Geometric parametrization of toe-out type vortex generators for energy-efficient capacity augmentation in finned-tube heat exchangers

Abstract

For compact sizing of finned-tube heat exchangers, improving the gas-side thermal conductance is essential, which is attempted by integrating longitudinal vortex generators. As geometry of the vortex generators is one of the principal design parameters, main thrust of this investigation is to identify their energy-efficient geometric design(s). Since spatial positioning of the generators as well as its attack angle have a strong bearing on the energy-efficiency of the said design, therefore, both are accounted over the entire effective range for a comprehensive and conclusive investigation. For the selection of best geometric designs, regression based phenomenological models are developed, and the selection is conducted based on a detailed thermo-hydraulic trade-off. After identifying the optimal design(s), a study is conducted to assess their robustness by making them perform under varying operating conditions. Although phenomenological models suffice the purpose of design optimization, they do not explain the physics of thermo-hydraulic augmentation. Therefore, a study investigating the effect of geometric variation on the flow and thermal characteristics is also conducted. Since tube wakes in plain finned-tube heat exchangers make a significant heat transfer area virtually unavailable for the stipulated task, a study discussing the effect of generator’s geometry on the thermal management of wake affected surfaces is also reported. It is observed that heat transfer augmentation over the surfaces wetted by the baseline wakes has a dominant role to play whenever energy-efficient compact sizing of the heat exchanger is desirable. For a selected position of generators in the tube aft region, the highest Colburn j-factor augmentation over the wake-affected fin equals 207.1 % at the specified Reynolds number.