Wind tunnel blockage

Corrections by Morgan [66] for the combined effects of free-stream turbulence and wind tunnel blockage on Nusselt numbers measured in air are shown in Fig. 6.27. Here, dldT is the ratio of the cylinder diameter to the wind tunnel height or diameter, 8 Nu<( is the increase in the Nusselt number over Hilpert s measurements (Fig. 6.26), and Tu is the intensity of the longitudinal turbulent fluctuations in the free stream. In addition to these corrections, Morgan also proposed the Nusselt number correlations shown in Table 6.7 that are applicable to an extremely wide range of Reynolds numbers in air. At the higher Reynolds numbers, these Nusselt numbers are quite consistent with those of Fig. 6.26. 

FIGURE 6.27 Correction factors to Nusselt numbers for combined effects of wind tunnel blockage and free-stream turbulence [66]. (Reprinted by permission of Academic Press.)... 

Experimental data on heat transfer from spheres to an air stream are shown in Fig. 5.20. Despite the large number of studies over the years, the amount of reliable data is limited. The data plotted correspond to a turbulence intensity less than 3%, negligible effect of natural convection (i.e., Gr/Re <0.1 see Chapter 10), rear support or freefloating, wind tunnel area blockage less than 10%, and either a guard heater on the support or a correction for conduction down the support. Only recently has the effect of support position and guard heating been appreciated a side support causes about a 10% increase in Nu... 

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