Large Eddy Simulations of three-dimensional turbulent compressible flows in curved duct of rectangular cross-section are presented. Strong secondary flows and turbulent structures are the typical features of this type of flow: these are responsible for most of the heat and mass transfer within the duct. Our goal is here to investigate the influence of the duct aspect ratio on the flow development. The results show that this parameter modifies the flow structures in terms of intensity and localisation. Thus it significantly influences the heat transfers when a higher temperature is imposed on the duct convex wall.