A CFD-based investigation of flow boiling of R32 in helical pipe evaporator: Parametric effects, phase-change behavior, and thermohydraulic benchmarking

Helical pipe evaporators (HPEs), commonly used in compact thermal devices, enhance boiling heat transfer through secondary flow effects from their helical shape. For further improvements in heat transfer performance, systematic study of the individual influence of key operating (mass flux and wall temperature) and geometrical (pipe diameter, coil diameter, and helical length) parameters on the R32 multiphase flow boiling under consistent conditions is necessary, as prior investigations, largely limited to other refrigerants such as R134a and R1234yf, covered only few parameters under inconsistent conditions, leaving their relative effects unclear. Moreover, the influence of these parameters on vapor-fraction distribution and phase-change processes in HPE is poorly understood. To address these issues, this study focuses on multiphase flow boiling in R32 in HPEs and provides a detailed numerical analysis of local and average HTC, ΔP, vapor fraction distribution, and phase change behavior. After gradually decreasing along the inlet straight section, HTC peaks at the first helical turn due to developing secondary flows, then decreases downstream as vapor fraction increases and mixing weakens. Higher wall temperatures shift the HTC peak to higher values of vapor fraction, while low mass flux allows quicker vaporization due to higher residence time. Increasing pipe diameter has a minor effect on phase change, coil diameter slightly enhances vaporization, while helical length has negligible impact. A CFD-informed Nu correlation is developed for interpolation within the investigated operating range. Finally, HPE is compared against straight, U-bend and multiple U-bend evaporators, showing the highest HTC improvement, along with larger ΔP.