The aim of the present study is to investigate thermoconvective instabilities in a mixed convection phenomenon. The flow of an incompressible fluid (water) in a horizontal rectangular duct, uniformly heated from below (Poiseuille-Benard flow) was considered. Many experiments were conducted for a wide range of fluid velocities (Reynolds number) and heat fluxes supplied to the wall (Richardson number) in order to describe and to analyse these instabilities. Experimental results enabled us to distinguish several regimes of flow and particularly the domains of thermoconvective instability. Using fluid temperature measurement, a stability diagram was established in a [Re, Re Ri] plane; this diagram shows the various regimes. In parallel, a direct numerical simulation, which solves the Navier-Stokes equations coupled with energy equation, was carried out. The computations helped us to identify and thus better understand the possible basic mechanisms leading to the quite complex fluid flow observed, which results in the combination of the forced fluid flow and the buoyancy-induced flow. In the range of parameters under consideration, the fluid flow structure is mainly composed of two longitudinal rolls, which interact dynamically once a threshold has been reached.