Application Of Planar Laser-induced Fluorescence Research Articles

Potential Analysis of Defossilized Operation of a Heavy-Duty Dual-Fuel Engine Utilizing Dimethyl Carbonate/Methyl Formate as Primary and Poly Oxymethylene Dimethyl Ether as Pilot Fuel

<div>This study demonstrates the defossilized operation of a heavy-duty port-fuel-injected dual-fuel engine and highlights its potential benefits with minimal retrofitting effort. The investigation focuses on the optical characterization of the in-cylinder processes, ranging from mixture formation, ignition, and combustion, on a fully optically accessible single-cylinder research engine. The article revisits selected operating conditions in a thermodynamic configuration combined with Fourier transform infrared spectroscopy.</div> <div>One approach is to quickly diminish fossil fuel use by retrofitting present engines with decarbonized or defossilized alternatives. As both fuels are oxygenated, a considerable change in the overall ignition limits, air–fuel equivalence ratio, burning rate, and resistance against undesired pre-ignition or knocking is expected, with dire need of characterization.</div> <div>Two simultaneous high-speed recording channels granted cycle-resolved access to the natural flame luminosity, which was recorded in red/green/blue and OH chemiluminescence.</div> <div>Selected conditions were investigated in more detail with the simultaneous application of planar laser-induced fluorescence of OH and HCHO and recording natural flame luminescence in a cycle-averaged manner.</div> <div>Poly oxymethylene dimethyl ether was used as pilot fuel, building on prior investigations. The mixture of 65 vol% Dimethyl Carbonate and 35 vol% Methyl Formate with prior verification on a passenger-car-sized engine substitutes synthetic natural gas in this study.</div> <div>Thermodynamically, the increased compression ratio up to 17.6 resulted in feasible operation and increased indicated efficiency. On the lower compression ratio of 15.48, a more comprehensive range of applicable air–fuel equivalence ratios and increased degrees of freedom regarding the pilot’s total energy share are observed compared to the base configuration with natural gas and EN590 as pilot fuel.</div> <div>The air–fuel equivalence ratio sweep from λ = 1.0–2.0 revealed predominantly premixed and high-temperature heat release via OH*. The temporal and spatial evolution shifts while leaning out the mixture with increasing gradients on the radial distribution and decouples for lean mixtures from the initial spray trajectory.</div>

Application of Planar Laser-Induced Fluorescence for Interfacial Transfer Phenomena

The present review describes the current achievements in the applications of a planar laser-induced fluorescence (PLIF) method for the diagnostics of liquid films, bubbles, individual droplets, and sprays. Such flows are related with strongly curved interphases, which often results in additional high errors during the PLIF data quantification because of laser light reflection, refraction, and absorption. The present review demonstrates that a two-color PLIF approach and a PLIF modification for regularly structured illumination resolves the reflection- and refraction-caused errors. The latter modification ensures proper phase separation in the measurement cross-section and visualization of the interface dynamics. The former approach provides the accurate evaluation of the local temperature and concentration both in liquid and gaseous phases even in the case of strong variations of the laser sheet intensity. With intensified cameras, the PLIF method is used for multi-parameter diagnostics of the two-phase combustion of sprays in combustion chambers with optical access. It visualizes and quantifies the liquid fuel evaporation and mixing, to measure temperature in the gas and liquid phases and to reveal the regions of pollutant formation. The PLIF technique can also be easily combined with a particle image (or tracking) velocimetry method, to evaluate local heat and mass transfer.

Spatiotemporally resolved heat transfer measurements in falling liquid-films by simultaneous application of planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography

We present an optical technique that combines simultaneous planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography for the space-and time-resolved measurement of the film-height, 2-D velocity and 2-D free-surface temperature in liquid films falling over an inclined, resistively-heated glass substrate. Using this information and knowledge of the wall temperature, local and instantaneous heat-transfer coefficients (HTCs) and Nusselt numbers, Nu, are also recovered along the waves of liquid films with Kapitza number, Ka=180, and Prandtl number, Pr=77. By employing this technique, falling-film flows are investigated with Reynolds numbers in the range Re = 18–66, wave frequencies set to fw=7, 12 and 17 Hz, and a wall heat flux set to q̇=2.5 W cm−2. Complementary data are also collected in equivalent (i.e., for the same mean-flow Re) flows with q̇=0 W cm−2. Quality assurance experiments are performed that reveal deviations of up to 2–3% between PLIF/PTV-derived film heights, interfacial/bulk velocities and flow rates, and both analytical predictions and direct measurements of flat films over a range of conditions, while IR-based temperature measurements fall within 1 °C of thermocouple measurements. Highly localized film height, velocity, flow-rate and interface-temperature data are generated along the examined wave topologies by phase/wave locked averaging. The application of a heat flux (q̇=2.5 W cm−2) results in a pronounced “thinning” of the investigated films (by 18%, on average), while the mean bulk velocities compensate by increasing by a similar extent to conserve the imposed flow rate. The axial-velocity profiles that are obtained in the heated cases are parabolic but “fuller” compared to equivalent isothermal flows, excluding any wave-regions where the interface slopes are high. As the Re is reduced, the heating applied at the wall penetrates through the film, resulting in a pronounced coupling between the HTC and the film height in thinner film regions. When the imposed wave frequency is increased, a narrower range of HTCs is observed, which we link to the evolution of the film topology and the associated redistribution of the fluid flow upstream of the imaging location, as the liquid viscosity decreases. The local and instantaneous Nu is strongly coupled to the film height and experiences variations that increase as fw is reduced.

Conditional flow field statistics of jet-in-hot-coflow flames

The conditional velocity field of the Delft jet-in-hot-coflow flames is studied by the simultaneous application of planar laser-induced fluorescence of the OH radical (OH-PLIF) and particle image velocimetry (PIV). The goal of this study is to assess the role of turbulence on chemical processes in the Delft jet-in-hot-coflow flames and similar setups, that are characterised by the presence of a turbulent jet that issues into a slower-moving and much less turbulent surroundings of combustion products. An important result is that the conditional velocity field (conditional on the presence of a strong OH signal) is very different from the non-conditional velocity field at the same location. These jet-in-hot coflow flames are thus characterised by a strong intermittency, and experience a very different flow field and flow time scales than one would expect from the local mean velocity and scalar field. An other result following from the local velocity data is that the average conditional strain perpendicular to the flame surface is neither compressive nor extensive. This is caused by to the orientation of the principle strains and that of the flame interface. One millimetre inward to the jet, from the edge of the fuel-rich side of the OH contours, the average strains do become compressive, and the pdf of normal strain becomes considerably wider. In light of these results, the closure of the mean chemical source term by introducing a single (unconditional) turbulent time scale seems to be an invalid approach. Furthermore, it demonstrates that the fluid dynamic properties of the coflow stream are decisive for the applicability of these setups to the study of turbulence-chemistry interaction in industrial combustion processes.

Application of planar laser-induced fluorescence measurement techniques to study the heat transfer characteristics of parallel-plate heat exchangers in thermoacoustic devices

This paper describes the development of an experimental arrangement and the application of acetone-based planar laser-induced fluorescence (PLIF) measurement techniques to study the unsteady characteristics of heat transfer processes in the parallel-plate heat exchangers of thermoacoustic devices. The experimental rig is a quarter-wavelength acoustic resonator where a standing wave imposes oscillatory flow conditions. Two mock-up heat exchangers, ‘hot’ and ‘cold’, have their fins kept at constant temperatures by electrical heating and water cooling, respectively. A purpose-designed acetone tracer seeding mechanism is used for PLIF temperature measurement. Acetone concentration is optimized from the viewpoint of PLIF signal intensity. Two-dimensional temperature distributions in the gas surrounding the heat exchanger plates, as a function of phase angle in the acoustic cycle, are obtained. Local and global (instantaneous and cycle-averaged) heat flux values on the fin surface are estimated and used to obtain the dependence of the space-cycle averaged Nusselt versus Reynolds number. Measurement uncertainties are discussed.

Two-component Doppler-shift fluorescence velocimetry applied to a generic planetary entry probe model

This study discusses the development and application of planar laser-induced fluorescence of nitric oxide (NO PLIF) to measure velocities in an axisymmetric hypersonic near-wake flow field around a model planetary-entry vehicle configuration. Shapes and positions of NO spectral lines at every location in the flow are determined over several successive shock tunnel runs. The lines experience Doppler shifts proportional to the local flow velocity component in the direction of the fluorescence-generating laser. A Gaussian line shape function is then fitted to the acquired wavelength-dependent fluorescence measurements, the line center of which is correlated to the time-averaged velocity at each pixel location. The flow field is probed successively by a laser in two orthogonal directions, which yields the velocity magnitude and direction everywhere in the illuminated plane. The accuracy of the measurement technique is discussed, and various strategies to characterize systematic errors are presented. The variation of random uncertainties in different regions of the flow field provides information about the local steadiness of the flow. To the authors’ knowledge, the measurements represent the first two-component velocity map of a hypersonic near-wake flow.

Experimental Determination of the Sub-grid Scale Scalar Flux in a Non-Reacting Jet-Flow

This work presents an experimental approach to the direct determination of the sub-grid scale scalar flux used in Large Eddy Simulation (LES). Two non-reacting jet-flow configurations were examined, one with the jet temperature similar to the ambient air temperature and one preheated jet. Hence data were obtained for varying viscosities. Each case was investigated for two different, high Reynolds numbers of 17,471 and 31,714. For the measurements of mixture fraction and flow velocity field a finest resolution of almost 100 μm was achieved. Thus both the sub-grid scale scalar flux as well as the spatially filtered quantities can be obtained from the same data set. The data were acquired by the simultaneous application of planar laser-induced fluorescence (LIF) and particle image velocimetry (PIV).

OH Planar Laser-Induced Fluorescence and Emission Imaging in High Pressure LOx/Methane Flames

The application of planar laser-induced fluorescence of OH to high-pressure liquid oxygen/gaseous methane flames is investigated in this article. As pressure is increased, the maximum level of OH fluorescence decreases while an interfering light intensity increases. It is shown that suitable data can only be obtained by properly tuning the detection scheme. Narrowband filtering of OH fluorescence is required to reduce the level of interfering signals. An analysis of the interfering light indicates that it is associated with polycyclic aromatic hydrocarbon fluorescence originating from a region surrounding the flame. OH and polycyclic aromatic hydrocarbon fluorescence signal amplitudes become comparable at a pressure of 2.5 MPa which constitutes an upper bound for standard imaging. Below that limit the flame is well characterized and features thin, wrinkled OH layers developing in the vicinity of the liquid oxygen jet. The initial flame sheet is continuous but it becomes highly corrugated further downstream when the liquid oxygen jet breaks down. The flame edge standoff distance is greater than a few LOx post lip sizes indicating that stabilization is less well achieved than in the case of liquid oxygen/hydrogen flames where combustion typically begins at less than one lip size from the injector.

The Application of Planar Laser-Induced Fluorescence as a Diagnostic in Plasma Processing

ABSTRACTThis paper reports an application of the planar laser-induced fluorescence (PLIF) technique to visualize species in a plasma jet. For the experiment, 10 μm manganese particles were injected through a center feed in the cathode of a 8 kW plasma torch. A tunable laser beam (−20 mJ/pulse) was focused by a cylinderical lens into a sheet, approximately 2.4 cm high and 550 μm thick, which passed through the center of the plasma plume, exciting manganese lying within the plane defined by the laser. The fluorescence generated was imaged onto a gated, intensified CID camera, converted to digital format, and processed by an image analyzer to form a 2-D map of the relative manganese vapor concentration within the plasma. The results of this proof-of-principle experiment demonstrate that the PLIF measurement holds considerable promise for obtaining simultaneous spatially resolved measurements of species concentration.