Response of Laminar Premixed Flame to Mass Flow Rate and Pressure Fluctuations

Laminar flamelets are often used to model premixed turbulent combustion. The libraries of rates of conversion from chemical to thermal enthalpies used for flamelets are typically based on counter-flow, stained laminar planar flames under steady conditions. The current research seeks further understanding of the effect of stretch on premixed flames by considering laminar flame dynamics in a cylindrically-symmetric outward radial flow geometry (i.e., inwardly propagating flame). This numerical model was designed to study the flame response when the flow and scalar fields align (i.e., no tangential strain on the flame) while the flame either expands (positive stretch) or contracts (negative stretch, which is a case that has been seldom explored) radially. The transient response of a laminar premixed flame has been investigated by applying a sinusoidal variation of mass flow rate at the inlet boundary with different frequencies to compare key characteristics of a steady unstretched flame to the dynamics of an unsteady stretched flame. An energy index (EI), which is the integration of the source term in the energy equation over all control volumes in the computational domain, was selected for the comparison. The transient response of laminar premixed flames, when subjected to positive and negative stretch, results in amplitude decrease and phase shift increase with increasing frequency. Other characteristics, such as the deviation of the EI at the mean mass flow rate between when the flame is expanding and contracting, are nonmonotonic with frequency. Also, the response of fuel lean flames is more sensitive to the frequency of the periodic stretching compared to a stoichiometric flame. An analysis to seek universality of transient flame responses across lean methane-air flames of different equivalence ratios (i.e., 1.0 to 0.7) using Damköhler Numbers (i.e., the ratio of a flow to chemical time scales) had limited success.

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CFD Investigation of Turbulent Premixed Flame Response to Transverse Forcing

Volume 1A: Combustion, Fuels and Emissions ◽

10.1115/gt2013-94312 ◽

2013 ◽

Cited By ~ 4

Author(s):

C. Y. Lee ◽

R. S. Cant

Keyword(s):

Surface Area ◽

Large Scale ◽

Bluff Body ◽

Transverse Velocity ◽

Premixed Flame ◽

High Frequency Oscillation ◽

Limit Cycle Oscillation ◽

Flame Surface ◽

Harmonic Forcing ◽

Flame Response

Screech is a high frequency oscillation that is usually characterized by instabilities caused by large-scale coherent flow structures in the wake of bluff-body flameholders and shear layers. Such oscillations can lead to changes in flame surface area which can cause the flame to burn unsteadily, but also couple with the acoustic modes and inherent fluid-mechanical instabilities that are present in the system. In this study, the flame response to hydrodynamic oscillations is analyzed in a controlled manner using high-fidelity Computational Fluid Dynamics (CFD) with an unsteady Reynolds-averaged Navier-Stokes approach. The response of a premixed flame with and without transverse velocity forcing is analyzed. When unforced, the flame is shown to exhibit a self-excitation that is attributed to the anti-symmetric shedding of vortices in the wake of the flameholder. The flame is also forced using two different kinds of low-amplitude out-of-phase inlet velocity forcing signals. The first forcing method is harmonic forcing with a single characteristic frequency, while the second forcing method involves a broadband forcing signal with frequencies in the range of 500–1000 Hz. For the harmonic forcing method, the flame is perturbed only lightly about its mean position and exhibits a limit cycle oscillation that is characteristic of the forcing frequency. For the broadband forcing method, larger changes in the flame surface area and detachment of the flame sheet can be seen. Transition to a complicated trajectory in the phase space is observed. When analyzed systematically with system identification methods, the CFD results, expressed in the form of the Flame Transfer Function (FTF) are capable of elucidating the flame response to the imposed perturbation. The FTF also serves to identify, both spatially and temporally, regions where the flame responds linearly and nonlinearly. Locking-in between the flame’s natural self-excited frequency and the subharmonic frequencies of the broadband forcing signal is found to alter the dynamical behaviour of the flame.

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An Approximate Solution for the Thermal Performance of a Stirling-Engine Regenerator

Journal of Engineering for Power ◽

10.1115/1.3574680 ◽

1969 ◽

Vol 91 (2) ◽

pp. 109-112 ◽

Cited By ~ 10

Author(s):

E. B. Qvale ◽

J. L. Smith

Keyword(s):

Experimental Data ◽

Approximate Solution ◽

Mass Flow ◽

Flow Rate ◽

Thermal Performance ◽

Closed Form Solution ◽

Form Solution ◽

Pressure Variation ◽

Stirling Engine ◽

The One

An approximate closed-form solution for the thermal performance of a Stirling-engine regenerator is derived. The solution is for sinusoidal mass flow rate and sinusoidal pressure variation with a phase angle relative to the mass flow. The solution provides the net enthalpy flux along the regenerator and the change of phase between the mass flow and the pressure. The method is similar to the one developed by Rea [1], and the results agree well with his experimental data.

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Transient response of laminar premixed flame to a radially diverging/converging flow

10.32920/14636262 ◽

2021 ◽

Author(s):

Meysam Sahafzadeh ◽

Larry W. Kostiuk ◽

Seth B. Dworkin

Keyword(s):

Mass Flow Rate ◽

Transient Response ◽

Mass Flow ◽

Flow Rate ◽

Laminar Flame ◽

Premixed Flames ◽

Scalar Fields ◽

Premixed Flame ◽

Computational Domain ◽

Tangential Strain

Laminar flamelets are often used to model premixed turbulent combustion. The libraries of rates of conversion from chemical to thermal enthalpies used for flamelets are typically based on counter-flow, stained laminar planar flames under steady conditions. The current research seeks further understanding of the effect of stretch on premixed flames by considering laminar flame dynamics in a cylindrically-symmetric outward radial flow geometry (i.e., inwardly propagating flame). This numerical model was designed to study the flame response when the flow and scalar fields align (i.e., no tangential strain on the flame) while the flame either expands (positive stretch) or contracts (negative stretch, which is a case that has been seldom explored) radially. The transient response of a laminar premixed flame has been investigated by applying a sinusoidal variation of mass flow rate at the inlet boundary with different frequencies to compare key characteristics of a steady unstretched flame to the dynamics of an unsteady stretched flame. An energy index (EI), which is the integration of the source term in the energy equation over all control volumes in the computational domain, was selected for the comparison. The transient response of laminar premixed flames, when subjected to positive and negative stretch, results in amplitude decrease and phase shift increase with increasing frequency. Other characteristics, such as the deviation of the EI at the mean mass flow rate between when the flame is expanding and contracting, are nonmonotonic with frequency. Also, the response of fuel lean flames is more sensitive to the frequency of the periodic stretching compared to a stoichiometric flame. An analysis to seek universality of transient flame responses across lean methane-air flames of different equivalence ratios (i.e., 1.0 to 0.7) using Damköhler Numbers (i.e., the ratio of a flow to chemical time scales) had limited success.

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