Cold-State Investigation on a Flame Holder

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Figure 1. Definition of the co-ordinate and the geometrical

parameters for the flow past a slitty bluff body.

cal geometries are generally bluff body and require spe-

cial attention when attempting to predict the associated

flow. In most problems, the flow is unsteady and turbu-

lent with vortex shedding. It is theoretically possible to

directly resolve the whole spectrum of turbulent scales

using an approach known as direct numerical simulation

(DNS) to capture the fluctuations. DNS is not feasible for

practical engineering problem, as DNS approaches are

too computationally inten se. An alternative f or numerical

simulation of complex turbulent flows is the large eddy

simulation (LES). It should, however, be stressed that the

application of LES to engineering simulations is in its

infancy. Evidence shows that LES is improper to calcu-

late the 2D bluff body flow.

Traditionally, numerical si mulations of these flows are

performed using the Reynolds-averaged Navier-Stokes

(RANS) equations and using phenomenological model to

fully represent the turbulence. The available turbulence

models vary extensively in complexity, in particular, from

simple algebraic eddy viscosity relationships to complex

formulations involving several additional differential equa-

tions. In each model, values for the empirical constants

are obtained from turbulent flows that are fundamentally

simple.

The RANS equations represent transport equations for

the mean flow quantities only, with all the scales of the

turbulence being modelled. A computational advantage is

seen in transient situations, since the time step will be

determined by the global unsteadiness in the mean flow

rather than by the turbulence. The Reynolds-averaged ap-

proach is generally adopted for practical engineering cal-

culations.

The RNG k-ε model is one of the k-ε variants of

RANS derived using a rigorous statistical technique

called renormalization group theory. It is similar in form

to the standard k-ε model, but includes the following

refinements:

• The RNG model has an additional term in its ε equa-

tion that significantly improves the accuracy for ra-

pidly strained flows.

• The effect of swirl on turbulence is included in the

RNG model, enhancing accuracy for swirling flows.

These features make the RNG more accurate and reli-

able for a wider class of flows than the standard k-ε

model. Thus RNG k-ε model in FLUENT 6.0 is used in

the present investigation using.

The computational meshes employed are non-uniform

grids. The number of grid cells is about 140,000 (slightly

varied with different gap ratio), these quadrilateral cells

are obtained with interval size 1mm using the Quad/

Tri-Pave meshing Scheme in GAMBIT, which creates a

paved mesh that consists primarily of quadrilateral ele-

ments but employs triangular mesh elements in any cor-

ners, the edges of which form a very small angle with

respect to each other.

Velocity Inlet boundary conditions include the flow

velocity, and all relevant scalar properties of the flow.

The total properties of the f low are not fixed, so they will

rise to whatever value necessary to provide the pre-

scribed velocity dis tribution. Figure 1 defined the inf low

velocity Re = 470,000 (nor mal to the boundary).

Pressure Outlet boundary conditions specify the static

pressure at flow outlets (and also other scalar variables,

in case of backflow). The use of a pressure outlet boun-

dary condition instead of an outflow condition often re-

sults in a higher rate of convergence when backflow oc-

curs during iteration. The outlet boundary is far enough

from the bluff body (x = Lx) with constant static pressure

which can be measured experimentally.

In order to model the natural perturbations in any real

flow, many numerical simulation s usually use an explicit

perturbation at the onset of the trans ient calculation. This

numerical disturbance exists in the form of a deranged

initial flow field often formed by applying slip velocity.

This explicit perturbation is said to be necessary in order

to disturb the Navier-Stokes equations and provoke or

“kick-start” the vortex shedding process by Anderson

(1993) [2]. D.G.E. Grigoriadis et al. (2003) investigated

incompressible turbulent flow past a long square cylinder

using LES [3]. They used a uniform stream U superim-

posed with Gaussian random divergence-free perturba-

tions of intensity 2% - 5% w.r.t. the local value. After a

transient time the flow rejects the initiate unrealistic co n-

dition and the shear layers at the cylinders’ faces initiated

vortex shedding.

In our investigation all flow fields were firstly calcu-

lated with a “stable” solver on the assumption that the

flow field can be time-independent. This assumption is of

signality, although it is undoubted that the real flow past

a bluff body is a ti me-dependent problem. To investigate

the intrinsic mechanism in the w ake flow of a slitty bluff

body, the inflow perturbation is unwanted. The perturba-

tion can be reduced to a certain level, but it can never be

removed completely or be diminished to very small level

in a real flow. However the simulation can initiate the

inflow perturbation to zero. All further time-dependent

simulations are on the basis of the stable results.