Numerical simulation of heat transfer enhancement in fin and tube compact heat exchanger with longitudinal vortex generator

Abstract

Over the last decades several studies have searched for improved Tube and Fin Heat Exchanger (TFHE) designs capable of providing the best thermohydraulic performance at the lowest possible cost. Such studies have proven relevant due to the various domestic and industrial applications of TFHEs. In order to enhance the heat transfer rate, they emphasize the use of passive methods involving the implementation of different types of Vortex Generators (VGs) and tubes, as well as their arrangements and parameters.

Every research has suggested at least one new configuration for TFHE; therefore, the number of available studies is large. In fact, each proposed configuration is composed of different parameters and arrangements for Heat Exchangers (HEs), tubes and VGs. This is why they cannot be compared. The present study aims at quantifying and comparing the thermohydraulic performance of HE configurations that were recognized as successful at enhancing heat transfer in previous research. Six new designs were proposed, all of them with identical parameters so they could be compared. Each design presented a more complex configuration than the previous one. The first case consisted of an in-line circular tube arrangement and the last one was a staggered oval tube with two pairs of Delta Winglet Vortex Generators (DWVGs) in CFU-CFD orientation.

Designing a validation model was key to achieving the objectives of this study and ensuring an appropriate procedure leading to appropriate results. For achieving this purpose, the heat transfer coefficient and friction factor were compared with a previous study. Once the model was validated and the grid independence was achieved, six different designs based on previous research were proposed. These TFHEs configurations were simulated and their flow behaviours were analysed.

Their performance was compared based on the Nusselt number and the friction factors obtained. Every geometry showed a better performance than the preceding design. Superior performances were achieved with lower Reynolds and the best performance was observed in the HE with staggered oval tubes and two pairs of DWVGs in CFU-CFD orientation (Case 5), the last configuration proposed. This configuration in particular enabled a 90% increase of the thermal performance factor when compared with the base case.

Every new design had a better performance than the previous one: staggered tubes improved the flow mix in dead water zones, oval tubes presented a smaller stagnation zone and DWVGs interrupted boundary layers. The sum of all of these implies a better performance.