R1336mzz(Z) droplet evaporation mode change by pressure and water condensation

Droplet evaporation in humid environments can significantly affect spray cooling performance, yet how ambient water condensation influences the evaporation of refrigerant droplets remains largely unexplored. This study experimentally and theoretically investigates single droplet evaporation of the refrigerant cis-1,1,1,4,4,4-hexafluoro-2-butene(R1336mzz(Z)), covering a range of ambient temperatures (50–250 °C) and pressures (0.1–2 MPa). At atmospheric pressure (0.1 MPa), the squared-diameter evolution deviates from the classical d ² law and exhibits two distinct linear quasi-steady stages separated by a turning point. This turning point corresponds to the moment when the droplet temperature rises above the water dew point and pronounced water vapor condensation ceases. The condensed water volume reaches approximately 40% of the initial droplet volume. Correspondingly, the droplet temperature passes through three characteristic phases, and its first inflection coincides with the diameter-squared turning point. At elevated pressures (1.0 and 2.0 MPa), only a single linear stage is observed, because higher pressure intensifies convective heating and lowers the refrigerant latent heat, rapidly lifting the droplet temperature above the dew point and thereby suppressing condensation. To validate this mechanism, a two-component evaporation model incorporating water condensation is formulated. The model reproduces the second-stage evaporation rate well, whereas a conventional single-component model tends to overestimate it noticeably. These findings indicate a pressure-regulated evaporation mode transition driven by ambient water condensation, extending the current understanding of droplet evaporation beyond the d ² law and highlighting the importance of water condensation for spray cooling applications in humid environments.