Flow and heat transfer characteristics of air and n-decane in eccentric tube-in-tube helically coiled heat exchangers

Present work proposed an eccentric tube-in-tube helically coiled heat exchanger (TTHC), used in Aero engine turbine film cooling which can precool compressed air more efficiently. The supercritical n-decane (3 MPa) at inner tube is used to precool the compressed air at annulus channel. The effects of mass flow rate, eccentric angle (phi) and eccentric ratio (e) of the inner tube on flow and heat transfer performance are investigated systematically by the RNG k - epsilon numerical model. The results showed that the Nusselt number (Nu(a)) of concentric and eccentric TTHC (phi = 0 degrees, e = 0.4) was 1.15 and 1.19 times of that for straight concentric tube-in-tube heat exchanger, respectively. The synergy effect at annulus side for the eccentric TTHC is more strengthened than the concentric one. When eccentric angle (phi) increased from 0 degrees to 180 degrees (e = 0.4), the Nua and f(a) decreased by 20% and 3%, and the performance evaluation factor (PEFa) decreased 10%. Moreover, when eccentric ratio (e) increased from 0 to 0.8 (phi = 0 degrees), the Nu(a) and the PEFa increased by 10.7% and 14%. Therefore, the optimal geometric parameters in eccentric TTHC can be selected as cp = 0 degrees, e = 0.8.