This paper aims to explain the effects of radial solid concentration distributions (RSCDs) [T.A. Ozbelge, A. Beyaz, Dilute solid-liquid upward flows through a vertical annulus in a closed-loop system, Int. J. Multiphase Flow (in press); T.A. Ozbelge, A. Beyaz, Seyreltik Sivi-Kati Karisimlarinin AkiS Ozellikleri, TUBITAK Project INTAG-822, Report No: 196 I 010, Ankara, 1999] on the mechanism of heat transfer enhancement achieved in water-feldspar upward flows [T.A. Ozbelge, S.H. Koker, Int. J. Heat Mass Transfer 39 (1) (1996) 135] through vertical annuli having different aspect ratios (kappa = 0.31, 0.42 and 0.54) at different operating conditions. The increasing trend of local solid concentrations from the inner wall to the outer wall of the annulus [Ozbelge and Beyaz (loc. cit.), Ozbelge et al. (loc. cit.)] is found to be favorable for the enhancement of heat transfer, since the heat transfer between the hot and the cold streams occurs across the inner wall [Ozbelge and Koker (loc. cit.)]. Also, the applicability of the fluid-particle interaction model [C.A. Brandon, D.G. Thomas, in: Proceedings of the Fourth International Heat Transfer Conference, Paris, Paper CT-2.1, 1970] to turbulent liquid-solid flows in the vertical annuli [Ozbelge and Koker (loc. cit.)] is checked here. The location of peak heat transfer enhancement is determined at the dimensionless grouping of d(p)(*) = (d(p)/D-h)(Re-s)(11/16) = 4.2 for water-feldspar slurries flowing turbulently upward in the vertical annuli which is the same value as that obtained in horizontal pipes [T.A. Ozbelge, Int. J. Multiphase Flow 19 (3) (1993) 535], regardless of the geometry. This is in accordance with the interaction model [Brandon and Thomas (loc. cit.); Ozbelge (loc. cit.)]. (C) 2001 Elsevier Science Ltd. All rights reserved.