Schlieren System Research Articles

Spatio-temporal filter bank for visualizing audible sound field by Schlieren method

Visualization of sound field using optical techniques is a powerful tool for understanding acoustical behaviors. It uses light waves to examine the acoustical quantities without disturbing the sound information of the field under investigation. Schlieren imaging is an optical method that uses a camera to visualize the density of transparent media. As it uses a single shot to capture the information without scanning, it can observe both reproducible and non-reproducible sound field. Conventionally, the Schlieren system is applied to high-pressure ultrasound and shock waves. However, since the density variation of air caused by the audible sound field is very small, this method was not applicable for visualizing these fields. In this paper, a spatio-temporal filter bank is proposed to overcome this problem. As the sound is a very specific signal, the spatio-temporal spectrum (in two-dimensional space and time) of the audible sound is concentrated in a specific region. The spatio-temporal filter bank is designed for extracting the sound field information in the specific region and removing noise. The results indicate that the visibility of the sound fields is enhanced by using the proposed method.

Out-of-focus effects on microscale schlieren measurements of mass transport in a microfluidic device

The microscale schlieren technique provides a means for a non-invasive, full-field measurement for mixing microfluidics with excellent sensitivity and resolution. Nevertheless, an out-of-focus effect due to microscopic optics may lead to undesirable errors in quantifying the gradient information at high degrees of magnification. If the channel in the microfluidic device under study is too deep, light deflection caused by inhomogeneity located far from the focal plane may contributes little to the intensity change on the image plane. To address this issue, we propose the use of a weighting function that approximates a Gaussian profile with an optical-system-dependable width. We assume that the resultant intensity change is proportional to a weighted sum of the gradient across the channel depth and acquire micro-schlieren images of fluid mixing in a T-junction microchannel at various positions along the optical axis. For each objective, the width of the weighting function is then determined iteratively by curve fitting the ratio of changes in grayscale readouts for out-of-focus and focus micro-schlieren images. The standard deviation in the Gaussian distribution facilitates the quantification of the out-of-focus effect. In addition, we measure the sensitivities of a microscale schlieren system equipped with different objectives and compare the values to the model. Despite its better resolution, we find that an objective with higher magnification suffers from a more severe out-of-focus effect and a loss of sensitivity. Equations are proposed for estimations of the standard deviation and the sensitivity of microscale schlieren measurements. The outcome will facilitate the selection of proper microchannel depths for various microscale schlieren systems or vice versa, thus improving the precision of micro-schlieren measurements in microfluidic devices.

Exhaust gas recirculation effects on flame structure and laminar burning speeds of H2/CO/air flames at high pressures and temperatures

Experimental studies have been performed in conjunction with a novel differential based multi-shell model to investigate the flame structure and measure laminar burning speeds of H2/CO/air/diluent premixed flames at high pressures and temperatures. This paper focuses on synthetic gas (syngas) as the fuel blend, which is a mixture of H2 and CO, and investigates the effect of synthetic exhaust gas recirculation (SEGR) as the diluent on flame structure and laminar burning speed. SEGR is a mixture of 14% CO2 and 86% N2. In these experiments two different SEGR concentrations of 5% and 10% have been used. The experiments were performed in two constant volume spherical and cylindrical chambers. The cylindrical chamber was set up in a schlieren system equipped with a high speed CMOS camera, capable of taking pictures up to 40,000 frames per second, which was used to study the structure and stability of the flame. The laminar burning speed of the combustion process was calculated from the pressure rise measurement during flame propagation in spherical chamber. Power law correlations have been developed for laminar burning speeds of smooth H2/CO/air/SEGR flames over a wide range of temperatures (298K up to 450K), pressures (from sub-atmospheric up to 5.5atm), equivalence ratios (ϕ=0.6–3) and three different hydrogen concentration of 5%, 10% and 25% in the fuel mixture. SEGR lowers the laminar burning speed and has significant effect on the flame stability compared to H2/CO/air, especially for very lean and very rich mixtures. Experimental burning speeds of H2/CO/air/SEGR mixtures have been compared with available measurements as well as computed values obtained by 1D free flame simulations using two chemical kinetics mechanisms. Very good agreements have been observed with the experimental data available in the scientific literature as well as computational burning speeds.

EXPERIMENTAL STUDY OF COMBUSTION CHARACTERISTICS OF ISOLATED POCKETS OF HYDROGEN-AIR MIXTURES

This paper examines the dynamics of unconfined hydrogen–air flames and the criterion for flame propagation between neighbouring pockets of reactive gas separated by air using the soap bubble technique. The combustion events were visualized using high-speed schlieren or large-scale shadowgraph systems. It was revealed that for sufficiently lean hydrogen–air mixtures characterized by low flame speeds, buoyancy effects become important at small scales. The critical radius of hemispherical flame that will rise due to buoyancy is highly sensitive to the hydrogen concentration. The test results demonstrate that for transition of a flame between neighbouring pockets, the separation distance between the bubbles is mainly determined by the expansion ratio for near stoichiometric mixture, but it becomes much smaller for leaner mixtures because the flame kernel rises due to buoyant effects before the flame can reach the second bubble, thus the separation distance is no longer governed by the expansion ratio.

On colliding spherical flames: Morphology, corner dynamics, and flame-generated vorticity

The collision of two identical spherical flames was studied for its relevance in flame interactions in various practical situations including those within turbulent flames. Experimentally, premixtures of fuel (H2, C3H8) and oxidizer were ignited by two synchronized sparks in a closed chamber under atmospheric pressure, with the subsequent flame-front evolution tracked by a Schlieren system with high-speed camera, as well as computationally simulated. Results show that development of the morphology of flame collision can be distinguished in three stages: independent propagation, strong interaction with flattened interfaces, and corner dynamics after merging of the interfaces. It is further demonstrated that propagation of the flame corner is controlled by the interacting mechanisms of preferential diffusion and kinematic restoration, that vorticity of considerable strength is generated downstream of the corner flame segment, and that the kinetic energy of the flame-generated vorticity is substantial as compared to those of the Kolmogorov eddies such that it could have strong influence on the turbulence energy spectrum and flame burning rate.

Wide-range average temperature measurements of convective fluid flows by using a schlieren system.

In the schlieren method, the deflection of light by the presence of an inhomogeneous medium is proportional to the gradient of its refractive index. In the presence of temperature variations in a fluid flow, the refraction index is related to the gas density by the Gladstone-Dale constant, which depends on the nature of the gas and the wavelength of light propagating in the medium. The deflection of light in a schlieren system is represented by intensity variations on the observation plane. Then, for a digital camera, the intensity level registered in each pixel depends mainly on the refractive index variation of the medium and exposure time. Therefore, if we regulate the intensity value of each pixel by controlling the exposure time, it is possible to adjust the temperature value measurements. In this way, a specific exposure time of a digital camera allows us to measure a determined range of temperature values. For that reason, in this study we determine the range of temperatures that can be measured with a digital camera for different exposure times. By doing this, a wide range of average temperature value fields can be obtained by summing up the temperature contribution of each exposure time. The basic idea in our approach to measure temperature by using a schlieren system is to relate the intensity level of each pixel in a schlieren image to the corresponding knife-edge position measured at the exit focal plane of the system. Our approach is applied to the measurement of temperature fields of the air convection caused by a heated rectangular metal plate (7.3 cm×12 cm) and a candle flame. We found that the maximum temperature values obtained for exposure times of 31.3, 15.7, 7.9, 3.9, and 2ms were 67.3°C, 122.6°C, 217.4°C, 364.3°C, and 524.0°C, respectively.

20方向シュリーレンカメラによる不足膨張超音速噴流のセル構造の瞬間密度分布の三次元CT計測

Non-scanning 3D-CT technique using a multi-directional quantitative schlieren system with flash light source is proposed to obtain instantaneous 3D density distributions of unsteady flow phenomina. This "Schlieren 3D-CT" is based on (i)simultaneous acquisition of flash-light schlieren images taken from numerous directions, and (ii) 3D-CT reconstruction of the images by an appropriate CT algorithm. In this technique, fast flash-light source enables the observations of high-speed flows, e.g. super sonic flows. In this paper, with custom-made 20 directional schlieren camera, an instantaneous density distribution of steady under-expanded supersonic jets, which issue from an axisymmetric miniature nozzle of exit diameter of 1.0 mm, designed Mach number 1.5, have been CT-reconstructed.

Effect of dilution on laminar burning characteristics of H2/CO/CO2/air premixed flames with various hydrogen fractions

The laminar burning characteristics of syngas-premixed (H2/CO/CO2/air) laminar spherical flames have been systematically investigated through an experiment using Schlieren system. In the experiment, the volume fraction of H2 in the H2/CO part of the H2/CO/CO2/air mixtures was varied from 30% to 100%, the volume fraction of CO2 in the H2/CO/CO2 mixtures was varied from 0% to 60%, and the equivalence ratio was varied from 0.4 to 1.0. The effects of hydrogen fraction, carbon dioxide fraction and equivalence ratio on flame propagation, Markstein length, and laminar burning velocities have been studied. The obtained laminar burning velocities were compared with existing data from the literature and excellent agreement was obtained. The flames become more unstable by decreasing the equivalence ratio and increasing dilution fraction. The value of laminar burning velocity decreases apparently with the increase of carbon dioxide dilution fraction due to combined effect of decrease in adiabatic temperature, thermal diffusivity and the CO2 participation in the chemical reaction of CO+OH↔CO2+H. With the increase of hydrogen fraction, the laminar burning velocity increases significantly due to the chemical effect and diffusion effect rather than thermal effect.

3D-CT(Computer Tomography) Measurement of an Instantaneous Density Distribution of Turbulent Flames with a Multi-Directional Quantitative Schlieren Camera (Reconstructions of High-Speed Premixed Burner Flames with Different Flow Velocities)

In order to provide a suitable technique for 3D observation of high speed turbulent flames, non-scanning 3D-CT(Computer Tomography) technique using a multi-directional quantitative schlieren system with flash light source, is proposed for instantaneous density distribution of unsteady premixed flames. This “schlieren 3D-CT” is based on (i)simultaneous acquisition of flash-light schlieren images taken from numerous directions, and (ii) 3D-CT reconstruction of the images by an appropriate CT algorithm. In this paper, first, as a preliminary research, 3D-CT reconstruction of non-axisymmetric steady flame is made with a single-directional quantitative schlieren system. Next, with custom-made 20 directional schlieren camera, instantaneous density distributions of a high-speed turbulent flames of nozzle exit velocities of 8.0 and 10.0 m/s has been CT-reconstructed. The 3D-views of the reconstructed flame front shape clearly give the information of the flame structure with fine scale corrugations. Based on the distributions, area-enlargement rates of the flame front area are derived, and investigated.

Visualisation of alternating shielding gas flow in GTAW

The alternating shielding gas technique is a method of achieving transient arc characteristics during arc welding; however the complex flow that occurs through its use has not been investigated previously. A schlieren system was used to image density gradients that arise when alternating argon and helium shield gases, under varying flow parameters, with gas tungsten arc welding (GTAW). A theoretical analysis was carried out to determine the conditions under which the technique facilitates arc pulsing, in particular to avoid mixing of the shield gases in the delivery pipe prior to the welding nozzle. At appropriate pulsing frequency and flow rates, a stable horizontal region of helium was observed in the weld region, maintained in position by the denser argon from the preceding pulse. This higher than average mass fraction of helium when applying the shielding gases alternately, compared to a premixed gas with the same volume of argon and helium, increased the weld penetration by 13% on average, suggesting a modest improvement in heat transfer.

Reducing the radiation influence of self-luminous object on schlieren imaging via spatial filtering

It is generally accepted that increasing the object distance can decrease the radiation influence of the self-luminous object in a schlieren system. This paper shows that a larger object distance results in a brighter image that exerts a greater influence on schlieren imaging because of overwhelming. To decrease the radiation influence, we propose to put the self-luminous object nearer to the second schlieren mirror, e.g. in the front focal plane, and spatially filter the radiation at the back focal plane of the second schlieren mirror by a diaphragm. An experiment was carried out, showing that the image irradiance contributed by self-luminescence can be reduced by several orders by this scheme.

Schlieren visualization of water natural convection in a vertical ribbed channel

Schlieren techniques are valuable tools for the qualitative and quantitative visualizations of flows in a wide range of scientific and engineering disciplines. A large number of schlieren systems have been developed and documented in the literature; majority of applications involve flows of gases, typically air. In this work, a schlieren technique is applied to visualize the buoyancy-induced flow inside vertical ribbed channels using water as convective fluid. The test section consists of a vertical plate made of two thin sheets of chrome-plated copper with a foil heater sandwiched between them; the external sides of the plate are roughened with transverse, square-cross-sectioned ribs. Two parallel vertical walls, smooth and unheated, form with the heated ribbed plate two adjacent, identical and asymmetrically heated, vertical channels. Results include flow schlieren visualizations with colour-band filters, reconstructions of the local heat transfer coefficient distributions along the ribbed surfaces and comparisons with past experiments performed using air as working fluid.

Experimental study on the effects of the ignition parameters on the spark characteristics and the flame propagation of premixed methane–air mixtures

The charging time and the electrode gap distance are very important ignition parameters for the spark ignition process. The effects of these two parameters on the spark characteristics and the flame propagation of premixed methane–air mixtures were investigated experimentally at a constant pressure and a constant temperature by using a constant-volume bomb and a high-speed schlieren system. The parameters of the spark characteristics, including the spark voltage, the spark current, the spark duration and the lean burn ignition limit, were experimentally measured, and the spark resistance and the ignition energy were calculated on the basis of these parameters. By comparing the schlieren photographs of laminar flame propagation in different conditions taken by the high-speed schlieren devices, the effects of the ignition parameters on the cross-sectional area of the flame, the flame radius and the speed of flame propagation were analysed. The results show that the charging time and the electrode gap distance have important influences on the early kernel formation. Increasing the charging time or the electrode gap distance can significantly increase the influences of the effective radius of ignition and the original kernel size, can increase the ignition energy and can promote kernel development and flame propagation. Increasing the charging time or the electrode gap distance can effectively increase the lean burn ignition limit but, when the electrode gap distance increases to a certain extent, only increasing the charging time can increase the lean burn ignition limit further.

Study on Flame Stabilization in a Dual-Mode Combustor Using Optical Measurements

Flame stabilization in a dual-mode scramjet combustor was studied using simultaneous detection of flowfield and reaction zone. The instantaneous reactive flowfield was clearly visualized using an improved pulsed schlieren system, whereas the reaction zone was marked by chemiluminescence. Experiments were performed in a Mach 2.5 model combustor with the total temperature of and the total pressure of 1.0 MPa. Ethylene was used as fuel with equivalence ratios varied from 0.258 to 0.411. Four typical flame stabilized locations and corresponding flowfields are presented. When no choke occurs, the flame is stabilized in the cavity or the shear layer. Schlieren images show that the flow in the reaction zone of the latter case is supersonic. When the combustor is choked at the injection location, the flame oscillates between the shear layer and the jet wake, and the flow in the reaction zone becomes subsonic. When the flame is stabilized in the jet wake, autoignition can occur due to the compression of precombustion shock train. The short-lived aerodynamic throat formed near the injection location may be the possible cause for triggering the unsteady flame oscillation. The effects of heat release on flame stabilization modes are discussed.

Experimental Study of the Transient Response of Bunsen Flame to Nanosecond Pulsed Discharges

The transient processes associated with the interaction of a Bunsen flame and nanosecond pulsed discharges (NPD) are explored experimentally with two optical methods. A nanosecond-gated schlieren system is employed to image the shockwave propagation and the hydrodynamic response of the flame to NPD while the time-resolved optical emission spectroscopy measurements are carried out to determine active species and temperature in the plasma region created by the discharges. Therefore, the unsteady process of the interaction of the flame with the discharges is recorded in real-time by the combined measurements. Numbers of experimental evidences for understanding the dynamics of non-equilibrium plasma produced by NPD and performing further numerical simulation are offered.

Study of Schlieren Optical Visualization Basics Technique and the Principle

This paper presents a schlieren optical visualization technique and its application in observation of the changes of intensity in real images. Schlieren optical visualization technique system is the unique technique because it produces a neutral image easily-interpretable image of refractive-index-gradient fields. The schlieren technique remains to be one of the most powerful techniques to visualize the flow and useful as a tool in order to observe the flow characteristics, fuel-air mixing, spray evaporation and flame development. The schlieren system provides a method to viewing the flow through the transparent media. This paper present the basics technique of schlieren system especially for Z-type and two mirror schlieren system. This optical visualization photography together with digital video camera will capture the detail spray evaporation, mixture formation and flame process.

Application of Schlieren Optical Visualization System in External Combustion and Internal Combustion Engine: A Review

Schlieren optical visualization technique system is the unique technique due to the ability in producing a neutral image easily-interpretable image of refractive-index-gradient areas. The Schlieren system provides a method for viewing the flow through the transparent media and the most using this technique is to photograph the flow. This paper presents the review of the application of the Schlieren optical visualization system external and internal combustion engine in order to observe the fuel-air mixing and flame development during the burning process. The basic technique of Schlieren system, especially for Z-type and two mirror Schlieren system provide a powerful and clearly image to visualize the changes of the density in a transparent medium. This method can capture spray evaporation, spray interference and mixture formation clearly with real images. Analysis of optical image visualization observations reveals that the mixture formation of fuel and air exhibits the influence of the ignition and flame development. Thus, the observation of systematic control of the creation of a mixture of experimental apparatus allows us to achieve significant progress in the combustion process and will present the information to understanding the basic terms of reduced fuel consumption and exhaust emissions.

Flammability limits and burning characteristics of CO–H2–H2O–CO2–N2 mixtures at elevated temperatures

The paper presents the results of an experimental evaluation of the flammability limits and maximum pressure rise rate for multi-component (CO–H2)–O2–(H2O–CO2–N2)-mixtures at elevated temperatures of 170 °C and 250 °C at ambient pressure of 1 bar. A spherical explosion chamber with a volume of 8.2 dm3 was used for the experiments. A pressure method and a high-speed camera combined with a schlieren system for flame visualization were used in this work. Flammability limits and pressure rise rates are presented for multi-component mixtures with a steam concentration of 60% and a wide variation of fuel and oxidizer concentrations in which the H2 concentrations was limited to 3%.

Analyzing Mixing Inhomogeneity in a Microfluidic Device by Microscale Schlieren Technique.

In this paper, we introduce the use of microscale schlieren technique to measure mixing inhomogeneity in a microfluidic device. The microscale schlieren system is constructed from a Hoffman modulation contrast microscope, which provides easy access to the rear focal plane of the objective lens, by removing the slit plate and replacing the modulator with a knife-edge. The working principle of microscale schlieren technique relies on detecting light deflection caused by variation of refractive index. The deflected light either escapes or is obstructed by the knife-edge to produce a bright or a dark band, respectively. If the refractive index of the mixture varies linearly with its composition, the local change in light intensity in the image plane is proportional to the concentration gradient normal to the optical axis. The micro-schlieren image gives a two-dimensional projection of the disturbed light produced by three-dimensional inhomogeneity. To accomplish quantitative analysis, we describe a calibration procedure that mixes two fluids in a T-microchannel. We carry out a numerical simulation to obtain the concentration gradient in the T-microchannel that correlates closely with the corresponding micro-schlieren image. By comparison, a relationship between the grayscale readouts of the micro-schlieren image and the concentration gradients presented in a microfluidic device is established. Using this relationship, we are able to analyze the mixing inhomogeneity from associate micro-schlieren image and demonstrate the capability of microscale schlieren technique with measurements in a microfluidic oscillator(4). For optically transparent fluids, microscale schlieren technique is an attractive diagnostic tool to provide instantaneous full-field information that retains the three-dimensional features of the mixing process.

Analyzing Mixing Inhomogeneity in a Microfluidic Device by Microscale Schlieren Technique

In this paper, we introduce the use of microscale schlieren technique to measure mixing inhomogeneity in a microfluidic device. The microscale schlieren system is constructed from a Hoffman modulation contrast microscope, which provides easy access to the rear focal plane of the objective lens, by removing the slit plate and replacing the modulator with a knife-edge. The working principle of microscale schlieren technique relies on detecting light deflection caused by variation of refractive index1-3. The deflected light either escapes or is obstructed by the knife-edge to produce a bright or a dark band, respectively. If the refractive index of the mixture varies linearly with its composition, the local change in light intensity in the image plane is proportional to the concentration gradient normal to the optical axis. The micro-schlieren image gives a two-dimensional projection of the disturbed light produced by three-dimensional inhomogeneity. To accomplish quantitative analysis, we describe a calibration procedure that mixes two fluids in a T-microchannel. We carry out a numerical simulation to obtain the concentration gradient in the T-microchannel that correlates closely with the corresponding micro-schlieren image. By comparison, a relationship between the grayscale readouts of the micro-schlieren image and the concentration gradients presented in a microfluidic device is established. Using this relationship, we are able to analyze the mixing inhomogeneity from associate micro-schlieren image and demonstrate the capability of microscale schlieren technique with measurements in a microfluidic oscillator4. For optically transparent fluids, microscale schlieren technique is an attractive diagnostic tool to provide instantaneous full-field information that retains the three-dimensional features of the mixing process.