Partially Premixed Flame Stabilization in the Presence of a Combined Swirl and Bluff Body Influenced Flowfield: An Experimental Investigation

2020 ◽  
Vol 142 (7) ◽  
Author(s):  
Rajesh Sadanandan ◽  
Aritra Chakraborty ◽  
Vinoth Kumar Arumugam ◽  
Satyanarayanan R. Chakravarthy
Keyword(s):  
Flow Field ◽  
Bluff Body ◽  
Reverse Flow ◽  
Fluid Dynamic ◽  
Thermal Power ◽  
Swirl Flow ◽  
Flame Stabilization ◽  
Dynamic Strain ◽  
Low Velocity ◽  
Divergent Flow

Abstract Optical and laser diagnostic measurements in a nonpremixed model gas turbine (GT) burner have been performed to investigate the effect of an increase in thermal power on the flame stabilization. The model GT burner has a large bluff body base with an annular swirl region, leading to a convergent-divergent flow field at the burner exit. Under the investigated conditions, the flame stabilizes predominantly in the diverging section characterized by the swirl flow with a central recirculation zone. With increasing thermal power, the reverse flow of hot burned gases is strengthened, with the hydroxyl radical (OH) planar laser induced fluorescence (PLIF) images indicating an increase in the temperature of the burned gases. The preferred flame stabilization location coincides with the inner shear layer between the reactant inflow and the reverse flow of hot burned gases. At high thermal power, the flame seems to stabilize in regions of high fluid dynamic strain rate, highlighting the influence of the reverse flowing burned gases in the evolution of the flammable mixture upstream. However, simultaneous and time-resolved measurements of the flow-field and scalar field are needed for direct quantification of this. The results are in agreement with the flame stabilization theories based on partial fuel-air mixing and streamline divergence. The flow is seen to decelerate upstream of the flame front and the flame stabilizes in a region of low velocity, created as a result of heat release diverging the streamlines ahead of it.

Simulation of Two-Dimensional Turbulent Recirculating Flow Field Behind a V-Shaped Bluff Body

1994 ◽  
Author(s):  
Z. Gu ◽  
M. A. R. Sharif
Keyword(s):  
Flow Field ◽  
Bluff Body ◽  
Flame Stabilization ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Combustion Chambers ◽  
Recirculating Flow ◽  
Conservative Form ◽  
Curvilinear Grid ◽  
Predictor Corrector

Abstract The two-dimensional turbulent recirculating flow fields behind a V-shaped bluff body have been investigated numerically. Similar bluff bodies are used in combustion chambers for flame stabilization. The governing transport equations in conservative form are solved by a pressure based predictor-corrector method. The standard k-ϵ turbulence closure model and a boundary fitted multi-block curvilinear grid system are used in the computation. The code is validated against turbulent flow over a backward facing step problem. The predicted flow field behind the bluff body is also compared with experiment. It is found that while the qualitative features of the flow are well predicted, there is quantitative disagreement between the measurement and prediction. This disagreement can be partially attributed to the k-ϵ turbulence model which is known to be inadequate for recirculating flows. Parametric investigation of the flow field by varying the shape and size of the bluff body is also performed and the results are reported.

Precession Effects on the Relationship Between Time-Averaged and Instantaneous Swirl Flow and Flame Characteristics

2015 ◽  
Author(s):  
I. Chterev ◽  
G. Sundararajan ◽  
J. M. Seitzman ◽  
T. C. Lieuwen
Keyword(s):  
Stagnation Point ◽  
Large Scale ◽  
Reverse Flow ◽  
Axial Flow ◽  
Swirl Flow ◽  
Flame Stabilization ◽  
Coherent Motion ◽  
Stagnation Points ◽  
Topological Features ◽  
The Relationship

Swirling flows exhibit a variety of unsteady fluid mechanic features, including large scale vortical structures and precessing recirculation zones. This paper considers the specific influence of precession on the relationship between time-averaged and instantaneous flow and flame features. The objective of this study is to aid in developing insight into high fidelity computations or experimental results. In particular, we describe how certain topological features in the time-averaged flow, such as centerline axial jets, centerline stagnation points, and symmetry of the flow about the centerline are influenced by precession. Insight is built by presenting results from a simplified model of a two-zone flow, consisting of a precessing reverse flow region embedded in a positive axial flow. A particularly significant result of this work is in regards to aerodynamically stabilized flames, which rely on the low velocity interior stagnation points in the vortex breakdown region for flame stabilization. We show how precession causes systematic differences between the location of the stagnation point of the time-averaged velocity and the time-averaged position of the instantaneous stagnation point. Indeed, an important implication of this point is that a perfect prediction of the time-averaged flow field could still lead to a completely erroneous time-averaged flame position prediction. Finally, we discuss the influence of precession and coherent motion on convergence of estimated averaged quantities.

Variations of Anchoring Pattern of a Bluff-Body Stabilized Laminar Premixed Flame as a Function of the Wall Temperature

2016 ◽  
Author(s):  
Sandrine Berger ◽  
Stéphane Richard ◽  
Florent Duchaine ◽  
Laurent Gicquel
Keyword(s):  
Flow Field ◽  
Boundary Conditions ◽  
Combustion Chamber ◽  
Wall Temperature ◽  
Bluff Body ◽  
Flame Stabilization ◽  
Premixed Flame ◽  
Strong Impact ◽  
Flame Holder ◽  
Thermal Boundary Conditions

Aircraft engine components are subject to hostile thermal environments. The solid parts in the hot stages encounter very high temperature levels and gradients that are critical for the engine lifespan. Combustion chamber walls in particular exhibit very heterogeneous thermal fields. The prediction of this specific thermal field is a very complex task as it results from complex interactions between fresh gas injections, cooling flow distributions, combustion, flame stabilization and thermal transfers to the solids. All these phenomena are tightly coupled and do not evolve linearly. Today, the design phase of a combustion chamber is strongly enhanced by the use of high fidelity computations such as Large Eddy Simulations (LES). However, thermal boundary conditions are rarely well known and are thus treated either as adiabatic or as approximated isothermal conditions. Such approximations on thermal boundary conditions can lead to several errors and inaccurate predictions of the combustion chamber flow field. With this in mind and to foresee the potential difficulties of LES based Conjugate Heat Transfer (CHT) predictions, the effect of the wall temperature on a laminar premixed flame stabilization is numerically investigated in this paper for an academic configuration. The considered case consists of a squared cylinder flame holder at a low Reynolds number for which several wall-resolved Direct Numerical Simulations (DNS) are performed varying the bluff-body wall thermal condition. In such a set-up, the reactive flow and the flame holder interact in a complex way with an underlying strong impact of the wall temperature. For a baseline configuration where the flame holder wall temperature is fixed at 700K, the flow field is steady with a flame stabilized thanks to the recirculation zone of the flame holder. As the wall temperature is decreased, the position of the stabilized flame moves further downstream. The flame remains steady until a threshold cold temperature is reached below which an instability appears. For solid temperatures above 700 K, the flame is seen to move further and further upstream. For very hot conditions, the flame even stabilizes ahead of the bluff-body. The various flow solution bifurcations as the flame stabilization evolves are detailed in this paper. Heat flux distribution along the bluff-body walls are observed to be dictated by the flame stabilization process illustrating different mechanisms while integration of these fluxes on the whole flame holder surface confirms that various theoretical equilibrium states may exist for this configuration. This suggests that computation of more realistic cases including thermal conduction in the bluff-body solid part could lead to different converged results depending on the initial thermal state.

Experimental and Numerical Analysis of Gas Premix Turbulent Flames Stabilized in a Swirl Burner with Central Bluff Body

2020 ◽  
Author(s):  
Fernando Biagioli ◽  
Alessandro Innocenti ◽  
Steffen Terhaar ◽  
Teresa Marchione
Keyword(s):  
Numerical Analysis ◽  
Bluff Body ◽  
Reverse Flow ◽  
Flame Stabilization ◽  
Turbulent Flames ◽  
Experimental Investigations ◽  
Swirl Burner ◽  
Stable Flame ◽  
Sudden Transition ◽  
Flame Behaviour

Abstract Lean premixed gas turbulent flames stabilized in the flow generated by an industrial swirl burner with a central bluff body are experimentally found to behave bi-stable. This bi-stable behaviour, which can be triggered via a small change in some of the controlling parameters, for example the bulk equivalence ratio, consists in a rather sudden transition of the flame from completely lifted to well attached to the bluff body. While several experimental investigations exist on this topic, numerical analysis is limited. The present work is therefore also of numerical nature, with a two-fold scope: a) simulation and validation with experiments of the bi-stable flame behaviour via Computational Fluid Dynamics (CFD) in the form of Large Eddy Simulation (LES) and b) analysis of CFD results to shed light on the flame stabilization properties. LES results, in case of the lifted flame, show that the vortex core is sharply precessing at a given frequency. Phase averaging these results at the frequency of precession clearly indicates a counter-intuitive and unexpected presence of reverse flow going all the way through the flame apex and the bluff body tip. A simple one-dimensional flame stabilization model is applied to explain the bi-stable flame behaviour.

Experimental and Numerical Analysis of Gas Premix Turbulent Flames Stabilized in a Swirl Burner With Central Bluff Body

2020 ◽  
Author(s):  
Fernando Biagioli ◽  
Alessandro Innocenti ◽  
Steffen Terhaar ◽  
Teresa Marchione
Keyword(s):  
Numerical Analysis ◽  
Burning Rate ◽  
Axial Velocity ◽  
Bluff Body ◽  
Reverse Flow ◽  
Flame Stabilization ◽  
Turbulent Flames ◽  
Swirl Burner ◽  
Lifted Flame ◽  
Flame Behaviour

Abstract Lean premixed gas turbulent flames stabilized in the flow generated by an industrial swirl burner with a central bluff body are experimentally found to behave bi-stable. This bi-stable behaviour, which can be triggered via a small change in some of the controlling parameters, for example the bulk equivalence ratio, consists in a rather sudden transition of the flame from completely lifted to well attached to the bluff body. This has impact on combustion dynamics, emissions and pressure losses. While several experimental investigations exist on this topic, numerical analysis is limited. The present work is therefore also of numerical nature, with a two-fold scope: a) simulation and validation with experiments of the bi-stable flame behaviour via Computational Fluid Dynamics (CFD) in the form of Large Eddy Simulation (LES) and b) analysis of CFD results to shed light on the flame stabilization properties. LES results, in case of the lifted flame, show that the vortex core is sharply precessing at a given frequency. Phase averaging these results at the frequency of precession clearly indicates a counter-intuitive and unexpected presence of reverse flow going all the way through the flame apex and the bluff body tip. The counter-intuitive presence of a lifted flame is explained here in terms of the phase averaged data which show that the flame apex is not placed at the centre of the spinning reverse flow region. It is instead slightly shifted radially outward where the axial velocity recovers to low positive values of the order of the turbulent burning rate. A simple one-dimensional flame stabilization model is applied to explain this peculiar flame behaviour. This model provides first an estimation of the flame radius of curvature in terms of axial velocity and turbulence quantities. This radius is therefore used to determine the total flux of reactants into the flame, given by an axial convection and a radial diffusion contributions. Subsequently the possibility of the flame positioned at the centre of the vortex is excluded based on the balance between this flux and the turbulent burning rate. A clear explanation of the mechanism leading to the sudden flame jump has instead not been identified and only some hypotheses are provided.

Numerical simulation of the fluid dynamic effects of laser energy deposition in air

2008 ◽  
Vol 605 ◽  
pp. 329-354 ◽  
Author(s):  
SHANKAR GHOSH ◽  
KRISHNAN MAHESH
Keyword(s):  
Flow Field ◽  
Energy Deposition ◽  
Laser Energy ◽  
Reverse Flow ◽  
Fluid Dynamic ◽  
Reynolds Numbers ◽  
Shock Propagation ◽  
Scaling Analysis ◽  
Plasma Core ◽  
The Impact

Numerical simulations of laser energy deposition in air are conducted. Local thermodynamic equilibrium conditions are assumed to apply. Variation of the thermodynamic and transport properties with temperature and pressure are accounted for. The flow field is classified into three phases: shock formation; shock propagation; and subsequent collapse of the plasma core. Each phase is studied in detail. Vorticity generation in the flow is described for short and long times. At short times, vorticity is found to be generated by baroclinic means. At longer times, a reverse flow is found to be generated along the plasma axis resulting in the rolling up of the flow field near the plasma core and enhancement of the vorticity field. Scaling analysis is performed for different amounts of laser energy deposited and different Reynolds numbers of the flow. Simulations are conducted using three different models for air based on different levels of physical complexity. The impact of these models on the evolution of the flow field is discussed.

scholarly journals Modelling of a Bluff-Body Stabilised Premixed Flames Close to Blow-Off

Computation ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 43
Author(s):  
Shokri Amzin ◽  
Mohd Fairus Mohd Yasin
Keyword(s):  
Bluff Body ◽  
Fluid Dynamic ◽  
Premixed Flames ◽  
Pollutant Emissions ◽  
Moment Closure ◽  
Recirculation Region ◽  
Scalar Dissipation ◽  
Average Error ◽  
Conditional Moment ◽  
Lean Premixed

As emission legislation becomes more stringent, the modelling of turbulent lean premixed combustion is becoming an essential tool for designing efficient and environmentally friendly combustion systems. However, to predict emissions, reliable predictive models are required. Among the promising methods capable of predicting pollutant emissions with a long chemical time scale, such as nitrogen oxides (NOx), is conditional moment closure (CMC). However, the practical application of this method to turbulent premixed flames depends on the precision of the conditional scalar dissipation rate,. In this study, an alternative closure for this term is implemented in the RANS-CMC method. The method is validated against the velocity, temperature, and gas composition measurements of lean premixed flames close to blow-off, within the limit of computational fluid dynamic (CFD) capability. Acceptable agreement is achieved between the predicted and measured values near the burner, with an average error of 15%. The model reproduces the flame characteristics; some discrepancies are found within the recirculation region due to significant turbulence intensity.

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