Flaming computers

Numerical models are offering fresh insight into flame physics, and are set to become much more than just a supplement to theory and experiment.

Desirable characteristics of a gas burner — be it for industrial or home use — are high energy efficiency and, at the same time, low pollutant emission. Progress in developing such devices is fuelled by findings in combustion chemistry and from turbulent flame experiments. Both approaches have limitations, respectively: because of the complexity of a turbulent flow-field, the understanding of its chemical kinetics remains elusive; and capturing the full three-dimensional structure of a flame and its internal processes poses considerable experimental challenges. In Proceedings of the National Academy of Sciences, John Bell and colleagues¹ show that state-of-the-art computer simulation could provide a valuable link between these fields and open the door to new insight.

Their investigations are based on a type of flame — the so-called turbulent V-flame —that has favourable properties with regard to NO_x emission. In the experimental part of the work, the flame is created by pushing a premixed methane–air fuel though a 5-cm-diameter nozzle, which has a thin rod fitted across it to stabilize the flame. A perforated grid placed some distance upstream introduces turbulent flow.

For the same scenario, Bell et al. simulate the transport process as well as the detailed chemical kinetics — 20 chemical species and 84 fundamental reactions are considered. The flame is allowed to blaze inside a cube with sides 12 cm long: earlier studies either covered only two dimensions or were restricted to three-dimensional domains extending one centimetre or less in each dimension. This 1,000-fold increase in size enables the authors to reproduce a full laboratory-scale flame in great detail.

Figure 1: Almost the real thing.

Bell et al. find remarkable agreement between their simulation of a turbulent V-flame (left) and its laboratory counterpart (right).

Figure (and thumbnail image) reproduced with permission from ref. 1. Copyright 2005 National Academy of Sciences, USA.

Comparison with experiment reveals that the model reproduces the basic morphology of the flame and the dynamics inside it with remarkable accuracy (Fig. 1). With this powerful tool in hand, the authors expect that they can explore the inner workings of flames in depth, and study aspects that cannot be analysed experimentally.