Laser Induced Exciplex Fluorescence Technique Research Articles

Experimental investigations of the phase change impacts on flash boiling spray propagations and impingements

Flash boiling atomization technique has been considered a potential method for spray breakup performance enhancement utilizing the micro-explosion of superheated liquid. But the strong evaporative features of flash boiling atomization are not thoroughly investigated and understood. For instance, the impact of flash boiling atomization on the liquid mass fraction inside the spray plume and the influence on the formation of spray impingement wall film are not clear. Therefore, this work focuses on the phase change impacts of flash boiling atomization and measures the volume flux of the liquid component during flash boiling atomization using phase Doppler interferometry. This work further investigates the consequence of flash boiling spray collapse in the aspect of spray impingement, with the two-dimensional thickness and temperature distribution measured by laser-induced exciplex fluorescence technique. Both free sprays and impinging sprays with the same settings (and different temperatures) are examined for compassion. This investigation verifies that due to the phase change of flash boiling atomization, the liquid component inside the plume drops significantly so that the wall impingement performance cannot be evaluated with spray penetration length only. Such observations also hold even spray collapse takes place using a multi-hole fuel injector.

Experimental Analysis of Mixture Formation Process in Gasoline Spray with Laser Induced Exciplex Fluorescence Technique

Experimental Analysis of Mixture Formation Process in Gasoline Spray with Laser Induced Exciplex Fluorescence Technique

Experimental study of flash boiling spray vaporization through quantitative vapor concentration and liquid temperature measurements

Flash boiling sprays of liquid injection under superheated conditions provide the novel solutions of fast vaporization and better air–fuel mixture formation for internal combustion engines. However, the physical mechanisms of flash boiling spray vaporization are more complicated than the droplet surface vaporization due to the unique bubble generation and boiling process inside a superheated bulk liquid, which are not well understood. In this study, the vaporization of flash boiling sprays was investigated experimentally through the quantitative measurements of vapor concentration and liquid temperature. Specifically, the laser-induced exciplex fluorescence technique was applied to distinguish the liquid and vapor distributions. Quantitative vapor concentration was obtained by correlating the intensity of vapor-phase fluorescence with vapor concentration through systematic corrections and calibrations. The intensities of two wavelengths were captured simultaneously from the liquid-phase fluorescence spectra, and their intensity ratios were correlated with liquid temperature. The results show that both liquid and vapor phase of multi-hole sprays collapse toward the centerline of the spray with different mass distributions under the flash boiling conditions. Large amount of vapor aggregates along the centerline of the spray to form a “gas jet” structure, whereas the liquid distributes more uniformly with large vortexes formed in the vicinity of the spray tip. The vaporization process under the flash boiling condition is greatly enhanced due to the intense bubble generation and burst. The liquid temperature measurements show strong temperature variations inside the flash boiling sprays with hot zones present in the “gas jet” structure and vortex region. In addition, high vapor concentration and closed vortex motion seem to have inhibited the heat and mass transfer in these regions. In summary, the vapor concentration and liquid temperature provide detailed information concerning the heat and mass transfer inside flash boiling sprays, which is important for the understanding of its unique vaporization process.

Simultaneous imaging of diesel spray atomisation and evaporation processes in a single-cylinder CR diesel engine

In direct injection diesel engines, combustion and formation of pollutants are directly influenced by the spatial and temporal distributions of the injected fuel. In this study mixture formation during the pre-combustion phase of a diesel engine was investigated using the laser-induced exciplex fluorescence (LIEF) technique. The main purpose of this investigation was to develop an experimental setup capable of providing the full-field view of both liquid and vapour phases of evaporating diesel sprays during the fuel injection process inside the combustion chamber of a diesel engine with optical access. An expanded laser beam was employed for full combustion chamber visualisation. In this study two model fuels were tested; one consisted of 89% decane, 10% α-methyl-naphthalene and 1% TMPD and the other 88% decane, 10% α-methyl-naphthalene and 2% TMPD. The spray atomisation and evaporation processes during the pre-combustion phase of a diesel engine were measured at an injection pressure of 1200bar and the engine speed of 1500rpm. The results demonstrated the capability of the full-field LIEF technique in simultaneous imaging of liquid fraction and fuel vapour distribution during high pressure fuel injection process. It also highlighted the effect of dopant concentration on the fluorescence intensity of liquid and vapour signals. The exciplex system containing 1% TMPD produced better visualisation of the liquid phase, though the crosstalk in the vapour phase precluded accurate detection of the vapour phase signal. In contrast, the exciplex system containing 2% TMPD resulted in satisfactory visualisation of the vapour phase; however the intensity of the liquid phase was compromised as a result. This was presumed to be mainly due to the spectral shift of the exciplex species and/or TMPD decomposition at elevated temperatures and pressures.

Quantitative Study of Concentration and Temperature of a Diesel Spray by Using Planar Laser Induced Exciplex Fluorescence Technique

Quantitative Study of Concentration and Temperature of a Diesel Spray by Using Planar Laser Induced Exciplex Fluorescence Technique

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.

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.

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.

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.

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.