This paper presents Large Eddy Simulations of a compressible turbulent flow through a duct of square cross-section with a localized heat source on its bottom wall. The potential applications are related to the accidental heat release in a road tunnel. A constant temperature TW is imposed on all duct walls except for the spot location. Two distinct heating configurations are considered. In the first configuration, the distribution of the spot temperature is uniform and varies suddenly from TW to 2TW at the spot borders. Two additional temperature levels are also considered for this configuration: Th /TW = 3 and 4. In the second configuration, the temperature changes linearly from TW to 2 TW at the spot centre. In both cases, the Reynolds number based on bulk velocity was maintained at 6000 and the Mach number at 0.5. Numerical experiments were conducted to investigate the spatial growth of the thermal field downstream of the spot and its influence on the velocity field. It was found that the heating on the lower wall induced a clear intensification of the secondary flow with a strong reduction in size of these vortices near the heated zone. In particular, a strong impinging motion has been observed just downstream of the spot.