Investigations on the heat transfer enhancement of converging minitubes for methane condensation

Liquid methane is a critical cryogenic fluid that is accelerating into widespread applications. Since the heat transfer process is continuously weakened during methane liquefaction, it is significant to conduct studies on heat transfer enhancement methods. By applying the VOF model, this study numerically simulates a designed converging minitube, investigates the mechanism of heat transfer enhancement intensively, and compares results with those of two straight minitubes with different diameters. An in-depth analysis combining several aspects of dimensionless velocity, turbulence intensity, and liquid film thickness is provided. It is found that the condensation heat transfer coefficient of the converging minitube decreases and then increases with declining vapor quality and shows a remarkable heat transfer enhancement compared to straight minitubes. The overall heat transfer efficiency of the converging tube is improved to a maximum of 94.72 % compared to the straight tube (D=1 mm, G=400 kg m⁻² s⁻¹). The turbulent intensity is mainly responsible for the heat transfer enhancement in the converging tube. For straight tubes, the combined effect of liquid film thickness and turbulent kinetic energy is required to determine the ultimate heat transfer performance, especially at low mass flux. The impact of flow directions is also discussed in the context of practical application scenarios. It is revealed that gravity could enhance heat transfer under specific conditions, enabling the horizontal arrangement to demonstrate better heat transfer performance. This research reveals the characteristics and mechanism of the condensation heat transfer enhancement of methane in the converging minitube, thus providing guidance for practical applications.