Mechanism analysis of multi-peak wall temperature of regasification heat transfer deterioration for supercritical methane in a horizontal tube

Supercritical heat transfer deterioration (HTD) phenomenon in tubes is frequently manifested as the distribution of multi-peak wall temperature along the tube length. Quantifying the HTD characteristics of supercritical methane (S-CH₄) in a horizontal tube is critical for improving thermal control of liquefied natural gas (LNG) vaporizers. In this paper, four different regasification heat transfer modes of S-CH₄ are firstly clarified, and the pseudo-phase distribution and vapor-like film (VLF) variations under different modes are also identified. Moreover, the mechanism of multi-peak HTD is deeply unveiled based on the progressive pseudo-boiling theory and virtual orifice contraction effect. Results demonstrate that one normal heat transfer and three HTD (single-peak, double-peak and triple-peak) modes are observed. The thickness of VLF (δ_VLF) near the top generatrix is much greater than that at the bottom generatrix, the former (0.505 mm) is 14 times that of the latter (0.035 mm) on the typical cross-section under the single-peak HTD mode. When HTD occurs, the expanding VLF will exert a virtual orifice on the core liquid-like flow. The alternating dominance of evaporation momentum force and inertia force mainly causes periodic δ_VLF. The average δ_VLF,bot decreases by 62.19% when mass flux increases from 370 kg/m²·s to 650 kg/m²·s, which attributes to the variations of thermal conductivity and specific heat of VLF. Rising pressure can yield smaller supercritical K number and thinner quantitative δ_VLF. The average δ_VLF,bot decreases 73.91% when pressure increases from 6.93 MPa to 12.5 MPa. Finally, novel assessment system and pattern maps of multi-peak HTD for S-CH₄ are established. Two new dimensionless correlations are proposed to predict the multi-peak wall temperature position and magnitude of S-CH₄ with the mean absolute relative deviations of 16.98% and 6.56%. The present findings not only benefit the understanding of supercritical heat transfer, but also provide crucial reference insights for thermal design and operational safety standards of LNG vaporizers or other related engineering equipment.