Investigation of the pipe diameter modulation effect and pressure drop prediction for R245fa gas-phase flow based on the Kriging surrogate mode

Abstract

In Organic Rankine Cycle (ORC) systems, the frictional pressure drop along transportation pipelines serves as a critical factor contributing to systemic irreversibility (exergy loss). This study focuses on R245fa as the working fluid and conducts a parametric investigation into its low-pressure gas-phase flow characteristics within a 5-meter straight transportation pipeline, utilizing Computational Fluid Dynamics (CFD) and Kriging surrogate models. By employing the control variable method, the modulation mechanisms of the cross-sectional width D (ranging from 200 mm to 500 mm) on the velocity field distribution, pressure gradient, and energy dissipation are comprehensively explored. The results indicate that as the pipe diameter increases, the working fluid velocity exhibits characteristic logarithmic decay due to the non-linear expansion of the flow cross-sectional area. This, in turn, triggers a quadratic reduction in shear stress, causing the total system pressure drop to decrease significantly from 3.23 Pa to 1.90 Pa, achieving a maximum drag reduction rate of 41.2%. Incorporating high-precision thermophysical property data from NIST REFPROP, the quasi-incompressibility of the low-pressure gas-phase working fluid under these specific operating conditions is theoretically demonstrated. Furthermore, the developed Kriging prediction model demonstrates exceptional robustness across the entire parameter space (R2=0.998), providing essential theoretical support for the lightweight design and real-time energy efficiency evaluation of long-distance pipeline networks in ORC systems.