Oleksandr Bibik

Acoustic absorption measurements for characterization of gas mixing

Controlling and/or monitoring the degree of mixing between constituents of a multicomponent media is a key problem in a variety of applications. Monitoring such mixing processes necessarily requires capabilities for quantification of the level of “mixedness.” However, quantification of molecular mixedness levels, as opposed to macroscale mixture uniformity, is difficult. This paper demonstrates the use of acoustic absorption measurements to characterize an average level of molecular mixedness between gases across the wave propagation path. This approach takes advantage of the fact that over a large frequency range, acoustic damping is dominated by vibrational relaxation processes. The vibrational relaxation frequency for a particular gas is often a strong function of the other species it is in molecular contact with. Thus, the relaxation frequency of each species in a multicomponent gas mixture varies with the level of molecular mixedness of the constituent species. This paper presents the results of exam...

Experimental Investigation of Spray Dynamics in Crossflow of Pre-heated air at Elevated Pressure

This paper describes an experimental investigation of the spray created by Jet A fuel injection from a plate containing sharp edged orifice 0.018 inches in diameter and L/D ratio of 10 into the crossflow of preheated air (555 K) at elevated pressure in the test section (4 ata) at Mach numbers 0.2 and 0.35. Investigation was carried out in a wide range of fuel to air momentum ratios between 5 and 180. Phase Doppler technique and macro and micro imaging technology were used for understanding of the breakup mechanism of the spray and investigating spray unsteadiness mechanism. Mechanism of spray formation was found to be shear breakup. The primary source of unsteadiness of the spray was confirmed to be the turbulence of the fuel jet itself.

Application of Planar Laser-Induced Phosphorescence to Investigate Jet-A Injection Into a Cross-Flow of Hot Air

This paper describes the development of the Planar Laser-Induced Phosphorescence (PLIP) technique for mapping the fuel temperature and concentration distributions in a jet-in-cross-flow (JICF) spray study. The spray was produced by injecting cold liquid Jet-A into hot cross-flowing air. The application of PLIP required the seeding of liquid fuel with micron-size thermographic phosphor particles before injection. The resulting spray produced phosphorescence and droplets Mie-scattering signals when illuminated by a 355nm planar UV laser sheet of 0.054J/pulse energy. The technique was investigated as a potential alternative to the use of Jet-A Planar Laser-Induced Fluorescence (PLIF) for the mapping of fuel concentration in sprays, because the low signal intensity of Jet-A’s fluorescence at high T prevents the use of the PLIF approach. In contrast, PLIP provides a strong signal at high T, and allows the simultaneous determination of local T and fuel concentration when two spectral bands of the phosphorescence emission are imaged and their ratio-of-intensities (RI) determined. In addition, the locations where liquid fuel droplets exist were imaged from the UV Mie-scattering of the laser-sheet (which can also be done in PLIF).In the present investigation, an optical system that imaged two spectral bands of phosphorescence and one wavelength of Mie-scattering was developed. It consisted of three CCD cameras with dichroic beam-splitters and interference narrow bandpass filters. The spray-pattern within a span of ∼80×30 orifice diameters was captured, with spatial resolution of about 0.1mm/px. The investigated jet-in-cross-flow spray was produced by injecting Jet-A fuel from a 0.671mm diameter orifice located on the wall of a rectangular channel (25.4×31.75mm cross-section). The cross-flow air was preheated to temperatures encountered in modern gas turbines (up to 480°C), while the temperature of the injected Jet-A fuel was in the T = 27–80°C range. YVO4:Eu phosphor particles with a median size of 1.8 microns were used to seed the fuel.Since the emissions of the commonly used Dy:YAG thermographic phosphor were found to be too weak and had wavelengths that overlapped with Jet-A fluorescence signals, YVO4:Eu was used for the JICF studies instead. It was observed that while the emissions of YVO4:Eu were stronger than Dy:YAG, the range of T where it can be applied in the PLIP technique was more limited — just sufficient for the investigated JICF. Preliminary results from the study showed rapid changes in fuel concentration and T from the injector up to z/dinj∼30 for momentum ratios of J = 5, 10 and 20, followed by a more gradual mixing/heat-up downstream. It was also found that deposition of phosphor particles on channel-walls interfered with the spray characterization, reducing the accuracy of the measurements.Copyright

Onset and Suppression of Instabilities in High Pressure Air- Breathing Combustor

Combustion instabilities were investigated in experiments where the fuel was rapidly heated close to critical temperatures, but the pressure in the combustion chamber was kept below the critical value for the injected fuel, n-heptane (C7H16). Two different unstable modes (~90Hz and ~400Hz) were excited depending on whether the intake air was preheated or not. Where the 90Hz mode was dominant, higher fuel preheat temperatures led to a lower level of registered instabilities. When the 400Hz mode was dominant, the fuel heater was set to operate at the highest productivity but severe combustion instabilities remained essentially unchanged. The observed qualitative trends agreed with the known data obtained in the high-power tests, where pressure in the combustor exceeded critical value.

Autoignition of a Jet-A Fuel Spray in a High Temperature Vitiated Air Flow

This study investigates the autoignition of a Jet-A fuel spray in a preheated air flow in a 75mm inner diameter quartz tube at atmospheric pressure. Fuel was injected via a pressure-swirl atomizing injector enclosed in an aerodynamically-shaped outer body (7mm diameter) that is installed coaxially with the flow using 3 water-cooled pylons. The air temperature and oxygen content were in the range of 1000–1400 K and 9–12%, respectively by controlling the equivalence ratio in the primary zone of the pre-burner/vitiator and by adding dilution air downstream. The co-flow air velocity ranged from 25–30 m/s for this study (Re = 10,000–12,000), with a trapezoidal profile. Fuel spray was characterized using PDPA. Shape of the spray was transitional between hollow and solid cone with the corner angle of 35° to the axis near injector which reduced to 22° downstream. Spray density varied significantly over cross-section of the tube with the minimum on the axes. Droplets produced have average diameters (D10) of 15–70μm on the axes and periphery, respectively, at 6 cm downstream from the injector. Character of the droplet size distribution was polydisperse. Auto-ignition time delays were captured using a time-averaged camera with CH* (432nm) filter. The measured values agree with delay times previously reported in literature. Two synchronized high-speed cameras with 432nm and 307nm filters were used to investigate dynamics of auto-ignition kernel initiation and convection by capturing of CH* and OH* chemiluminescence at 5000 f ps. This methodology allowed qualitative characterization of the equivalence ratio of kernels in process of their convection and growth. It was shown that kernels are always initiated on the axes of the spray where the average droplet size is minimum. Kernels were formed leaner and become richer as they grow down-stream as indicated by the increase of CH*/OH*intensity ratio. Additionally, kernel behavior depends greatly on air temperature with kernels transitioning from randomly appearing (i.e. single kernel), to periodic, to a constantly auto-igniting flame with the spatial scatter of ignition kernels decreasing with temperature.Copyright

Effect of Air-Assist on Liquid Jet Penetration and Dispersion in a Cross-Flow of Hot, High-Pressure Air

This paper describes an experimental investigation of the effects of air-assist upon the penetration and dispersion of a liquid fuel jet that is injected into cross-flowing air. The spray patterns across the central longitudinal plane were investigated at flow conditions similar to those encountered at the combustor inlet of a modern gas turbine engine. Temperatures of the cross-flow and assist air were at 316 and 427°C, while test-channel pressures were set at 2.02 and 2.53MPa. Jet-A fuel was injected through a wall-recessed plain orifice into a rectangular test-channel where the cross-flow air velocity was Ucross-flow=75m/s. Assist air was injected from four slots surrounding the fuel orifice within the wall-recessed well. The air-assist jets impinged upon the fuel jet at a 45° angle. Pressure drops across the air-slots were limited to ≤4% of test-channel pressure to simulate the difference between stagnation and static pressures on a typical fuel-air mixer/injector. Thus, the assist-air-to-liquid fuel mass-flow ratios (ALR) were limited to 0.41, which was much lower than those used in traditional airblast atomizers with ALR in the range of 1 to 10. Momentum-flux ratios (J) of the fuel jet to cross-flow were varied between J=5 to 40. A 355nm planar laser was used to illuminate the spray’s central plane to capture images of liquid droplets Mie-scattering. An attempt was made at correlating the trajectories of the jet using an effective momentum-flux ratio Jeff that accounts for air-assist jets’ momentum. It was discovered that air-assist had limited influence on the spray’s outer-edge penetration, while it strongly enhances the penetration of the inner-edge and spray centerline. Air-assist’s effects were also found to be proportional to ALR. Contrary to the results of airblast jet-in-cross-flow researches, it was found that at J∼5, when the sprays’ inner-edges were close to the wall, air-assist enhanced the inner-edge penetration in a manner that was not well-captured by Jeff. Finally, it was also observed that sprays at 2.53MPa were more sensitive to J and air-assist variations than sprays at 2.02MPa.Copyright

Liquid Fuel Jet in Crossflow -Trajectory Correlations based on the Column Breakup Point

The placement of the fuel in combustors is significant for combustor design. Hence the study of the liquid penetration into a crossflow has received attention from various researchers. There have been various correlations suggested for the upper surface of the liquid jet trajectories suggested by several researchers. However, many of these correlations are applicable to specific operating conditions, injector geometries and measurement techniques. This study is an attempt to develop spray trajectory correlations that is applicable to a wide range of operating conditions and injector geometries. Previous studies have shown that the penetration of a spray created by round edged orifice is higher than that created by a sharp edged orifice. The approach is to develop trajectory correlations to the spray created by a round edged orifice that is expected to give the highest penetration. For the injectors of various other geometries, a correction factor is used to obtain the spray trajectories. Preliminary study has shown that the location of the column breakup point obtained by using the liquid jet light guiding technique could possibly be a part of the correction factor to the penetration of the spray. The trajectories of the jet and the location of the column breakup points are obtained for four different injectors at several operating conditions. These results are used for obtaining the correlation for the spray trajectory and the correction factor for the various injectors.

The regimes of twin-fluid jet-in-crossflow at atmospheric and jet-engine operating conditions

The “Twin-Fluid Jet-in-Crossflow (TF-JICF)” is a nascent variation of the classical JICF, in which a liquid jet is co-injected with an annular sleeve of gas into a gaseous crossflow. Jet-engine designers are interested in using TF-JICF for liquid-fuel injection and atomization in the next-generation combustors because it is expected to minimize combustor-damaging auto-ignition and fuel-coking tendencies. However, experimental data of TF-JICF are sparse. Furthermore, a widely accepted TF-JICF model that correlates the spray’s penetration to the combined liquid-gas momentum-flux ratio (Jeff) is increasingly showing discrepancy with emerging results, suggesting a gap in the current understanding of TF-JICF. This paper describes an investigation that addressed the gap by experimentally characterizing the TF-JICF produced by a single injector across wide ranges of operating conditions (i.e., jet-A injectant, crossflow of air, crossflow Weber number = 175-1050, crossflow pressure Pcf = 1.8-9.5 atm, momentum-flu...

Prediction of Blow-Offs of Bluff Body Stabilized Flames Utilizing Close-Coupled Injection of Liquid Fuels

This paper describes the development of an empirical approach that attempts to predict blow-out of bluff body stabilized flames using global flow parameters in systems where liquid fuel injectors are located a short distance upstream of the wake. This approach was created on the hypothesis that flame stability in such a combustion system (referred to as a close-coupled injection) is determined by the strength of the heat source developed in the bluff body recirculation zone and by the availability of sufficient contact time with fresh mixture for its ignition, similar in nature to premixed combustion systems. Based on this concept, global equivalence ratio on the classical DeZubay stability map was replaced by local equivalence ratio in the recirculation zone of the bluff body. This local equivalence ratio was determined experimentally using a chemiluminescence measurement system. Tests were conducted using a single bluff body with a close-coupled injection system in a 76 × 152 mm (3 × 6 in.) combustion tunnel. A wide range of fuel–air ratios and velocities were achieved by variation of the global equivalence ratio, incoming flow velocity, and injector size. The obtained experimental dataset was used to develop a transfer function that allowed calculation of the local equivalence ratio in the recirculation zone based on the global flow parameters. Equivalence ratio in the recirculation zone was found to be exponentially dependent upon the square root of the fuel to air momentum flux ratio such that increasing the momentum flux ratio led to a reduction in the recirculation zone equivalence ratio. Additional adjustment of this general trend by the diameter of injector and air flow velocity was necessary to improve the quality of the prediction. The developed approach demonstrated a good prediction of the globally rich blow-out of the flame. In fact, the recirculation zone lean blow-out limit (corresponding with globally rich blow-out) predicted for close coupled injection using the developed transfer function closely coincided with the lean blow-out line of the classical DeZubay envelope and with results obtained with premixed injection using the same bluff body. On the contrary, globally lean (locally rich) blow-out was predicted ∼20% below the DeZubay rich blow-out line, possibly because of the limited range of the fuel flow rates on the experimental rig used.

Dynamics of V-gutter-stabilized Jet-A Flames in a Single Flame Holder Combustor with Full Optical Accesss

An investigation of flame oscillations in V-gutter-stabilized Jet-A combustion was conducted in a single flame holder augmenter (SFA) rig. The facility simulates conditions encountered in the exhaust of an aircraft gas turbine engine, with an elevated inlet temperature, T~2000R, and reduced oxygen content, ~14.5%. The rig consists of a 36-inch transparent test section with a 3-inch horizontal by 6-inch vertical cross-section and quartz windows for sidewalls, allowing full optical access to the flame. Four V-guttered flame holder widths (<46°; W=2.25, 2.00, 1.875 and 1.75 inches) were used to investigate flame stabilization. Fuel injectors were located 1.5 inch upstream of the flame holder trailing edge and injected Jet-A fuel across the main flow of the incoming vitiated air. Test runs were performed over a wide range of flow conditions (M=0.27 0.40, T=720°C 849°C, Φglobal = 0.2 – 1.2), and significant attention was given to oscillatory heat release measurements. This was done by capturing and processing a sequence of high-speed (HS) camera images of the flame and spray at a rate of 10,000 frames-per-second. More than 60 high-speed movies were captured at different operating conditions and V-gutter widths. Longitudinal instabilities occurred at frequencies of f=210-240 Hz and 107 Hz, which corresponded to 3⁄4 and 1⁄4 longitudinal waves in the SFA, respectively. These low frequency oscillations were observed by both routine (acoustic sensors and photomultiplier) and advanced (HS image processing) methodologies. Longitudinal instabilities were quite intense for many operating conditions, with peak-to-peak dynamic pressure amplitude of the 3⁄4 wave mode routinely exceeding 13% of the static pressure in the combustor. The rarely-excited 1⁄4 wave mode was even more intense, and pressure oscillations attained 20% of the static pressure. Interestingly, acoustic velocity oscillations associated with this mode attained ±100% of the mean. This was observed on the HS images of the spray by fuel jets “shooting” perpendicular to the main flow for a small portion of each cycle, indicative of zero crossing velocity. The same images revealed a certain time delay of the spray trajectory fluctuations with respect to pressure oscillations in the combustor, which indicated potential spray engagement into the thermo-acoustic loop of longitudinal instability. Von-Karman instability of the flame was observed only by the HS imaging technique in the form of a “snake-like” flame shape, resulting from flame stabilization in asymmetrically shed vortices downstream of the V-gutter (i.e. Von-Karman street). Frequencies of V-K flame instability were in the range of f=1000-1600 Hz at incoming flow Mach numbers of M=0.3 and M=0.4, respectively. Acoustic sensors were insensitive even to strong V-K flame oscillations because of a lack of thermo-acoustic coupling in the SFA test rig in this frequency range. Ievertheless, significant attention was given to this type of instability because of its potential thermo-acoustic coupling with one of the transverse acoustic modes in a larger combustor, thus creating destructive pressure oscillations.