Turbulent Bunsen burners

Instantaneous schlieren photographs of turbulent Bunsen burner flames at P = 0.1 MPa (left) and P = l.OMPa (right). The flow at U = 2.0m/s is made turbulent, thanks to a perforated plate with hole diameter d = 2.0mm. The burner exit diameter is 20mm. (Reprinted from Frank, J.H., Kalt, P.A., and Bilger, R.W., Combust. Flame, 116, 220, 1999. With permission. Figure 9, p. 238, copyright Elsevier editions.)... 

The knowledge of turbulent premixed flames has improved from this very simple level by following the progress made in experimental and numerical techniques as well as theoretical methods. Much employed in early research, the laboratory Bunsen burners are characterized by relatively low turbulence levels with flow properties that are not constant everywhere in the flame. To alleviate these restrictions, Karpov et al. [5] pioneered as early as in 1959 the studies of turbulent premixed flames initiated by a spark in a more intense turbulence, produced in a fan-stirred quasi-spherical vessel. Other experiments carried out among others by Talantov and his coworkers allowed to determine the so-called turbulent flame speed in a channel of square cross-section with significant levels of turbulence [6]. 

Methane-air Bunsen burner turbulent premixed flame. 

The values of laminar flame speeds for hydrocarbon fuels in air are rarely greater than 45cm/s. Hydrogen is unique in its flame velocity, which approaches 240cm/s. If one could attribute a turbulent flame speed to hydrocarbon mixtures, it would be at most a few hundred centimeters per second. However, in many practical devices, such as ramjet and turbojet combustors in which high volumetric heat release rates are necessary, the flow velocities of the fuel-air mixture are of the order of 50m/s. Furthermore, for such velocities, the boundary layers are too thin in comparison to the quenching distance for stabilization to occur by the same means as that obtained in Bunsen burners. Thus, some other means for stabilization is necessary. In practice, stabilization... 

The classical view is that a turbulent flame is equivalent to a distorted and wrinkled laminar flame. The turbulent flame brush is thus supposed to be an integrated picture of a rapidly fluctuating surface, and instantaneous schlieren pictures seem to support this interpretation 50). Grumer, however, has shown that schlieren snapshots of turbulent hot gas issuing from a Bunsen burner look very much like the flame pictures (34) the implication is that one sees, not the instantaneous flame surface, but the boundary of the hot gas. 

First, however, consider that in turbulent Bunsen flames the axial component of the mean velocity along the centerline remains almost constant with height above the burner but away from the centerline, the axial mean velocity increases with height. The radial outflow component increases with distance from the centerline and reaches a peak outside the flame. Both axial and radial components of tuibulent velocity fluctuations show a complex variation with position and include peaks and troughs in the flame zone. Thus, there are indications of both generation and removal of turbulence within the flame. With increasing height above the burner. 

Bunsen burners should not be used in biological safety cabinets. If one must be used, then one with a pilot light should be chosen because the continuous flame of other models can produce turbulence and the heat produced can damage the filter. See below. 

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