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Papers by Siva Muppala

Innovative Practice in Higher Education, 2018

This poster will focus on a problem based learning (PBL) method which was incorporated in the Yea... more This poster will focus on a problem based learning (PBL) method which was incorporated in the Year 2 (level 5) module `Care of the Older Person`. This method was used to encourage students to identify and develop a variety of clinical assessment skills based on a short 10 minute video of Peter Thursby`s (Innovations in Dementia, 2010) autobiographical account of his life and career.

Alexandria Engineering Journal

Aerospace Science and Technology, 2022

ABSTRACT The enhanced combustion properties of hydrogen as well as worldwide concerns due to glob... more ABSTRACT The enhanced combustion properties of hydrogen as well as worldwide concerns due to global warming are acting together as the major driving force to a future hydrogen-based economy. However this will not be a step change and the blends of hydrogen and hydrocarbons must be looked as a transition solution to a purely hydrogen-based economy [1]. In the present study, numerical investigations of a wider range of fuel/air mixtures which includes H 2 -hydrocarbon blends have been carried out for the unstretched laminar burning velocities, S L0 at STP conditions using detailed reaction mechanisms. For this purpose, different reaction mechanisms are combined with the combustion simulation package Cosilab [2]. Comparative studies with experimental data from several independent investigations show that San Diego mechanism [3] predicts S L0 of H 2 /air mixtures very well while Konnov v0.5 [4] is the most valid mechanism for CH 4 /air mixtures. QMech [5] shows good agreement for C 3 H 8 /air mixtures. For H 2 /CH 4 /air mixtures of overall equivalence ratio 1.0, kinetic mechanisms show a splitting behaviour for varied concentration levels of blended fuel hydrogen. For mixtures with volume concentrations of H 2 (Vol. H 2) < 60%, QMech predicts reasonable results while for Vol. H 2 ≥ 60% Appel [6] is comparatively better. QMech also shows valid predictions for H 2 /C 3 H 8 /air mixtures especially for equivalence ratios between 0.6 and 1.0. For H 2 /CO/air mixtures satisfactory agreement is found with CMech [7]. An extended evaluation of the influence of carbon dioxide in the latter mixture is conducted which shows that S L0 decreases almost linearly with increasing quantities of carbon dioxide.

IOP Conference Series: Materials Science and Engineering, 2021

In this study, we show numerical predictions of the Zimont and Peter’s turbulent flame speed mode... more In this study, we show numerical predictions of the Zimont and Peter’s turbulent flame speed models for low-swirl Methane/Air/Hydrogen flames [1]. These models are contained as default options within the ANSYS Fluent Premixed reaction model. Two distinct simulation tasks as part of the present study are – non-reacting and reacting flows, for with three different methane mixtures in compositions added with – 0, 40 and 60% hydrogen. The results show that the RANS approach provides a reasonable prediction of the cold flow conditions, whilst the reacting flow conditions, good agreement is reached up to 40% enrichment, except near the recirculation region,.

Education Quarterly Reviews, 2021

We propose a new approach to classroom learning based on sequential numeral-division. It builds o... more We propose a new approach to classroom learning based on sequential numeral-division. It builds on the concept of trichotomy – division of students based on creamy-level, middle-level and weaker-level students -- proposed by the present authors. A sequenced series of formative assessments can map student progress and achievement, particularly in the case of weaker students. The idea behind the development of this model is to study if weak students perform better on critical-thinking tests in a collaborative learning setting rather than when they study individually. We propose a mathematical model to measure group activity/achievement, which is a complex function of several parameters. We collect data on different parameters for validation of the model in the near future.

Methodical design of gas turbine premixed turbulent combustion systems for fuel flexibility is a ... more Methodical design of gas turbine premixed turbulent combustion systems for fuel flexibility is a necessity for higher efficiency and operational flexibility, due to the use of vast range of fuel types and fuel mixtures. Different fuels/fuel mixtures have different reactivity and varied molecular transport characteristics. The central theme of this paper is numerical investigation of these effects, along with the dynamics of turbulent premixed high-pressure flames. We studied these dominant factors in two different geometrical configurations, a cylindrical dump combustion system and an industrial gas turbine single burner combustion system. Here, we chose the algebraic flame surface density model as subgrid scale reaction closure for LES approach in conjunction with three widely employed turbulence models for dump combustor (F-1 configuration). For the second, the flame stability of a swirl burner with mixing section (F-2 configuration) for variation of fuel types is analysed in RANS...

The present RANS-combustion study focuses on the flame behaviour and turbulent flame speeds at di... more The present RANS-combustion study focuses on the flame behaviour and turbulent flame speeds at different equivalence ratios of methane/air mixtures typical of gas turbine conditions. It also highlights degree of accuracy of the Weller combustion model available in the XiFoam solver, for a set of conditions, for three equivalence ratios, for leaner mixtures (φ) 0.43, 0.50 and 0.56, at high pressure 5 bar and preheating temperature 673K, at bulk velocity of 40m/s. We present and discuss the flame position, turbulent flame speeds are in comparison with experimental data. This study uses two sub-models available in the Weller combustion model, to show the strong influence of equivalence ratio on above flame quantities. For lean mixture (φ = 0.43), the flame shape is elliptic with increased flame brush thickness whereas for rich mixture (φ= 0.56) produces a flame of conical shape of reduced flame brush thickness. We will show a good quantitative agreement between experiment and simulatio...

MATEC Web of Conferences, 2018

The present paper discusses the numerical investigation of turbulent premixed flames under lean c... more The present paper discusses the numerical investigation of turbulent premixed flames under lean conditions. Lean premixed combustion, a low NOx emission technique but are prone to instabilities, extinction and blow out. Such flames are influenced by preferential diffusion due to different mass diffusivities of reactants and difference between heat and mass diffusivities in the reaction zone. In this numerical study, we estimate non-reacting flow characteristics with implementation of an Algebraic Flame Surface Wrinkling Model (AFSW) in the open source CFD code OpenFOAM. In these flows, the mean velocity fields and recirculation zones were captured reasonably well by the RANS standard k-epsilon turbulence model. The simulated turbulent velocity is in good agreement with experiments in the shear-generated turbulence layer. The reacting flow study was done at three equivalence ratios of 0.43, 0.5 and 0.56 to gauge the ability of numerical model to predict combustion quantities. At equi...

Journal of Education and Learning, 2020

In this work we focussed on assessment and quantification of students’ prior knowledge at the sta... more In this work we focussed on assessment and quantification of students’ prior knowledge at the start of their classes and the learning and teaching feedback given by them after their classes. Using questionnaires, we collected data on prior-knowledge/Student Learning Abilities – SLAs and, students’ performance/Learning Outcomes – LOs. Our analysis shows that typically, in any classroom the SLAs follow a non-linear trend. This pattern, identified in group learning, requires proportionally distributed intervention by staff, with more support to those in need of help with learning. We show that the above approach, underpinned by an application of Multiple Intelligences and e-learning facilities, supports weaker students and helps them to achieve higher pass percentages and better LOs. This innovation in terms of evidence-based identification of need for support and selective intervention helps in optimal use of staff time and effort as compared to a one-method-fits all approach to learn...

SAE International Journal of Engines, 2019

Addition of hydrogen to hydrocarbons in premixed turbulent combustion is of technological interes... more Addition of hydrogen to hydrocarbons in premixed turbulent combustion is of technological interest due to their increased reactivity, flame stability and extended lean extinction limits. However, such flames are a challenge to reaction modelling, especially as the strong preferential diffusion effects modify the physical processes, which are of importance even for highly turbulent high-pressure conditions. In the present work, RANS modelling is carried out to investigate pressure and hydrogen content on methane/hydrogen/air flames. For this purpose, four different subclosures, used in conjunction with an algebraic reaction model, are compared with two independent sets of experimental data: 1. Orleans data consists of pressures up to 9 bar, with addition of hydrogen content up to 20% in hydrogen/methane mixture, for moderate turbulence intensities. 2. The Paul Scherrer Institute data includes same fuels with higher volume proportion of hydrogen (40%), at much higher turbulent intensities at 5 bar. The first model Model I is based solely on the increased reactivity of the hydrogen/methane mixture under laminar conditions. It shows that the increase of unstretched laminar burning velocity (S L0) is not sufficient to describe the increased reactivity in turbulent situations. This non-corroboration proves the importance of preferential diffusion effects in highly turbulent flames. Models II and III are formulated based on the localized increase in S L0 , local burning velocity which is a strong function of local curvature and flow strain. Model II over predicts the reactivity for higher pressures. Model III accurately predicts for nearly all studied flame conditions. Model IV is based on the leading point concept that the leading part of the turbulent flame brush is more important than rear part of premixed flame with the Lewis number less than unity. This model in its present formulation under predicts the average reaction rate compared with experiments.

ERCOFTAC Series, 2011

In this LES study, an algebraic flame surface wrinkling model based on the progress variable grad... more In this LES study, an algebraic flame surface wrinkling model based on the progress variable gradient approach is validated for lean premixed turbulent propane/air flames measured on VOLVO test rig. These combustion results are analyzed for uncertainty in the solution using two quality assessment techniques.

Progress in Computational Fluid Dynamics, An International Journal, 2004

ABSTRACT Effects of pressure are studied numerically for premixed turbulent flames under varied t... more ABSTRACT Effects of pressure are studied numerically for premixed turbulent flames under varied turbulence conditions up to 1.0 MPa. In order to model the pressure dependency of the reaction source term, a set of available experimental Bunsen flame data for methane/air mixtures from Kobayashi et al. are used as reference for a numerical optimisation study, where three parameters of a generalized algebraic closure relation for a reaction progress variable approach are determined. This approach is based on the algebraic turbulent flame speed closure model proposed by Zimont, which is subjected to modifications for influence of pressure and for low turbulence conditions. The net pressure dependency of the turbulent flame speed is slightly above zero, similar to the experimental fit of Kobayashi. Dependency of turbulent velocity is found proportional to u'<sup align="right"> 0.55 , similar to several experimental data fits by Bradley et al. If the resulting flame angles are represented in terms of s<sub align="right"> T /s<sub align="right"> L vs. u'/s<sub align="right"> L , then the fitted curve of the calculated results is closely related to that of the experiments. Both are showing a nonlinear bending behaviour especially at low turbulence intensities, which seems to be related to the influence of laminar flame instabilities.

In this numerical study three well-known flame surface density models for turbulent premixed comb... more In this numerical study three well-known flame surface density models for turbulent premixed combustion, namely, CFM1, MB and CPB, are evaluated. Calculated non-dimensional flame angles, described in terms of s T /s L and plotted vs. u'/s L , are compared with the broad set of experimental data of Kobayashi et al. of lean premixed Bunsen flames of three hydrocarbon fuels at 1 bar. The CFM1 and the MB models yield too low reaction rates for the tested range of data, but show qualitatively the right trend behaviour. Tuning of the model constants in these two cases offers suitability with respect to the experiments. For the CPB model parameter tuning was not fruitful, since it underestimates the mean reaction rate at low and overpredicts it at high turbulence intensities, showing a distinct ever-increasing-polynomial trend for all fuel-air mixtures. The original CPB model is reformed accordingly. With this, satisfactory quantitative results are achieved.

5th International Energy Conversion Engineering Conference and Exhibit (IECEC), 2007

(Abstract) It is widely recognized that stationary gas turbine combustors under lean turbulent pr... more (Abstract) It is widely recognized that stationary gas turbine combustors under lean turbulent premixed conditions offers the advantages of low temperature operation and thus low NOx emissions. At such ultralean conditions, hydrocarbon flames are inherently unstable, with low range of extinction limits, hampering wide range operability especially at high pressures. However, addition of small amounts of hydrogen, characterized by high burning velocities, has the potential to extend this extinction limit. Hydrogenated fuels offers higher turbulent flame speed ST compared to pure hydrocarbons under identical conditions. These issues are addressed using the existing Algebraic Flame Surface-Wrinkling reaction subclosure in which the influence of high-pressure and the Lewis number were explicitly included [1]. The fuel effects were derived from the flame ball concept by Zel'dovich [2]. In the first instance, comparative studies between calculations and experiments are based on turbulent flame speed of lean high-pressure premixed turbulent pure methane flames obtained on a typical sudden-expanding dump combustor [3]. The pure methane mixtures were preheated to 673 K with maximum operating pressure of 10 bar, for a range of equivalence ratios. Simulation studies carried out using the AFSW model predicts an increase in ST for pure methane mixtures in line with experiments. In the second step, reaction model correlation results for hydrogen enriched methane flames were compared with the corresponding measured data obtained on the same burner. Griebel et al. [4] observed a nonlinear increase in ST with hydrogen addition, especially for equivalence ratio 0.50. The maximum hydrogen weightage for mixed fuel mixtures is 50 % by volume. In this part of the study, for the hydrogen blended methane mixtures, the differences between and analytical results from the AFSW model in its basic form and measurements are found to be significant. This increase in S T is explained using critical chemical time scale taken from a numerical study of outwardly propagating spherical flames by Lipatnikov and Chomiak [5]. This time scale characteristic of the critically curved laminar flamelets based on leading point concept [5] addresses the combined preferential-thermo-diffusive effects.

Heat Transfer, Volume 2, 2006

In this study, we investigate some preliminary reaction model predictions analytically in compari... more In this study, we investigate some preliminary reaction model predictions analytically in comparison with experimental premixed turbulent combustion data from four different flame configurations, which include i) high-jet enveloped, ii) expanding spherical, iii) Bunsen-like, and iv) wide-angled diffuser flames. The special intent of the present work is to evaluate the workability range of the model to hydrogen and hydrogen-doped hydrocarbon mixtures, emphasizing on the significance of preferential diffusion, PD, and Le effects in premixed turbulent flames. This is carried out in two phases: first, involving pure hydrocarbon and pure hydrogen mixtures from two independent measured data, and second, with the blended mixtures from two other data sets. For this purpose, a novel reaction closure embedded with explicit high-pressure and exponential Lewis number terms developed in the context of hydrocarbon mixtures is used. These comparative studies based on the global quantity, turbulent...

Flow, Turbulence and Combustion, 2012

ABSTRACT In this numerical study, an algebraic flame surface wrinkling (AFSW) reaction submodel b... more ABSTRACT In this numerical study, an algebraic flame surface wrinkling (AFSW) reaction submodel based on the progress variable approach is implemented in the large-eddy simulation (LES) context and validated against the triangular stabilized bluff body flame configuration measurements i.e. in VOLVO test rig. The quantitative predictability of the AFSW model is analyzed in comparison with another well validated turbulent flame speed closure (TFC) combustion model in order to help assess the behaviour of the present model and to further help improve the understanding of the flow and flame dynamics. Characterization of non-reacting (or cold) and reacting flows are performed using various subgrid scale models for consistent grid size variation with 300,000 (coarse), 1.2 million (intermediate) and 2.4 million (fine) grid cells. For non-reacting flows at inlet velocity of 17 m/s and inlet temperature 288 K, coarse grid leads to over prediction of turbulence quantities due to low dissipation at the early stage of flow development behind the bluff body that convects downstream eventually polluting the resulting solution. The simulated results with the intermediate (and fine) grid for mean flow and turbulence quantities, and the vortex shedding frequency (fs) closely match experimental data. For combusting flows for lean propane/air mixtures at 35 m/s and 600 K, the vortex shedding frequency increase threefold compared with cold scenario. The predicted results of mean, rms velocities and reaction progress variable are generally in good agreement with experimental data. For the coarse grid the combustion predictions show a shorter recirculation region due to higher turbulent burning rate. Finally, both cold and reacting LES data are analyzed for uncertainty in the solution using two quality assessment techniques: two-grid estimator by Celik, and model and grid variation by Klein. For both approaches, the resolved turbulent kinetic energy is used to estimate the grid quality and error assessment. The quality assessment reveals that the cold flows are well resolved even on the intermediate mesh, while for the reacting flows even the fine mesh is locally not sufficient in the flamelet region. The Klein approach estimates that depending on the recirculation region in cold scenario both numerical and model errors rise near the bluff-body region, while in combusting flows these errors are significant behind the stabilizing point due to preheating of unburned mixture and reaction heat release. The total error mainly depends on the numerical error and the influence of model error is low for this configuration.

Flow, Turbulence and Combustion, 2005

Abstract. This numerical investigation carried out on turbulent lean premixed flames accounts for... more Abstract. This numerical investigation carried out on turbulent lean premixed flames accounts for two algebraic – the Lindstedt–Vaos (LV) and the classic Bray–Moss–Libby (BML) – reaction rate models. Computed data from these two models is compared with the experimental ...

Flow, Turbulence and Combustion, 2007

ABSTRACT This large eddy simulation (LES) study is applied to three different premixed turbulent ... more ABSTRACT This large eddy simulation (LES) study is applied to three different premixed turbulent flames under lean conditions at atmospheric pressure. The hierarchy of complexity of these flames in ascending order are a simple Bunsen-like burner, a sudden-expansion dump combustor, and a typical swirl-stabilized gas turbine burner–combustor. The purpose of this paper is to examine numerically whether the chosen combination of the Smagorinsky turbulence model for sgs fluxes and a novel turbulent premixed reaction closure is applicable over all the three combustion configurations with varied degree of flow and turbulence. A quality assessment method for the LES calculations is applied. The cold flow data obtained with the Smagorinsky closure on the dump combustor are in close proximity with the experiments. It moderately predicts the vortex breakdown and bubble shape, which control the flame position on the double-cone burner. Here, the jet break-up at the root of the burner is premature and differs with the experiments by as much as half the burner exit diameter, attributing the discrepancy to poor grid resolution. With the first two combustion configurations, the applied subgrid reaction model is in good correspondence with the experiments. For the third case, a complex swirl-stabilized burner–combustor configuration, although the flow field inside the burner is only modestly numerically explored, the level of flame stabilization at the junction of the burner–combustor has been rather well captured. Furthermore, the critical flame drift from the combustor into the burner was possible to capture in the LES context (which was not possible with the RANS plus k–ɛ model), however, requiring tuning of a prefactor in the reaction closure.

Innovative Practice in Higher Education, 2018

This poster will focus on a problem based learning (PBL) method which was incorporated in the Yea... more This poster will focus on a problem based learning (PBL) method which was incorporated in the Year 2 (level 5) module `Care of the Older Person`. This method was used to encourage students to identify and develop a variety of clinical assessment skills based on a short 10 minute video of Peter Thursby`s (Innovations in Dementia, 2010) autobiographical account of his life and career.

Alexandria Engineering Journal

Aerospace Science and Technology, 2022

ABSTRACT The enhanced combustion properties of hydrogen as well as worldwide concerns due to glob... more ABSTRACT The enhanced combustion properties of hydrogen as well as worldwide concerns due to global warming are acting together as the major driving force to a future hydrogen-based economy. However this will not be a step change and the blends of hydrogen and hydrocarbons must be looked as a transition solution to a purely hydrogen-based economy [1]. In the present study, numerical investigations of a wider range of fuel/air mixtures which includes H 2 -hydrocarbon blends have been carried out for the unstretched laminar burning velocities, S L0 at STP conditions using detailed reaction mechanisms. For this purpose, different reaction mechanisms are combined with the combustion simulation package Cosilab [2]. Comparative studies with experimental data from several independent investigations show that San Diego mechanism [3] predicts S L0 of H 2 /air mixtures very well while Konnov v0.5 [4] is the most valid mechanism for CH 4 /air mixtures. QMech [5] shows good agreement for C 3 H 8 /air mixtures. For H 2 /CH 4 /air mixtures of overall equivalence ratio 1.0, kinetic mechanisms show a splitting behaviour for varied concentration levels of blended fuel hydrogen. For mixtures with volume concentrations of H 2 (Vol. H 2) < 60%, QMech predicts reasonable results while for Vol. H 2 ≥ 60% Appel [6] is comparatively better. QMech also shows valid predictions for H 2 /C 3 H 8 /air mixtures especially for equivalence ratios between 0.6 and 1.0. For H 2 /CO/air mixtures satisfactory agreement is found with CMech [7]. An extended evaluation of the influence of carbon dioxide in the latter mixture is conducted which shows that S L0 decreases almost linearly with increasing quantities of carbon dioxide.

IOP Conference Series: Materials Science and Engineering, 2021

In this study, we show numerical predictions of the Zimont and Peter’s turbulent flame speed mode... more In this study, we show numerical predictions of the Zimont and Peter’s turbulent flame speed models for low-swirl Methane/Air/Hydrogen flames [1]. These models are contained as default options within the ANSYS Fluent Premixed reaction model. Two distinct simulation tasks as part of the present study are – non-reacting and reacting flows, for with three different methane mixtures in compositions added with – 0, 40 and 60% hydrogen. The results show that the RANS approach provides a reasonable prediction of the cold flow conditions, whilst the reacting flow conditions, good agreement is reached up to 40% enrichment, except near the recirculation region,.

Education Quarterly Reviews, 2021

We propose a new approach to classroom learning based on sequential numeral-division. It builds o... more We propose a new approach to classroom learning based on sequential numeral-division. It builds on the concept of trichotomy – division of students based on creamy-level, middle-level and weaker-level students -- proposed by the present authors. A sequenced series of formative assessments can map student progress and achievement, particularly in the case of weaker students. The idea behind the development of this model is to study if weak students perform better on critical-thinking tests in a collaborative learning setting rather than when they study individually. We propose a mathematical model to measure group activity/achievement, which is a complex function of several parameters. We collect data on different parameters for validation of the model in the near future.

Methodical design of gas turbine premixed turbulent combustion systems for fuel flexibility is a ... more Methodical design of gas turbine premixed turbulent combustion systems for fuel flexibility is a necessity for higher efficiency and operational flexibility, due to the use of vast range of fuel types and fuel mixtures. Different fuels/fuel mixtures have different reactivity and varied molecular transport characteristics. The central theme of this paper is numerical investigation of these effects, along with the dynamics of turbulent premixed high-pressure flames. We studied these dominant factors in two different geometrical configurations, a cylindrical dump combustion system and an industrial gas turbine single burner combustion system. Here, we chose the algebraic flame surface density model as subgrid scale reaction closure for LES approach in conjunction with three widely employed turbulence models for dump combustor (F-1 configuration). For the second, the flame stability of a swirl burner with mixing section (F-2 configuration) for variation of fuel types is analysed in RANS...

The present RANS-combustion study focuses on the flame behaviour and turbulent flame speeds at di... more The present RANS-combustion study focuses on the flame behaviour and turbulent flame speeds at different equivalence ratios of methane/air mixtures typical of gas turbine conditions. It also highlights degree of accuracy of the Weller combustion model available in the XiFoam solver, for a set of conditions, for three equivalence ratios, for leaner mixtures (φ) 0.43, 0.50 and 0.56, at high pressure 5 bar and preheating temperature 673K, at bulk velocity of 40m/s. We present and discuss the flame position, turbulent flame speeds are in comparison with experimental data. This study uses two sub-models available in the Weller combustion model, to show the strong influence of equivalence ratio on above flame quantities. For lean mixture (φ = 0.43), the flame shape is elliptic with increased flame brush thickness whereas for rich mixture (φ= 0.56) produces a flame of conical shape of reduced flame brush thickness. We will show a good quantitative agreement between experiment and simulatio...

MATEC Web of Conferences, 2018

The present paper discusses the numerical investigation of turbulent premixed flames under lean c... more The present paper discusses the numerical investigation of turbulent premixed flames under lean conditions. Lean premixed combustion, a low NOx emission technique but are prone to instabilities, extinction and blow out. Such flames are influenced by preferential diffusion due to different mass diffusivities of reactants and difference between heat and mass diffusivities in the reaction zone. In this numerical study, we estimate non-reacting flow characteristics with implementation of an Algebraic Flame Surface Wrinkling Model (AFSW) in the open source CFD code OpenFOAM. In these flows, the mean velocity fields and recirculation zones were captured reasonably well by the RANS standard k-epsilon turbulence model. The simulated turbulent velocity is in good agreement with experiments in the shear-generated turbulence layer. The reacting flow study was done at three equivalence ratios of 0.43, 0.5 and 0.56 to gauge the ability of numerical model to predict combustion quantities. At equi...

Journal of Education and Learning, 2020

In this work we focussed on assessment and quantification of students’ prior knowledge at the sta... more In this work we focussed on assessment and quantification of students’ prior knowledge at the start of their classes and the learning and teaching feedback given by them after their classes. Using questionnaires, we collected data on prior-knowledge/Student Learning Abilities – SLAs and, students’ performance/Learning Outcomes – LOs. Our analysis shows that typically, in any classroom the SLAs follow a non-linear trend. This pattern, identified in group learning, requires proportionally distributed intervention by staff, with more support to those in need of help with learning. We show that the above approach, underpinned by an application of Multiple Intelligences and e-learning facilities, supports weaker students and helps them to achieve higher pass percentages and better LOs. This innovation in terms of evidence-based identification of need for support and selective intervention helps in optimal use of staff time and effort as compared to a one-method-fits all approach to learn...

SAE International Journal of Engines, 2019

Addition of hydrogen to hydrocarbons in premixed turbulent combustion is of technological interes... more Addition of hydrogen to hydrocarbons in premixed turbulent combustion is of technological interest due to their increased reactivity, flame stability and extended lean extinction limits. However, such flames are a challenge to reaction modelling, especially as the strong preferential diffusion effects modify the physical processes, which are of importance even for highly turbulent high-pressure conditions. In the present work, RANS modelling is carried out to investigate pressure and hydrogen content on methane/hydrogen/air flames. For this purpose, four different subclosures, used in conjunction with an algebraic reaction model, are compared with two independent sets of experimental data: 1. Orleans data consists of pressures up to 9 bar, with addition of hydrogen content up to 20% in hydrogen/methane mixture, for moderate turbulence intensities. 2. The Paul Scherrer Institute data includes same fuels with higher volume proportion of hydrogen (40%), at much higher turbulent intensities at 5 bar. The first model Model I is based solely on the increased reactivity of the hydrogen/methane mixture under laminar conditions. It shows that the increase of unstretched laminar burning velocity (S L0) is not sufficient to describe the increased reactivity in turbulent situations. This non-corroboration proves the importance of preferential diffusion effects in highly turbulent flames. Models II and III are formulated based on the localized increase in S L0 , local burning velocity which is a strong function of local curvature and flow strain. Model II over predicts the reactivity for higher pressures. Model III accurately predicts for nearly all studied flame conditions. Model IV is based on the leading point concept that the leading part of the turbulent flame brush is more important than rear part of premixed flame with the Lewis number less than unity. This model in its present formulation under predicts the average reaction rate compared with experiments.

ERCOFTAC Series, 2011

In this LES study, an algebraic flame surface wrinkling model based on the progress variable grad... more In this LES study, an algebraic flame surface wrinkling model based on the progress variable gradient approach is validated for lean premixed turbulent propane/air flames measured on VOLVO test rig. These combustion results are analyzed for uncertainty in the solution using two quality assessment techniques.

Progress in Computational Fluid Dynamics, An International Journal, 2004

ABSTRACT Effects of pressure are studied numerically for premixed turbulent flames under varied t... more ABSTRACT Effects of pressure are studied numerically for premixed turbulent flames under varied turbulence conditions up to 1.0 MPa. In order to model the pressure dependency of the reaction source term, a set of available experimental Bunsen flame data for methane/air mixtures from Kobayashi et al. are used as reference for a numerical optimisation study, where three parameters of a generalized algebraic closure relation for a reaction progress variable approach are determined. This approach is based on the algebraic turbulent flame speed closure model proposed by Zimont, which is subjected to modifications for influence of pressure and for low turbulence conditions. The net pressure dependency of the turbulent flame speed is slightly above zero, similar to the experimental fit of Kobayashi. Dependency of turbulent velocity is found proportional to u'<sup align="right"> 0.55 , similar to several experimental data fits by Bradley et al. If the resulting flame angles are represented in terms of s<sub align="right"> T /s<sub align="right"> L vs. u'/s<sub align="right"> L , then the fitted curve of the calculated results is closely related to that of the experiments. Both are showing a nonlinear bending behaviour especially at low turbulence intensities, which seems to be related to the influence of laminar flame instabilities.

In this numerical study three well-known flame surface density models for turbulent premixed comb... more In this numerical study three well-known flame surface density models for turbulent premixed combustion, namely, CFM1, MB and CPB, are evaluated. Calculated non-dimensional flame angles, described in terms of s T /s L and plotted vs. u'/s L , are compared with the broad set of experimental data of Kobayashi et al. of lean premixed Bunsen flames of three hydrocarbon fuels at 1 bar. The CFM1 and the MB models yield too low reaction rates for the tested range of data, but show qualitatively the right trend behaviour. Tuning of the model constants in these two cases offers suitability with respect to the experiments. For the CPB model parameter tuning was not fruitful, since it underestimates the mean reaction rate at low and overpredicts it at high turbulence intensities, showing a distinct ever-increasing-polynomial trend for all fuel-air mixtures. The original CPB model is reformed accordingly. With this, satisfactory quantitative results are achieved.

5th International Energy Conversion Engineering Conference and Exhibit (IECEC), 2007

(Abstract) It is widely recognized that stationary gas turbine combustors under lean turbulent pr... more (Abstract) It is widely recognized that stationary gas turbine combustors under lean turbulent premixed conditions offers the advantages of low temperature operation and thus low NOx emissions. At such ultralean conditions, hydrocarbon flames are inherently unstable, with low range of extinction limits, hampering wide range operability especially at high pressures. However, addition of small amounts of hydrogen, characterized by high burning velocities, has the potential to extend this extinction limit. Hydrogenated fuels offers higher turbulent flame speed ST compared to pure hydrocarbons under identical conditions. These issues are addressed using the existing Algebraic Flame Surface-Wrinkling reaction subclosure in which the influence of high-pressure and the Lewis number were explicitly included [1]. The fuel effects were derived from the flame ball concept by Zel'dovich [2]. In the first instance, comparative studies between calculations and experiments are based on turbulent flame speed of lean high-pressure premixed turbulent pure methane flames obtained on a typical sudden-expanding dump combustor [3]. The pure methane mixtures were preheated to 673 K with maximum operating pressure of 10 bar, for a range of equivalence ratios. Simulation studies carried out using the AFSW model predicts an increase in ST for pure methane mixtures in line with experiments. In the second step, reaction model correlation results for hydrogen enriched methane flames were compared with the corresponding measured data obtained on the same burner. Griebel et al. [4] observed a nonlinear increase in ST with hydrogen addition, especially for equivalence ratio 0.50. The maximum hydrogen weightage for mixed fuel mixtures is 50 % by volume. In this part of the study, for the hydrogen blended methane mixtures, the differences between and analytical results from the AFSW model in its basic form and measurements are found to be significant. This increase in S T is explained using critical chemical time scale taken from a numerical study of outwardly propagating spherical flames by Lipatnikov and Chomiak [5]. This time scale characteristic of the critically curved laminar flamelets based on leading point concept [5] addresses the combined preferential-thermo-diffusive effects.

Heat Transfer, Volume 2, 2006

In this study, we investigate some preliminary reaction model predictions analytically in compari... more In this study, we investigate some preliminary reaction model predictions analytically in comparison with experimental premixed turbulent combustion data from four different flame configurations, which include i) high-jet enveloped, ii) expanding spherical, iii) Bunsen-like, and iv) wide-angled diffuser flames. The special intent of the present work is to evaluate the workability range of the model to hydrogen and hydrogen-doped hydrocarbon mixtures, emphasizing on the significance of preferential diffusion, PD, and Le effects in premixed turbulent flames. This is carried out in two phases: first, involving pure hydrocarbon and pure hydrogen mixtures from two independent measured data, and second, with the blended mixtures from two other data sets. For this purpose, a novel reaction closure embedded with explicit high-pressure and exponential Lewis number terms developed in the context of hydrocarbon mixtures is used. These comparative studies based on the global quantity, turbulent...

Flow, Turbulence and Combustion, 2012

ABSTRACT In this numerical study, an algebraic flame surface wrinkling (AFSW) reaction submodel b... more ABSTRACT In this numerical study, an algebraic flame surface wrinkling (AFSW) reaction submodel based on the progress variable approach is implemented in the large-eddy simulation (LES) context and validated against the triangular stabilized bluff body flame configuration measurements i.e. in VOLVO test rig. The quantitative predictability of the AFSW model is analyzed in comparison with another well validated turbulent flame speed closure (TFC) combustion model in order to help assess the behaviour of the present model and to further help improve the understanding of the flow and flame dynamics. Characterization of non-reacting (or cold) and reacting flows are performed using various subgrid scale models for consistent grid size variation with 300,000 (coarse), 1.2 million (intermediate) and 2.4 million (fine) grid cells. For non-reacting flows at inlet velocity of 17 m/s and inlet temperature 288 K, coarse grid leads to over prediction of turbulence quantities due to low dissipation at the early stage of flow development behind the bluff body that convects downstream eventually polluting the resulting solution. The simulated results with the intermediate (and fine) grid for mean flow and turbulence quantities, and the vortex shedding frequency (fs) closely match experimental data. For combusting flows for lean propane/air mixtures at 35 m/s and 600 K, the vortex shedding frequency increase threefold compared with cold scenario. The predicted results of mean, rms velocities and reaction progress variable are generally in good agreement with experimental data. For the coarse grid the combustion predictions show a shorter recirculation region due to higher turbulent burning rate. Finally, both cold and reacting LES data are analyzed for uncertainty in the solution using two quality assessment techniques: two-grid estimator by Celik, and model and grid variation by Klein. For both approaches, the resolved turbulent kinetic energy is used to estimate the grid quality and error assessment. The quality assessment reveals that the cold flows are well resolved even on the intermediate mesh, while for the reacting flows even the fine mesh is locally not sufficient in the flamelet region. The Klein approach estimates that depending on the recirculation region in cold scenario both numerical and model errors rise near the bluff-body region, while in combusting flows these errors are significant behind the stabilizing point due to preheating of unburned mixture and reaction heat release. The total error mainly depends on the numerical error and the influence of model error is low for this configuration.

Flow, Turbulence and Combustion, 2005

Abstract. This numerical investigation carried out on turbulent lean premixed flames accounts for... more Abstract. This numerical investigation carried out on turbulent lean premixed flames accounts for two algebraic – the Lindstedt–Vaos (LV) and the classic Bray–Moss–Libby (BML) – reaction rate models. Computed data from these two models is compared with the experimental ...

Flow, Turbulence and Combustion, 2007

ABSTRACT This large eddy simulation (LES) study is applied to three different premixed turbulent ... more ABSTRACT This large eddy simulation (LES) study is applied to three different premixed turbulent flames under lean conditions at atmospheric pressure. The hierarchy of complexity of these flames in ascending order are a simple Bunsen-like burner, a sudden-expansion dump combustor, and a typical swirl-stabilized gas turbine burner–combustor. The purpose of this paper is to examine numerically whether the chosen combination of the Smagorinsky turbulence model for sgs fluxes and a novel turbulent premixed reaction closure is applicable over all the three combustion configurations with varied degree of flow and turbulence. A quality assessment method for the LES calculations is applied. The cold flow data obtained with the Smagorinsky closure on the dump combustor are in close proximity with the experiments. It moderately predicts the vortex breakdown and bubble shape, which control the flame position on the double-cone burner. Here, the jet break-up at the root of the burner is premature and differs with the experiments by as much as half the burner exit diameter, attributing the discrepancy to poor grid resolution. With the first two combustion configurations, the applied subgrid reaction model is in good correspondence with the experiments. For the third case, a complex swirl-stabilized burner–combustor configuration, although the flow field inside the burner is only modestly numerically explored, the level of flame stabilization at the junction of the burner–combustor has been rather well captured. Furthermore, the critical flame drift from the combustor into the burner was possible to capture in the LES context (which was not possible with the RANS plus k–ɛ model), however, requiring tuning of a prefactor in the reaction closure.