Study of laminar thermal boundary layers occurring around the leading edge of a vertical isothermal wall using a specklegram technique
✍ Scribed by D. Kastell; K. D. Kihm; L. S. Fletcher
- Book ID
- 104656504
- Publisher
- Springer
- Year
- 1992
- Tongue
- English
- Weight
- 877 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0723-4864
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✦ Synopsis
The influence of a 45 degree leading edge on the develop-Xo ment of a thermal boundary layer on a vertical isothermal wall has been investigated by measuring detailed temperature profiles with a y He Ne laser specklegram technique. A digital image processor was z employed to improve the accuracy in evaluating the space of fringes, ct which were constructed from the specklegram. A matrix of five fl different temperatures and four different vertical locations was con-e sidered in the investigation. The leading edge effect was evident in 6 the vicinity of the edge (x = 7.8 mm and 14.2 mm) showing a strong nonsimilarity, and the thermal boundary layers occurred around the 0 leading edge. Classical theory did not properly describe the actual situation near the leading edge, where the theory assumed an abrupt starting of the boundary layer. In an attempt to improve the agree-q ment in temperature profiles, the Grashof number was modified by extending x to an equivalent plate length x 0. In the region farther Q 2 from the leading edge (x = 50.0 mm), where similarity in temperature A profiles was well established, agreement with theory was dramatical-v ly improved with substitution of x 0 for x. A linear decrease in the extension ratio Xo/X with Gr~/4, i.e., Xo/X=-0.4. Gr~/4+12.63 where the Grashof number was based on x, was observed for Grashof numbers up to 8.0 x 105. For Grx>8.0 x l0 s, the leading edge effect became negligibly small and the ratio Xo/X approached unity.
List of symbols a
distance between the test section and the second parabolic mirror (mm) b distance between the second mirror and the ground glass (mm) c distance between the ground glass and the focal plane of a camera used for specklegram recording (mm) d distance between the specklegram negative and the detector plane of a video camera used for the fringe spacing evaluation (ram) g acceleration of gravity (9.8 m/s 2)