Experimental Investigations into the Heat Transfer of a Standardized Bimetal Annular-Finned Tube with Various Height of the Fins Under the Conditions of Free Air Convection

The paper presents a generalization of the results of an experimental investigation into the heat transfer of a bimetal finned tube with spiral knurled annular fins (interfin pitch s = 2.5 mm) under the conditions of free convection.

The obtained results were approximated using two options for selecting defining characteristics to calculate similarity numbers, viz., the West European variant, where the interfin space s′ is taken as a characteristic dimension and the wall temperature t_w is taken as a defining temperature, and the East European variant, where the fin base-limited tube diameter d₀ is taken as a characteristic dimension and the air temperature inside the chamber t₀ is taken as a defining temperature. It has been established that in tubes with narrow finning, free-convection heat transfer is represented with better quality by the criterial equations of the East European variant.

A similarity equation has been obtained for calculating the average reduced heat transfer of a single finned tube with various fin height h = 0–14.6 mm at different angles of the tube inclination γ = 0–90° to the horizontal plane at the Rayleigh number Ra = (32–450)·10³. A heat flux optimal fin height (h_ef = 10.8–12.3 mm) has been determined that decreases with increase in the angle of inclination.