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| Summary: Being used in the nuclear reactors, gas turbines and drilling systems in the petrolium and natural gas industries, the heat transfer characteristics of heat exchangers are very important key in the performans analysis of that systems . there are many different ways, passive or active, to improve the flow and heat transfer characteristics of the heat exchangers. One of the ways is to use a variable cross-section duct. In this study, laminar flow with steady, incompressible, axisymetric and constant physical properties in concentric annular ducts having variable cross-sectional has been numerically investigated.Flow and heat transfer characteristics in concentric annular ducts for different entrance velocities; uniform and parabolic profiles have been modelled with finite elemnet method. The entrance velocity profiles, uniform and parabolic represent developing and developed flows in the annular duct,respectivelly. The velocity profiles acquiried along the duct have been compared with the analytical results. There are three geometric models which have been implemented in the applications. These geometries have same inlet diameter ratios, (d*g =0.348), but different outlet diameter ratios ,(d*ç =0.465, 0.581, 0.697), respectively. As the inner element of annular ducts expands conically,the inlet hydraulic diameter has been used in the Reynolds number. The Reynolds number used in the calculations was varied from 65 to 1950. According to inlet velocity profiles and constant temperature boundary condition;Ti=333 K and Tw=298 K velocity profiles, temperature profiles, local and average Nusselt number, wall shear stres and pressure in the duct are calculated with different Reynolds number and outlet diameter ratios.Key Words: Laminar Flow, Annular Ducts with Varying Cross-Sectional, Finite Element Method, Constant Wall Temperature, Concentric
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