Understanding and interpreting laser diagnostics in flames: a review of experimental measurement techniques

Published: 29 November 2019There is a wealth of existing experimental data of flames collected using laser diagnostics. The primary objective of this review is to provide context and guidance in interpreting these laser diagnostic data. This educational piece is intended to benefit those new to laser diagnostics or with specialization in other facets of combustion science, such as computational modeling. This review focuses on laser-diagnostics in the context of the commonly used canonical jet-in-hot-coflow (JHC) burner, although the content is applicable to a wide variety of configurations including, but not restricted to, simple jet, bluff body, swirling and stratified flames. The JHC burner configuration has been used for fundamental studies of moderate or intense low oxygen dilution (MILD) combustion, autoignition and flame stabilization in hot environments. These environments emulate sequential combustion or exhaust gas recirculation. The JHC configuration has been applied in several burners for parametric studies of MILD combustion, flame reaction zone structure, behavior of fuels covering a significant range of chemical complexity, and the collection of data for numerical model validation. Studies of unconfined JHC burners using gaseous fuels have employed point-based Rayleigh-Raman or two-dimensional Rayleigh scattering measurements for the temperature field. While the former also provides simultaneous measurements of major species concentrations, the latter has often been used in conjunction with planar laser-induced fluorescence (PLIF) to simultaneously provide quantitative or qualitative measurements of radical and intermediary species. These established scattering-based thermography techniques are not, however, effective in droplet or particle laden flows, or in confined burners with significant background scattering. Techniques including coherent anti-Stokes Raman scattering (CARS) and non-linear excitation regime two-line atomic fluorescence (NTLAF) have, however, been successfully demonstrated in both sooting and spray flames. This review gives an overview of diagnostics techniques undertaken in canonical burners, with the intention of providing an introduction to laser-based measurements in combustion. The efficacy, applicability and accuracy of the experimental techniques are also discussed, with examples from studies of flames in JHC burners. Finally, current and future directions for studies of flames using the JHC configuration including spray flames and studies and elevated pressures are summarized.Michael J. Evans and Paul R. Medwel

Temperature measurements in a turbulent spray flame using NTLAF

This paper reports the first measurements of temperature in turbulent dilute spray flames of acetone using non-linear excitation regime two-line atomic fluorescence (NTLAF), a technique well suited for temperature measurements in flames laden with particles and droplets. The NTLAF technique has previously been successful at measuring temperature in turbulent non-premixed flames of gaseous fuels in the presence of soot. Temperature is extracted from the fluorescence ratio collected of two lines of indium generated from indium chloride which is seeded into the flow. The non-linear excitation regime is exploited to improve the signal-to-noise ratio of the measurements. In the current arrangement, indium chloride is seeded with the acetone fuel and laminar premixed flames of methane are used for calibration. The preliminary results are promising and indicate that the presence of droplets does not affect the signal, making NTLAF particularly well-suited to measure temperature in turbulent, dilute spray flames.Paul R. Medwell, Phuong X. Pham and Assaad R. Masrihttp://cfe.uwa.edu.au/news/acs2013http://www.anz-combustioninstitute.org

Simultaneous imaging of OH, formaldehyde, and temperature of turbulent nonpremixed jet flames in a heated and diluted coflow

This paper reports measurements in turbulent nonpremixed CH4/H2 jet flames issuing into a heated and highly diluted coflow. These conditions emulate those of moderate or intense low-oxygen-dilution (MILD) combustion. The spatial distribution of the hydroxyl radical (OH), formaldehyde (H2CO), and temperature, imaged using planar laser-induced fluorescence and Rayleigh scattering laser diagnostic techniques, are measured and presented. Comparisons are made between three jet Reynolds numbers and two coflow O2 levels. Measurements are taken at two downstream locations. The burner used in this work facilitates the additional study on the effects of the entrainment of surrounding air on the flame structure at downstream locations. Reducing the coflow oxygen level is shown to lead to a suppression of OH as a result of the reduced temperatures in the reaction zone. Decreasing the oxygen level of the coflow also results in a broadening of the OH distribution. At downstream locations, the surrounding air mixes with the jet and coflow. The subsequent drop in the temperature of the oxidant stream can lead to a rupture of the OH layer. Localized extinction allows premixing of the fuel with the surrounding air. The result is an increase in the reaction rate, highlighting the need for homogeneous mixing to maintain MILD combustion conditions.Paul R. Medwell, Peter A.M. Kalt and Bassam B. Dall

Natural draft and forced primary air combustion properties of a top-lit up-draft research furnace

Worldwide, over four million people die each year due to emissions from cookstoves. To address this problem, advanced cookstoves are being developed, with one system, called a top-lit up-draft (TLUD) gasifier stove, showing particular potential in reducing the production of harmful emissions. A novel research furnace analogy of a TLUD gasifier stove has been designed to study the TLUD combustion process. A commissioning procedure was established under natural draft and forced primary air conditions. A visual assessment was performed and the temperature and emissions profiles were recorded to identify the combustion phases. The efficiency was evaluated through the nominal combustion efficiency (NCE = CO2/(CO2 + CO)), which is very high in the migrating pyrolysis phase, averaging 0.9965 for the natural draft case. Forced primary air flows yield similar efficiencies. In the lighting phase and char gasification phase the NCE falls to 0.8404 and 0.6572 respectively in the natural draft case. When providing forced primary air flows, higher NCE values are achieved with higher air flows in the lighting phase, while with lower air flows in the char gasification phase. In the natural draft case high H2 emissions are also found in the lighting and char gasification phases, the latter indicating incomplete pyrolysis. From the comparison of the natural draft with the forced draft configurations, it is evident that high efficiency and low emissions of incomplete combustion can only be achieved with high controllability of the air flow in the different phases of combustion.Thomas Kirch, Paul R. Medwell, Cristian H.Birze

Modeling lifted jet flames in a heated coflow using an optimized Eddy dissipation concept model

Moderate or intense low oxygen dilution (MILD) combustion has been established as a combustion regime with improved thermal efficiency and decreased pollutant emissions, including NOx and soot. MILD combustion has been the subject of numerous experimental studies, and presents a challenge for computational modeling due to the strong turbulence–chemistry coupling within the homogeneous reaction zone. Models of flames in the jet in hot coflow (JHC) burner have typically had limited success using the eddy dissipation concept (EDC) combustion model, which incorporates finite-rate kinetics at low computational expense. A modified EDC model is presented, which successfully simulates an ethylene-nitrogen flame in a 9% O2 coflow. It is found by means of a systematic study in which adjusting the parameters and from the default 0.4082 and 2.1377 to 3.0 and 1.0 gives significantly improved performance of the EDC model under these conditions. This modified EDC model has subsequently been applied to other ethylene- and methane-based fuel jets in a range of coflow oxidant stream conditions. The modified EDC offers results comparable to the more sophisticated, and computationally expensive, transport probability density function (PDF) approach. The optimized EDC models give better agreement with experimental measurements of temperature, hydroxyl (OH), and formaldehyde (CH2O) profiles. The visual boundary of a chosen flame is subsequently defined using a kinetic mechanism for OH* and CH*, showing good agreement with experimental observations. This model also appears more robust to variations in the fuel jet inlet temperature and turbulence intensity than the standard EDC model trialed in previous studies. The sensitivity of the newly modified model to the chemical composition of the heated coflow boundary also demonstrates robustness and qualitative agreement with previous works. The presented modified EDC model offers improved agreement with experimental data profiles than has been achieved previously, and offers a viable alternative to significantly more computationally expensive modeling methods for lifted flames in a heated and vitiated coflow. Finally, the visually lifted flame behavior observed experimentally in this configuration is replicated, a phenomenon that has not been successfully reproduced using the EDC model in the past.M. J. Evans, P. R. Medwell & Z. F. Tian

The influence of coal particle and air jet momenta on MILD combustion in a recuperative furnace

The moderate or intense low-oxygen dilution (MILD) combustion regime is a promising technology that operates at high combustion efficiency and lessens pollutant emissions. This numerical study of a parallel jet recuperative MILD combustion furnace investigates the effects of coal particle size and inlet air momentum on furnace dynamics and global CO emissions. It is found that coal particle size affects the coal penetration depth within the furnace and the location of a particle stagnation point. The effects of air inlet momentum are tested in two ways, first by raising the inlet temperature at constant mass flow rate, and second by increasing the mass flow rate at constant temperature. In both cases, increasing the air jet momentum broadens the reaction zone and facilitates MILD combustion, but also increases CO emissions due to lowered reaction rates.Emmet M. Cleary, Paul R. Medwell, Bassam B. Dallyhttp://cfe.uwa.edu.au/news/acs2013http://www.anz-combustioninstitute.org

Parametric study of EDC model constants for modelling lifted jet flames in a heated coflow

Moderate or Intense Low oxygen Dilution (MILD) combustion offers improved thermal efficiency and a reduction of NOx pollutants and soot emissions compared to conventional combustion. Previously MILD combustion in the Jet in Hot Coflow (JHC) burner, using methane-based fuels, has been simulated with the Eddy Dissipation Concept (EDC) turbulence-chemistry interaction model. In this paper, the EDC model is used with a modified standard k-ε (SKE) turbulence model to simulate an ethylene-nitrogen flame in a hot, 9% O2 coflow. Modifications to the parameters used in the EDC model are investigated and a parametric study of Cτ and Cξ is undertaken. The combination of Cτ=3 and Cξ=1, modified from the default Cτ=0.4082 and Cξ=2.1377 used in the original model, shows better agreement with experimentally measured radial profiles than any previous implementation of the EDC model and replicates the visually lifted properties observed experimentally in this configuration which have not been modelled successfully in the past.M.J. Evans, P.R. Medwell, Z.F. Tianhttp://cfe.uwa.edu.au/news/acs2013http://www.anz-combustioninstitute.org

Influences of fuel bed depth and air supply on small-scale batch-fed reverse downdraft biomass conversion

The producer gas composition and the thermochemical conversion process of a small-scale reverse downdraft reactor has been investigated under ten operating conditions with different fuel bed depths and air supply rates. The operating principle of this research reactor is a batch-fed reverse downdraft process, using wood pellets as the solid biomass fuel. The oxygen-limited regime, where the fuel consumption increases nearly linearly with the air supply, has been identified, and four flow rates over the range of this regime have been investigated. The fuel bed depth was varied between one and four reactor diameters (1D (100 mm)–4D (400 mm)). The results demonstrate that increasing the primary air mass flux leads to both greater fuel consumption and higher temperatures as well as heating rates in the reaction front. Greater air supply rates and the resulting higher temperatures lead to a substantial increase in fuel conversion into permanent gases, rather than tars or char, and a rise in the cold gas efficiency (CGE) from 33% to 73%, from the lowest to highest air flow rate at a 4D fuel bed depth. However, the temporal producer gas heating value is similar in all configurations. With increasing depth, it is evident that H₂ production is promoted by the char layer downstream of the reaction front and that a certain layer thickness is necessary to achieve the potential near steady-state product flow at a specific flow rate. Interestingly, a greater fuel bed depth enhances the hydrogen conversion rate to permanent gases by more than 20% and the CGE from 48% to 53%, while the fuel consumption and temperature profiles remain similar. A general trend of increasing performance was identified at the 3D and 4D depths, when compared with the 1D and 2D fuel bed depths. The produced char exhibits a high fixed and elemental carbon content. Therefore, the conversion efficiency of this process can be increased both through increasing the fuel bed depth and, even more, through adjusting the air supply, promoting the yield of permanent gases and the conversion of produced tars.Thomas Kirch, Paul R. Medwell, Cristian H. Birzer and Philip J. van Ey

Influence of primary and secondary air supply on gaseous emissions from a small-scale staged solid biomass fuel combustor

The emissions from traditional biomass combustion systems for cooking and heating are, globally, the main cause for premature mortality as a result of air pollution. A staged combustion process that separates the thermochemical conversion of the solid fuel and the combustion of the released products offers potential to reduce harmful emissions for solid fuel combustion and could, therefore, help mitigate the issue. In the present study, the fundamental combustion behavior of a small-scale staged combustor was investigated, with a focus on an independent systematic analysis of relevant parameters. Natural and forced draft conditions as well as a combination of both were tested. The relative location of primary to secondary air was also varied. When lighting the fuel, higher air flows lead to faster ignition and lower emissions. A steady-state combustion phase is achieved when gasification products are burned with secondary air, which occurs mainly while the solid fuel is being pyrolyzed. After the steady-state phase, char remains as the solid pyrolysis product. Gasification of the remaining char was found to release great amounts of CO, which are emitted from the combustor, in the case of natural draft secondary air (SA). With higher air flows of forced SA, an exceptionally high nominal combustion efficiency [NCE = XCO2/(XCO + XCO2)] can be achieved in the steady-state phase. Forced SA flows cause a longer duration of the steady-state phase from the combustion of raw biomass gasification products into the combustion of char gasification products. This extension leads to a significant reduction of emissions of incomplete combustion. Additionally, smaller distances between the SA inlet and the fuel stack caused lower emissions of incomplete combustion. The combination of forced draft primary air and natural draft SA presented worse combustor performance than under natural draft conditions.Thomas Kirch, Cristian H. Birzer, Philip J. van Eyk, and Paul R. Medwel

Particulate emissions from a wood-fired improved biomass stove

Worldwide, approximately three billion people cook their food using biomass fuels such as wood, charcoal, crop residues, and animal dung. The emissions produced by these smoky fires lead to four million premature deaths annually and large environmental consequences. While the negative effects of biomass burning have spurred much research into designing less polluting cookstoves, researchers need a knowledge-base to draw from when making design decisions. However, previous research has measured these emissions relatively far away from the source, not exploring how the design modifications affect the actual combustion. In this study, the emissions from an improved cookstove, the Berkeley-Darfur Stove, and single blocks of wood are examined in-situ using laser extinction. The pollutant production, measured by the opacity or soot volume fraction, was compared between the two systems to gain a deeper understanding of combustion in the stove while providing initial steps towards a non-intrusive sampling system for pollutant production in cookstove combustion chambers.Kathleen M. Lask, Paul R. Medwell, Cristian H. Birzer, Ashok J. Gadgilhttp://cfe.uwa.edu.au/news/acs2013http://www.anz-combustioninstitute.org