Numerical study of guide vane effects on reacting flow characteristics in a trapped vortex combustor

ABSTRACT

Trapped Vortex Combustor (TVC) is a simple and promising concept for flame stabilizations in aero-engines. In this concept, cavity trapped vortices are used to establish pilot flames for robust combustion performance. The main objective of this study is to numerically investigate the effects of guide vanes on the performance of a TVC installed in a small ramjet. Three dimensional steady and unsteady simulations are performed by solving the Reynolds-Averaged Navier-Stokes (RANS) equations with the commercial flow solver ANSYS FLUENT v16.1. The shear stress transport (SST) model is selected for the turbulence closure, and the eddy dissipation model (EDM) is used for the combustion modeling of gaseous propane (C₃H₈) fuel. In the current work, four cases are studied: (1) TVC without vanes, (2) TVC with a square-tip vane, (3) TVC with a sharp-tip vane, and (4) TVC with a sharp-tip vane and additional cavity fuel injection. Numerical results reveal that the introduction of guide vanes significantly changes the cavity vortex structure. Unlike the previous single cavity vortex, a counter-rotating dual-vortex structure is formed inside the cavity when the vane is implemented: one vortex is generated by the air jet guided by the vane and the other is the wake vortex downstream the vane. Because of the additional air introduced into the cavity by the guide vane, a fuel-lean mixture is obtained inside the cavity. This indicates fuel can be further injected inside to make full use of the air. Combustion results show that compared with the TVC without vanes, case 4 increases the combustion efficiency by 14% (to 99.3%) at the exit of combustor. Meanwhile, the combination of the sharp tip vane and cavity fuel injection only provides around 5% increase in aerodynamic drag and 3.9% in overall total pressure loss.

KEYWORDS:

• trapped vortex combustion
• cavity flame-holder
• ramjet
• flame stablization

Nomenclature

	=	width of the guide vane, 15 mm
	=	thickness of the guide vane, 1 mm
	=	specific heat at constant pressure
	=	gap between the guide vane and fore-body, 3 mm
	=	diameter of combustor, 100 mm
	=	diameter of fore-body and after-body, respectively
	=	mass diffusion coefficient
	=	pressure drag coefficient
	=	Soret diffusion coefficient
	=	specific enthalpy of mixtures and the k-th species per unit mass
	=	enthalpy of formation at the standard reference temperature
	=	length of the guide vane, 15 mm
Ma	=	Mach number
	=	molecular weight of the k-th species
	=	fuel mass flow rate
	=	static pressure
	=	heat source term
	=	momentum flux ratio
	=	universal gas constant
	=	cylindrical coordinates (radial, tangential, and axial directions, respectively)
	=	turbulent Schmidt number, 0.7
	=	static temperature and the standard reference temperature (298.15K)
	=	mean velocity in direction
	=	mass fraction of the k-th species
	=	turbulent viscosity
	=	mean density
	=	thermal conductivity and turbulent effective thermal conductivity
	=	species source term for the k-th species
	=	viscous stress
	=	combustion efficiency

Additional information

Funding

This work was financially supported by Singapore Ministry of Education funding with grant No. [M4011228.050] and the University of Canterbury, New Zealand with grant No. [452STUPDZ].