Optimization of in-tube enhancement for large evaporators and condensers

Introducing roughness in tubes increases the turbulent heat-transfer coefficient but there is usually a much larger increase in friction factor for constant geometry and flow conditions. In this study, generalized rib-tube correlations are used in optimizing the roughness geometry for two different objective functions or performance evaluation criteria (PEC). Considered are the maximum heat transfer for given pumping power and surface area (R-3) and the minimum surface area for constant heat load and pumping power (R-5). Only tube-side flow was considered. The PEC, with their associated constraint equations, were evaluated by using a simple iterative procedure over a wide range of parameters (Re = 10,000–100,000, Pr = 0.7–35, e/d = 0.01–0.1, p/d = 0.1–1.0, and α/90 = 0–1.0). The optimum rib geometry had e/d = 0.02, p/d = 0.1, and α/90 = 1.0. The Reynolds and Prandtl numbers had little influence on heat-transfer augmentation. The improvement in the effectiveness due to the ribbed tubes has been illustrated for different tube and flow variables to aid in the preliminary design of enhanced heat exchangers.