Schlieren technique

In the 1960s, Oppenheim et al. [10,19,20] succeeded in obtaining photographs with better resolution by means of schlieren technique with microsecond flash and then with the very short (less than 10 s) laser light pulses. This facilitated the attainment of a stroboscopic set of essentially still photographs that revealed many details of DDT. At the same time, Soloukhin [21] published a series of streak photographs taken with schlieren system and Denisov and Troshin [22] discovered that detonation leaves a record of its passage in the form of imprint on a wall coated with the thin layer of soot. 

Many photographic records of fhe flow made using the schlieren technique have revealed its very high... 

The shadow cone is used by many experimenters because it is much simpler than the Schlieren techniques. Moreover, because the shadow is on the cooler side, it certainly gives more correct results than the visible cone. However, the flame cone can act as a lens in shadow measurements, causing uncertainties to arise with respect to the proper cone size. 

Concentration changes are observed optically from time to time either by light absorption (e.g. ultraviolet absorption for protein solutions) or, more usually, by schlieren or interference methods. These optical methods for examining concentration changes in liquid columns (especially the schlieren technique in its various forms) are also employed in the ultracentrifuge and for studying movingboundary electrophoresis. The schlieren method is based on the fact that, at a boundary between two transparent liquids of different... 

Belinger, Schlieren and Shadowgraph Equipment , JOpticalSocAmer 35(8), Aug 1945 11) W.R. Keagy et al, "Schlieren Techniques for the Quantitative Study of Gas Mixing , Rand Corp, Santa Monica, Calif, R-164 (1949) 11a) A.J. Zaehringer, RocketScience... 

B2. Schlieren Techniques for the Quantitative Study of Gas Mixing. Battelle Mem. 

The unimolecular decomposition of 1,3,5-trioxane into formaldehyde has been studied experimentally in the temperature range of 700-800 K under atmospheric pressure <1992JPC295> the rate constant measured in this temperature range is k (s )= exp(-(47.5 2.4) kcalmoP (iJ7) ). The laser schlieren technique was... 

Channel formation experiments have been conducted with a small Prototype machine. This machine is a 600 kV, 5 60 ns water transmission line charged with a Marx generator. With fill pressures of 1 to 4 atmospheres, these experiments have created channels of 0.1 mm radius at currents up to 100 kA. The density distribution of a channel is measured by a Moire-Schlieren technique with illumination by a short pulse N2 laser (3373 A). An example of such a channel photograph is shown in Fig. 20. In these experiments no sausage or kink instabilities have been observed out to 120 ns. 

Turning to optical evidence of the breakdown process, laser-Schlieren techniques, which are capable of detecting microscopic local changes in liquid density on a nanosecond time scale, have been used. 

It is possible, however, to get more useful information on streamer initiation the method consists of, under a rectangular voltage, measuring t, defined as the time at which a small optical perturbation (e.g., < 10 ym in diameter) is detectable by the Schlieren technique, or by light emission, or also by current measurements characteristic of the early streamer propagation. It was shown that when the rise time of the voltage was much lower than t, for several liquids, initiation times were shorter with the positive point than with the negative point, but from the V-t curves it was only deduced that one mechanism appeared to be rejected-cavitation, as will be discussed later - and it was not feasible... 

Using the schlieren technique we have shown that patterns were present in the system before irradiation, and that after irradiation the coloured structures appeared where the pre-patterns had already been localized. The photochemical reac -tions merely serve to reveal the convective pre-patterns induced by evaporative cooling. There exists a linear relationship between the average wavelength of the structures and the depth of the layer. 

Then they have clearly demonstrated the importance of evaporation in the generation of these structures. Indeed on closure of the reactor the shapes fade away but reform on opening. Furthermore, using the schlieren technique they obtained photographs of stria-tions (prepatterns) [6 ] existing in the fluid prior to irradiation. The existence of these vermiculated convective rolls was decisively proven by depositing ink on the bottom of the dish. 

Since last century, many researchers have undertaken the observation of Marangoni convection, especially using the laser Schlieren technique. In this section, some results of our experimental study on interface structure are presented for illustration [5-9]. The experimental installation is shown in Fig. 8.1. The experiment was undertaken at constant temperature so that the temperature-induced Marangoni convection can be eliminated. The liquid-gas contactor can be placed in horizontal position for horizontal liquid-gas flow, or in vertical position for falling liquid film and uprising gas flow. The mass transfer process to be study is either absorption or desorption of CO2 by various kinds of absorbent. Nitrogen is served as CO2 carrier. The liquid phase can be in either stagnant or countercurrent flow with the gas phase. 

Fig. 2. Sketch of the optical setup for the Schlieren technique with the system in the drop configuration in the Petri dish.

---