A scaling law for the recirculation zone length behind a bluff body in reacting flows

Abstract

The recirculation zone length behind a bluff body is influenced by the turbulence intensity at the base of the body in isothermal flows and also the heat release and its interaction with turbulence in reacting flows. This relationship is observed to be nonlinear and is controlled by the balance of forces acting on the recirculation zone, which arise from the pressure and turbulence fields. The pressure force is directly influenced by the volumetric expansion resulting from the heat release, whereas the change in the turbulent shear force depends on the nonlinear interaction between turbulence and combustion. This behaviour is elucidated through a control volume analysis. A scaling relation for the recirculation zone length is deduced to relate the turbulence intensity and the amount of heat release. This relation is verified using the large eddy simulation data from 20 computations of isothermal flows and premixed flames that are stabilised behind the bluff body. The application of this scaling to flames in an open environment and behind a backward facing step is also explored. The observations and results are explained on a physical basis.

Experimental investigation of isothermal and reacting flow characteristics downstream of axisymmetric bluff body stabilizers under stratified or fully premixed inlet mixture conditions

“…However, in fully premixed flames, the increase in δ reduces the length of the recirculation, contrary to what was demonstrated in Figure 7-10. This trend is similar to other studies such as [29]. In [29], the role of the equivalence ratio and turbulence levels on the RZ size were discussed.…”

“…This trend is similar to other studies such as [29]. In [29], the role of the equivalence ratio and turbulence levels on the RZ size were discussed. There, the authors provide an estimation for The presently identified behavior of the recirculation length is in accordance with those estimations for both stratified and fully premixed methane flames.…”

“…However, a controversial trend appears for propane flames. Under stratification, an increase of Φpeak results in RZ reduction, but under fully premixed conditions the RZ either marginally increases or maintains its length, likely implying an effect of fuel type and Lewis number [29].This response of the variations in recirculation zone and structure under different inlet mixture conditions merits further investigations through future experimental and computational efforts.…”

“…Also, in many attempts, the researchers highlighted the entrainment effect and suggested theoretical or practical approaches for its estimation. In [18,25,28,29] experimental and computational efforts have also been performed to correlate recirculation zone length with turbulent intensity, heat release and bluff body characteristics for both unconfined and confined configurations. The results showed that recirculation length decreased with increasing φ for various fuel-air mixtures with…”

Mitigating combustion instability and NOx emissions with annular oxyfuel jets into the flame shear layer region

“…Although there are numerous studies about eliminating combustion instability by open‐loop injection, 1,5–8 such as altering the flame anchoring zone or using different injection medium, to date, there is no research on suppressing combustion instability through changing the inner or outer shear layer of the flame. First of all, the shear layer of the burner could significantly affect the combustion process 34,45–47 . Besides, the CO 2 /O 2 atmosphere could damp the flame combustion instability and NO x emissions at the same time 8,48,49 .…”