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Due to the non-premixed nature of diesel combustion, mixing prior to the reaction zone has proven to be one of the primary factors in emissions formation. Therefore, the advancement of diagnostics used to measure mixing fields in diesel applications is imperative for a greater understanding of how in-cylinder emissions mitigation techniques operate. Towards this goal, we have recently demonstrated the use of a high-speed two-wavelength extinction imaging measurement, UV-VIS DBI, for time-resolved measurements of mixing in a diesel spray. This diagnostic operates by back-lighting the spray with ultra-violet and visible illumination. The visible illumination is selected at a non-absorbing wavelength, such that the visible light is only attenuated by liquid droplet scattering, enabling discrete detection of the liquid-vapor mixture and pure vapor phases of the spray. For this work, Ultraviolet and visible light are generated using a ND:YAG pumped frequency-doubled tunable dye laser operating at 9.9 kHz . The simultaneous UV-Visible illumination is used to back-illuminate a vaporizing diesel spray, and the resulting extinction of each signal is recorded by a pair of high-speed cameras. Using an aromatic tracer (naphthalene, BP = 218 °C) in a base fuel of dodecane (BP = 215–217 °C), the UV illumination (280 nm) is absorbed along the illumination path through the spray, yielding a projected image of line-of-sight optical depth that is proportional to the projected fuel vapor concentration in the pure vapor region of the spray. In this paper, a new method of determining the absorption coefficient for the pure-vapor phase of the spray will be discussed, along with showing how an Inverse-Abel transform can be used to compute planar concentration data from the projected concentration data yielded by the diagnostic. This diagnostic and data processing is applied to diesel sprays from two Bosch CRI3-20 ks1.5 single-orifice injectors (140 μm and 90 μm orifice diameters) injecting into a nonreacting high-pressure and temperature nitrogen environment using a constant-flow, optically-accessible spray chamber operating at 60 bar and 900 K. The mixing data produced agrees well with previously existing mixing data, which further instills confidence in the diagnostic, and gives the diesel combustion community access to mixing field data for a 140 μm orifice diameter injector at a 60 bar and 900 K condition.

This paper presents measurements of 10 kHz OH planar laser induced fluorescence (PLIF) with an objective to study the interaction of effusion cooling with the flame and hot combustion products in the liquid fueled combustor. The combustor rig is a single sector representation a rich-burn/quick-quench/lean-burn (RQL) configuration. It consists of a swirl nozzle, dilution, and effusion jets. The rig is operated under realistic aircraft conditions, including elevated combustor inlet temperature, and elevated pressure. The PLIF laser sheet was arranged perpendicular and parallel to the liner at distinct liner locations. Parametric variations of important parameters, namely equivalence ratio, and effusion cooling air blowing ratio are conducted to investigate their effect on flame-effusion jet interactions. The PLIF images were analyzed using several data reduction techniques to de-noise the images and identify patterns in the effusion jet-flame interactions. Results show that the effusion jets are highly unsteady, interacting strongly with the turbulent flame from the swirl nozzle and the dilution jets. This work is an extension of recent effusion film mixing studies that were performed with acetone PLIF under non-reacting conditions.

In this paper, we discuss the development of a non-intrusive surface temperature sensor based on long-wavelength infrared (LWIR) hyperspectral technology. The LWIR detection enables to minimize optical interferences from hot combustion gases (emission mostly within UV-MWIR region). Utilization of hyperspectral detection allows to further improve temperature measurement accuracy and precision. The developed sensor with fiber coupling provides the required flexibility to be maneuvered around/through combustor hardware. The LWIR fiber probe is fully protected by the custom-designed water-cooled probe housing. This device is designed to sustain temperature of 2400 K at pressure of 50 bar, which enables long-term optical diagnostics inside the practical high-pressure combustion facilities where extreme thermal acoustic perturbation and intense heat fluxes are present. The housing featured a diamond window to selectively measure spectra in the LWIR region to get accurate surface temperature exclusively of the combustor wall. The probe was installed into a RQL style combustor to get surface temperature of both hot and cold side of the combustor wall. Further, pointwise heat flux estimates across the combustion liner wall was derived using the temperature measurements.

Diesel sprays present a challenging environment for detailed quantitative measurement of the liquid field, and to date, there have been only a few efforts to characterize drop sizes within the family of Engine Combustion Network (ECN) diesel sprays. Drop sizing diagnostics, including optical microscopy and Ultra-Small Angle X-ray Scattering (USAXS), have been recently demonstrated in Spray A/D ECN activities, but little data exist to validate these results. This work therefore seeks to extend the available ECN data on the liquid phase field and provide a new comparative data set for assessment of previous ECN drop sizing measurements. In particular, this work presents the development of a two-wavelength, line-of-sight extinction measurement to examine liquid volume fraction and the corresponding droplet field in high-pressure fuel sprays. Here, extinction of lasers emitting at 10.6 μm and 0.633 μm are used for the measurement. To enable quantification of the liquid field in optically dense regions of the spray, a transfer function is developed to account for the influence of multiple scattering. The developed diagnostic is then applied to n-dodecane sprays from the ECN Spray A and Spray D injectors at varying fuel rail pressures and atmospheric chamber condition. Overall, the results show a reasonable agreement with droplet sizes measured using USAXS, as well as from more recent measurements using a Scattering-Absorption Measurement Ratio (SAMR) technique also developed in our group. This is particularly the case near the spray periphery, where on average, less than 40% difference in the measured Sauter mean diameter is observed. Nonetheless, an apparent discrepancy is observed between drop sizes from different diagnostics close to the jet centerline (i.e. nearly 100% difference between available data for Spray D injector). Moreover, the presented diagnostic shows an improved capability in the dilute regions of the spray, where x-ray-based diagnostics are generally subject to high noise and low signal sensitivity.

This paper presents analysis of results from optical diagnostics in a high-pressure combustor burning gaseous oxygen (GOX) and liquid kerosene RP-2 fuel through a jet-swirl coflow injector. The obj...

The mixing of fuel and air in the combustion chamber of an IC engine is crucial to emissions formation. Therefore, developing effective diagnostic techniques for measuring mixing is critical for progressing IC engines. Existing methodologies for the optical measurement of air-fuel mixing, including Rayleigh scattering and Laser Induced Fluorescence (LIF), have demonstrated various diagnostic-implementation challenges, high uncertainties under engine-relevant environments, and strong interferences from the liquid spray which prevents their use in near-spray measurements. This work presents the use of an alternative approach based on a laser-absorption/scattering technique called Ultraviolet-Visible Diffuse Back-Illumination (UV-Vis DBI) to quantify local equivalence ratio in a vaporizing diesel spray. Ultraviolet and visible light are generated using a ND:YAG pumped frequency-doubled tunable dye laser operating at 9.9 kHz. The simultaneous UV-Visible illumination is used to back-illuminate a vaporizing diesel spray, and the resulting extinction of each signal is recorded by a pair of high-speed cameras. Using an aromatic tracer (naphthalene, BP = 218 °C) in a base fuel of dodecane (BP = 215–217 °C), the UV illumination (280 nm) is absorbed along the illumination path through the spray, yielding a projected image of line-of-sight optical depth that is proportional to the path-average fuel vapor concentration in the vapor region of the spray. The visible illumination is chosen at a non-absorbing wavelength (560 nm), such that the light extinction is only due to liquid scattering, yielding a projected image of the liquid spray. A key advantage of the method is that the absorption coefficient of the selected tracer is relatively independent of temperature and pressure for 280-nm illumination, reducing measurement uncertainties at engine-relevant conditions. Measurements are also achievable in near-spray vapor regions since there is no mie-scattering interference from the liquid spray. The diagnostic is applied to measure the fuel-air mixing field of a diesel spray produced by a Bosch CRI3-20 ks1.5 single-orifice injector (90 μm diameter) similar to ECN Spray A. Measurements are conducted in a non-reacting high-pressure and temperature nitrogen environment using a constant-flow, optically-accessible spray chamber operating at 60 bar and 900 K. The results are evaluated against existing ECN mixing measurements based on Rayleigh scattering. The diagnostic yields centerline and radial mixture fraction measurements that match the ECN Rayleigh measurements within uncertainty bounds.

Simultaneous high-speed stereo-particle image velocimetry, OH planar-laser-induced fluorescence (PLIF), and C H 2 O PLIF measurements in a vitiated bluff body combustor are considered. An ex situ, simultaneous, time-resolved laser sheet intensity profile correction procedure is introduced. This procedure is easily implemented experimentally and is capable of correcting multiple sheets at the same time. As a proof of concept, the procedure is applied to perform correction of the C H 2 O PLIF images in vitiated and unvitiated conditions. The challenges associated with C H 2 O PLIF under these combustor operating conditions are also discussed.

The Moments-algorithm was developed to post-process images of sprays with the aim of characterizing the sprays’ complex features (e.g., trajectory, dispersions and dynamics) in terms of simple curves, which can be used for developing correlation models and design tools. To achieve this objective, the algorithm calculates the first moments of pixel intensity values in instantaneous images of the spray to determine its center-of-gravity (CG) trajectory (i.e., the spray density-weighted centerline trajectory). Thereafter, the second moments (i.e., standard-deviations, σ) of intensities are calculated to describe the dispersion of spray materials around the CG. After the instantaneous CG's and σ's for the instantaneous images have been obtained, they are arithmetically averaged to produce the average spray trajectories and dispersions. Additionally, the second moments of instantaneous CG's are used to characterize the spray’s fluctuation magnitude. The Moments-algorithm has three main advantages over threshold-based edge-tracking and other conventional post-processing approaches: (1) It simultaneously describes the spray’s instantaneous and average trajectories, dispersions and fluctuations, instead of just the outer/inner-edges, (2) the use of moments to define these spray characteristics is more physically meaningful because they reflect the statistical distribution of droplets within the spray plume instead of relying on an artificially interpreted “edge”, and (3) the use of moments decreases the uncertainties of the post-processed results because moments are mathematically defined and do not depend upon user-adjustments/interpretations.

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...

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 © 2015 by ASME

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 © 2015 by ASME

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...

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.

This paper describes recent learning on the flame structure associated with bluff-body stabilized flames and the influence of the fuel distribution with nonpremixed, jet-in-crossflow fuel injection. Recent experimental and analytical results disclosing the flame structure are discussed in relation to classical combustion reaction zone regimes. Chemiluminescence and planar fluorescence imaging of OH* radicals as an indicator of the flame zone are analyzed from various tests conducted at Georgia Tech using a two-dimensional vane-type bluff-body with simple wall-orifice fuel injectors. The results described in this paper support the view that combustion occurs in separated flame zones aligned with the nonpremixed fuel distribution associated with the fuel jets that are very stable and contribute to flame stability at low fuel flow rates. The experimental data is also compared with computational reacting flow large-eddy simulations and interpreted in terms of the fundamental reaction zone regimes for premixed flames. For the conditions of the present experiment, the results indicate combustion occurs over a wide range of flame regimes including the broken reaction zone or separated flamelet regimes.

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.

Injection of the liquid fuel across the incoming air flow is widely used in gas turbine engine combustors. Thus it is important to understand the mechanisms that control the breakup of the liquid jet and the resulting penetration and distribution of fuel droplets. This understanding is needed for validation of Computational Fluid dynamics (CFD) codes that will be subsequently incorporated into engine design tools. Additionally, knowledge of these mechanisms is needed for interpretation of observed engine performance characteristics at different velocity/altitude combinations of the flight envelope and development of qualitative approaches for solving problems such as combustion instabilities (Bonnel et al., 1971). This chapter provides an introduction and literature review into the subject of cross-flow fuel injection and describes the fundamental physics involved. Additionally highlighted are experimental technique and recent experimental data describing the variables involved in fuel spray penetration and fuel column disintegration. In recent years, there has been a great drive to reduce harmful emissions of oxides of Nitrogen oxides (NOx) from aircraft engines. One of the several approaches to achieve low emissions is to avoid hot spots in combustors by creating a lean homogeneous fuel-air mixture just upstream of the combustor inlet. This concept is termed as Lean Premixed Prevaporized (LPP) combustion. Creating such a mixture requires fine atomization and careful placement of fuel to achieve a high degree of mixing. Liquid jet in cross flow, being able to achieve both of these requirements, has gained interest as a likely candidate for spray creation in LPP ducts (Becker & Hassa, 2002). Since the quality of spray formation directly influences the combustion efficiency of engines, it is important to understand the fundamental physics involved in the formation of spray. As seen in Fig. 1, the field of a spray created by a jet in cross flow can be divided into three modes: 1) Intact liquid column, 2) Ligaments, and 3) Droplets. The liquid column develops hydrodynamic instabilities and breaks up into ligaments and droplets (Marmottant & Villermaux, 2004; Madabushi, 2003; Wu et al., 1997). This process is referred to as primary breakup. The location where the liquid column ceases to exist is known as the column breakup point (CBP) or the fracture point. The ligaments breakup further into smaller droplets and this process is called secondary breakup. The most relevant parameter for drop breakup criterion is the Weber number

This study demonstrates a new approach to liquid rocket engine (LRE) design, which permits the suppression of the severe 1-T spinning tangential combustion instability. Presently, control of the spinning tangential wave with a frequency of f ≈ 5000 Hz has been demonstrated in this laboratory reproduced liquid rocket engine combustor using an asymmetric fuel injector distribution. This combustor burns liquid fuel (Jet-A) with preheated air, and is equipped with six “smart” injectors, which have the ability to modify characteristics of their sprays without changing the fuel and oxidizer flow rates. To control the symmetry of the reaction zone, the combustor is equipped with capabilities to operate the spray characteristics of five of the injectors simultaneously, while varying the spray of the sixth injector separately. A high speed camera operating at 30,000 fps was used to take images of the tangential wave, while focusing on the reaction zone from the exhaust. Aside from the high speed camera, three pressure sensors (P’1, P’2 & P’3) and three fiber optic probes (FOP1,2,3) were strategically placed around the combustor. These diagnostic tools provided insight into the characterization associated with controlling the tangential mode instability. During testing, it was found that the high amplitude spinning tangential wave was able to be transformed into a low amplitude standing wave by altering the spray characteristics of the single injector. Across the tests, it was found that this transition in mode was characterized by an approximate 6X decrease in oscillatory pressure amplitude, while remaining at a constant frequency of f ≈ 5000 Hz. From this investigation, it has been suggested that breaking the symmetry of the reaction zone by controlling the spray pattern of a single injector in the group represents a viable LRE design approach to control the detrimental 1-T spinning tangential instability.

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.

Spray created by Jet A fuel injection from a plate containing sharp edged orifice 0.018 inches (457 µm) in diameter and L/D ratio of 10 into the crossflow of preheated air (555 K) at elevated pressure in the test section (5 atm) was compared to that created by a smooth countersunk injector with a L/D ratio of approximately unity under the same flow conditions. The Weber number (We) of the spray and momentum flux ratio (q) of the spray were also investigated with measurements taken at We=500, 1000, and 1500 and q=10, 20, and 40. The smooth countersink design in the second injector is meant to decrease the turbulence within the fuel injection column. A two component Phase Doppler Particle Analyzer was used for measuring the characteristics of the spray along its centerline. Macro images of the spray were also taken for spray trajectory (outer-edge) measurements. It was found that the core of the spray produced by the smooth countersunk injector penetrates further into the test section away from the injector orifice by approximately 2mm. It was seen that large scale turbulent structures on the order of 0.5-1.0 diameters form on the surface of the liquid column using the sharp edged injector while the liquid jet column produced by the smooth injector has smaller surface wave structures that were of the order of 0.2d. The smooth injector also produces droplets with a smaller mean diameter (D10), especially close to the orifice. Meanwhile, droplets from this injector that are close to the wall have a higher average velocity in the direction of fuel injection.

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.

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.

This paper describes a study of the onset of severe combustion instabilities (i.e., with peak to peak amplitudes of up to 0.97MPa) in a high pressure (PC>500psia or 3.45MPa) air breathing combustor as the inlet temperature of the injected liquid fuel (n-heptane - C7H16) was varied over the 20–335°C range, achieving supercritical conditions when the temperature exceeded 300°C. The attainment of supercritical operation was determined by a specially developed probe that illuminated the liquid spray with a laser beam and collected the light scattered off the spray. As the temperature of the fuel was increased, the spray disappeared when the fuel attained a supercritical state and the scattered signal could be no longer detected. Two different unstable modes (i.e., ∼100 and ∼400Hz) were excited in the combustor as the fuel temperature was increased from low to supercritical conditions and then cooled again. The lower frequency instability (∼100Hz) was excited and then disappeared when the fuel temperature was well below supercritical values (i.e., TFUEL250°C). It’s shown that the dynamics of the excitation and disappearance of these modes as well as their limit cycle amplitudes strongly depend upon the direction in which fuel temperature varies and also upon the temperature of the combustion air. Significantly, the results of this study strongly suggest that combustion instabilities may be excited in future high performance aircraft combustor that will operate at very high-pressures and with supercritical fuel injection.