David Forliti

Fluidic Flame Stabilization in a Planar Combustor Using a Transverse Slot Jet

T HE stabilization of a premixed flame in a high-speed internal environment has received a considerable amount of interest from many researchers, for example, [1–3]. Flame stabilization behind a bluff body represents the most common strategy for flame anchoring [4–8]. Bluff bodies and rearward-facing step flows stabilize the flame by introducing a low-velocity recirculation zone containing combustion products that act as a continuous ignition source. Although these flame-holding devices provide an environment suitable for flame holding, a drag penalty is incurred. An alternative fluidic-based approach using a transverse slot jet to generate a “virtual bluff body” would reduce the thrust penalties through the removal of form drag while producing a flowfield with flame-holding potential. Figure 1 shows two schematics for combustors employing bluff body and fluidic methods for flame holding. The dashed box in the figure represents the control volume that will be used to demonstrate that form drag imposes a penalty. A momentum balance in the streamwise direction for the two situations, employing the assumption of negligible viscous shear, indicates that the drag force on the bluff body FD will result in a loss in either streamwise momentum or an increased pressure drop across the burner. For the fluidic case, a balancewill be maintained between pressure drop and an increase in streamwise momentum. A simple one-dimensional analysis of the systems, shown in Fig. 1, using a drag coefficient based on the inlet flow properties was conducted to determine the additional total pressure losses due to flame-holder drag. The calculation employs constant specific heats, models the fluid as air, and employs conservation of mass, momentum, the ideal gas equation, Mach number definition, and stagnation relations. Figure 2 shows the additional total pressure loss due to form drag on the flame holder as a function of inlet Mach number and drag coefficient. In addition to thrust penalty reduction, the fluidic flame holder allows active control of the recirculation zone size, providing dynamic control of the stabilization characteristics that will allow a broader operating envelope and improved off-design performance. Dynamic control would allow for optimization of the tradeoff between combustion efficiency and flame stability. The cost of the fluidic actuation is considered in terms of the required mass flow rate to achieve the necessary recirculation zone. For the present experiments, the fluidic flow ratewas nominally 7% of themain flow rate. The objective of the current note is to document the operating characteristics of a fluidic flame holder consisting of a planar transverse jet issuing into a channel flow. The influence of the test chamber initial conditions on the scaling of the induced recirculation zonewill be shown. It will also be shown that the jet equivalence ratio can be used to manipulate the rich and lean blowout limits.

Near-Wall Velocity Field Measurements of a Very Low Momentum Flux Transverse Jet

Abstract : Transverse jets have been the subject of study for several decades due to their relevance in a number of flows in nature and engineering applications. The momentum flux ratio between the jet and crossflow is generally considered to be the leading independent parameter, and has been explored extensively over a nominal range from unity to several hundred. The current study considers the flow field of a transverse jet at momentum flux ratios much smaller than unity. A modulated absorption/emission thermometry technique under development at AFRL requires a narrow film barrier to maintain a clean optical access window, while limiting the transverse penetration of the jet into the crossflow to avoid corruption of the optical measurement. The current effort examined the near field behavior of very low momentum flux ratio transverse jets to elucidate the jet and crossflow interactions. Under low momentum flux ratio conditions, the crossflow has sufficient local momentum near the injection location, exceeding that of the jet, which leads to crossflow ingestion. It was expected that reduced area and consequent increase in mean jet velocity would increase the effective momentum flux ratio which would lead to increased momentum flux and penetration into the crossflow. However, the complex 3-dimensional interaction between the jet and crossflow suggests penetration is far reduced for these very low momentum flux ratio cases. Particle image velocimetry was used to investigate the jet and cross flow interactions near the jet exit over a range of low momentum flux ratios, 0.0013 or = J or = 0.075. A fully developed laminar jet was injected into a fully developed turbulent channel crossflow. The two lowest momentum flux ratio cases were found to penetrate the least into the crossflow with regard to mean behavior of the jet.

On the Flame and Vorticity Characteristics of a Fluidically Stabilized Premixed Turbulent Flame

Flame stabilization is the act of maintaining combustion in the presence of a high-speed premixed flow, and continues to be an important process that influences the performance and limitations for propulsion applications. A common approach for current generation flame holders involves the employment of a low-speed recirculation zone where hot combustion products are maintained and act as a continuous ignition source. The recirculation zone is often induced using a wake-generating bluff body that is submerged in the flow, or through the use of a rearward-facing step. A fluidic-based flame holder using a transverse slot jet issuing into a cross flow offers potential thrust and efficiency benefits for propulsion. The transverse slot jet flame holder has been shown to develop a low-speed recirculation zone capable of stabilizing a stationary flame, analogous to a rearward-facing step (i.e. a wall-bounded bluff body). The fluidic flame holder provides competitive flame holding performance to the mechanical counterpart, while having enhanced combustion rates that result in higher combustor efficiencies and/or shorter burners. The mechanisms contributing to the enhanced combustion rates are discussed. Turbulent flame structures were investigated for various flame holders with emphasis on the downstream shear region. The role of baroclinic torque on turbulent flame structure evolution and the flowfield will be described. Comparisons will be made to a rearward-facing step flame holder. The details of the turbulent flow with combustion will be described, showing the potential advantages achieved using fluidics.

Exploring Mechanisms of Fluidic Thrust Vectoring Using a Transverse Jet and Suction

Fluidic flow control methods, such as transverse injection or suction, have been shown to be effective for thrust vector angle control of the flow issuing from a rectangular channel. The Reynolds number based on the channel height was 39,000. The objective of the current work was to explore the operating mechanism of these independent fluidic approaches and to consider the performance when they are combined in a single configuration. Suction vectors the flow via static pressure control near the jet exit, where the pressure boundary condition extends into the channel to produce finite vectoring at the exit of the jet. Transverse injection within the channel produces a recirculation zone that contributes to the vectoring effect due to the alteration of the wall static pressure distribution. The influence of suction on the vectoring performance is dependent on the level of transverse injection, with a degraded suction effect at higher transverse injection rates. Suction is effective when it is able to disturb and enhance the mixing of the jet shear layer.

Combustor Flowfield Measurements of a Transverse Jet Flame Holder

A fluidic-based flame holder offers potential thrust and efficiency benefits for propulsion. The capability of fluidics for flame stabilization in a high-speed premixed reactant flow has been established. A transverse slot jet issuing into a channel flow has been shown to develop a low-speed recirculation zone capable of stabilizing a stationary flame, analogous to typical geometrical flame holders. The current study documents detailed reacting flowfield measurements to help understand the fluidic flame holder. The fluidic stream of the fluidic flame holder consisted of a mixture of methane fuel and air at an equivalence ratio matching that of the main combustor flow. Digital particle image velocimetry was used to study the flowfield of a fluidic based dump combustor. The effects of combustion on the mean and turbulent flowfields for different momentum ratio cases are described. Dilatation was used to asses a relative heat release rate for the fluidic dump combustor. Comparisons of the turbulence length and velocity scales as well as flame topology for different blowing ratios are made to help understand the performance of the fluidic dump combustor.

Enhancing Combustion in a Dump Combustor Using Countercurrent Shear: Part 2 — Heat Release Rate Measurements and Geometry Effects

Research to advance our understanding of the countercurrent shear flow has been conducted, with particular emphasis on those characteristics of countercurrent shear that are beneficial for combustion applications. Studies carried out in a backward-facing step combustor burning prevaporized JP10-air mixtures, have examined the implementation of counterflow as a means to enhance turbulent burning velocities, with the overall objective of increasing volumetric heat release rates and thereby create a more compact combustion zone. The dump combustor is characterized by a nominally two-dimensional primary flow mixture of prevaporized fuel and air, entering a rectangular channel before encountering a 2:1 single-sided step expansion. Flow separation over the sudden expansion and the resulting recirculation set up a countercurrent shear layer downstream of the dump plane and a low velocity zone conducive to flame anchoring. Combustion control strategies aim to increase turbulent kinetic energy and flame three-dimensionality in an effort to increase flame surface area and thus burning rates. A secondary flow is created via suction at the dump plane as a fluidic control mechanism to enhance the naturally occurring countercurrent shear layer. Counterflow is shown to elevate turbulence levels and volumetric heat release rates downstream of the step in the base geometry while concomitantly reducing the scale of the recirculation zone[1]. Modifications to the rearward-facing step geometry are investigated using Particle Image Velocimetry (PIV) under isothermal flow conditions in an effort to extend the near field interaction between the recirculation zone and the incoming primary flow, thus exploiting the benefits of counterflow as seen in the base geometry. Using chemiluminescence, relative heat release rates are shown to increase by 90% with a counterflow level of 6% of the primary flow by mass in the base geometry, and a 150% increase with a counterflow level of 2.4% in the modified step geometry.Copyright

Enhancing Combustion in a Dump Combustor Using Countercurrent Shear: Part 1 — Nonreacting Flow Control and Preliminary Combustion Results

During the last decade, countercurrent shear has been established as an effective flow control technique for increasing turbulent mixing in a variety of flow configurations and operating regimes. Based on the robust mixing enhancement observed for jets and shear layers, the technique appears to have many potential benefits for enhancement and control for turbulent combustion flows. Countercurrent shear flow control has been applied to a planar asymmetric rearward-facing step dump combustor. A nonreacting flow study on the implementation of suction-based countercurrent shear at the dump plane provided insight into the flow control mechanisms. Control of turbulence velocity and length scales occurs through two mechanisms, the development of a countercurrent shear layer near the dump plane, and enhanced global recirculation caused by the removal of mass at the dump plane. Parametric studies on the geometry of the suction slot indicate that the enhancement of the global recirculation zone is the primary mechanism for increasing global turbulence levels within the combustor. Turbulence energy and length scales both increase in a manner such that the spatially-filtered strain rates as measured with particle image velocimetry remain nominally constant, a desirable characteristic for premixed turbulent combustion. Connections will be made to a recent study on fully-developed turbulent countercurrent shear layers showing additional attractive features of countercurrent shear including enhanced turbulent energy production, entrainment, and three dimensionality. Preliminary reacting flow results for the dump combustor operating while burning premixed/prevaporized JP-10 illustrate qualitative changes in the turbulent combustion process within the combustor. The companion paper will describe the quantitative effects of countercurrent shear on the global heat release rates within the combustor.Copyright