Laminar flamelet model

The laminar flamelet model is one of the methods of modelling turbulent combustions apart from SCRS, eddy flamelet model and others.[1] Combustion is a very important thermochemical process with significant material and aerodynamic implications and thus CFD modeling of combustion has become indispensable. The laminar flamelet model is basically for non pre-mixed fuel (the system in which the fuel and oxygen is supplied from two different pipes).

The concept of ensemble of laminar flamelets was first introduced by Forman A. Williams in 1975[2], while the theoretical foundation was developed by Norbert Peters in the early 80s[3][4][5].

Theory

The flamelet concept considers the turbulent flame as an aggregate of thin, laminar (Re < 2000), locally one-dimensional flamelet structures present within the turbulent flow field. Counterflow diffusion flame is a common laminar flame which is used to represent a flamelet in a turbulent flow. Its geometry consists of opposed and axi-symmetric fuel and oxidizer jets. As the distance between the jets is decreased and/or the velocity of the jets is increased, the flame is strained and departs from its chemical equilibrium until it eventually extinguishes. The mass fraction of species and temperature fields can be measured or calculated in laminar counterflow diffusion flame experiments. When calculated, a self-similar solution exists, and the governing equations can be simplified to only one dimension i.e. along the axis of the fuel and oxidizer jets. It is in this direction where complex chemistry calculations can be performed affordably.[6]

Assumptions

The following assumptions are made in the study of all the flamelet models:[7]

1. While modelling only a single mixture fraction are allowed. Modelling of two-mixture-fraction flamelet models is not possible.

2. It is assumed that the mixture fraction follow the β-function PDF, and scalar dissipation fluctuations are not considered.

3. Empirically-based streams cannot be used.

Logic and formulae

To model a non-premixed combustion, governing equations for fluid elements are required. The conservation equation for the species mass fraction is as follows:-

${\displaystyle {\partial Y_{k} \over \partial t}={\chi \over 2}{1 \over Le_{k}}{\biggl (}{\partial ^{2}Y_{k} \over \partial Z^{2}}{\biggr )}+{\omega _{k} \over \rho }}$

(1)

Le_k → lewis number of kth species and the above formula was derived with keeping constant heat capacity. The energy equation with variable heat capacity:-

${\displaystyle {c_{p}}{\partial T \over \partial t}={\chi \over 2}{\biggl (}{\partial ^{2}h \over \partial Z^{2}}{\biggr )}-{\sum _{k=1}^{N}h_{k}}{\biggl \{}{\chi \over 2}{\partial ^{2}Y_{k} \over \partial Z^{2}}+{\omega _{k} \over \rho }{\biggr \}}}$

(2)

As can be seen from above formulas that the mass fraction and temperature are dependent on

1.Mixture fraction Z

2.Scalar dissipation χ

3.Time

Many a times we neglect the unsteady terms in above equation and assume the local flame structure having a balance between steady chemical equations and steady diffusion equation which result in Steady Laminar Flamelet Models (SLFM). For this, an average value of χ is computed known as favre value[8]

${\displaystyle {{\widetilde {\chi }}_{st}}={{\widetilde {\chi }} \over \int \limits _{0}^{1}{\frac {F(Z)}{F(Z_{st})}}\ P(Z)\,dZ}}$

(3)

${\displaystyle F(Z)=\exp {\bigl (}-2{\bigl [}\operatorname {erfc} ^{-1}(2Z){\bigr ]}^{2}{\bigr )}}$

(3)

The basic assumption of a SLFM model is that a turbulent flame front behaves locally as a one dimensional, steady and laminar which proves to be a very useful while reducing the situation to a much simpler terms but it does create problems as few of the effects are not accounted for.

Advantages

The advantages of using this combustion model are as follows:-

1. They have the advantage of showing strong coupling between chemical reactions and molecular transport.

2. The steady laminar flamelet model is also used to predict chemical non-equilibrium due to aerodynamic straining of the flame by the turbulence.

Disadvantages

The disadvantages of Steady Laminar Flamelet model due to above mentioned reason are:[9]

1.It does not account for the curvature effects which can change the flame structure and is more detrimental while the structure hasn’t reached the quasi-steady state.

2.Such transient effects also arise in turbulent flow, the scalar dissipation experience a sudden change. As the flame structure take time to get stabilize.

To improve the above SLFM models, few more models has been proposed like Transient laminar flamelet model ( TLFM) by Ferreira .

References

1. "Combustion". Online CFD website. CFD Online. Retrieved 5 November 2014.
2. Williams, F. A. (1975). Recent advances in theoretical descriptions of turbulent diffusion flames. In Turbulent mixing in nonreactive and reactive flows (pp. 189–208). Springer, Boston, MA.
3. Peters, N. (1983). Local quenching due to flame stretch and non-premixed turbulent combustion. Combustion Science and Technology, 30(1–6), 1–17.
4. Peters, N., & Williams, F. A. (1983). Liftoff characteristics of turbulent jet diffusion flames. AIAA journal, 21(3), 423–429.
5. Peters, N. (1984). Laminar diffusion flamelet models in non-premixed turbulent combustion. Progress in energy and combustion science, 10(3), 319–339.
6. ANSYS, Fluent. "Laminar flamelet models theory". FLUENT. ANSYS. Archived from the original on 6 November 2014. Retrieved 6 November 2014.
7. ANSYS, FLUENT. "Assumptions". Aerojet.eng. ANSYS. Archived from the original on 6 November 2014. Retrieved 6 November 2014.
8. Pfuderer, D.G.; Neuber, A.A.; Fruchtel, G.; Hassel, E.P.; Janicka, J. (1996). "Combustion flame". 106: 301–317. Cite journal requires |journal= (help)
9. Pitsch, H.; Peters, N. (1998). "Unsteady Flamelet Modelling Of Turbulent Hydrogen-Air Diffusion Flames". Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute. pp. 1057–1064.

Further reading

1. Versteeg H.K. and Malalasekera W., An introduction to computational fluid dynamics, ISBN 978-81-317-2048-6.

2. Stefano Giuseppe Piffaretti, Flame Age Model: a transient laminar flamelet approach for turbulent diffusion flames, A dissertation submitted to the Swiss Federal Institute of Technology in Zurich.

3. N. Peters, Institut für Technische Mechanik RWTH Aachen, Four Lectures on turbulent Combustion.