Aqib Chishty | Luleå University of Technology (original) (raw)

Papers by Aqib Chishty

Research paper thumbnail of Effect of acoustic perturbation on particle dispersion in a swirl-stabilized pulverized fuel burner: Cold-flow conditions

Fuel Processing Technology

Research paper thumbnail of TRANSITION PREDICTION IN LOW PRESSURE TURBINE (LPT) USING GAMMA THETA MODEL & PASSIVE CONTROL OF SEPARATION

The boundary layer of low-pressure turbine blades has received a great deal of attention due to a... more The boundary layer of low-pressure turbine blades has received a great deal of attention due to advent of high lift and ultra high lift LP turbines. At cruising condition, Reynolds number is very low in engine and LP turbine performance suffers mainly from losses due to the laminar separation bubble on suction surface. In this paper, T106A low pressure turbine profile has been used to study the behavior of boundary layer and subsequently, flow is controlled using the passive technique. Unsteady Reynolds Averaged Navier Stokes equations were solved using SST Gamma-Theta transition model for turbulence closure. Hybrid mesh topology has been used to discretize the computational domain, with highly resolved structured mesh in boundary layer (Y < 1) and unstructured mesh in the rest of domain. Simulations were performed using commercial CFD code ANSYS FLUENT ® at Reynolds number 91000 (based on inlet velocity and chord length) and turbulence intensity of 0.4%. To study the effect of d...

Research paper thumbnail of On the Flame Shape in a Premixed Swirl Stabilised Burner and its Dependence on the Laminar Flame Speed

Flow, Turbulence and Combustion, 2021

Gas turbines for power generation are optimised to run with fossil fuels but as a response to tig... more Gas turbines for power generation are optimised to run with fossil fuels but as a response to tighter pollutant regulations and to enable the use of renewable fuels there is a great interest in improving fuel flexibility. One interesting renewable fuel is syngas from biomass gasification but its properties vary depending on the feedstock and gasification principle, and are significantly different from conventional fuels. This paper aims to give an overview of the differences in combustion behaviour by comparing numerical solutions with methane and several different synthesis gas compositions. The TECFLAM swirl burner geometry, which is designed to be representative of common gas turbine burners, was selected for comparison. The advantage with this geometry is that detailed experimental measurements with methane are publicly available. A two-stage approach was employed with development and validation of an advanced CFD model against experimental data for methane combustion followed b...

Research paper thumbnail of Correction to: On the Flame Shape in a Premixed Swirl Stabilised Burner and its Dependence on the Laminar Flame Speed

Flow, Turbulence and Combustion, 2021

Research paper thumbnail of Soot formation modelling for n-dodecane sprays using the transported PDF model

Soot formation in an n-dodecane spray flame under diesel engine conditions, known as Spray A, is ... more Soot formation in an n-dodecane spray flame under diesel engine conditions, known as Spray A, is modelled with the transported probability function (TPDF) method. The approach employs an acetylene-based two-equation soot model coupled with a Reynolds-averaged turbulence model and a Lagrangian discrete phase spray model. The aims are to evaluate, in the context of soot, the predictive capability of the model, the effects of turbulence–chemistry interactions (TCI), and various available chemistry mechanisms. TCI effects are evaluated by comparisons between the TPDF model and simulations using a well-mixed model neglecting turbulent fluctuations. Five test cases having variations in ambient temperature and oxygen concentration are considered. Five chemical mechanisms are first compared to experiments in terms of their ignition delay (ID) and lift-off length (LOL) under ambient O2 and temperature variations. Three relatively new mechanisms exhibit good ID performance (with both TCI appr...

Research paper thumbnail of Numerical simulation of a biomass cyclone gasifier: Effects of operating conditions on gasifier performance

Fuel Processing Technology

Abstract In Nordic countries, biomass gasification in a cyclone gasifier combined with a gas engi... more Abstract In Nordic countries, biomass gasification in a cyclone gasifier combined with a gas engine has been employed to generate small scale heat and power. Numerical simulations were carried out to analyze the effect of different operating conditions on the functioning of the gasifier. Reynolds-Averaged Navier-Stokes equations are solved together with the eddy-break up combustion model in conjunction with a modified k − ϵ model to predict the temperature and the flow field inside the gasifier. Results were compared with the experimental measurements in a 4.4 MW cyclone gasifier constructed by Meva Energy AB at Hortlax, Pitea, Sweden. The predicted results were in good agreement with the experimental data and the model provides detailed information about the gas compositions, cold gas efficiency and temperature field. Furthermore, the model allows different operating scenarios to be examined in an efficient manner such as the number of inlets, fuel to air velocity difference (slip-velocity) and moisture content in the fuel feedstock. The cold gas efficiency, composition of product gases and outlet temperature were monitored for each test case. These findings help to understand the importance of geometry modification, feedstock contents and make it possible to scale-up the gasifier for future applications.

Research paper thumbnail of The effect of Stefan flow on Nusselt number and drag coefficient of spherical particles in non-isothermal gas flow

International Journal of Multiphase Flow

Research paper thumbnail of Modeling combustion under engine combustion network Spray A conditions with multiple injections using the transported probability density function method

International Journal of Engine Research, 2017

In this study, numerical simulations of an n-dodecane spray flame—known as Spray A—with multiple ... more In this study, numerical simulations of an n-dodecane spray flame—known as Spray A—with multiple injections (0.5 ms injection/0.5 ms dwell/0.5 ms injection) have been carried out using the transported probability density function method in the Reynolds-averaged Navier–Stokes framework. In terms of the methodology employed, the transported probability density function method can handle the multiple-injections case without any modification because the model does not assume that thermodynamical states lie on a low-dimensional manifold such as the mixture fraction manifold, as is the case for many other turbulent combustion models, for example, the representative interactive flamelet model and the conditional moment closure model. Simulation results have been compared with recent experimental data in terms of inert and reactive jet tip penetration and vapor boundary (from schlieren imaging), ignition delay and flame base location of the first and second fuel injection, spatial distribution of formaldehyde (CH2O) and polyaromatic hydrocarbon (from 355-nm planar-laser-induced fluorescence). Particular attention has been paid to the ignition behavior of the second fuel injection. The timing and progression of the first- and second-stage ignition events are qualitatively well reproduced by the model. Simulation results have been further analyzed to assess the validity of the beta-function as the presumed shape of the mixture fraction probability density function, which is typically employed in mixture fraction–based models. The beta-function probability density function was found to provide a good approximation throughout the jet region apart from a brief period of around 100 µs when the second fuel stream encounters the pre-existing fuel–air mixture from the first fuel injection. Overall, it is shown that the transported probability density function model is able to capture the main features related to auto-ignition involved with multiple injections, and simulation results can be used to assess some of the underlying assumptions invoked by other models.

Research paper thumbnail of Study of Flow Controlling on LP Turbine at Different Reynolds Number

Volume 1: Symposia, Parts A and B, 2012

ABSTRACT Active and passive techniques have been used in the past, to control flow separation. Nu... more ABSTRACT Active and passive techniques have been used in the past, to control flow separation. Numerous studies were published on controlling and delaying the flow separation on low pressure turbine. In this study, a single dimple (i.e. passive device) is engraved on the suction side of LP turbine cascade T106A. The main aim of this research is to find out the optimum parameters of dimple i.e. diameter (D) and depth (h) which can produce strong enough vortex that can control the flow either in transition or fully turbulent phase. Furthermore, this optimal dimple is engraved to suppress the boundary layer separation at different Reynolds number (based on the chord length and inlet velocity). The dimple of different depth and diameter are used to find the optimal depth to diameter ratio. Computational results show that the optimal ratio of depth to diameter (h/D) for dimple is 0.0845 and depth to grid boundary layer (h/δ) is 0.5152. This optimized dimple efficiently reduces the normalized loss coefficient and it is found that the negative values of shear stresses found in uncontrolled case are being removed by the dimple. After that, dimple of optimized parameters are used to suppress the laminar separation bubble at different Re~25000, 50000 and 91000. It was noticed that the dimple did not reduce the losses at Re~25000. But at Re~50000, it produced such a strong vortex that reduced the normalized loss coefficient to 25%, while 5% losses were reduced at Re~91000. It can be concluded that the optimized dimple effectively controlled flow separation and reduced normalized loss coefficient from Re 25000 to 91000. As the losses are decreased, this will increase the low pressure turbine efficiency and reduce its fuel consumption.

Research paper thumbnail of Transition Prediction in Low Pressure Turbine (LPT) Using Gamma Theta Model & Passive Control of Separation

The boundary layer of low-pressure turbine blades has received a great deal of attention due to a... more The boundary layer of low-pressure turbine blades has received a great deal of attention due to advent of high lift and ultra high lift LP turbines. At cruising condition, Reynolds number is very low in engine and LP turbine performance suffers mainly from losses due to the laminar separation bubble on suction surface.

Research paper thumbnail of A Progress Review on Soot Experiments and Modeling in the Engine Combustion Network (ECN)

The 4 th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, J... more The 4 th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, Japan. This manuscript presents a summary of the progress in experiments and modeling among ECN contributors leading to a better understanding of soot formation under the ECN " Spray A " configuration and some parametric variants. Relevant published and unpublished work from prior ECN workshops is reviewed. Experiments measuring soot particle size and morphology, soot volume fraction (f v), and transient soot mass have been conducted at various international institutions providing target data for improvements to computational models. Multiple modeling contributions using both the Reynolds Averaged Navier-Stokes (RANS) Equations approach and the Large-Eddy Simulation (LES) approach have been submitted. Among these, various chemical mechanisms, soot models, and turbulence-chemistry interaction (TCI) methodologies have been considered.

Research paper thumbnail of Assessing the Importance of Radiative Heat Transfer for ECN Spray A Using the Transported PDF Method.pdf

The importance of radiative heat transfer on the combustion and soot formation characteristics un... more The importance of radiative heat transfer on the combustion and soot formation characteristics under nominal ECN Spray A conditions has been studied numerically. The liquid n-dodecane fuel is injected with 1500 bar fuel pressure into the constant volume chamber at different ambient conditions. Radiation from both gas-phase as well as soot particles has been included and assumed as gray. Three different solvers for the radiative transfer equation have been employed: the discrete ordinate method, the spherical-harmonics method and the optically thin assumption. The radiation models have been coupled with the transported probability density function method for turbulent reactive flows and soot, where unresolved turbulent fluctuations in temperature and composition are included and therefore capturing turbulence-chemistry-soot-radiation interactions.
Results show that the gas-phase (mostly CO2 ad H2O species) has a higher contribution to the net radiation heat transfer compared to soot. The effect of radiation absorption was found to be important and the typical radiation time scale is observed to overlap with the long injection duration, leading to a moderate influence on the temperature distribution. The flame lift-off length is not affected by radiation and differences in soot formation are perceivable but only minor. The performance of the DOM and P1 models is comparable, whereas the optically thin assumption leads to a higher cooling effect. It is anticipated that NOx formation rates are expected to be influenced by radiative heat transfer in a more pronounced manner.

Research paper thumbnail of The effect of radiation heat transfer under ECN Spray A conditions

This article examines the effect of radiation in a model of a canonical diesel spray flame known ... more This article examines the effect of radiation in a model of a canonical diesel spray flame known as Spray A. In spray A, liquid n-dodecane spray was injected with 1500 bar fuel pressure into a constant volume chamber at temperature, oxygen mole fraction and density of 900K, 15% and 22.8 kg/m 3 , respectively. The unsteady Reynolds-averaged Navier-Strokes (RANS) model equations coupled with a reduced 53 species n-dodecane chemical mechanism and a two-equation soot model are solved. The effect of radiation has been introduced using three different radiation models (spherical-harmonics and discrete ordinate method) along with the optically thin (i.e. emission only, no absorption) assumption and results are compared with the case without radiation. At Spray A conditions the contribution of gas phase radiation (mainly from CO 2 and H 2 O) is more dominant than the soot radiation. Moreover, the effect of radiation absorption is found to be important and a moderate influence on the temperature distribution of the order of 10-20K at the most in the fuel lean region has been noticed. The radiative emission rate is similar for all three radiation models. However, the higher absorption rate has been noticed for the DOM model in comparison with the P1 model. The flame lift-off length is not affected by the radiation and differences in the soot formation are perceivable but only minor. The performance of the DOM and P1 models is comparable, whereas the optically thin assumption leads to a higher cooling effect. Nomenclature ECN= Engine Combustion Network RTE = Radiative Transfer Equation TRI = Turbulence Radiation Interaction TCI = Turbulence Chemistry Interaction Y s = Soot mass fraction N s = Particle number density ISAT = In-Situ Adaptive Tabulation S qrad = Radiation source term α total = Total absorption coefficient (m-1) G = Radiation Intensity (Wm-2) σ = Stefan-Boltzmann const. 5.67×10 −8 W m −2 K −4 fv soot = Soot volume fraction (ppm) α soot = Soot absorption coefficient (m-1) T rate = Temperature rate of change (Ks-1) T No Rad. = Temperature without radiation (K) T Rad.. = Temperature with radiation (K) ΔT = T Rad.-T No Rad. (K)

Research paper thumbnail of Soot Formation Modelling of Spray-A Using a Transported PDF Approach

Numerical simulations of soot formation were performed for n-dodecane spray using the transported... more Numerical simulations of soot formation were performed for n-dodecane spray using the transported probability density function (TPDF) method. Liquid n-dodecane was injected with 1500 bar fuel pressure into a constant-volume vessel with an ambient temperature, oxygen volume fraction and density of 900 K, 15% and 22.8 kg/m 3 , respectively. The interaction by exchange with the mean (IEM) model was employed to close the micro-mixing term. The unsteady Reynolds-averaged Navier-Stokes (RANS) equations coupled with the realizable k-ϵ turbulence model were used to provide turbulence information to the TPDF solver. A 53-species reduced n-dodecane chemical mechanism was employed to evaluate the reaction rates. Soot formation was modelled with an acetylene-based two-equation model which accounts for simultaneous soot particle inception, surface growth, coagulation and oxidation by O 2 and OH. The modelling results for ignition delay, lift-off length, flame length evolution and distribution of soot volume fraction () are compared with the corresponding experimental data. Good predictions of the temporal evolution and spatial extent of the soot volume fraction have been observed. The findings suggest that the transported probability density function approach for soot modelling is a promising framework.

Research paper thumbnail of A Numerical Study of 'Spray A' with Multiple-Injections Using the Transported PDF Method

1 Abstract In this study, numerical simulations of 'Spray A' with multiple-injections have been c... more 1 Abstract In this study, numerical simulations of 'Spray A' with multiple-injections have been carried out using the transported probability density function (TPDF) method. At the nominal Spray A condition, the ambient temperature, oxygen volume fraction and density are 900 K, 15% and 22.8 kg/m 3 , respectively. Liquid n-dodecane is injected at a pressure of 1500 bar following a split-injection schedule: 0.5/0.5 dwell/0.5 ms. Numerical simulations were carried out using the unsteady Reynolds-averaged Navier-Stokes (RANS) equations. The realizable í µí±˜ − í µí¼– turbulence model was used to provide the turbulence information to the TPDF solver. The micro-mixing term was enclosed using the interaction by exchange with the mean (IEM) model. The reaction rates were evaluated using a 53-species reduced n-dodecane chemical mechanism. The vapor penetration length, timing and progression of the first-and second-stage ignition events are compared with the corresponding experimental data. The temporal evolution of the cool flame structure (CH 2 O) in conjunction with the high temperature reaction zone (OH) is also compared with the available experimental results from 355 nm planar laser-induced fluorescence (PLIF). Good predictions of the ignition delay time and lift-off lengths for the first-and second-stage ignitions have been reported. Moreover, the spatial extent of the computed mass fractions of C 2 H 2 , CH 2 O and OH are consistent with the experimental observations. The findings suggest that the TPDF approach is a promising framework for the modelling of multiple injections.

Research paper thumbnail of Investigation of the Flame Structure of Spray-A Using the Transported Probability Density Function

An n-dodecane spray (Spray A) injected into a high-pressure and high-temperature combustion vesse... more An n-dodecane spray (Spray A) injected into a high-pressure and high-temperature combustion vessel at typical diesel engine conditions is investigated using the transported probability density function (TPDF) method. The Lagrangian Monte Carlo method is used to treat the TPDF equation. The unsteady Reynolds averaged Navier-Stokes (URANS) k-ε turbulence model is used to provide the turbulence information to the TPDF solver. The micro-mixing term is closed by the Euclidean minimum spanning trees (EMST) model. A reduced chemical mechanism with 88 species is implemented. The spray is treated by a traditional Lagrangian discrete phase model. The cool flame structure (CH 2 O) together with the high temperature reaction zone (OH) are examined at different ambient temperatures and ambient oxygen conditions, and compared with the available experimental results from 355 nm planar laser-induced fluorescence (PLIF) and OH PLIF. A good agreement has been found for some of these measurements, while agreement for others leaves room for improvement.

Research paper thumbnail of Flow controlling on low pressure turbine using passive methods

Flow control in low pressure turbine using passive devices is efficient technique to raise the ef... more Flow control in low pressure turbine using passive devices is efficient technique to raise the efficiency of engine. In this study T106A LP Turbine geometry is used for controlling the flow separation using passive device. A lot of work is done for flow controlling using different kind of passive devices like dimples, bumps, riblets etc. Work on the backward step geometry for controlling the flow separation is not done effectively. Fine structured mesh is created with boundary layer (y + < 1) and unstructured meshing in the rest of the domain. Unsteady RANS are solved using the Gamma-Theta Model Transitional Model for capturing the laminar separation bubble. In this study a smooth backward step of different height to length ratios (h/l) are applied on the 65% axial chord length for the reduction of loss coefficient which is another way to specify the efficiency of turbine using pressure values. After specifying the best h/l ratio at 65% axial chord where the losses are minimum, this specific h/l ratio step is shifted at different axial locations to find out the optimal location where the maximum losses are reduced. By making the coefficient of pressure plots and boundary layer profiles, it has been confirmed that backward step completely control the laminar separation bubble and also reduce the loss coefficient.

Research paper thumbnail of Effect of acoustic perturbation on particle dispersion in a swirl-stabilized pulverized fuel burner: Cold-flow conditions

Fuel Processing Technology

Research paper thumbnail of TRANSITION PREDICTION IN LOW PRESSURE TURBINE (LPT) USING GAMMA THETA MODEL & PASSIVE CONTROL OF SEPARATION

The boundary layer of low-pressure turbine blades has received a great deal of attention due to a... more The boundary layer of low-pressure turbine blades has received a great deal of attention due to advent of high lift and ultra high lift LP turbines. At cruising condition, Reynolds number is very low in engine and LP turbine performance suffers mainly from losses due to the laminar separation bubble on suction surface. In this paper, T106A low pressure turbine profile has been used to study the behavior of boundary layer and subsequently, flow is controlled using the passive technique. Unsteady Reynolds Averaged Navier Stokes equations were solved using SST Gamma-Theta transition model for turbulence closure. Hybrid mesh topology has been used to discretize the computational domain, with highly resolved structured mesh in boundary layer (Y < 1) and unstructured mesh in the rest of domain. Simulations were performed using commercial CFD code ANSYS FLUENT ® at Reynolds number 91000 (based on inlet velocity and chord length) and turbulence intensity of 0.4%. To study the effect of d...

Research paper thumbnail of On the Flame Shape in a Premixed Swirl Stabilised Burner and its Dependence on the Laminar Flame Speed

Flow, Turbulence and Combustion, 2021

Gas turbines for power generation are optimised to run with fossil fuels but as a response to tig... more Gas turbines for power generation are optimised to run with fossil fuels but as a response to tighter pollutant regulations and to enable the use of renewable fuels there is a great interest in improving fuel flexibility. One interesting renewable fuel is syngas from biomass gasification but its properties vary depending on the feedstock and gasification principle, and are significantly different from conventional fuels. This paper aims to give an overview of the differences in combustion behaviour by comparing numerical solutions with methane and several different synthesis gas compositions. The TECFLAM swirl burner geometry, which is designed to be representative of common gas turbine burners, was selected for comparison. The advantage with this geometry is that detailed experimental measurements with methane are publicly available. A two-stage approach was employed with development and validation of an advanced CFD model against experimental data for methane combustion followed b...

Research paper thumbnail of Correction to: On the Flame Shape in a Premixed Swirl Stabilised Burner and its Dependence on the Laminar Flame Speed

Flow, Turbulence and Combustion, 2021

Research paper thumbnail of Soot formation modelling for n-dodecane sprays using the transported PDF model

Soot formation in an n-dodecane spray flame under diesel engine conditions, known as Spray A, is ... more Soot formation in an n-dodecane spray flame under diesel engine conditions, known as Spray A, is modelled with the transported probability function (TPDF) method. The approach employs an acetylene-based two-equation soot model coupled with a Reynolds-averaged turbulence model and a Lagrangian discrete phase spray model. The aims are to evaluate, in the context of soot, the predictive capability of the model, the effects of turbulence–chemistry interactions (TCI), and various available chemistry mechanisms. TCI effects are evaluated by comparisons between the TPDF model and simulations using a well-mixed model neglecting turbulent fluctuations. Five test cases having variations in ambient temperature and oxygen concentration are considered. Five chemical mechanisms are first compared to experiments in terms of their ignition delay (ID) and lift-off length (LOL) under ambient O2 and temperature variations. Three relatively new mechanisms exhibit good ID performance (with both TCI appr...

Research paper thumbnail of Numerical simulation of a biomass cyclone gasifier: Effects of operating conditions on gasifier performance

Fuel Processing Technology

Abstract In Nordic countries, biomass gasification in a cyclone gasifier combined with a gas engi... more Abstract In Nordic countries, biomass gasification in a cyclone gasifier combined with a gas engine has been employed to generate small scale heat and power. Numerical simulations were carried out to analyze the effect of different operating conditions on the functioning of the gasifier. Reynolds-Averaged Navier-Stokes equations are solved together with the eddy-break up combustion model in conjunction with a modified k − ϵ model to predict the temperature and the flow field inside the gasifier. Results were compared with the experimental measurements in a 4.4 MW cyclone gasifier constructed by Meva Energy AB at Hortlax, Pitea, Sweden. The predicted results were in good agreement with the experimental data and the model provides detailed information about the gas compositions, cold gas efficiency and temperature field. Furthermore, the model allows different operating scenarios to be examined in an efficient manner such as the number of inlets, fuel to air velocity difference (slip-velocity) and moisture content in the fuel feedstock. The cold gas efficiency, composition of product gases and outlet temperature were monitored for each test case. These findings help to understand the importance of geometry modification, feedstock contents and make it possible to scale-up the gasifier for future applications.

Research paper thumbnail of The effect of Stefan flow on Nusselt number and drag coefficient of spherical particles in non-isothermal gas flow

International Journal of Multiphase Flow

Research paper thumbnail of Modeling combustion under engine combustion network Spray A conditions with multiple injections using the transported probability density function method

International Journal of Engine Research, 2017

In this study, numerical simulations of an n-dodecane spray flame—known as Spray A—with multiple ... more In this study, numerical simulations of an n-dodecane spray flame—known as Spray A—with multiple injections (0.5 ms injection/0.5 ms dwell/0.5 ms injection) have been carried out using the transported probability density function method in the Reynolds-averaged Navier–Stokes framework. In terms of the methodology employed, the transported probability density function method can handle the multiple-injections case without any modification because the model does not assume that thermodynamical states lie on a low-dimensional manifold such as the mixture fraction manifold, as is the case for many other turbulent combustion models, for example, the representative interactive flamelet model and the conditional moment closure model. Simulation results have been compared with recent experimental data in terms of inert and reactive jet tip penetration and vapor boundary (from schlieren imaging), ignition delay and flame base location of the first and second fuel injection, spatial distribution of formaldehyde (CH2O) and polyaromatic hydrocarbon (from 355-nm planar-laser-induced fluorescence). Particular attention has been paid to the ignition behavior of the second fuel injection. The timing and progression of the first- and second-stage ignition events are qualitatively well reproduced by the model. Simulation results have been further analyzed to assess the validity of the beta-function as the presumed shape of the mixture fraction probability density function, which is typically employed in mixture fraction–based models. The beta-function probability density function was found to provide a good approximation throughout the jet region apart from a brief period of around 100 µs when the second fuel stream encounters the pre-existing fuel–air mixture from the first fuel injection. Overall, it is shown that the transported probability density function model is able to capture the main features related to auto-ignition involved with multiple injections, and simulation results can be used to assess some of the underlying assumptions invoked by other models.

Research paper thumbnail of Study of Flow Controlling on LP Turbine at Different Reynolds Number

Volume 1: Symposia, Parts A and B, 2012

ABSTRACT Active and passive techniques have been used in the past, to control flow separation. Nu... more ABSTRACT Active and passive techniques have been used in the past, to control flow separation. Numerous studies were published on controlling and delaying the flow separation on low pressure turbine. In this study, a single dimple (i.e. passive device) is engraved on the suction side of LP turbine cascade T106A. The main aim of this research is to find out the optimum parameters of dimple i.e. diameter (D) and depth (h) which can produce strong enough vortex that can control the flow either in transition or fully turbulent phase. Furthermore, this optimal dimple is engraved to suppress the boundary layer separation at different Reynolds number (based on the chord length and inlet velocity). The dimple of different depth and diameter are used to find the optimal depth to diameter ratio. Computational results show that the optimal ratio of depth to diameter (h/D) for dimple is 0.0845 and depth to grid boundary layer (h/δ) is 0.5152. This optimized dimple efficiently reduces the normalized loss coefficient and it is found that the negative values of shear stresses found in uncontrolled case are being removed by the dimple. After that, dimple of optimized parameters are used to suppress the laminar separation bubble at different Re~25000, 50000 and 91000. It was noticed that the dimple did not reduce the losses at Re~25000. But at Re~50000, it produced such a strong vortex that reduced the normalized loss coefficient to 25%, while 5% losses were reduced at Re~91000. It can be concluded that the optimized dimple effectively controlled flow separation and reduced normalized loss coefficient from Re 25000 to 91000. As the losses are decreased, this will increase the low pressure turbine efficiency and reduce its fuel consumption.

Research paper thumbnail of Transition Prediction in Low Pressure Turbine (LPT) Using Gamma Theta Model & Passive Control of Separation

The boundary layer of low-pressure turbine blades has received a great deal of attention due to a... more The boundary layer of low-pressure turbine blades has received a great deal of attention due to advent of high lift and ultra high lift LP turbines. At cruising condition, Reynolds number is very low in engine and LP turbine performance suffers mainly from losses due to the laminar separation bubble on suction surface.

Research paper thumbnail of A Progress Review on Soot Experiments and Modeling in the Engine Combustion Network (ECN)

The 4 th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, J... more The 4 th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, Japan. This manuscript presents a summary of the progress in experiments and modeling among ECN contributors leading to a better understanding of soot formation under the ECN " Spray A " configuration and some parametric variants. Relevant published and unpublished work from prior ECN workshops is reviewed. Experiments measuring soot particle size and morphology, soot volume fraction (f v), and transient soot mass have been conducted at various international institutions providing target data for improvements to computational models. Multiple modeling contributions using both the Reynolds Averaged Navier-Stokes (RANS) Equations approach and the Large-Eddy Simulation (LES) approach have been submitted. Among these, various chemical mechanisms, soot models, and turbulence-chemistry interaction (TCI) methodologies have been considered.

Research paper thumbnail of Assessing the Importance of Radiative Heat Transfer for ECN Spray A Using the Transported PDF Method.pdf

The importance of radiative heat transfer on the combustion and soot formation characteristics un... more The importance of radiative heat transfer on the combustion and soot formation characteristics under nominal ECN Spray A conditions has been studied numerically. The liquid n-dodecane fuel is injected with 1500 bar fuel pressure into the constant volume chamber at different ambient conditions. Radiation from both gas-phase as well as soot particles has been included and assumed as gray. Three different solvers for the radiative transfer equation have been employed: the discrete ordinate method, the spherical-harmonics method and the optically thin assumption. The radiation models have been coupled with the transported probability density function method for turbulent reactive flows and soot, where unresolved turbulent fluctuations in temperature and composition are included and therefore capturing turbulence-chemistry-soot-radiation interactions.
Results show that the gas-phase (mostly CO2 ad H2O species) has a higher contribution to the net radiation heat transfer compared to soot. The effect of radiation absorption was found to be important and the typical radiation time scale is observed to overlap with the long injection duration, leading to a moderate influence on the temperature distribution. The flame lift-off length is not affected by radiation and differences in soot formation are perceivable but only minor. The performance of the DOM and P1 models is comparable, whereas the optically thin assumption leads to a higher cooling effect. It is anticipated that NOx formation rates are expected to be influenced by radiative heat transfer in a more pronounced manner.

Research paper thumbnail of The effect of radiation heat transfer under ECN Spray A conditions

This article examines the effect of radiation in a model of a canonical diesel spray flame known ... more This article examines the effect of radiation in a model of a canonical diesel spray flame known as Spray A. In spray A, liquid n-dodecane spray was injected with 1500 bar fuel pressure into a constant volume chamber at temperature, oxygen mole fraction and density of 900K, 15% and 22.8 kg/m 3 , respectively. The unsteady Reynolds-averaged Navier-Strokes (RANS) model equations coupled with a reduced 53 species n-dodecane chemical mechanism and a two-equation soot model are solved. The effect of radiation has been introduced using three different radiation models (spherical-harmonics and discrete ordinate method) along with the optically thin (i.e. emission only, no absorption) assumption and results are compared with the case without radiation. At Spray A conditions the contribution of gas phase radiation (mainly from CO 2 and H 2 O) is more dominant than the soot radiation. Moreover, the effect of radiation absorption is found to be important and a moderate influence on the temperature distribution of the order of 10-20K at the most in the fuel lean region has been noticed. The radiative emission rate is similar for all three radiation models. However, the higher absorption rate has been noticed for the DOM model in comparison with the P1 model. The flame lift-off length is not affected by the radiation and differences in the soot formation are perceivable but only minor. The performance of the DOM and P1 models is comparable, whereas the optically thin assumption leads to a higher cooling effect. Nomenclature ECN= Engine Combustion Network RTE = Radiative Transfer Equation TRI = Turbulence Radiation Interaction TCI = Turbulence Chemistry Interaction Y s = Soot mass fraction N s = Particle number density ISAT = In-Situ Adaptive Tabulation S qrad = Radiation source term α total = Total absorption coefficient (m-1) G = Radiation Intensity (Wm-2) σ = Stefan-Boltzmann const. 5.67×10 −8 W m −2 K −4 fv soot = Soot volume fraction (ppm) α soot = Soot absorption coefficient (m-1) T rate = Temperature rate of change (Ks-1) T No Rad. = Temperature without radiation (K) T Rad.. = Temperature with radiation (K) ΔT = T Rad.-T No Rad. (K)

Research paper thumbnail of Soot Formation Modelling of Spray-A Using a Transported PDF Approach

Numerical simulations of soot formation were performed for n-dodecane spray using the transported... more Numerical simulations of soot formation were performed for n-dodecane spray using the transported probability density function (TPDF) method. Liquid n-dodecane was injected with 1500 bar fuel pressure into a constant-volume vessel with an ambient temperature, oxygen volume fraction and density of 900 K, 15% and 22.8 kg/m 3 , respectively. The interaction by exchange with the mean (IEM) model was employed to close the micro-mixing term. The unsteady Reynolds-averaged Navier-Stokes (RANS) equations coupled with the realizable k-ϵ turbulence model were used to provide turbulence information to the TPDF solver. A 53-species reduced n-dodecane chemical mechanism was employed to evaluate the reaction rates. Soot formation was modelled with an acetylene-based two-equation model which accounts for simultaneous soot particle inception, surface growth, coagulation and oxidation by O 2 and OH. The modelling results for ignition delay, lift-off length, flame length evolution and distribution of soot volume fraction () are compared with the corresponding experimental data. Good predictions of the temporal evolution and spatial extent of the soot volume fraction have been observed. The findings suggest that the transported probability density function approach for soot modelling is a promising framework.

Research paper thumbnail of A Numerical Study of 'Spray A' with Multiple-Injections Using the Transported PDF Method

1 Abstract In this study, numerical simulations of 'Spray A' with multiple-injections have been c... more 1 Abstract In this study, numerical simulations of 'Spray A' with multiple-injections have been carried out using the transported probability density function (TPDF) method. At the nominal Spray A condition, the ambient temperature, oxygen volume fraction and density are 900 K, 15% and 22.8 kg/m 3 , respectively. Liquid n-dodecane is injected at a pressure of 1500 bar following a split-injection schedule: 0.5/0.5 dwell/0.5 ms. Numerical simulations were carried out using the unsteady Reynolds-averaged Navier-Stokes (RANS) equations. The realizable í µí±˜ − í µí¼– turbulence model was used to provide the turbulence information to the TPDF solver. The micro-mixing term was enclosed using the interaction by exchange with the mean (IEM) model. The reaction rates were evaluated using a 53-species reduced n-dodecane chemical mechanism. The vapor penetration length, timing and progression of the first-and second-stage ignition events are compared with the corresponding experimental data. The temporal evolution of the cool flame structure (CH 2 O) in conjunction with the high temperature reaction zone (OH) is also compared with the available experimental results from 355 nm planar laser-induced fluorescence (PLIF). Good predictions of the ignition delay time and lift-off lengths for the first-and second-stage ignitions have been reported. Moreover, the spatial extent of the computed mass fractions of C 2 H 2 , CH 2 O and OH are consistent with the experimental observations. The findings suggest that the TPDF approach is a promising framework for the modelling of multiple injections.

Research paper thumbnail of Investigation of the Flame Structure of Spray-A Using the Transported Probability Density Function

An n-dodecane spray (Spray A) injected into a high-pressure and high-temperature combustion vesse... more An n-dodecane spray (Spray A) injected into a high-pressure and high-temperature combustion vessel at typical diesel engine conditions is investigated using the transported probability density function (TPDF) method. The Lagrangian Monte Carlo method is used to treat the TPDF equation. The unsteady Reynolds averaged Navier-Stokes (URANS) k-ε turbulence model is used to provide the turbulence information to the TPDF solver. The micro-mixing term is closed by the Euclidean minimum spanning trees (EMST) model. A reduced chemical mechanism with 88 species is implemented. The spray is treated by a traditional Lagrangian discrete phase model. The cool flame structure (CH 2 O) together with the high temperature reaction zone (OH) are examined at different ambient temperatures and ambient oxygen conditions, and compared with the available experimental results from 355 nm planar laser-induced fluorescence (PLIF) and OH PLIF. A good agreement has been found for some of these measurements, while agreement for others leaves room for improvement.

Research paper thumbnail of Flow controlling on low pressure turbine using passive methods

Flow control in low pressure turbine using passive devices is efficient technique to raise the ef... more Flow control in low pressure turbine using passive devices is efficient technique to raise the efficiency of engine. In this study T106A LP Turbine geometry is used for controlling the flow separation using passive device. A lot of work is done for flow controlling using different kind of passive devices like dimples, bumps, riblets etc. Work on the backward step geometry for controlling the flow separation is not done effectively. Fine structured mesh is created with boundary layer (y + < 1) and unstructured meshing in the rest of the domain. Unsteady RANS are solved using the Gamma-Theta Model Transitional Model for capturing the laminar separation bubble. In this study a smooth backward step of different height to length ratios (h/l) are applied on the 65% axial chord length for the reduction of loss coefficient which is another way to specify the efficiency of turbine using pressure values. After specifying the best h/l ratio at 65% axial chord where the losses are minimum, this specific h/l ratio step is shifted at different axial locations to find out the optimal location where the maximum losses are reduced. By making the coefficient of pressure plots and boundary layer profiles, it has been confirmed that backward step completely control the laminar separation bubble and also reduce the loss coefficient.