Abstract
The thermal-hydraulic performance in a novel annular tube formed by outer straight and inner twisted oval tubes is numerically investigated. An annular tube formed by two straight oval tubes is also studied for comparison. Inner twisted oval tubes with different aspect ratios and twist ratios are studied. The heat transfer is well improved by the symmetrical secondary flow in the annulus. The Nusselt number generally increases when the inner oval tube becomes flatter and the twists stronger in the studied range of geometrical parameters. The largest Nusselt number Nu of the inner twisted tube increases by 116% while the friction factor f increases by only 46% compared with that of the inner straight tube, and the largest value of the thermal performance factor (JF) can be up to 1.9. Correlations of the Nusselt number and friction factor are proposed for laminar and turbulent flows, and the deviations of the correlations are within ±5% and ±4% for Nu and f, respectively.
Highlights
The double-pipe heat exchanger (DPHE) is widely applied in numerous energy fields due to its simplicity, convenient cleaning and wide range of applications [1]
Magnetic fields [3,4,5], need external power. The passive techniques, such as tube inserts [6,7], turbulators [8], vortex generators [9,10,11] and special shaped tubes [12,13,14], can induce strong secondary flow, which has been proven to be beneficial to heat transfer [15,16,17,18,19]
The results showed that the Nu increases as Reynold’s number (Re) and Pr increase, and decreases as the twist ratio increases
Summary
The double-pipe heat exchanger (DPHE) is widely applied in numerous energy fields due to its simplicity, convenient cleaning and wide range of applications [1]. Magnetic fields [3,4,5], need external power The passive techniques, such as tube inserts [6,7], turbulators [8], vortex generators [9,10,11] and special shaped tubes [12,13,14], can induce strong secondary flow, which has been proven to be beneficial to heat transfer [15,16,17,18,19]. Wang et al [27] numerically studied a DPHE by employing an outward helical corrugated tube as the inner tube They found that secondary and swirling flows are generated on the sides of the tube and shell, and the optimum geometric parameter is obtained at the shell diameter of 38 mm. The results have significance for the design of DPHEs for the purpose of high thermal performance
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