Numerical Analysis of Heat Transfer Inhomogeneity of Supercritical Pressure Carbon Dioxide in Horizontal Minichannels

To prevent local overshoot of heat exchanger wall temperature in the Brayton cycle system, this study numerically investigated the heat transfer inhomogeneity of turbulent mixed convection of SCO₂ inside flow passages with different cross-sections of circular, semicircular, and rectangular cross-sections. The operating parameters corresponded to the low-pressure side of a heat exchanger (p = 8 MPa, T_in = 303 K, q/G = 41.67–250 J·kg⁻¹). It was found that the variation of density with temperature led to the secondary flow in the cross-sections. The heat transfer inhomogeneity emerged with the development of secondary flow, and increased during the peaks of isobaric specific heat capacity (c_p) and thermal conductivity (λ) moved from the buffer layer to the logarithmic-law layer. When the peaks of c_p and λ converged in the logarithmic-law layer, the heat transfer inhomogeneity decreased. In a circular tube, the heat transfer inhomogeneity can be ignored when q/G = 41.67 (heat transfer enhancement), but when q/G ⩾ 125 (heat transfer deterioration) the heat transfer inhomogeneity cannot be ignored. Under the same q/G condition, the intensity of secondary flow was inversely proportional to G, so the heat transfer inhomogeneity decreased with increasing G. Affected by channel geometry, the variation of maximum wall temperature difference (ΔT_max) coincided with the wall temperature inhomogeneity (δ_T) for a circular channel, but not for semicircular and rectangular channels. ΔT_max was the largest for a semicircular channel while δ_T was the largest for a rectangular channel under identical operating conditions.