Laser-induced Exciplex Fluorescence Research Articles

Dense-field spray droplet size quantification of flashing boiling atomization using structured laser illumination planar imaging technique

The statistics of droplets formed by the primary jet breakup of a liquid atomization system are critical since it determines the atomization quality and resultant spray characteristics. However, quantifying the droplet statistics by applying optical diagnostic instruments is challenging because of the dense population of droplets near the nozzle exit. In a flash boiling spray, the more rapid breakup produces a higher optical dense droplet cloud, which is more challenging. Besides, the interference from the vapor phase heavily evaporated is more problematic. In this study, we incorporated an in-house developed laser-induced exciplex fluorescence (LIEF/Mie) based droplet sizing technique to quantify the near-field droplet characteristics of a single jet flash boiling spray, which can separate the liquid spray from the vapor phase to eliminate the influence of the vaporization. To mitigate the interference of multiple scattering in the dense region, a 3-phase structured laser illumination planar imaging (3p-SLIPI) facility was implemented into the measuring system to modulate the laser beam. The combination of these two unique techniques succeeded in obtaining accurate quantification of a planar droplet size distribution of this flash boiling spray. The 3p-SLIPI-LIEF/Mie measurements were validated against phase Doppler interferometry (PDI) droplet size measurement results. It is found from this study that the SLIPI-LIEF/Mie results are more accurate and more appropriate for simulation or theoretical analysis compared to LIEF/Mie results. The investigation of the flash boiling sprays under various superheat conditions was conducted using this technique. The quantitative near-field droplet size information reveals the prompt atomization mechanism of the flash boiling sprays. Comparing to the non-flash boiling sprays, the SMD is much smaller, the distribution is more uniform, and the tendency becomes more significant as the superheat degree increases. The SLIPI-LIEF/Mie results show that there is no liquid core region or sudden droplet size reduction due to flash boiling secondary micro-explosion breakups. Rather, a central dense region formed due to rapid expansion and evaporation at the exterior zones of the flash boiling spray was found.

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

Gasoline spray characteristics using a high pressure common rail diesel injection system by the method of laser induced exciplex fluorescence

The liquid and vapor phases of gasoline sprays were investigated using a common rail diesel injection system in a constant volume vessel by laser induced exciplex fluorescence (LIEF) and compared with diesel sprays. The effects of injection pressure and ambient pressure on gasoline sprays were measured under evaporating conditions. The results reveal that: the liquid phase penetration has a little change as the fuel is injected, while the vapor phase penetration increases constantly with obvious branch-like structures generated at the downstream vapor phase. The spray volume and mass of entrained gas both increase roughly linearly versus the mass of injected fuel. Compared with diesel sprays in similar conditions, gasoline sprays display a significantly shorter liquid penetration, while a smaller difference between the vapor penetration lengths of gasoline and diesel sprays is observed. The maximum equivalence ratio of gasoline sprays is still more than 2 at selected conditions. As the ambient pressure increases, the penetration lengths of liquid and vapor phases both reduce, and the mean equivalence ratio also decreases with a drop of spray volume. Conversely, the mass of entrained gas rises continually. With the increase of injection pressure, the liquid phase penetration length maintains almost no change. However, the vapor penetration length shows a declined trend during the injection pressure rise at iso-injected fuel mass condition. The spray volume and mass of entrained gas give a slight reduction when the injection pressure enhances. Oppositely, the mean equivalence ratio presents an increased tendency with similar mass of injected fuel.

Quantitative Analysis of Vapor Formation Process in Gasoline Spray in Evaporation Field with Laser Induced Exciplex Fluorescence Method (LIEF Method)

Quantitative Analysis of Vapor Formation Process in Gasoline Spray in Evaporation Field with Laser Induced Exciplex Fluorescence Method (LIEF Method)

Evaporation and condensation of flash boiling sprays impinging on a cold surface

Flash boiling spray is considered a potential solution for addressing spray impingement related issues such as pool fire caused by spray-wall interactions, in addition to its benefit of improving fuel-air mixing in engine cylinders. In comparison to typical sub-cooled spray impingement, flash boiling spray impingement can be affected by more complicated heat transfer and the mass transfer processes, including fuel vapor condensation near the wall when the temperature is very low. This work aims to investigate the wall film formation when flash boiling sprays impinge on a cold surface. Laser-induced exciplex fluorescence (LIEF) technique was used in this investigation to separate the fluorescent signals from the liquid phase and the gas phase during spray impingement. The simultaneous measurement of the thickness and temperature distributions of the wall film on a transparent plate were conducted, and corresponding heat and mass transfer of the film were analyzed. Generally, flash boiling impingement on cold surface would deposit less film mass on the wall than sub-cooled sprays, however it was still much larger than flash boiling spray impinging on room temperature plate. It was found that under flash boiling conditions, the combination of fine droplets and a large quantity of vapor impinged on the wall to form much less film. As wall temperature decreased, evaporation suppression and condensation effects became increasingly significant to dominate the wall film formation and development.

Quantitative analysis of vapor concentration in unsteady evaporating gasoline spray with LIEF method (2nd Report：Analysis of mixture-liquid two-phase characteristics)

This paper reports the quantitative analysis of vapor concentration in the mixture-liquid two-phase region with Laser Induced Exciplex Fluorescence（LIEF）for the unsteady evaporating gasoline type spray. These methods were used to analyze the mixture-liquid two-phase characteristics in the process of mixture formation. The exciplex fluorescence method, developed by Melton, is a method to optically separate the fluorescence of the liquid and vapor phases, but quantitative analysis of vapor concentration in the mixture-liquid two-phase e region is difficult due to fluorescence cross talk. In this study, the quantitative analysis of vapor concentrations using the average temperature of mixing was used to quantify the vapor concentrations in the mixture-liquid two-phase region by image processing that considers the fluorescence cross talk between the liquid and vapor phases. As a result, the vapor concentration, the temperature of the mixture, and the concentration distribution of the liquid phase in the mixture-liquid two-phase region of the spray cross section were obtained. Based on these results, the correlation diagram between vapor concentration and fluorescence intensity of the liquid phase and the relative frequency distribution of vapor concentration were calculated. A comparison was made between the characteristic values of the fuel and the temperature under the experimental conditions. It was found that the spray consisted mainly of a region where the liquid phase was almost absent and dilute vapors were formed, and a region where the liquid phase was present and formed dense vapors. The same analysis was performed to evaluate the process of vaporization of the spray mixture at different times from the start of injection.

Lifetime-filtered laser-induced exciplex fluorescence for crosstalk-free liquid-vapor imaging.

Laser-induced exciplex fluorescence is a well-established technique for liquid-vapor imaging in evaporating sprays that offers phase-dependent spectrally separated emission. However, the accuracy of this approach is limited by substantial crosstalk from the liquid to vapor phase signals. This Letter shows the use of a combination of spectral and temporal filtering to reduce this crosstalk by three orders of magnitude and eliminate the need for temperature-dependent crosstalk corrections in the N,N-diethylmethylamine/fluorobenzene system. The relative decay rates of the liquid and vapor signals are quantified and show crosstalk-free imaging for monodisperse evaporating droplets over a wide range of exciplex tracer concentrations.

Quantitative Evaluation of Gasoline Spray in Evaporation Fields Using Laser Induced Exciplex Fluorescence Method

Quantitative Evaluation of Gasoline Spray in Evaporation Fields Using Laser Induced Exciplex Fluorescence Method

Analysis of evaporation characteristics and heat transfer for flash-boiling sprays

In order to improve engine thermal efficiency with reduced emissions, substantial studies have been carried out to improve the atomization and evaporation of fuel sprays, as well as fuel-air mixing in the engine cylinder. It has been found that flash-boiling spray is an effective way to achieve these goals. To further understand the evaporation characteristics and hear transfer phenomena between flash-boiling sprays and ambient gas, liquid temperature and vapor concentration of flash-boiling spray were measured quantitatively using laser induced exciplex fluorescence (LIEF) technique. The effects of fuel temperature, ambient pressure and superheated index (Pa/Ps) on the evaporation characteristics and heat transfer of fuel spray were investigated. The experimental results demonstrate that compared with non-flash-boiling spray, the rapid evaporation of flash-boiling spray leads to a higher vapor concentration along the center line. Meanwhile, the remarkable amount of energy absorbed from liquid phase of flash-boiling spray results in a faster liquid temperature decrease along the center line. When hot fuel is injected into cold environment, increasing fuel temperature leads to an increase of heat transfer from fuel spray to ambient gas because of the higher temperature difference between them. On the other hand, increasing ambient pressure decreases the evaporation rate of fuel spray due to the higher Pa/Ps. As such, the energy absorbed for fuel evaporation from liquid phase of spray is decreased, the heat transfer from fuel spray to ambient gas is increased.

Spray and flame characteristics of wall-impinging diesel fuel spray at different wall temperatures and ambient pressures in a constant volume combustion vessel

The qualitative mixture concentration distributions and the flame characteristics of wall-impinging diesel fuel spray at different wall temperatures and ambient pressures were investigated in a high-temperature high-pressure constant volume combustion vessel. The techniques of laser induced exciplex fluorescence (LIEF) and flame natural luminosity imaging have been used to visualize the liquid and vapor phases of the spray and the flame development process respectively. Results reveal that, as the wall temperature increases, the vapor-rich field extends to the region close to the wall and the area of high vapor fluorescence intensity increases. However, the wall temperature only has a little influence on the liquid phase of the spray. The flame luminosity intensity increases and the ignition delay decreases as the wall temperature increases. Cases at higher wall temperature show more soot emissions, higher soot formation and oxidation rate. In the condition of Pa = 4 MPa, the fuel has evaporated completely before impinging on the wall. The flame area and height at different wall temperatures are nearly the same. The ignition position is observed near the impingement point. However, in the condition of Pa = 2 MPa, there are some liquid phase of the fuel impinging on the wall. The flame area and height increase as the wall temperature increases. The ignition position is always observed in the wall jet region. The distance between ignition position and the axis of the spray decreases as the wall temperature increases.

Study on Fuel Distribution of Wall-Impinging Diesel Spray under Different Wall Temperatures by Laser-Induced Exciplex Fluorescence (LIEF)

Spray wall-impingement has large effects on the pollutant emissions and thermal efficiency of engines. Different wall temperatures can affect the gas–liquid phase transition of the spray, and the temperature gradient from the ambient to the wall will cause a different mixture process in the region nearby the wall. However, there are few studies on spray wall-impingement with different wall temperatures, particularly on the liquid–gas phase transition of the spray. Therefore, in this paper, the effects of different wall temperatures on spray-impingement have been investigated in a high-temperature, high-pressure constant volume combustion vessel. Cooling equipment was used to adjust the temperature difference between the wall and the ambient gas. n-Dodecane was chosen as the diesel surrogate to study the spray process. The spray wall-impingement was tested by changing the injection pressures (Pi) and wall temperatures (Tw). The ambient temperature (Ta) and ambient pressure (Pa) were kept constant at 773 K and 4 MPa, and the distance (L) between wall and injector was set to 35 mm to mimic the radius of the combustion chamber in heavy-duty diesel engines. A laser-induced exciplex fluorescence (LIEF) technique was used to probe the vapor and liquid phases of the injected fuel. Results show that the liquid phases of the spray do not reach the wall except in the condition of low wall temperature and high injection pressure. The liquid penetration develops and then becomes constant after 1 ms from the start of injection. With the increase of injection pressures (600–1600 bar), the liquid concentration of the spray decreases; however the liquid penetration decreases insignificantly. The wall temperature and the injection pressure have little influence on the liquid interpenetration process. For the vapor phase of the spray, the high concentration regions (equivalence ratio (φ) > 1) mainly distribute in the area of 10 mm away from the impact point on the wall. With the decrease of wall temperatures, the high-concentration regions are enlarged at the near wall regions. However, at the injection pressure of 1600 bar, the influence of wall temperatures on the equivalence ratio is small. The decreasing wall temperature deteriorates the mixing process of the fuel and ambient gas, but the effect is weakened with the increase of injection pressure.

Numerical study of the impact on high-pressure and evaporating spray behavior of nozzle cavitation at typical diesel engine conditions

In order to more accurately reproduce diesel sprays a strategy including measurement of nozzle inlet pressure at the realistic diesel injection condition, modeling of nozzle cavitating flow and detailed coupling of nozzle exit flow and spray was presented, moreover, the validity of this strategy was firstly verified against the quantitative spray data obtained by planar laser induced exciplex fluorescence (PLIEF) technique. Based on the above strategy, the effect of cavitation phenomenon on spray formation at the typical diesel engine condition was further evaluated. The final numerical results mainly clarified that the contribution of cavitation phenomena to primary breakup is quite appreciable, and subsequently the evolution of the high-pressure and evaporating diesel spray structure greatly changes as cavitation occurs inside fuel injection nozzles. Moreover, evaluating the effects of cavitation phenomena on realistic diesel spray cannot be only confined to primary breakup or near-nozzle field.

Evaporation of Gasoline-Like Sprays from an Outwards-Opening Injector Studied with LIEF

The design and operation of the fuel injection system plays an important role to achieve a suitable fuel distribution in the cylinder. The liquid fuel should evaporate and mix well with air, and avoid overly rich or lean zones as well as wall and piston wetting. Spray imaging in a constant-volume, constant-pressure chamber, provides a well-controlled environment for detailed characterization of spray development under different operational conditions. In this study planar laser-induced exciplex fluorescence is used since it enables separate imaging of both vapor and liquid phase of fuel simultaneously. As a surrogate fuel for gasoline n-hexane is used with fluorobenzene and diethylmethylamine added as exciplex-forming tracers. Fluorescence excitation is carried out with a parallel laser sheet from the fourth harmonic light of a Nd: YAG-laser (266 nm). Exciplex fluorescence images from liquid phase and monomer fluorescence images from vapor phase can be acquired by a single UV-sensitive CMOS camera equipped with a stereoscope having filters selectively transmitting monomer fluorescence at 295 nm and exciplex fluorescence at 355 nm. Sprays from an outwards-opening injector with an initial hollow-cone shape were investigated at gas pressures above atmospheric. The expanding conical fuel sheet breaks up and a torus-shaped vortex structure is formed with the vortices continuing to expand after the end of injection, which is the typical development using this type of injector. Fuel vapor is firstly observed at the same locations as the liquid drops, and is then accumulated into the center of the vortices. The fuel evaporation was found to be very slow at room temperature, whereas the vapor fraction was significant already at the end of injection above 40°C, and an even faster evaporation was observed above 55°C.

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.

Droplet temperature measurement based on 2-color laser-induced exciplex fluorescence

Measurements of liquid phase temperature distributions in liquid–vapor co-existing conditions (such as in evaporating sprays) are important to understand the physics of droplet evaporation. The techniques based on laser-induced fluorescence are not suitable for evaporating case since both liquid and vapor phases emit fluorescence with the same wavelength. In this study, the fluorescence from liquid and vapor phases was separated by use of laser-induced exciplex fluorescence (LIEF) technique. Two fluorescence bands from the liquid phase fluorescence spectra were detected simultaneously, and their intensity ratio was correlated to the liquid phase temperature. For the LIEF imaging system, FB-DEMA-n-hexane was selected as it was a typical LIEF system for the vapor concentration diagnostic, and thus easily to be extended to a simultaneous diagnostic on the vapor concentration and the droplet temperature. The fluorescence spectra were obtained in the temperature range from 303 to 423 K. The effects of liquid temperature, liquid pressure, dopant concentration and laser energy on the temperature measurement were investigated. The results show a good linear relationship between the fluorescence ratio and the temperature function. Increasing the dopant concentration can raise the signal-to-noise ratio but deteriorate temperature sensitivity. The optimal range of the dopant concentration was found between 0.1 % and 0.5 %. After calibration, the technique was applied to a monosized droplet stream, and the measurement results demonstrated excellent measurement accuracy with error below 1 % in the range of 303–423 K.

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.

Atomization and vaporization for flash-boiling multi-hole sprays with alcohol fuels

The spray structural changes and vaporization processes for flash-boiling multi-hole sprays over a broad range of superheated conditions were investigated using Mie-scattering and Laser-induced-exciplex-fluorescence (LIEF) optical techniques. The fuel property effects were examined by characterizing n-hexane, methanol and ethanol fluids over a wide range of conditions consistent with that found in today’s spark-ignition-direct-injection (SIDI) engines. The macroscopic spray structure was quantified using spray penetration, spray-plume width and normalized distance between spray plumes. These structural parameters were correlated to the ratio of the ambient pressure to saturation pressure (Pa/Ps) that represents the superheated degree. Three continuous regions were identified by quantifying the spray transformation with increasing superheated degree; namely the non-flash-boiling, transitional flash-boiling and flare flash-boiling regions. Two critical values of Pa/Ps were identified, where the flash-boiling and spray collapsing transitions occurred at Pa/Ps values of 1.0 and 0.3, respectively. The evaporation process was examined using the LIEF optical technique for n-hexane, providing the relative vapor quantity throughout the spray transformation process. The correlations of the spray structural change and extent of vaporization with increasing superheated degree provided good insight into the mechanisms responsible for the observed behaviors during flash-boiling conditions.

Direct microscopic image and measurement of the atomization process of a port fuel injector

The main objective of this study is to observe and investigate the phenomena of atomization, i.e. the fuel break-up process very close to the nozzle exit of a practical port fuel injector (PFI). In order to achieve this objective, direct microscopic images of the atomization process were obtained using an ultra-high-speed video camera that could record 102 frames at rates of up to 1 Mfps, coupled with a long-distance microscope and Barlow lens. The experiments were carried out using a PFI in a closed chamber at atmospheric pressure. Time-series images of the spray behaviour were obtained with a high temporal resolution using backlighting. The direct microscopic images of a liquid column break-up were compared with experimental results from laser-induced exciplex fluorescence (LIEF), and the wavelength obtained from the experimental results compared with that predicated from the Kelvin–Helmholtz break-up model. The droplet size diameters from a ligament break-up were compared with results predicated from Weber's analysis. Furthermore, experimental results of the mean droplet diameter from a direct microscopic image were compared with the results obtained from phase Doppler anemometry (PDA) experimental results. Three conclusions were obtained from this study. The atomization processes and detailed characterizations of the break-up of a liquid column were identified; the direct microscopic image results were in good agreement with the results obtained from LIEF, experimental results of the wavelength were in good agreement with those from the Kelvin–Helmholtz break-up model. The break-up process of liquid ligaments into droplets was investigated, and Weber's analysis of the predicated droplet diameter from ligament break-up was found to be applicable only at larger wavelengths. Finally, the direct microscopic image method and PDA method give qualitatively similar trends for droplet size distribution and quantitatively similar values of Sauter mean diameter.

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