In the past, fin-and tube heat exchanger (FTHE) tube pattern ratios have been largely based on ad-hoc design principles. Here, we investigate the optimal tube arrangements for a FTHE with plain fins in marine environments represented by two different air types; one for unfiltered air with high condensation rate and one for clean dry filtered air conditions. The thermal-hydraulic efficiency of the FTHE design is measured by comparing a modified ratio of Colburn j-factor and Fanning friction factor. The regression model generated from the CFD data is then used to identify the maximum efficiency for two design specific fin pitches separately. We identified two optimal tube patterns: one for a large fin pitch for unfiltered air, and another for a small fin pitch for filtered air. Manufacturing restrictions were found to significantly limit the maximum achievable efficiency of a tube pattern. By neglecting the related manufacturing restrictions, 4% higher efficiency for a fin pitch of 1.5 mm and 23% higher efficiency for a fin pitch of 3.5 mm is achieved. Without any application specific limitations or manufacturing restrictions the fin pitch 1.5 mm can have a 36% increased efficiency than fin pitch 3.5 mm. These novel results show that development in manufacturing have potential for significant improvements in thermal-hydraulic efficiency.

Polymer additives are commonly utilized to manipulate bubbly flows in various applications. Here we investigate the effects of clean and contaminated bubbles driven upward (upflow) in Newtonian and viscoelastic turbulent channel flows. Interface-resolved direct numerical simulations are performed to examine sole and combined effects of soluble surfactant and viscoelasticity using an efficient three-dimensional finite-difference-front-tracking method. The incompressible flow equations are solved fully coupled with the FENE-P viscoelastic model and the equations governing interfacial and bulk surfactant concentrations. The latter coupling is accomplished by a nonlinear equation of state that relates the surface tension to the surfactant concentration. For Newtonian turbulent bubbly flows, the effects of Triton X-100 and 1-pentanol surfactant are examined. It is observed that the sorption kinetics highly affect the dynamics of bubbly flow. A minute amount of Triton X-100 is found to be sufficient to prevent the formation of bubble clusters restoring the single-phase behavior while even two orders of magnitude more 1-pentanol surfactant is not adequate to prevent the formation of layers. For viscoelastic turbulent flows, it is found that the viscoelasticity promotes formation of the bubble wall-layers and thus the polymer drag reduction is completely lost for the surfactant-free bubbly flows, while the addition of small amount of surfactant (Triton X-100) in this system restores the polymer drag reduction resulting in 25% drag reduction for the Wi=4 case.