43689-G9
Combustion of Methane in Countercurrent Shear Layers
The application of countercurrent shear for turbulent flows is well known to enhance mixing and entrainment. The motivation of the current project is to explore the role of countercurrent shear flow in the presence of combustion to compete with the detrimental influence of heat release on entrainment. A planar countercurrent shear layer experiment has been constructed and tested under a wide range of operating conditions. This configuration consists of a high-speed stream of air and fuel premixed to a given equivalence ratio, and a countercurrent parallel stream of pure air. The primary flow is fuel rich, resulting in a combustion process that is generally stabilized in the shear layer mixing region. During the past twelve months, the facility has been optimized in terms of geometry to allow the development of reacting countercurrent shear layers with minimized streamwise pressure gradients and confinement effects.
An experimental study was conducted to document the influence of both primary stream equivalence ratio and the velocity ratio across the shear layer. Particle image velocimetry (PIV) measurements were made through the use of aluminum oxide particles seeded in the primary and counterflowing streams. Sets of 800 instantaneous velocity field measurements were collected to allow for the quantification of the local and spatial characteristics of the velocity statistics.
The local shear layer thickness measurement was made as the transverse width that contained the central 80% of the velocity variation within the shear layer. The spatial development of the shear layer thickness for different conditions was documented. The shear layer width typically shows a nonlinear growth region near the shear layer origin, eventually becoming linear within a short downstream distance. The slope of the downstream linear region is called the growth rate and is an important characteristic that represents entrainment and mixing.
The most striking result to date is the trend of the growth rate with respect to the primary stream equivalence ratio at a fixed shear layer velocity ratio. The equivalence ratio was varied from 1.4 to 2.0. The growth rate as a function of the equivalence ratio is shown in Fig. 1. The figure illustrates that the growth rate experiences a dramatic
Fig.1 Growth rate as a function of
equivalence ratio
reduction in growth rate as the equivalence
ratio increases. Two potential physical
explanations exist for the observed trend.
The heat release within the shear layer increases proportionally with
increasing equivalence ratio due to the presence of more fuel. The second possible influence relates to the
position of the flame relative to the shear layer vorticity. For the lower equivalence ratio cases, the
primary stream needs a small amount of air to reach stoichiometric conditions,
resulting a flame that would tend to prefer a position near the primary
stream. As the equivalence ratio
increases, more air is required to reach the stoichiometric condition,
hence the flame would tend to be located near the middle of the shear
layer. The flame position relative to
the vorticity field has been seen in other studies to have a strong impact on
the resultant turbulent structure.
It should be noted that the
probability distribution function of local equivalence ratio within the shear
layer will depend on the relative entrainment from each stream, called the entrainment
ratio, which is expected to depend on the velocity ratio. Figure 2 shows the growth rate of the shear
layer as a function of velocity ratio parameter λ for a fixed equivalence
ratio of 1.4; the velocity ratio parameter is defined as
where U is the freestream velocity
and subscripts 1 and 2 represent the primary and counterflowing stream,
respectively. An interesting trend is
represented in Fig. 2 where
Fig. 2: Growth rate as a function of velocity ratio
an increase in velocity ratio
actually results in a decrease in the shear layer growth rate. This is the opposite trend observed for
nonreacting flow. The trends shown in
Fig. 1 and 2 suggest that an increase in velocity ratio under combustion
conditions may actually reduce the entrainment ratio, bringing the flame
towards the center of the shear layer.
Related to
the entrainment ratio, a unique phenomenon has been observed for the recent
experiments where shear layer fluid is repeatedly ejected from the shear layer
into the counterflowing stream, resulting in a secondary stream that consists
of inhomogeneous regions of air and combustion products. This phenomenon interferes with the
entrainment of air from the counterflowing stream. Current efforts are focused on characterizing
this ejection process and exploring the presence of this phenomenon on the
operating conditions (e.g. counterflowing vs. coflowing shear layers).
The project
has benefited the PI's research program and has identified new shear layer
phenomenon that are currently being explored and will result in
publications. The graduate student
supported by this work has acquired an engineering position in industry.
