Two-Phase Flow Visualization of Evaporating Liquid Fuels at Atmospheric Pressure

Two-phase flow visualization of fuel sprays is important for the design of better engines because it determines the efficiency and emissions of the combustion process. Simultaneous two-phase flow imaging using techniques such as planar laser-induced fluorescence (PLIF) has been a challenge due to the large variation in LIF signals from the gas and liquid phases. After laser excitation, the liquid signal initially overwhelms the gas phase signal due to its higher number density. However, the liquid signal quenches dramatically due to quenching effects that dominate the liquid LIF signal. By applying the novel concept of temporal filtering, separation of liquid and vapor signal can be achieved using different time delayed camera systems. The optical measurement provides a non-intrusive means of obtaining the liquid and vapor distributions in a spray. The experiment is performed using an ultraviolet beam from a burst-mode Nd:YAG laser in combination with two intensified cameras that are timed to maximize either the liquid or vapor phase signal. The setup is complemented by a drop generator and vaporizer flow system to allow studies of aviation fuels such as Jet-A or JP10, as well as reciprocating engine fuels such as diesel or toluene (as a surrogate for gasoline)

Probe-pulse optimization for nonresonant suppression in hybrid fs/ps coherent anti-Stokes Raman scattering at high temperature

Hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) offers accurate thermometry at kHz rates for combustion diagnostics. In high-temperature flames, selection of probe-pulse characteristics is key to simultaneously optimizing signal-to-nonresonant-background ratio, signal strength, and spectral resolution. We demonstrate a simple method for enhancing signal-to-nonresonant-background ratio by using a narrowband Lorentzian filter to generate a time-asymmetric probe pulse with full-width-half-maximum (FWHM) pulse width of only 240 fs. This allows detection within just 310 fs after the Raman excitation for eliminating nonresonant background while retaining 45% of the resonant signal at 2000 K. The narrow linewidth is comparable to that of a time-symmetric sinc2 probe pulse with a pulse width of ~2.4 ps generated with a conventional 4-f pulse shaper. This allows nonresonant-background-free, frequency-domain vibrational spectroscopy at high temperature, as verified using comparisons to a time-dependent theoretical fs/ps CARS model

Quantitative measurement of binary liquid distributions using multiple-tracer x-ray fluorescence and radiography

The complex geometry and large index-of-refraction gradients that occur near the point of impingement of binary liquid jets present a challenging environment for optical interrogation. A simultaneous quadruple-tracer x-ray fluorescence and line-of-sight radiography technique is proposed as a means of distinguishing and quantifying individual liquid component distributions prior to, during, and after jet impact. Two different pairs of fluorescence tracers are seeded into each liquid stream to maximize their attenuation ratio for reabsorption correction and differentiation of the two fluids during mixing. This approach for instantaneous correction of xray fluorescence reabsorption is compared with a more time-intensive approach of using stereographic reconstruction of x-ray attenuation along multiple lines of sight. The proposed methodology addresses the need for a quantitative measurement technique capable of interrogating optically complex, near-field liquid distributions in many mixing systems of practical interest involving two or more liquid streams

Dual-pump vibrational/rotational femtosecond/ picosecond coherent anti-Stokes Raman scattering temperature and species measurements

A method for simultaneous ro-vibrational and pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) is presented for multi-species detection and improved temperature sensitivity from room temperature to flame conditions. N2∕CH4 vibrational and N2∕O2∕H2 rotational Raman coherences are excited simultaneously using fs pump pulses at 660 and 798 nm, respectively, and a common fs Stokes pulse at 798 nm. A fourth narrowband 798 nm ps pulse probes all coherence states at a time delay that minimizes nonresonant background and the effects of collisions. The transition strength is concentration dependent, while the distribution among observed transitions is related to temperature through the Boltzmann distribution. The broadband excitation pulses and multiplexed signal are demonstrated for accurate thermometry from 298 to 2400 K and concentration measurements of four key combustion species

Femtosecond Laser Electronic Excitation Tagging, FLEET, for Combustion and Flow

Femtosecond Laser Electronic Excitation Tagging, known as FLEET, can be used to measure the nitrogen gas content within a gaseous mixture. FLEET does not require any trace particles that could affect the combustion reaction or physical properties of the flow. Another advantage is the simple experimental implementation. In this work a 120-femtosecond laser pulse was focused in to the probe volume to dissociate the nitrogen gas via multiphoton process. The intensity of the light emitted after the recombination is proportional to the nitrogen gas to oxygen mass ratio as the dissociated nitrogen bonds with oxygen to form nitric oxide and other reactions, which does not emit light. Intensity of the light from FLEET within a methane-air diffusion flame was used to determine fuel/air ratio.. The intensity of FLEET signal was calibrated for different mixtures in test cell in the ranges of pressures to simulate the change of number density due to increase of the temperature in the flame

A High-Speed X-Ray Detector System for Noninvasive Fluid Flow Measurements

The opaque nature of many multiphase flows has long posed a significant challenge to the visualization and measurement of desired characteristics. To overcome this difficulty, X-ray imaging, both in the form of radiography and computed tomography, has been used successfully to quantify various multiphase flow phenomena. However, the relatively low temporal resolution of typical X-ray systems limit their use to moderately slow flows and time-average values. This paper discusses the development of an X-ray detection system capable of high-speed radiographic imaging that can be used to visualize multiphase flows. Details of the hardware will be given and then applied to sample multiphase flows in which X-ray radiographic images of up to 1,000 frames per second were realized. The sample flows address two different multiphase flow arrangements. The first is a gas-liquid system representative of a small bubble column. The second is a gas-solid system typically found in a fluidized bed operation. Sample images are presented and potential challenges and solutions are discussed

Effects of repetitive pulsing on multi-kHz planar laser-induced incandescence imaging in laminar and turbulent flames

Planar laser-induced incandescence (LII) imaging is reported at repetition rates up to 100 kHz using a burst-mode laser system to enable studies of soot formation dynamics in highly turbulent flames. To quantify the accuracy and uncertainty of relative soot volume fraction measurements, the temporal evolution of the LII field in laminar and turbulent flames is examined at various laser operating conditions. Under high-speed repetitive probing, it is found that LII signals are sensitive to changes in soot physical characteristics when operating at high laser fluences within the soot vaporization regime. For these laser conditions, strong planar LII signals are observed at measurement rates up to 100 kHz but are primarily useful for qualitative tracking of soot structure dynamics. However, LII signals collected at lower fluences allow sequential planar measurements of the relative soot volume fraction with a sufficient signal-to-noise ratio at repetition rates of 10-50 kHz. Guidelines for identifying and avoiding the onset of repetitive probe effects in the LII signals are discussed, along with other potential sources of measurement error and uncertainty

Fiber-Optic Imaging in an Internal Combustion Engine Test Rig

The formation of particulate matter (PM/soot), nitrogen oxides (NOx), and other byproducts of the combustion process in diesel engines is controlled by spatiotemporally varying quantities within the engine cylinders which traditional sensors cannot resolve. This study explores the use of an advanced sensing technique using an optical probe which can be used to produce highly spatiotemporally resolved in cylinder images of the flame formation during the combustion stroke. Using a fiber optic cable and custom lensing system adapted to fit a pre-existing pressure transducer port, light from within the cylinder can be transmitted through the imaging probe to a high speed camera where high resolution images of the flame are captured. This method enables no modifications to the engine geometry or materials, which ensures that the combustion and heat transfer characteristics are the same in the operating engine as they would be without the sensor implementation. Simulation results of an optical system to meet the desired design constraints are presented and discussed in addition to models of the imaging probe design. These results showed that an effective imaging system could be packaged within a narrow design envelope and produce high resolution images for a wide field of view

Simultaneous high-speed measurement of temperature and lifetime-corrected OH laserinduced fluorescence in unsteady flames

A means of performing simultaneous, high-speed measurements of temperature and OH lifetime-corrected laser-induced fluorescence (LIF) for tracking unsteady flames has been developed and demonstrated. The system uses the frequency-doubled and frequency-tripled output beams of an 80 MHz mode-locked Ti:sapphire laser to achieve ultrashort laser pulses (order 2 ps) for Rayleigh-scattering thermometry at 460 nm and lifetime-corrected OH LIF at 306.5 nm, respectively. Simultaneous, high-speed measurements of temperature and OH number density enable studies of flame chemistry, heat release, and flame extinction in unsteady, strained flames where the local fluorescence-quenching environment is unknown