Laser-induced Exciplex Fluorescence Research Articles

Quantitative liquid and vapor distribution measurements in evaporating fuel sprays using laser-induced exciplex fluorescence

Fully quantitative two-dimensional measurements of liquid- and vapor-phase fuel distributions (mass per unit volume) from high-pressure direct-injection gasoline injectors are reported for conditions of both slow and rapid vaporization in a heated, high-pressure spray chamber. The measurements employ the coevaporative gasoline-like fluorobenzene (FB)/diethylmethylamine (DEMA)/hexane exciplex tracer/fuel system. In contrast to most previous laser-induced exciplex-fluorescence (LIEF) experiments, the quantitative results here include regions in which liquid and vapor fuel coexist (e.g. near the injector exit). A unique aspect is evaluation of both vapor- and liquid-phase distributions at varying temperature and pressure using only in situ vapor-phase fluorescence calibration measurements at room temperature and atmospheric pressure. This approach draws on recent extensive measurements of the temperature-dependent spectroscopic properties of the FB–DEMA exciplex system, in particular on knowledge of the quantum efficiencies of the vapor-phase and liquid-phase (exciplex) fluorescence. In addition to procedures necessary for quantitative measurements, we discuss corrections for liquid–vapor crosstalk (liquid fluorescence that overlaps the vapor-fluorescence bandpass), the unknown local temperature due to vaporization-induced cooling, and laser-sheet attenuation by scattering and absorption.

Spectroscopic characterization of the fluorobenzene/DEMA tracer system for laser-induced exciplex fluorescence for the quantitative study of evaporating fuel sprays

Quantitative analysis of laser-induced exciplex fluorescence (LIEF) requires knowledge of the spectral characteristics of all the involved substances. The temperature dependence of the ratio of exciplex and monomer in the liquid phase as well as the temperature-dependent fluorescence intensity from monomers in the gas phase must be known when quantifying signals and correcting for cross talk between liquid and vapor phase. In this work we present an extensive characterization of the fluorescence of the fluorobenzene/diethyl-methyl-amine (DEMA)/n-hexane exciplex system. We use a mixture of 2% fluorobenzene, 9% DEMA, and 89% n-hexane, which was tested before for its coevaporative behavior. The temperature dependent fluorescence of the liquid exciplex was studied in a thin layer cell with UV-light (266 nm) excitation. Cross talk of a known combination of bandpass filters was quantified for the relevant temperature range. The temperature dependence of the gas-phase absorption and fluorescence signal (fluorobenzene) was studied in a heated static cell and a heated gas stream in a nitrogen coflow, respectively. These measurements provide the background for the application of LIEF for quantitative analysis of vapor and liquid distributions in fuel sprays.

Numerical and experimental study on effect of wall geometry on wall impingement process of hollow-cone fuel spray under various ambient conditions

The spray–wall impingement process in gasoline direct injection (GDI) engines, which is caused by the interaction among spray, wall and air to move the air–fuel mixture near the spark plug, directly influences the engine performance and emissions. Therefore, a detailed understanding of this process is very important in designing an injection system and controlling a strategy of GDI engines. The purpose of this study is to understand the spray–wall impingement characteristics for more efficient designing of the injection system in GDI engines and to supply the fundamental data under engine operation conditions. The wall impingement processes of hollow-cone fuel spray according to ambient gas conditions and wall geometry are calculated by validated spray models. The calculated results were compared with the experimental results obtained by the laser-induced exciplex fluorescence (LIEF) technique. It was found that the spray and vortex cloud at the high ambient pressure were distributed at inner area of cavity and the more fuel film mass observed at this condition. The fuel film mass decreased with the increase of ambient temperature, while the fuel film mass increased at high cavity angles.

Air–fuel mixing and combustion in a small-bore direct injection optically accessible diesel engine using a retarded single injection strategy

In this paper, the air–fuel mixing and combustion in a small-bore direct injection optical diesel engine were studied for a retarded single injection strategy. The effects of injection pressure and timing were analyzed based on in-cylinder heat release analysis, liquid fuel and vapor fuel imaging by Laser induced exciplex fluorescence technique, and combustion process visualization. NOx emissions were measured in the exhaust pipe. Results show that increasing injection pressure benefits soot reduction while increases NOx emissions. Retarding injection timing leads to simultaneous reduction of soot and NOx emissions with premixed homogeneous charge compression ignition (HCCI) like combustion modes. The vapor distribution in the cylinder is relatively homogeneous, which confirms the observation of premixed combustion in the current studies. The postulated path of these combustion modes were analyzed and discussed on the equivalence ratio-temperature map.

Numerical modeling of hollow-cone fuel atomization, vaporization and wall impingement processes under high ambient temperatures

In the following paper, a numerical study of the atomization, vaporization and wall impingement processes of hollow-cone fuel spray from high-pressure swirl injectors under various ambient temperature conditions was carried out. Also, the availability of applied models and the effect of ambient temperature on spray characteristics is discussed. The Linearized Instability Sheet Atomization (LISA) model combined with the Aerodynamically Progressed Taylor Analogy Breakup (APTAB) model, the improved Abramzon model and the Gosman model are used to calculate the atomization, vaporization and wall impingement processes of hollow-cone fuel spray, respectively. Spray models are implemented with the modified KIVA code. The calculation results of the spray characteristics under two ambient temperatures, including spray tip penetration, spray structure and radial distance after spray-wall impingement are compared to the experimental results obtained by the Laser Induced Exciplex Fluorescence (LIEF) technique. The droplet size distribution, ambient gas velocity field, vapor phase distribution and fuel film mass generated by spray-wall impingement, measurements which are generally difficult to obtain by experimental methods, are also calculated and discussed. Quantitative discussions on the effect of the ambient temperature on the spray development process are conducted. It is shown that the applied models are applicable even in the high ambient temperature condition.

Numerical and experimental study on hollow-cone fuel spray of highpressure swirl injector under high ambient pressure condition

A hybrid breakup model was proposed as a trustworthy prediction of hollow-cone fuel spray in the present study and the atomization process of the hollow-cone fuel spray of a high-pressure swirl injector in a Gasoline Direct Injection (GDI) engine under high ambient pressure conditions was studied by a new hybrid breakup model. The proposed hybrid breakup model is composed of the Linearized Instability Sheet Atomization (LISA) model as a primary breakup process. The Aerodynamically Progressed Taylor Analogy Breakup (APTAB) model, instead of the Taylor Analogy Breakup (TAB) model, was used as a secondary breakup process. The effects of the droplet deformation on a droplet aerodynamic external force are considered in the APTAB model. In addition, we replaced the x2 distribution function used in previous the APTAB model by the Rosin-Rammler distribution function to improve the prediction precision. The Laser Induced Exciplex Fluorescence (LIEF) technique and the Phase Doppler Anemometry (PDA) system were used to produce a set of experimental data for the model validation. The estimation of the prediction ability of the LISA+APTAB model was carried out, and spray characteristics, which are difficult to obtain by experimental method, were calculated and discussed. The suggested hybrid breakup model showed better prediction capability compared with the previous model (LISA+TAB model). From the calculated results, the effect of the ambient pressure on the SMD (Sauter Mean Diameter) and droplet velocity could be discussed quantitatively.

Development and validation of a new hybrid break-up model for the modelling of hollow-cone fuel spray

In the present study, a numerical study and an experimental study on the break-up process of a hollow-cone fuel spray were performed. This was done to develop a new hybrid break-up model and to validate this model under various ambient pressure conditions. The new hybrid break-up model was composed of the linearized instability sheet atomization (LISA) model for the primary break-up and the aerodynamically progressed Taylor analogy break-up (APTAB) model, with consideration for the deformed droplet shape, for the secondary break-up. The APTAB model was used instead of the Taylor analogy break-up (TAB) model used in the previous hybrid break-up model (the LISA—TAB model). The hybrid break-up models were implemented in modified KIVA code. The calculated results of the spray characteristics, such as the spray tip penetration, spray width, Sauter mean diameter, and droplet velocity, by means of the LISA—APTAB model were compared with the calculated results via the LISA—TAB model and the experimental results found with the laser-induced exciplex fluorescence technique and the phase Doppler anemometry (PDA) system. The calculations and the experiment were performed under ambient pressures of 0.1 MPa and 0.5 MPa and an ambient temperature of 293 K. It was found that the calculated results from the LISA—APTAB model showed good agreement with the experimental results. The discrepancies between the calculated results and the experimental results in spray characteristics were reduced by a considerable margin by adopting the APTAB model instead of the TAB model for the secondary break-up process.

벽면 캐비티 각에 따른 GDI 분무의 벽 충돌 과정에 대한 수치적 연구

A spray-wall impingement process of a hollow-cone fuel spray from the high-pressure swirl injector in the Gasoline Direct Injection (GDI) engine were experimented and calculated at various wall geometries. The Linearized Instability Sheet Atomization (LISA) & the Aerodynamically Progressed Taylor Analogy Breakup (APTAB) model and the Gosman model were applied to model the breakup and the wall impingement process of the hollow-cone fuel spray. The numerical modelings were implemented in the modified KIVA code. The calculation results of spray characteristics, such as a spray development process and a radial distance after wall impingement, compared with the experimental results by the Laser Induced Exciplex Fluorescence (LIEF) technique. The droplet size distribution and the ambient gas velocity field, which are generally difficult to obtain by the experimental methods, were also calculated and discussed. It was found that the radial distance after wall impingement and Sauter Mean Diameter (SMD) decreased with increasing a cavity angle.

Two-phase fuel distribution measurements in a gasoline direct injection engine with an air-assisted injector using advanced optical diagnostics

The fuel distribution and mixture formation processes within an internal combustion (IC) engine can greatly affect its ignition, combustion, pollutant formation, and fuel consumption. For more than two decades the laser-induced fluorescence (LIF) technique has been successfully applied to the study of mixture formation and distribution. However, in a gasoline direct injection (GDI) engine the fuel may exist in either the liquid or vapour phase, and it is therefore advantageous to be able to observe both phases simultaneously. One means of achieving this is the laser-induced exciplex fluorescence (LIEF) technique. This paper details its application to a single-cylinder GDI engine equipped with an air-assisted fuel injection system and full optical access. Naphthalene and N, N-dimethlylaniline (DMA) tracers were used to track the fuel and were excited using the expanded beam from an XeCl excimer laser at 308 nm. An image doubler and optical filtering system was used to collect the images that were recorded from the front of the cylinder. Early and late injection timings were investigated, and the spray characteristics and mixture formation from the two strategies were found to differ considerably. In order to investigate the effect of absorption on the fluorescence signal, the beam was introduced into the cylinder from three directions. Crank angle resolved results, from the start of injection to the end of the compression stroke, are presented, with an image for each illumination direction and injection timing.

Diesel Spray Analysis by Means of Planar Laser-Induced Exciplex Fluorescence

The planar laser-induced exciplex fluorescence (PLIEF) technique has been used to visualize and measure the fuel/air concentration fields in both liquid and vapour phases of direct injection (DI) diesel sprays. Experiments have been performed in a hot rig, which provides a thermodynamic environment realistic for current DI diesel engines, using a blend of 90 per cent hexadecane, 9 per cent α-methyl-naphthalene, and 1 per cent N,N,N',N'-tetramethyl- p-phenylenediamine to separate spectrally the fluorescence signal from the liquid and vapour phases of the diesel. In this paper, measurements have been performed for three injection pressures and three combustion chamber density values. The results on penetration and fuel concentration in liquid and vapour phases are analysed, providing evidence of the similarities between diesel sprays and gaseous turbulent jets.

Mixture Preparation and Combustion in an Optically-Accessible HCCI, Diesel Engine

Mixture Preparation and Combustion in an optically-accessible HCCI, Diesel EnginePlanar laser-induced fluorescence (LIF) imaging techniques have been applied in order to study the mixture preparation and combustion process in a single cylinder, optically-accessible homogeneous charge, compression ignition (HCCI) engine. In particular, the influence of piston bowl geometry on the in-cylinder mixture distribution and subsequent combustion process has been investigated. A new optically-accessible piston design enabled the application of LIF diagnostics directly within the combustion chamber bowl. Firstly, laser-induced exciplex fluorescence (LIEF) was exploited in order to characterise the in-cylinder fuel spray and vapour distribution. Subsequently a detailed study of the twostage CCI combustion process was conducted by a combination of direct chemiluminescence imaging, laser-induced fluorescence (LIF) of the intermediate species formaldehyde (CH2O) which is present during the cool flame and LIF of the OH radical which is subsequently present in the reaction and burned gas zones at higher temperature. Finally, spectrometry measurements were performed with the objective of determining the origin of the emitting species of the chemiluminescence signal. The experiments were performed on a single cylinder optical engine equipped with a direct-injection, common rail injection system and narrow angle injector. The experimental results presented reveal the significant role of the combustion chamber geometry on the mixture preparation and combustion characteristics for late HCCI injection strategies particularly in such cases where liquid impingement is unavoidable. Planar LIF 355 imaging revealed the presence of the intermediate species formaldehyde allowing the temporal and spatial detection of auto-ignition precursors prior to the signal observed by chemiluminescence in the early stages of the cool flame. Formaldehyde was then rapidly consumed at the start of main combustion which was marked not only by the increase in the main heat release but also by the first detection of OH, present within the reaction and burned gas zones. In the case of the flat piston geometry, soot precursors were also detected, as indicated by the strong polycyclic aromatic hydrocarbon (PAH) fluorescence signal observed later in the cycle (from 375 CAD).

Limitations on the use of the planar laser induced exciplex fluorescence technique in diesel sprays

The Planar Laser Induced Exciplex Fluorescence (PLIEF) technique is widely used to visualize and measure the fuel concentration fields in both liquid and vapor phases of DI Diesel sprays. However, the real limitations of the PLIEF technique in Diesel sprays and the accuracy of the results obtained are still a source of controversy. In this work, a complete methodology for maximum penetration and fuel concentration measurements in evaporating conditions in Diesel sprays has been developed and the reliability of the results obtained has been investigated. The methodology includes new procedures for measuring both liquid and vapor phases, adapting, when necessary, correlations available in the literature for calibration. An experimental matrix of nine test points with different injection pressures and combustion chamber densities has been performed. A critical analysis of the different error sources for proper quantification is made. Results have shown that macroscopic features can be accurately determined using the PLIEF technique, but for fuel concentration measurements special considerations have to be taken into account, particularly in the regions where liquid and vapor coexist.

LIEE와 Mie 산란 방법을 이용한 직분식 가솔린 엔진의 스월 및 슬릿 인젝터의 분무 특성

The spray characteristics of DISI injector have a great role in gasoline engine efficiency and emission. Thus, many researchers have studied to investigate the spray characteristics of swirl and slit injectors that are used in a DISI engine. In this study, we tried to provide spray parameters, which affect on the spray characteristics such as injection pressure, ambient pressure and ambient temperature. In addition, we calculated to investigate the break up mechanism of test injectors and obtained to evaluate the spray characteristics. As the ambient pressure increases in case of slit injector, decreases. The laser-induced exciplex fluorescence (LIEF) technique, which is based on spectrally resolved two-color fluorescent emissions, has applied to measure the liquid and vapor phases for on evaporating spray simultaneously. The TMPD/naphthalene proposed by Melton is used as a dophant to detect exciplex signal. The temporal and spatial distribution of liquid and vapor phases during the mixture formation process was measured by this technique. In the LIEF technique, the vapor phase is detected by the monomer fluorescence while the liquid phase is tracked by the exciplex fluorescence. From this experiment, we found that the spray area of the vapor phase is increased with elapsed time after injection and the area of liquid is decreased when the ambient pressure is 0.1MPa. However, the area tends to increase until the end of injection when the ambient pressure is 1.0MPa.

Characterization of an evaporating direct-injected gasoline spray using laser-induced exciplex fluorescence and particle image velocimetry techniques

The purpose of this study is to characterize an evaporating direct-injected (DI) gasoline spray from a high-pressure swirl injector using the laser-induced exciplex fluorescence (LIEF) technique and particle image velocimetry (PIV). A fluorobenzene/diethylmethylamine (DEMA) system was used as the exciplex-forming dopants. The behaviour of the liquid and vapour phases was analysed by image processing. For the analysis of vorticity inside the spray, droplet velocity data obtained by PIV were used. The experiments were performed at two ambient temperatures (293 and 473 K) and three different ambient pressures (0.1, 0.5 and 1.0 MPa). It was found that ambient temperature had a significant effect on the axial and radial growth of the liquid phase of the evaporating spray at atmospheric pressure while it had little effect under elevated pressures. Radial growth of the vapour phase of the evaporating spray was more dominant than axial growth under high temperature and pressure conditions. As the ambient pressure was elevated, the liquid phase of the spray transformed from a hollow cone to a solid cone of bell shape, while the vapour phase varied from a widespread distribution to a compact shape with a locally richer mixture. The evaporating spray could be divided into two spray regions from the analysis of vorticity and the distributions of liquid and vapour phases. The cone region (penetrations of 0.3-0.5) was mainly liquid phase and disappeared rapidly at the end of injection. The mixing region contained the active interaction between entrained air and fuel vapour.

Effects of piston shapes and intake flow on the behavior of fuel mixtures in a GDI engine

The purpose of this study is to investigate the stratification of fuel vapor with different in-cylinder flow, piston cavity and injection timings in an optically accessible engine. Three different piston shapes that are F (Flat), B (Bowl) and R (Re-entrance) types were used. The images of liquid and vapor fuel were captured under the motoring condition using Laser Induced Exciplex Fluorescence technique. As a result, at early injection timing of 270° BTDC, liquid fuel was evaporated faster by tumble flow than swirl flow, where most of fuel vapor were transported by tumble flow to the lower region and both sides of cylinder for the F-type piston. At late injection timing of 90° BTDC, tumble flow appears to be moving the fuel vapor to the intake side of the cylinder, while swirl flow convects the fuel vapor to the exhaust side. The concentration of mixture in the center region was highest in the B-type piston, while fuel vapor was transported to the exhaust side by swirl flow in F and R-type pistons. At the injection timing of 60° BTDC, the R -type piston was better for stratification due to a relatively smaller bowl diameter than the others.

Spray structures and vaporizing characteristics of a GDI fuel spray

The spray structures and distribution characteristics of liquid and vapor phases in non-evaporating and evaporating Gasoline Direct Injection (GDI) fuel sprays were investigated using Laser Induced Exciplex Fluorescence (LIEF) technique. Dopants were 2% fluorobenzene and 9% DEM A (diethyl-methyl-amine) in 89% solution of hexane by volume. In order to study internal structure of the spray, droplet size and velocity under non-evaporating condition were measured by Phase Doppler Anemometry (PDA). Liquid and vapor phases were visualized at different moments after the start of injection. Experimental results showed that the spray could be divided into two regions by the fluorescence intensity of liquid phase: cone and mixing regions. Moreover, vortex flow of vapor phase was found in the mixing region. About 5μn diameter droplets were mostly distributed in the vortex flow region. Higher concentration of vapor phase due to vaporization of these droplets was distributed in this region. Particularly, higher concentration of vapor phase and lower one were balanced within the measurement area at 2ms after the start of injection.

Two-dimensional imaging of fuel-vapour concentration byuse of LIEF technique during mixture formation process in a DIdiesel engine

The laser-induced exciplex fluorescence (LIEF)technique, which is based on spectrally resolved two-colour fluorescentemissions, has been applied to anevaporating diesel spray for the simultaneous separation ofliquid and vapour phases on the two-dimensional images. Theexciplex system is based on the TMPD/naphthalene systemproposed by Melton. The temporal and spatial distribution ofliquid and vapour phases during the mixture formation process was detected by this technique. In the LIEF technique, the vapourphase is detected by the monomer fluorescence while the liquidphase is tracked by the exciplex fluorescence. It is able toprovide quantitative information on the diffusion state forapplying the statistical entropy concept. This concept was putinto practice in the image sets obtained through LIEFmeasurement in order to investigate the diffusion process of vapour phase in the evaporating diesel spray. The lasershadowgraph was taken to deduce the effect of turbulent mixingon the ignition. The experiments were conducted in a directinjection diesel engine with optical access required forthe two-dimensional laser imaging.From the results, it is found that ignition occurs in the regionwhere turbulent mixing has advanced a nearly homogeneousmixture of fuel vapour and air. Multi-sectional images obtainedby LIEF indicate that there is a spatial dependence ofignition on the vapour-phase fuel in the combustion chamber.

Application of Exciplex Chemical Sensor for Visualization of Liquid- and Vapor-Phase Fuel Distribution

Two-dimensional slices of the cross-sectional distributions of fuel images in the combustion chamber were visualized quantitatively using a laser-induced exciplex fluorescence technique. A new exciplex visualization system consisting of 5% N,N-dimethylaniline (DMA)·5%1,4,6-trimethylnaphthalene (TMN) in 90% isooctane (2,2,4-trimethylpentane) fuel was employed. In this method, the vapor phase was tagged by the monomer fluorescence while the liquid phase was tracked by the exciplex fluorescence with good spectral and spatial resolution. The 308 nm (XeCl) line of an excimer laser was used to excite the doped molecules in the fuel and the resulting fluorescence images were obtained with an intensified charge coupled device (ICCD) detector as a function of time. The results demonstrate that at the end of the compression stroke, the liquid fuel was almost vaporized and a dense stratified vapor phase charge developed near the two spark plugs.

A probe to estimate the local fuel concentration in spark ignition engines: design and validation study of catalytic hot wire probe

A catalytic hot wire probe (CHWP) technique has been developed to estimate the local fuel concentration near the spark plug of a spark ignition engine. Knowledge of this local concentration is highly useful in studying the combustion process. The small fuel concentration variation is measured by superimposing a catalysis effect in the thermal balance of the hot wire. Various parameters such as pressure, temperature and sampling velocity have been tested for their effect on the hot wire catalytic response. To validate this CHWP technique, local fuel concentration was also measured by two optical diagnostic techniques: planar laser-induced fluorescence (PLIF) and fuel/air ratio laser-induced exciplex fluorescence (FARLIEF). Comparison with PLIF measurements shows good agreement, and the capabilities and limitations of both techniques. With the FARLIEF/CHWP study, we can characterise the time and spatial variation of the fuel/air ratio in the vicinity of the spark plug in the gasoline direct injection engine.

Quantitative vapour-liquid visualization using laser-induced exciplex fluorescence

We describe an optical technique for quantitative evaluation of the concentrations of coexisting vapour and liquid phases in a spray. A laser-induced exciplex fluorescence visualization system allows time-resolved two-dimensional fluorescence images of vapour and liquid distributions to be acquired simultaneously using two-colour emission. The method is based on the exciplex formers DMA (N,N-dimethylaniline) and 1,4,6-TMN (trimethylnaphthalene) in a non-polar solvent. We report here methods for quantitatively calibrating the liquid and vapour phases of a solution containing 90% isooctane, 5% DMA and 5% 1,4,6-TMN. The evaluation of fluorescence intensities yields quantitative two-dimensional concentration maps of liquid and vapour phases. This technique is expected to find applications in studying mixture formation in diesel or spark ignition engines with spectrally well-separated fluorescence images obtained from the monomer and exciplex constituents dissolved in a fuel.