Bluff bodies

Unpredictable. Disks and plates tend to wobble, while fuller bluff bodies tend to rotate. 

Gas Burners Gas burners may be classified as premixed or non-premixed. Many types of flame stabilizer are employed in gas burners (see Fig. 27-32). Bluff body, swirl, and combinations thereof are the predominant stabilization mechanisms. 

Tsuchiya, M., S. Murakami, A. Nochida, K. Kondo, and Y. Ishida. 1997. Development of a new i-e model for flow and pressure fields around bluff body. /. Witid Engineering and Industrial Aerodynamics, vols. 67-68, pp. 169-182. 

Utilizing a forced-draft fan, the burner has a gas head arranged to mix the fuel and air in a blast tube which controls the stability and shape of the flame. Gas exits from nozzles or holes in the head and is mixed partly in the high-velocity air stream and partly allowed to exit into an area downstream of a bluff body. Behind the bluff body, a relatively quiescent zone forms which provides a means for flame stability. Many configurations exist, but the most... 

When a bluff body is interspersed in a fluid stream, the flow is split into two parts. The boundary layer (see Chapter 11) which forms over the surface of the obstruction develops instabilities and vortices are formed and then shed successively from alternate sides of the body, giving rise to what is known as a von Karman vortex street. This process sets up regular pressure variations downstream from the obstruction whose frequency is proportional to the fluid velocity, as shown by Strouai. 9. Vortex flowmeters are very versatile and can be used with almost any fluid — gases, liquids and multi-phase fluids. The operation of the vortex meter, illustrated in Figure 6.27, is described in more detail in Volume 3, by Gjnesi(8) and in a publication by a commercial manufacturer, Endress and Hauser.10 ... 

F.H. Wright and E.E. Zukowsky 1962, Flame spreading from bluff-body flame holders, Proc. Combust. Inst. 8 933-943. 

A bluff-body stabilized flame of CH4/H2 in air (designated HMl by Dally et al. [22]) (a) time-averaged photograph of flame luminosity, (b) time-averaged streamlines from LES, (c) instantaneous visualization of OH "luminosity" from LES, and (d) instantaneous temperature field from LES. (b and d are adapted from Raman, V. and Pitch, H., Combust. Flame, 142,329,2005. With permission.)... 

Dally, B. B., Masri, A. R., Barlow, R. S., and Fiechtner, G. J., Instantaneous and mean compositional structure of bluff-body stabilized nonpremixed flames. Combust. Flame, 114, 119, 1998. 

Raman, V. and Pitsch, H., Large-eddy simulation of a bluff-body-stabilized non-premixed flame using a recursive filter-refinement procedure. Combust. Flame, 142, 329, 2005. 

Correa, S. M. and S. B. Pope (1992). Comparison of a Monte Carlo PDF/finite-volume mean flow model with bluff-body Raman data. In Twenty-fourth Symposium (International) on Combustion, pp. 279-285. Pittsburgh, PA The Combustion Institute. 

Dally, B. B., D. F. Fletcher, and A. R. Masri (1998). Measurements and modeling of a bluff-body stabilized flame. Combustion Theory and Modelling 2, 193-219. 

Jenny, P., M. Muradoglu, S. B. Pope, and D. A. Caughey (2001a). PDF simulations of a bluff-body stabilized flow. Journal of Computational Physics 169, 1-23. 

Recirculation of combustion products can be obtained by several means (1) by inserting solid obstacles in the stream, as in ramjet technology (bluff-body stabilization) (2) by directing part of the flow or one of the flow constituents, usually air, opposed or normal to the main stream, as in gas turbine combustion chambers (aerodynamic stabilization), or (3) by using a step in the wall enclosure (step stabilization), as in the so-called dump combustors. These modes of stabilization are depicted in Fig. 4.52. Complete reviews of flame stabilization of premixed turbulent gases appear in Refs. [66, 67],... 

FIGURE 4.54 Recirculation zone and flame-spreading region for a fully developed turbulent wake behind a bluff body (after Williams [57]). 

FIGURE 4.55 Flame-spreading interaction behind multiple bluff-body flame stabilizers. 

In either case, bluff body or aerodynamic, blowout is the primary concern. In ramjets, the smallest frontal dimension for the highest flow velocity to be used is desirable in turbojets, it is the smallest volume of the primary recirculation zone that is of concern and in dump combustors, it is the least severe step. 

There were many early experimental investigations of bluff-body stabilization. Most of this work [69] used premixed gaseous fuel-air systems and typically plotted the blowoff velocity as a function of the air-fuel ratio for various stabilized sizes, as shown in Fig. 4.56. Early attempts to correlate the data appeared to indicate that the dimensional dependence of blowoff velocity was different for different bluff-body shapes. Later, it was shown that the Reynolds number range was different for different experiments and that a simple independent dimensional dependence did not exist. Furthermore, the state of turbulence, the temperature of the stabilizer, incoming mixture temperature, etc., also had secondary effects. All these facts suggest that fluid mechanics plays a significant role in the process. 

Stirred reactor theory was initially applied to stabilization in gas turbine combustor cans in which the primary zone was treated as a completely stirred region. This theory has sometimes been extended to bluff-body stabilization, even though aspects of the theory appear inconsistent with experimental measurements made in the wake of a flame holder. Nevertheless, it would appear that stirred reactor theory gives the same functional dependence as the other correlations developed. In the previous section, it was found from stirred reactor considerations that... 

From these correlations it would be natural to expect that the maximum blowoff velocity as a function of air-fuel ratio would occur at the stoichiometric mixture ratio. For premixed gaseous fuel-air systems, the maxima do occur at this mixture ratio, as shown in Fig. 4.56. However, in real systems liquid fuels are injected upstream of the bluff-body flame holder in order to allow for mixing. Results [60] for such liquid injection systems show that the maximum... 

Recess stabilization appears to have two major disadvantages. The first is due to the large increase in heat transfer in the step area, and the second to flame spread angles smaller than those obtained with bluff bodies. Smaller flame spread angles demand longer combustion chambers. 

Establishing a criterion for blowoff during opposed-jet stabilization is difficult owing to the sensitivity of the recirculation region formed to its stoichiometry. This stoichiometry is well defined only if the main stream and opposed jet compositions are the same. Since the combustor pressure drop is of the same order as that found with bluff bodies [76], the utility of this means of stabilization is questionable. 

It is interesting to note that Eq. (7.47) is essentially the condition used in bluff-body stabilization conditions in Chapter 4, Section F. This result gives the intuitively expected answer that the higher the ambient temperature, the shorter is the ignition time. Hydrocarbon droplet and gas fuel injection ignition data correlate well with the dependences as shown in Eq. (7.47) [8,9],... 

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