RESUMEN
This study investigated the laminar convective heat transfer and fluid flow of Al2O3 nanofluid in a counter flow double-pipe heat exchanger equipped with overlapped twisted tape inserts in both inner and outer tubes. Two models of the same (co-swirling twisted tapes) and opposite (counter-swirling twisted tapes) angular directions for the stationary twisted tapes were considered. The computational fluid dynamic simulations were conducted through varying the design parameters, including the angular direction of twisted tape inserts, nanofluid volume concentration, and Reynolds number. It was found that inserting the overlapped twisted tapes in the heat exchanger significantly increases the thermal performance as well as the friction factor compared with the plain heat exchanger. The results indicate that models of co-swirling twisted tapes and counter-swirling twisted tapes increase the average Nusselt number by almost 35.2-66.2% and 42.1-68.7% over the Reynolds number ranging 250-1000, respectively. To assess the interplay between heat transfer enhancement and pressure loss penalty, the dimensionless number of performance evaluation criterion was calculated for all the captured configurations. Ultimately, the highest value of performance evaluation criterion is equal to 1.40 and 1.26 at inner and outer tubes at the Reynolds number of 1000 and the volume fraction of 3% in the case of counter-swirling twisted tapes model.
RESUMEN
BACKGROUND AND OBJECTIVE: In most countries, the higher death rates are due to cardiovascular disease and stroke. These problems often derive from irregular blood flow and the circulatory system disorder. METHODS: In this paper, the blood flow is simulated in a created aneurysm in the artery upon using Lattice Boltzmann Method (LBM). Blood is selected as a non-Newtonian fluid which was simulated with power-law model. The lattice Boltzmann results for non-Newtonian fluid flow with power-law model and the curved boundary are compared and validated with previous studies which show a good agreement. In this study, simulations are carried out for two types of aneurysms. For the first aneurysm, three power-law exponents of 0.6, 0.8 and 1.0 at Reynolds number of 100 for three different cases are investigated. RESULTS: The results show that the wall shear stress increases with increasing the power-law exponent. In addition, in the main duct of artery where the velocity is larger, shear stress is lower due to the smaller velocity gradient. For the second Aneurysm, the simulations are done for three Reynolds numbers of 100, 150 and 200, and three Womersley numbers of 4, 12 and 20. The blood flow is pulsating at the inlet such as the real pulsating wave in the blood. Results show that with increasing the Womersley number, the velocity profiles in the middle of the aneurysm are closer at a constant Reynolds number. CONCLUSIONS: With increasing the Reynolds number, the range of vortices and values of velocity and tension grow in the aneurysm.