Christopher F. Powell

Quantitative analysis of highly transient fuel sprays by time-resolved x-radiography

Microsecond time-resolved synchrotron x-radiography has been used to elucidate the structure and dynamics of optically turbid, multiphase, direct-injection gasoline fuel sprays. The combination of an ultrafast x-ray framing detector and tomographic analysis allowed three-dimensional reconstruction of the dynamics of the entire 1-ms-long injection cycle. Striking, detailed features were observed, including complex traveling density waves, and unexpected axially asymmetric flows. These results will facilitate realistic computational fluid dynamic simulations of high-pressure sprays and combustion.

Time-resolved measurements of supersonic fuel sprays using synchrotron X-rays.

A time-resolved radiographic technique has been developed for probing the fuel distribution close to the nozzle of a high-pressure single-hole diesel injector. The measurement was made using X-ray absorption of monochromatic synchrotron-generated radiation, allowing quantitative determination of the fuel distribution in this optically impenetrable region with a time resolution of better than 1 micros. These quantitative measurements constitute the most detailed near-nozzle study of a fuel spray to date.

The 7BM beamline at the APS: a facility for time-resolved fluid dynamics measurements

The 7BM beamline, a facility for time-resolved fluid dynamics measurements at the Advanced Photon Source, is described.

The Effects of Diesel Injector Needle Motion on Spray Structure

The internal structure of diesel fuel injectors is known to have a significant impact on the steady-state fuel distribution within the spray. However, little experimental or computational work has been performed on the dynamics of fuel injectors. Recent studies have shown that it is possible to measure the three-dimensional geometry of the injector nozzle, and to track changes in that geometry as the needle opens and closes in real time. This has enabled the dynamics of the injector to be compared with the dynamics of the spray, and allows computational fluid dynamics (CFD) simulations to use realistic time-dependent flow passage geometries. In this study, X-ray phase-enhanced imaging has been used to perform time-resolved imaging of the needle seat area in several common-rail diesel injection nozzles. The fuel distributions of the sprays emitted by these injectors were also studied with fast X-ray radiography. Correlations between eccentric motions of the injector needle valve and oscillations in the fuel density as it emerges from the nozzle are examined. CFD modeling is used to interpret the effect of needle motion on fuel flow.

Pixel array detectors for time resolved radiography (invited)

Intense x-ray sources coupled with efficient, high-speed x-ray imagers are opening new possibilities of high-speed time resolved experiments. The silicon pixel array detector (PAD) is an extremely flexible technology which is currently being developed as a fast imager. We describe the architecture of the Cornell PAD, which is capable of operating with submicrosecond frame times. This 100×92 pixel prototype PAD consists of a pixelated silicon diode layer, for direct conversion of the x rays to charge carriers, and a corresponding pixellated complementary metal–oxide–semiconductor electronics layer, for processing and storage of the generated charge. Each pixel diode is solder bump bonded to its own pixel electronics consisting of a charge integration amplifier, an array of eight storage capacitors and an output amplifier. This architecture allows eight complete frames to be stored in rapid succession, with a minimum integration time of 150 ns per frame and an interframe deadtime of 600 ns. We describe the ...

Effect of nozzle transients and compressibility on the penetration of fuel sprays

A study has been performed using a combination of high speed optical imaging and a synchrotron based technique to obtain a time history of nozzle exit velocity, discharge coefficient, and spray tip velocity of high pressure fuel sprays. The results support a recently proposed theoretical model of spray propagation that suggests a compressible region of flow immediately ahead of the spray has a strong influence on the evolution of the tip velocity profile. Coupled with this is the variation in discharge coefficient due to injector needle movement which largely governs the spray exit velocity immediately after start of injection.

Unraveling the Geometry Dependence of In-Nozzle Cavitation in High-Pressure Injectors

Cavitation is an intricate multiphase phenomenon that interplays with turbulence in fluid flows. It exhibits clear duality in characteristics, being both destructive and beneficial in our daily lives and industrial processes. Despite the multitude of occurrences of this phenomenon, highly dynamic and multiphase cavitating flows have not been fundamentally well understood in guiding the effort to harness the transient and localized power generated by this process. In a microscale, multiphase flow liquid injection system, we synergistically combined experiments using time-resolved x-radiography and a novel simulation method to reveal the relationship between the injector geometry and the in-nozzle cavitation quantitatively. We demonstrate that a slight alteration of the geometry on the micrometer scale can induce distinct laminar-like or cavitating flows, validating the multiphase computational fluid dynamics simulation. Furthermore, the simulation identifies a critical geometric parameter with which the high-speed flow undergoes an intriguing transition from non-cavitating to cavitating.

High-resolution large eddy simulations of cavitating gasoline–ethanol blends

Cavitation plays an important role in the formation of sprays in fuel injection systems. With the increasing use of gasoline–ethanol blends, there is a need to understand how changes in fluid properties due to the use of these fuels can alter cavitation behavior. Gasoline–ethanol blends are azeotropic mixtures whose properties are difficult to model. We have tabulated the thermodynamic properties of gasoline–ethanol blends using a method developed for flash-boiling simulations. The properties of neat gasoline and ethanol were obtained from National Institute of Standards and Technology REFPROP data, and blends from 0% to 85% ethanol by mass have been tabulated. We have undertaken high-resolution three-dimensional numerical simulations of cavitating flow in a 500-µm-diameter submerged nozzle using the in-house HRMFoam homogeneous relaxation model constructed from the OpenFOAM toolkit. The simulations are conducted at 1 MPa inlet pressure and atmospheric outlet pressure, corresponding to a cavitation number range of 1.066–1.084 and a Reynolds number range of 15,000–40,000. For the pure gasoline case, the numerical simulations are compared with synchrotron X-ray radiography measurements. Despite significant variation in the fluid properties, the distribution of cavitation vapor in the nozzle is relatively unaffected by the gasoline–ethanol ratio. The vapor remains attached to the nozzle wall, resulting in an unstable annular two-phase jet in the outlet. Including turbulence at the conditions studied does not significantly change mixing behavior, because the thermal nonequilibrium at the vapor–liquid interfaces acts to low-pass filter the turbulent fluctuations in both the nozzle boundary layer and jet mixing layer.

Application of X-ray fluorescence to turbulent mixing.

Combined measurements of X-ray absorption and fluorescence have been performed in jets of pure and diluted argon gas to demonstrate the feasibility of using X-ray fluorescence to study turbulent mixing. Measurements show a strong correspondence between the absorption and fluorescence measurements for high argon concentration. For lower argon concentration, fluorescence provides a much more robust measurement than absorption. The measurements agree well with the accepted behavior of turbulent jets.

Determination of Diesel Spray Axial Velocity Using X-Ray Radiography

Present knowledge of the velocity of the fuel in diesel sprays is quite limited due to the obscuring effects of fuel droplets, particularly in the high-density core of the spray. In recent years, x-ray radiography, which is capable of penetrating dense fuel sprays, has demonstrated the ability to probe the structure of the core of the spray, even in the dense near-nozzle region. In this paper, x-ray radiography data was used to determine the average axial velocity in diesel sprays as a function of position and time. Here, we report the method used to determine the axial velocity and its application to three common-rail diesel sprays at 250 bar injection pressure. The data show that the spray velocity does not reach its steady state value near the nozzle until approximately 200 {micro}s after the start of injection. Moreover, the spray axial velocity decreases as one moves away from the spray orifice, suggesting transfer of axial momentum to the surrounding ambient gas.