Temperature Measurement And Natural Convection Heat Transfer Of Cylinder Using Laser Interferometry

Temperature measurement and natural convection heat transfer from a horizontal cylinder make considerable promotion in energy, power and production safety. The inhomogeneous property of refractive index caused by different temperature is indicated from the interference fringes intensity using laser interferometry. Density, concentration or temperature can be deduced from the refractive index by applying the Gladstone-Dale equation in a non-intrusive and full-field way.Firstly, a comparison is made on principle and application among various optical techniques. Lateral shearing interferometry(LSI) and holographic interferometry(HI) are the most powerful. LSI belongs to a common optical path arrangement, which is capable of compact design, good stability, low-quality optical components and real-time measurement. An interferogram is generated by the wavefront and its laterally shifted wavefront in the overlap area. When the two wavefronts shift completely, the fringe pattern can be simplified together with two parts, in which each part is similar to that obtained using HI. In double-exposure HI, the original and distorted wavefronts strike on the same holographic plate. The plate is then repositioned and illuminated to produce the interferogram of the distorted wavefront. The additional phase is refrained and only one interferogram is required for reconstruction. Therfore, in this paper, temperature fields around two different horizontal cylinders in natural convection are experimentally studied using LSI with a large amount and double-exposure HI.Secondly, fringes in the shearogram without a cylinder(called the background) were simulated and thinned. The optical aberration in the background was analyzed. The actual optical defocus and spherical aberration were deduced numberically and verified experimently. Based on the deflection between the incident and the emergent angles, effect on the shearogram was discussed by variation of the shear amount. The optimum conventional angle was calculated as 60 deg. Experimental dark fringes were thinned by MATLAB software with the image morphology processing algorithm. Another fringe center method was presented.Thirdly, 420 shearograms of No.1 cylinder(the ratio of the length and the diameter L/D equal to 8.4) were recorded with large shearing amount. The cylinder wall temperature was naturally dropped from 425℃ to 15.88℃. Fringe centers were extracted from the peaks and valleys of Red channel in a color shearogram. Two interferograms(with and without the cylinder) was utilized to obtain the phase map and then reconstruct the 2D temperature field. This method overcomes systematic errors. Compared with that measured by thermocouples, the derivation was less than 0.19%. Curve-fitting of the disrete temperature data was done using exponential and six-degree-polynomial functions, respectively. For the wall temperatures 50℃, both the two functions were in a good agreement. While for the wall temperatures 420℃, the six-degree-polynomial showed a considerable advantage.Finally, natural convection heat transfer from No.2 cylinder(L/D=14.0) was studied experimentally using the double-exposure holography. The wall temperature was varied from 82℃ to 22.1℃. Fringe centers were identified by MATLAB procedures. The phase map is obtained from one hologram and then to calculate temperature field. Temperature gradient normal to the wall was estimated by extrapolation and then to evaluate the local and the average Nusselt, which indicate the heat transfer rate. A new Nusselt equation by curve-fitting was compared fairly well with the previous works.