Turbulent Characteristics and Entrainment Characteristics in a Reacting Countercurrent Shear Layer

Over the past decade, countercurrent shear has been established as a robust flow control technique for enhancing mixing and entrainment in a wide range of shear flows.  Recent studies have shown that countercurrent shear enhances premixed combustion rates by over 100%, providing an example of the potential benefits of this flow control technique to combustion systems. 

Although the performance of countercurrent shear has been documented for some flow configurations, the dynamical changes in the flow are not well understood.  Past studies on the physics of shear layers has been somewhat enigmatic, often raising as many questions as they answer.  An area that has very little experimental information is the area that we are currently focusing on which is countercurrent shear flow control to a planar shear layer experiencing combustion of methane. 

A recent study on nonreacting countercurrent shear layers documented a bifurcation yielding enhanced entrainment and turbulent energy production, a transition that will be beneficial under combustion conditions.  The present work will focus on the developing region of the countercurrent shear layer with heat release.  The following aspects will be explored:  1) The ability of countercurrent shear to enhance mixing and entrainment in the presence of practical heat release levels. 2) An examination of the effects of velocity ratio on entrainment and flame attachment location.  One of the velocity ratios that we looked at was l=1.38.  Figures 1 depicts the Umean and Streamlines which helps to elucidate the physical paths that are traveled by the two streams.  Figure 2 Shows the V components of velocity which are of interest when examining entrainment into the mixing layer.  Figures 3 and 4 show that the turbulence levels are greatest in the mixing region which makes sense since that is the location with the greatest velocity gradients.  Figure 5 shows combustion at a mainstream stoichometric mixture of 1.9.  The effects of entrainment in Fig. 2 will be expanded upon and compared to those in Fig. 5.

Preliminary data shows that during combustion process a recirculation zone forms and compresses with increasing velocity ratio while shifting closer to the splitter plate tip.  It is believed that this recirculation zone is the result of a triple flame developing under the influence of a counter current shear.  An interesting finding preliminarily is that the volume flux on the counter current side appears to be out of the shear layer.  Future work is going to delve into the mass flux which should give more quantifiable results. It is the hope of this study that the improved chemical mixing resulting from the increased understanding of the entrainment processes will have a revolutionary effect on future combustion processes. 

Figure 1.  Contours of X-Component of Velocity profile for Countercurrent Shear Layer with streamlines for a velocity ratio of l=1.38.

Figure 2.  Contours of Y-Component of Velocity profile for Countercurrent Shear Layer for a velocity ratio of l=1.38.

Figure 3. Contours of X-Component of Turbulence levels profile for Countercurrent Shear Layer for a velocity ratio of l=1.38.

Figure 4. Contours of Y-Component of Turbulence levels profile for Countercurrent Shear Layer for a velocity ratio of l=1.38.

Figure 5.  Combustion in Countercurrent Shear Layer with a mainstream stoichiometric mixture ratio of 1.9.