Alper Ercan

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.

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 ...

Four dimensional visualization of highly transient fuel sprays by microsecond quantitative x-ray tomography

An ultrafast x-ray microtomography technique based on synchrotron x rays and a fast-framing x-ray detector was developed to reconstruct the highly transient sprays in four dimensions with microsecond-temporal resolution in the near-nozzle region. The time-resolved quantitative fuel distribution allowed a realistic numerical fluid dynamic simulation with initial conditions based on the measurement, which demonstrates that the fuel has completed the primary breakup upon exiting the nozzle. The secondary-breakup-based simulation agrees well with the experimental fuel-volume fraction distribution, which challenges most existing simulation assumptions and results.

A high-speed area detector for novel imaging techniques in a scanning transmission electron microscope.

A scanning transmission electron microscope (STEM) produces a convergent beam electron diffraction pattern at each position of a raster scan with a focused electron beam, but recording this information poses major challenges for gathering and storing such large data sets in a timely manner and with sufficient dynamic range. To investigate the crystalline structure of materials, a 16x16 analog pixel array detector (PAD) is used to replace the traditional detectors and retain the diffraction information at every STEM raster position. The PAD, unlike a charge-coupled device (CCD) or photomultiplier tube (PMT), directly images 120-200keV electrons with relatively little radiation damage, exhibits no afterglow and limits crosstalk between adjacent pixels. Traditional STEM imaging modes can still be performed by the PAD with a 1.1kHz frame rate, which allows post-acquisition control over imaging conditions and enables novel imaging techniques based on the retained crystalline information. Techniques for rapid, semi-automatic crystal grain segmentation with sub-nanometer resolution are described using cross-correlation, sub-region integration, and other post-processing methods.

Development of ultrafast computed tomography of highly transient fuel sprays

The detailed analysis of the fuel sprays has been well recognized as an important step for optimizing the operation of internal-combustion engines to improve efficiency and reduce emissions. However, the structure and dynamics of highly transient fuel sprays have never been visualized or reconstructed in three dimensions (3D) previously due to numerous technical difficulties. By using an ultrafast x-ray detector and intense monochromatic x-ray beams from synchrotron radiation, the fine structures and dynamics of 1-ms direct-injection gasoline fuel sprays were elucidated for the first time by a newly developed, ultrafast computed microtomography technique. Due to the time-resolved nature and the intensive data analysis, the Fourier transform algorithm was used to achieve an efficient reconstruction process. The temporal and spatial resolutions of the current measurement are 5.1 μs and 150 μm, respectively. Many features associated with the transient liquid flows are readily observable in the reconstructed spray. Furthermore, an accurate 3D fuel density distribution was obtained as the result of the computed tomography in a time-resolved manner. These results not only reveal the characteristics of automotive fuel sprays with unprecedented details, but will also facilitate realistic computational fluid dynamic simulations in highly transient, multiphase systems.

Quantitative Characterization of Near-Field Fuel Sprays by Multi-Orifice Direct Injection Using Ultrafast X-Tomography Technique

A low-pressure direct injection fuel system for spark ignition direct injection engines has been developed, in which a high-turbulence nozzle technology was employed to achieve fine fuel droplet size at a low injection pressure around 2 MPa. It is particularly important to study spray characteristics in the nearnozzle region due to the immediate liquid breakup at the nozzle exit. By using an ultrafast x-ray area detector and intense synchrotron x-ray beams, the interior structure and dynamics of the direct injection gasoline sprays from a multi-orifice turbulence-assisted nozzle were elucidated for the first time in a highly quantitative manner with µs-temporal resolution. Revealed by a newly developed, ultrafast computed x-microtomography technique, many detailed features associated with the transient liquid flows are readily observable in the reconstructed spray. Furthermore, an accurate 3dimensional fuel density distribution, in the form of fuel volume fraction, was obtained by the time-resolved computed tomography. The time-dependent fuel density distribution revealed that the fuel jet is well broken up immediately at the nozzle exits. These results not only reveal the near-field characteristics of the partial atomized fuel sprays with unprecedented detail, but also facilitate the development of an advanced multi-orifice direct injector. This ultrafast tomography capability also will facilitate the realistic computational fluid dynamic simulations in highly transient and multiphase fuel spray systems.

Shock waves generated by high-pressure fuel sprays directly imaged by x-radiography.

Synchrotron x-radiography and a novel fast x-ray detector are used to visualize the detailed, time-resolved structure of the fluid jets generated by a high pressure diesel-fuel injection. An understanding of the structure of the high-pressure spray is important in optimizing the injection process to increase fuel efficiency and reduce pollutants. It is shown that x-radiography can provide a quantitative measure of the mass distribution of the fuel. Such analysis has been impossible with optical imaging due to the multiple-scattering of visible light by small atomized fuel droplets surrounding the jet. In addition, direct visualization of the jet-induced shock wave proves that the fuel jets become supersonic under appropriate injection conditions. The radiographic images also allow quantitative analysis of the thermodynamic properties of the shock wave.

A High Frame Rate Hybrid X-Ray Image Sensor

This paper describes a solid-state image sensor for high-speed X-ray imaging. The sensor is made up of a light sensitive detector layer bump-bonded to a readout integrated circuit (ROIC). The detector layer is high resistivity n-type silicon and is fully depleted in operation. The p-implanted islands are used to define pixel regions with 100-μm × 100-μm area. The detector layer contains 852 × 209 pixels indium bump-bonded to four identical CMOS ROICs. Each ROIC contains 213 × 209 pixels and is fabricated using a 0.25-μm CMOS process. The ROIC utilizes a capacitive transimpedance amplifier-type front-end coupled to a switched capacitor analog memory. This architecture allows storage of eight frames for high-speed burst imaging of up to a million frames per second, followed by a slower multiplexed readout. Details of the sensor design and operation are presented together with characterization results.