Turbulence models for CFD in the 21st century

Brian Spalding, of CHAM Ltd

Invited lecture presented at ACFD 2000, Beijing

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.