Kareem Ahmed

Flowfield Characteristics of a Confined Transverse Slot Jet

The current study explored the mean and turbulent flowfield features of a confined transverse slot jet. The slot jet spans 95 % of the full channel spanwise dimension, a geometrical feature found to result in a highly three-dimensional mean flowfield. The transverse slot jet produces a recirculation bubble that has similarities to that found for the flow downstream of a rearward-facing step. The flowfield results were compared with a 2:1 expansion ratio step flow and the observations were discussed in the context of how the transverse slot jet may provide advantages compared with a sudden expansion for subsonic combustors. High turbulence levels are achieved and large turbulent length scales are produced for strong transverse slot jets. The momentum ratio of the jet to that of the channel is found to be a governing parameter, and the dimensions of the recirculation zone scale with this parameter. A series of models constructed in ANSYS/CFX-10 were done to complement the experimental work and showed that the three-dimensionality of the mean flow disappears when the slot jet extends fully across the channel.

Flowfield Characteristics of Oblique Shocks Generated using Microjet Arrays

High momentum microjet arrays are used to generate single and/or multiple oblique shocks in a supersonic crossflow. The properties of these microjet generated shocks can be tailored to be parallel or coalescing, at variable supply pressures, depending on the application. Flowfield measurements using Particle Image Velocimetry were obtained for a range of test conditions. The velocity field obtained clearly shows the global effect of the microjet generated oblique shocks on the flowfield. The jump in the velocity field across the oblique shock and the extent of the influence of the shock and expansion fan are also clearly seen from the results. The microjet generated shocks were compared with a ramp shock and it was found that the properties of the two share some similarity. The strength of the microjet shocks was found to be constant along the length of the shock. The results obtained were compared to previous results obtained using Background Oriented Schlieren and shadowgraph techniques and were found to be consistent. The parallel and coalescing properties of the multiple microjet shocks were also investigated.

Flame Extinction Dynamics of Lean Premixed Bluff-Body Stabilized Flames

There is a crucial need to improve energy conversion efficiencies and minimize the environmental impact of turbulent combustion systems for energy production. Lean premixed turbulent combustion operation will significantly reduce efficiency-based thermal losses and combustion emissions. However, the performance of lean turbulent combustion technology is inevitably limited by the effects of flame extinction. Flames operating at lean conditions are susceptible to stabilization dynamics caused by local reaction extinction; this leads to global flame blowout and termination of the combustion energy production process. An improved understanding and prediction of flame extinction and stability will guide strategies to enhance efficiency, reduce emissions and improve performance of turbulent combustion systems. This research is focused on understanding the physical mechanisms of flame extinction using a newly-developed physics-based model. The novelty of this model is that it interactively couples the physics of the turbulent flow through a dynamic Lagrangian vortex method and the strained flame reaction kinetics using a one-dimensional opposed-jet flame. This innovative modeling strategy effectively captures the dynamic flame stability and extinction for turbulent premixed combustion.

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.

Measuring Shear Stress with a Microfluidic Sensor to improve Aerodynamic Efficiency

In aerodynamic structures, shear stress is the greatest contributor to a bodys total parasitic skin friction drag. This drag is proportional to the local wall shear stress on a surface. Measurement difficulties, high errors and the cost of fabrication have motivated innumerable efforts to develop precise and inexpensive methods for measuring the local shear stress in fluid structures. This is especially important in the supersonic aerodynamic environment, where the interaction between the sensor and air flow induces even higher errors. In order to further improve the efficiency of aircraft and other aerodynamic bodies, sensitive measurements on small scales are required. The present study introduces a novel electrochemical microfluidic shear stress sensor enabling the measurement of the wall shear stress in wind tunnel models. Our company proposes a paradigm shift in shear stress measurements which will take advantage of the complete sensing package offered in micro electro-mechanical systems (MEMS) without the need for moving mechanical parts or expensive manufacturing. The sensor contains a cavity, capped by a thin membrane. The air flow above the membrane deflects the membrane and induces fluid motion within the cavity, which accordingly changes the conductance of the electrolyte solution inside the cavity. This allows the direct measurement of the shear stress by measuring the electrical current under a fixed voltage applied. The proposed sensor is tested inside a subsonic wind tunnel at different air flow rates, using optical experiments and image processing techniques. These measurements enable the comparison of the shear stress measured by the sensor to that obtained by boundary layer measurements and cavity convection measurements. These results imply that the electrochemical shear stress sensor offers a precise and robust measurement system capable of quantifying wall shear stress in air flows.

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.

Flame Holding and Combustion Characteristics of a Geometrical Flame Holder

Flame Stabilization in a high-speed premixed environment requires the presence of a mechanism to stabilize the flame. Bluff bodies or geometrical flame holders introduce a recirculation zone that anchor the flame. The current study considers the influence of equivalence ratio and the boundary layer state at the trailing edge of the flame holder on the flowfield and combustion characteristics. It was found that the recirculation zone is shortened as the equivalence ratio increases towards unity. A secondary shear region emerges downstream of the recirculation zone and is caused by the accelerated low-density combustion products. The emergence of the secondary shear region moves upstream with increasing equivalence ratio. Tripping the boundary layer causes a dramatic reduction in the length of the recirculation zone, and the secondary shear region is greatly augmented. Visualizations show that tripping the boundary layer resulted in a greatly disturbed flame near the trailing edge and large flame scales. Flowfield measurements suggest that the heat release is increased by approximately 50% when the boundary layer tripped.Copyright

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.

Shock tube demonstration of acousto-optically modulated quantum cascade laser as a broadband, time-resolved combustion diagnostic

We provide the first demonstration of an acousto-optically modulated quantum cascade laser (AOM QCL) system as a diagnostic for combustion by measuring nitric oxide (NO), a highly regulated emission produced in gas turbines. The system provides time-resolved broadband spectral measurements of the present gas species via a single line of sight measurement, offering advantages over widely used narrowband absorption spectroscopy (e.g., the potential for simultaneous multispecies measurements using a single laser) and considerably faster (>15 kHz rates and potentially up to MHz) than sampling techniques, which employ fourier transform infrared (FTIR) or GC/MS. The developed AOM QCL system yields fast tunable output covering a spectral range of 1725–1930 cm 1 with a linewidth of 10–15 cm . For the demonstration experiment, the AOM QCL system has been used to obtain time-resolved spectral measurements of NO formation during the shock heating of mixture of a 10% nitrous oxide (N2O) in a balance of argon over a temperature range of 1245–2517 K and a pressure range of 3.6–5.8 atm. Results were in good agreement with chemical kinetic simulations. The system shows revolutionary promise for making simultaneous time-resolved measurements of multiple species concentrations and temperature with a single line of sight measurement. [DOI: 10.1115/1.4040381]