BCs for buoyant flow with outlet parallel to gravity vector
 

I have been unable to see the effects of buoyancy on a flame simulation, although the case is chosen as one for which buoyancy should be significant (Fr ≈ 0.3, where the Froude number is the ratio of inertial to gravitational forces). The flow and flame results are approximately what I expect from 1D simulations, but there is no change to the vertical flame position with gravity on or off. I get a slightly different flame position with the outflow and pressure outlet boundary conditions, but no change with gravity for either one. I've also been able to show buoyancy effects within a domain with only horizontal outlets, but this vertical (parallel to gravity) outlet seems problematic.

I'm new to Fluent, and would be grateful for any suggestions on possible problems with boundary conditions or elsewhere.

The flow and temperature for this counterflow diffusion flame are shown below.

• Left side: Axis of 2D axisymmetric domain (yellow)
• Bottom: Fuel inlet (velocity inlet)
• Top: Air inlet (velocity inlet)
• Right side: Outlet

Attachment 98523

Simulation overview:

• - Fluent 2D axisymmetric; steady, laminar flow.
• - Species transport with volumetric gas phase reactions: 30 species, 200 reactions, stiff chemistry solver.
• - Species thermodynamic properties from Chemkin database, with multicomponent & thermal diffusion.
• - Pressure-based solver; pressure-velocity coupled, Rhie-Chow distance-based flux type.
• - Mesh 350 (X) by 45 (Y) rectangular grid, checked for mesh-independence.

Boundary condition details:

• 1. fuel_inlet
• - Velocity Specification Method: Magnitude, Normal to Boundary
• - Reference Frame: Absolute
• - Velocity Magnitude [m/s]: 0.2084
• - Supersonic/Initial Gauge Pressure [Pa]: 0
• - Temperature [K]: 318.7
• - Specify Species in Mole Fractions? yes[/INDENT]
• 2. air_inlet
• - Velocity Specification Method: Magnitude, Normal to Boundary
• - Reference Frame: Absolute
• - Velocity Magnitude [m/s]: 0.1125
• - Supersonic/Initial Gauge Pressure [Pa]: 0
• - Temperature [K]: 368.6
• - Specify Species in Mole Fractions? yes
• 3. outlet
• - Gauge Pressure [Pa]: 0
• - Pressure Profile Multiplier: 1
• - Build artificial walls to prevent reverse flow? yes
• - Average Pressure Specification? no
• - Specify targeted mass flow rate: no
• 4. axis
• - axis: axis