tailieunhanh - solar collectors and panels theory and applications Part 7

Tham khảo tài liệu 'solar collectors and panels theory and applications part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 172 Solar Collectors and Panels Theory and Applications a c b d Fig. 2. a The hyperbolic surface with the null screen b Flat printed null screen with grid lines for qualitative testing c resultant image of the screen shown in b reflection on the test surface and d resultant image by a null screen with drop shaped spots for quantitative testing. For a quantitative testing of the surface a null screen with drop-shaped spots is used Fig. 2d to simplify the measurement of the positions of the spots on the CCD plane which are estimated by the centroids of the spots on the image of the null screen. Spherical convex surface The spherical convex surface used was a steel ball with a diameter of 40 mm the proposed cylindrical null screen was 60 mm in diameter. For a qualitative evaluation of the shape of the surface we designed a screen to produce a square array of 19x19 lines on the image plane. Figure 3a shows the spherical surface in Fig. 3b the flat printed null screen is shown and the image of the cylindrical screen after reflection on the spherical surface is shown in Shape Measurement of Solar Collectors by Null Screens 173 Fig. 3c the image is almost a perfect square grid but in this case the departures from a square grid which can be seen are probably due to a defocus of the surface and some printing errors and not to deformations of the surface. a b c Fig. 3. a Spherical surface steel ball b flat printed null screen with grid lines for qualitative testing and c the resultant image of the screen after reflection on the test surface. Surface shape evaluation The shape of the test surface can be obtained from measurements of the positions of the centroids of the spot images on the CCD plane through the formula Díaz-Uribe 2000 z - z0 1 d dy 3 where Hx Hy and Hz are the Cartesian components of the normal vector N on the test surface and Z0 is the sagitta for one point of the surface. The value of Z0 is not obtained from the test but it is only a constant value

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