TITLE : EXPLOSION IN A BAFFLED ENCLOSURE

BY: D J FREEMAN - CHAM Development Team

PHOENICS VERSION: 2.1

DATE: SEPTEMBER 1995

DETAILS :

A methane explosion in a vented enclosure 1.2m long is simulated. There are 5 pairs of baffles equally spaced along the length of the enclosure, generating turbulence.
The simulation is two-dimensional and the LVEL turbulence model is used in conjunction with the Multi-Fluid Turbulent Combustion model (see next page).
Some preliminary simulations involved simulation of only the first baffle.

The Multi-Fluid Turbulent Combustion (MFTC) Model:

The MFTC has been developed within CHAM to simulate the complex combusting flows associated with explosions and flames more accurately.
In this example the reacting mixture consists of four fragments, of different (and fixed) composition and temperature (see next page). Mixing of the hot and cold fragments creates the intermediate fragments, with the hottest being hot enough to react (i.e. burn) to form hot products.
The rate of combustion is therefore controlled not only by the rate of mixing (as in the Eddy-break-up model) but also by the kinetic rate of reaction.
The MFTC may be extended to any number of fluids, depending on the requirements of the simulation.

The Multi-Fluid Turbulent Combustion Model (cont.)

The four fluids used in this simulation were: A, comprising of fully unburned fuel/air mixture, B, a mixture of unburned gas and products at a temperature too low for chemical combustion to proceed, C, the same mixture but at a temperature at which the chemical rate is significant, and D, fully burned gas.

A and D mix to form B and C, while A mixes with C and B with D to form B and C respectively. C reacts to form D.

The overall rate of combustion therefore depends on the mixing rate and the rate of reaction.

The effect of changing the mixing rate constants was investigated in this work.

Pictures are as follows:

1. Contours of constant flame front arrival time (low value of CMIX)

2. Contours of constant flame front arrival time (medium value of CMIX)

3. Contours of constant flame front arrival time (high value of CMIX)

4. Preliminary simulation: Concentration contours of A

5. Preliminary simulation: Concentration contours of B

6. Preliminary simulation: Concentration contours of C

7. Preliminary simulation: Concentration contours of D

8. Full simulation: Concentration contours of D (products)showing the development of the flame front with time:

at 60 and 80 microseconds

at 100 and 120 microseconds

at 140 and 160 microseconds

DISCUSSION

Although the MFTC is still (1995) in the early stages of development, the results shown here are in reasonable agreement with the experimental data, especially the prediction of the shape of the flame front.
Further work needs to be done to evaluate the best values for the mixing rate constants, for a variety of conditions.
The MFTC has been used within CHAM for several different combustion scenarios, including stationary and transient flames and large scale explosions.

wbs