Large Lateral Shearing Interferometry And Its Applications In Physical Measurement

Laser interferometry has an inherent virtue in measuring the index of refraction in a non-destructive way. Moreover, the index of refraction can be related to many other parameters such as density, concentration and temperature by use of the Lorentz-Lorenz equation and its simplified format (namely the Gladstone-Dale equation) for gas. Modern interferometer systems which afford multi-directional projections and real-time measurements are all constructed from the double paths style interferometry. Although the common paths style interferometry has many virtues such as compact layout, insensitivity to vibration, less hardware demand, easy adjustment and adaptation to real-time measurement, the characters of its fringe pattern which is more complex than the double paths style interferometry's has limited its application.Although the lateral shearing displacements in the traditional studies about the lateral shearing interferometry are totally small and seem to be unimportant, the author found that when the lateral shearing displacement exceeds half width of the test distorted wave-front, the two sides of the fringe pattern seem similar to the simple one obtained in the double exposure holographic interferometry. Once the lateral shearing displacement exceeds full width of the test distorted wave-front, this fringe pattern would be torn apart and form two separate parts while each part equals the double exposure holographic interferometric fringe pattern. Considering that the primary drawback which limits its applications in comparison with other interferometry could be overcome in the above way, the large lateral shearing displacement interferometry would be a more economic choice to build up a practicable multi-directional interferometric system and has an enormous potential to replace traditional interferometry either in laboratorial conditions or in industrial conditions.In fact, there exist many kinds of aberration (such as the spherical aberration) in the optics components so the interferometric system still export a kind of special background fringe pattern even though no test object exists. Fortunately, the character of this special background fringe pattern is regular and can be described accurately using simple expressions. When the lateral shearing displacement is large enough, the phase difference between two states of fringe patterns, for example, one state goes with the test object while the other state doesn't, would directly lead to the test distorted wave-front without the use of a complex reductive algorithm which is essential in a condition with small lateral shearing displacement.Firstly, this large lateral shearing displacement interferometry has been used to measure the full field film thickness of the soap bubble, which is a typical 1-D test object, during the process from its formation to burst. We found that the film thickness distribution along the vertical ordinate of the test soap bubble could be calculated with a maximum error less than 3/8λ. Moreover, this film thickness distribution shows a very good agreement with a kind of exponential model at all times and the maximum discrepancy is less than 0.4λin most of the field. When the compensating effect from the plastic cannulation (used to blow the soap bubble and keep its suspense) is weaker than the drained effect from the top of the soap bubble to its bottom, a special zone where the water layer has drained away but the surfactant solution layer remains would appear at the top of the soap bubble and expand towards the bottom, until its burst. Considering the above exponential model with three parameters could also be simplified into an acceptable exponential model with two parameters, the quotient between the equivalent film thickness and the vertical gradient of the fringe order can be deduced along the vertical ordinate. In condition that the resolution of the fringe pattern is known, the measurement limitation of the film thickness along the vertical ordinate can be evaluated by this quotient.Considering that the flame of the co-axial burner usually departs slightly from its axis, here we update the above single-directional system and propose a tri-directional large lateral shearing interferometric system. Then this system has been used in the experimental research about the temperature reconstruction of a quasi-axisymmetric diffused ethylene flame under 2-D and 3-D methods.The 2-D method is based on axisymmetric character in an inverse Radon transform step and the primary result has been corrected by a particular coefficient field from the numerical simulation work. In comparison with the thermocouple results, the final reconstructed result shows a good agreement either outside the peak temperature location or in the further part inside the peak temperature location, with an average discrepancy between 20 and 40 K. However, the discrepancy is obviously higher in the closer part inside the peak temperature location with a maximum discrepancy up to 200 K. Considering that a high soot volume fraction due to the insufficient combustion only exists in the closer part inside the peak temperature location, the thermocouple results in fact correspond to the continuously growing soot ball around the node rather than to the gas flame. When the deposition effect of soot has been excluded, the 2-D reconstructed result would be a credible and quantitative measurement result for the axisymmetric test flame.The 3-D method doesn't utilize the axisymmetric character and has only been corrected by a universal coefficient value as 1.05, which is the average statistical value of the particular coefficient field. This result is qualitatively similar to the 2-D result but also exhibits some asymmetrical character. However, this reconstructed temperature field at 3 cm height exhibits obvious disturbance because the relative intensity ratio between the laser beam and the flame radiation is the weakest at 3 cm height. The relative discrepancies between the two kinds of reconstructed temperature values along the axis are generally less than 3.3% except at 1 cm height, where it has a high concentration of ethylene and is not suitable to use the universal correct coefficient. Therefore the 3-D method might still afford qualitative analysis for an asymmetrical flame, such as online combustion monitoring for turbulent flow flame in industry.