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Compact sizing of fin-and-tube heat exchangers is quite possible by integrating longitudinal vortex generators with the baseline fins. As geometry of the vortex generators is an important design parameter, the thrust of this three-dimensional numerical investigation is to identify the most favorable geometry of winglet type generators, if it exists. For that purpose, a thermo-hydraulic assessment of all the geometric designs, without any bias in the selection of potential contenders, is obligatory. Although past studies have reported the effect of winglet geometry on the performance augmentation, the selected designs were either biased or too less to draw conclusions. Each winglet design may be identified based on the geometric aspect ratio of the generators, which is varied over a wide range (0.5–1.5) in this study. Multiple favorable positions are considered, for installing the winglets around the tubes, before arriving at the favorable geometric design(s). Although highest thermal augmentation is delivered by the generators bearing smallest aspect ratio, linearized highest thermo-hydraulic augmentation is achieved when the ratio equals 0.85 irrespective of the generators’ position. After ascertaining the promising designs, their applicability is examined over a range of Reynolds number which spans from 2122 to 6367. Later part of the study discusses the change in the thermal and flow characteristics of the heat exchanger, due to variations in the winglet geometry. A study of the flow structures makes it evident that the spread of the longitudinal vortices widens, with eventual dissipation, in the flow direction, which has a discernible effect on the heat flux distribution over the fin surface. The study pertaining to the local changes suggests that the surfaces of both fins as well as the tubes undergo heat transfer augmentations, and the degree of augmentation grows with the decreasing aspect ratio of the vortex generators.

A microchannel with a 90° bend is one of the essential components of a complex microchannel in microdevices. Therefore, a fundamental understanding of flow through a microchannel with bends is required. This paper investigates gaseous flow experimentally and numerically through a microchannel with bends in the slip regime. A microdevice with 20 microchannels of dimensions 393.96 × 1.03 × 25 200 µm3 with double 90° bends has been fabricated using standard silicon microfabrication processes. These channels have a common inlet and outlet plenum, and nitrogen gas is used as the working fluid. The mass flow rate through the microchannel is measured for various pressure ratios ranging between 1.30 and 1.55 at different outlet Knudsen numbers in the slip regime. Three-dimensional simulations are performed for rarefied isothermal flow using Ansys fluent 18.1 with slip boundary conditions on the top and bottom walls. The slip effect is quantified by comparing the measured data of the mass flow rates with the corresponding no-slip boundary conditions. To the authors’ best knowledge, experimental data in the slip regime covering the Knudsen number range 0.0662–0.0805 have not been reported previously. The gas flow was modelled numerically using the first-order model with an appropriate slip coefficient (C 1 = 1.15), and the equivalent straight length per bend in the slip regime was also calculated.