Experimental investigation of the turbulent boundary layer associated with mixed, free and forced convection inside a vertical cylinder

Laser Doppler anemometry was used to measure characteristics of the boundary layers formed in free or combined free and forced air convection in a vertical hollow cylinder. The study covered the Re range 10,000-2 million and Grashof (Gr) numbers from 3 billion to 100 billion. Air temperature was kept around 40 C, wall temperature about 100 C, and flow velocities of 0-6 m/sec resulted. A decrease in the heat transfer rate was proportional to changes in the Gr. The velocity profile quickly changed from that of natural to mixed convection during nearly natural convective conditions. Adding a forced flow induced laminarization and reduced the heat transfer rate.