Schlieren System Research Articles

Laminar flame characteristcs of ethanol-air mixture: Experimental and simulation study

Experimental test for laminar combustion of ethanol-air mixture was investigated in a constant volume combustion bomb. The laminar burning velocity and Markstein length were determined over an extensive range of equivalence ratios from 0.7-1.6 under an initial condition of 0.1 MPa pressure and 358 K temperature, with high-speed Schlieren system. The methods of linear extrapolation and non-linear extrapolation are compared and discussed. Apart from experiments, simulation was carried out in Chemkin by using the Marinov ethanol oxidation mechanism. Results indicate that non-linear extrapolation is more suitable to calculate the laminar burning velocity of ethanol-air mixture. The overall trends of laminar burning velocity vs. equivalence ratio are consistent between the experiment and simulation. The peak values of the laminar burning velocity from present experiment and simulation are 531.2 mm/s and 565.3 mm/s, both appearing at the equivalence ratio of 1.1. Moreover, the Markstein length of ethanol-air mixtures generally decreases with increasing equivalence ratio.

Impulse and mass removal rate of aluminum target by nanosecond laser ablation in a wide range of ambient pressure

Impulse and mass removal rate of a spherical target irradiated by a nanosecond 1-μm-wavelength pulse laser are measured experimentally in air at ambient pressures from 10 Pa to 100 kPa. Shock waves resulting from expansion of the laser ablation plume are visualized by a Schlieren system. At the pressures between 100 Pa and 10 kPa, the shock wave is found to expand anisotropically affecting the impulse that is generated from both of ablation jet and blast wave energy. The mass removal rate is found constant up to 4 kPa and then peaking at 20 kPa because of the confinement effect.

Relaxation process of the discharge channel near the anode in long air gaps under positive impulse voltages

The relaxation process of the discharge channel near the anode in a long air gap was observed using a Schlieren system with a temporal resolution of 5 µs and a spatial resolution of 70 µm. The dynamic characteristics of the decay process in the vicinity of the anode are obtained. The discharge channel evolves just as a growing mushroom in nature during the relaxation phase. Two physical quantities, angle θ and velocity v, are defined to describe the process in this paper. The average value of the angle and velocity under lightning impulses are 71.7° and 3.3 m s−1 respectively, while 7.7 m s−1 under switching impulses. A simplified model was established to simulate the formation of mushroom-shaped channel. The simulation and experimental results show that the formation and development of the mushroom-shaped channel are due to two factors. One is the convection of the high temperature and high pressure air near the anode produced by the first corona discharge; the other is the ionic migration. These two factors result in the phenomena that the cooling process in the vicinity of the anode is much more efficient than further into the gap, whereas the thermal conductivity of the anode may have little contribution to that.

The thermocapillary deformations in the locally heated shear-driven liquid film flowing in minichannel

Using phase schlieren system we have measured the shape of thermocapillary bump in the gas-driven (nitrogen) locally heated water film flowing in the minichannel. The measurements were conducted for liquid and gas flow rates corresponding to Rel=5.6 and Reg=73.5, respectively, and heat flux of 2 W/cm2. The paper shows a qualitative coincidence of the experimental results obtained for the locally heated flowing liquid film and the numerical results on the deformation in a locally heated shear-driven water film flowing in the minichannel.

Effect of a transverse plasma jet on a shock wave induced by a ramp

We conducted experiments in a wind tunnel with Mach number 2 to explore the evolution of a transverse plasma jet and its modification effect on a shock wave induced by a ramp with an angle of 24°. The transverse plasma jet was created by arc discharge in a small cylindrical cavity with a 2 mm diameter orifice. Three group tests with different actuator arrangements in the spanwise or streamwise direction upstream from the ramp were respectively studied to compare their disturbances to the shock wave. As shown by a time-resolved schlieren system, an unsteady motion of the shock wave by actuation was found: the shock wave was significantly modified by the plasma jet with an upstream motion and a reduced angle. Compared to spanwise actuation, a more intensive impact was obtained with two or three streamwise actuators working together. From shock wave structures, the control effect of the plasma jet on the shock motion based on a thermal effect, a potential cause of shock modification, was discussed. Furthermore, we performed a numerical simulation by using the Improved Delayed Detached Eddy Simulation (IDDES) method to simulate the evolution of the transverse plasma jet plume produced by two streamwise actuators. The results show that flow structures are similar to those identified in schlieren images. Two streamwise vortices were recognized, which indicates that the higher jet plume is the result of the overlap of two streamwise jets.

Effect of Opposing Multiphase Jet on Hypersonic Flow Around Blunt Body

An experimental research has been conducted to investigate the flow control method using opposing jet with air and water as a fluid spike on a bunt body in hypersonic freestream at Mach number 7, using the Kashiwa hypersonic wind tunnel. The dynamic characteristics and instability of opposing jet were clarified. It can be seen from the comparison between the air opposing jet and the water opposing jet that a liquid jet seems more effective in generating fluid spike for reduction in the drag force and the heating rate on the body. Details of the flow field and the jet oscillation have been captured by Schlieren system with high-speed cameras. The experimental data reveal the interaction of the jet oscillation with the shock wave fluctuation. Such interaction has strong impact on the whole flow field structure and should be taken into consideration in the design of the active flow control system using the opposing jet method.

Flame structure and laminar burning speed of gas to liquid fuel air mixtures at moderate pressures and high temperatures

Gas to liquid (GTL) fuel, synthesized from natural gas through Fisher–Tropsch (F–T) process, has gained significant attention due to its cleaner combustion characteristics when compared to conventional fuels. Combustion properties such as flame structure and laminar burning speed of GTL/air mixture premixed flames have been investigated. The GTL fuel used in this research was provided by Air Force Research Laboratory (AFRL), designated by Syntroleum S-8, which was derived from natural gas via F–T process. A mixture of 32% iso-octane, 25% n-decane, and 43% n-dodecane by volume is considered as the surrogates of GTL fuel for filling process. Experiments were conducted using a cylindrical chamber to study the flame structure and a spherical chamber for laminar burning speeds measurement. The cylindrical chamber was set up in a Z-shape schlieren system coupled with a high-speed CMOS camera that was used to capture evolutionary behavior of flames at up to 40,000 frames per second. Pressure rise as a function of time during the flame propagation in the spherical vessel was the primary input of the multi-shell thermodynamic model used to calculate the laminar burning speed for the smooth flames. Power law correlations over a wide range of pressures (from 0.5atm up to 4.3atm), temperatures (from 490K up to 620K), and equivalence ratios (from 0.7 to 1.2) have been developed for laminar burning speeds of GTL/air flames. Experimental burning speed results have been compared with simulation values obtained by the solution of one dimensional steady premixed flame code from CANTERA using Ranzi’s chemical kinetics mechanisms. Comparisons show very good agreement with the available experimental data in this study.

Flow of hot gases in vertical shafts with natural and forced ventilation

This paper presents the results from an experimental investigation on the flow of hot gases induced by pressure and buoyancy in vertical shafts. This flow circumstance simulates the propagation of smoke and other combustion products in vertical elevator shafts due to fire in high-rise, multistoried, buildings. The effect of natural and forced ventilation, occurring through shaft openings or vents, on the spread of smoke and hot gases is of particular interest with respect to the development of hazardous conditions in the building. Different configurations are investigated, such as natural and forced ventilation at the top or the bottom of the vertical shaft. The inflow conditions for the hot gases, at a vertical opening near the base of the shaft, are varied and characterized in terms of the inlet Grashof and Reynolds numbers. Extensive temperature measurements are taken within the shaft. These data are used to investigate the steady state thermal field that arises for various scenarios. These results may then form the basis for developing appropriate approaches for smoke mitigation in high-rise building fires. Flow visualization, using smoke and a Schlieren system, is also used to study the flow characteristics in the shaft and near the hot gas inlet. The results obtained indicate that a wall plume arises in several cases and a relatively well-mixed flow in others. A strong wall flow arises at high Grashof numbers and low Reynolds numbers, while increasing Reynolds numbers leads to greater mixing in the shaft. The resulting flow and its effect on the spread of hot gases and temperature decay are investigated for different shaft configurations and inlet conditions. It is found that the best smoke or hot gas removal and lowest shaft temperatures occur with natural ventilation at the top and forced ventilation up from the shaft bottom. It is also shown that forced downward flow at the top can be used to arrest smoke spread, as well as to minimize the effects of the fire.

Smartphone schlieren and shadowgraph imaging

Schlieren and shadowgraph techniques are used throughout the realm of scientific experimentation to reveal transparent refractive phenomena, but the requirement of large precise optics has kept them mostly out of reach of the public. New developments, including the ubiquity of smartphones with high-resolution digital cameras and the Background-Oriented Schlieren technique (BOS), which replaces the precise optics with digital image processing, have changed these circumstances. This paper demonstrates a number of different schlieren and shadowgraph setups and image examples based only on a smartphone, its software applications, and some inexpensive accessories. After beginning with a simple traditional schlieren system the emphasis is placed on what can be visualized and measured using BOS and digital slit-scan imaging on the smartphone. Thermal plumes, liquid mixing and glass are used as subjects of investigation. Not only recreational and experimental photography, but also serious scientific imaging can be done.

Initiation of homogeneous combustion in a high-velocity jet of a fuel–air mixture by an optical discharge

The effect of a focused pulsed-periodic beam of a CO2 laser on initiation and evolution of combustion in subsonic and supersonic flows of homogeneous fuel–air mixtures (H2 + air and CH4 + air) is experimentally studied. The beam generated by the CO2 laser propagates across the flow and is focused by a lens at the jet axis. The flow structure is determined by a schlieren system with a slot and a plane knife aligned in the streamwise direction. The image is recorded by a high-speed camera with an exposure time of 1.5 μs and a frame frequency of 1000 s−1. The structure of the combustion region is studied by an example of inherent luminescence of the flame at the wavelengths of OH and CH radicals. The distribution of the emission intensity of the mixture components in the optical discharge region is investigated in the present experiments by methods of emission spectroscopy.

The use of optical methods for measuring irregular reflection of weak acoustic shock pulses from a solid surface in air

Irregular reflection ofacoustic weak shockwaves (acoustic Mach numbers less than 0.01) is known as the von Neumann paradox and could not be described by the three-shock theory. In this work, nonlinear reflection regimes were studied experimentally using spark-generated spherically divergent N-waves reflecting from a plane rigid surface. Two optical methods were used in the measurements: a Schlieren system to visualize reflection patterns and a Mach-Zehnder interferometer to reconstruct pressure waveforms. The reconstruction was performed by applying the inverse Abel transform to the phase of the signal measured by the interferometer. The Mach stem formation was observed close to the surface as a result of collision of the incident and reflected front shocks of the N-wave and further away from the surface where the reflected front shock interacted with the incident rear shock. It was shown that irregular reflection occurred in a dynamic way and the length of the Mach stem increased while the N-wave propagated along the surface. In addition, reflection patterns were analyzed for several rough plane surfaces with different roughness size. The height of the Mash stem was found shorter for surfaces with larger dimension of the roughness.

Natural Convection in Redox Electrochemistry: Model, Simulations and Experiments

A natural convection model has been implemented on redox species TMPD with supporting electrolyte TBAP in acetonitrile solvent. The cross section geometry used for the two-dimensional simulations is 20 mm (W) x 10 mm (H). The working electrode is a 2 mm wide strip at the center of the cell floor and the counter electrode is the entire cell ceiling. For the three-dimensional simulations, a 3mm diameter working electrode in the semi-infinite configuration was used. A uniform magnetic field was applied in the direction perpendicular the working electrode surface. The first case we have studied has an initial concentration of 10.3 mM TMPD- and no oxidized form of TMPD. The supporting electrolyte concentration was 0.5 M. The cell was operated in the potential step mode and the applied potential was high enough for operation in the diffusion-limited regime. Simulations were run for two values of the magnetic field intensity. The electrical conductivity of the bulk solution was 0.625 S/m. The density was calculated using the formulation in our natural convection model based on electroneutrality, where non-electroactive ions migrate to the diffusion layer to neutralize the charge buildup due to the redox reaction. Cases with gravity not present, and gravity applied in the +x, -x, +y and -y directions in 2D were investigated. For the 3D simulations, gravity was applied in the direction normal to the electrode, toward and away from the electrode surface. To study the effect of pure natural convection, cases were run without applying the magnetic field. For this electrochemical system, the density changes from the initial bulk density (785 kg/m3) is very small with a maximum value of 0.0231 kg/m3, which gives a difference of 0.00294% between the highest and the lowest density. Even under this small difference, the evolution of the flow field is quite remarkable. With increasing time from the start, the effect of natural convection on the flow field becomes greater. The maximum velocity increased by an order of magnitude in ~30s from the start. At larger times, the maximum velocity tends toward an asymptotic value along with the electrode current also leveling off and reaching an asymptotic value. The behavior of mixed convection characterized by competing forces of buoyancy and Lorentz force was also investigated, where two values of the magnetic field were chosen. The natural convection-only cases show that the effect is stronger when the established flow is away from the electrode than when it is toward the electrode. This can be attributed to the electrode surface acting to provide a stable flow field configuration, confirming published results. The velocity is an order of magnitude higher when the flow is away from the electrode surface. Note that the cell we used is an order of magnitude higher than a typical microfluidic cell. Work is in progress to study the effect of geometry on natural convection where buoyancy is the only body force, and mixed convection where both buoyancy and MHD are present.Experimental results have been obtained on the ferri-ferro-KCL system. Quantitative Micro-schlieren (QMS) is an instrument suitable for measuring density gradients. A lens-based design makes packaging more compact. An LED light source is used to reduce cooling requirements. All the optics are research grade, thereby improving resolution and reducing aberrations. The electrochemical cell was mounted on a three-axis stage that is mounted on the optical rail for easy alignment. This schlieren system differs from conventional schlieren systems in its compact size (~6m X 2m vs. 1.5m x 0.5m, in footprint), its use of a more uniform intensity LED light source, variable density filter, bandpass filter, lenses instead of mirrors, a digital frame camera, and modern image processing techniques to extract quantitative information from the images. While conducting the redox electrochemistry experiments, it’d be important to ensure that the data are not corrupted by natural convection induced by temperature gradients that might be introduced by sources such as LED illumination. While it is not possible to completely eliminate this source of error, it can be minimized by keeping the LED intensity as low as possible and having the LED on only when necessary. The acquired sequence of images during a time period up to ~300s from the start show a column of high density fluid that starts from the surface and grows in length with time. At larger times (>100s) the column develops a transverse instability with a wave length of ~30 mm. Figure 1

Experimental investigation of effects of super high injection pressure on diesel spray and induced shock waves characteristics

Modern diesel engines employ higher injection pressure in the common-rail injection system that utilizes high-speed fuel jet to achieve rapid fuel-air mixing for spray atomization improvement. However, the spray characteristics under super high pressure are not well understood. To provide more information on spray characteristics, both the macro- and micro-characteristics of spray have been investigated under super high pressures (up to 300MPa) via an experimental method. The spray penetration and spray cone angle were measured using a high speed camera and the microscopic characteristic of Sauter mean diameter (SMD) was obtained via Malvern Spray Tec to analyze the influence of super high injection pressure on diesel spray atomization. Induced shock waves were captured using a Schlieren system when the spray tip velocity was greater than the local sound velocity accompanied by diesel spray. The results obtained show that higher injection pressures lead to larger spray velocities and a smaller shock wave angle if it exists, with connection between spray and shock waves, and generates a smaller SMD with less decreasing amplitude. Additionally, two different types of leading shock waves—bow and oblique shock wave appeared sequentially with increasing injection pressure.

Wide range instantaneous temperature measurements of convective fluid flows by using a schlieren system based in color images

In the schlieren method, the deflection of light by the presence of an inhomogeneous medium is proportional to the gradient of its refractive index. Such deflection, in a schlieren system, is represented by light intensity variations on the observation plane. Then, for a digital camera, the intensity level registered by each pixel depends mainly on the variation of the medium refractive index and the status of the digital camera settings. Therefore, in this study, we regulate the intensity value of each pixel by controlling the camera settings such as exposure time, gamma and gain values in order to calibrate the image obtained to the actual temperature values of a particular medium. In our approach, we use a color digital camera. The images obtained with a color digital camera can be separated on three different color-channels. Each channel corresponds to red, green, and blue color, moreover, each one has its own sensitivity. The differences in sensitivity allow us to obtain a range of temperature values for each color channel. Thus, high, medium and low sensitivity correspond to green, blue, and red color channel respectively. Therefore, by adding up the temperature contribution of each color channel we obtain a wide range of temperature values. Hence, the basic idea in our approach to measure temperature, 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 was applied to the measurement of instantaneous temperature fields of the air convection caused by a heated rectangular metal plate and a candle flame. We found that for the metal plate temperature measurements only the green and blue color-channels were required to sense the entire phenomena. On the other hand, for the candle case, the three color-channels were needed to obtain a complete measurement of temperature. In our study, the candle temperature was took as reference and it was found that the maximum temperature value obtained for green, blue and red color-channel was ∼275.6, ∼412.9, and ∼501.3°C, respectively.

An investigation on supersonic bevelled nozzle jets

This paper reports upon a numerical and experimental study on supersonic jets exhausting from bevelled nozzles with 30° and 60° exit inclination angles. To begin with, a simple but effective method to design a shock-free convergent–divergent circular jet nozzle to produce a well-conditioned supersonic Ma = 1.5 jet based on simple circular fillets is presented. Subsequently Reynolds-Averaged Navier–Stokes simulations of circular non-bevelled and bevelled nozzle jets were performed at over-expanded, perfectly expanded and under-expanded conditions. Lastly, these supersonic jets were visualized experimentally using a modified Z-type Schlieren system. Results show that the shock cell structure within the jet potential core changes from a diamond pattern to triangular and rectangular patterns as the nozzle pressure ratio and inclination angle are varied. Furthermore, jet plumes are deflected differently when the bevelled nozzles were operated at off-design conditions, with changes to the supersonic jet potential core length. Finally, quantitative analysis of the results reveals that bevelled nozzles are able to reduce the intensity of the supersonic jet shock cell structure considerably, which is potentially useful for broadband shock-associated noise mitigation purposes.

Optical Measurements of Shock Waves in Critical Nozzles at Low Reynolds Numbers

Two-dimensional critical nozzle flows at low Reynolds numbers are visualized by the rainbow schlieren deflectometry. Experiments have been performed in a region of overexpanded nozzle flow. The variation of the shock structure against the back pressure ratio can be clearly visible with color gradation. Static pressure rises due to the shock-induced flow separation are compared with the previous theories. The unsteady characteristics of overexpanded critical nozzle flows at low Reynolds numbers are quantitatively and qualitatively visualized using laser schlieren and Mach-Zehnder interferometer systems combined with a high-speed digital camera. It was found that an oscillating normal shock wave appears inside the nozzle, and that the shock wave has a specified dominant frequency. Also the time-history of the oscillating shock wave is obtained from both the systems and compared with each other.

Investigations on driving flow expansion characteristics inside ejectors

This research investigates the Mach wave structure of the driving flow under off-design working conditions by both numerical and experimental methods. By adopting the method of characteristics as the simulation model, prediction of the driving flow regime inside an ejector is obtained. The simulation results are further validated by an experimental visualization method conducted using a Schlieren system. Through this investigation, the influence of Mach wave on the driving flow boundary development is discussed. The expansion wave from the nozzle exit increases the driving flow regime in the under-expanded condition, which has a negative impact on ejector performance. The results show that the Mach wave should be considered when the ejector is operated under off-design working conditions. The results also demonstrate that an appropriate nozzle structure design was able to restrain the effect of the expansion wave, which improves ejector performance. The results are significant for achieving a comprehensive understanding of the mechanism of an ejector, as well as for the applications, such as ejection refrigeration cycles.

On the flame stability and laminar burning speeds of syngas/O2/He premixed flame

Fundamental properties such as flame structure and laminar burning speed of syngas/O2/He premixed flames have been investigated. Synthetic gas, also known as syngas, is a mixture of hydrogen and carbon monoxide that has been used in the present study. Three different mole fractions of 5%, 10% and 25% of hydrogen in the syngas were used in this research. Experiments were done in a cylindrical and a spherical chambers. Flame structure studies were made in the cylindrical vessel. The cylindrical vessel was coupled with a Z-shape schlieren system, equipped with a high speed CMOS camera, which has the capability of capturing pictures up to 40,000 frames per second, in order to study flame structure and instability. Hydrodynamic and thermo-diffusive instabilities on the flame front during propagation of flames have been observed and studied. Helium’s smaller molar heat capacity and higher thermal conductivity than nitrogen results in a more stable and smooth flames. The increase range of smooth flames can be used to validate chemical kinetics mechanisms at higher pressures and temperatures. Pressure rise data as a function of time during the combustion process was obtained through a pressure transducer in the spherical vessel was the primary input of the multi-shell model used to calculate the laminar burning speed for the smooth flames. Power law correlations over a wide range of pressures (from sub-atmospheric up to 7.3atm), temperatures (298K up to 650K), and equivalence ratios (0.6–3.0) have been developed for laminar burning speeds of smooth H2/CO/O2/He flames. At equivalence ratios corresponding to maximum laminar burning speeds, the experimental laminar burring speeds show a positive relationship with pressure. Experimental laminar burning speeds of H2/CO/O2/He mixtures have been compared with numerical values calculated by free flat flame simulation using two chemical kinetics mechanisms and some discrepancies discovered for equivalence ratios greater than 2. The effect of helium as a diluent on flame morphology and laminar burning speeds of syngas are compared with two diluents, nitrogen as well as nitrogen that is further diluted with synthetic exhaust gas recirculation (SEGR).

Visualization of hot flows of tall space fires in model experiments with Schlieren Photography technique.

The natural smoke exhaust system for tall spaces is more advantageous than the mechanical type of exhaust. In Taiwan, the effectiveness of natural smoke exhaust systems is inspected only by checking the vent area size. However, the air flow field in a tall space is very complicated, both at ordinary times or during fires. This study used Schlieren Photography technique, on the principle that light rays are refracted when penetrating materials of different densities, to test and simulate the dynamic measurement of hot air in tall space model. A single-mirror Schlieren system, including an 838 mm (H) × 736 mm (W) square concave mirror, as well as the focal length of 3100 mm was adopted. The experimental process of six smokeless candles were used for 1/12.5 model experiment to record the dynamic distribution and accumulation of air flow in the abovementioned space. FDS software was used to simulate various fire scenarios. The different locations of openings in some cases were studied with the maximum temperature scales of 40 and 45 °C, separately. The simulation results and experimental images showed highly similar hot air flow patterns. Schlieren Photography was proved capable of recording and visualizing the dynamic flow of hot air immediately, directly and accurately.

Focusing on breath

When I recorded the breath of patients with chronic obstructive pulmonary disease at the Royal Brompton and Harefield Hospitals, several of them talked about how difficult they found it to exhale. In looking for tools that were capable of registering even the most subtle of expirations, Peter Hobson (Department of Electronic and Computer Engineering, Brunel University, London, UK) and I decided to explore the potential of Schlieren imaging which can visually depict the flow of fluids or gases of varying density. Prof Hobson provided an appropriate optical setup at Brunel University (London, UK) where we explored the possibilities of this process to record the exhaled breath. The optical Schlieren system is capable of recording subtle differences in refractive indices, such as those created by warm breath in cold air. Light diverging from a laser source is focused by a spherical, concave mirror onto a knife edge (which provides a sharp line to precisely allow specific elements of the laser beam to be selected). Breathing warm air close to the surface of the mirror disturbs the light rays so that, when focussed, some bypass the knife edge and can be recorded by a camera positioned behind the system. Prof Hobson provided the breath, which I photographed to record the patterns produced by the warm breath turbulently mixing with the cool laboratory air. The image was recorded digitally directly into the camera and is therefore circular.