Optical Diagnostic Techniques Research Articles

Investigations of high-speed projectile impact on symmetric sandwich structures containing solid propellant with core perforations

The response of solid rocket motors (SRMs) to high-speed fragment impacts is crucial for their safety design and operational use in scenarios such as rocket launches and space applications. The visualized Burn to Violent Reaction (BVR) test is used to observe intense reactions induced by high-speed projectile impacts. Employing a two-stage light gas gun and optical diagnostic techniques including high-speed schlieren imaging and direct photography, the impact-induced deflagration/explosion behavior, and reaction growth behavior were investigated. The damage mechanisms of the casing and propellant samples were assessed, and the reaction growth and afterburn effects of the impact-induced fragment cloud were quantitatively analyzed. The results indicate that the ignition delay time is inversely correlated with the impact velocity, decreasing from ms to μs scale. Across a wide range of velocities (1050–2058 m/s), higher projectile velocities induce more sustained and vigorous combustion reactions within the propellant. Furthermore, increasing the propellant air gap to 7.8 cm does not trigger further reactions under the studied configurations. The reaction mechanisms are closely linked to the characteristics of the fragment cloud induced by the impact. The developed Smoothed Particle Hydrodynamics (SPH) method, incorporating material constitutive models, ignition criteria, and reaction growth models, was used to study the influence of projectile velocity on the reaction mechanisms. The simulation results were compared with experimental data, demonstrating satisfactory accuracy.

Design and Assembly of a Miniature Catheter Imaging System for In Vivo Heart Endoscopic Imaging.

In this paper, we present the design and fabrication of a novel chip-on-tip catheter, which uses a microcamera and optical fibres to capture in vivo images in a beating porcine heart thanks to a saline flush to clear the blood field. Here, we demonstrate the medical utility and mechanical robustness of this catheter platform system, which could be used for other optical diagnostic techniques, surgical guidance, and clinical navigation. We also discuss some of the challenges and system requirements associated with developing a miniature prototype for such a study and present assembly instructions. Methods of clearing the blood field are discussed, including an integrated flush channel at the distal end. This permits the capture of images of the endocardial walls. The device was navigated under fluoroscopic guiding, through a guiding catheter to various locations of the heart, where images were successfully acquired. Images were captured at the intra-atrial septum, in the left atrium after a trans-septal cross procedure, and in the left ventricle, which are, to the best of our knowledge, the first images captured in an in vivo beating heart using endoscopic techniques.

Soot volume fraction measurements in aero-engine model combustor outlet using two-color laser-induced incandescence

Quantitative measurement of Soot Volume Fraction (SVF) is an essential prerequisite for controlling soot particle emissions from aero-engine combustors. As an in-situ and non-intrusive optical diagnostic technique, Laser-Induced Incandescence (LII) has been increasingly applied for soot concentration quantification in various combustion environments such as laminar flame, vehicle exhaust, internal combustion chamber as well as aero-engine combustor. In this work, we experimentally measured the spatial and temporal distribution of SVF using two-color LII technique at the outlet of a single-sector dual-swirl aero-engine model combustor. The effect of inlet pressure and air preheat temperature on the SVF distribution was separately investigated within a pressure range of 241–425 kPa and a temperature range of 292–500 K. The results show that soot production increases with the inlet pressure but generally decreases with the air preheat temperature. Qualitative analysis was provided to explain the above results of parametric studies. The LII experiments were also conducted under 3 designed conditions to evaluate soot emission under practical operations. Particularly, weak soot emission was detected at the outlet under the idle condition. Our experimental results provide a valuable benchmark for evaluating soot emission in the exhaust plume of this aero-engine combustor during practical operations.

Integrated Optical Diagnostics Of Cyclopentane Sprays To Characterize Transcritical Phase Transitions

This research investigates spray phase transitions under subcritical and transcritical conditions relevant to high-pressure combustion systems. Using advanced optical diagnostic techniques, including shadowgraphy (SH), Mie scattering (MS), and infrared radiation (IR) measurements, the study focuses on cyclopentane sprays in a high-pressure, high-temperature chamber filled with gaseous nitrogen. Three cases are studied to understand spray behavior under varying injection conditions. SH and MS analyses reveal significant differences across the cases. As the chamber temperature increases, the liquid core length shortens, indicating enhanced vaporization. This is particularly evident in the MS images, where step-like transitions in the axial distribution highlight abrupt phase changes from liquid to gas in subcritical conditions. Such transitions are absent under supercritical conditions, suggesting a smoother phase transition process. IR measurements provide additional insights into the spray dynamics, especially under transcritical conditions. The IR images display complex downstream (at x/D > 125) signal patterns with bimodal distributions. The application of inverse Abel deconvolution highlights negative intensities near the spray axis downstream, indicative of regions near the pseudo-critical temperature. This phenomenon, characterized by blocked background radiation due to critical opalescence, offers indirect evidence of the pseudo-critical temperature and the associated phase transition dynamics. Dynamic Mode Decomposition (DMD) analysis identifies dominant frequencies around 4.5 kHz, corresponding to injection pressure fluctuations. This correlation suggests that injection pressure variations significantly influence spray fluctuations. Differences in mode shapes among the cases are also observed; lateral fluctuations near the injector nozzle exit are present in Cases 1 and 2 but not in Case 3. Axial fluctuations beyond a certain length are consistent in all cases. The lateral fluctuations are related to the chamber pressure and the potential large-scale vortex formation or cavitation in the nozzle. The combined use of SH, MS, and IR methodologies provides a comprehensive understanding of transcritical spray dynamics, emphasizing the effectiveness of combining multiple optical diagnostic techniques. These findings will improve numerical simulations and contribute to the development of more efficient high-pressure combustion systems. The research underscores the complexity of phase transitions in sprays and the critical role of precise diagnostics in the advancement of combustion technology.

The combined use of serum Raman spectroscopy and D dimer testing for the early diagnosis of acute aortic dissection

ObjectivesAcute aortic dissection (AAD) is an extremely life-threatening medical emergency, often misdiagnosed in its early stages, resulting in prolonged wait times for rescue. This study aims to identify potential serum biomarkers that can assist in the accurate diagnosis of AAD and effectively differentiate it from other conditions causing severe chest pain. MethodsA total of 122 patients with AAD and 129 patients with other severe chest pain disorders were included in the study. Serum samples were analyzed by measuring the peak intensities of Raman spectra. For each measurement, the Raman spectrum was accumulated by two accumulations (3 s per acquisition). Logistic regression and nomogram models were developed using these peak intensities as well as D-dimer levels to predict the occurrence of AAD. The clinical utilities of these models were assessed through receiver operating characteristics (ROC) curve analysis, net reclassification improvement (NRI), integrated discrimination improvement (IDI), and decision curve analysis (DCA) in both training and internal test cohorts. ResultsThe D-dimer levels of AAD patients were significantly increased, as well as higher intensities at specific Raman peaks, including 505 cm−1, 842 cm−1, 947 cm−1, 1254 cm−1, 1448 cm−1, and 1655 cm−1 when compared to non-AAD patients. Conversely, decreased intensities were observed at Raman peaks such as 750 cm−1, 1004 cm−1, 1153 cm−1, 1208 cm−1, and 1514 cm−1 in AAD patients. Least absolute shrinkage and selection operator regression analysis on the training cohort identified eight potential predictors: D-dimer along with intensity measurements at peaks such as 505 cm−1, 750 cm−1, 1153 cm−1, 1208 cm−1, 1254 cm−1, 1448 cm−1, and 1655 cm−1. The combination of these eight potential predictors demonstrated a good discriminatory performance, with an area under the curve (AUC) value of 0.928 in the training cohort and an AUC of 0.936 in the internal test cohort, outperforming the use of D-dimer alone. Furthermore, DCA curve analysis revealed that leveraging this combination of eight potential predictors would provide substantial net benefits for clinical application. Moreover, this combination significantly augmented discrimination power, as evidenced by a continuous NRI of 39.8 % and IDI of 9.95 % in the training cohort, as well as a continuous NRI of 27.1 % and IDI of 9.95 % in the internal test cohort. ConclusionsThe employment of this combination of eight potential predictors effectively rules out AAD to a greater extent. This study presents a promising diagnostic strategy for early detection using optical diagnostic techniques such as Raman spectroscopy.

Investigating the origin of the far-field reflection interference fringe (RIF) of microdroplets

We show that the reflection interference fringe (RIF) is formed on a screen far away from the microdroplets placed on a prism-based substrate, which have low contact angles and thin droplet heights, caused by the dual convex–concave profile of the droplet, not a pure convex profile. The geometric formulation shows that the interference fringes are caused by the optical path difference when the reflected rays from the upper convex profile at the droplet–air interface interfere with reflection from the lower concave profile at oblique angles lower than the critical angle. Analytic solutions are obtained for the droplet height and the contact angle out of the fringe number and the fringe radius in RIF from the geometric formulation. Furthermore, the ray tracing simulation is conducted using the custom-designed code. The geometric formulation and the ray tracing show excellent agreement with the experimental observation in the relation between the droplet height and the fringe number and the relation between the contact angle and the fringe radius. This study is remarkable as the droplet's dual profile cannot be easily observed with the existing techniques. However, the RIF technique can effectively verify the existence of a dual profile of the microdroplets in a simple setup. In this work, the RIF technique is successfully developed as a new optical diagnostic technique to determine the microdroplet features, such as the dual profile, the height, the contact angle, the inflection point, and the precursor film thickness, by simply measuring the RIF patterns on the far-field screen.

Hydrophilic surface modification of polypropylene by AC-DBD and NS-DBD

Dielectric barrier discharge (DBD) is a method to modify material surfaces by creating low-temperature plasma at atmospheric pressure. However, in comparison to low-pressure discharge, atmospheric DBD has the drawback of uneven surface treatment due to the ease of forming filamentary channels. This limitation hinders the widespread application of DBD technology in material surface modification. In this study, the surface of polypropylene (PP) was treated using high frequency alternating current (AC) and nanosecond (NS) pulsed argon DBDs to improve surface hydrophilicity. The changes of the PP surface hydrophilicity and the treatment uniformity were characterized by water contact angle (WCA) measurement. The relationships between the DBD characteristics, the treatment effect and the treatment uniformity of the PP surface were investigated using electrical and optical diagnostic techniques. The PP material surfaces before and after plasma treatments were evaluated by the physical morphology and chemical composition measurements to examine the influence of DBD on the treatment effect and uniformity on the PP material surface. The findings show that the modification by AC-DBD reduces the WCA of PP material surface by 40°∼50° and exhibits a variance of 1.77, indicating an improvement in hydrophilicity to some extent. Furthermore, the modification by NS-DBD further decreases the WCA of PP material surface, reducing the variance to 1.14. Compared with AC-DBD, NS-DBD has a better uniformity, a higher concentration of active particles in plasma, a greater surface roughness of PP material after treatment, a better hydrophilicity and a more uniform effect with less energy consumption. The results are meaningful to improve the application of DBD in material surface modification and optimize the power source election and operation parameters.

Experimental study on the structure and propagation of the stretched hydrogen-rich turbulent premixed flames in the thin-reaction-zone regime

An experimental study on the structure and propagation of the stretched hydrogen-rich premixed turbulent flames in the thin-reaction-zone regime was conducted. A set of optical diagnostic techniques including the 2D/3D-particle image velocimetry, hot wire anemometer, intensified charge-coupled device (ICCD), and laser tomography visualization (LTV) methods were adopted to quantitatively measure the velocity field, turbulent intensity, and thickness of the stretched flame of the under various conditions. A new method to improve the measurements of the flame preheat zone thickness was developed by using the low-temperature available LTV method to identify the onset of the preheat zone and using the peak location of CH* to identify the reaction zone. The results showed that the flow field was in the inertial turbulent subrange and the turbulent flames obtained in the present counterflow configuration were categorized into the thin reaction zone regime in Borghi/Peters turbulent flame diagram. The experimental results showed that the flame structure of a turbulent H2-rich premixed flame was synergistically influenced by the hydrodynamic stretch, turbulence, and preferential diffusion. New evidence of the phenomenon accumulated that large-scale vortices wrinkled the surface of the flame while small-scale vortices penetrated into the flame preheat zone. Scaling laws for the turbulent flame propagation speed, apparent turbulent thermal conductivity, and were given with the consideration of the synergetic effect of hydrodynamic stretch, turbulence, and preferential diffusion. The scaling of the turbulent flame preheat zone thickness turned out to be sensitive, while those of the turbulent flame propagation speed and apparent turbulent thermal conductivity were insensitive, to the bulk flow strain rate.

Understanding the ignition process and flame structure of conventional and oxygenated fuels under engine relevant conditions – An optical study

Renewably generated synthetic fuels such as poly-oxymethylene ethers (OME) have a significant potential to effectively break the soot-NOX trade-off in compression ignition engines by using exhaust gas recirculation (EGR) to maintain low nitrogen oxide (NOX) emissions while maintaining good efficiency and simultaneously contributing to circular carbon economy. However, owing to the fundamental differences in properties of OME when compared to fossil-based diesel fuels, it is critical to fully understand its ignition and combustion phenomenology to take advantage of this fuel to its utmost potential. In this context, this work outlines the results of a systematic experimental study performed in a heavy-duty, single-cylinder, optical engine probing the spatial and temporal progression of fuel decomposition and ignition behavior of OME when compared to n-dodecane, a diesel-fuel surrogate. Thermodynamic analysis and optical diagnostics techniques including simultaneous HCHO-PLIF and OH-PLIF complemented by high-speed OH* chemiluminescence were employed along with parametric sweeps of intake temperature and EGR dilution rates. OME does not exhibit any observable low temperature heat release irrespective of the ambient oxygen concentration. Differences in the observed diffusive flame structure such as longer flame lift-off length, less pronounced combustion recession, faster premixed burn at ignition (“volumetric” ignition), non-sooting behavior suggest that the inherent presence of fuel-bound oxygen in OME can skew the air-fuel ratio (AFR) distribution within the jet thereby reducing the reliance of combustion on mixing and air entrainment. This leads to rapid late-cycle oxidation leading to shorter combustion duration and favorable combustion phasing. Results also suggest that OME exhibits relatively weak negative temperature coefficient (NTC) behavior, however, the OME fuel-decomposition kinetic-pathways produce significant concentration of HCHO, which might be erroneously interpreted as a product of cool-flames.

The relative position of pyrolysis onset and flame front location for downward flame spread

The interaction of solid and gas phase phenomena in downward flame spread is studied under buoyant conditions. Phosphor thermometry (PT) was used to determine the spatiotemporally resolved surface temperatures both ahead and beneath the flame while CH* chemiluminescence allowed for the quantification of the flame location relative to the fuel surface. The combination of these measurements allowed for the superposition of the flame front location and the surface temperature distribution. The standoff distance (∼1.1 mm) and preheated distance along the surface (∼3.5 mm) were quantified. The application of PT allowed for the continued measurement of surface temperatures in the pyrolyzing region beneath the flame, where the surface temperature was found to remain relatively constant between 300 and 350 °C. The combination of measurements allowed for the calculation of the gas-phase heat flux distribution ahead of the flame front using a point-source approximation, which compared well with previous studies. PT and CH* measurements were used to expand previous work to quantify the thermal gradients through the solid both ahead of and behind the flame front. Surface temperature measurements were coupled with thermogravimetric analysis (TGA) and a simple pyrolysis model to determine that the onset of pyrolysis occurs approximately 1 mm ahead of the flame front. This approach was also used to illustrate that pyrolysis gases were generated in sufficient volume to result in a flammable mixture at the leading edge of the flame. This study highlights the benefit of optical diagnostic techniques in exploring the controlling mechanisms of flame spread, and provides insight into the interaction of solid and gas-phase phenomena in downward flame spread.

Optical Diagnostics in Periodontology

In terms of clinical performance, the conventional methods for diagnosing periodontitis fall short of ideal. By adding optical techniques to ocular inspection and periodontal probing, these procedures can be made better. The minor changes in periodontal tissues that cannot be visually inspected can be identified and measured constantly over a period of time with the help of optical diagnostic techniques. The removal of subgingival microbial biofilms and dental calculus deposits during debridement of teeth and dental implants can be made more efficient by using magnifying loupes, operating microscopes, and cutting-edge methods based on how light interacts with bacterial deposits, such as differential reflectometry and light-induced fluorescence. The absence of ionizing radiation from optical devices, which enables frequent usage on the same patient in clinical practice, is one of the greatest reasons in favor of their use as diagnostic adjuncts. The periodontium’s soft tissues can be examined using three-dimensional scanning, fluorescence spectroscopy, and optical coherence tomography to see how they relate to natural teeth and dental implants. By limiting the under- and overuse of treatment alternatives, an accurate prognosis-determining system would enable a more targeted deployment of finances, enhancing the appropriateness and quality of dental care. The clinical management of people with periodontal disease may be enhanced by a novel approach to periodontal diagnostics.

A cost-effective label-free biosensor for rapid detection of multiple viral respiratory infections base on liquid crystals: Fabrication and modeling

The high transmission rate and incurability of recurrent Herpes Simplex Virus (HSV) infection require an accurate and fast detection approach for its efficient prevention. In an attempt to find a novel sensitive and simple optical diagnostic technique which enables on-spot testing, here we developed label-free biosensors based on E7 nematic liquid crystals (LCs) for specific detection of HSV DNA. For this purpose, homeotropically oriented LCs as smart materials are spread on the textile grid to make their both upper and lower surfaces be in contact with ambient air. HSV DNA of concentration between 1.4 nM and 5.5 nM is added to the platform, and DNA elution buffer is used as a control. Then, the reorientation of LCs occurs due to the coupling of electrical dipoles between LCs and DNA molecules. Creating a unique pattern that is projected onto CCD by putting the sample in a cross-polarizer and using white light. To model the LC's director distribution, LC's free energy equation is solved using the variational method and finite difference method (FDM). Overall, our LC-based sensor is a user-friendly, cost-effective, highly sensitive, and fast way for specific detection of HSV without the need for experts and expensive equipment.

Meteorite temperature measurements during ground testing

Ground testing meteorite samples offer in-situ measurements of known materials in conditions that occur during entry into Earth’s atmosphere. 22 meteorite samples with a wide range of origins and classifications were tested in the plasma wind tunnel facility PWK1 at the Institute of Space Systems in Stuttgart. These tests recreate the flow condition of a meteoroid during entry into earth’s atmosphere at an altitude of 78.8km altitude and a velocity of 11.7kms-1. Four optical diagnostic techniques were used to measure the surface temperature above 1000K. 2-D methods showed that the surface temperature is evenly distributed over the sample surface, while time-resolved analyses show that the samples reach a steady state temperature within 0.5s. The steady-state temperature for chondritic samples was consistent but varied significantly for achondrites and iron meteorite samples. The composition data showed a strong dependency of the surface temperature on the silicon content. The surface temperatures were shown to be dependent on the material and a database of temperatures was set up. The Planck fit methodology could be directly adapted to spectral meteor observation systems. A comparison of the method to established methods showed an offset between the methods. This data could be applied to thermal models to better understand the energy transfer processes during meteor flight.

Arrays of glass wedges for multi-dimensional optical diagnostics.

There is a common need in the advancement of optical diagnostic techniques to increase the dimensionality of measurements. For example, point measurements could be improved to multi-point, line, planar, volumetric, or time-resolved volumetric measurements. In this work, a unique optical element is presented to enable multi-dimensional measurements, namely, an array of glass wedges. A light source is passed through the wedges, and different portions of the illumination are refracted by different amounts depending on the glass wedge angle. Subsequent optics can be used to focus the light to multiple points, lines, or planes. Basic characterization of a glass wedge array is presented. Additional wedge-array configurations are discussed, including the use of a periodic intensity mask for multi-planar measurements via structured illumination. The utility of this optical element is briefly demonstrated in (a)multi-planar flame particulate measurements, (b)multi-point femtosecond-laser electronic excitation tagging for flow velocimetry, and (c)multi-line nitric oxide molecular tagging velocimetry in a hypersonic shock-tunnel. One significant advantage of this optical component is its compatibility with high-energy laser sources, which may be a limiting factor with other beam-splitting or beam-forming elements such as some diffractive optics. Additionally, an array of glass wedges is simple and easily customizable compared to other methods for forming multiple closely spaced illumination patterns. Suggestions for further development and applications are discussed.

Near blowout instabilities and identification based on multivariate analysis with high-speed multi-species optical diagnostic techniques

Early prediction and depth analysis of lean blowouts are desirable for both aeroengine and stationary gas turbines. This study used a combination of high-speed multi-species optical diagnostic techniques and multivariate analysis methods to investigate near-blowout instabilities and identify combustion conditions. The flame root dynamic behavior and heat release pulsation characteristics were investigated using advanced modal decomposition methods. Results showed that a near-blowout flame exhibited a characteristic frequency (∼200 Hz) or “fingerprint” related to the temporal and spatial coupling characteristics of CH and OH radicals. Furthermore, flame image feature screening and multivariate fusion for combustion condition identification are reported for the first time. Significant differential features and effective fusion models were investigated and discussed. The identification accuracy of the multivariate fusion feature model compared with that of the geometric feature model improved by more than 24 %. Moreover, simplified fusion models of low-order moment and geometric features were constructed with relatively high identification accuracy (99 %) and low relative standard deviation value (14.1 %). These results demonstrate the significant potential of combining multi-species optical diagnostic techniques and multivariate analysis methods, thereby expanding their application to the field of combustion science.

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.

Failure characterisation of sandwich beams using integrated acoustic emission and digital image correlation techniques

The paper presents the experimental study of the failure behaviour of sandwich beams subjected to bending. The samples examined are sandwich beams made of polyethylene terephthalate foam core and glass fibre-reinforced polymer laminate face sheets. In a series of experiments, it has been proposed to integrate diagnostic techniques with acoustic emission and digital image correlation to accurately track the cracking process on the surface as well as in the entire volume of the beam. The research programme carried out allowed observing various modes of failure in composite specimens. The results obtained showed that the integration of acoustic and optical diagnostic techniques provided complementary results and can be used successfully for failure monitoring in sandwich beams.

Ability of Li4Ti5O12 to suppress Li metal deposition under overpotential conditions confirmed by electrochemical surface plasmon resonance spectroscopy

Undesirable Li metal deposition on the negative electrode increases the risks associated with the use of Li-ion batteries. Generally, spinel-type lithium-titanium oxide Li4Ti5O12 (LTO)-based negative electrodes are well known as safe dendrite-free options, due to their higher working potential. However, the ability of LTO-based materials to prevent Li metal deposition has not been well confirmed under sudden overpotential application. This study examines the behavior of Li metal electrodeposition under various overpotential conditions for unloaded and LTO-loaded Cu electrodes. Electrochemical surface plasmon resonance spectroscopy (EC-SPR) is used as the optical diagnostic technique, owing to its sensitivity in detecting Li metal deposition. Li metal deposition is easily detected for the unloaded Cu electrode at overpotential of − 0.2 V (voltage) versus Li counter electrode (LiCE), whereas no Li metal deposition occurs for the LTO-loaded Cu electrode under the same overpotential. The safety of LTO is investigated by analyzing the reaction kinetics.

Experimental study on the combustion and soot emission of twin-spray collision at various collision angles and injection pressures under marine engine’s conditions

In a large-bore marine engine, the larger cylinder diameter and space correspond to more fuel injection in a limited time of each working cycle. The fuel injection of multiple injectors can achieve fast atomization and homogeneous fuel-gas mixing performance, which is liable to cause the spray collision problem, but the twin-spray collision phenomenon is rarely observed and studied. Therefore, this work aims to explore the effect of various collision angles (60°, 90°, 120°, and 150°) and injection pressures (60, 100, 140, and 180 MPa) on the combustion and soot emission of twin-spray collision process. Based on a large-bore constant volume chamber and two optical diagnostic techniques, the ignition delay time, initial ignition location, flame collision structure, and soot emission luminosity are obtained in a reactive atmosphere. The results show that near the collision point, there exists a fan-shaped flame structure. Due to the mutual entrainment from twin sprays, the ignition delay time of twin-spray collision is shorter than that of single free spray. As a result, at the constant diesel injection mass, the soot emission intensity of twin-spray collision becomes higher. With the collision angle increasing, the ignition delay time becomes shorter, and the initial ignition location gradually moves upstream, while the soot emission intensity becomes higher. The time integrated natural luminosity difference between 60 and 180 MPa gradually decreases and then reverses. Moreover, the wider collision angle also makes the ignition delay time less sensitive to the injection pressure.

Experimental investigation of a post-combustion pool fire phenomenon due to the spray-wall interaction under DI engine-like cold-start conditions

Spray-wall interactions (SWI) have remained nowadays as an active area of research and the complex multi-physical field processes between the spray and the wall are still not well understood. In this study, a post-combustion pool fire phenomenon due to the SWI was found by optical diagnostic techniques: pool fire appeared near the stagnation point after spray combustion. The spray combustion and pool fire are not continuous. This abnormal combustion characteristic under low ambient temperature conditions (680–720 K) was investigated and the effect of ambient temperature on the macroscopic characteristics of pool fires was quantified and analyzed. The results show that the wall-attached fuel film will be re-ignited and continue to combust for a longer period after the impingement spray combustion. The appearance moment, dwell time, fire duration, and macroscopic characteristics of this post-combustion pool fire are non-monotonic as a function of the ambient temperature. When the ambient temperature is 700 K, the post-combustion pool fire appears earliest, the dwell time is the smallest, and the pool fire duration, flame length, and flame height of the pool fire are the largest. This is mainly dominated by the gas temperature around the wall-attached fuel film and fuel film mass.