Investigation of flame stretch in turbulent lifted jet flame

DNS data of a laboratory-scale turbulent lifted hydrogen jet flame has been analysed to show that this flame has mixed mode combustion not only at the flame base but also in downstream locations. The mixed mode combustion is observed in instantaneous structures as in earlier studies and in averaged structure, in which the predominant mode is found to be premixed combustion with varying equivalence ratio. The nonpremixed combustion in the averaged structure is observed only in a narrow region at the edge of the jet shear layer. The analyses of flame stretch show large probability for negative flame stretch leading to negative surface averaged flame stretch. The displacement speed-curvature correlation is observed to be negative contributing to the negative flame stretch and partial premixing resulting from jet entrainment acts to reduce the negative correlation. The contribution of turbulent straining to the flame stretch is observed to be negative when the scalar gradient aligns with the most extensive principal strain rate. The physics behind the negative flame stretch resulting from turbulent straining is discussed and elucidated through a simple analysis of the flame surface density transport equation.The authors are grateful for the inspiring discussion with Prof. K.N.C. Bray, and financial support from Mitsubishi Heavy Industries (MHI) is gratefully acknowledged. A part of this work is performed under the collaborative research between Cambridge University and JAXA.This is an Accepted Manuscript of an article published by Taylor & Francis in Combustion Science and Technology on 24 February 2014, available online: http://wwww.tandfonline.com/10.1080/00102202.2013.877335

An experimental investigation into the role of autoignition in turbulent flame stabilisation

This thesis presents experimental and complimentary numerical results based on a turbulent jet in a hot coflow burner (JHC). The thesis focuses on understanding and exploring the relative importance of autoignition in the flame stabilisation process for the conditions, temperatures and fuels considered. The influence of fuel type is explored using a range of gaseous fuels including: alkanes, alkenes, H2 and dimethyl ether (DME). High-speed (10 kHz) measurements of chemiluminescence and sound are applied to all flame cases, for all fuels, the measurements are used to temporally resolve the interaction of the flame base with ignition kernels. Similar flame-base and ignition kernel interaction characteristics are found for all fuels where the formation and merging of rapidly growing ignition kernels stabilise these flames. A measurement campaign employing 10 kHz OH and CH2O Planar Laser Induced Fluorescence combined with volumetric chemiluminescence imaging is applied to the ignition kernel formation region in DME flames. The measurements identify regions of low and high-temperatures respectively, with their spatial overlap representing heat release. The kernel heat release measurements indicate that differing degrees of autoignition stabilisation occurs for DME flames, specific to high and low coflow temperature flames. High coflow temperature flames produce lower heat release ignition-kernels; hence these flames are believed to have reduced dependence on autoignition for stability. Zero-dimensional and one-dimensional numerical simulation results, obtained in this thesis, agree with the findings from the hot coflow experiments. The 0-D ignition delay times are shown to successfully capture the different fuels lift-off height sensitivities with coflow temperature. The sensitivity of relatively low coflow temperatures are particularly well represented by delay times, with a linear correlation between delay times and experimental lift-off heights. To replicate the strained and diffusive conditions induced by the JHC burner, unsteady 1-D counter-flow simulations were applied. These simulations, using DME, identify that for high coflow temperature flames, the ignition kernels produce lower heat release, since they are igniting leaner. Using CH4 with the same counter-flow setup, the effect of strain-rate was explored. It was found that increased strain rate delays ignition, since the unity balance between diffusion and production fluxes of CH2O is also delayed. Furthermore, under autoignition conditions, the counter-flow solver, in addition to a premixed solver, show CH2O convection and production fluxes increase, with a corresponding diffusive decrease

Lean partially premixed turbulent flame equivalence ratio measurements using laser-induced breakdown spectroscopy

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.The creation of a more stable flame along with the extension of flammability limits under lean mixture combustion was the main motivation to develop a new burner design, which has been investigated in this research. The current burner configuration was utilized to create a wide range of higher turbulent intensities and to produce different degrees of mixture inhomogeneity, which acted to promote minimum pollution, highest performance and higher flame stability. The burner stability assessment was investigated using two types of fuel: natural gas (NG) and liquefied petroleum gas (LPG). They were tested under different degrees of partial premixing, and two turbulence generator disks for lean mixture at an equivalence ratio of φ = 0.8 were used. Following this, the Laser Induced Breakdown Spectroscopy (LIBS) technique was utilized to characterize and quantify the impact of changing the disk slit diameter on the distributions profiles of equivalence ratio or mixture fraction for a NG/air partially premixed flame. A series of homogeneous NG/air mixtures with different equivalence ratios were used to obtain the correlations between the measured emission lines of LIBS spectra and the global flame equivalence ratio. Consequently, the emission spectral lines ratios of H/N, H/O and C/N + O were utilized to predict the equivalence ratio distributions. The results demonstrated that for all of the mixing lengths, NG/air mixture with larger disk generator diameter yielded the maximum burner stability, whilst the LPG/air mixture with a larger disk generator diameter resulted in the minimum burner stability. Furthermore, the flame associated with the larger disk slit diameter had a uniform local equivalence ratio distribution and lower RMS fluctuation profiles of equivalence ratio in comparison to the lower disk slit diameter

Experimental and theoretical investigation of the flashback of a swirling, bluff-body stabilised, premixed flame

Flashback of an open turbulent, premixed flame in a swirl burner with central bluff-body is considered. The aim is to obtain further understanding of the physical mechanisms responsible for the upstream flame propagation. Previous studies on the same configuration hypothesised that there is an adverse pressure gradient in the direction of flame propagation. In this paper this is further investigated experimentally and theoretically. Static gauge pressure is measured on the surface of the bluff-body during flame flashback. Simultaneously, flame luminosity is imaged at 5 kHz. The results indicate that the static pressure rises downstream of the propagating reactive front. This is, then, discussed in the context of the theory of vortex bursting. An existing theory of flame propagation in the core flow is extended to a configuration similar to that investigated experimentally. The theory, although highly simplified, explains the generation of adverse pressure gradient across the flame and is qualitatively consistent with the experiment

A priori investigation of subgrid correlation of mixture fraction and progress variable in partially premixed flames

Subgrid correlation of mixture fraction, Z, and progress variable, c, is investigated using Direct Numerical Simulation (DNS) data of a hydrogen lifted jet flame. Joint subgrid behaviour of these two scalars are obtained using a Gaussian-type filter for a broad range of filter sizes. Joint probability density function (JPDF) constructed using single-snapshot DNS data is compared qualitatively with that computed using two independent β-PDFs and a copula method. Strong negative correlation observed at different streamwise locations in the flame is well captured by the copula method. The subgrid contribution to the Z-c correlation becomes important if the filter is of the size of laminar flame thickness or larger. A priori assessment for the filtered reaction rate using flamelet approach with independent β-PDFs and correlated JPDF is then performed. The comparison with the DNS data shows that both models provide reasonably good results for a range of filter sizes. However, the reaction rate computed using copula JPDF is found to have a better agreement with the DNS data for large filter sizes because the subgrid Z-c correlation effect is included

Turbulent partially premixed combustion: DNS analysis and RANS simulation

Increasingly stringent regulation of pollutant emission has motivated the search for cleaner and more efficient combustion devices, which remain the primary means of power generation and propulsion for all kinds of transport. Fuel-lean premixed combustion technology has been identified to be a promising approach, despite many difficulties involve, notably issues concerning flame stability and ignitability. A partially premixed system has been introduced to remedy these problems, however, our understanding on this combustion mode needs to be greatly improved to realise its full potential. This thesis aims to further the understanding of various fundamental physical processes in turbulent partially premixed flames. DNS data of a laboratory-scale hydrogen turbulent jet lifted flame is analysed in this study. The partially premixed nature of this flame is established by examining the instantaneous and averaged reaction rates and the "Flame Index", which indicate premixed and diffusion burning modes coexisting. The behaviour of turbulent flame stretch and its relation to other physical processes, in particular the scalar-turbulence interaction, the effects of partial premixing on the displacement speed of iso-scalar surface and its correlation with the surface curvature are explored using DNS data. The scalar gradient alignment characteristics change from aligning with the most compressive strain to aligning with the most extensive one in regions of intensive heat release. This alignment change creates negative normal strain rate which can result in negative surface averaged tangential strain rate. The partial premixing affects the flame surface displacement speed through the mixture fraction dissipation rate and a second derivative in the mixture fraction space. The correlation of curvature and displacement speed is found to be negative in general and the effects of partial premixing act to reduce this negative correlation. The combined effects of the normal strain rate and the displacement speed/curvature correlation contribute to the negative mean flame stretch observed in the flame brush. Scalar dissipation rates (SDR) of the mixture fraction ẼZZ, progress variable Ẽcc and their cross dissipation rates (CDR) ẼcZ are identified as important quantities in the modelling of partially premixed flames. Their behaviours in the lifted flame stabilisation region are examined in a unified framework. It is found that SDR of mixture fraction is well below the quenching value in this region while SDR of progress variable is smaller than that in laminar flames. The CDR changes from weakly positive to negative at the flame leading edge due to the change in scalar gradient alignment characteristics. Axial and radial variation of these quantities are analysed and it is found that Ẽcc is an order of magnitude bigger than ẼZZ. ẼcZ is two orders of magnitude smaller than Ẽcc and it can be either positive or negative depending on local flow and flame conditions. Simple algebraic models show reasonable agreement compared to DNS when a suitable definition of c is used. Further statistics of the scalar gradients are presented and a presumed lognormal distribution is found to give reasonable results for their marginal PDFs and a bivariate lognormal distribution is a good approximation for their joint PDF. Four mean reaction rate closures based on presumed PDF and flamelets are assessed a priori using DNS data. The turbulent flame front structure is first compared with unstrained and strained laminar premixed and dif fusion flamelets. It is found that unstrained premixed flamelets give overall reasonable approximation in most parts of this flame. A joint PDF model which includes the correlation between mixture fraction and progress variable using a "copula" method shows excellent agreement with DNS results while their statistical independence does not hold in the burning regions of this partially premixed flame. The unstrained premixed flamelet with the correlated joint PDF method is identified to be the most appropriate model for the lifted jet flame calculation. This model is then used in the RANS simulation of turbulent jet lifted flames. A new model to include the contribution from diffusion burning and the effects of partial premixing due to SDR of mixture fraction is also identified and included in the calculation. These models are implemented in a commercial CFD code "Fluent" with user defined scalars and functions. It is found that both the correlated joint PDF model and the model accounting for the diffusive burning in partial premixing are important in order to accurately predict flame lift-off height compared to the experiments

Numerical simulation of flames using flamelet models

The thesis topic is located in the domain of numerical simulation of laminar flames. The principal aim of the presented research is the study of numerical techniques for the multidimensional simulation of flames with low computational costs. Present work is divided into three parts: First part is related to the development of a C++ simulation code for 1D laminar premixed flames. In the second part, a new technique to account for differential diffusion effects is proposed, which is based on tabulated chemistry methods. The third part focuses on the analysis of partially premixed flames. A dedicated one-dimensional flame code is discussed for the simulation of complex/detailed chemistry and diffusion processes in premixed laminar flames. This code is written in C++ and is able to use different diffusion models (Fickian, Hirschfelder and Curtiss). The code yields accurate solutions of the major parameters as well as pollutant formation, both in the flame zone as well as downstream in the post-flame region. Results prove the accuracy of the code when compared to experimental data. Following, a new technique is proposed to include differential diffusion effects into flamelet models. This approach is developed in the context of tabulated chemistry methods.The technique is based on correcting the progress-variable of flamelet models. The main feature of the proposed technique is the use of only one progress variable equation (1D manifold) without requiring a second parameter. This correction technique allows including detailed chemistry effects at low-cost in numerical simulation of multidimensional flames. A series of simulations are carried out for various flames. The results are excellently matched with full model solutions/detailed chemistry solutions. The flamelet solutions databases, namely premixed and non-premixed, are further tested for partially premixed flames. This study is based on the investigation of partially premixed flame using single mode flamelet database solutions. For the verification of database solutions, finite rate chemistry simulations are also carried out to solve partially premixed flames. 3D jet coflow simulations are performed for three different level of premixing and results are compared with experimental data. The results show good agreement along with capabilities and limitations of flamelet databases solutions.El tema de la tesi es troba en el domini de la simulació numèrica de les flames laminars. L'objectiu principal de la investigació presentada és l'estudi de tècniques numèriques per a la simulació multidimensional de flames amb baixos costos computacionals. El treball actual es divideix en tres parts: La primera part es relaciona amb el desenvolupament d'un codi 1D de simulació C ++ per flames laminars premesclades. A la segona part, es proposa una nova tècnica per explicar els efectes de difusió diferencial, que es basa en mètodes de química tabulada. La tercera part se centra en l'anàlisi de flames parcialment premesclades. Es discuteix un codi de flama unidimensional dedicat per a la simulació de processos complexos i detallats de química i difusió en flames laminars premesclades. Aquest codi està escrit en C ++ i és capaç d'utilitzar diferents models de difusió (Fickian, Hirschfelder i Curtiss). El codi proporciona solucions precises dels paràmetres principals, així com la formació de contaminants, tant a la zona de la flama com a la regió posterior a la flama. Els resultats demostren l'exactitud del codi en comparació amb les dades experimentals. A continuació, es proposa una nova tècnica per a incloure efectes de difusió diferencial en models de flamelet. Aquest enfocament es desenvolupa en el context dels mètodes tabulats de química. La tècnica es basa en la correcció de la variable de progrés dels models de flamelet. La característica principal de la tècnica proposada és l'ús d'una única equació de variable de progrés (espai 1D) sense necessitat d'un segon paràmetre. Aquesta tècnica de correcció permet incloure efectes químics detallats a baix cost en la simulació numèrica de flames multidimensionals. Una sèrie de simulacions es realitzen per diverses flames. Els resultats mostren una concordància excel·lentment amb solucions completes/detallades de la química. Les bases de dades de solucions de flamelet, a saber premesclades i no premesclades, es sotmeten a proves addicionals per flames parcialment premesclades. Aquest estudi es basa en la investigació de flames parcialment prebarrejades utilitzant solucions de base de dades monomode. Per a la verificació d'aquestes bases de dades, es realitzen simulacions de química de velocitat finita per resoldre flames parcialment premesclades. Es realitzen simulacions de flux d'aire en 3D per a tres nivells diferents de premescla i els resultats es comparen amb dades experimentals. Els resultats mostren un bon acord juntament amb les capacitats i limitacions de les solucions de bases de dades de flamelet.Postprint (published version

Large eddy simulation of an oscillating flame using the stochastic fields method

Large eddy simulation (LES) of a partially pre-mixed, swirl-stabilised flame is performed using atransported Probability Density Function approachsolved by the stochastic fields method to accountfor turbulence-chemistry interaction on the sub-gridscales. The corresponding sub-grid stresses and scalarfluxes are modelled via a dynamic version of theSmagorinsky model and a gradient diffusion approx-imation, respectively.A 15-step reduced methanemechanism including 19 species is employed for thedescription of all chemical reactions. The test case in-volves a widely studied gas turbine model combustorwith complex geometry and the simulation is carriedout for a specific operating condition involving an os-cillating flame. Overall, results of the velocity, temper-ature and major species mass fractions as well as theinstantaneous thermochemical properties are shown tobe in good agreement with experimental data, demon-strating the capabilities of the applied stochastic fieldsmethod. The inclusion of wall heat transfer in the com-bustion chamber is found to improve temperature pre-dictions, especially in the near-wall regions. In sum-mary, this work showcases the LES method’s accuracyand robustness - none of the default model parametersare adjusted - for an application to complex, partiallypremixed combustion problem

A numerical method for the prediction of combustion instabilities

This thesis describes one of the first computational works to investigate the physical feedback mechanisms associated with self-excited, combustion-driven instabilities in gas turbines. For this purpose, a novel numerical method based on large eddy simulation is devised. The method (called BOFFIN) uses a fully compressible formulation to account for acoustic wave propagation and applies a transported probability density function approach for turbulence-chemistry interactions. The latter is solved by the Eulerian stochastic fields method and is complemented by two different 15-step / 19 species chemical reaction schemes. This approach is shown to be flame burning regime independent and therefore highly applicable in the context of partially premixed gas turbine combustion. Combustion instabilities are a phenomenon often encountered in the late design stages of modern gas turbine combustors. Under certain conditions, these types of instabilities can develop into sustained limit-cycle oscillations with potentially severe consequences on a combustor's operating behaviour. In order to study the various physical feedback mechanisms driving such limit-cycle oscillations, two different test cases are simulated in the present work. Firstly, the combined effects of thermo-acoustic and hydrodynamic instabilities are examined in the lab-scale PRECCINSTA model combustor. Secondly, the superposition of a longitudinal and azimuthally spinning instability mode is investigated in the industrial SGT-100 combustor. Amongst the different feedback mechanisms identified and studied in these cases are: mass flow rate and equivalence ratio oscillations, as well as hydrodynamic phenomena such as flame angle oscillations, periodic vortex shedding and a precessing vortex core. It is further demonstrated that in addition to reproducing longitudinal instability modes, the applied LES approach is capable of accounting for modes acting in the transverse direction. Overall, the findings of this research project strongly suggest that BOFFIN is a reliable and accurate method for the prediction of self-excited combustion instabilities in gas turbines.Open Acces