PREDICTIONS OF TURBULENT FLOW AND HEAT TRANSFER IN INTERNALLY FINNED TUBES

A two-equation turbulence model was developed to investigate both fluid flow (including secondary flow) and heat transfer characteristics of internally finned tubes under fully-developed turbulent air (Pr = 0.71) flow conditions. Finning configurations consisted of identical, equi-spaced, longitudinal fins of trapezoidal profile. The thermal boundary conditions were uniform heat input per unit axial length with constant peripheral (including fin surfaces) temperature. Predictions were obtained and are presented for fin height-to-lube radius ratios (H) of 0.2 to 0.8, fin numbers (M) from 6 to 14, and a Reynolds number (Re) range of (2.5-15)10(4). The predicted results provide extensive information on the effects of M, H and Re on secondary flow pattern, the cross-sectional distributions of mean axial velocity and turbulent kinetic energy, and the distributions of local wall shear stress (and local heat flux) on the fin surface and tube wall. The secondary flow pattern was found to consist of two counter-rotating cells of secondary flow in each primary flow cell for all M. W combinations. The strength and extent of each cell was strongly dependent on H and M. and peak secondary velocities ranged from-about 1 1/2% to 4% of the bulk velocity. For the mean axial velocity for H > similar to 0.6. in addition to the global maximum in velocity at the tube centreline, a second peak was present in each of the bay regions formed by adjacent fins. In general. the predicted results are in satisfactory agreement with the available (limited) experimental data. In particular, predicted friction factors and average Nusselt numbers for the investigated range, were round to be compatible with, and form an extension to, the experimental results of previous investigators.