ABSTRACT A parametric study was carried out to predict the turbulent Prandtl number for the two-phase gas-solid dilute suspension flow in pipes. The effect of the presence of solid particles with different sizes at different solid-loading ratios (solid to gas mass flow rate ratio) on both the thermal eddy diffusivity and the heat transfer coefficient in a turbulent flow of air through a horizontal tube was investigated. Experiments were conducted using a test-section of a brass tube (40 mm inner diameter and 3 m long) heated with a uniform heat flux. Sand particles with three different average-sizes of 200, 120 and 30 microns were suspended into a turbulent stream of air within a range of Reynolds number from 20000 up to 80000. The solid-loading ratio was varied from 0.0 (case of clean-air flow) up to 1.0. In the analytical part of the present work, a parametric study based on a two-equation model using the suspension-flow thermal eddy diffusivity concept was performed. The energy equations for both gaseous and particulate phase were numerically solved by using the finite difference technique. New correlations for the suspension-flow turbulent Prandtl number and the asymptotic Nusselt number were obtained as functions of Reynolds number, solid-loading ratio and particle to tube diameter ratio.