Rainbow Schlieren Deflectometry Research Articles

Schlieren measurements and analysis of concentration field in self-excited helium jets

A low-density gas jet injected into a high-density ambient gas is known to exhibit self-excited global oscillations accompanied by large vortical structures interacting with the flow field. In this study, the formation and evolution of vortices and scalar structure of the flow field are investigated in buoyant helium jets discharged from a vertical tube into quiescent air. This is accomplished by applying the quantitative rainbow schlieren deflectometry technique to optically measure the local helium mole percentage across the whole field. Data were acquired over downstream locations extending from tube exit to about 3.0d (d=31.8 or 19.1 mm is the jet tube inside diameter) at spatial resolution of 0.14 mm and temporal resolution of 16.7 ms. Oscillations at identical frequency were observed throughout the flow field. The evolving flow structure is described by helium mole percentage contours during an oscillation cycle. Instantaneous, mean, and rms concentration profiles are presented to describe interactions of the vortex with the jet flow. Oscillations in a narrow wake region near the jet exit are shown to spread through the jet core near the downstream location of the vortex formation. The effects of jet Richardson number on characteristics of vortex and flow field are investigated and discussed.

Gravitational Effects on Near-Field Flow Structure of Low-Density Gas Jets

Experiments were conducted in Earth gravity and microgravity to acquire quantitative data on near field flow structure of helium jets injected into air. Microgravity conditions were simulated in the 2.2-second drop tower at NASA Glenn Research Center. The jet flow was observed by quantitative rainbow schlieren deflectometry, a non-intrusive line of site measurement technique for the whole field. The flow structure was characterized by distributions of angular deflection and helium mole percentage obtained from color schlieren images taken at 60 Hz. Results show that the jet flow was significantly influenced by the gravity. The jet in microgravity was up to 70 percent wider than that in Earth gravity. The jet flow oscillations observed in Earth gravity were absent in microgravity, providing direct experimental evidence that the flow instability in the low density jet was buoyancy induced. The paper provides quantitative details of temporal flow evolution as the experiment undergoes a change in gravity in the drop tower.

Application of rainbow schlieren deflectometry to measure temperature and oxygen concentration in a laminar gas-jet diffusion flame

The quantitative rainbow schlieren deflectometry (RSD) technique was applied to measure temperature and oxygen concentration in an axisymmetric hydrogen gas-jet diffusion flame burning in quiescent air at fuel jet exit Reynolds number of 70. Schlieren measurements were compared with conventional measurements using a thermocouple and a gas-sampling probe. Good agreement between the two measurement techniques was achieved on the fuel-lean side of the flame.

Temperature measurements in steady axisymmetric partially premixed flames by use of rainbow schlieren deflectometry.

We focus on the utility of rainbow schlieren as a tool for measuring the temperature of axisymmetric partially premixed flames (PPFs). Methane-air PPFs are established on a coannular burner. The flames involve two spatially distinct reaction zones, one in an inner premixed region that has a curved tip and a spatially planar wing portion and another that involves an outer nonpremixed zone in which intermediate species burn in air. Schlieren images are found to visualize clearly these PPF characteristics through light deflection by steep refractive-index gradients in the two reaction zone fronts. The temperature distributions of two flames established at fuel-rich mixture equivalence ratios of phi(r) = 1.5 and 2.0, with bulk-averaged velocities, Vreac = 60 cm s(-1) and Vair = 50 cm s(-1), are inferred from color schlieren images, and a measurement error analysis is performed. Errors arise from two sources. One lies in the process of inferring the temperature from the refractive-index measurement by making assumptions regarding the local composition of the flame. We have shown through simulations that the average temperature deviations due to these assumptions are 1.7% for the phi(r) = 1.5 flame and 2.3% for the phi(r) = 2.0 flame. Another source involves the local uncertainty in the measurement of the transverse ray displacement at the filter plane that is used to determine the refractive index and thereafter the flame temperature. We have ascertained that a maximum error of 4.3% in the temperature determination can be attributed to this local measurement uncertainty. This investigation demonstrates the capability of the schlieren technique for providing not only qualitative displays of the PPFs but also full-field-of-view temperature measurements that are accurate, spatially resolved, and nonintrusive.

Effect of varying composition on temperature reconstructions obtained from refractive index measurements in flames

Optical methods (such as holographic interferometry, speckle photography, speckle shearing interferometry, moiré deflectometry, rainbow Schlieren deflectometry and Talbot interferometry, and so forth) have the potential for accurately measuring the entire temperature field associated with multidimensional flames, which may be difficult to do using other techniques. These interferometric or deflectometric techniques first determine the refractive index in flames, and thereafter infer the temperature distribution. The relationship between the refractive index and temperature is obtained by using a state equation and the Gladstone-Dale relation. However, a potential source of error arises since the local composition of the flame being studied is usually unknown. In a previous investigation, we examined the occurrence of this error by assuming the local flame composition to be the same as that of air at the local temperature. This was examined through one-dimensional simulations of counterflow flames. We found that while calculating the temperature from the measured refractive index this assumption could lead to significant errors for some flames and to minimal errors in other flames. This investigation quantifies those errors in the context of two-dimensional flames in both planar and axisymmetric geometries. It is found that the refractive index values of a mixture are nearly identical with those of the refractive index of air for partially premixed flames (PPFs). The maximum error lies in the range of 6.3 to 10.7% for one-dimensional (1-D) counterflow PPFs, and between 6.1 to 8.0% for two-dimensional (2-D) planar and axisymmetric PPFs (for equivalence ratios in the range of 1.5 ≤ φr ≤ 2.0). For nonpremixed flames, however, the maximum error can have values up to 33.8% and 34.5% for the 1-D and 2-D configurations, respectively. Therefore, the accurate inference of the temperature of nonpremixed flames from the measured refractive index distribution requires that an alternative approach be developed. We have developed an interpolation method that reduces the maximum error from 34.5% to 9.8% for these flames, and is associated with even smaller errors in most other regions of these flames.

Effects of ambient pressure and burner scaling on the flame geometry and structure of hydrogen jet flames in cross-flow

The effects of ambient pressure and burner scaling on flame geometry and structure of a hydrogen jet flame in a cross-flow are presented. To delineate the effects of buoyancy, the fuel was injected horizontally into a vertically moving air stream, thus, buoyancy was aligned with the direction of the cross-flow. The combustion chamber pressure was varied from 1 to 1.45 bar, and the jet-to-cross-flow velocity ratio from 25 to 240. For flow visualization, Rainbow Schlieren Deflectometry (RSD) along with conventional flame photography was used. The centerline flame trajectory showed three distinct regions which were different from the previously reported findings where buoyancy acted along the jet axis. The dimensionless coordinate of the jet centerline in the jet direction varied with 0.5, 0.3, 0 power of the coordinate in the vertical cross-flow direction, and was correlated with 0.15 power of the burner diameter. The effect of pressure on flame trajectories was not significant. Flame chord length varied with −0.38 power of the jet-to-cross-flow velocity ratio.

Quantitative measurements of laminar hydrogen gas-jet diffusion flames in A 2.2 s drop tower

Drop tower experiments were performed to identify buoyancy effects in laminar, hydrogen gas jet diffusion flames. Quantitative rainbow schlieren deflectometry was utilized to optically measure distributions of refractive index and, thus, temperature and oxygen molar percentage assuming chemical equilibrium in the flame. Test conditions consisted of atmospheric pressure flames burning in quiescent air at jet exit Reynolds numbers of 40 and 70, burner diameters of 0.30 and 1.19 mm, and jet exit Froude numbers of 1150 to 219,900 in earth gravity. Results show that the stoichiometric flame surface reached steady microgravity conditions, although the surrounding flow was evolving at the end of the 2.2 s drop period. In microgravity, the axial diffusion was important throughout the flow field. The maximum temperature and flame length were independent of gravity. Gravity affected the flame width, although the effect diminished at higher jet exit Froude numbers. For a given jet exit Reynolds number, the shape of the microgravity flame was independent of the burner diameter.

Schlieren analysis of an oscillating gas-jet diffusion flame

Flow structure of a flickering gas-jet diffusion flame was investigated using quantitative rainbow schlieren deflectometry. The fuel was pure hydrogen exiting from a burner tube at 43 m/s. Angular deflection data were obtained across the whole field of color schlieren images taken at a spatial resolution of 0.14 mm and a temporal resolution of 60 Hz. These line-of-sight data were analyzed to determine power spectra and flame flicker frequency at different operating pressures to scale the effects of buoyancy. The measurements were tomographically inverted to obtain the refractive index and, hence, temperature distributions assuming chemical equilibrium in the flame. The oscillating flow is described quantitatively in terms of temporal evolution of the temperature field. The flame structure is also described statistically by mean, root-mean-square (RMS), and probability density function profiles of temperature. Results reveal global oscillations in the flow field of the flame. The oscillations were stronger in the outer flow as compared to those at the flame surface. The flame was clipped-off at a downstream location, where the outer vortical structures intensified and entrained cold fluid toward the center region.

Effects of Buoyancy on Steady Hydrogen Gas-jet Diffusion Flames

Laminar gas-jet diffusion flames of a nonsooting, fast reacting fuel were investigated at atmospheric pressure in microgravity and at different pressures in normal gravity. These weakly luminous flames were visualized and quantified using the rainbow schlieren deflectometry. The results show that the overall features of the microgravity flame are replicated in normal gravity experiments at sub-atmospheric pressures. The flame width in microgravity correlated with that in normal gravity, when the normalized Froude number at the burner exit was used as a parameter. Normal gravity experiments at low pressures produced nonbuoyant flames that remained parallel to the burner tube regardless of the tube's orientation with respect to the gravitational vector.

Quantitative Evaluation of Flow Computations by Rainbow Schlieren Deflectometry

Rainbow schlieren deflectometry, a line-of-sight optical measurement technique, was used to evaluate computations of steady nonreaeting and reacting flows in axisymmetric systems. The nonreacting flow was that of an isothermal helium jet discharged vertically in air, and the reacting flow was that of a hydrogen jet diffusion flame. The computed results were processed to obtain a color schlieren image for direct visual comparison with the corresponding experimental color schlieren image. The computed and measured angular deflections of light rays were also compared. In addition, the computed field distributions of scalar flow properties including the refractive index, species concentration, and temperature were compared quantitatively by inverting the experimental schlieren measurements. The results demonstrate the effectiveness of the rainbow schlieren technique in evaluating the physical models used for flow computations

Application of rainbow schlieren deflectometry for concentration measurements in an axisymmetric helium jet

The rainbow schlieren deflectometry technique was used to measure oxygen concentrations in a laminar, isothermal helium jet discharged vertically into ambient air. The concentration distributions were inferred from the color schlieren image by taking into consideration the sampling interval and noise in measurements, especially near the jet center. Excellent quantitative agreement was reached between measurements from schlieren and a continuous sampling probe. This work demonstrates the capability of the schlieren technique for providing accurate, spatially-resolved, nonintrusive, full-field of view measurements of species concentration in an isothermal binary system. Because the basic quantity measured is the refractive index, the present schlieren technique can be extended for quantitative measurements of other scalar flow properties related to the refractive index.

Three-dimensional rainbow schlieren tomography of a temperature field in gas flows

We present quantitative rainbow schlieren deflectometry with tomography for measurements of temperature in three-dimensional gas flows. The schlieren apparatus with a continuously graded spectral filter of known transmissivity was used to create color schlieren images of the test media. These images at multiple viewing angles were used to infer beam deflection angles by the medium. The deflection data were used with a tomographic technique to reconstruct the refractive index and thus the temperature field. The temperature distributions obtained by the rainbow schlieren tomography agreed with those measured by a thermocouple probe. This research demonstrates that tomography can be used with full-field schlieren deflectometry to measure quantitatively temperature in asymmetric gas flows. The technique could be used to obtain related properties such as pressure, density, and gas composition.

Quantitative evaluation of flow computations by rainbow schlieren deflectometry

Quantitative evaluation of flow computations by rainbow schlieren deflectometry

Flame spread across liquid pools with very low-speed opposed or concurrent airflow

The consequences of forced airflow on flame spread over liquids is to a large extentunexplored, especially for low air speeds on the order of that due to buoyant convection driven by the flame. Yet small gas flows can influence heat and mass transfer ahead of the flame and alter its spreading characteristics. In this paper we present results from normal and microgravity experiments on flame spread over 1-butanol with either an opposed or a co-current forced airflow ranging from 5 to 30 cm/s. A small flow duct was used to control the air flowing over a laboratory-scale fuel tray. Video flame imaging was used to determine the flame shapes and spread rates, and Rainbow Schlieren Deflectometry (RSD) and infrared thermography of the liquid surface were employed to measure the liquid thermal fields during spread. Large differences in the liquid temperature field were noted depending on the flow direction, and for some concurrent flow rates, the initial flame spread rate was affected. Slow opposed airflows serve mainly to alter the flame pulsation frequency, while small concurrent flows eliminate the pulsations in agreement with numerical modeling that indicates a gas-phase recirculation cell is necessary for flame pulsations. In microgravity the flame spreads slowly and uniformly for higher opposed velocities, and a lower bound of between 10 and 20 cm/s is found below which the flame begins to spread and then extinguishes.

Detailed experiments of flame spread across deep butanol pools

Experiments using simultaneous rainbow schlieren deflectometry, particle image velocimetry (PIV), infrated (IR) thermography, standard flame imaging, and thermocouples are conducted to reveal detailed physics of low-speed, forced-opposed-flow flame spread over liquid pools. Our effort concentrates on the effects of gravity and air flow speed on the flame-spread behavior across a deep, rectangular tray filled with 1-butanol. The microgravity (μg) part of the experiment is the first combustion experiment conducted aboard a sounding rocket. Several novel observations are made. Even with forced-air velocities of the same order of magnitude as that induced naturally by buoyancy in normal gravity, the μg flame behavior is completely different. Whereas the normal gravity (1 g) flames we studied are soot-producing, and pulsate and spread rapidly, the μg flames are free of soot and spread steadily and very slowly. The rainbow schlieren measurements show that heat penetrates far deeper into the pool in μg, suggesting that the major effect of liquid-phase buoyancy is its stratification of the temperature field in 1 g: its absence in μg may lead to a very different liquid-phase surface temperature, flow field, and flame-spread character. The IR thermography reveals hetetofore unobserved liquid surface temperature and side-flow phenomena in both 1 g and μg. PIV has been used for the first time to obtain quantitative, full-field, liquid-phase velocity fields ahead of the flame, revealing significantly more surface flow in μg than in 1 g. A state-of-the-art model's predictions are qualitatively confirmed in regard to gravitational effects on flame shape, flame extinction in μg when there is no opposed air flow, and fuel consumption rate. Disagreement is found in the flame spread character in μg, unless hot gas expansion is artificially set to zero.

Compact color schlieren optical system

A compact optical system for use with rainbow schlieren deflectometry is described. Both halves of the optical system consist of well-corrected telescopes whose refractive elements are all from manufacturer's stock catalogs, with the reflective primary being a spherical surface. As a result, the system is relatively easy to construct and meets the requirement of long focal length for quantitative rainbow schlieren measurements.