Investigation of the influence of complex geometry of pipelines and heat exchange surfaces on the dynamics and parameters of heat exchange of gas-liquid flows in industrial equipment

Abstract

The article examines how complex geometries of pipelines and heat-transfer surfaces affect the hydrodynamics and heat transfer of two-phase gas-liquid flows in industrial equipment, addressing the high uncertainty of calculations for elbows, helical coils, and finned channels where traditional correlations yield large errors. The methodology combines validated CFD modeling (ANSYS Fluent 2021 R2, VOF for the interfacial boundary, k-w SST for turbulence; geometries: straight pipe, elbow $R/D=1.5$, coil $S/D=2.0$, transverse finning $h/D=0.1$; meshes of 3,5-6,2 million cells, $y+n$; heat flux q const; air-water medium) with a laboratory experiment on a closed loop rig (transparent sections, visualization up to 5000 fps, Coriolis flowmeters $\le0.5\%$, differential pressure transducers up to 50 kPa, 24 K-type thermocouples with $\le0,15^{\circ}C$ error; Nu and computed in MATLAB). It is shown that at $Re\_mix=18000$ and $\beta=50\%$ the mean Nusselt number increases relative to a straight pipe by 65,7% for the elbow, 105, 1% for the coil, and 126,6% for finning, with Nu sensitivity to rising gas content in complex geometries exceeding that in a straight pipe, indicating synergistic enhancement of turbulence. Simultaneously, the friction factor increases by multiples: 3,9x (elbow), 4,78x (coil), and 5,89x (finning) at $\beta=50\%$, necessitating a techno-economic trade-off between compactness and pumping power. For the coil at $\beta=50\%$ increasing Re_mix from 8000 to 40000 raises Nu by 3,23x, while decreases moderately, reflecting growing inertia dominance over viscosity and tempering the rise in pump power at high loads. Local analysis of finning reveals extreme nonuniformity of heat transfer: peak values on the fin's upper face $(\approx2,3\times Nu\_avg)$ and a trough in the recirculation zone immediately behind the fin $(\approx0,42\times Nu\_avg)$, defining critical regions for strength and fouling risk. It is concluded that optimal geometry must be selected multicritically (Nu-λ-PEC): at $\beta=40-60\%$ and $Re>20000$ helical coils provide the best balance between heat-transfer gain and losses, whereas finning is justified under stringent compactness requirements; future work should incorporate phase change and unsteady phenomena to improve reliability and efficiency.