Determining of the thermo-hydraulic characteristics and exergy analysis of a triple helical tube with inner twisted tube

Abstract

An innovative technique to enhance heat transfer as a passive method was investigated. The combination of the twisted tube and helical coil to increase the swirl intensity is presented. The current investigation showed experimentally and numerically the exergy and thermal performance analysis of a new design of a triple tube design called a triple helical tube with inner twisted tube, THTITT. The new design is a modified design of a double helical tube with inner twisted tube, DHTITT which is created by twisting the inner tube. Besides the benefits of adding the third fluid to DHTITT that achieve extra contact surface area per unit length between the intermediate tube and the new passage in the outer tube. In which expected to enhance the temperature gradient between the three fluids and consequently, the thermal characteristics augment occurred. The twisted tube increased the intensity of the swirl flow and more intense disturbance of the fluid in the tube occurred. A 3d CFD model was established to get more insight at a level of detail not always available in the experiment. The effects of twisted pitch ratio, ξ hydraulic diameter, ω, helical coil torsion, α, helical coil inclination angle, φ, as well as Dean number, N_{Dn, h} were explored. Four groups of test specimens include various ξ of 5.32, 7.97, and ∞, various ω of 3.8, 6.8, and 9.9 mm, various α of 0.068, 0.095, and 0.121, and various φ of 0°, 45°, and 90° were established, manufactured and examined in this investigation. The investigation covered Reynolds number, N_Re,h, for a range of 2450:29300 corresponding to Dean number, N_{Dn, h}, for a range of 570:4800. The results showed a higher Nusselt number, N_{Nu, h}, of THTITT compared to DHTITT by 61.8 %%, while the increase in f_h is approximately negligible. Also, by decreasing ξ from ∞ to 5.32 increases N_{Nu, h} by 33.4 %, with an increase in friction factor, f_h by 57.6 %. Furthermore, with a decreasing ω from 9.9 mm to 3.8 mm, a significant increase in N_{Nu, h} occurs by 20.6 %, at the expense of increasing f_h by 36.4 %. In addition, with decreasing α from 0.121 to 0.068, a significant increase in N_{Nu, h} occurs by 27.6 %, at the expense of increasing f_h by 17.1 %. Finally, the THTITT decreases the heat loss (exergy destruction) between the hot and cold fluids. New correlations to predict N_Nu,h, and f_h are presented.