The phase-down of high-GWP hydrofluorocarbons (HFCs) is accelerating the adoption of low-GWP A1 refrigerant blends such as R515B, renewing the need for flow regime evidence to support reliable condenser design in compact enhanced tubes and non-horizontal layouts. This study experimentally investigates R515B flow condensation in helical micro-fin tubes with outer diameters of 4, 5, and 7 mm operated at downward inclinations (β = -30° and −60°), extending earlier horizontal and upward datasets obtained in the same geometry family. Flow patterns were identified in an outlet visualization chamber using high-speed imaging (2000 fps), supported by circumferential wall temperature indicators, over mass fluxes of 50–400 (Formula presented) and outlet vapor qualities of approximately x = 0.1–0.9. Four regimes were consistently observed: wavy-stratified, intermittent, transitional, and annular. A clear inclination sign effect emerges in the gravity-controlled region, where downward orientations stabilize stratified configurations and suppress the instability route toward churn-like structures, while annular flow remains comparatively robust and primarily governed by vapor shear and inertia. The inclination response is diameter dependent, being most pronounced for the 7 mm OD tube and progressively constrained as diameter decreases. Finally, the database is assessed against established flow-map frameworks, showing that modified force-balance representations provide the most transferable separation between gravity and shear controlled regimes across tube diameters and inclination sign, offering a practical basis for regime-aware interpretation of inclined micro-fin condensation. The six additional downward tube-inclination maps complement the horizontal and upward configurations previously available for this geometry family, substantially enlarging the sign-resolved database and extending the usable inclination envelope to cover the full −60° to +60° range.
Experimental flow characterization of R515B condensation in downward inclined micro-fin tubes
Irannezhad, Nima;Diani, Andrea
2026
Abstract
The phase-down of high-GWP hydrofluorocarbons (HFCs) is accelerating the adoption of low-GWP A1 refrigerant blends such as R515B, renewing the need for flow regime evidence to support reliable condenser design in compact enhanced tubes and non-horizontal layouts. This study experimentally investigates R515B flow condensation in helical micro-fin tubes with outer diameters of 4, 5, and 7 mm operated at downward inclinations (β = -30° and −60°), extending earlier horizontal and upward datasets obtained in the same geometry family. Flow patterns were identified in an outlet visualization chamber using high-speed imaging (2000 fps), supported by circumferential wall temperature indicators, over mass fluxes of 50–400 (Formula presented) and outlet vapor qualities of approximately x = 0.1–0.9. Four regimes were consistently observed: wavy-stratified, intermittent, transitional, and annular. A clear inclination sign effect emerges in the gravity-controlled region, where downward orientations stabilize stratified configurations and suppress the instability route toward churn-like structures, while annular flow remains comparatively robust and primarily governed by vapor shear and inertia. The inclination response is diameter dependent, being most pronounced for the 7 mm OD tube and progressively constrained as diameter decreases. Finally, the database is assessed against established flow-map frameworks, showing that modified force-balance representations provide the most transferable separation between gravity and shear controlled regimes across tube diameters and inclination sign, offering a practical basis for regime-aware interpretation of inclined micro-fin condensation. The six additional downward tube-inclination maps complement the horizontal and upward configurations previously available for this geometry family, substantially enlarging the sign-resolved database and extending the usable inclination envelope to cover the full −60° to +60° range.
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