Study on heat transfer and hydraulic resistance in a narrow annular channel

Experimental investigations of heat transfer and hydraulic resistance in turbulent flow of air in narrow annular channels (d1/d2=0.74 and 0.84) with a heated internal tube and small absolute values of a gap (β < 1.2 mm) have been performed. To avoid the eccentricity in channels spacers have been applied. The investigations have been performed in the range of Re=(6-60)x103, Tsub(wall)=450-770 K, q=60-350 kW(th)/m2(heat load), Tsub(wall)/Tsub(gas)=1.2-1.7. From the results obtained it follows, that in calculations of pressure losses in narrow annular channels with small gaps (β < 1 mm) and wall roughness Rsub(z) (>=) 0.003 mm for Re > 6x103, it is necessary to introduce corresponding corrections for wall roughness. On the base of the experimental data it is concluded that the Petukhov and Rojzen formula can be used for narrow annular chanels (d1/d2 > 0.7) with small values of gaps in calculations of average coefficients of heat transfer for the air. When various spacers are used, corrections determined experimentally are recommended