Heat transfer enhancement of twisted tape inserts in supercritical carbon dioxide flow conditions based on CFD and vortex kinematics

Abstract

Twisted tape inserts (TTI) are often employed to enhance heat transfer in tubes but their enhancement for supercritical carbon dioxide (SCO 2 ) is unknown, and the thermal performance of TTIs is never studied by vortex kinematics. The TTIs with three twist ratios (TR) were designed for an experimental SCO 2 water-cooled counter-flow tube-in-tube heat exchanger, and the convective heat transfer of SCO 2 in the exchangers with the TTIs was simulated with ANSYS CFX based on the three-dimensional, Reynolds-averaged Navier-Stokes equations, energy equation and shear stress transport turbulence model. Effects of TR, mass flux, inlet pressure and wall heat flux on the performance were clarified. The optimal TRs were sought by using the performance evaluation criterion curve and criterion of friction factor ratio ≤5. The thermal performance was characterised by using cross-sectional average absolute helicity. The friction factor ratio, Nusselt number ratio and performance evaluation criterion vary in the ranges of 3.63–7.29, 3.43–5.75 and 1.90–2.94 at the pseudocritical point, inlet pressure of 8, 9 MPa, mass flux of 200, 400 kg/m 2 s and wall heat flux of 12, 24 kW/m 2 as TR = 2.17, 3.78, 5.39. The best heat transfer enhancement caused by TTIs appears at the point with 2–3 times better than that in water or air flow. The enhancement reduces dramatically on the left- and right-hand sides of the point. The increased mass flux and inlet pressure can reduce but a large wall flux can raise the enhancement on both sides of the point. The optimal TR depends on SCO 2 operational conditions and TR = 3.78 is the optimal TR for most cases herein.