Application of Dynamic Bubble Departure Model for New, Low-GWP Refrigerants on Enhanced, Structured Surfaces

Structured surfaces consisting of sub-surface tunnels and openings in the form of pores or gaps are used to enhance boiling heat transfer resulting into compact heat exchangers. One of the applications of enhanced surface tubes is in flooded evaporators in water chillers. The fundamental mechanisms in nucleate boiling on structured surfaces are not still well understood, especially for new, low-GWP refrigerants. In this study, the focus is on bubble departure models. Most of the nucleate boiling models consider the static force model for calculating bubble diameter at the departure. However as per flow visualization studies in published literatures, the process of bubble growth and departure is dynamic and hence three more forces (in addition to buoyancy and surface tension) need to be accounted for while calculating the instantaneous bubble departure diameter. In this study, numerical results are presented for bubble departure diameter for four refrigerants, viz. R134a (the currently used, high GWP refrigerant) and its targeted low-GWP replacements, viz. R1234ze (E), R513A and R450A on enhanced, structured surfaces. Results from the dynamic force model show the bubble departure diameter in the range of 0.78 mm to 0.85 mm for all the four refrigerants. The unsteady growth force ranges from 4.8 × 10⁻⁶ N to 1.35 × 10⁻⁵ N while the surface tension force ranges from 2.49 × 10⁻⁶ N to 1.975 × 10⁻⁶ N. Similar results are provided for other forces as a function of wall superheat.