Bassam B. Dally

Structure of turbulent non-premixed jet flames in a diluted hot coflow

Moderate and intense low oxygen dilution combustion is a newly implemented and developed concept to achieve high thermal efficiency and fuel savings while maintaining emission of pollutants at very low levels. It utilizes the concept of heat and exhaust gas recirculation to achieve combustion at a reduced temperature, a flat thermal field, and low turbulence fluctuations. An experimental burner is used in this study to simulate the heat and exhaust gas recirculation applied to a simple jet in a hot coflow. Temporally and spatially resolved measurements of reactive scalars are conducted on three different turbulent nonpremixed flames of a H 2 /CH 4 fuel mixture at a fixed-jet Reynolds number, and different oxygen levels in the hot oxidant stream. The data were collected using the single-point Raman-Rayleigh-laser-induced fluorescence technique. The results show substantial variation in the flame structure and appearance with the decrease of the oxygen level. By reducing the oxygen level in the hot coflow, the flame becomes less luminous, the temperature increase in the reaction zone can get as low as 100 K, and the levels of CO and OH are substantially lowered. The levels of NO also decrease with decreasing the oxygen levels and at 3% by mass, it is less that 5 ppm. For this case, a widely distributed NO profile is found which is not consistent with profiles for other oxygen levels.

Performance Variations of Leading-Edge Tubercles for Distinct Airfoil Profiles

An experimental investigation has been undertaken to determine the influence of sinusoidal leading-edge protrusions on the performance of two NACA airfoils with different aerodynamic characteristics. Force measurements on full-span airfoils with various combinations of tubercle amplitude and wavelength reveal that when compared to the unmodified equivalent, tubercles are more beneficial for the NACA 65-021 airfoil than the NACA 0021 airfoil. It was also found that for both airfoil profiles, reducing the tubercle amplitude leads to a higher maximum lift coefficient and larger stall angle. In the poststall regime, however, the performance with largeramplitude tubercles is more favorable. Reducing the wavelength leads to improvements in all aspects of lift performance, including maximum lift coefficient, stall angle, and poststall characteristics. Nevertheless, there is a certain point at which further reduction in wavelength has a negative impact on performance. The results also suggest that tubercles act in a manner similar to conventional vortex generators.

PROBABILITY DENSITY FUNCTION COMPUTATIONS OF A STRONGLY SWIRLING NONPREMIXED FLAME STABILIZED ON A NEW BURNER

Turbulent nonpremixed swirl-stabilized flames are common in practical combustors and form the next level of complexity after piloted and bluff-body stabilized flames. Modeling swirling flows remains a challenge especially when the swirl level is high enough to induce vortex breakdown and recirculation. This paper presents experimental results on the velocity field and the stability characteristics of a new swirl burner which has well-defined boundary conditions. This burner is capable of stabilizing turbulent nonpremixed flames which have high swirl numbers and which may have a significant degree of turbulence-chemistry interactions. A Monte Carlo-based probability density function (PDF) method is also used to compute the same turbulent, highly swirling flame using the simplest models for velocity (SLM), turbulent frequency (JPM), and molecular mixing (IEM). A single flamelet library is used here to represent chemistry. These simple computations reproduce the correct flow structure and compare well with the measured velocity field. Refinements to the computations and more extensive measurements in such flows are forthcoming.

Development of temperature imaging using two-line atomic fluorescence

This work aims to advance understanding of the coupling between temperature and soot. The ability to image temperature using the two-line atomic fluorescence (TLAF) technique is demonstrated. Previous TLAF theory is extended from linear excitation into the nonlinear fluence regime. Nonlinear regime two-line atomic fluorescence (NTLAF) provides superior signal and reduces single-shot uncertainty from 250 K for conventional TLAF down to 100 K. NTLAF is shown to resolve the temperature profile across the stoichiometric envelope for hydrogen, ethylene, and natural gas flames, with deviation from thermocouple measurements not exceeding 100 K, and typically ≲30 K. Measurements in flames containing soot demonstrate good capacity of NTLAF to exclude interferences that hamper most two-dimensional thermometry techniques.

Reaction Zone Weakening Effects under Hot and Diluted Oxidant Stream Conditions

This paper reports computational results, to complement experimental observations, on the turbulence–chemistry interaction of nonpremixed jet flames issuing into a heated and highly diluted oxidant stream. It is found experimentally that large-scale vortices and flame stretch can lead to spatial thinning and a decrease in OH concentration. This reduction in OH is described as a weakening of the reaction zone. Accompanying reaction zone weakening is also an increase in H2CO levels. The reduction in reaction rates is most noticeable at low oxidant stream O2 levels. The heated and low oxygen oxidant conditions typify those of moderate or intense low oxygen dilution (MILD) combustion. The computational results indicate that the effects of the low oxygen levels of MILD combustion leads to both a reduction in reaction rates and an increase in transport of O2 across the reaction zone. The relationship between the reaction rate and level of O2 permeation suggests that a form of partial premixing can occur under MILD combustion conditions. This partial premixing leads to the formation of flame intermediates, which contribute to the stabilization of the flames. The permeation effects are most pronounced at high strain rates, which are commonly encountered in practical MILD combustors.

The formation mechanism and impact of streamwise vortices on NACA 0021 airfoil's performance with undulating leading edge modification

Wings with tubercles have been shown to display advantageous loading behavior at high attack angles compared to their unmodified counterparts. In an earlier study by the authors, it was shown that an undulating leading-edge configuration, including but not limited to a tubercled model, induces a cyclic variation in circulation along the span that gives rise to the formation of counter-rotating streamwise vortices. While the aerodynamic benefits of full-span tubercled wings have been associated with the presence of such vortices, their formation mechanism and influence on wing performance are still in question. In the present work, experimental and numerical tests were conducted to further investigate the effect of tubercles on the flow structure over full-span modified wings based on the NACA 0021 profile, in the transitional flow regime. It is found that a skew-induced mechanism accounts for the formation of streamwise vortices whose development is accompanied by flow separation in delta-shaped regions n...

Two-photon laser-induced fluorescence measurement of CO in turbulent non-premixed bluff body flames

Abstract Detailed measurements of reactive and passive scalars in axisymmetric bluff-body flames of CH4/H2, CH4/air, and CO/H2 have been reported in the past [1] and have been used in several studies directed at developing and validating turbulent combustion models [2] , [3] , [4] , [5] . Comparisons of measured and modeled results for flames with CH4/H2 as fuel have shown significant disagreement in the mass fractions of CO, with the Raman scattering measurements of [CO] being much higher than the modeling calculations. Previously published data [1] were collected using the Raman scattering technique for all major species including CO. Raman scattering measurements of [CO] in methane flames suffer from laser-excited interference from higher hydrocarbons formed on the fuel-rich side of the reaction zone. It is well established [6] , [7] , [8] that measurements of [CO] based on two-photon laser-induced fluorescence (TPLIF) can provide much better accuracy than those based on Raman scattering in methane and natural gas flames.Keywords: Bluff body; CO; LIF; Raman; Flame

LOx Jet Atomization Under Transverse Acoustic Oscillations

Testing has been conducted with the BKH rocket combustor at the European Research and Technology Test Facility P8 for cryogenic rocket engines at DLR Lampoldshausen. BKH has multiple shear coaxial injectors and an exhaust modulation system for forcing excitation of acoustic resonances in the combustion chamber. Optical access windows allow the application of parallel high-speed shadowgraph and flame emission imaging of the near-injector region. This paper reports measurements of the intact liquid oxygen core during forced excitation of the first transverse acoustic mode. High-speed shadowgraph images show that the mechanism of core breakup and atomization differs between off-resonance and first transverse excitation conditions. The core length is found to decrease with increasing amplitude of acoustic pressure, or equivalently with transverse acoustic velocity, with a core length reduction of up to 70% for conditions approaching those of naturally occurring high frequency combustion instabilities. This de...

The effect of undulating leading-edge modifications on NACA 0021 airfoil characteristics

In spite of its mammoth physical size, the humpback whales manoeuvrability in hunting has captured the attention of biologists as well as fluid mechanists. It has now been established that the protrusions on the leading-edges of the humpbacks pectoral flippers, known as tubercles, account for this species’ agility and manoeuvrability. In the present work, Prandtls nonlinear lifting-line theory was employed to propose a hypothesis that the favourable traits observed in the performance of tubercled lifting bodies are not exclusive to this form of leading-edge configuration. Accordingly, a novel alternative to tubercles was introduced and incorporated into the design of four airfoils that underwent wind tunnel force and pressure measurement tests in the transitional flow regime. In addition, a Computation Fluid Dynamics study was performed using the Shear Stress Transport transitional model in the context of unsteady Reynolds-Averaged Navier-Stokes at several attack angles. The results from the numerical investigation are in reasonable agreement with those of the experiments, and suggest the presence of features that are also observed in flows over tubercled foils, most notably a distinct pair of streamwise vortices for each wavelength of the tubercle-like feature.

Solvent effects on two-line atomic fluorescence of indium

We aim to investigate the potential of four different organic solvents, namely, acetone, ethanol, methanol, and isopropanol, and the organic-solvent-water mixtures as a seeding medium for the two-line atomic fluorescence technique. Water is used as the reference case. Indium, which has been previously shown to have suitable spectroscopic attributes, is chosen as the thermometry species in the present study. Acetone and methanol are shown to enhance the fluorescence signal intensity the most (approximately threefold to fivefold at stoichiometric conditions) when used. Acetone and methanol are shown to improve the fluorescence emission over the entire stoichiometric envelope of flame, most significantly in the rich combustion region, as well as a twofold enhancement in the signal-to-noise ratio.