A depth understanding of fluid flow past a curved duct having rectangular cross-section with different aspect ratios (l) are essential for various engineering applications such as in chemical, mechanical, bio-mechanical and bio-medical engineering. So highly ambitious researchers have given significant attention to study new characteristics of fluid flow in a curved duct. The flow characterization in the rectangular duct has been studied over a wide range of numerical and selective experimental studies. However, proper knowledge with the effects of Coriolis force for different aspect ratios is important for better understanding of the transitional behaviour and the subsequent heat generation, which is required to improve further. The purpose of this study is to reveal insight into the transitional flow pattern and heat transfer in a curved rectangular domain. The Navier-Stokes equations are solved using the spectral method, while the Crank-Nicolson method is used to solve the energy equation. An in-house FORTRAN code is developed to get the numerical solution. For post-processing purposes, Tecplot-360 and Ghost-script tools are used. The present study exposes development of Dean vortices that affect heat generation as well as thermal enhancement in the flow with underlying the flow controlling parameters, the Dean number (Dn), the Grashof number (Gr) and the Taylor number (Tr). Time-dependent results followed by phase spaces show that transient flow undergoes in the scenario ‘chaotic → multi-periodic→ periodic→ steady-state’ generating 2- to 8- vortices for the periodic/multi-periodic flow at 2000 ≤ Tr ≤ 2205 for l = 2, whereas similar sort of flow is observed in the range of 3100 ≤ Tr ≤ 3195 for l = 3. More complicated 4- to 13-vortex solutions are obtained for the chaotic flow regime at l = 2 in the range of 0 ≤ Tr < 2200 and at l = 3 in the range of 0 ≤ Tr < 3100. The chaotic flow that occurs at the certain range of Tr proficiently intensifies the heat transfer than the unperturbed, periodic or multi-periodic flow. The overall investigation reveals that in the rotating duct, the temperature-influenced buoyancy compulsion and centrifugal-coriolis joint forces are dominant, influencing the characteristic of the fluid and thus optimizing the transfer of heat. The present investigation will contribute to enhancing the understanding of fluid flow and heat transfer of internal heating/cooling/gas turbines, electric generators, biological systems, and some separation processes.