K.K.J. Ranga Dinesh

Large eddy simulation of isothermal turbulent swirling jets

Abstract This article investigates the application of the large eddy simulation (LES) technique to turbulent isothermal swirling flows. The aim was to improve our understanding of the flow physics and turbulence structure of unconfined swirling flows and examine the capability of LES to predict the formation of the vortex breakdown (VB) and recirculation zones. In this study, the filtered Navier-Stokes equations are closed using the Smagorinsky eddy viscosity model with the localized dynamic procedure of Piomelli and Liu (1995). The Sydney University swirl burner experiments are simulated as test cases. Three different test cases have been investigated covering a range of swirl numbers and stream wise annular velocities. The cases considered have swirl numbers ranging from 0 to 1.59 and Reynolds numbers from 32400 to 59000. The LES calculations confirm that the combination of lower swirl number and higher axial velocity of the primary annulus leads to the establishment of the downstream vortex breakdown region. For the cases considered, the LES calculations were successful in predicting observed recirculation zones, vortex breakdown and showed good agreement with experimentally measured mean velocities, their rms fluctuations and Reynolds shear stresses.

Effects of Swirl on Intermittency Characteristics in Non-Premixed Flames

Swirl effects on velocity, mixture fraction, and temperature intermittency have been analyzed for turbulent methane flames using large eddy simulation (LES). The LES solves the filtered governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modeling based on the localized dynamic Smagorinsky and the steady laminar flamelet models, respectively. Probability density function (PDF) distributions demonstrate a Gaussian shape closer to the centerline region of the flame and a delta function at the far radial position. However, non-Gaussian PDFs are observed for velocity and mixture fraction on the centerline in a region where center jet precession occurs. Non-Gaussian behavior is also observed for the temperature PDFs close to the centerline region of the flame. Due to the occurrence of recirculation zones, the variation from turbulent to nonturbulent flow is more rapid for the velocity than the mixture fraction and therefore indicates how rapidly turbulence affects the molecular transport in these regions of the flame.

Identification and analysis of instability in non-premixed swirling flames using LES

Large eddy simulations (LES) of turbulent non-premixed swirling flames based on the Sydney swirl burner experiments under different flame characteristics are used to uncover the underlying instability modes responsible for the centre jet precession and large scale recirculation zone. The selected flame series known as SMH flames have a fuel mixture of methane-hydrogen (50:50 by volume). The LES solves the governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky model and the steady laminar flamelet model respectively. The LES results are validated against experimental measurements and overall the LES yields good qualitative and quantitative agreement with the experimental observations. Analysis showed that the LES predicted two types of instability modes near fuel jet region and bluff body stabilised recirculation zone region. The mode I instability defined as cyclic precession of a centre jet is identified using the time periodicity of the centre jet in flames SMH1 and SMH2 and the mode II instability defined as cyclic expansion and collapse of the recirculation zone is identified using the time periodicity of the recirculation zone in flame SMH3. Finally frequency spectra obtained from the LES are found to be in good agreement with the experimentally observed precession frequencies.

A study of mixing and intermittency in a coaxial turbulent jet

A large eddy simulation study of mixing and intermittency of a coaxial turbulent jet discharging into an unconfined domain has been conducted. The work aims to gain insight into the mixing and intermittency of turbulent coaxial jet configurations. The coaxial jet considered has relatively high jet velocities for both core and annular jets with an aspect ratio (core jet to annular jet) of 1.48. The computations resolved the temporal development of large-scale flow structures by solving the transport equations for the spatially filtered mass, momentum and passive scalar on a non-uniform Cartesian grid and employed the localized dynamic Smagorinsky eddy viscosity as a sub-grid scale turbulence model. The results for the time-averaged mean velocities, associated turbulence fluctuations and mean passive scalar fields are presented. The initial inner and outer potential cores and the shear layers established between two cores have been resolved, together with the establishment of high turbulence regions between the shear layers. The passive scalar fields developing from the core and the bypass flow were found to exhibit differences at near and far field locations. Probability density distributions of instantaneous mixture fraction and velocity have been created from which intermittency has been calculated and the development of intermittency from the probability density distributions for instantaneous velocity follows similar variations as for the passive scalar fields.

Influence of bluff-body and swirl on mixing and intermittency of jets

Abstract In this paper we present the modelled results of turbulence, scalar mixing and intermittency for three different basic fluid dynamical problems using large eddy simulation (LES). The modelled problems are a turbulent round jet, a bluff body stabilised jet, and a bluff body stabilised swirl jet in a co-flow environment. Both instantaneous and time averaged results along with the probability density functions (pdf) and intermittency of velocity and passive scalar are presented. Simulations well captured the flow features of jet, bluff body stabilised jet and bluff body stabilised swirl jet. The instantaneous and time averaged data show the differences in turbulence and mixing and also an improvement of mixing in the presence of a bluff body and swirl. The addition of bluff body and swirl affect the structure of pdfs for both velocity and passive scalar at different axial and radial locations. The radial variation of intermittency at locations close to the centreline indicates turbulent to non-turbulent phenomena with respect to bluff body and swirl at both upstream and downstream recirculation regions.

Analysis of impinging wall effects on hydrogen non-premixed flame

Investigations of the flame–vortex and flame–wall interactions have been performed for hydrogen impinging non-premixed flame at a Reynolds number of 2000 and a nozzle-to-plate distance of 4 jet diameters by direct numerical simulation (DNS) and flamelet generated manifold (FGM) based on detailed chemical kinetics. The results presented in this study were obtained from simulations using a uniform Cartesian grid with 200 × 600 × 600 points. The spatial discretization was carried out using a sixth-order accurate compact finite difference scheme, and the discretized equations were advanced using a third-order accurate fully explicit compact-storage Runge–Kutta scheme. The results show that the inner vortical structures dominate the mixing of the primary jet for the nonbuoyant case, while outer vortical structures dominate over the inner vortical structures in the flow fields of the buoyant cases. The formation of vortical structures due to buoyancy has a direct impact on the flow patterns in both the primary and wall jet streams, which in turn affects the flame temperature and the near-wall heat transfer. It has been found that the buoyancy instability plays a key role in the formation of the much wider and higher value wall heat flux compared with the nonbuoyant case, while external perturbation does not play a significant role. The computational results show an increased wall heat flux with the presence of buoyancy.

Large eddy Simulation of a turbulent swirling coaxial jet

This work uses the Large Eddy Simulation (LES) technique to study velocity and passive scalar mixing along with intermittency of a spatially evolving turbulent coaxial swirl jet. The simulations captured the potential core and also predicted high level turbulence intensities in the inner mixing regions. The Probability Density Functions (PDFs) and radial intermittency plots revealed an intermittent mixing behaviour especially in the outer region of the flow where the fluctuations of velocity rapidly change from rotational to irrotational and vice versa. The PDF and radial intermittency profiles exhibit Gaussian and non-Gaussian distributions close to the jet centreline and away from the centreline, respectively.

Simulations of Unsteady Oscillations in Turbulent Non-Premixed Swirling Flames

Simulations of turbulent non-premixed swirling flames based on the Sydney swirl burner experiments under different flame characteristics are conducted using large eddy simulations (LES). The simulations attempt to capture the unsteady flame oscillations and explore the underlying instability modes responsible for a centre jet precession and the large scale recirculation zone oscillation. The selected flame series known as SMH flames have a fuel mixture of methane-hydrogen (50:50 by volume). The LES program solved the governing equations on a structured Cartesian grid using finite volume method and the subgrid turbulence and combustion models used the localized dynamic form of Smagorinsky eddy viscosity model and the steady laminar flamelet model respectively. The results show that the LES predicts two types of instability modes near fuel jet region and the bluff body stabilized recirculation zone region. The Mode I instability defined as cyclic precession of a centre jet is identified using time periodicity of the centre jet in flames SMH1 and SMH2. The Mode II instability defined as cyclic expansion and collapse of the recirculation zone is identified using time periodicity of the recirculation zone in flame SMH3. The calculated frequency spectrums found reasonably good agreement with the experimental precession frequencies. Overall, the LES yield a good qualitative and quantitative agreement with the experimental observations, although some discrepancies are apparent.

DNS of Turbulent Lean Premixed Syngas Flames at Elevated Pressures

The use of low carbon content fuels as replacements or supplement for petroleum fuels offer the advantage of a much cleaner fuel with little added atmospheric carbon dioxide greenhouse burden with a low potential sulfer content.

Mixing, intermittency and Large Eddy Simulation of a turbulent round jet

Large Eddy Simulation (LES) is a promising technique for accurate prediction of turbulent free shear flows in a wide range of applications. Here the LES technique has been applied to study the intermittency in a high Reynolds number turbulent round jet in a co-flow environment. The objective of this work is to study the external intermittency of velocity and scalar fields. LES based time averaged statistics show good agreement with the experimental measurements. The calculated probability density distributions show a Gaussian shape near the centreline region and a delta function at the furthest radial locations. The agreement between LES results and experimental data are good for the external intermittency indicating that the dynamic LES subgrid model accurately predicts the external intermittency.