Numerical simulation of fluid flow and heat transfer of two-phase slug flow in inclined pipes

Gas-liquid two-phase slug flow is characterized by significant stochasticity and uncertainty while widely used in practical engineering applications. However, for inclined downward pipes, the fluid flow and fluid heat transfer mechanisms deserve further investigation. Therefore, in this study, hydrodynamic models and heat transfer models applicable to inclined pipes are considered together. First, a hydrodynamic model of a two-phase slug flow in an inclined regular-size channel was created using the slug cell method. Each slug cell was assumed to consist of a liquid slug and a Taylor bubble region. In addition, variables such as two-phase pressure drop and overall void fraction were used to derive the local heat transfer coefficients of the liquid plug and Taylor bubble regions and thus to integrate the total heat transfer coefficient. Experimental verification confirms that the model accurately predicts the heat transfer coefficients. Finally, the heat transfer performance of slug flow was analyzed and synthesized by six characteristic parameters. The results indicate that under laminar flow, the enhancement of heat transfer through pipe inclination was more important. The apparent gas velocity and the two-phase heat transfer multiplier show an approximate quadratic relationship in the small inclination (0°∼40°) and low Reynolds number intervals ( Re : 124∼332). In terms of heat transmission, the two-phase slug flow outperformed the liquid single-phase flow. Additionally, compared to the thin thermal boundary layer with large inclination, the heat transfer coefficient of thick thermal boundary layer with low inclination is only 37%-56%. Pipe inclination promotes the heat transfer coefficient better than increasing the gas-phase velocity alone. The maximum increase in the two-phase heat transfer coefficient due to an increase in inclination was 3520 W/(m·K), while the maximum increase in two-phase heat transfer coefficient due to increase in superficial gas Reynolds number is 1050 W/(m·K). The optimum pipe inclination to maximize the heat transfer coefficient and minimize the pressure loss is in the range of (50°∼60°).