© 2018 Elsevier Masson SAS The behavior of the flow through a straight square duct is examined using Large-Eddy Simulations. Both isothermal and non-isothermal conditions were considered, the latter generated with either the wall temperature or the wall heat flux held constant. Fully-developed conditions were attained by initializing the velocity field with perturbed streamwise streaks, and by employing a method for efficiently applying cyclic boundary conditions to the velocity field while minimizing aliasing errors. Results were obtained for two values of bulk Reynolds numbers, Reb=6,000 and 10,000. The numerical accuracy was checked via several alternative methods that included performing computations on grids with different resolutions, both with and without sub-grid scale models. The results were used to test some of the assumption underlying the use of RANS approaches to predict the flow and thermal fields. Of particular interest was the examination of the effects of the turbulence-driven secondary motions on the near-wall processes, especially on existence and extent of the thermal logarithmic law of the wall, and on departures from the Reynolds analogy. Their effect on the turbulent Prandtl number was quantified, and the implications on the use of Fourier's law to relate the turbulent heat fluxes to the gradients on mean temperature are discussed.