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### Example 4. A study of smoke generation in a gas-turbine combustor.

• The smoke-generation model has been activated for case 492 of the PHOENICS library, which represents a (rather simple) three-dimensional gas-turbine-like combustor, which is fed by a rich-fuel-air vapour mixture, and by primary-, secondary- and dilution-air streams.

• The graphical convergence monitor for the 40-fluid run shown here gives proof of a satisfactorily converging calculation;
and computer times are seen to be small.

### The results

The table shows how the number of fluids influences the predicted rate of smoke production and the computer time.

 number smoke seconds 1 0.74 100 10 2.38 139 20 2.28 217 30 2.31 267 40 2.26 485 50 2.27 599

Note that:

• On this occasion MFM predicts more smoke production than the conventional single-fluid model; and
• the 10-fluid model provides a good approximation.

The following figures show the computed PDFs for a location in the middle of the outlet plane of the combustor, for 10 fluids, 40 fluids, 50 fluids.

The shapes are all similar; and the root-mean-square and population-average values do not differ much.

The following contour plots show various aspects of the 50-fluid calculation:

1. The very different smoke distributions on an axial plane according to:
(a) the single-fluid (no fluctuations) model and
(b) the multi-fluid model

The flow is from right to left.

2. The somewhat different distributions of population-average temperature according to:
(a) the single-fluid model
and
(b) the multi-fluid model

The highest temperature encountered is (understandably) greater for the single-fluid than for the multi-fluid model.

3. The concentrations of fluids: fluid 1, (pure air) fluid 6, (fuel-lean) , and fluid 11 (approximately stoichiometric).

• Obviously, a large amount of valuable insight into combustor behaviour can be derived from such studies.

• The author commends them to the attention of designers.