On the Correct Modeling of Flow Characteristics in Double Pipe Heat Exchangers With Inner Dimpled Tube

Abstract

Double pipe heat exchangers (DPHXs), which are made up of two concentric or eccentric ducts, are generally used in industrial applications due to the simplicity of the geometry. Passive heat transfer improvement techniques have been investigated to improve the heat transfer ability of the DPHX by the orientation of flow propagation and changing heat transfer area. Dimpled surfaces are considered as the promising passive heat transfer improvement method because of their low weight, small pressure drop penalty, simple fabrication, and small maintenance costs. Since dimples improve the convective heat transfer by flow reattachment, flow impingement, and upwash flow at the downstream region of the dimples, the correct determination of the flow regime is important for accurate prediction of the heat transfer performance especially in small Re numbers. This numerical study presents a sensitivity analysis on the correct modeling of flow characteristics for the investigation of heat transfer performance of DPHXs working with small Re numbers (Re = 500) in low-temperature applications. The effects of laminar flow and turbulent flow solutions on flow propagation and heat transfer performance have been investigated by developing a transient 3D numerical model. The k - omega model was employed to evaluate the influence of dimples on turbulent flow. A constant temperature (T-c=253.15 K) boundary condition was applied at the inner pipe. Ethylene glycol-water mixture (50:50) enters the annuli at T-i=293.15 K. The influence of the dimpled geometry in annular flow propagation is discussed by considering velocity and temperature distributions at the critical cross-sections of the geometry.