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Numerical Simulations of Turbulent Combustion and Combustion Engines

Researchers: M. Valentino, L. Cao, Dr. Y. Zhu

Supervisors: Dr. X. Jiang, Prof. H. Zhao

Sponsors: EPSRC, Ford, BRIEF

Turbulent Modelling

Possible Approaches

Reynolds-Averaged Navier Stokes (RANS)

Solve for the mean values of all quantities, the predominant approach in engineering CFD packages.

Large Eddy Simulations (LES)

The turbulent large scales are explicitly calculated whereas the effects of smaller ones are modelled using subgrid closure rules. LES is particularly appealing for IC engine applications, which is attracting more research efforts.

Direct Numerical Simulations (DNS)

Solve the full instantaneous Navier-Stokes equations without any model for turbulent motions. DNS of IC engine flows is possible, but has not been performed.

Comparison of Approaches

Approach	Advantages	Disadvantages
RANS	"Coarse" numerical grid Geometrical simplifications (2D Flows...) "Reduced" numerical cost	Only mean flow field Models required
LES	Unsteady features Reduced modelling impact (compared to RANS)	Subgrid models required 3D simulations required Needs precise codes High numerical cost
DNS	No models for turbulence / combustion interaction Tool to study models	Prohibitive numerical cost (fine grids, precise codes) Limited to academic problems

(Left) Time evolutions of local temperature computed with DNS, LES and RANS in a turbulent flow field.
(Right) Turbulent energy spectrum plotted as a function of wave numbers.

Example of DNS:

Instantaneous 3D visualisations of a rectangular buoyant reacting jet. (a) Reaction rate at t=24 (b) Vorticity magnitude at t=24 (Jiang & Luo, 2001)

CFD of IC engine flow and combustion: RANS vs. LES

(Left) Kiva-3V (RANS)
(Right) Kiva-LES
A comparison between KIVA-LES and KIVA-3V (RANS approach) of the droplet "parcels" inside the cylinder and the stoichiometric surface due to spray evaporation at 90 degree after injection. (Sone et al., 2000)

Computational Mesh

 

Resolution requirements of DNS: piston en, may be accessed with DNS. (Poinsot & Veynante 2001)

In-Cylinder Equivalence Ratio & Temperature at 1600rpm

	18° ATDC	24° ATDC
Bowl 1
Bowl 2
Bowl 3