Heat exchangers are critical components in thermal systems used across power generation, refrigeration, and process industries. This paper presents a Computational Fluid Dynamics (CFD) investigation of a concentric double pipe heat exchanger, carried out to study its thermal and flow behaviour under steady-state conditions. A three-dimensional model of the exchanger, comprising an inner tube (34 mm ID / 42 mm OD) and an outer annulus (66 mm ID / 70 mm OD) of length 1250-1400 mm, was developed in SolidWorks and discretised using a structured hexahedral mesh in ANSYS Workbench. The governing Navier-Stokes and energy equations were solved using a finite volume based solver (ANSYS Fluent) with the standard k-ε turbulence model, with hot water flowing through the inner tube and cold water through the annulus. Boundary conditions included specified inlet velocities and temperatures for both fluids, a pressure outlet, no-slip walls, and an adiabatic outer surface. The temperature contour obtained from the simulation shows a clear thermal gradient developing along the length of the exchanger, with the hot fluid cooling from inlet to outlet as heat is transferred across the tube wall to the cold annular stream, consistent with expected double pipe heat exchanger behaviour. Design relations for heat transfer rate, effectiveness, the NTU method, and frictional pressure drop are also presented to support interpretation of the flow field, together with an extended discussion of how each governing parameter ( tube diameters, length, mass flow rate, fluid properties, and turbulence intensity) individually influences the resulting thermal performance. The results confirm that CFD is a practical and cost-effective tool for visualising internal temperature and flow distributions in a heat exchanger prior to physical fabrication, and they lay the groundwork for a full comparative study of parallel and counter flow configurations and experimental validation in the next phase of this work.

Key Words: Heat Exchanger, Computational Fluid Dynamics, Double Pipe, ANSYS Fluent, k-ε Turbulence Model, Counter Flow, Parallel Flow.