Delineation of critical mass flux and thermal equilibrium quality: Heat transfer disparity in helically coiled tubes of different orientations under gravity/centrifugal force dominance

Helically coiled tubes (HCTs) are extensively employed in compact heat exchangers and high-parameter energy systems for superior thermal-hydraulic performance. However, orientation effects on boiling heat transfer remain insufficiently understood. This study conducted subcritical water flow boiling experiments in vertical and horizontal helically coiled tubes (VHCT and HHCT) under pressures of 8–14 MPa, mass fluxes of 375–1500 kg⋅m−2⋅s−1, and heat fluxes of 100–300 kW⋅m−2, and analyzed the flow-path-averaged heat transfer coefficient (havg). The results indicate higher havg in HHCT at low thermal equilibrium quality (x), whereas havg in VHCT approaches or exceeds that in HHCT as x increases. This disparity diminishes at high mass flux, with havg differences within 5.51%. To elucidate the observed heat transfer disparities, fluid dynamics analysis identified two critical parameters: the critical mass flux (Gcr), indicating whether differences exist in heat transfer between HCT orientations, and the critical thermal equilibrium quality (xcr), representing the turning point in heat transfer evolution along the tube, thereby defining the flow boiling transition from gravity to centrifugal force (Fc) dominance. Numerical simulations reveal liquid accumulation in the lower section of the HHCT at low mass flux, while increasing the mass flux from 375 to 1200 kg⋅m−2⋅s−1 increases liquid phase Fc by 11.3 times and results in Fc-dominated vapor-liquid phase distribution for both orientations. Further analysis of flow-path-averaged inner wall temperature identified Gcr and xcr. These findings provide a scientific basis for HCT orientation effects on boiling heat transfer and guide arrangement strategies in high-parameter energy systems.