704 research outputs found
Investigation of the influence of swirl on a confined coannular swirl jet
No full textLarge Eddy Simulations are used to model a turbulent confined coannular combustor and examine the effects of swirl on the flow field and mixing. Three separate simulations with relatively high mesh resolutions and different swirl numbers have been carried out using a finite volume method on a Cartesian non-uniform structured grid. A localised dynamic Smagorinsky model is used to parameterize the subgrid scale turbulence. The snapshots of the axial and swirl velocities and velocity vector fields show the complex flow patterns developing with increased swirl number and the rapid decay of axial momentum. Precessing vortex cores (PVC) were identified for all three cases and the mean axial velocity plots indicate that the upstream extremity of the vortex breakdown bubble shifts towards the inlet as the swirl number increases. The calculated power spectra indicate the distinct precession frequency for high swirl number. Probability density functions of axial velocity showed the changes of their distributions from approximately Gaussian to non-Gaussian with increased swirl number. The swirl has a large effect on the rate of decay of the axial velocity throughout the domain, whereas only has a significant effect on the decay of swirl velocity in the near field close to the jet inlet. The relation between swirl number and the axial extent of the recirculation zone is approximately linear. Radial plots of mean passive scalar and its variance also demonstrate an increase in the rate of mixing with increasing swirl number
Study of jet precession, recirculation and vortex breakdown in turbulent swirling jets using LES.
No full textLarge eddy simulations (LES) are used to investigate turbulent isothermal swirling flows with a strong emphasis on vortex breakdown, recirculation and instability behaviour. The Sydney swirl burner configuration is used for all simulated test cases from low to high swirl and Reynolds numbers. The governing equations for continuity and momentum are solved on a structured Cartesian grid, and a Smagorinsky eddy viscosity model with the localised dynamic procedure is used as the sub-grid scale turbulence model. The LES successfully predicts both the upstream first recirculation zone generated by the bluff body and the downstream vortex breakdown bubble. The frequency spectrum indicates the presence of low frequency oscillations and the existence of a central jet precession as observed in experiments. The LES calculations well captured the distinct precession frequencies. The results also highlight the precession mode of instability in the center jet and the oscillations of the central jet precession, which forms a precessing vortex core. The study further highlights the predictive capabilities of LES on unsteady oscillations of turbulent swirling flow fields and provides a good framework for complex instability investigations
Large Eddy Simulation of a turbulent swirling coaxial jet
No full textThis 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
A study of mixing and intermittency in a coaxial turbulent jet
No full textA 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
LES Of intermittency in a turbulent round jet with different inlet conditions
No full textLarge 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 jet with and without a bluff body. The objective of this work is to study the turbulence intermittency of velocity and scalar fields and its variation with respect to different inlet conditions. Probability density function distributions (pdf) of instantaneous mixture fraction and velocity have been created from which the intermittency has been calculated. The time averaged statistical results for a round jet are first discussed and comparisons of velocity and passive scalar fields between LES calculations and experimental measurements are seen to be good. The calculated probability density distributions show changes from a Gaussian to a delta function with increased radial distance from the jet centreline. The effect of introducing a bluff body into the core flow at the inlet changes the structure of pdfs, but the variation from Gaussian to delta distribution is similar to the jet case. However, the radial variation of the intermittency indicates differences between the results with and without a bluff body at axial locations due the recirculation zone created by the bluff body
Effects of swirl on intermittency characteristics in turbulent non-premixed flames
No full textSwirl effects on velocity, mixture fraction and temperature intermittency have been analysed for turbulent methane flames using LES. The LES solves the filtered governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky and the steady laminar flamelet models respectively. Probability density function (pdf) distributions demonstrate a Gaussian shape closer to the centreline 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 centreline in a region where centre jet precession occurs. Non-Gaussian behaviour is also observed for the temperature pdfs close to the centreline region of the flame. Due to the occurrence of recirculation zones, the variation from turbulent to non turbulent 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
Flame dynamics of swirling nonpremixed hydrogen-carbon monoxide syngas flames
No full textFlame characteristics of swirling non-premixed H2/CO syngas fuel mixtures have been simulated using large eddy simulation and detailed chemistry. The selected combustor configuration is the TECFLAM burner which has been used for extensive experimental investigations for natural gas combustion. The large eddy simulation (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 predictions for H2-rich and CO-rich flames show considerable differences between them for velocity and scalar fields and this demonstrates the effects of fuel variability on the flame characteristics in swirling environment. In general, the higher diffusivity of hydrogen in H2-rich fuel is largely responsible for forming a much thicker flame with a larger vortex breakdown bubble (VBB) in a swirling flame compared to the H2-lean and CO-rich syngas flames
Simulations of unsteady oscillations in turbulent nonpremixed swirling flames
No full textSimulations 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
Computational fluid dynamics modelling toward clean combustion
No full textA turbulent hydrogen-air nonpremixed jet flame is studied using three-dimensional large eddy simulation (LES) and laminar flamelet model based on detailed chemical kinetics. 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 gives good prediction of the flow field, flame temperature and major species. The three-dimensional transient LES demonstrates the variation of low and high temperature structures and both transient and mean predictions show that the high temperature regions and combustion product appear close to the jet centreline. The present findings provide useful details on fundamental issues of turbulence-chemistry interactions of hydrogen combustion and help to identify potential pathways for combustion modelling towards clean combustion
LES of turbulent swirling jets: study of jet precession, recirculation and vortex breakdown
No full textLarge eddy simulations (LES) of turbulent isothermal swirling flows have been investigated. The Sydney swirl burner configuration has been used for all simulated test cases from a low to a high swirl and Reynolds numbers. Four test cases based on different swirl numbers have been considered and the influence of the swirl number for producing recirculation, vortex breakdown, precession vortex core and the precession frequencies have been investigated. The governing equations for the continuity and momentum are solved on a structured Cartesian grid, and a Smagorinsky eddy viscosity model with the localised dynamic procedure is used as the subgrid scale turbulence model. The results show that the LES successfully predicts both the upstream first recirculation zone generated by the bluff body and the downstream vortex breakdown bubble (VBB) induced by swirl. The plots reveal that the expansion of the upstream recirculation zone is almost similar for each test case. LES results revealed that the increasing swirl number affect to form the VBB in the downstream region, however it promotes the shear layer instability in the recirculation zones. The frequency spectrums indicate the presence of low frequency oscillations and the existence of a central jet precession. Results demonstrated distinct precession frequencies at the considered spatial jet locator and agreed well with the experimental values. The results also highlight the formation of a precessing vortex core (PVC)
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