-weighted average temperature from laser absorption spectroscopy (LAS) at three different burner operating conditions, we combine ABC and large eddy simulations (LES) to generate posterior distributions of inflow speeds and heat addition characteristics above the burner. We show that the ABC method correctly estimates trends in the inflow speed for different conditions, and we find that there is a strong likelihood of incomplete combustion for higher equivalence ratios. We evaluate the predictive capability of the approach using an observing system experiment, indicating that the ABC method, when combined with LES, is able to accurately predict LAS measurements. We thus demonstrate that ABC is an effective tool for obtaining additional insights from available experimental measurements, thereby improving understanding of real-world engineering systems.
Graphic abstract
The current state and challenges of advanced experimental and modeling methods for a better understanding of heterogeneous chemical reactions are discussed using examples from developing and future technologies in the area of emission reduction of local pollutants and greenhouse gases. In situ and operando experimental techniques using laser and X-ray absorption spectroscopy, for instance, are able to resolve spatial and temporal concentration and temperature profiles in the near-wall gas phase, the interphase and inside the solid bulk. They have been exploited for a better understanding of the interaction of chemical reactions and transport processes. The experimental elucidation of chemical conversion on the microscopic scale leads to elementary step-like surface reaction mechanisms. The microkinetic description of gas-surface reactions is still challenging due to the complex influence of the modification of the solid material itself on the microscopic scale during the chemical reaction, which is caused by intrinsic materials’ modifications due to adsorbed species and temperature variations. Furthermore, transient inlet and boundary conditions on the reactor scale have a strong impact on the material and reaction rate. In addition to thermochemical reactions, an additional complexity comes into play with electrochemical ones. This paper will discuss heterogeneous chemical reactions in the light of emerging technologies such as emission control of natural gas and hydrogen fueled engines, use of CO2 in chemical (methanation, dry reforming) and steel industry (off-gas reforming), hydrogen production by pyrolysis of methane, small-scale ammonia synthesis and use, and recyclable carbon-free energy carriers. Hence, this article will also reveal a new playground and the potential of methods, know-how, and skills of the combustion community to significantly contribute to the solution of climate-change relevant challenges.
In this work, a generic exhaust gas test bench is introduced on which reproducible experiments can be performed to gain a deeper understanding of processes during exhaust gas aftertreatment of internal combustion engines. We present the design and initial flow characterization as well as tomographic measurement results of gaseous water distributions. The aim of the development was to provide a generic geometry as well as highly reproducible process boundary conditions for numerical simulation of exhaust aftertreatment phenomena. The presented initial measurements are intended to demonstrate the qualification of the test bench for extensive experimental characterization ranging from measurements of the spray injection, film evaporation, and reaction kinetics to the highly complex multiphase flow conditions during selective catalytic reduction (SCR) processes, which are characterized by high mass flows and temperatures, pronounced transients, and a corrosive atmosphere.
High-resolution absorption spectroscopy is a promising method for non-invasive process monitoring, but the computational effort required to evaluate the data can be prohibitive in high-speed, real-time applications. This study presents a fast method to estimate absorbance spectra from transmitted intensity signals. We employ Bayesian statistics to combine a measurement model with prior information about the shape of the baseline intensity and absorbance spectrum. The resulting linear least-squares problem shifts most of the computational effort to a preparation step, thereby facilitating quick processing and low latency for any number of measurements. The method is demonstrated on simulated tunable diode laser absorption spectroscopy data with additive noise and a fluctuating fringe. Results were highly accurate and the method was computationally efficient, having a processing time of only 2 ms per spectrum.
Chemical species tomography enables non-invasive measurements of temperatures andconcentrations in gas phase processes. In this work, we demonstrate the recently introducedlinear hyperspectral absorption tomography (LHAT) on an axisymmetric counterflow burner usedfor speciation studies of Oxyfuel combustion. As LHAT reconstructs spectrally resolved localabsorption coefficient spectra, the physical plausibility of these reconstructed spectra degradeswith an over-regularization of the tomographic problem. As presented in this work, this behaviorcan be employed in a novel regularization parameter choice method based on the residuals of localspectroscopic fits to the reconstructed spectra. After determining the regularization parameter,the reconstructions of the temperature and water mole fraction profiles of different flames arecompared to numerical simulations, showing a good agreement.
This paper describes the contents of the 2016 edition of the HITRAN molecular spectroscopic compilation. The new edition replaces the previous HITRAN edition of 2012 and its updates during the intervening years. The HITRAN molecular absorption compilation is composed of five major components: the traditional line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, infrared absorption cross-sections for molecules not yet amenable to representation in a line-by-line form, collision-induced absorption data, aerosol indices of refraction, and general tables such as partition sums that apply globally to the data. The new HITRAN is greatly extended in terms of accuracy, spectral coverage, additional absorption phenomena, added line-shape formalisms, and validity. Moreover, molecules, isotopologues, and perturbing gases have been added that address the issues of atmospheres beyond the Earth. Of considerable note, experimental IR cross-sections for almost 300 additional molecules important in different areas of atmospheric science have been added to the database. The compilation can be accessed through www.hitran.org. Most of the HITRAN data have now been cast into an underlying relational database structure that offers many advantages over the long-standing sequential text-based structure. The new structure empowers the user in many ways. It enables the incorporation of an extended set of fundamental parameters per transition, sophisticated line-shape formalisms, easy user-defined output formats, and very convenient searching, filtering, and plotting of data. A powerful application programming interface making use of structured query language (SQL) features for higher-level applications of HITRAN is also provided.
We present a novel framework and experimental method for the quantification of spatial resolution of a tomography system. The framework adopts the "black box" view of an imaging system, considering only its input and output. The tomography system is locally stimulated with a step input, viz., a sharp edge. The output, viz., the reconstructed images, is analysed by Fourier decomposition of their spatial frequency components, and the local limiting spatial resolution is determined using a cut-off threshold. At no point is an observer involved in the process. The framework also includes a means of translating the quantification region in the imaging space, thus creating a spatially resolved map of objectively quantified spatial resolution. As a case-study, the framework is experimentally applied using a gaseous propane phantom measured by a well-established chemical species tomography system. A spatial resolution map consisting of 28 regions is produced. In isolated regions, the indicated performance is 4-times better than that suggested in the literature and varies by 57% across the imaging space. A mechanism based on adjacent but non-interacting beams is hypothesised to explain the observed behaviour. The mechanism suggests that, as also independently concluded by other methods, a geometrically regular beam array maintains maximum objectivity in reconstructions. We believe that the proposed framework, methodology, and findings will be of value in the design and performance evaluation of tomographic imaging arrays and systems.
This article reports the application of optical tomography and chemical species tomography to the characterisation of the in-cylinder mixture preparation process in a gasoline, direct-injection, single-cylinder, motored research engine. An array of 32 near-infrared beams is transmitted in a horizontal plane across the cylinder bore near the top of the cylinder, through a circular quartz annulus. A novel approach to enable the optical alignment of the transmitting and receiving optics is utilised. The engine is operated at a stoichiometric condition at 1200 r/min, with negative valve overlap timing. Two tomographic measurement schemes (optical attenuation and chemically specific absorption) were used to acquire data on the spatial and temporal distribution of fuel throughout the engine cycle. Optimised data pre-processing methods are described for maximal beam count and data reliability. The presence of fuel during the intake stroke was detected by the optical beam attenuation due to scattering from the liquid gasoline droplets. Optical tomographic reconstruction of the spatial distribution of these droplets was achieved at an imaging rate of 7200 frames per second, revealing rapid intra-cycle spatial variations that were consistent between consecutive cycles. During the compression stroke, chemical species tomography images of fuel vapour were reconstructed from data acquired using chemically selective spectral absorption by the hydrocarbon molecules, at an imaging rate of 2400 frames per second. Later in the compression stroke, the temporal evolution of the fuel vapour distribution in the plane of observation is relatively slow and displays inhomogeneities that are consistent between consecutive cycles. This is the first report of the use of tomography to image, within individual engine cycles, the in-cylinder evolution of both fuel spray droplet distribution and fuel vapour distribution.
Growing customer demands and more stringent regulations to reduce harmful air emissions from ships have resulted in an increased interest for the installation of shipboard abatement technologies. Specifically, the selective catalytic reduction (SCR) technology to reduce emissions of nitrogen oxides (NOx) was early adopted by several Swedish shipping companies. The potential NOx reduction efficiency of the SCR is well established, but the practical experiences of shipboard installations have been less documented. This paper reviews from a systems perspective the practical experiences of marine SCR installations in Swedish shipping. The aim is to identify important not only technical but also human and organizational conditions necessary for safe, efficient, and sustainable SCR operations at sea. Further, to investigate to what extent the capabilities and limitations of human operators and maintainers are taken into account in the design and installation phase of the systems. Two focus group interviews (n = 10) and five individual interviews were held with relevant stakeholders in the industry, following a semi-structured schedule on the themes installation, operation, maintenance, and training. The results show that deficiencies in the overall system design—with a combination of technical issues, maintenance access problems, and untrained operators with inadequate understanding of the SCR process—have led to inefficient, costly, and unsafe operations. It is concluded that installations and operations of marine SCR systems, and possibly other forthcoming abatement technologies, would benefit from the use of traditional ergonomic principles and methods. This would in turn contribute towards increased sustainability and a reduced environmental impact from shipping.
Creating a mesh is the first step in a wide range of applications, including scientific computing and computer graphics. An unstructured simplex mesh requires a choice of meshpoints (vertex nodes) and a triangulation. We want to offer a short and simple MATLAB code, described in more detail than usual, so the reader can experiment (and add to the code) knowing the underlying principles. We find the node locations by solving for equilibrium in a truss structure (using piecewise linear force-displacement relations) and we reset the topology by the Delaunay algorithm. The geometry is described implicitly by its distance function. In addition to being much shorter and simpler than other meshing techniques, our algorithm typically produces meshes of very high quality. We discuss ways to improve the robustness and the performance, but our aim here is simplicity. Readers can download (and edit) the codes from http://math.mit.edu/~persson/mesh.
Hyperspectral absorption tomography (HAT) reconstructs the distribution of key gas parameters, including composition, pressure, and temperature, from multi-beam absorbance data with numerous spectral resolution elements. There is a nonlinear relationship between the parameters of interest and the spectral absorption coefficient, which must be incorporated into the tomography algorithm. Nonlinear HAT simultaneously reconstructs the composition and temperature of a gas by minimizing a single nonconvex objective function, which combines the light attenuation and spectroscopy models, using a metaheuristic technique. The time required for this computation depends, strongly, on the assumed heuristics, but the high computational cost limits the problem size and, hence, the obtainable spatial resolution. Conversely, linear HAT reconstructs the absorption coefficient for each measurement wavenumber, individually, exploiting the linear structure of the underlying tomography problem. Local spectra are then post-processed with a spectroscopic model to recover multiple parameters. The linear technique enables accurate reconstructions on a high-resolution grid by way of an established statistical imaging algorithm. Moreover, local spectra can be employed to gauge phenomena such as multi-species broadening and line mixing with a calibrated regression model. We simulate linear and nonlinear HAT and reconstruct experimental absorbance data using the former approach to demonstrate its superior performance. Nonlinear reconstructions required a 100-fold computational effort compared to linear HAT. In our experimental test, we reconstructed the mole fraction, pressure, and temperature of water vapor in a stagnation flow, which represents the first three-parameter laser absorption tomography experiment. Simulated and experimental results in our paper make a comprehensive case for linear HAT compared to the nonlinear method.
A multichannel tunable diode laser absorption spectrometer is used to measure absolute ammonia concentrations and their distributions in exhaust gas applications with intense CO2 and H2O background. Designed for in situ diagnostics in SCR after treatment systems with temperatures up to 800 K, the system employs a fiber coupled near-infrared distributed feedback diode laser. With the laser split into eight coplanar beams crossing the exhaust pipe, the sensor provides eight concentration measurements simultaneously. Three ammonia ro-vibrational transitions coinciding near 2200.5 nm with rather weak temperature dependency and negligible CO2/H2O interference were probed during the measurements. The line-of-sight averaged channel concentrations are transformed into 2-D ammonia distributions using limited data IR species tomography based on Tikhonov regularization. This spectrometer was successfully applied in the exhaust system of a 340 kW heavy duty diesel engine operated without oxidation catalyst or particulate filter. In this harsh environment the multi-channel sensor achieved single path ammonia detection limits of 25 to 80 ppmV with a temporal resolution of 1 Hz whereas, while operated as a single-channel sensor, these characteristics improved to 10 ppmV and 100 Hz. Spatial averaging of the reconstructed 2-D ammonia distributions shows good agreement to cross-sectional extractive measurements. In contrast to extractive methods more information about spatial inhomogeneities and transient operating conditions can be derived from the new spectrometer.
A new developed tunable diode laser spectrometer for the measurement of ammonia (NH3) mole fractions in exhaust gas matrices with strong CO2 and H2O background at temperatures up to 800 K is presented. In-situ diagnostics in harsh exhaust environments during SCR after treatment are enabled by the use of ammonia transitions in the v2+v3 near-infrared band around 2300 nm. Therefore three lines have been selected, coinciding near 2200.5 nm (4544.5 cm-1) with rather weak temperature dependency and minimal interference with CO2 and H2O. A fiber-coupled 2.2 μm distributed feedback laser diode was used and attached to the hot gas flow utilizing adjustable gas tight high-temperature fiber ports. The spectrometer spans four coplanar optical channels across the measurement plane and simultaneously detects the direct absorption signal via a fiber-coupled detector unit. An exhaust simulation test rig was used to characterize the spectrometer’s performance in ammonia doped hot gas environments. We achieved a temporal resolution of 13 Hz and temperature dependent precisions of NH3 mole fraction ranging from 50 ppmV to 70 ppmV. There the spectrometer achieved normalized ammonia detection limits of 7-10 ppm∙m and 2-3 ppm∙m/√Hz.
We present a numerical study demonstrating the simultaneous reconstruction of the temperature, species concentration, and pressure of molecular water vapor, by tomographic means, from near-infrared laser absorption measurements over beam paths transecting the gaseous region. The synthetic measurement data is predicted from a set of randomized distributions of the reconstructed variables using spectroscopic theory and the HITRAN parameter database. Design optimization geometric orientation of the beams is performed to maximize reconstruction accuracy.
Acoustic travel-time tomography allows one to reconstruct temperature and wind velocity fields in the atmosphere. In a recently published paper [S. Vecherin et al., J. Acoust. Soc. Am. 119, 2579 (2006)], a time-dependent stochastic inversion (TDSI) was developed for the reconstruction of these fields from travel times of sound propagation between sources and receivers in a tomography array. TDSI accounts for the correlation of temperature and wind velocity fluctuations both in space and time and therefore yields more accurate reconstruction of these fields in comparison with algebraic techniques and regular stochastic inversion. To use TDSI, one needs to estimate spatial-temporal covariance functions of temperature and wind velocity fluctuations. In this paper, these spatial-temporal covariance functions are derived for locally frozen turbulence which is a more general concept than a widely used hypothesis of frozen turbulence. The developed theory is applied to reconstruction of temperature and wind velocity fields in the acoustic tomography experiment carried out by University of Leipzig, Germany. The reconstructed temperature and velocity fields are presented and errors in reconstruction of these fields are studied.