Schlieren Imaging Research Articles

Experimental study on the axial growth characteristics of streamer stem during dark period in long spark discharge

This paper presents an original investigation into the axial evolution of streamer stem during a dark period in long spark discharge. To obtain thermodynamic morphology and temperature distribution of stems, we set up a quantitative schlieren system with the temporal and spatial resolutions of 0.37 μs and 31 μm/pixel, respectively. The quantitative schlieren observation experiments of positive leader discharge with a 1.0 m rod-plate gap were carried out, and the time-resolved quantitative schlieren images were captured. Furthermore, the temperature distribution of stems and its morphology evolution in the axial direction during a dark period were obtained. Due to the dispersion of first streamer discharge, the gas temperature in stem roots shows two evolutionary trends, namely, rising and falling. It was found that the gas temperature in stem decreased along the axis with the increase in the distance from stem root, and the gas temperature of a thermal thin channel was between 400 and 800 K. There is a significant dependency between axial development parameters of thermal thin channels and the first streamer discharge parameters. The phenomenon of channel abrupt elongation triggered by secondary streamer discharge was observed by the schlieren system, and the influence of characteristic parameters on the inception of secondary streamer was statistically analyzed. The ion current waveform in leader relaxation phase was measured, and it is clarified that the generation mechanism of thermal thin channels is due to the energy transfer between positive ions and neutral particles, which finally leads to the increase in gas temperature in the channels.

Specific-heat ratio effects on the interaction between shock wave and heavy-cylindrical bubble: Based on discrete Boltzmann method

Specific-heat ratio effects on the interaction between a planar shock wave and a two-dimensional heavy-cylindrical bubble are studied by the discrete Boltzmann method. Snapshots of schlieren images and evolutions of characteristic scales, being consistent with experiments, are obtained. The specific-heat ratio effects on some relevant dynamic behaviors such as the bubble shape, deformation process, average motion, vortex motion, mixing degree of the fluid system are carefully studied, as well as the related Thermodynamic Non-Equilibriums (TNE) behaviors including the TNE strength, entropy production rate of the system. Specifically, it is found that the influence of specific-heat ratio on the entropy production contributed by non-organized energy flux (NOEF) is more significant than that caused by non-organized momentum flux (NOMF). Effects of specific-heat ratio on entropy production caused by NOMF and NOEF are contrary. The effects of specific-heat ratio on various TNE quantities show interesting differences. These differences consistently show the complexity of TNE flows which is still far from clear understanding.

Supersonic Flow Separation in Front of a Forward-Facing Step

A detailed experimental study has been done to understand the Shock Wave-Boundary Layer Interaction (SWBLI) over a Forward-Facing Step (FFS) of height (h) in a Mach 2.5 flow using Particle Image Velocimetry (PIV). In addition to PIV, unsteady wall pressure measurements, high-speed schlieren and surface oil flow visualization have been studied. Flow separation using surface oil flow visualization was observed around 4.1 step heights upstream of the step face. Shock motions extracted from the schlieren image sequence shows a peak frequency of approximately 1000 Hz, which corresponds to the dominant (low) frequency at which the shock oscillates; this being two orders in magnitude smaller than the characteristics frequency of the incoming boundary layer. Also, spectra of the unsteady wall pressure data shows that the shock oscillates with a peak frequency of approximately 1000 Hz. The effect of shock location (xs) from instantaneous velocity PIV fields in the cross-stream plane due to an upstream influence parameter (line-averaged boundary layer streamwise velocity, ul) and a downstream parameter (separation bubble or recirculation region area, A) is studied. Instantaneous cross-stream PIV measurements show significant variations of the separation shock location, incoming boundary layer velocity fluctuations and recirculation region. Besides, conditionally averaged velocity field based on incoming boundary layer streamwise velocity fluctuations and separation bubble area in the cross-stream plane shows that the shock location (xs) is correlated to the size of the separation bubble and not to boundary layer velocity fluctuations.

Aerothermodynamic Analysis of Space Recovery Experiment Capsule at Mach 6

The main focus of this paper is to compute the flowfield and assess the aerodynamic drag of Space Recovery Experiment (SRE) modules at freestream Mach 6 at zero angle of incidence. Time-dependent Compressible axisymmetric Navier-Stokes equations are numerically simulated using a finite volume discretization method with a three-stage Runge-Kutta time-stepping scheme. The computed shock stand-off distance, pressure ratio across normal shock and stagnation point heat flux are showing satisfactory results with the analytical solutions.Available wind tunnel data such as Schlieren images, oil flow pictures and fore-body aerodynamic drag are compared with numerical results.Influence of various ramp angles of the SRE modules on surface pressure, flow separation, skin friction coefficient, wall heat flux and fore and base aerodynamic drag coefficients are investigated using present numerical data.

Fiber-based high-speed 3D schlieren imaging.

This Letter reports the first demonstration of a high-speed three-dimensional (3D) schlieren technique based on the combination of fiber imaging, Toepler's lens-type schlieren, and computed tomography (CT). The technique uses a single high-speed camera, two xenon lamps, and a series of fiber bundles to simultaneously capture the schlieren images of turbulent flames from seven orientations with a framerate beyond tens of kHz. The presented method complements the existing technique with advantages of being flexible, high speed, and low cost. The 3D schlieren technique is first demonstrated and validated on the turbulent premixed flame and stable laminar premixed flame, respectively. Then, the 3D schlieren technique is used to measure the transient, dynamic ignition process. The results show that time-resolved 3D fundamental properties of ignition kernels (i.e., structure and edge speed) can be obtained by the technique.

An Inexpensive, Portable Shield to Improve Healthcare Worker Safety During Intubation Procedures.

Healthcare workers (HCW) are exposed to risk of infection during intubation procedures, in particular, in the prehospital setting. Here, we demonstrate a novel shield that can be used during intubation to block aerosols and droplets from reaching the HCW. The device is mounted on the patient's head and provides a barrier between patient and HCW. It incorporates a self-sealing port through which an endotracheal tube can be inserted. The port "floats" in the plane of the shield to facilitate maneuvering of the endotracheal tube. The shield is fabricated from transparent materials to enable the HCW to visualize the procedure. Using two complementary imaging methods, background oriented Schlieren imaging and laser sheet droplet imaging, we show that the device prevents detectable transmission of gas flow and droplets through the shield both before and after endotracheal tube insertion.Clinical Relevance- This device has the potential to protect HCWs from infections during intubation procedures, especially in the prehospital setting.

Comparative study of droplet and liquid film velocities in sprays

This study presents a comparative analysis of droplet and conical liquid film velocities in sprays by a pressure swirl injector with Abramovich geometric constant K = 1.986. The velocities were measured simultaneously by schlieren image velocimetry and sequential images of the liquid film, using a high-speed camera with 8192 fps and shutter of 2 μs. Other parameters, such as discharge coefficients, spray cone angles and breakup lengths were also determined, using water as test fluid, for injection pressures from 0.05 to 0.5 MPa. Experimental velocity data were compared to results from different semi-empirical equations. The breakup lengths decreased continuously from around 20 mm to 15 mm for injection pressures from 0.1 to 0.5 MPa, while spray cone angles increased continuously from about 42°–53°, for pressures from 0.2 to 0.5 MPa. Mean axial droplet velocities varied from 4.7 m s−1 to 14.5 m s−1, while the mean total droplet velocities varied from 5 m s−1 to 16.2 m s−1 and the total liquid film velocity increased from 5.7 m s−1 to 20.2 m s−1, approximately, for increasing injection pressure. Liquid film velocities were about 15%–28% higher than the droplet velocities in the pressure range considered, due to the energy required for liquid film breakup and the air drag on the droplets. The current findings underscore that significant discrepancies may arise when relying on inadequate velocity data, particularly when employed in the computation of key parameters such as the Reynolds and Weber numbers.

Background oriented schlieren image displacement estimation method based on global optical flow

Background oriented schlieren (BOS) technology is a recent non-contact optical diagnostic technology, which can be used for the visualization of variable density flow and the quantitative measurement of the refractive index field. The accuracy of BOS image displacement estimation directly determines the accuracy of refractive index field reconstruction. For BOS images, the optical path construction method, vibration and uneven illumination, blurring and defocusing effects, and the refocusing effect of volume refraction make it difficult for the BOS image sequence to satisfy the constant brightness assumption. In this study, the optical flow algorithm based on the assumption of a constant gradient in PIV is improved and applied to BOS image displacement estimation. A data item based on the assumption of constant brightness combined with the assumption of constant gradient is proposed. The first-order div-curl regularization constraint is used as the spatial smoothing condition, and the non-convex Charbonnier penalty function is used to determine the displacement field by the variational method to minimize the energy function. The proposed method is tested on the synthetic images obtained by two-dimensional turbulent direct numerical simulation (DNS), the BOS synthetic images with brightness changes, and the actual experimental BOS images of the flow field above a premixed butane/air combustion flame with different equivalence ratios. Compared with the current advanced optical flow algorithm, this method has higher accuracy and robustness. Overall, the proposed method can not only obtain more accurate results when the brightness of the BOS synthetic image and experimental image changes, but also helps in retaining the flow details such as small divergence and vorticity structure of the heat flow field while reducing the outliers.

Ignition detection with the breakdown voltage measurement during nanosecond repetitively pulsed discharges

Nanosecond Repetitively Pulsed Discharge (NRPD) is a promising ignition concept for introducing diesel-like process parameters for hard-to-ignite renewable fuels in Spark Ignition (SI) engines. Knowing whether an ignition event initiated by a series of nanosecond electrical discharges was successful or not gives the possibility of using this information for closed-loop ignition control. This paper presents a methodology for detecting successful ignition under NRPD ignition.After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse is proportional to the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. Lower breakdown voltages of the control pulse are present if a robust early flame kernel is present. The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to place additional ignition events, if necessary.This technique was experimentally analyzed in a Constant Volume ignition Cell (CVC) and in a Rapid Compression Expansion Machine (RCEM). In the CVC at the ignitability limit, lower breakdown voltages of the control pulse are mostly measured when no pressure rise is measured. In the RCEM, the heat release rate is analyzed with a two-zone thermodynamic model, and the early flame kernel formation is monitored with Schlieren imaging. Some overlap exists in the control pulses' breakdown voltages for the ignition and quenching experiments; nevertheless, the Schlieren videos outline that the overlapping cases have a similar flame kernel formation, and the difference arises thereafter.

Investigation on auto-ignition and chemical energy release characteristics of pilot hydrogen in supersonic combustion flow

This study investigates auto-ignition and heat release characteristics of pilot hydrogen chemical energy in a scramjet combustor equipped with a single cavity. Experiments are conducted in a direct-connected facility simulating Mach 6.0 flight conditions with a total temperature of 1350 K and total pressure of 1.75 MPa. Data are obtained from schlieren imaging, hydroxyl planar laser-induced fluorescence, flame emission, and 10-kHz static pressure transducers. The present investigation extends the pilot hydrogen ignition delay experimental dataset and clarifies the instabilities present in the ignition process. The results show that the supersonic internal flow of a confined cavity exhibits self-oscillating behavior with a dominant frequency of approximately 141.3 Hz. The primary chemical reaction occurs at mid-cavity, where the chemical energy of the pilot hydrogen begins to be converted into heat energy, then approaches the cavity ramp before finally being distributed across the whole cavity. The combustion mode is the cavity-stabilized scramjet mode. The distribution of hydroxyl radicals varies significantly because the combustion in the cavity is unsteady. The ignition delay time increases as the injection pressure rises. However, an injection pressure of 4.0 MPa produces an ignition delay of 24.7 ms, which is apparently shorter than the delay under an injection pressure of 3.5 MPa and similar to that under an injection pressure of 3.0 MPa. The injection of pilot hydrogen under high pressures induces greater heat release and more intense blockage effects, thus enhancing the probability of successful ignition and stable combustion.

Assessment of injector-flow characteristics of additised and renewable diesel blends through high-speed imaging

The influence of additives inducing viscoelasticity in diesel fuel, on the in-nozzle cavitation evolution and the expelled spray morphology has been quantified by high-speed, diffused back-light and schlieren imaging applied to two single-hole true-scale transparent injectors of straight and tapered orifice layouts (so-called Spray C and D of the engine Combustion Network), as well as a five-hole configuration (Spray M). More specifically, the in-nozzle cavitating flow and its effect on near-nozzle spray formation of a non-Newtonian diesel fuel sample treated with Quaternary Ammonium Salt (QAS) additives and exhibiting viscoelastic effects, as well as biodiesel (FAME), are compared against conventional diesel fuel for the first time. The operating conditions corresponded to injection and ambient pressures in the range of 500–900 bar and 1–20 bar, respectively. It was found that viscoelasticity has an overall suppressing effect on wall-attached, or so-called geometrical, cavitation. Furthermore, the investigation revealed that the action of viscoelastic additives has the capability to enhance the magnitude of well-established longitudinal vortices, with the subsequent after-effect of leading to increased cone angles of the expelled spray. On the contrary, it tends to suppress turbulence-induced transient instabilities in a manner similar to turbulence suppression.

Review of Visual Measurement Methods for Metal Vaporization Processes in Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) is of great importance for the visual measurement and analysis of the metallization process, which is the process of solid, liquid, and gas phase transformations of metal powders under high-energy laser irradiation due to the low boiling point/high saturated vapor pressure. Since the evaporation of metals involves the interaction of driving forces such as vapor back pressure, surface tension, and gravity, the movement of the melt pool is not stable. At the same time, it also produces vaporization products such as vapor plumes and sprays, which cause defects such as bubbles, porosity, lack of fusion, inclusions, etc., during the manufacturing process of the parts, affecting the performance and manufacturing quality of the parts. More and more researchers are using imaging technologies, such as high-speed X-ray, high-speed visible light cameras, and high-speed schlieren imaging, to perform noncontact visual measurements and analyses of the melt pool, vapor plume, and spatter during the metal evaporation process, and the results show that the metal evaporation process can be suppressed by optimizing the process parameters and changing the processing atmosphere, thereby reducing part defects and improving part performance and built part quality. This paper reviews the research on metal evaporation mechanisms and visual measurement methods of metal evaporation, then discusses the measures of metal evaporation, and finally summarizes and prospects the future research hotspots of LPBF technology, according to the existing scholars' research on numerical simulation analysis and visual measurement methods of the metal evaporation process.

Effect of acoustic excitation on the combustion and emission characteristics of methane-ammonia-air swirling flame

The blending characteristics of ammonia with conventional fuels have recently received much attention from researchers. This paper investigated the effect of acoustic excitation on the combustion and emission characteristics of methane-ammonia-air swirling flame on a laboratory-scale swirl burner. The high-speed schlieren imaging system was used to visualize the flame flow under acoustic excitation. The emission characteristics of CO and NOx at different excitation frequencies (70–270 Hz) and sound pressure levels (SPL, 117–128 dB) were determined. Results show that adding ammonia causes a significant increase in NOx emission and an increase in CO emission under fuel-rich conditions. The flame shows a layered V-shape with periodic development under weak low-frequency (70 Hz) acoustic excitation and an inverted cone with step distribution under high-frequency (270 Hz) acoustic excitation. Low-frequency acoustic excitation can reduce CO and NOx emissions under fuel-lean conditions. Especially for the excitation condition of 120 Hz (Prel = 0.3), NOx and CO emissions are reduced by 203 ppmvd and 170 ppmvd, respectively. However, when the excitation is stronger, the CO emission will rise sharply due to the destruction of the flame structure. Besides, the more intense acoustic excitation will cause a decrease in CO emission and an increase in NOx emission under fuel-rich conditions.

A deep learning approach for velocity field prediction in a scramjet isolator from Schlieren images

Accurate measurements of physical parameters in a scramjet isolator are very important to promote the design and optimization of the isolator and even the scramjet. In a ground experiment, limited by the inherent characteristics of measurement technology and equipment, it is a big challenge to obtain the velocity field inside an isolator. In this study, a deep learning approach was introduced to combine data obtained from ground experiments and numerical simulations, and a velocity field prediction model was developed for obtaining the velocity field inside an isolator based on experimental Schlieren images. The velocity field prediction model was designed with convolutional neural networks as the main structure. Ground experiments of a scramjet isolator under continuous Mach number variation were carried out, and Schlieren images of the flow field inside the isolator were collected. Numerical simulations of the isolator were also carried out, and the velocity fields inside the isolator under various Mach numbers were obtained. The velocity field prediction model was trained using flow field datasets containing experimental Schlieren images and velocity field, and the mapping relationship between the experimental Schlieren images and the predicted velocity field was successfully established.

Deflagration and detonation induced by shock wave focusing at different Mach numbers

Shock wave focusing is an effective way to create a hot spot or a high-pressure and high-temperature region at a certain place, showing its unique usage in detonation initiation, which is beneficial for the development of detonation-based engines. The flame propagation behavior after the autoignition induced by shock wave focusing is crucial to the formation and self-sustaining of the detonation wave. In this study, wedge reflectors with two different angles (60° and 90°) and a planar reflector are employed, and the Mach number of incident shock waves ranging from 2.0 to 2.8 is utilized to trigger different flame propagation modes. Dynamic pressure transducers and the high-speed schlieren imaging system are both employed to investigate the shock-shock collision and ignition procedure. The results reveal a total of four flame propagation modes: deflagration, DDT (Deflagration-to-Detonation Transition), unsteady detonation, and direct detonation. The detonation wave formed in the DDT and unsteady detonation mode is only approximately 75%−85% of the Chapman-Jouguet (C-J) speed; meanwhile, the directly induced detonation wave speed is close to the C-J speed. Transverse waves, which are strong evidence for the existence of detonation waves, are discovered in experiments. The usage of wedge reflectors significantly reduces the initial pressure difference ratio needed for direct detonation ignition. We also provide a practical method for differentiating between detonation and deflagration modes, which involves contrasting the speed of the reflected shock wave with the speed of the theoretically nonreactive reflected shock wave. These findings should serve as a reference for the detonation initiation technique in advanced detonation propulsion engines.

Velocity Mapping of an H2 − O2 Exhaust Jet in Air by Means of Schlieren Image Velocimetry (SIV)

Visualization methods have always been used to inspect flows that are invisible to the naked eye. Seedless velocimetry has been regarded as an alternative to other intrusive quantitative methods and adapted to fit many applications in the industrial or scientific field. Schlieren image velocimetry (SIV) uses the general working principle of a schlieren system to acquire flow images, while relying on a particle image velocimetry (PIV)-like algorithm to obtain quantitative data related to the studied flow. The test case of this study consists of a turbulent round exhaust jet generated by a micro-thruster that uses H2−O2 as a propellent. Mapping the local velocities of the flow is achieved by initially performing a lagrangian tracking method which makes use of a direct image correlation algorithm. These results are then compared to the velocity map obtained from a kymograph applied to a series of images. The velocity profiles obtained through SIV will be compared to the velocity profile of the jet provided by the CFD simulation. The schlieren investigation of the jet’s local velocity map is set to determine the thruster’s capabilities, and conclude if the thruster reaches the desired Mach for which it has been designed.

Spatiotemporal signatures of elastoinertial turbulence in viscoelastic planar jets

The interplay between viscoelasticity and inertia in dilute polymer solutions at high deformation rates can result in inertio-elastic instabilities. We show how fluid elasticity has a nonmonotonic effect on jet stability depending on magnitude, creating two distinct regimes. The nonlinear evolution of these instabilities generates a state of elasto-inertial turbulence (EIT) with different spatiotemporal features than Newtonian turbulence. We use high-speed digital schlieren imaging and dynamic mode decomposition to quantify EIT and identify two modes of instability which can lead to a transition to turbulence at a lower Reynolds number with flow-aligned structures in the turbulent region.

An experimental facility for detailed studies on energy absorbing components subjected to blast loading

AbstractDeformable components such as sandwich structures possess promising properties for use in protection systems. Detailed studies on energy absorption and fluid–structure interaction effects are necessary for the application of deformable sandwich structures in blast resistant design. In this paper, an existing shock tube facility has been extended with a transparent section to observe and measure fluid flow and the structural response of deformable components during transient dynamic loading. The extension was instrumented with pressure sensors and load cells to measure the pressure and force transmitted through the component during testing. The transparent design allows the use of optical measurement techniques. Here, high‐speed cameras were used both for digital image correlation and background‐oriented schlieren imaging. Tests with free‐standing plates and sandwich components were performed. A strong dependency was observed between the plate mass, and thus the velocity of the plates, and the pressure measured upstream and downstream of the components. The tests were simulated with a one‐dimensional numerical model for compressible shock flow with fluid–structure interaction. The numerical model accurately reproduced the shock flow and component displacements measured experimentally. Overall, the experimental set‐up presented in this study proved to be suitable for the detailed examination of deformable components subjected to airblast loading.

Boundary layer suction timing and location effects on unstarting flows in a model scramjet

The effects of the suction timing and location on inlet unstart were investigated to determine the necessary response time and appropriate location of a flow control device. Inlet unstart is one of the causes for scramjet-powered engine failure, and boundary layer suction could suppress or delay the phenomenon. A model scramjet having a rectangular cross-section was tested in a Mach 4.5 freestream via nitrogen-jet-injection-induced inlet unstart. High-speed schlieren imaging and pressure measurements were performed to study the characteristic behaviors of the unstarting and unstarted flows. Suction extracted approximately 1–2% of the freestream flow depending on the test conditions. Boundary-layer suction successfully prevented inlet unstart with the most upstream suction location for the jet momentum ratio near the unstart threshold condition but lost the control authority as the suction delay time increased. Suction was not able to prevent unstart but was delayed with downstream suction locations, and a similar deterioration in the suction control performance was observed an increasing suction delay time. For a jet momentum 10% higher than the threshold, suction could not prevent unstart but suppressed the propagation speed of the unstart shockwave. Fluctuations in the wall pressure were significantly reduced with suction. For all tested cases, better control performance was achieved with a faster suction trigger timing. The results from the flow visualization and pressure measurements indicated that suction must be applied before the unstart shockwave passes through the suction location to achieve effective flow control.

Characteristics of flame acceleration and deflagration-to-detonation transition enhanced by SF6 jet-in-cross-flow/flame interaction

Jet-in-crossflow (JICF), as a turbulence generator, is proposed to promote the flame acceleration and deflagration-to-detonation transition (DDT) process. Previous work has suggested that JICF using CO2 or Ar was in favor of flame acceleration under optimal conditions. The competition mechanism between the positive combustion-enhancement effect caused by jet-induced turbulence and the negative combustion-inhibition caused by non-reactive gas has been demonstrated. It is worth noting that the difference in molar weight between the medium of JICF and the combustible mixture may significantly impact deflagration propagation prior to DDT. Thus, in this work, to investigate the flame propagation perturbed by the enhancement on the difference in molar weight between the JICF medium and combustible mixture, an experimental study of the flame propagation perturbed by SF6 JICF is carried out. By adjusting the injection pressure (Pj) of the SF6 JICF, the characteristics of flame propagation and DDT are analyzed. The results showed that the flame can accelerate to the onset of detonation within a short run-up distance in the cases with SF6 JICF. Schlieren images of flame evolution show that the flame disturbed by SF6 JICF rapidly transitions to a wrinkled flame. It is suggested that the SF6 JICF-enhanced turbulence promotes flame acceleration and DDT. The effect of SF6 JICF on the flame characteristics can be concluded in two aspects: one is that the inhomogeneously distributed vortices that are induced by JICF cause a nonuniform diffusion of SF6 in the tube, resulting in a nonuniform dilution of the combustible mixture near the jet; another one is that the vortices induced by JICF stretch the flame surface to increase the flame area. Increasing the injection pressure could promote the formation of complex wrinkles in the flame, however, it shows few further improvements in promoting DDT. Different jet mediums are also tested, and the results show that SF6 JICF can shorten the run-up distance significantly compared with helium and nitrogen JICF. The small and densely distributed wrinkles significantly increase the flame area to enhance heat release. The flame disturbed by SF6 JICF could accelerate to the local sound speed of the unburnt mixture, which results in the facilitation of the precursor shock wave formation. After the flame passes through the SF6 JICF, it remains a larger acceleration rate and then transitions to quasi-detonation rapidly. The conclusion of this work could help to qualitatively understand the positive effect of JICF-induced turbulence on flame acceleration and DDT. It also provides a relatively efficient and safe technology to promote detonation initiation in the design of detonation engines.