In this work mixed convection in a horizontal channel with the lower wall heated at uniform heat flux is studied numerically. A three dimensional problem is modeled and solved by means of the FLUENT code. The domain is made of a principal channel and two channels with adiabatic walls, one upstream the principal channel and the other downstream. The principal channel is formed by a uniformly heated horizontal wall, a parallel wall located above and two adiabatic vertical side walls. The aim of this paper is to investigate the effect of Reynolds and Rayleigh numbers on thermal and fluid dynamic behavior in mixed convection in a horizontal channel heated from below. The analysis is carried out in transient regime in order to evaluate the thermal and fluid dynamic parameters as functions of the time. The Reynolds and Rayleigh numbers investigated are between 5 and 500 and 1.37×106 and 2.75×106 respectively. The corresponding Richardson number, Ri = Gr/Re2, holds values in the range 7.76 – 1.55 × 105. Wall temperature distributions and profiles along longitudinal and transversal coordinates are reported for different time values. Air velocity and temperature in the principal channel are presented along the longitudinal and transversal sections for some time values.

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