Long-distance Microscopy Research Articles

Fuel temperature and injection pressure influence on the cold start GDI sprays

Cold start in gasoline direct injection engines (GDI) is a critical issue that significantly impacts fuel consumption and emissions. Therefore, it is essential to investigate and improve the spray and air-fuel mixing processes during cold starts. This study employed a complimentary set of optical diagnostic techniques, including line-of-sight(extinction, Schlieren, and long-distance microscopy) and 3D computed tomography (CT), to characterize and understand the cold-start spray dynamics under various fuel temperature and injection pressure conditions. The experiments were conducted in a constant volume spray vessel and the fuel temperature was varied using a coolant circulator, with temperatures reaching as low as -7 °C to simulate cold-start conditions. The cold fuel exhibited longer liquid/vapor penetration lengths compared to hot fuel under low injection pressure conditions. This deterioration in spray characteristics was attributed to the attenuated fuel evaporation and reduced entrainment of ambient air. The 3D spray visualization obtained through the CT algorithm, particularly the cut plane images, revealed that plumes with low fuel temperatures had narrower individual plume widths, resulting in minimized plume-to-plume interaction. Microscopic imaging further confirmed this observation which showed separate plumes in the near-nozzle region for cold fuel conditions. Meanwhile, hot fuel under high injection pressure conditions exhibited complete plume collapsing, leading to a significant amount of liquid fuel remaining in the spray core. The liquid penetration reached 70 mm during the injection period, potentially can cause wall wetting on the piston top or cylinder wall. Based on the experimental findings, this study suggests the application of multiple injections with a moderate level of injection pressure for optimized engine performance and reduced emissions during cold starts.

Experimental study of Gas-To-Liquid (GTL) and diesel fuel blends evaporation behaviour and droplet lifetime through Leidenfrost effect

Gas to Liquid (GTL) fuel is considered a clean alternative fuel and has been given much attention to replacing conventional fuel. Evaporation behaviour and droplet lifetime are critical aspects that need to be determined as these aspects can affect the fuel spray properties and improve the combustion process. In this study, a set of GTL–diesel fuel blends are prepared, which are G20, G50, G80, and G100 (the number represents the GTL percentage fuel ratio in the fuel blends). Subsequently, using a single droplet drop test on the hot plate (Leidenfrost effect), the GTL fuel blends droplet lifetime and evaporation behaviour are visualised using a high-speed camera connected to long-distance microscopy. An image processing system and a MatLab were used to measure and analyse the droplet evaporation data. Comparing the fuel characterisation of the GTL fuel and conventional diesel fuel, the GTL fuel shows a lower value in all properties tested except for the calorific value, cetane number, and vapour pressure. The qualitative results through the Leidenfrost effect have shown that increasing the GTL fuel ratio (20% to 100%) decreases the heating phase duration (37.3 % to 14.4 %) and evaporation rate (1.29 mm 2/s to 1.10 mm 2/s) while increasing their steady evaporation phase duration and droplet lifetime. The shorter period to achieve steady evaporation is beneficial as the spray inside the chamber has a limited time to evaporate for combustion.

Long-distance polarizing microscope system combined with solenoid-type magnet for microscopy and simultaneous measurement of physical parameters.

We have developed a long-distance polarizing microscope system combined with a solenoid-type superconducting magnet. By inserting an infinity-corrected objective lens into the magnet, direct or polarizing microscope images are observed in magnetic fields of up to 12T at various temperatures down to 2K. Through magneto-optical measurements in the transmission geometry, the local magnetization process of a transparent magnet is evaluated in areas of 10 × 10 µm2. This system enables simultaneous measurements of other physical properties over a wide range of temperatures and magnetic fields. The basic principle of the proposed long-distance microscopy can be applied to imaging experiments in various research fields, particularly biology and chemistry.

A phenomenological model for near-nozzle fluid processes: Identification and qualitative characterisations

It is well established that emissions and inefficiencies primarily arise from localised fuel rich regions, which do not undergo complete combustion. In order to achieve significant reductions in NOx and soot, modern engines employ multiple injection and flow rate profiling strategies. However, during the end of each injection event, the needle restricts the internal flow of fuel, rapidly reducing the inertia of the emerging spray. Atomisation is inhibited and large fluid structures are released into the cylinder. Once the flow subsides, fuel films remain on the nozzle surface well into the cycle. Fuel residing in the nozzle cavities emerges as the cycles progresses, amalgamating with the spray deposited films. The surface-bound fuel presents an ideal environment for deposit forming reactions and a medium for precursors to adhere onto. In order to better understand these processes we performed measurements of injection transients inside an optical reciprocating rapid compression machine, using high-speed long-distance microscopy to obtain detailed characterisations of fuel films on the surface of an injector. We summarise our observations into a new phenomenological model which describes the uncontrolled release of fuel over an entire engine cycle. This model identifies the micro-scale processes that lead to the ejection, accumulation and vaporisation of fuel in-between injection events. Characterisation of these critical, transient processes can support mitigation strategies that inhibit pollutants and the formation of injector deposits.

Fluidization of lactose carrier powders through normally directed airflow: The effect of recirculation and particle size

This study investigates the fluidization of lactose carriers from a powder bed subjected to a normal force in a channel flow using high-speed imaging, particle image velocimetry (PIV), and high-speed, long-distance microscopy (HS-LDM). Pharmaceutical lactose carriers (LH200 and SV010) with different cohesiveness and fines percentages were examined in this study. Airflow velocities in the range of1.4 m/s and7m/s were tested, corresponding to flow rates ranging from 20 to 100 L/min. The use of HS-LDM in tandem with PIV has enabled measurement of the slip factor between particles and conveying airflow as well as metrics that help to identify dose homogeneity as a function of location in the channel flow.The results indicate a lower slip ratio and a larger change in powder particle size bands percentages along with channel height in the region near the powder bed, because of flow recirculation and higher velocity fluctuation observed in that region.

Local dynamics of pharmaceutical powder fluidization using high speed long distance microscopy and particle image velocimetry

The local dynamics of fluidized pharmaceutical carrier powders in a turbulent channel flow was studied using particle image velocimetry (PIV) and High-speed, long-distance microscopy (HS-LDM). Four different lactose powders which have been used as a drug carrier in dry powder inhalers were used in this study. These powders have median powder particle diameters ranging between 61 and 121 µm. Air flow velocities ranging between 13.3 m/s and 66.7 m/s were examined. In addition, the effect of grid blockage ratio (ranging from ~25% to ~40% of the area of channel cross-section) was also investigated. Results show that the high-speed, long-distance microscopy (HS-LDM) technique was able to capture the mean velocity of the particles, and the results corresponded well with the PIV measurements. Results from the high-speed, long-distance microscopy (HS-LDM) method also demonstrate that the span of particle velocity closely follows that of the particle size distribution both for cohesive and non-cohesive powders. This study contributes towards an improved understanding of pharmaceutical carrier dynamics in turbulent channel flows and demonstrates how advanced image processing can be used to capture local particle dynamics.

A thermally-limited bubble growth model for the relaxation time of superheated fuels

We propose a novel approach to evaluate the relaxation time of vapor bubble growth in the context of the flash boiling of a superheated liquid. In alternative to the empirical correlation derived from superheated water experiments almost fifty years ago, the new model describes the thermally-dominated growth of vapor bubbles in terms that are dependent on the local Jakob number (the ratio of sensible heat to latent heat during phase change) and the number density of vapor bubbles. The model is tested by plugging the resulting relaxation time into the Homogenous Relaxation Model (HRM). Flash-boiling simulations carried out with HRM are compared with n-pentane (C5H12) injection and boil-off experiments conducted with a real-size, axial-hole, transparent gasoline injector discharging into a constant-pressure vessel. The long-distance microscopy images from the experiments, processed to derive the projected liquid volume (PLV) of the spray, provide a unique set of time-resolved validation data for direct fuel injection simulations. At conditions ranging from flaring to mild and minimal flash boiling, we show that switching to the new relaxation time improves the agreement with the measured PLV profiles with respect to the standard empirical model. Particularly at flaring conditions, the predicted increase in gas cooling caused by rapid vapor production is shown to be more consistent with the observed boil-off.

Volume measurement of atomizing fragments using image slicing

The complete structure of atomizing liquid fragments can be analysed by employing multi-dimensional visualization techniques. These techniques require advanced image processing, where objects from multiple views are matched to extract information. However, in higher Weber number sprays, owing to a population of randomly shaped atomizing fragments, matching objects from different imaging frames is very challenging. This paper presents a novel image processing technique to extract volumetric information without a need for matching. The spray is visualized using high speed long-distance microscopy from two perpendicular angles. The developed in-house script employs the principle of image discretisation, where each image is divided into a number of slices and the individual slice from each camera is matched to compute the liquid volume fraction in each image. The volume of individual objects is calculated based on their planar area and orientation. An error analysis is performed using dozens of three-dimensional virtual models of fragment like shapes with known volume. The paper concludes with an analysis of spray volume measured for a range of real twin-fluid atomized cases. The proposed methodology enables the measurement of volume fragments with reasonable accuracy in atomizing sprays with exit Weber numbers of up to around 100. This is a significant extension from an earlier limit of We ~ 20 [1].

A Holistic View on Urea Injection for NOx Emission Control: Impingement, Re-atomization, and Deposit Formation

The injection of urea-water solution (UWS) sprays into engine-close selective catalytic reduction (SCR) systems faces developers with complex challenges regarding two-phase flow and deposit formation. Potential spray impact on the static mixing elements inside the exhaust duct can lead to accumulation of liquid film and solid deposits, which may result in detachment of large UWS droplets at the trailing edges of the mixer blades. Present work focuses on the mechanisms of film and deposit formation within the scope of a joint study on spray preparation in the mixing section of SCR systems. A production type SCR exhaust system is used for optical investigations on gas flow and UWS droplets at the mixing element using Laser Doppler Anemometry (LDA) and Long Distance Microscopy (LDM) with a high-speed camera. Measured shear flows inside the mixing element are up to three times higher than the flow velocity upstream the mixer. Here, a decrease of droplet diameters of detached droplets is revealed with increased shear flow velocity. Characteristic urea deposits are created in a laboratory test bench and their surface structure is measured by use of Confocal Microscopy. Results show highly rough surface structures, which are used as input parameters together with the flow measurements to define the computational domain at the trailing edge of the mixer blades. Smoothed Particle Hydrodynamics (SPH) method is adopted for the numerical simulation of the transient two-phase flow and validated by LDM high-speed recordings of droplet detachment. Comparisons reveal a distinct influence of solid depositions on the re-atomization of the liquid film at the rear edge of the mixer blade. By interface resolving simulations with a phase-field method, the wall impingement of a sequence of large secondary droplets and associated film formation are studied. The results indicate that UWS film formation at the exhaust pipe wall may be reduced by hydrophobic surfaces and high gas velocities near the wall.

Sterilization of Biofilm in Foam Using a Single Cavitation Bubble

This article presents the sterilization of bacteria using cavitation bubbles. Cavitation generated by ultrasound creates a cavitation cloud. Therefore is more advantageous to generate the cavitation bubbles by laser-induced breakdown, because it is possible to generate individual bubbles for the purpose of study single impact and physical mechanism of acting. The cavitation bubble is generated by a Nd: YAG 532nm laser beam, a short 10ns pulse. Here, we used optics to focus the laser beam and a high-speed camera to visualize characteristics the bubble. We used the method of long-distance microscopy and shadowgraph lightening for the visualization. We used the particle image velocimetry (PIV) method to determine the interaction of the bubble with the surrounding liquid and solid surface. The main goal of the research is to use cavitation to sterilize bacteria and biofilm in impact of single bubble collapse on living cells.

Experimental and numerical study on the initial tip structure evolution of diesel fuel spray under various injection and ambient pressures

The formation and evolution of the initial spray structure of diesel fuel in the near-nozzle region under different injection and ambient pressures were studied. A visualisation experiment study on diesel fuel using long-distance microscopy and nanosecond-pulse flashlight was performed. Four types of spray tip structure were identified and named as ‘needle’, ‘bubble’, ‘mushroom’ and ‘umbrella’. The obtained high-resolution images revealed that both injection pressure and ambient pressure had a significant influence on the evolution of the spray tip structure. The measurement of the spray penetration and spray angle showed that the increase of injection pressure enhanced spray dispersion while the increase of ambient pressure exhibited an opposite effect. In order to provide a better understanding on the formation mechanism, a numerical study based on large eddy simulation (LES) and volume of fluid (VOF) interface capturing technique was conducted.

High-precision in-situ size measurements of single microparticles in an RF plasma

An in-situ method to measure the radii of single microparticles in plasmas with high precision is presented. The particles are trapped in the plasma sheath and illuminated with laser light. Using out-of-focus imaging and polarizing optics, the angle- and polarization-resolved scattering intensities are measured and compared to Lorenz-Mie theory. A two-stage fit procedure is introduced to obtain the complex refractive index in addition to the particle radius. Complementary long-distance microscopy measurements are performed to compare with fit results. The method is applied to particles of different materials used in complex plasma research to measure etch rates due to plasma inherent processes.

A combination of imaging techniques reveals a new method of measuring irregular dust in plasmas

By bringing together long-distance microscopy with phase-resolve resonance, researchers have measured the charge properties of zinc oxide double tetrapod dust particles.

Charging of an irregularly shaped particle in the sheath of an rf plasma

The charging process of micrometer-sized irregularly shaped particles in the sheath of a radio frequency discharge is measured using a combination of long-distance microscopy and the phase-resolved resonance method. The applicability of the method is shown for a cylindrical zinc oxide particle by measuring its mass density. A particle with more complex geometry is compared to a spherical polymethyl methacrylate particle to investigate the charging of irregularly shaped particles in detail. The results are similar to the charging of the smallest enclosing sphere suggesting that the charging process is independent of the shape of the particle. Furthermore, molecular dynamics simulations were performed, which support the experimental results.

Size and density evolution of a single microparticle embedded in a plasma

This article presents two measurement techniques to determine the diameter of a single dust particle during plasma operation. Using long-distance microscopy (LDM), the particle is imaged from outside the plasma chamber. In combination with phase-resolved resonance measurements, the development of volume-averaged particle mass density is measured over several hours. The measurements show a significant decrease of mass density for polymethyl methacrylate particles due to a plasma etching process on the surface. This is explained by a core-shell model and is supported by a surface roughness effect seen in the LDM images, an out-of-focus imaging of the angular Mie scattering pattern and ex-situ laser scattering microscopy measurements.

On the transcritical mixing of fuels at diesel engine conditions

Whilst the physics of both classical evaporation and supercritical fluid mixing are reasonably well characterized and understood in isolation, little is known about the transition from one to the other in the context of liquid fuel systems. The lack of experimental data for microscopic droplets at realistic operating conditions impedes the development of phenomenological and numerical models. To address this issue we performed systematic measurements using high-speed long-distance microscopy, for three single-component fuels (n-heptane, n-dodecane, n-hexadecane), into gas at elevated temperatures (700–1200K) and pressures (2–11MPa). We describe these high-speed visualizations and the time evolution of the transition from liquid droplet to fuel vapour at the microscopic level. The measurements show that the classical atomization and vaporisation processes do shift to one where surface tension forces diminish with increasing pressure and temperature, but the transition to diffusive mixing does not occur instantaneously when the fuel enters the chamber. Rather, subcritical liquid structures exhibit surface tension in the near-nozzle region and then, after time surrounded by the hot ambient gas and fuel vapour, undergo a transition to a dense miscible fluid. Although there was clear evidence of surface tension and primary atomization for n-dodecane and n-hexadecane for a period of time at all the above conditions, n-heptane appeared to produce a supercritical fluid from the nozzle outlet when injected at the most elevated conditions (1200K, 10MPa). This demonstrates that the time taken by a droplet to transition to diffusive mixing depends on the pressure and temperature of the gas surrounding the droplet as well as the fuel properties. We summarise our observations into a phenomenological model which describes the morphological evolution and transition of microscopic droplets from classical evaporation through a transitional mixing regime and towards diffusive mixing, as a function of operating conditions. We provide criteria for these regime transitions as reduced pressure–temperature correlations, revealing the conditions where transcritical mixing is important to diesel fuel spray mixing.

The Potentiodynamic Polarisation of a High-Alloy Steel—An Analysis by Acoustic Emission Testing and Long-Distance Microscopy

The potentiodynamic polarisation behaviour of a high-alloy Cr-Mn-Ni steel in its as-cast state was investigated in a 0.1M H2SO4 solution by means of acoustic emission (AE) testing. Simultaneously, the evolution of hydrogen and oxygen gas bubbles was observed by long-distance microscopy. Three regions of acoustic activity were identified during the polarisation process from the cathodic range up to transpassivity. Bubble release at the pores and gas-assisted exfoliation of the passive oxide were identified as the AE sources. An evaluation of the gas stream revealed two types of oxygen bubbles that differed significantly in size. The effect was attributed to different points of origin at the electrode.

Boundary condition and fuel composition effects on injection processes of high-pressure sprays at the microscopic level

Detailed imaging of n-dodecane and ethanol sprays injected in a constant-flow, high-pressure, high-temperature optically accessible chamber was performed. High-speed, diffused back-illuminated long-distance microscopy was used to resolve the spray structure in the near-nozzle field. The effect of injection and ambient pressures, as well as fuel temperature and composition have been studied through measurements of the spray penetration rates, hydraulic delays and spreading angles. Additional information such as transient flow velocities have been extracted from the measurements and compared to a control-volume spray model. The analysis demonstrated the influence of outlet flow on spray development with lower penetration velocities and wider spreading angles during the transients (start and end of injection) than during the quasi-steady period of the injection. The effect of fuel composition on penetration was limited, while spreading angle measurements showed wider sprays for ethanol. In contrast, varying fuel temperature led to varying penetration velocities, while spreading angle remained constant during the quasi-steady period of the injection. Fuel temperature affected injector performance, with shorter delays as fuel temperature was increased. The comparisons between predicted and measured penetration rates showed differences suggesting that the transient behavior of the spreading angle of the sprays modified spray development significantly in the near-field. The reasonable agreement between predicted and measured flow velocity at and after the end of injection suggested that the complete mixing assumptions made by the model were valid in the near nozzle region during this period, when injected flow velocities are reduced.

Microscopic investigation of the atomization and mixing processes of diesel sprays injected into high pressure and temperature environments

Atomization and mixing of sprays are key parameters to successfully describe and predict combustion in direct-injection engines. Understanding these processes at the conditions most relevant to engines (high pressures and temperatures) is of primary interest. The present work investigates the atomization and mixing processes of sprays injected into environments with progressively higher pressure and temperature. An Engine Combustion Network Spray A single-hole injector is used to inject n-dodecane fuel into an optically accessible combustion chamber. High-speed imaging is performed using long-distance microscopy and diffused back-illumination to resolve ligament structures and droplets in the near-nozzle region. A unique aspect of this study is the application of high-speed imaging of sprays in the near-field under realistic diesel conditions, particularly those near top-dead-center. Imaging showed droplet and ligament dynamics at low pressure and temperature, but not at high pressure and temperature typical of engine operation, indicating diminished effects of surface tension. Relating the ambient conditions to the fuel critical pressure and temperature showed that when the ambient conditions are slightly above the critical point of the fuel, surface tension remains in effect at gas–liquid interfaces. However, for higher pressure and temperature conditions, the surface tension appears to diminish significantly as expected for supercritical fuel–air mixtures.

Pyrolytic Mercury Removal from Coal and Its Adverse Effect on Coal Swelling

Coal combustion is one of the largest anthropogenic contributors of mercury emissions to the environment. The high level of mercury toxicity and its potential to bioaccumulate have prompted strict regulations for mercury emission control in many countries. While end-of-pipe technologies for mercury control are currently undergoing increased international development, precombustion pyrolytic mercury removal has also been proposed as a possible option. Mercury is highly volatile at low temperatures, and its removal from coal can be controlled by low-temperature pyrolysis. This work investigated the effect of temperature on mercury release while monitoring the adverse effects of the thermal pretreatment process on coal using coal swelling as an indicative parameter for combustion quality and char burnout rate. Three coal samples were pyrolyzed for 20 min to selected temperatures ranging between 200 and 600 °C at a heating rate of 50 °C/min. Mercury content in the coal chars was determined by a cold vapor atomic fluorescence method. The total mercury in coal was found to decrease between 5 and 25% when the coal samples were pyrolyzed at 200 °C, while at 300 °C, the amount of mercury removed from coal ranged between 55 and 90%. Representative coal char samples were also observed by long-distance microscopy in a single-particle reactor in which they were pyrolyzed at a heating rate of 200 °C/s. The results showed a significant decrease in the maximum transient swelling behavior when the char samples were thermally treated at temperatures above 400 °C for one coal and 500 °C for the remaining two coal samples. The results suggested that mercury removal from coal by prior thermal upgrading to around a maximum temperature of 300 °C may provide an option for partial mercury control for some coals. However, a better understanding and quantification of the strongly bonded fraction of mercury in the coal is required to model a feasible mercury retention process using the option of thermal upgrading.