Condensation and boiling in the entrance region of horizontal and inclined smooth tubes

Abstract

This manuscript presents the results of an experimental investigation into ammonia condensation and boiling in the entrance region of horizontal and inclined smooth tubes with an inner diameter (ID) of 11 mm. The study was conducted under saturation temperatures ranging from 55 °C to 65 °C, mass velocities between 60 and 120 kg m⁻² s⁻¹, and vapour qualities from 0.1 to 0.7. Two-phase flow pattern transitions were also examined using a transparent tube of ID 7.5 mm, equipped with ID 2 mm throttling orifice. Perturbation lengths were quantitatively evaluated for saturation temperatures between 35 °C and 55 °C, mass velocities of 80–160 kg m⁻² s⁻¹, and vapour qualities from 0.1 to 0.7. The results indicate that perturbation length is primarily determined by the initial two-phase flow pattern. The perturbation length remains unaffected by variations in saturation temperature, mass velocity, or vapour quality within the given flow pattern. In horizontal flow, the perturbation length does not exceed 23 hydraulic diameters (D) for stratified-wavy flows, and 15D for annular and annular-wavy flows. For vertical upflow, plug and churn flow regimes exhibit perturbation lengths of up to 24D, whereas annular and annular-wavy patterns remain stable beyond 20D. An L-junction does not significantly affect the flow pattern beyond a distance of 12D. In the entrance region, average condensation heat transfer coefficients (HTCs) are generally lower than those in the fully developed region at low mass velocities (<120 kg m⁻² s⁻¹) and vapour qualities (<0.7). For boiling, performance in the entrance region is comparable to that in the fully developed region at low mass velocities. However, at higher mass velocities (120 kg m⁻² s⁻¹), a slight reduction in boiling HTCs—approximately 10 %—was observed, which can be explained by the suppression of the nucleate boiling mechanism.