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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...

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.

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

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.

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.