Search

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.

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 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 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...

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 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.

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.